gold ball mill for closed circuit grinding station

5 gold extraction methods to improve your recovery rate | fote machinery

5 gold extraction methods to improve your recovery rate | fote machinery

The crushing and screening stage in the industry is mainly composed of three-stage and a closed-circuit process. Gold ores need to go through coarse, medium, and fine crushing processes to be minimized into smaller pieces. The screening equipment is used to sieving the smaller gold ores into the proper size for the next steps.

The grinding operation usually adopts one or two ball mills with types of lattice and overflow. The second stage grinding operation forms a closed circuit with a spiral classifier or a hydro cyclone to ensure the grinding fineness.

Since traditional ball milling equipment appears some shortcomings such as fast wear and large energy consumption, many manufacturers adopt new wear-resisting rubber lining boards, sliding bearing to improve a mill operation efficiency and prolong a machine's service life.

The beneficiation stage is a crucial part of gold extraction during the whole gold ore processing plant. Placer gold mine and rock gold mine are most widely processed to extract gold concentration.

The gold slurry process of the carbon slurry method (CIP and CIL) is to put activated carbon into cyanide ore slurry, adsorb dissolved gold on activated carbon, and finally to extract gold from activated carbon.

Equipment required for carbon slurry gold mining process: Leaching mixing tank, activated carbon screen, Two-layer (three-layer) washing and thickening machine, fast desorption electrolysis system with high-efficiency and low-consumption, high-frequency dewatering screen.

It means that by ion exchange resin, gold also can be extracted from ore pulp. Like carbon, the process makes gold absorbed onto solid spherical polystyrene resin beads instead of activated carbon grains.

According to different physical and chemical properties of different types of gold ores, flotation separation utilizes various reagents to make the gold attached to the bubbles then scraping these gold particles from blades to get the concentrate.

A jigger is one of the main pieces of equipment in the gravity separation process. The jigging process mixes gold ore particles of different specific gravity together, then stratifying these particles. The minerals with small specific gravity will be on the upper layer while the minerals with large specific gravity will be on the lower layer.

A shaking table is used to process gold ores in the horizontal medium flow. The motor drives the surface of the shaker to perform the longitudinal reciprocating motion, as well as the differential motion of the washing stream and the surface of the bed. Gold ore particles are stratified perpendicular to the surface of the bed, then being separated parallel to the surface of the bed in reciprocating motion which allows gold ores with different particle sizes to be discharged from different parts to achieve separation.

It adopts lope water flow to achieve separation. With the effect of the combined force of water flow, mineral gravity, the friction created by the bottom of the tank, and ore particles, the gold ore particles will settle in different areas of the tank. The ore particles with small specific gravity will flow away with the water, while ore particles with larger specific gravity would stay.

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Henan Fote Heavy Machinery Co., Ltd. (FTM) has more than 40-year experience in the design of gold mining equipment processes. Its beneficiation equipment and plants sales to many countries including Tanzania, India, South Africa, the United Kingdom and other regions. According to the actual needs of customers, all machines can be customized here.

As a leading mining machinery manufacturer and exporter in China, we are always here to provide you with high quality products and better services. Welcome to contact us through one of the following ways or visit our company and factories.

Based on the high quality and complete after-sales service, our products have been exported to more than 120 countries and regions. Fote Machinery has been the choice of more than 200,000 customers.

closed circuit grinding vsopen circuit grinding

closed circuit grinding vsopen circuit grinding

The simplest grinding circuit consists of a ball or rod mill in closed circuit with a classifier; the flow sheet is shown in Fig. 25 and the actual layout in Fig. 9. This single-stage circuit is chiefly employed for coarse grinding when a product finer than 65 mesh is not required, but it can be adapted for fine grinding by substituting a bowl classifier for one of the straight type so as to enable the W/S ratio of the overflow to be kept below the 4/1 limit usually necessary for flotation. On account of the greater efficiency of the bowl classifier the trend of practice is towards its installation in plants grinding as coarse as 65 mesh.

Single-stage grinding is generally to be recommended for small plants on account of its simplicity. Variations in the size and character of the ore are unavoidable in most plants, but they are, as a rule, very much more noticeable when operations are on a small than when they are on a large scale. Multi-stage grinding as practised in large installations may, therefore, prove impossible to control on a very small scale, and for this reason the simplicity of single-stage grinding is likely to result in a greater overall efficiency than would be obtained with a multi-stage arrangement. When, however, the capacity of a plant approaches or exceeds 1,000 tons per day, two-stage grinding becomes preferable because the effect of normal variations of the ore is less marked and control becomes correspondingly easier.

The usual type of two-stage circuit is shown in Fig. 26, and is one that can be employed for any degree of grinding, although a straight must be substituted for a bowl classifier in the second stage when a 48-mesh product is required. It used to be the practice at one time to omit the first classifier and to pass the feed straight through the primary mill to the secondary circuit, but it was not a good method because either the secondary mill received pieces of ore that were too big or else the primary mill overground a large proportion of the feed. Much better results are obtainable by keeping the coarse ore circulating round the primary circuit, which is set for the efficient grinding of such material, until it is fine enough to be sent to the secondary circuit where the machines are set to grind fine ore more efficiently than coarse. It should be noted that the overflow of the primary classifier is sent to the secondary classifier, not direct to the mill, in order that all material which has been ground fine enough in the primary circuit can be discharged immediately without any chance of its entering and being overground in the secondary ball mill.

So important is it from the point of view of efficiency to get the undersize out of the circuit at the earliest possible moment, whether it is produced in the primary or in the secondary mill, that a special intermediate bowl classifier is often installed between the two stages. Such an arrangement has been found very useful in plants in which improvements in dry crushing practice have resulted in a reduction in the size of the feed to the grinding mills with the result that they have been able to take larger tonnages; the classifiers have then becomeoverloaded, especially in the case of older installations in which both stages were equipped with straight classifiers.

The method of installing a bowl classifier to overcome the difficulty is shown in Fig. 27. This circuit is usually adopted in modern practice, but with a bowl instead of a straight classifier, if necessary, in the closed circuit of the secondary ball mill.

Any increase in the efficiency of classification gives greater economy of power by reducing the amount of ore that is overground, so making a larger proportion of the power required to turn the mill available forgrinding the particles that are still too large. From a theoretical point of view, the ideal method of grinding would consist of a series of ball mills, each in closed circuit with a classifier and each so short that the ore in its passage through the mill would be struck only two or three times by the balls, any undersize produced being removed at once by the classifier ; in this way the chance of a particle being struck again after it had reached the required size would be reduced to a minimum. Practice circuit design approaches the ideal:

Open circuit grinding consists of one or more grinding mills, either parallel or in series, that discharges a finalground product without classification equipment and no return of coarse discharge back to the mill. Some very simplistic examples of open circuit grinding are see below and are made of aRod mill,Ball Mill or aRod mill, ball mill combination.

Not all ores can be ground in an open circuit type ofarrangement. Some conditions which do favor open circuit grinding such assmall reduction ratios,reduction of particles to a coarse, natural grainsize,recirculation of cleaner flotation middlings forregrinding anda non-critical size distribution of the final groundproduct.

Closed circuit grinding consists of one or more mills discharging ground product to classifiers which in turn return the coarse product from the size separation back to the mill for further grinding. In this circuit, grinding efficiency is very dependent upon the size separation effected so care should be exercised in selecting the type and size of classifier used to close the system.

This type of grinding is the most common circuit found in mineral processing facilities, mainly because a lot of ores and product requirements are not suitable for open circuit grinding. Some advantages presented by grinding in closed circuit are that this arrangement usually results in higher mill capacity and lower power consumption per ton of product, it eliminates overgrinding by removing fines early and it avoids coarse material in the final ground product by returning this material to the mill.

Although closed circuit grinding offers many choices for arrangement of the equipment as well as combinations of equipment, some of the more common circuits arerod mill/classifier,Ball mill/Classifier,Rod mill/Ball mill/Classifier and Rod mill/Classifier/Ball mill/Classifier.

The importance of the grinding circuit to overall production in any facility should be obvious by now. Because of the responsibilities assigned to grinding it becomes essential that a grinding mill accepts a certain required tonnage of ore per day while yielding a product that is of a known and controllable particle size. This leads to the conclusion that close control over the grinding circuit is extremely important.

There are many factors which can contribute to fluctuationsin performance of a mill, but some of the most common found inindustrial practice are thechanges in ore taken from different parts of the mine,changes in crusher settings,wear in the crushers,screen damage in the crusher circuit.

These are a few things that operators should look for when changes in mill performance are noticed. Stockpiling of ore ahead of the mill can aid in smoothing out some of the fluctuations although it must be stored in such a manner that no segregation occurs.

The reduction ratio in the grinding section is so much greater than in the crushing plant that labour becomes a relatively small item and the power and steel consumption the largest items of cost. Table 18 gives the average total consumption of power that may be expected in modern ball mill installations of various capacities up to 4,000 tons per day, the figures being based on an average medium-hard ore.

The cost of grinding is more difficult to predict than that of crushing because variations in the hardness and toughness of the ore produce proportionately wider variations in the consumption of power and steel. An approximate guide to grinding costs; they are direct costs and include no overhead charges. Power is assumed to cost 0.075 per kilowatt-hour in the case of the smallest plant and to decrease to a minimum per kilowatt-hour for the largest.

As the tonnage rises up to 1,000 tons per day the costs fall rapidly. In plants of greater capacity, however, they do not decrease in the same proportion with increase of tonnage, because the extra capacity is not obtained by increasing the size of the individual machines but by installing two or more similar units side by side, each of equal efficiency.Reduction of costs then becomes more a matter of organization than of plant design.

As already stated, the power and steel costs are the two largest items, those of labour and supplies being small by comparison ; it is on this account that recent progress has been mainly directed towards reducing the consumption of power and steel by means of greater efficiency in classification and by the use of mills of larger diameter.

The way in which the efficiency of classification has been increased has been described in the paragraph headed Grinding Circuits. An increase in the diameter of a mill gives greater economy in two ways : In the first place, the balls do more effective work in a large than in asmall mill, because, falling from a greater height, they shatter the pieces of ore with greater force ; in the second place, the ratio of the deadweight of the mill to the weight of the ball charge decreases as the diameter increases and thus in a large mill the useless weight to be moved is distributed over a greater weight of useful ball load than in a small mill, with the result that a larger proportion of the total power consumption is available to give the balls the cascading and rolling action necessary to break up the ore.

It is essential for the grinding and flotation sections of a plant to be run continuously. It takes nearly half an hour to clear the circuit of even a small grinding unit preparatory to stopping it, and often an hour is necessary to get the circuit fully loaded after restarting ; most of the power required to keep the machines running during the stopping and starting periods is wasted. Moreover, the operation of the flotation machines is poor during these periods so that much of the power required to keep them running is also wasted. In addition, modern practice aims at the elimination of everything likely to cause fluctuating conditions. For these reasons it is the universal custom to run the grinding and flotation plants for 24 hours per day.

flash flotation with closed circuit grinding

flash flotation with closed circuit grinding

The reason why you need Flash Flotation in a Closed Grinding Circuit relates toRecovering your mineral as soon as free which has long been recognized in ore dressing practice. This not only applies to gravity treatment but also to flotation. For this application the FlashFlotation Cell was developed for use in the grinding circuit and has done a remarkable job in many plants.

A greater amount of granular higher grade concentrates can be produced and, in general, overall plant recovery is improved by reducing slime losses due to overgrinding and colliding of high specific gravity minerals.

Typical flowsheets are shown to indicate a few of the possible applications of FlashCells in grinding circuits. In recent years the successful application of hydraulic cyclones, rubber lined pumps, and two stage grinding circuits have enhanced the feasibility of unit cell applications. Cyclones in particular have increased the flexibility of such applications by permitting positive and continuous gravity flow of unit cell tailings to subsequent treatment steps.

Molded rubber wearing parts are used exclusively in unit cells. If wear is severe due to coarse abrasive solids, a special molded soft rubber compound is available which greatly extends the impeller and wearing plate life. Conical disk impellers and wearing plates are standard for unit cell applications.

