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ball mill maintenance & installation procedure

ball mill maintenance & installation procedure

Am sure your BallMill is considered the finest possible grinding mill available. As such you will find it is designed and constructed according to heavy duty specifications. It is designed along sound engineering principles with quality workmanship and materials used in the construction of the component parts. YourBallMill reflects years of advancement in grinding principles, materials, and manufacturing techniques. It has been designed with both the operators and the erectors viewpoints in mind. Long uninterrupted performance can be expected from it if the instructions covering installation and maintenance of the mill are carried out. You may be familiar with installing mills of other designs and manufacture much lighter in construction. YourBallis heavy and rugged. It should, therefore, be treated accordingly with due respect for its heavier construction.

The purpose of this manual is to assist you in the proper installation and to acquaint you a bit further with the assembly and care of this equipment. We suggest that these instructions be read carefully and reviewed by everyone whenever involved in the actual installation and operation of the mill. In reading these general instructions, you may at times feel that they cover items which are elementary and perhaps not worthy of mention; however in studying hundreds of installations, it has been found that very often minor points are overlooked due to pressure being exerted by outside influences to get the job done in a hurry. The erection phase of this mill is actually no place to attempt cost savings by taking short cuts, or by-passing some of the work. A good installation will pay dividends for many years to come by reduced maintenance cost.With the modern practice of specialized skills and trades, there is often a line drawn between responsibilities of one crew of erectors and another. Actually the responsibility of installation does not cease with the completion of one phase nor does it begin with the starting of another. Perhaps a simple rule to adopt would be DO NOT TAKE ANYTHING FOR GRANTED. This policy of rechecking previously done work will help guarantee each step of the erection and it will carefully coordinate and tie it into subsequent erection work. To clarify or illustrate this point, take the example of concrete workers completing their job and turning it over to the machinist or millwright. The latter group should carefully check the foundation for soundness and correctness prior to starting their work.

Sound planning and good judgement will, to a great extent, be instrumental in avoiding many of the troublesome occurrences especially at the beginning of operations. While it is virtually impossible to anticipate every eventuality, nevertheless it is the intention of this manual to outline a general procedure to follow in erecting the mill, and at the same time, point out some of the pitfalls which should be avoided.

Before starting the erection of the mill, adequate handling facilities should be provided or made available, bearing in mind the weights and proportions of the various parts and sub-assemblies. This information can be ascertained from the drawings and shipping papers.

The gearing, bearings, and other machined surfaces have been coated with a protective compound, and should be cleaned thoroughly with a solvent, such as Chlorothene, (made by Dow Chemical). Judgement should be exercised as to the correct time and place for cleaning the various parts. Do not permit solvents, oil or grease to come in contact with the roughened top surfaces of the concrete foundation where grouting is to be applied; otherwise proper bonding will not result.

After cleaning the various parts, the gear and pinion teeth, trunnion journals and bearings, shafting and such, should be protected against rusting or pitting as well as against damage from falling objects or weld splatter. All burrs should be carefully removed by filing or honing.

Unless otherwise arranged for, the mill has been completely assembled in our shop. Before dismantling, the closely fitted parts were match marked, and it will greatly facilitate field assembly to adhere to these match marks.

The surfaces of all connecting joints or fits, such as shell and head flanges, trunnion flanges, trunnion liner and feeder connecting joints, should be coated with a NON-SETTING elastic compound, such as Quigley O-Seal, or Permatex to insure against leakage and to assist in drawing them up tight. DO NOT USE WHITE LEAD OR GREASE.

Parts which are affected by the hand of the mill are easily identified by referring to the parts list. In general they include the feeder, feed trunnion liner, discharge trunnion liner if it is equipped with a spiral, spiral type helical splitter, and in some cases the pan liners when they are of the spiral type. When both right and left hand mills are being assembled, it is imperative that these parts which involve hand be assembled in the correct mill.

Adequate foundations for any heavy equipment, and in particular grinding mills, are extremely important to assure proper operation. The foundation should preferably be in one piece, that is, with a reinforced slab footing (so called mat) extending under both trunnion bearing foundations as well as the pinion bearing foundation. If possible or practical, it should be extended to include also the motor and drive. With this design, in the event of some movement, the mill and foundation will tend to move as a unit. ANY SLIGHT SETTLING OF FOUNDATIONS WILL CAUSE BEARING AND GEAR MISALIGNMENT, resulting in excessive wear and higher maintenance costs. It has been found that concrete foundations on a weight basis should be at least 1 times the total weight of the grinding mill with its grinding media.

Allowable bearing pressure between concrete footings and the soil upon which the foundation rests should first be considered. The center of pressure must always pass through the center of the footing. Foundations subject to shock should be designed with less unit pressure than foundations for stationary loads. High moisture content in soils reduces the amount of allowable specific pressure that the ground can support. The following figures may be used for preliminary foundation calculations.

Portland cement mixed with sand and aggregate in the proper proportions has come to be standard practice in making concrete. For general reference cement is usually shipped in sacks containing one cubic foot of material. A barrel usually holds 4 cubic feet. Cement will deteriorate with age and will quickly absorb moisture so it should be stored in a dry place. For best results the sand and gravel used should be carefully cleaned free of humus, clay, vegetal matter, etc.

Concrete may be made up in different mixtures having different proportions of sand and aggregate. These are expressed in parts for example a 1:2:4 mixture indicates one bag of cement, 2 cubic feet of sand, and 4 cubic feet of gravel. We recommend a mixture of 1:2:3 for ball mill and rod mill foundations. The proper water to sand ratio should be carefully regulated since excess water increases the shrinkage in the concrete and lends to weaken it even more than a corresponding increase in the aggregate. Between 5 to 8 gallons of water to a sack of cement is usually recommended, the lower amount to be used where higher strength is required or where the concrete will be subject to severe weathering conditions.

Detailed dimensions for the concrete foundation are covered by the foundation plan drawing submitted separately. The drawing also carries special instructions as to the allowance for grouting, steel reinforcements, pier batter, foundation bolts and pipes. During concrete work, care should be taken to prevent concrete entering the pipes, surrounding the foundations bolts, which would limit the positioning of the bolts when erecting the various assemblies. Forms should be adequately constructed and reinforced to prevent swell, particularly where clearance is critical such as at the drive end where the gear should clear the trunnion bearing and pinion bearing piers.

For convenience in maintenance, the mill foundations should be equipped with jacking piers. These will allow the lifting of one end of the mill by use of jacks in the event maintenance must be carried out under these conditions.

Adequate foundations for any heavy equipment, and in particular Marcy grinding mills, are extremely important to assure proper operation of that equipment. Any slight settling of foundations will cause bearing and gear misalignment, resulting in excessive wear and higher maintenance costs. It has been found that concrete foundations on a weight basis should be approximately 1 times the total weight of the grinding mill with its grinding media.

Allowable bearing pressure between concrete footings and the soil upon which the foundation rests should first be considered. The center of pressure must always pass through the center of the footing. Foundations subject to shock should be designed with less unit pressures than foundations for stationary loads. High moisture content in soils reduces the amount of allowable pressure that that material can support. The following figures may be used for quick foundation calculations:

Portland cement mixed with sand and aggregate in the proper proportions has come to be standard practice in making concrete. For general reference cement is usually shipped in sacks containing one cubic foot of material. A barrel usually consists of 4 cubic feet. Cement will deteriorate with age and will quickly absorb moisture so it should be stored in a cool, dry place. The sand and gravel used should be carefully cleaned for best results to be sure of minimizing the amount of sedimentation in that material.

Concrete may be made up in different mixtures having different proportions of sand and aggregate. These are expressed in parts for example a 1:2:4 mixture indicates one bag of cement, 2 cubic feet of sand, and 4 cubic feet of gravel. We recommend a mixture of 1:2:3 for ball mill and rod mill foundations. The proper water to sand ratio should be carefully regulated since excess water will tend to weaken the concrete even more than corresponding variations in other material ratios. Between 5 to 8 gallons of water to a sack of cement is usually recommended, the lower amount to be used where higher strength is required or where the concrete will be subject to severe weathering conditions.

We recommend the use of a non-shrinking grout, and preferably of the pre-mixed type, such as Embeco, made by the Master Builders Company of Cleveland, Ohio. Thoroughly clean the top surfaces of the concrete piers, and comply with the instructions of the grouting supplier.

1. Establish vertical and horizontal centerline of mill and pinion shaftagainst the effects of this, we recommend that the trunnion bearing sole plate be crowned so as to be higher at the center line of the mill. This is done by using a higher shim at the center than at the endsand tightening the foundation bolts of both ends.

After all shimming is completed, the sole plate and bases should be grouted in position. Grouting should be well tamped and should completely fill the underside of the sole plate and bases. DO NOT REMOVE THE SHIMS AFTER OR DURING GROUTING. When the grout has hardened sufficiently it is advisable to paint the top surfaces of the concrete so as to protect it against disintegration due to the absorption of oil or grease.

If it is felt that sufficient accuracy in level between trunnion bearing piers cannot be maintained, we recommend that the grouting of the sole plate under the trunnion bearing opposite the gear end be delayed until after the mill is in place. In this way, the adjustment by shimming at this end can be made later to correct for any errors in elevation. Depending on local climatic conditions, two to seven days should he allowed for the grouting to dry and set, before painting or applying further loads to the piers.

Pinion bearings are provided of either the sleeve type or anti-friction type. Twin bearing construction may use either individual sole plates or a cast common sole plate. The unit with a common sole plate is completely assembled in our shop and is ready for installation. Normal inspection and cleaning procedure should be followed. Refer to the parts list for general assembly. These units are to be permanently grouted in position and, therefore, care should be taken to assure correct alignment.

The trunnion bearing assemblies can now be mounted on their sole plates. If the bearings are of the swivel type, a heavy industrial water-proof grease should be applied to the spherical surfaces of both the swivels and the bases. Move the trunnion bearings to their approximate position by adjustment of the set screws provided for this purpose.

In the case of ball mills, all internal wearing parts will pass through the manhole, whereas in the case of open end rod mills they will pass through the discharge trunnion opening. When lining the shell, start with the odd shaped pieces around the manhole opening if manholes are furnished. Rubber shell liner backing should be used with all cast type rod mills shell liners. If the shell liners are of the step type, they should be assembled with the thin portion, or toe, as the leading edge with respect to rotation of the mill.

Lorain liners for the shell are provided with special round head bolts, with a waterproof washer and nut. All other cast type liners for the head and shell are provided with oval head bolts with a cut washer and nuts. Except when water proof washers are used, it is advisable to wrap four or five turns of candle wicking around the shank of the bolt under the cut washer. Dip the candle wicking in white lead. All liner bolt threads should be dipped in graphite and oil before assembly. All liner bolt cuts should be firmly tightened by use of a pipe extension on a wrench, or better yet, by use of a torque wrench. The bolt heads should be driven firmly into the bolt holes with a hammer.