Two stage classification is shown in this flowsheet with a Unit Cell between the classifiers. The primary classifier may overflow as coarse as 20 mesh and at densities up to 50% solids. This is ideal feed for the unit cell. Unit cell tailings are classified through a cyclone and the oversize returned to the ball mill for further grinding. The cyclone classifier overflow, 65 mesh or finer, is treated by regular bulk or selective flotation.

With this two stage classification system the unit cell can be conveniently located to deliver a positive gravity discharge of pulp to the pump feeding the cyclone. The pump sump box can be made a part of the unit cell tank if desired.

Two stage grinding and classification is provided in this flowsheet which is generally applicable to larger tonnage installations in which a substantial percentage of the values can be recovered directly from the grinding circuit. The primary Rod Mill in open circuit will reduce crushed ore to approximately minus 10 mesh.A Mineral Jig is recommended if coarse mineral and metallics are present.

The spiral or rake classifier overflow passes to the unit cell and on to classification and regrinding. High grade unit cell concentrates can be produced with this system, and on low ratio of concentration ores a substantial increase in mill capacity is possible. Slime losses are greatly minimized with this combination jig and unit cell circuit.

The trend in many of the large tonnage millingcircuits is to completely eliminate conventional rake or spiral classifiers by going to two stage grinding with a rod and ball mill in series. The rod mill discharge goes direct to the ball mill and then on to a pump and hydraulic cyclone classifier. These modern grinding and classification circuits are ideal for including the unit cell as the primary mineral recovery step.

One large copper operation with two stage grinding and cyclone classification, actually treats cyclone underflow, 20 plus 100 mesh, through Unit Cells. The unit cells recover a substantial percentage of the total copper in final concentrate form. The unit cell tailings at 55% solids return by gravity to the regrind ball mill feed. Since incorporating the unit cells and by careful checking between parallel circuits, it has been established by recovering the mineral as soon as free that final mill tailings were reduced by lb. copper per ton.

Unit Flash Cell flotation tests should be made before planning an installation. This will establish if the ore will respond to such treatment to advantage.A 100 lb. representative sample of the ball mill feed is sufficient for the unit cell flotation tests.

The simplest flotation circuit is a comparatively recent innovation. It consists of the introduction ofa flotation cell into the grinding circuit between ball mill and classifier as shown below.The discharge end of the mill is fitted with a trommel screen with openings about 4 mesh in size to separate out coarse material, which is laundered direct to the classifier ; the remainder of the pulp passes to the flotation cell where most of the mineral which has been released from the gangue is taken off as a concentrate, the necessary reagents having been added at some previous point in the circuit, usually at the mill feed box. Flotation can be carried out in a pulp containing as much as 65% of solids, but water is added in most cases to bring the W/S ratio to about 1/1. In such a thick, heavy pulp it is possibleto float particles as coarse as 10 mesh. At this size, however, only pure mineral will adhere to a bubble, for which reason the concentrate is generally of unusually high grade.

The advantage of this method of flotation is that the valuable minerals are removed from the circuit as soon as they have been released from the gangue, so that their accumulation in the classifier is prevented and the possibility of overgrinding them is reduced. Moreover, the granular nature of the particles floated assists considerably in the subsequent filtration of the combined flotation concentrate. It is a simple matter to instal the cell in the ball mill circuit, since it fits readily intothe space between mill and classifier and occasions no loss of head. The only disadvantage is the heavy wear to which the interior of the cell is subjected by the coarse material passing through it, but the modern method of lining both moving and stationary parts with rubber reduces this difficulty to minor proportions.

Mcintyre Porcupine Mines, Ltd., was one of the first companies to practise flotation in the grinding circuit. Their installation is described later in the paragraph headed Flotation of Gold and Silver Ores . Others soon also adopted the method. In their plant grinding is carried out in a Hardinge Ball Mill in closed circuit with a Dorr Classifier, and a Sub-A Cell is employed as the flotation unit between the two, the pulp being maintained at a density of 65% solids. Under normal operating conditions 60-70% of the copper and 40% of the nickel are recovered in the grinding circuit. Subsequent flotation in Sub-A Machines gives a total recovery of nearly 99% of the copper and over 94% of the nickel. No attempt is made to separate the copper from the nickel minerals.

The process can be adapted to the selective flotation of complex ores. Another mill for instance, where a lead-zinc ore is treated by two-stage selective flotation, the method being very similar to the standard procedure described in the paragraph entitled Flotation of Lead-Zinc Ores , the installation of a cell between ball mill and classifier has resulted in the removal of most of the galena before the pulp passes to the main flotation circuit. The sphalerite in the ore is so finely intermixed with a portion of the galena that, although a large proportion of the latter mineral is actually liberated at a comparatively coarse mesh, it is necessary to reduce the whole tonnage to 87% minus 200 mesh in order to separate the two minerals at all completely. Before flotation in the grinding circuit was tried, three stages of cleaning were required to make a high- grade lead concentrate, and 96% of the finished product would pass through a 325-mesh screen, only a trace remaining on 200 mesh. The introduction of a Sub-A Cell into the grinding circuit enabled over 70% of the lead to be recovered in one operation without cleaning in a concentrate running 65-70% lead. The concentrate contains only 53% of minus 325-mesh material, 25% remaining on 200 mesh, and, being more granular than that obtained in the main flotation circuit, it gives better filtration. The total recovery remains much the same as before. Reagents are added in the ball mill feed box, and the pulp is maintained in the cell at about 58% solids.

There is no necessity to limit the size of the flotation machine between ball mill and classifier to a single cell, through the use of a multi-cell or a long pneumatic machine would involve changing the relative positions of the three units from the present standard arrangement. The trendof progress indicates that the flotation machine may become in some cases as important a factor in the proper classification of the ore during grinding as the classifier itself is at present.

Hydrocyclones are used in many grinding circuits to make a size separation which ideally sends the fine ore fraction to conventional flotation and the coarse fraction back to the mill for further grinding. The separation results not only from particle size but also from particle specific gravity. The result is a cyclone underflow which is higher in grade than the cyclone feed. When comparing sulphides and precious metals to silicates, the floatable size fraction in the cyclone underflow contains higher mineral values than gangue. This results because the lower specific gravity gangue particles tend to follow the water in the cyclone overflow.

Typical grinding mill circulating loads range from 200% to 500% with a large part being particles which should have reported to conventional flotation. These particles, mainly heavy sulphides and precious metals, are being reground sometimes several times before eventually making it to the conventional flotation circuit. With each pass through the mill, the particles are ground finer until they are overground. This produces slimes which are usually lost to tailings. The attempt has been made in the past to recover valuable minerals from grinding circuits by flotation. This method, referred to as a unit cell operation, treated grinding mill discharge with a conventional flotation machine. Until now the success of this method has been limited by the inability of a conventional machine to treat the very coarse, high density slurries associated with grinding mill discharges.

Outokumpu developed a specially designed tank to work in conjunction with its flotation agitation mechanism to recover valuable minerals from grinding and classification circuits before they are overground and lost as slimes. The Outokumpu Skim-air coarse flotation machine has been used successfully to recover values from both grinding mill discharge and hydrocyclone underflow.

The Skim-Air Flash Flotation machine floats only those liberated valuable particles which are quick floating. There is not sufficient residence time in the machine to float the slower floating middlings and gangue particles. Thus, each time a particle with ideal fast floating characteristics reports to the cyclone underflow, it is removed in the Skim-Air.

The feed to the Skim-Air machine is normally in the range of 65% to 85% solids. In most cases optimum results are achieved with little or no dilution water being added to the machine. The concentrate produced in the Skim-Air is normally sent as final concentrate. Because only quick floating mineral particles have time to float in the Skim-Air, the concentrate from the machine is usually higher grade than that produced from the conventional flotation circuit. The higher feed density allows coarser particles to be floated resulting in an overall coarser concentrate being produced in the Skim-Air than in conventional flotation.

The overall recovery of valuable minerals can be increased through reduced overgrinding. This results because the quick floating liberated valuable particles in the cyclone underflow are removed from the circuit by the Skim-Air and sent directly as final concentrate. If left in the circuit as part of the recirculating load, these particles will be further ground and reduced in size until they become part of the slow floating slimes fraction. At this point they can easily be lost to tailings. Overgrinding and slimes losses are particularly a problem when processing heavy sulphides such as copper, lead and zinc or precious metals such as gold and silver.

In operations where coarse fraction losses are a problem, the use of a Skim-Air allows the ore to be ground finer without fear of overgrinding the valuable mineral. This is particularly helpful in lead/zinc concentrators where a finer grind may liberate more zinc but also increase lead slimes losses.

Figure 2 shows the narrowing of the particle size range being sent to conventional flotation. The difference between the two curves corresponds to a decrease in relative losses of both the slimes and oversize fractions.

When being fed from the cyclone underflow, the Skim-Air is able to produce concentrate with a grade equal to or better than that being produced in conventional flotation. Much of the floatable size gangue in the hydrocyclone feed passes with the water to the cyclone overflow and on to conventional flotation. This results in a feed to the Skim-Air which is very low in floatable gangue. Coupling this with a particle residence time in the machine of 1-2 minutes which allows only liberated values to float, enables the Skim-Air to produce a high grade concentrate.

ball mill classification | henan deya machinery co., ltd

ball mill classification | henan deya machinery co., ltd

Ball mills are also classified by the nature of the discharge. They may be simple trunnion over-flow mills, operated in open or closed circuit, or grate discharge (low-level discharge) mills. The latter type is fitted with discharge grates between the cylindrical mill body and the discharge trunnion. The pulp can flow freely through the openings in the grate and is then lifted up to the level of the discharge trunnion. These mills have a lower pulp level than overflow mills, thus reducing the dwell time of particles in the mill. Very little over grinding takes place and the product contains a large fraction of coarse material, which is returned to the mill by some form of classifying device. Closed-circuit grinding, with high circulating loads, produces a closely sized end product and a high output per unit volume compared with open circuit grinding. Grate discharge mills usually take a coarser feed than overflow mills and are not required to grind so finely, the main reason being that with many small balls forming the charge the grate open area plugs very quickly. The trunnion overflow mill is the simplest to operate and is used for most ball mill applications, especially for fine grinding and regrinding. Energy consumption is said to be about 15% less than that of a grate discharge mill of the same size, although the grinding efficiencies of the two mills are the same.

how to operate a grinding circuit

how to operate a grinding circuit

How hard a ball mill operator has to work depends partly on himself, and partly on the kind of muck the mine sends over to the mill. In some plants, the ore may change two or three times a shift, and a ball mill operator has to keep on his toes.

Thats why it would be just as well for you, as a ball mill operator, to study out a few ways of doing your job easier and better, because there will be times, even in the best of mills, when youll run into a lot of trouble. Collected here you will find some practical suggestions, contributed by a number of good mill men, that might give you an idea or two that would help get around some of that trouble.

To be sure we understand each other, lets begin with the equipment. In a simple grinding circuit there will be a ball mill and a classifier. Some circuits, especially in large mills, have more units or two or three stages of grinding, but whatever is said here will apply to the complicated circuits as well as the simple ones.

The two types of ball mill in general use are the grate mill and the open-end mill. Most manufacturers make both kinds; the difference between them is that the grate mill has a steel grid clear across the discharge end, but the open-end mill has only an open trunnion at the discharge end, through which pulp flows freely. If you dont already know all about the inside of your ball mill and what it is supposed to do, it would be a good idea to ask the shift boss, the metallurgist, or the superintendent to tell you about it.

Mechanical classifiers make use of rakes, spirals, or a simple drag belt. For our purposes it doesnt matter which type you are working with, because you would handle them all in pretty much the same way.

In operation, you add water to the ball mill along with the ore. Flowing out of the ball mill, the ground ore pulp pours into the classifier pool, where the finished material is separated from the coarse sand. You do that by adding a lot more water to the pool, so that the finer sand overflows the weir and goes on to the next step (flotation perhaps), and the coarse sand settles to the classifier bottom and is raked back into the mill to be ground finer. You, the operator, aresupposed to control these actions in order to send on to the machines below you the right amount of ore, ground just fine enough, and with just the right amount of water with it.