In order to minimise the effect of pulp race, we recommend that the spaces between the ends of the shell liners and the head liners or grates be filled with suitable packing. This packing may be in the form of rubber belting, hose, rope or wood.

If adequate overhead crane facilities are available, the heads can be assembled to the shell with the flange connecting bolts drawn tightly. Furthermore, the liners can be in place, as stated above, and the gear can be mounted, as covered by separate instructions. Then the mill can be taken to its location and set in place in the trunnion bearings.

If on the other hand the handling facilities are limited it is recommended that the bare shell and heads be assembled together in a slightly higher position than normal. After the flange bolts are tightened, the mill proper should be lowered into position. Other intermediate methods may be used, depending on local conditions.

In any event, just prior to the lowering of the mill into the bearings the trunnion journal and bushing and bases should be thoroughly cleaned and greased. Care should be taken not to foul the teeth in the gear or pinion. Trunnion bearing caps should be immediately installed, although not necessarily tightened, to prevent dirt settling on the trunnions. The gear should be at least tentatively covered for protection.

IMPORTANT. Unless the millwright or operator is familiar with this type of seal, there is a tendency to assume that the oil seal is too long because of its appearance when held firmly around the trunnion. It is not the function of the brass oil seal band to provide tension for effective sealing. This is accomplished by the garter spring which is provided with the oil seal.

Assemble the oil seal with the spring in place, and with the split at the top. Encircle the oil seal with the band, keeping the blocks on the side of the bearing at or near the horizontal center line so that when in place they will fit between the two dowel pins on the bearing, which are used to prevent rotation of the seal.

Moderately tighten up the cap screws at the blocks, pulling them together to thus hold the seal with its spring in place. If the blocks cannot be pulled snuggly together, then the oil seal may be cut accordingly. Oil the trunnion surface and slide the entire seal assembly back into place against the shoulder of the bearing and finish tightening. Install the retainer ring and splash ring as shown.

In most cases the trunnion liners are already mounted in the trunnions of the mills. If not, they should be assembled with attention being given to match marks or in some cases to dowel pins which are used to locate the trunnion liners in their proper relation to other parts.

If a scoop feeder, combination drum scoop feeder or drum feeder is supplied with the mill, it should be mounted on the extended flange of the feed trunnion liner, matching the dowel pin with its respective hole. The dowel pin arrangement is provided only where there is a spiral in the feed trunnion liner. This matching is important as it fixes the relationship between the discharge from the scoop and the internal spiral of the trunnion liner. Tighten the bolts attaching the feeder to the trunnion liner evenly, all around the circle, seating the feeder tightly and squarely on its bevelled seat. Check the bolts holding the lips and other bolts that may require tightening. The beveled seat design is used primarily where a feeder is provided for the trunnion to trunnion liner connection, and the trunnion liner to feeder connection. When a feeder is not used these connecting joints are usually provided by a simple cylindrical or male and female joint.

If a spout feeder is to be used, it is generally supplied by the user, and should be mounted independently of the mill. The spout should project inside the feed trunnion liner, but must not touch the liner or spiral.

Ordinarily the feed box for a scoop tender is designed and supplied by the user. The feed box should be so constructed that it has at least 6 clearance on both sides and at the bottom of the scoop. This clearance is measured from the outside of the feed scoop.

The feed box may be constructed of 2 wood, but more often is made of 3/16 or plate steel reinforced with angles. In the larger size mills, the lower portion is sometimes made of concrete. Necessary openings should be provided for the original feed and the sand returns from the classifiers when in closed circuit.

A plate steel gear guard is furnished with the mill for safety in operation and to protect the gear and pinion from dirt or grit. As soon as the gear and pinion have been cleaned and coated with the proper lubricant, the gear guard should be assembled and set on its foundation.

Most Rod Mills are provided with a discharge housing mechanism mounted independently of the mill. This unit consists of the housing proper, plug door, plug shaft, arm, and various hinge pins and pivot and lock pins. The door mechanism is extra heavy throughout and is subject to adjustment as regard location. Place the housing proper on the foundation, level with steel shims and tighten the foundation bolts. The various parts may now be assembled to the housing proper and the door plug can be swung into place, securing it with the necessary lock pins.

After the mill has been completely assembled and aligned, the door mechanism centered and adjusted, and all clearances checked, the housing base can be grouted. The unit should be so located both vertically and horizontally so as to provide a uniform annular opening between the discharge plug door and the head liners.

In some cases because of space limitation, economy reasons, etc., the mill is not equipped with separate discharge housing. In such a case, the open end low discharge principal is accomplished by means of the same size opening through the discharge trunnion but with the plug door attached to lugs on the head liner segments or lugs on the discharge trunnion liner proper. In still other cases, it is sometimes effected by means of an arm holding the plug and mounted on a cross member which is attached to the bell of the discharge trunnion liner. In such cases as those, a light weight sheet steel discharge housing is supplied by the user to accommodate the local plant layout in conjunction with the discharge launder.

TRUNNION BEARING LUBRICATION. For the larger mills with trunnion bearings provided with oil seals, we recommend flood oil lubrication. This can be accomplished by a centralized system for two or more mills, or by an individual system for each mill. We recommend the individual system for each mill, except where six or more mills are involved, or when economy reasons may dictate otherwise.