To help you do this, and to make a. record of how things are going, you will have to take samples of the pulp regularly. Different mills have different ideas on sampling, but all of them take at least hourly samples of the classifier overflow. What it amounts to is weighing a certain volume of the pulp to determine its density. Higher density means thicker pulp and usually coarser sand. Lower density means thinner pulp and finer sand. The shift boss will tell you what the density ought to be, and it will be up to you to hold it there.

You may also have to take density samples of the ball mill discharge, which runs a lot thicker than the classifier overflow, and some mills also expect you to take measured samples of the ball-mill feed and weigh them.

Another sample you may take is one for pH, which is a term that takes a little explaining. You can find out exactly what pH means from a chemistry book if you want to; but for all practical purposes, it is enough to know that pH is a number that tells you how much acid or alkali there is in the pulp. A pH of 7 is alkaline.

If you add acid, the pH goes down below 7; if you add an alkali like lime, the pH will go up, say to 9 or 10, depending on how much lime you add. In any case, you can bank on it that if the brass hats want you to watch the pH at all, they have good reasons for wanting you to hold it steady.

You may also have the job of adding balls to the mill each shift. The shift boss tells you how many or what weight, and you put them in. Drop them into the scoop if you have a grate mill, or put them in through the discharge trunnion if it is an open-end mill.

The controls you will have to work with are given in Table I, and are also indicated in the drawing. As to which one of these controls is most important, mill men dont all agree. Probably it depends on what kind of ore you are grinding. Most good operators, though, say that the classifier water valve should be the first one to adjust, because it controls directly the kind of finished material you send on down the line to the next man.

The most important point is this: You cannot adjust any one of these controls without paying some attention to the other two. For example, if you change the feed rate, you will probably have to reset the two water valves. They all work together. In fact, the whole grinding circuit acts like a team of horses, and as thetime at first.

In Table II you will find some suggestions on what to look for to help you decide how to use these controls. In the column headed if you find, there are set down the things youll run into if something is wrong with the circuit. That is, if the ball-mill feed gets finer than it usuallyis, the top line tells you what to expect and what to do. But dont think you have to do all these things all the time. Do only as much as you are sure you have to do.

The classifier overflow is really the most important spot in the circuit, because whatever comes over that weir is out of your hands, and your work will be judged by how good a product it is. Most operators believe that if there is any change in setting to be made, the density of the classifier overflow is where you make it first. Remember, more water to the classifier means thinner pulp and finer overflow; less water means thicker pulp and coarser overflow.

The matter of feed to the ball mill brings up a point that is important in keeping you out of trouble. You can find out by asking the old- timers how each kind of ore is going to act when it hits the mill, and if youfor each change as it comes along.

For example, suppose you are working in a lead-zinc flotation mill where there are two kinds of ore one that is coarse and low grade, and another that is finer and higher grade. Keep the feed to the ball mill lower when the coarse stuff comes along, because it takes longer to grind and you dont want to overload the mill. Then when the fine muck shows up, increase the feed and also run the classifier density higher. That will throw the high grade over into the flotation cells where it belongs.

You see, the high-grade mineral is heavier than the low grade, and it takes a little higher density in the classifier to lift it out. If you carry a low density, too much lead and zinc mineral keeps going back into the ball mill, and eventually may be ground into slime and lost altogether. Doing extralittle things like this is what marks a really good operator, and you can learn these things only by study and asking questions.

Keep ahead of trouble is good advice for flotation operators, and it is just as good for bail-mill men. A good operator can take care of even big changes in muck so smoothly and easily that if you were watching him, youd never know anything was running differently.

On the other hand, consider Joe Blow, the Wonder Boy. Thats him down there sitting on the rail near the ball mill, swinging his heels and probably wondering whatever happened to that little blonde hasher over at the Greasy Spoon. Suddenly Joe looks up. He has heard a splashing sound that doesnt belong there. The ball mill is strangely quiet. Joe looks at the feed box, and finds pulp pouring out on the floor.

Joe can tell right away what has happened. The mill has been overloaded and the grate has plugged. Quick as a flash, Joe races around and shuts the feed off, then whips open the valve pouring water into the mill. Hes fast; he wants action.

He gets it. The mill comes unstuck with a vengeance and belches sand into the classifier like a tidal wave. Joe, the dope, flushes water into the classifier, too, and you can almost hear it groan as the rakes get buried. The flotation man down below is tearing his hair and spinning valves. What he says about Joe blisters the paint on the concentrate launders, but Joe cant hear him. Joe is up under the mill shovellingcleaned up before the shifter comes.

Watching the mill discharge (2) will tell you what goes on inside the mill. Some operators note how high on the side of the discharge flange the wave of pulp is carried when the mill is running right. Then if the wave runs higher or lower than that, they know something is wrong.

If the mill is low on muck, (3) it rattles and bangs like a boiler factory, and a lot of good steel goes to waste. But if the mill is too full of muck, you can hardly hear it. Keep your ear peeled for the sound of the mill that you know is right.

Many operators feel the classifier overflow (6) by nibbling their fingers together with their hands in the stream, and with a little experience, you can tell pretty accurately whether or not the overflow is fine enough.

The amp-meter (4) is really as good a guide to the condition of the mill as the sound or the discharge. It tells you how much power the mill drive motor is drawing, but remember that if you overload the mill, or if you underload it, it draws less power.

You check on the circulating load (7) by watching the height of the sand on the rakes or spiral flights as they push it back to the mill feed launder. The shift boss will tell you about how high the sand ought to come.

What was wrong? He shouldnt have let the mill plug in the first place. But suppose it plugged anyway, he should have cut off the feed all right, but he should also have shut down the classifier, and increased the head water only a little. Then he should have cut down on the classifier water and then increased it, little by little, when the mill opened up. He should have done his best to keep things balanced instead of slamming everything out of adjustment at once. Well, hell learn. He will, or the boss will murder him some dark night.

Now, just because all these things to look for and to do have been put down in a table, dont think you ought to walk your shift carrying this operating manual in one hand and a density sample can in the other. It is no use trying to run a mill out of a cookbook. But what we did want to do was to set these things down here so you could think about them, and keep thinking about them, while you are working.

Just go at the problem the way things are arranged in the table. When something in the circuit begins to change, make sure you know exactly what is happening; then ask yourself what is causing it. Then, when you have answered that question, decide what to do about it. Think out each thing you do, and dont do things in a rush or without knowing why you are doing them. Dont be a Joe Blow, in other words.

One thing more, and a very important thing: When you do make a change, allow a little time, say 15 or 20 minutes, for the effect to show up before you make another change. Dont over-control. For instance, if the density in the classifier is up a little and you add more water, dont expect the density to change right away, and dont go back and open the valve even wider just because nothing seems to have happened. It will; just wait a while. A superoperator who cant let well enough alone gets on everybodys nerves.

In starting the grinding circuit after anything but a very short shut spitars enough to clear the samepacked on the tank bottom, start the classifier overflow pump, then start the classifier, and after that, the ball mill. But dont throw in all these switches at once. Youll get the electricians down on you if you do. Keep the water fed to the circuit down low until the load builds up a little; then set the valve hand wheels at about thepoint they should be for normal operation. You can check on this setting by marking one spot on the rim of the wheel and counting turns, or by counting exposed threads on the valve stem. Dont forget to lower the classifier rakes again as the load builds up.

In shutting down the mill, cut off the feed a few minutes before the shutdown is due. That will give time to grind out some of the circulating load and will make starting easier. Then when you are ready to stop, shut down the mill, then the classifier (raise the rakes), then the pump for the classifier overflow, if there is one.

If the power fails suddenly, shut off the water valves and raise the classifier rakes. And for goodness sake, dont forget to shut off any drip cans or siphon feeders of pine oil or other reagent you may have running somewhere in the circuit.

So far as mechanical trouble goes, there will probably be little of that if the equipment is reasonably good. Ball mills spring leaks from time to time because the bolts holding in the liner plates work loose. If a leak develops near the discharge end of the mill, shut down right away and fix it. This is especially true of an open-end mill. The point is that you dont want sand getting into theout in short order.

Now a word about safety, a subject that I am putting last because I want to leave it first in your minds. Whatever else you do, dont go poking aimlessly around the mill or the classifier, sticking your nose or your fingers in here and there to see how the machinery works. I wouldnt make that statement if I hadnt seen a man or two doing just that. Nor have I forgotten the time I was routed out of bed at two a.m. to help bandage a man whose right-hand fingers had just been taken off by the ball-mill scoop as effectively, though not as neatly, as a surgeon could have done it.He had been just poking around, too. Remember your company, and your country, need you on that ball-mill floor, and you wouldnt be happy holding down a hospital bed these days. So just be careful.

This Public DomainRobert Ramsey article is based in large part on experiences and opinions generously supplied by the following mill men: Clyde Simpson, Bagdad Copper Co., Hillside, Ariz.; E. J. Duggan and M. E. Kennedy, Climax Molybdenum Co., Climax, Colo.; John Palecek, Keystone Copper Corp., Copperopolis, Calif.; Frank M. McKinley, Bunker Hill & Sullivan M. & C. Co., Kellogg, Idaho; Malcolm Black, Wright-Hargreaves Mines, Ltd., Kirkland Lake, Ont.; and the concentrator staffs at Hudson Bay Mining & Smelting Co., Flin Flon, Manitoba, and Sherritt Gordon Mines, Ltd., Sherridon, Manitoba.https://archive.org/details/malozemoffmining00platrich

ball mill | henan deya machinery co., ltd

ball mill | henan deya machinery co., ltd

The final stages of comminution are performed in tumbling mills using steel balls as the grinding medium and so designated ball mills. Since balls have a greater surface area per unit weight than rods, they are better suited for fine finishing. The term ball mill is restricted to those having a length to diameter ratio of 1.5 to 1 and less. Ball mills in which the length to diameter ratio is between 3 and 5 are designated tube mills. These are sometimes divided into several longitudinal compartments, each having a different charge composition; the charges can be steel balls or rods, or pebbles, and they are often used dry to grind cement clinker, gypsum, and phosphate. Tube mills having only one compartment and a charge of hard, screened ore particles as the grinding medium are known as pebble mills. They are widely used in the South African gold mines. Since the weight of pebbles per unit volume is 35-55% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly higher operating cost. However, it is claimed that the increment in capital cost can be justified economically by a reduction in operating cost attributed to the lower cost of the grinding medium. This may, however, be partially offset by higher energy cost per tonne of finished product.

Ball mills are also classified by the nature of the discharge. They may be simple trunnion overflow mills, operated in open or closed circuit, or grate discharge (low-level discharge) mills. The latter type is fitted with discharge grates between the cylindrical mill body and the discharge trunnion. The pulp can flow freely through the openings in the grate and is then lifted up to the level of the discharge trunnion (see attached figure 1 grate discharge mill). figure 1 Grate discharge mill These mills have a lower pulp level than overflow mills, thus reducing the dwell time of particles in the mill. Very little overgrinding takes place and the product contains a large fraction of coarse material, which is returned to the mill by some form of classifying device. Closed-circuit grinding (see attached figure 2 Simple closed-grinding circuit), with high circulating loads, produces a closely sized end product and a high output per unit volume compared with open circuit grinding. figure 2 Simple closed grinding circuit Interested in Ball Mill? PRODUCT DETAILS CONTACT NOW Grate discharge mills usually take a coarser feed than overflow mills and are not required to grind so finely, the main reason being that with many small balls forming the charge the grate open area plugs very quickly. The trunnion overflow mill is the simplest to operate and is used for most ball mill applications, especially for fine grinding and regrinding. Energy consumption is said to be about 15% less than that of a grate discharge mill of the same size, although the grinding efficiencies of the two mills are the same Ball mills are rated by power rather than capacity. Today the largest overflow ball mill is 7.9 in diameter length 13.6m, it was produced by CITIC Heavy Industries(see attached figure 3 largest ball mill ). figure 3 Largest ball-mill The trend in recent years has been to use fewer comminution machines per grinding line with the result that units have increased considerably in capacity. For example, in the 1980s, the largest operating ball mill was 5.5 m in diameter by 7.3 m in length driven by a 4 MW motor. Today, ball mills of 5 m plus are commonplace, and 7 m ball mills are currently employed on at least two sites. However, there are several cases where large ball mills have not achieved design capabilities. One example was the 5.5 m diameter by 6.4 m ball mills at Bougainville Copper Ltd where the coarse material grinding was particularly inefficient. The post-Bougainville Copper literature has been reviewed recently. Operational data from a wide range of large diameter ball mills were collected and analysed. The issues related to sizing of large diameter ball mills included power draw, residence time, feed size, and the applicability of Bonds equations. It was concluded that the power draw of large diameter mills follows the same relationships that hold for smaller diameter mills, as described in Morrells power equation. Ball-mill scale-up studies have been conducted in Australia and the United States, the results emphasizing that there are limitations to conventional procedures for estimating large mill requirements from small-scale results. Attempts have been made to use laboratory ball-mill test results to calibrate a suitable ball mill mathematical model, and a set of scale-up criteria have been developed for scaling the parameters of the model to predict full-scale ball-mill performance. Further validation of these ball-mill scale-up procedures is under way. If proved, it offers a useful tool in greenfield design of ball milling circuit. Grinding in a ball mill is effected by point contact of balls and ore particles and, given time, any degree of fineness can be achieved. The process is completely random- the probability of a fine particle being struck by a ball is the same as that of a coarse particle. The product from an open-circuit ball mill therefore exhibits a wide range of particle size, and overgrinding of at least some of the charge becomes a problem. Closed-circuit grinding in mills providing low residence time for the particles is almost always used in the last stages to overcome this. Several factors influence the efficiency of ball mill grinding. The pulp density of the feed should be as high as possible, consistent with ease of flow through the mill. It is essential that the balls are coated with a layer of ore; too dilute a pulp increases metal-to-metal contact, giving increased steel consumption and reduced efficiency. Ball mills should operate between 65 and 80% solids by weight, depending on the ore. The viscosity of the pulp increases with the fineness of the particles, therefore fine-grinding circuits may need lower pulp densities. The major factors affecting the pulp rheology and its effects on grinding circuits have been discussed by a number of researchers. It was found that not only the viscosity of the pulp but also the rheological type (Newtonian or non-Newtonian) would affect ball milling performance. The efficiency of grinding depends on the surface area of the grinding medium. Thus, balls should be as small as possible and the charge should be graded such that the largest balls are just heavy enough to grind the largest and hardest particles in the feed. A seasoned charge will consist of a wide range of ball sizes and new balls added to the mill are usually of the largest size required. Undersize balls leave the mill with the ore product and can be removed by passing the discharge over screens. Various formulae have been proposed for the required ratio of ball size to ore size, none of which is entirely satisfactory. The correct sizes are often determined by trial and error, primary grinding usually requiring a graded charge of 10-5 cm diameter balls, while secondary grinding requires 5-2cm. Concha et al. (1988) have developed a method to calculate ball-mill charge by using a grinding circuit simulator with a model of ball wear in a tumbling mill. Segregation of the ball charge within the mill is achieved in the Hardinge mill(see attached figure 4 Hardinge mill). figure 4 Hardinge mill The conventional drum shape is modified by fitting a conical section, the angle of the cone being about 30 degree . Due to the centrifugal force generated, the balls are segregated so that the largest are at the feed end of the cone, i.e. the largest diameter and greatest centrifugal force, and the smallest are at the discharge. By this means, a regular gradation of ball size and of size reduction is produced. Grinding balls are usually made of forged or rolled high-carbon or alloy steel, or cast alloy steel, and consumption varies between 0.1 and as much as 1 kg/t of ore depending on hardness of ore, fineness of grind, and medium quality. Medium consumption can be a very high proportion, sometimes as much as 40% of the total milling cost, so is an area that often warrants special attention. Good quality grinding media may be more expensive, but may be economic due to lower wear rates. Very hard media, however, may lead to lower grinding efficiencies due to slippage, and this also should be taken into account. Finer grinding may lead to improved metallurgical efficiency, but at the expense of higher grinding energy and media consumption. Therefore, particularly with ore of low value, where milling costs are crucial, the economic limit of grinding has to be carefully assessed. As the medium consumption contributes significantly to the total milling cost, great effort has been expended in the study of medium wear. Three wear mechanisms are generally recognised: abrasion, corrosion, and impact. Abrasion refers to the direct removal of metal from the grinding media surface. Corrosion means the less resistant corrosion product films being abraded away during wet grinding. Impact wear refers to pitting, spalling, breaking, or flaking caused in the ore-metal-environment contact. Operational data show that abrasion is the major cause of metal loss in grinding, while corrosion represents less than 10% of the total loss. In recent years, attempts have been extended to predict media wear by developing a total media wear model incorporating the abrasive, corrosive, and impact wear mechanisms. The model parameters were determined from three ore-metal-environment-specific laboratory tests and validated with full-scale grinding operation data. The charge volume is about 40-50% of the internal volume of the mill, about 40% of this being void space. The energy input to a mill increases with the ball charge, and reaches a maximum at a charge volume of approximately 50% (see attached figure 5 Energy input versus charge volume), but for a number of reasons, 40-50% is rarely exceeded. The efficiency curve is in any case quite flat about the maximum. In overflow mills the charge volume is usually 40%, but there is a greater choice in the case of grate discharge mills. The optimum mill speed increases with charge volume, as the increased weight of charge reduces the amount of cataracting taking place. figure 5 Energy input versus charge volume Ball mills are often operated at higher speeds than rod mills, so that the larger balls cataract and impact on the ore particles. The work input to a mill increases in proportion to the speed, and ball mills are run at as high a speed as is possible without centrifuging. Normally this is 70-80% of the critical speed, the higher speeds often being used to increase the amount of cataracting taking place in order to break hard or coarse feeds. ball-mill-deya-machinery-06ball-mill-deya-machinery-05ball-mill-deya-machinery-04ball-mill-deya-machinery-03ball-mill-deya-machinery-02ball-mill-deya-machinery-01

These mills have a lower pulp level than overflow mills, thus reducing the dwell time of particles in the mill. Very little overgrinding takes place and the product contains a large fraction of coarse material, which is returned to the mill by some form of classifying device. Closed-circuit grinding (see attached figure 2 Simple closed-grinding circuit), with high circulating loads, produces a closely sized end product and a high output per unit volume compared with open circuit grinding. figure 2 Simple closed grinding circuit Interested in Ball Mill? PRODUCT DETAILS CONTACT NOW Grate discharge mills usually take a coarser feed than overflow mills and are not required to grind so finely, the main reason being that with many small balls forming the charge the grate open area plugs very quickly. The trunnion overflow mill is the simplest to operate and is used for most ball mill applications, especially for fine grinding and regrinding. Energy consumption is said to be about 15% less than that of a grate discharge mill of the same size, although the grinding efficiencies of the two mills are the same Ball mills are rated by power rather than capacity. Today the largest overflow ball mill is 7.9 in diameter length 13.6m, it was produced by CITIC Heavy Industries(see attached figure 3 largest ball mill ). figure 3 Largest ball-mill The trend in recent years has been to use fewer comminution machines per grinding line with the result that units have increased considerably in capacity. For example, in the 1980s, the largest operating ball mill was 5.5 m in diameter by 7.3 m in length driven by a 4 MW motor. Today, ball mills of 5 m plus are commonplace, and 7 m ball mills are currently employed on at least two sites. However, there are several cases where large ball mills have not achieved design capabilities. One example was the 5.5 m diameter by 6.4 m ball mills at Bougainville Copper Ltd where the coarse material grinding was particularly inefficient. The post-Bougainville Copper literature has been reviewed recently. Operational data from a wide range of large diameter ball mills were collected and analysed. The issues related to sizing of large diameter ball mills included power draw, residence time, feed size, and the applicability of Bonds equations. It was concluded that the power draw of large diameter mills follows the same relationships that hold for smaller diameter mills, as described in Morrells power equation. Ball-mill scale-up studies have been conducted in Australia and the United States, the results emphasizing that there are limitations to conventional procedures for estimating large mill requirements from small-scale results. Attempts have been made to use laboratory ball-mill test results to calibrate a suitable ball mill mathematical model, and a set of scale-up criteria have been developed for scaling the parameters of the model to predict full-scale ball-mill performance. Further validation of these ball-mill scale-up procedures is under way. If proved, it offers a useful tool in greenfield design of ball milling circuit. Grinding in a ball mill is effected by point contact of balls and ore particles and, given time, any degree of fineness can be achieved. The process is completely random- the probability of a fine particle being struck by a ball is the same as that of a coarse particle. The product from an open-circuit ball mill therefore exhibits a wide range of particle size, and overgrinding of at least some of the charge becomes a problem. Closed-circuit grinding in mills providing low residence time for the particles is almost always used in the last stages to overcome this. Several factors influence the efficiency of ball mill grinding. The pulp density of the feed should be as high as possible, consistent with ease of flow through the mill. It is essential that the balls are coated with a layer of ore; too dilute a pulp increases metal-to-metal contact, giving increased steel consumption and reduced efficiency. Ball mills should operate between 65 and 80% solids by weight, depending on the ore. The viscosity of the pulp increases with the fineness of the particles, therefore fine-grinding circuits may need lower pulp densities. The major factors affecting the pulp rheology and its effects on grinding circuits have been discussed by a number of researchers. It was found that not only the viscosity of the pulp but also the rheological type (Newtonian or non-Newtonian) would affect ball milling performance. The efficiency of grinding depends on the surface area of the grinding medium. Thus, balls should be as small as possible and the charge should be graded such that the largest balls are just heavy enough to grind the largest and hardest particles in the feed. A seasoned charge will consist of a wide range of ball sizes and new balls added to the mill are usually of the largest size required. Undersize balls leave the mill with the ore product and can be removed by passing the discharge over screens. Various formulae have been proposed for the required ratio of ball size to ore size, none of which is entirely satisfactory. The correct sizes are often determined by trial and error, primary grinding usually requiring a graded charge of 10-5 cm diameter balls, while secondary grinding requires 5-2cm. Concha et al. (1988) have developed a method to calculate ball-mill charge by using a grinding circuit simulator with a model of ball wear in a tumbling mill. Segregation of the ball charge within the mill is achieved in the Hardinge mill(see attached figure 4 Hardinge mill). figure 4 Hardinge mill The conventional drum shape is modified by fitting a conical section, the angle of the cone being about 30 degree . Due to the centrifugal force generated, the balls are segregated so that the largest are at the feed end of the cone, i.e. the largest diameter and greatest centrifugal force, and the smallest are at the discharge. By this means, a regular gradation of ball size and of size reduction is produced. Grinding balls are usually made of forged or rolled high-carbon or alloy steel, or cast alloy steel, and consumption varies between 0.1 and as much as 1 kg/t of ore depending on hardness of ore, fineness of grind, and medium quality. Medium consumption can be a very high proportion, sometimes as much as 40% of the total milling cost, so is an area that often warrants special attention. Good quality grinding media may be more expensive, but may be economic due to lower wear rates. Very hard media, however, may lead to lower grinding efficiencies due to slippage, and this also should be taken into account. Finer grinding may lead to improved metallurgical efficiency, but at the expense of higher grinding energy and media consumption. Therefore, particularly with ore of low value, where milling costs are crucial, the economic limit of grinding has to be carefully assessed. As the medium consumption contributes significantly to the total milling cost, great effort has been expended in the study of medium wear. Three wear mechanisms are generally recognised: abrasion, corrosion, and impact. Abrasion refers to the direct removal of metal from the grinding media surface. Corrosion means the less resistant corrosion product films being abraded away during wet grinding. Impact wear refers to pitting, spalling, breaking, or flaking caused in the ore-metal-environment contact. Operational data show that abrasion is the major cause of metal loss in grinding, while corrosion represents less than 10% of the total loss. In recent years, attempts have been extended to predict media wear by developing a total media wear model incorporating the abrasive, corrosive, and impact wear mechanisms. The model parameters were determined from three ore-metal-environment-specific laboratory tests and validated with full-scale grinding operation data. The charge volume is about 40-50% of the internal volume of the mill, about 40% of this being void space. The energy input to a mill increases with the ball charge, and reaches a maximum at a charge volume of approximately 50% (see attached figure 5 Energy input versus charge volume), but for a number of reasons, 40-50% is rarely exceeded. The efficiency curve is in any case quite flat about the maximum. In overflow mills the charge volume is usually 40%, but there is a greater choice in the case of grate discharge mills. The optimum mill speed increases with charge volume, as the increased weight of charge reduces the amount of cataracting taking place. figure 5 Energy input versus charge volume Ball mills are often operated at higher speeds than rod mills, so that the larger balls cataract and impact on the ore particles. The work input to a mill increases in proportion to the speed, and ball mills are run at as high a speed as is possible without centrifuging. Normally this is 70-80% of the critical speed, the higher speeds often being used to increase the amount of cataracting taking place in order to break hard or coarse feeds. ball-mill-deya-machinery-06ball-mill-deya-machinery-05ball-mill-deya-machinery-04ball-mill-deya-machinery-03ball-mill-deya-machinery-02ball-mill-deya-machinery-01