In any event oil flow to each trunnion bearing should be between 3 to 5 gallons per minute. The oil should be adequately filtered and heaters may be used to maintain a temperature which will provide proper filtration and maintain the necessary viscosity for adequate flow. The lines leading from the filter to the bearing should be of copper tubing or pickled piping. The drain line leading from the bearings to the storage or sump tank should be of adequate size for proper flow, and they should be set at a minimum slope of per foot, perferably per foot. Avoid unnecessary elbows and fittings wherever possible. Avoid bends which create traps and which might accumulate impurities. All lines should be thoroughly cleaned and flushed with a solvent, and then blown free with air, before oil is added.

It is advisable to interlock the oil pump motor with the mill motor in such a way that the mill cannot be started until after the oil pump is operating. We recommend the use of a non-adjuslable valve at each bearing to prevent tampering.

When using the drip oil system it is advisable to place wool yarn or waste inside a canvas porous bag to prevent small pieces of the wool being drawn down into the trunnion journal. If brick grease is used, care should be taken in its selection with regard to the range of its effective temperature. In other words, it should be pointed out that brick grease is generally designed for a specific temperature range. Where the bearing temperature does not come up to the minimum temperature rating of the brick grease, the oil will not flow from it, and on the other hand if the temperature of the bearing exceeds the maximum temperature rating of the brick grease, the brick is subject to glazing; therefore, blinding off of the oil. This brick should be trimmed so that it rests freely on the trunnion journal, and does not hang up, or bind on the sides of the grease box.

When replacing the brick grease, remove the old grease completely. Due to the extended running time of brick grease, there is usually an accumulation of impurities and foreign matter on the top surface, which is detrimental to the bearing.

Where anti-friction bearings are supplied, they are adequately sealed for either grease or oil lubrication. If a flood system is used for the trunnion bearings and it is adequately filtered, it can then be used for pinion bearings with the same precautions taken as mentioned above, with a flow of to 1 gallons per minute to each bearing.

These lubricants can be applied by hand, but we highly recommend some type of spray system, whether it be automatic, semi-automatic or manually operated. It has been found that it is best to lubricate gears frequently with small quantities.

Start the lubrication system and run it for about ten minutes, adjusting the oil flow at each bearing. Check all of the bolts and nuts on the mill for tightness and remove all ladders, tools and other obstructions prior to starting the mill.

Before starting the mill, even though it is empty, we recommend that it be jogged one or two revolutions for a check as to clearance of the gear and its guard, splash rings, etc. The trunnion journal should also be checked for uniform oil film and for any evidence of foreign material which might manifest itself through the appearance of scratches on the journal. If there are any scratches, it is very possible that some foreign material such as weld splatter may have been drawn down into the bushing, and can be found imbedded there. These particles should be removed before proceeding further.

If everything is found to be satisfactory, then the mill should be run for ten to fifteen minutes, and stopped. The trunnion bearings should be checked for any undue temperature and the gear grease pattern can be observed for uniformity which would indicate correct alignment.

It should be noted that with an empty mill the reactions and operating characteristics of the bearings and gearing at this point are somewhat different than when operating with a ball or rod charge. Gear noises will be prominent and some vibration will occur due to no load and normal back-lash. Furthermore, it will be found that the mill will continue to rotate for some time after the power is shut off. Safety precautions should therefore he observed, and no work should be done on the mill until it has come to a complete stop.

We have now reached the point where a half ball or rod charge can be added, and the mill run for another six to eight hours, feeding approximately half the anticipated tonnage. The mill should now be stopped, end the gear grease pattern checked, and gear and pinion mesh corrected, if necessary, according to separate instructions.

The full charge of balls or rods can now be added, as well as the full amount of feed, and after a run of about four to six days, ALL BOLTS SHOULD AGAIN BE RETIGHTENED, and the gear and pinion checked again, and adjusted if necessary.

Where starting jacks are provided for the trunnion bearings of the larger sized mills, they should be filled with the same oil that is used for the lubrication of the trunnion bearings. Before starting the mill they should be pumped so as to insure having an oil film between the journal and the bushing.

When relining any part of the mill, clean away all sand from the parts to be relined before putting in the new liners. For the head liners and shell liners you may then proceed in the same manner used at the time of the initial assembly.

Before relining the grate type discharge head, it is advisable to refer to the assembly drawings and the parts list.Because of such limitations as the size of the manhole opening, and for various other reasons, it will be found that the center discharge liner and cone designs vary. The cone may be a separate piece or integral with either the trunnion liner, or the router discharge liner. Furthermore, it will be found in some mills that the center discharge liner is held by bolts through the discharge head, whereas in other cases it depends upon the clamping effect of grates to hold it in position. In any event, the primary thing to remember in assembling the discharge grate head parts is the fact that the grate should be first drawn up tightly towards the center discharge liner by adjusting the grate set screws located at the periphery of the discharge head. This adjustment should be carried out in progressive steps, alternating at about 180 if possible and in such a manner that, the center discharge liner does not become dislodged from its proper position at the center of the mill. These grate set screws should be adjusted with the side clamp bar bolts loosened. After the grates have been completely tightened with the set screws, check for correct and uniform position of each grate section. The side clamp bar bolts may now be lightened, again using an alternate process. This should result in the side clamp bars firmly bearing against the beveled sides of the grates. The side clamp bars should not hear against the lifter liners.

When new pan liners are installed, they should be grouted in position so as to prevent pulp race in the void space between the discharge head and the pan liner. Another good method of preventing this pulp race is the use of the sponge rubber which can be cemented in place.

After the mill is erected, in order to avoid overlooking both obvious and obscure installation details, we recommend the use of a check list. This is particularly recommended for multiple mill installations where it is difficult to control the different phases of installation for each and every mill. Such a check list can be modeled after the following:

No. 1 Connecting Bolts drawn tight. A. Head and Shell flange bolts. B. Gear Connecting, bolts. No. 2 Trunnion studs or bolts drawn up tight. No. 3 Trunnion liner and feeder connecting bolts or studs drawn up tight. No. 4 Feeder lip bolts tightened. No. 5 Liner bolts drawn up tight. No. 6 Gear. A. Concentric B. Backlash C. Runout D. Joint bolts drawn up tight. No. 7 Coupling and Drive alignment and lubrication. No. 8 Bearings and Gearing cleaned and lubricated. No. 9 Lubrication system in working order with automatic devices including alarms and interlocking systems.

We further recommend that during the first thirty to sixty days of operation, particular attention be given to bolt tightness, foundation settlement and condition of the grouting. We suggest any unusual occurrence be recorded so that should trouble develop later there may be a clue which would simplify diagnosing and rectifying the situation.

As a safety precaution, and in many cases in order to comply with local safety regulations, guards should be used to protect the operators and mechanics from contact with moving parts. However, these guards should not be of such a design that will prevent or hinder the close inspection of the vital parts. Frequent inspection should be made at regular intervals with particular attention being given to the condition of the wearing parts in the mill. In this way, you will be better able to anticipate your needs for liners and other parts in time to comply with the current delivery schedules.

When ordering repair or replacement parts for your mill, be sure to identify the parts with the number and description as shown on the repair parts list, and specify the hand and serial number of the mill.

By following the instructions outlined in this manual, mechanical malfunctions will be eliminated. However, inadvertent errors may occur even under, the most careful supervision. With this in mind, it is possible that some difficulties may arise. Whenever any abnormal mechanical reactions are found, invariably they can be attributed to causes which though sometimes obvious are often hidden. We sight herewith the most common problems, with their solutions.

Cause A GROUT DISINTEGRATION. Very often when the grouting is not up to specification the vibration from the mill tends to disintegrate the grouting. In most instances the disintegration starts between the sole plate and the top surface of the grouting near or at the vertical centerline of the mill. As this continues, the weight of the mill causes the sole plate and trunnion bearing base to bend with a resultant pinching action at the side of the bearing near the horizontal center line of the mill. This pinching will cut off and wipe the oil film from the journal and will manifest itself in the same manner as if the lubrication supply had been cut off. If the grout disintegration is limited to about . 050 and does not appear to be progressing further, the situation can be corrected by applying a corresponding amount of shimming between the trunnion bearing base and the sole plate near the centerline of the mill in such a fashion that the trunnion bearing base has been returned to its normal dimensional position. If, on the other hand, the grouting is in excess of . 050 and appears to be progressing further, it is advisable to shut down operations until the sole plate has been re grouted.

Cause B HIGH SPOT ON THE BUSHING. While all BallMill bushings are scraped in the shop to fit either a jig mandrel or the head proper to which it is to be fitted, nevertheless there is a certain amount of seasoning and dimensional change which goes on in the type of metals used. Therefore if high spots are found, the mill should be raised, the bushings removed and rescraped. Bluing may be used to assist in detecting high spots.

Cause C INSUFFICIENT OIL FLOW. Increase the oil supply if it is a flood oil system. If brick grease is used, it is possible that the particular grade of brick may not be applicable to the actual bearing temperature. Refer to the remarks in this manual under the paragraph entitled Lubrication.

Cause E EXCESSIVE RUBBING ON THE SIDE OF THE BUSHING. This comes about due to the improper setting of the bearings in the longitudinal plane. In some cases, particularly on dry grinding or hot clinker grinding mills, the expansion of the mills proper may account for this condition. In any event, it can be remedied by re-adjusting the bearing base on the sole plate longitudinally at the end opposite the drive.

There are many more lubricant suppliers, such as E. F. Houghton and Co. , or Lubriplate Division of Fiske Bros. Refining Co. In making your final selection of lubricants, you should consider the actual plant conditions as well as the standardization of lubricants. New and improved lubricants are being marketed, and we, therefore, suggest that you consult your local suppliers.

spiral classifier for mineral processing

spiral classifier for mineral processing

In Mineral Processing, the SPIRAL Classifier on the other hand is rotated through the ore. It doesnt lift out of the slurry but is revolved through it. The direction of rotation causes the slurry to be pulled up the inclined bed of the classifier in much the same manner as the rakes do. As it is revolved in the slurry the spiral is constantly moving the coarse backwards the fine material will flow over the top and be travelling fast enough to be able to work its way downwards to escape. The Variables of these two types of classifiers are The ANGLE of the inclined bed, this is normally a fixed angle the operator will not be able to adjust it.

The SPEED of the rakes or spirals, the DENSITY of the slurry, the TONNAGE throughput and finally the SETTLING RATE of the ore itself.To be effective all of these variables must be balanced. If the incline is too steep the flow of slurry will be too fast for the rakes or spirals to separate the ore. If the angle is too flat the settling rate will be too high and the classifier will over load. The discharge rate will be lower than the feed rate, in this case. The load on the rakes will continue to build until the weight is greater than the rake or spiral mechanism is able to move. This will cause the classifier to stop and is known as being SANDED UP. If the speed of the rakes or spirals are too fast, too much will be pulled, out the top. This will increase the feed to the mill and result in an overload in either the mill or classifier as the circuit tries to process the increased CIRCULATING LOAD.