Grate discharge mills usually take a coarser feed than overflow mills and are not required to grind so finely, the main reason being that with many small balls forming the charge the grate open area plugs very quickly. The trunnion overflow mill is the simplest to operate and is used for most ball mill applications, especially for fine grinding and regrinding. Energy consumption is said to be about 15% less than that of a grate discharge mill of the same size, although the grinding efficiencies of the two mills are the same Ball mills are rated by power rather than capacity. Today the largest overflow ball mill is 7.9 in diameter length 13.6m, it was produced by CITIC Heavy Industries(see attached figure 3 largest ball mill ). figure 3 Largest ball-mill The trend in recent years has been to use fewer comminution machines per grinding line with the result that units have increased considerably in capacity. For example, in the 1980s, the largest operating ball mill was 5.5 m in diameter by 7.3 m in length driven by a 4 MW motor. Today, ball mills of 5 m plus are commonplace, and 7 m ball mills are currently employed on at least two sites. However, there are several cases where large ball mills have not achieved design capabilities. One example was the 5.5 m diameter by 6.4 m ball mills at Bougainville Copper Ltd where the coarse material grinding was particularly inefficient. The post-Bougainville Copper literature has been reviewed recently. Operational data from a wide range of large diameter ball mills were collected and analysed. The issues related to sizing of large diameter ball mills included power draw, residence time, feed size, and the applicability of Bonds equations. It was concluded that the power draw of large diameter mills follows the same relationships that hold for smaller diameter mills, as described in Morrells power equation. Ball-mill scale-up studies have been conducted in Australia and the United States, the results emphasizing that there are limitations to conventional procedures for estimating large mill requirements from small-scale results. Attempts have been made to use laboratory ball-mill test results to calibrate a suitable ball mill mathematical model, and a set of scale-up criteria have been developed for scaling the parameters of the model to predict full-scale ball-mill performance. Further validation of these ball-mill scale-up procedures is under way. If proved, it offers a useful tool in greenfield design of ball milling circuit. Grinding in a ball mill is effected by point contact of balls and ore particles and, given time, any degree of fineness can be achieved. The process is completely random- the probability of a fine particle being struck by a ball is the same as that of a coarse particle. The product from an open-circuit ball mill therefore exhibits a wide range of particle size, and overgrinding of at least some of the charge becomes a problem. Closed-circuit grinding in mills providing low residence time for the particles is almost always used in the last stages to overcome this. Several factors influence the efficiency of ball mill grinding. The pulp density of the feed should be as high as possible, consistent with ease of flow through the mill. It is essential that the balls are coated with a layer of ore; too dilute a pulp increases metal-to-metal contact, giving increased steel consumption and reduced efficiency. Ball mills should operate between 65 and 80% solids by weight, depending on the ore. The viscosity of the pulp increases with the fineness of the particles, therefore fine-grinding circuits may need lower pulp densities. The major factors affecting the pulp rheology and its effects on grinding circuits have been discussed by a number of researchers. It was found that not only the viscosity of the pulp but also the rheological type (Newtonian or non-Newtonian) would affect ball milling performance. The efficiency of grinding depends on the surface area of the grinding medium. Thus, balls should be as small as possible and the charge should be graded such that the largest balls are just heavy enough to grind the largest and hardest particles in the feed. A seasoned charge will consist of a wide range of ball sizes and new balls added to the mill are usually of the largest size required. Undersize balls leave the mill with the ore product and can be removed by passing the discharge over screens. Various formulae have been proposed for the required ratio of ball size to ore size, none of which is entirely satisfactory. The correct sizes are often determined by trial and error, primary grinding usually requiring a graded charge of 10-5 cm diameter balls, while secondary grinding requires 5-2cm. Concha et al. (1988) have developed a method to calculate ball-mill charge by using a grinding circuit simulator with a model of ball wear in a tumbling mill. Segregation of the ball charge within the mill is achieved in the Hardinge mill(see attached figure 4 Hardinge mill). figure 4 Hardinge mill The conventional drum shape is modified by fitting a conical section, the angle of the cone being about 30 degree . Due to the centrifugal force generated, the balls are segregated so that the largest are at the feed end of the cone, i.e. the largest diameter and greatest centrifugal force, and the smallest are at the discharge. By this means, a regular gradation of ball size and of size reduction is produced. Grinding balls are usually made of forged or rolled high-carbon or alloy steel, or cast alloy steel, and consumption varies between 0.1 and as much as 1 kg/t of ore depending on hardness of ore, fineness of grind, and medium quality. Medium consumption can be a very high proportion, sometimes as much as 40% of the total milling cost, so is an area that often warrants special attention. Good quality grinding media may be more expensive, but may be economic due to lower wear rates. Very hard media, however, may lead to lower grinding efficiencies due to slippage, and this also should be taken into account. Finer grinding may lead to improved metallurgical efficiency, but at the expense of higher grinding energy and media consumption. Therefore, particularly with ore of low value, where milling costs are crucial, the economic limit of grinding has to be carefully assessed. As the medium consumption contributes significantly to the total milling cost, great effort has been expended in the study of medium wear. Three wear mechanisms are generally recognised: abrasion, corrosion, and impact. Abrasion refers to the direct removal of metal from the grinding media surface. Corrosion means the less resistant corrosion product films being abraded away during wet grinding. Impact wear refers to pitting, spalling, breaking, or flaking caused in the ore-metal-environment contact. Operational data show that abrasion is the major cause of metal loss in grinding, while corrosion represents less than 10% of the total loss. In recent years, attempts have been extended to predict media wear by developing a total media wear model incorporating the abrasive, corrosive, and impact wear mechanisms. The model parameters were determined from three ore-metal-environment-specific laboratory tests and validated with full-scale grinding operation data. The charge volume is about 40-50% of the internal volume of the mill, about 40% of this being void space. The energy input to a mill increases with the ball charge, and reaches a maximum at a charge volume of approximately 50% (see attached figure 5 Energy input versus charge volume), but for a number of reasons, 40-50% is rarely exceeded. The efficiency curve is in any case quite flat about the maximum. In overflow mills the charge volume is usually 40%, but there is a greater choice in the case of grate discharge mills. The optimum mill speed increases with charge volume, as the increased weight of charge reduces the amount of cataracting taking place. figure 5 Energy input versus charge volume Ball mills are often operated at higher speeds than rod mills, so that the larger balls cataract and impact on the ore particles. The work input to a mill increases in proportion to the speed, and ball mills are run at as high a speed as is possible without centrifuging. Normally this is 70-80% of the critical speed, the higher speeds often being used to increase the amount of cataracting taking place in order to break hard or coarse feeds. ball-mill-deya-machinery-06ball-mill-deya-machinery-05ball-mill-deya-machinery-04ball-mill-deya-machinery-03ball-mill-deya-machinery-02ball-mill-deya-machinery-01

The trend in recent years has been to use fewer comminution machines per grinding line with the result that units have increased considerably in capacity. For example, in the 1980s, the largest operating ball mill was 5.5 m in diameter by 7.3 m in length driven by a 4 MW motor. Today, ball mills of 5 m plus are commonplace, and 7 m ball mills are currently employed on at least two sites. However, there are several cases where large ball mills have not achieved design capabilities. One example was the 5.5 m diameter by 6.4 m ball mills at Bougainville Copper Ltd where the coarse material grinding was particularly inefficient. The post-Bougainville Copper literature has been reviewed recently. Operational data from a wide range of large diameter ball mills were collected and analysed. The issues related to sizing of large diameter ball mills included power draw, residence time, feed size, and the applicability of Bonds equations. It was concluded that the power draw of large diameter mills follows the same relationships that hold for smaller diameter mills, as described in Morrells power equation.

Ball-mill scale-up studies have been conducted in Australia and the United States, the results emphasizing that there are limitations to conventional procedures for estimating large mill requirements from small-scale results. Attempts have been made to use laboratory ball-mill test results to calibrate a suitable ball mill mathematical model, and a set of scale-up criteria have been developed for scaling the parameters of the model to predict full-scale ball-mill performance. Further validation of these ball-mill scale-up procedures is under way. If proved, it offers a useful tool in greenfield design of ball milling circuit. Grinding in a ball mill is effected by point contact of balls and ore particles and, given time, any degree of fineness can be achieved. The process is completely random- the probability of a fine particle being struck by a ball is the same as that of a coarse particle. The product from an open-circuit ball mill therefore exhibits a wide range of particle size, and overgrinding of at least some of the charge becomes a problem. Closed-circuit grinding in mills providing low residence time for the particles is almost always used in the last stages to overcome this.

Several factors influence the efficiency of ball mill grinding. The pulp density of the feed should be as high as possible, consistent with ease of flow through the mill. It is essential that the balls are coated with a layer of ore; too dilute a pulp increases metal-to-metal contact, giving increased steel consumption and reduced efficiency. Ball mills should operate between 65 and 80% solids by weight, depending on the ore. The viscosity of the pulp increases with the fineness of the particles, therefore fine-grinding circuits may need lower pulp densities. The major factors affecting the pulp rheology and its effects on grinding circuits have been discussed by a number of researchers. It was found that not only the viscosity of the pulp but also the rheological type (Newtonian or non-Newtonian) would affect ball milling performance.

The efficiency of grinding depends on the surface area of the grinding medium. Thus, balls should be as small as possible and the charge should be graded such that the largest balls are just heavy enough to grind the largest and hardest particles in the feed. A seasoned charge will consist of a wide range of ball sizes and new balls added to the mill are usually of the largest size required. Undersize balls leave the mill with the ore product and can be removed by passing the discharge over screens. Various formulae have been proposed for the required ratio of ball size to ore size, none of which is entirely satisfactory. The correct sizes are often determined by trial and error, primary grinding usually requiring a graded charge of 10-5 cm diameter balls, while secondary grinding requires 5-2cm. Concha et al. (1988) have developed a method to calculate ball-mill charge by using a grinding circuit simulator with a model of ball wear in a tumbling mill.