The DENSITY of the slurry is very important, too high the settling will be hampered by too many solids. Each particle will support each other preventing the heavier material from quickly reaching the bottom of the slurry. This will not allow a separation to take place quickly. The speed at which the slurry will be travelling will be slow and that will hamper effective classification. Another variable is the TONNAGE. All equipment has a limit on the throughput that anyone is able to process, classifiers are no different. This and the other factors will have to be adjusted to compensate for the last variable, the ore itself. Every ore type has a different rate of settling. To be effective each of the previous variables will have to be adjusted to conform to each ones settling characteristics.

The design of these classifiers (rake, spiral, screw) have inherent problems, First, they are very susceptible to wear, caused by the scrubbing action of the ore, that plus all of the mechanical moving parts create many worn areas to contend with. The other problem that these classifiers have is that they are easily overloaded. An overloaded classifier can quickly deteriorate into a sanded-up classifier. Once that happens the results are lost operating time, spillage and a period of poor Mineral Processing and Separation performance.

Another mechanical classifier is the spiral classifier. The spiral classifier such as the Akins classifier consists of a semi-cylindrical trough (a trough that is semicircular in cross-section) inclined to the horizontal. The trough is provided with a slow-rotating spiral conveyor and a liquid overflow at the lower end. The spiral conveyor moves the solids which settle to the bottom upward toward the top of the trough.

The slurry is fed continuously near the middle of the trough. The slurry feed rate is so adjusted that fines do not have time to settle and are carried out with the overflow .liquid. Heavy particles have time to settle, they settle to the bottom of the trough and the spiral conveyor moves the settled solids upward along the floor of the trough toward the top of the trough/the sand product discharge chute.

gravity spiral concentrator working principle

gravity spiral concentrator working principle

The gravity spiral circuit is designed to extract and concentrate coarse gold from the recirculating load in the mill grinding circuit and hence prevent a build up within that circuit and the eventual escape of some of that gold into the C.I.L. tanks and thereon into the final tails. (See fig. 4)

For the spirals to work efficiently the feed supply must have consistent characteristics and be of a constant rate. Variations in the flow rate, the feed size distribution and percentage solids will have adverse effects upon separation. Generally the solids tonnage should give adequate loading of the concentrate and middlings areas and the pulp density should be low enough to ensure mobility of particles in these areas. BUY SPIRALS

Feed to the spirals may be adjusted by the moving of two splitter arms on either side of the cyclone underflow discharge box, this altering the volume of the feed passing over the splitter screen. (See fig. 5)

The feed may also be adjusted by varying the speed of the gravity feed pump. This is necessary when the mill feed has been dropped and it is impossible to get sufficient feed for the gravity pump by adjustment of the splitter arms. At such times the speed will need to be dropped and the water additionadjusted to provide optimum feed density.

The pulp density may be altered by the addition of water, before the splitter screen, in the gravity feed pump hopper or to the concentrate launder beneath the primary spirals. The latter option adjusts the density of the feed to the cleaner spiral only.

The static distributor (See fig. 6) at the head of the primary spirals ensures an accurate division of the pulp stream to the spirals. For maximum efficiency a constant head should be maintained in the head pot. The head can be adjusted by either altering the flow rate from the splitter screen and/or altering the annular gap between the head pot and the distributor body, by moving the head pot up or down as required.

Feed from the splitter screen passes down into the gravity feed pump hopper and from there it is pumped to the static distributor above six triplex type primary spirals. As the pulp passes down these spirals; separaration of particles occurs according to specific gravity and the heavier minerals progress to the inner profile while lighter minerals are forced towards the outer profile, along with most of the water and slimes. At the bottom of each spiral layer there are splitters which can be adjusted to ensure the optimum recovery of coarse gold. (See fig. 7)

The middlings and tailings from the primary spirals are directed to both the mill feed and the mill discharge pump. The proportion going to either may be adjusted so as to help achieve optimum grinding conditions.

The concentrate from the inner outlet of the cleaner spiral is fed directly on to the Wilfley table and the middlings and tailings report to the gravity sump pump which feeds into the mill discharge pump feed hopper.

The Humphreys Spiral Concentrator, which was invented by I. B. Humphreys and first used in 1943 for concentrating chromite in Oregon beach sands, consists of five or six spiral turns of a modified semicircular launder which is about the size of a conventional automobile tire. Feed enters the top spiral and the tailing discharges from the bottom one, while concentrate and middlings are cut off by outlet ports regularly spaced at each turn of the spiral, and the products passed through rubber hoses to common launders which run the full length of a bank of spirals. Wash water is supplied from a small wash-water channel paralleling the main channel.

Operating entirely by gravity flow and involving no mechanical parts, the separation of the heavy constituents of the feed is effected by the same centrifugal forces and flow gradients encountered in ordinary river or stream concentration.

A capacity of 38 tons per spiral was obtained in the 1000-ton per 24 hr. Oregon plant operating on about a minus 40-mesh feed and in the 5000-ton plant recently installed near Jacksonville to concentrate ilmenite 174 roughing, and 12 finishing spirals have replaced an installation of tables and flotation cells.

The Humphreys Spiral has been successfully applied to recovery of chromite from chrome sands, rutile, ilmenite, and zircon from sand deposits, tantalum minerals and lepidolite from their ores, gravity concentration of base metal ores and in the cleaning of fine coal.

How it works: Pulp is introduced at the top of the spiral and flows downward. As the pulp follows the spiral channel the light particles in the pulp stream move outward and upward into the fast flowing portion of the stream while the heavy particles move to the inner slow moving portion of the stream, where they are drawn off through concentrate ports.

Adjustable splitters allow any portion to be removed through the ports. Tailing is discharged at the bottom of the spiral. Spirals are usually installed in double units, two spirals to a frame, in rows of two to twelve. Feed is split evenly to all spirals. At one plant 21 rows of 12 spirals each are fed by one pump.

The Humphreys Spiral Concentrator is a simple, efficient gravity concentrator which effects a separation between minerals of the proper size range that have sufficient difference in their specific gravity.

This concentrator is a spiral conduit of modified semi-circular cross-section, with outlets for removal of concentrate and middling. Pulp is introduced at the top of the spiral. As the pulp follows the spiral channel, lighter particles in the pulp stream move outward and upward into the fast moving part of the pulp stream. The heavy particles move to the inner, slow moving portion of the stream, where they are drawn off through concentrate or middling outlets. Adjustable splitters allow any portion of the concentrate or middling to be diverted through the outlets. Tailing discharges from lower end of spiral. A full- size spiral is used for laboratory testing. Two arrangements are suggested for test work.

In the closed circuit test unit illustrated, although a full-size spiral is used, as little as 20 pounds of material will indicate the possibility of spiral concentration in a batch test. By removing measured quantities of products, and adding like amounts of feed in repeated steps, substantial samples may be taken for analysis and estimates of capacity. Results from this procedure, using 100 to 300 pounds of material, are close to pilot test results.

Another arrangement, also using a full-size spiral, is a small pilot plant, and is suitable for test work where a larger quantity of material can be handled. The storage tanks may be built on the job from drawings supplied. This unit allows continuous feeding of material and permits accumulation of concentrate and tailing in separate tanks, which may then be re-run as feed for second stage concentration or scavenging of tailing.

Spiral concentrators are modem, high capacity, low costunits developed for the concentration of low grade ores. Spirals consist of a single or double helical sluice wrapped around a central support with a wash water channel and a series of concentrate take-off ports placed at regular intervals along the spiral (Figure 17). To increase the amount of material that can be processed by one unit, two or more starts are constructed around one central support. New spirals have been developed that do not use wash water. These new units have modified cross sections and only one concentrate-take-off port, which is

rapid wear of the rubber lining and irregular wash water distribution resulted in major production problems. Although still in use, the Humphreys cast iron spirals have been largely superseded by a variety of other types, notably the fiberglass Reichert spirals and new, lightweight Humphreys spirals.

The processes involved in mineral concentration by spirals are similar for all models. As feed containing 25-35% solids by volume is fed into the channel, minerals immediately begin to settle and classify. Particles with the greatest specific gravity rapidly settle to the bottom of the spiral and form a slow-moving fluid film. Thus the flow divides vertically: one level is a slow-moving fluid film composed of heavy and coarse minerals; the other level, the remainder of the stream, is composed of lighter material and comprises the bulk of the wash water. The slow-moving fluid film, its velocity reduced by friction and drag, flows towards the lowest part of the spiral cross-section (nearest the central support) where removal ports are located. The stream containing the lighter minerals and the wash water develops a high velocity, and is thrust against the outside of the channel (Figure 18). Separation is enhanced by the differences in centrifugal forces between the two: the lighter, faster flowing material is forced outward towards the surface, and the heavier, slower material remains inward towards the bottom.

Spiral concentrators are capable of sustained recoveries of heavy minerals in the size range of 3 mm down to 75 microns (6 to 200 mesh). They are suitable for use as roughers, cleaners, or scavengers. Feed rates may vary from 0.5 to 4 tons per hour per start, depending on the size, shape, and density of the valuable material. Some factors that affect recovery are the diameter and pitch of the spiral, the density of the feed, the location of splitters and take-off points, and the volume and pressure of thewash water. Individual spirals are easily monitored and controlled, but a large bank of spirals requires nearly constant attention.

Advantages of spiral concentrators include low cost, long equipment life, low space requirements, and good recovery of fine material. They can also be checked visually to determine if the material is separating properly. For maximum operating efficiency, feed density should remain constant, the particle-size distribution of the feed should be uniform, and fluctuations in feed volume should be minimized. Spiral concentrators will tolerate minor feed variations without requiring adjustment. Spiral concentrators, like cone concentrators, are efficient, low-maintenance units that should be considered for any large- scale gravity separation system.

The newer Humphreys spirals are capable of recovering particles as small as 270 mesh (53 microns). In a test at CSMRI, a new Mark VII Reichert spiral recovered 91.3% of the free gold contained in the feed in a concentrate representing only 5.4% of the feed weight. The unit showed little decrease in gold recovery efficiency with material down to 325 mesh (45 microns) (Spiller, 1983).

spiral (concentrators) - an overview | sciencedirect topics

spiral (concentrators) - an overview | sciencedirect topics

The spiral concentrator is a modern high-capacity and low-cost device. It is developed for concentration of low-grade ores and industrial minerals in slurry form. It works on a combination of solid particle density and its hydrodynamic dragging properties. The spirals consist of a single or double helical conduit or sluice wrapped around a central collection column. The device has a wash water channel and a series of concentrate removal ports placed at regular intervals. Separation is achieved by stratification of material caused by a complex combined effect of centrifugal force, differential settling, and heavy particle migration through the bed to the inner part of the conduit (Fig.13.31). Extensive application is the treatment of heavy mineral beach sand consisting of monazite, ilmenite, rutile, zircon, garnet, and upgrade chromite concentrate. Two or more spirals are constructed around one central column to increase the amount of material that can be processed by a single integrated unit.