Segregation of the ball charge within the mill is achieved in the Hardinge mill(see attached figure 4 Hardinge mill). figure 4 Hardinge mill The conventional drum shape is modified by fitting a conical section, the angle of the cone being about 30 degree . Due to the centrifugal force generated, the balls are segregated so that the largest are at the feed end of the cone, i.e. the largest diameter and greatest centrifugal force, and the smallest are at the discharge. By this means, a regular gradation of ball size and of size reduction is produced. Grinding balls are usually made of forged or rolled high-carbon or alloy steel, or cast alloy steel, and consumption varies between 0.1 and as much as 1 kg/t of ore depending on hardness of ore, fineness of grind, and medium quality. Medium consumption can be a very high proportion, sometimes as much as 40% of the total milling cost, so is an area that often warrants special attention. Good quality grinding media may be more expensive, but may be economic due to lower wear rates. Very hard media, however, may lead to lower grinding efficiencies due to slippage, and this also should be taken into account. Finer grinding may lead to improved metallurgical efficiency, but at the expense of higher grinding energy and media consumption. Therefore, particularly with ore of low value, where milling costs are crucial, the economic limit of grinding has to be carefully assessed. As the medium consumption contributes significantly to the total milling cost, great effort has been expended in the study of medium wear. Three wear mechanisms are generally recognised: abrasion, corrosion, and impact. Abrasion refers to the direct removal of metal from the grinding media surface. Corrosion means the less resistant corrosion product films being abraded away during wet grinding. Impact wear refers to pitting, spalling, breaking, or flaking caused in the ore-metal-environment contact. Operational data show that abrasion is the major cause of metal loss in grinding, while corrosion represents less than 10% of the total loss. In recent years, attempts have been extended to predict media wear by developing a total media wear model incorporating the abrasive, corrosive, and impact wear mechanisms. The model parameters were determined from three ore-metal-environment-specific laboratory tests and validated with full-scale grinding operation data. The charge volume is about 40-50% of the internal volume of the mill, about 40% of this being void space. The energy input to a mill increases with the ball charge, and reaches a maximum at a charge volume of approximately 50% (see attached figure 5 Energy input versus charge volume), but for a number of reasons, 40-50% is rarely exceeded. The efficiency curve is in any case quite flat about the maximum. In overflow mills the charge volume is usually 40%, but there is a greater choice in the case of grate discharge mills. The optimum mill speed increases with charge volume, as the increased weight of charge reduces the amount of cataracting taking place. figure 5 Energy input versus charge volume Ball mills are often operated at higher speeds than rod mills, so that the larger balls cataract and impact on the ore particles. The work input to a mill increases in proportion to the speed, and ball mills are run at as high a speed as is possible without centrifuging. Normally this is 70-80% of the critical speed, the higher speeds often being used to increase the amount of cataracting taking place in order to break hard or coarse feeds. ball-mill-deya-machinery-06ball-mill-deya-machinery-05ball-mill-deya-machinery-04ball-mill-deya-machinery-03ball-mill-deya-machinery-02ball-mill-deya-machinery-01

The conventional drum shape is modified by fitting a conical section, the angle of the cone being about 30 degree . Due to the centrifugal force generated, the balls are segregated so that the largest are at the feed end of the cone, i.e. the largest diameter and greatest centrifugal force, and the smallest are at the discharge. By this means, a regular gradation of ball size and of size reduction is produced.

Grinding balls are usually made of forged or rolled high-carbon or alloy steel, or cast alloy steel, and consumption varies between 0.1 and as much as 1 kg/t of ore depending on hardness of ore, fineness of grind, and medium quality. Medium consumption can be a very high proportion, sometimes as much as 40% of the total milling cost, so is an area that often warrants special attention. Good quality grinding media may be more expensive, but may be economic due to lower wear rates. Very hard media, however, may lead to lower grinding efficiencies due to slippage, and this also should be taken into account. Finer grinding may lead to improved metallurgical efficiency, but at the expense of higher grinding energy and media consumption. Therefore, particularly with ore of low value, where milling costs are crucial, the economic limit of grinding has to be carefully assessed.

As the medium consumption contributes significantly to the total milling cost, great effort has been expended in the study of medium wear. Three wear mechanisms are generally recognised: abrasion, corrosion, and impact. Abrasion refers to the direct removal of metal from the grinding media surface. Corrosion means the less resistant corrosion product films being abraded away during wet grinding. Impact wear refers to pitting, spalling, breaking, or flaking caused in the ore-metal-environment contact. Operational data show that abrasion is the major cause of metal loss in grinding, while corrosion represents less than 10% of the total loss. In recent years, attempts have been extended to predict media wear by developing a total media wear model incorporating the abrasive, corrosive, and impact wear mechanisms. The model parameters were determined from three ore-metal-environment-specific laboratory tests and validated with full-scale grinding operation data. The charge volume is about 40-50% of the internal volume of the mill, about 40% of this being void space. The energy input to a mill increases with the ball charge, and reaches a maximum at a charge volume of approximately 50% (see attached figure 5 Energy input versus charge volume), but for a number of reasons, 40-50% is rarely exceeded. The efficiency curve is in any case quite flat about the maximum. In overflow mills the charge volume is usually 40%, but there is a greater choice in the case of grate discharge mills. The optimum mill speed increases with charge volume, as the increased weight of charge reduces the amount of cataracting taking place. figure 5 Energy input versus charge volume Ball mills are often operated at higher speeds than rod mills, so that the larger balls cataract and impact on the ore particles. The work input to a mill increases in proportion to the speed, and ball mills are run at as high a speed as is possible without centrifuging. Normally this is 70-80% of the critical speed, the higher speeds often being used to increase the amount of cataracting taking place in order to break hard or coarse feeds. ball-mill-deya-machinery-06ball-mill-deya-machinery-05ball-mill-deya-machinery-04ball-mill-deya-machinery-03ball-mill-deya-machinery-02ball-mill-deya-machinery-01

Ball mills are often operated at higher speeds than rod mills, so that the larger balls cataract and impact on the ore particles. The work input to a mill increases in proportion to the speed, and ball mills are run at as high a speed as is possible without centrifuging. Normally this is 70-80% of the critical speed, the higher speeds often being used to increase the amount of cataracting taking place in order to break hard or coarse feeds.

construction of ball mill/ ball mill structure | henan deya machinery co., ltd

construction of ball mill/ ball mill structure | henan deya machinery co., ltd

Structurally, each ball mill consists of a horizontal cylindrical shell, provided with renewable wearing liners and a charge of grinding medium. The drum is supported so as to rotate on its axis on hollow trunnions attached to the end walls (attached figure 1 ball mill). The diameter of the mill determines the pressure that can be exerted by the medium on the ore particles and, in general, the larger the feed size the larger needs to be the mill diameter. The length of the mill, in conjunction with the diameter, determines the volume, and hence the capacity of the mill.

The feed material is usually fed to the mill continuously through one end trunnion, the ground product leaving via the other trunnion, although in certain applications the product may leave the mill through a number of ports spaced around the periphery of the shell. All types of mill can be used for wet or dry grinding by modification of feed and discharge equipment.

Mill shells are designed to sustain impact and heavy loading, and are constructed from rolled mild steel plates, buttwelded together. Holes are drilled to take the bolts for holding the liners. Normally one or two access manholes are provided. For attachment of the trunnion heads, heavy flanges of fabricated or cast steel are usually welded or bolted to the ends of the plate shells, planed with parallel faces which are grooved to receive a corresponding spigot on the head, and drilled for bolting to the head.

The mill ends, or trunnion heads, may be of nodular or grey cast iron for diameters less than about 1 m. Larger heads are constructed from cast steel, which is relatively light, and can be welded. The heads are fibbed for reinforcement and may be flat, slightly conical, or dished. They are machined and drilled to fit shell flanges(attached figure 2 tube mill end and trunnion). figure 2 Tube mill end and trunnion Trunnions and bearings The trunnions are made from cast iron or steel and are spigoted and bolted to the end plates, although in small mills they may be integral with the end plates. They are highly polished to reduce bearing friction. Most trunnion bearings are rigid highgrade iron castings with 120-180 degree lining of white metal in the bearing area, surrounded by a fabricated mild steel housing, which is bolted into the concrete foundations (attached figure 3 oil-lubricated trunnion bearing). figure 3 oil-lubricated trunnion bearing The bearings in smaller mills may be grease lubricated, but oil lubrication is favoured in large mills, via motor-driven oil pumps. The effectiveness of normal lubrication protection is reduced when the mill is shut down for any length of time, and many mills are fitted with manually operated hydraulic starting lubricators, which force oil between the trunnion and trunnion bearing, preventing friction damage to the beating surface, on starting, by re-establishing the protecting film of oil (attached figure 4 Hydraulic starting lubricator). figure 4 Hydraulic starting lubricator Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

The trunnions are made from cast iron or steel and are spigoted and bolted to the end plates, although in small mills they may be integral with the end plates. They are highly polished to reduce bearing friction. Most trunnion bearings are rigid highgrade iron castings with 120-180 degree lining of white metal in the bearing area, surrounded by a fabricated mild steel housing, which is bolted into the concrete foundations (attached figure 3 oil-lubricated trunnion bearing). figure 3 oil-lubricated trunnion bearing The bearings in smaller mills may be grease lubricated, but oil lubrication is favoured in large mills, via motor-driven oil pumps. The effectiveness of normal lubrication protection is reduced when the mill is shut down for any length of time, and many mills are fitted with manually operated hydraulic starting lubricators, which force oil between the trunnion and trunnion bearing, preventing friction damage to the beating surface, on starting, by re-establishing the protecting film of oil (attached figure 4 Hydraulic starting lubricator). figure 4 Hydraulic starting lubricator Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

The bearings in smaller mills may be grease lubricated, but oil lubrication is favoured in large mills, via motor-driven oil pumps. The effectiveness of normal lubrication protection is reduced when the mill is shut down for any length of time, and many mills are fitted with manually operated hydraulic starting lubricators, which force oil between the trunnion and trunnion bearing, preventing friction damage to the beating surface, on starting, by re-establishing the protecting film of oil (attached figure 4 Hydraulic starting lubricator). figure 4 Hydraulic starting lubricator Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Some manufacturers install large roller bearings, which can withstand higher forces than plain metal bearings (attached figure 5 Trunnion with roller-type bearings ). Trunnion with roller-type bearings Drive Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated. Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing. The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry. Liners The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Ball mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. The pinion shaft is driven from the prime mover through vee-belts, in small mills of less than about 180 kW. For larger mills the shaft is coupled directly to the output shaft of a slow-speed synchronous motor, or to the output shaft of a motor-driven helical or double helical gear reducer. In some mills thyristors and DC motors are used to give variable speed control. Very large mills driven by girth gears require two to four pinions, and complex load sharing systems must be incorporated.

Large ball mills can be rotated by a central trunnion drive, which has the advantage of requiting no expensive ring gear, the drive being from one or two motors, with the inclusion of two-or three-speed gearing.

The larger the mill, the greater are the stresses between the shells and heads and the trunnions and heads. In the early 1970s, maintenance problems related to the application of gear and pinion and large speed reducer drives on dry grinding cement mills of long length drove operators to seek an alternative drive design. As a result, a number of gearless drive (ring motor) cement mills were installed and the technology became relatively common in the European cement industry.

The internal working faces of mills consist of renewable liners, which must withstand impact, be wear-resistant, and promote the most favourable motion of the charge. Rod mill ends have plain fiat liners, slightly coned to encourage the selfcentring and straight-line action of rods. They are made usually from manganese or chromemolybdenum steels, having high impact strength. Ball-mill ends usually have ribs to lift the charge with the mill rotation. These prevent excessive slipping and increase liner life. They can be made from white cast iron, alloyed with nickel (Ni-hard), other wear-resistant materials, and rubber. Trunnion liners are designed for each application and can be conical, plain, with advancing or retarding spirals. They are manufactured from hard cast iron or cast alloy steel, a rubber lining often being bonded to the inner surface for increased life. Shell liners have an endless variety of lifter shapes. Smooth linings result in much abrasion, and hence a fine grind, but with associated high metal wear. The liners are therefore generally shaped to provide lifting action and to add impact and crushing, the most common shapes being wave, Lorain, stepped, and shiplap (attached figure 6 ball mill shell liners). The liners are attached to the mill shell and ends by forged steel countersunk liner bolts. figure 6 ball mill shell liners Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used. Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost. Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings. The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts. A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported. To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines. Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner. Mill feeders Spout feeder The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Rod mill liners are also generally of alloyed steel or cast iron, and of the wave type, although Nihard step liners may be used with rods up to 4 cm in diameter. Lorain liners are extensively used for coarse grinding in rod and ball mills, and consist of high carbon rolled steel plates held in place by manganese or hard alloy steel lifter bars. Ball mill liners may be made of hard cast iron when balls of up to 5 cm in diameter are used, but otherwise cast manganese steel, cast chromium steel, or Ni-hard are used.

Ball Mill liners are a major cost in mill operation, and efforts to prolong liner life are constantly being made. There are at least ten wear-resistant alloys used for ball-mill linings, the more abrasion-resistant alloys containing large amounts of chromium, molybdenum, and nickel being the most expensive. However, with steadily increasing labour costs for replacing liners, the trend is towards selecting liners which have the best service life regardless of cost.

Rubber liners and lifters have supplanted steel in some operations, and have been found to be longer lasting, easier and faster to install, and their use results in a significant reduction of noise level. However, increased medium consumption has been reported using rubber liners rather than Ni-hard liners. Rubber lining may also have drawbacks in processes requiring the addition of flotation reagents directly into the mill, or temperatures exceeding 80. They are also thicker than their steel counterparts, which reduces mill capacity, a particularly important factor in small mills. There are also important differences in design aspects between steel and rubber linings.

The engineering advantage of rubber is that, at relatively low impact forces, it will yield, resuming its shape when the forces are removed. However, if the forces are too powerful, or the speed of the material hitting the rubber is too high, the wear rate is dramatic. In primary grinding applications, with severe grinding forces, the wear rate of rubber inhibits its use. Even though the wear cost per tonne of ore may be similar to that of the more expensive steel lining, the more frequent interruptions for maintenance often make it uneconomical. The advantage of steel is its great hardness, and steel-capped liners have been developed which combine the best qualities of rubber and steel. These consist of rubber lifter bars with steel inserts embedded in the face, the steel providing the wear resistance and the rubber backing cushioning the impacts.

A concept which has found some application for ball mills is the angular spiral lining. The circular cross-section of a conventional mill is changed to a square cross-section with rounded corners by the addition of rubber-lined, flanged frames, which are offset to spiral in a direction opposite to the mill rotation. Double wave liner plates are fitted to these frames, and a sequential lifting of the charge down the length of the mill results, which increases the grinding ball to pulp mixing through axial motion of the grinding charge, along with the normal cascading motion. Substantial increases in throughput, along with reductions in energy and grinding medium consumptions, have been reported.

To avoid the rapid wear of rubber liners, a new patented technology for a magnetic metal liner has been developed by China Metallurgical Mining Corp. The magnets keep the lining in contact with the steel shell and the end plates without using bolts, while the ball scats in the charge and magnetic minerals are attracted to the liner to form a 30-40mm protective layer, which is continuously renewed as it wears. Over 10 years the magnetic metal liner has been used in more than 300 full-scale ball mills at over 100 mine sites in China. For example, one set of the magnetic metal liner was installed in a 3.2m (D) x 4.5 m (L) secondary ball mill (60mm ball charge) at Waitoushan concentrator of Benxi Iron and Steel Corp. in 1992. Over nine years, 2.6 Mt of iron ore were ground at zero additional liner cost and zero maintenance of the liners. The magnetic metal liner has also found applications in large ball mills, such as the 5.5 m (D) x 8.8 m (L) mills installed at Diaojuntai concentrator in Qidashan Iron Ore Mines.

Another advantage of the magnetic metal liner is that as the liners are thinner and lighter than conventional manganese steel, the effective mill volume is larger, and the mill weight is reduced. An 11.3% decrease in mill power draw at the same operational conditions has been realised in a 2.7m (D) x 3.6m (L) ball mill by using the magnetic metal liner.

The type of feeding arrangement used on the mill depends on whether the grinding is done in open or closed circuit and whether it is done wet or dry. The size and rate of feed are also important. Dry mills are usually fed by some sort of vibratory feeder. Three types of feeder are in use in wet-grinding mills. The simplest form is the spout feeder (attached figure 7 Spout feeder), consisting of a cylindrical or elliptical chute supported independently of the mill, and projecting directly into the trunnion liner. Material is fed by gravity through the spout to feed the mills. They are often used for feeding rod mills operating in open circuit or mills in closed circuit with hydrocyclone classifiers. figure 7 Spout feeder Drum feeders Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

Drum feeders (attached figure 8 Drum feeder on ball mill) may be used as an alternative to a spout feeder when headroom is limited. The entire mill feed enters the drum via a chute or spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient method of adding grinding balls to a mill. figure 8 Drum feeder on ball mill Combination drum-scoop feeders These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

These (attached figure 9 Drum-scoop feeder) are generally used for wet grinding in closed circuit with a spiral or rake classifier. New material is fed directly into the drum, while the scoop picks up the classifier sands for regrinding. Either a single or a double scoop can be used, the latter providing an increased feed rate and more uniform flow of material into the mill; the counter-balancing effect of the double-scoop design serves to smooth out power fluctuation and it is normally incorporated in large-diameter mills. Scoop feeders are sometimes used in place of the drum-scoop combination when mill feed is in the fine-size range. figure 9 Drum-scoop feeder

gold leaching equipment, circuits & process plants

gold leaching equipment, circuits & process plants

In Leaching for Gold, there is often a tendency to overlook or minimize the importance of the small mine. The small mine of today may develop into the large mine of tomorrow. Under proper management and financing it has as good a chance of yielding a profit as the larger property. Unfortunately large capital is seldom interested in them and they are left to the small groups who are not in a position to obtain the best engineering service. Mills are often erected without proper metallurgical tests and expensive Gold Leachingplant equipment are installed at a time when such large expenditures of capital on the surface is not justified by the underground developments. Careful metallurgical testing on the ore might have disclosed the fact that a simple method of amalgamation or concentration could have been employed and the mill built for a third the cost of a Gold Leaching plant.

By taking advantage of the fact that gold is one of the heaviest metals known and readily forms an amalgam with mercury, an effective but simple and inexpensive plant can be built for most small gold mines. Usually the major percentage of the gold values are in the native or metallic state and are free at commercial fineness of grinding and can be recovered by some combination of amalgamation and concentration.

Plate amalgamation, where the gold values are caught and held in the quicksilver film on a copper plate is the only step required for a commercial recovery on some few ores. In most cases a portion of the gold is filmed so that it does not amalgamate readily or is contained in ores with other minerals that also amalgamate or foul the quicksilver sufficiently to destroy its effectiveness for gold recovery. Here a form of selective concentration such as the Mineral Jigs and blanket tables, is used to concentrate the gold values in a small bulk of high grade concentrates for treatment in an amalgamation barrel or other amalgamator, where the gold is amalgamated and recovered as bullion.

The advantages of these simple plants are many and are not only attractive to the proved small mine but also to those under development. Within recent years many of our well known mines have been developed and brought into large scale production from revenue secured from a small milling plant operating on development ore.

A study of a large number of mills using amalgamation and concentration has disclosed bullion recoveries ranging from 60 per cent to 90 per cent and total recoveries, including concentrates, from 85 per cent to 97 per cent. The average bullion recovery will be about 70 per cent and very often this is of utmost importance as geographic location makes the shipping of the concentrate to a smelting plant undesirable.

While cyanidation is usually favored for treating gold ores to get maximum recovery of the values in bullion form, nevertheless, the fact that an amalgamation plantcan be built for approximately one- third of a complete Gold Leaching mill, together with the lower operating costs of the simpler plant, partially offsets the lower recovery. It is customary to impound the tailings from the amalgamation plant and these are cheaply treated when mine developments have justified the erection of the more complete Gold Leachingplant. An amalgamation and concentration plant can be operating intermittently without sacrificing efficiency, and this allows the operation of the plant for only one or two shifts per day to keep the peak power requirements at a minimum as mine compressors can be operated or the hoisting done while the mill is not in operation. The fact that 60 to 90 per cent of the values can be recovered by amalgamation will usually supply sufficient revenue from the mill to pay for development charges andbuild a reserve for the construction of the complete Gold Leaching plant.

With reasonable care in the design and construction of the original amalgamation and concentration plant all of the equipment can be utilized in the later complete Gold Leaching mill. By using standard equipment it is possible to add the Gold Leaching equipment following the already installed amalgamation and concentration units as these are an essential part of the completed plant.

Other advantages of these simple and inexpensive amalgamation and concentration plants are that they can be successfully operated with unskilled labor as no chemical knowledge or previous experience is necessary. Even flotation has been simplified through the use of Sub-A Flotation Cells; this addition of flotation means no marked increase in milling costs, but often a large increase in recovery due to the saving of extremely fine mineral values.

It is interesting to note the numerous dividend paying gold properties, particularly those in Eastern Canada, which have followed the treatment methods shown in the following flowsheets during the development stage and they have gradually added to the equipment as the profits and ore developments warranted. The use of standard proved equipment eliminates the biggest element of chance, and from this nucleus a more efficient and complete plant can be acquired as the flexibility of the equipment permits the change from one flowsheet to another.

We are giving five typical flowsheets used in treating gold ores and are describing the possible applications of these flowsheets, together with their fields of usefulness, and while in each case there is a similarity in equipment, you will note the changes necessary for various type ores. In each case we have endeavored to show the simplest possible plant for best results on each type of ore and to show the improvements that can be made to further increase recoveries at slight additional cost.

This flowsheet is the lowest in price, and can be used on what are commonly termed as free milling gold ores where a high percentage of the values are free and where these values are unlocked at reasonably coarse grinding.This flowsheet is often used for treating high grade pockets. The ball mill is in open circuit and the size of the product to amalgamation plates is controlled by a Spiral Screen on the ball mill discharge. The concentrating table also functions as a classifier and the middling is returned as oversize product for further grinding.

Flowsheet BB has a Mineral Jig and amalgamator in addition to the equipment shown for Flowsheet AA, and is used for an inexpensive plant where values are coarse but minerals are coated or filmed, and will not amalgamate readily on plates. The jig recovers the rusty values in a high grade concentrate for forcedamalgamation treatment in the Amalgamator. Onthe ores where this flowsheet is applicable, blankets, corduroy, or Gold Matting are usually substituted for amalgamation plates and their concentrate also is treated in the amalgamator with the jig product.

This flowsheet with the ball mill in closed circuit with a classifier, and with the jig in this circuit, will give the highest recovery possible for amalgamation and gravity concentration. The addition of the classifier allows finer grinding and the efficiency of the jig is greatly increased by using it in the closed grinding circuit. This flowsheet not only improves recoveries on ores as described in the previous flowsheets, but is also useful where the minerals are fine and where metallic values are in auriferous sulphides as well as in the free state in the gangue.

The addition of flotation to Flowsheet CC brings recovery to the highest point in Flowsheet DD as flotation recovers the slime values that are normally lost where gravity concentration only is used. The values that can be amalgamated are secured in bullion form from the high grade jig and table concentrates, and the remaining values are recovered in the flotation concentrate. This flowsheet is also necessary where a minor percentage of the gold values are present as metallics at commercial fineness of grinding or where the minerals are friable and easily slimed in fine grinding such as galena or the various telluride minerals.

The addition of flotation does not increase greatly the first cost of the plant, nor does it increase the operating expenses more than a few cents per ton. In a great many cases the additional recovery made by flotation means the difference between operating at a profit and at a loss. Flotation is responsible for the success of many small mining properties today.

Where the isolated location of the mill makes shipping of concentrates prohibitive, many properties store their product until they are justified in installing a complete treatment plant on the ground; current expenses are thus paid through bullion recovered by amalgamation ahead of flotation.

The equipment in this flowsheet is identical to that of DD. Here the ability of the Sub-A Flotation Machine to effectively handle a coarse feed is capitalized on to allow the handling of greatly increased tonnages. The ball mill discharge passes in open circuit over the jig, amalgamation plates or blanket tables and the flotationmachine. A middling product is returned from theconcentrating table and is dewatered in the classifier and returned for regrinding. On tailings, dumps, or low grade ores where it is necessary to handle a larger tonnage, this flowsheet is very effective, and while the recoveries would not be as high as in Flowsheet DD, the loss in recovery is more than offset by the greatly increased tonnage handled and the resultant lower milling cost. With this flowsheet a coarse tailing can be discarded, but slime losses are entirely eliminated as these, together with the granular minerals, are recovered in the flotation machine.

This flexibility of flowsheet is possible only where the Sub-A Flotation Machine is used. The (Selective) Mineral Jig is a valuable addition here as the excessive dilution would make it impossible to use any other type of gravity concentration device ahead of flotation. The change from Flowsheet DD to Flowsheet EE can be very easily made to accommodate changes in ore and to allow greater profits from the treatment of any type gold ore encountered.

No two ores are exactly alike. What method of treatment will give you the greatest net profit in milling your ore? This can be determined by proper metallurgical tests. They will show the recoveries which may be obtained by various methods of treatment; and the type and cost of equipment required, and the operating cost for each method are then easily established.

Ore tests are conducted on the basis of obtaining the simplest possible flowsheet, using standard, proved equipment. Also, as you will note in the flowsheets shown, this fundamental principle is always followed: Recover the mineral as soon as it is free.

A study of a large number of mills using amalgamation and concentration has disclosed bullion recoveries ranging from 60 per cent to 90 per cent and total recoveries, including concentrates, from 85 per cent to 97 per cent. The average bullion recovery will be about 70 per cent and very often this is of utmost importance as geographic location makes the shipping of the concentrate to a smelting plant undesirable.

While cyanidation is usually favored for treating gold ores to get maximum recovery of the values in bullion form, nevertheless, the fact that an amalgamation plant can be built for approximately one-third of a complete cyanide mill, together with the lower operating costs of the simpler plant, partially offsets the lower recovery. It is customary to impound the tailings from the amalgamation plant and these are cheaply treated when mine developments have justified the erection of the more complete cyanide plant. An amalgamation and concentration plant can be operating intermittently without sacrificing efficiency, and this allows the operation of the plant for only one or two shifts per day to keep the peak power requirements at a minimum as mine compressors can be operated or the hoisting done while the mill is not in operation. The fact that 60 to 80 per cent of the values can be recovered by amalgamation will usually supply sufficient revenue from the mill to pay for development charges and build a reserve for the construction of the complete cyanide plant.

With reasonable care in the design and construction of the original amalgamation and concentration plant all of the equipment can be utilized in the later complete cyanide mill. By using standard equipment it is possible to add the cyanide equipment following the already installed amalgamation and concentration units as these are an essential part of the completed plant.

Other advantages of these simple and inexpensive amalgamation and concentration plants are that they can be successfully operated with unskilled labor as no chemical knowledge or previous experience is necessary. Gold ore bodies can be accurately sampled by milling all of the ore from mine development work and the errors resulting from ordinary sampling methods can be entirely eliminated.

It is interesting to note the numerous dividend paying gold properties, particularly those in Eastern Canada, which have followed the treatment methods shown in the following flowsheets during the development stage and they have gradually added to the equipment as the profits and ore developments warranted. The use of standard proved equipment eliminates the biggest element of chance, and from this nucleus a more efficient and complete plant can be acquired as the flexibility of the equipment permits the change from one flowsheet to another.

We are giving four typical flowsheets used in treating gold ores and are describing the possible applications of these flowsheets, together with their fields of usefulness, and while in each case there is a similarity in equipment, you will note the changes necessary for various type ores. In each case we have endeavoured to show the simplest possible plant for best results on each type of ore and to show the improvements that can be made to further increase recoveries at slight additional cost.

This flowsheet is the lowest in price, and can be used on what are commonly termed as free milling gold ores where a high percentage of the values are free and where these values are unlocked at reasonably coarse grinding. This flowsheet is often used for treating high grade pockets. The ball mill is in open circuit and the size of the product to amalgamation plates is controlled by a Spiral Screen on the ball mill discharge. The concentrating table also functions as a classifier and the middling is returned as oversize product for further grinding.

Flowsheet BB has a Mineral Jig and amalgamator in addition to the equipment shown for Flowsheet AA, and is used for an inexpensive plant where values are coarse but minerals are coated or filmed, and will not amalgamate readily on plates. The jig recovers the rusty values in a high grade concentrate for forced amalgamation treatment in the Amalgamator. On the ores where this flowsheet is applicable, blankets, corduroy, or Gold Matting are usually substituted for amalgamation plates and their concentrate also is treated in the amalgamator with the jig product.

This flowsheet with the ball mill in closed circuit with a classifier, and with the jig in this circuit, will give the highest recovery possible for amalgamation and gravity concentration. The addition of the classifier allows finer grinding and the efficiency of the jig is greatly increased by using it in the closed grinding circuit. This flowsheet not only improves recoveries on ores as described in the previous flowsheets, but is alo useful where the minerals are fine and where metallic values are in auriferous sulphides as well as in the free state in the gangue.

The addition of flotation to Flowsheet CC brings recovery to the highest point in Flowsheet DD as flotation recovers the slime values that are normally lost where gravity concentration only is used. The values that can be amalgamated are secured in bullion form from the high grade jig and table concentrates, and the remaining values are recovered in the flotation concentrate. This flowsheet is also necessary where a minor percentage of the gold values are present as metallics at commercial fineness of grinding or where the minerals are friable and easily slimed in fine grinding such as galena or the various telluride minerals.

The addition of flotation does not increase greatly the first cost of the plant, nor does it increase the operating expenses more than a few cents per ton. In a great many cases the additional recovery made by flotation means the difference between operating at a profit and at a loss. Flotation is responsible for the success of many small mining properties today.

Where the isolated location of the mill makes shipping of concentrates prohibitive, many properties store their product until they are justified in installing a complete treatment plant on the ground; current expenses are thus paid through bullion recovered by amalgamation ahead of flotation. The equipment in this flowsheet is identical to that of DD. Here the ability of the Flotation Machine to handle a coarse feed is capitalized on to allow the handling of greatly increased tonnages. The ball mill discharge passes in open circuit over the jig, amalgamation plates or blanket tables and the flotation machine. A middling product is returned from the concentrating table and is dewatered in the classifier and returned for regrinding. On tailings, dumps, or low grade ores where it is necessary to handle a larger tonnage, this flowsheet is very effective, and while the recoveries would not be as high as in Flowsheet DD, the loss in recovery is more than offset by the greatly increased tonnage handled and the resultant lower milling cost. With this flowsheet a coarse tailing can be discarded, but slime losses are entirely eliminated as these, together with the granular minerals, are recovered in the flotation machine.

This flexibility of flowsheet is possible only where the standard Sub-A Type Flotation Machine is used. The Mineral Jig is a valuable addition here as the excessive dilution would make it impossible to use any other type of gravity concentration device ahead of flotation. The change from Flowsheet DD to Flowsheet EE can be very easily made to accommodate changes in ore and to allow greater profits from the treatment of any type gold ore encountered.

The 5 Gold Leaching Equipment Flowsheets illustrated above indicate the equipment essential for small cyanide mills of five different tonnages. These flowsheets are all similar with equipment sized for the tonnages shown. They are typical flowsheets for continuous counter-current decantation cyanidation plus a Mineral Jig in the grinding circuit with provisions for amalgamation of the jig concentrates.

The Mineral Jig and Amalgamation Unit have a definite place in cyanide plants as the coarse and granular gold can be readily recovered which may not be completely dissolved by the cyanide solution during the treatment time given to the pulp. The cyanide process has the advantage of producing precious metals in bullion form with the highest net return from those gold and silver ores amenable to cyanidation. The counter current decantation washing circuit has been found to be a most economical method for removing dissolved precious metals. Washing Tray Thickeners require the minimum floor space and capital costs. In counter current decantation wash water and barren solution are added in the last thickener units and flow counter to pulp flows, becoming enriched and are finally passed to clarification and precipitation where precious metals are precipitated and recovered.

The above flowsheets illustrate a method of increasing both capacity and recovery in a small gold plant by several stages. This is typical of the Pay As You Grow method of increasing capacity and profits essential in so many small operations. Because each ore has its own individual characteristics it is wise to first start with reliable test data. This is just as important in developing a flowsheet for a small mill as it is for a large plant.

Gold Flowsheet No. 1 shows a typical simple mill for the recovery of gold by amalgamation and by concentrating tables. However, on many ores such a flowsheet gives high losses of both fine gold and sulfide minerals.

Gold Flowsheet No. 3 indicates the addition of a required mill, classifier and extra Sub-A Flotation cells to provide for an increase in capacity and improvement in recoveries by regrinding of middling products.

Gold Flowsheet No. 4 shows an increase in flotation capacity to further improve recovery. The additions as illustrated allow an operation to be started on limited capital and gradually to be expanded as conditions warrant.

** Extracted from Memorandum Series No. 47, by C. S. Parsons, Engineer, Ore Dressing and Metallurgical Division, Mines Branch, Department of Mines, Ottawa. Published by permission of the Director, Mines Branch.

Source: This article is a reproduction of an excerpt of In the Public Domain documents held in 911Metallurgy Corps private library.[/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]

closed circuit ball mill basics revisited - sciencedirect

closed circuit ball mill basics revisited - sciencedirect

Since the early days, there has been a general consensus within the industry and amongst grinding professionals that classification efficiency and circulating load both have a major effect on the efficiency of closed circuit ball mills. However, the effect of each is difficult to quantify in practice as these two parameters are usually interrelated. Based on experience acquired over the years and the investigative work conducted by F.C. Bond, it was established that the optimum circulating load for a closed ball mill cyclone circuit is around 250%. This value is used as guideline for the design of new circuits as well as to assess the performance of existing circuits.

The role of classification in milling appears to have been neglected in the current efforts to reduce the energy consumption of grinding. Two past approaches, experimental and modelling, for quantifying the effects of classification efficiency and circulating load on the capacity of closed ball mill circuits, are revisited and discussed in this paper. Application to the optimisation of existing circuits and design of new circuits is also discussed, with special attention to the development of more energy efficient circuits.

Circulating load and classification efficiency effect on ball mill capacity revisited. Relative capacity model introduced and validated. Relationship between circulating load and classification efficiency verified by industrial data. Existing fine screening technology could increase ball mill circuit capacity 1525%.

ball mill for sale | grinding machine - jxsc mining

ball mill for sale | grinding machine - jxsc mining

Ball mill is the key equipment for grinding materials. those grinding mills are widely used in the mining process, and it has a wide range of usage in grinding mineral or material into fine powder, such as gold, ironzinc ore, copper, etc.

JXSC Mining produce reliable effective ball mill for long life and minimum maintenance, incorporate many of the qualities which have made us being professional in the mineral processing industry since 1985. Various types of ball mill designs are available to suit different applications. These could include but not be restricted to coal mining grate discharge, dry type grinding, wet mineral grinding, high-temperature milling operations, stone & pebble milling.

A ball mill grinds ores to an end product size of thirty-five mesh or finer. The feeding material to a ball mill is treated by: Single or multistage crushing and screening Crushing, screening, and/or rod milling Primary crushing and autogenous/semi-autogenous grinding.

Normal feed sizes: eighty percent of six millimeters or finer for hard rocker eighty percent of twenty-five millimeters or finer for fragile rocks (Larger feed sizes can be tolerated depending on the requirements).

The ratio of machine length to the cylinder diameter of cylindrical type ball mills range from one to three through three to one. When the length to diameter ratio is two to one or even bigger, we should better choose the mill of a Tube Mill.

Grinding circuit design Grinding circuit design is available, we experienced engineers expect the chance to help you with ore material grinding mill plant of grinding circuit design, installation, operation, and optimization. The automatic operation has the advantage of saving energy consumption, grinding media, and reducing body liner wear while increasing grinding capacity. In addition, by using a software system to control the ore grinding process meet the requirements of different ore milling task.

The ball mill is a typical material grinder machine which widely used in the mineral processing plant, ball mill performs well in different material conditions either wet type grinding or dry type, and to grind the ores to a fine size.

Main ball mill components: cylinder, motor drive, grinding medium, shaft. The cylinder cavity is partial filling with the material to be ground and the metal grinding balls. When the large cylinder rotating and creating centrifugal force, the inner metal grinding mediums will be lifted to the predetermined height and then fall, the rock material will be ground under the gravity force and squeeze force of moving mediums. Feed material to be ground enters the cylinder through a hopper feeder on one end and after being crushed by the grinding medium is discharged at the other end.

Mining Equipment Manufacturers, Our Main Products: Gold Trommel, Gold Wash Plant, Dense Media Separation System, CIP, CIL, Ball Mill, Trommel Scrubber, Shaker Table, Jig Concentrator, Spiral Separator, Slurry Pump, Trommel Screen.

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