The spiral concentrator first appeared as a production unit in 1943 in the form of the Humphrey Spiral, for the separation of chrome-bearing sands in Oregon. By the 1950s, spirals were the standard primary wet gravity separation unit in the Australian mineral sands industry.

In the spiral concentrator the length of the sluicing surface required to bring about segregation of light from heavy minerals is compressed into a smaller floor space by taking a curved trough and forming into a spiral about a vertical axis. The slurry is fed into the trough at the top of the spiral and allowed to flow down under gravity. The spiralling flow of pulp down the unit introduces a mild centrifugal force to the particles and fluid. This creates a flow of pulp from the centre of the spiral outwards to the edge. The heaviest and coarsest particles remain near the centre on the flattest part of the cross-section, while the lightest and finest material is washed outwards and up the sides of the launder (Fig. 15.15). This separation may be assisted by the introduction of additional water flowing out from the centre of the spiral either continuously or at various locations down the length of the spiral. This wash water may be distributed through tubes or by deflection from a water channel that runs down the centre of the spiral. Some present designs have overcome the need for this wash water. Once the particle stream has separated into the various fractions, the heavy fraction can be separated by means of splitters at appropriate positions down the spiral. A concentrate, middlings mid tailing fraction can be recovered.

In practice spirals are arranged in stacks or modules of roughers, scavengers and cleaners, where the initial concentrate is retreated to upgrade the fraction to its final grade. Spiral length is usually five or more turns for roughing duty and three turns in some cleaning unite. For coal concentration, 6 turns providing a gentler slope with longer residence time for the more difficult separation.

The performance of spirals is dependent on a number of operating parameters, summarised in Table 15.9. Spirals generally a chieve an upgrade ratio of 3:1 (heavy fraction:feed grade) and hence multiply treatments are required [13]. The presence of slimes adversely affects the spiral performance. More than 5% of 45m slimes will affect the separation efficiency.

With the steep pitch of a spiral, two or three spirals can be wound around the same common column and these types of spirals have been used in Australia for more than 20years. The multistart spirals conserve floor space and launder requirements. These triple-start spirals are built into a twelve spiral module and for these modules, the design of the distributor is critical to ensure that each spiral has a uniform feed.

The splitter blades on these spirals are all adjustable to direct the heavy fraction into pipes or a collecting launder. The current range of spirals available consist of a number of different profiles which have individual separation characteristics. The dimensions of some of the available spirals range from 270 406mm pitch, 590 700mm diameter and 2.1 2.4m high.

The advantages that modem spirals offer are simple construction requiring little maintenance, low capital cost and low operating cost - no reagents required, no dense media losses occur, low operating personnel required.

This is another variation of gravity separation, using density differences and centrifugal force; Figure 3.13. Originally known as Humphreys spiral (after the inventor) a wide range of devices are now available. A spiral concentrator consists of a helical conduit of semi-circular cross-section. Feed pulp of between 15 and 45 percent solids in the size range 3 mm to 75 m is introduced at the top of the spiral. As it flows downwards, the particles stratify due to the combined action of centrifugal force, the differential settling rates of the particles, and the effect of interstitial trickling through the flowing particle bed. The higher specific gravity particles are removed through the port located at the lowest point in the cross-section. Wash water added at the inner edge of the stream, flows outwardly across the concentrate band. Adjustable splitters control the width of the concentrate band removed at the ports. The grade of concentrate drawn from descending ports decreases progressively, with tailings discharged from the lower end of the spiral conduit.

Gravity concentration is a proven process for mineral beneficiation. The gravity concentration techniques are often considered where flotation practice is less efficient and operational costs are high due to extremely complicated physical, chemical and mechanical considerations. The gravity separations are simple and separate mineral particles of different specific gravity. This is carried out by their relative movements in response to gravity along with one or more forces adding resistance to motion offered by viscous media such as air or water. Particle motion in a fluid depends on specific gravity, size and shape of the moving material. The efficiency increases with coarser size to move sufficiently but becomes sensitive in presence of slimes. There are many types of gravity separators suitable for different situations. There are many devices for gravity concentration. The common methods are manual pans, jigs, pinched sluice and cones, spiral concentrator and shaking table to name a few.

Panning as a mineral or metal recovery technique was known to ancients since centuries past. Gold panning was popular and extensively practiced in California, Argentina, Australia, Brazil, Canada, South Africa and India during the nineteenth century. Panning is done by manual shaking of tray containing riverbed sand and gravels, alluvial deposits containing precious metals like gold, silver, tin, tungsten etc. The shaking of the tray separates sand, stones and fine-grained metals into different layers by differential gravity concentration (Fig. 12.28). The undesired materials are removed. This is primitive practice at low cost and generally in practice at small scale by the local tribal people.

Jigs are continuous pulsating gravity concentration devices. Jigging for concentrating minerals is based exclusively on differences in the density of the particles. The elementary jig (Fig. 12.29) is an open tank filled with water. A thick bed of coarse heavy particles (ragging) is placed on a perforated horizontal jig screen. The feed material is poured from the top. Water is pulsated up and down (the jigging action) by pneumatic or mechanical plunger. The feed moves across the jig bed. The heavier particles penetrate through the ragging and screen to settle down faster as concentrate. The concentrate is removed from the bottom of the device. Jigging action causes the lighter particles to be carried away by the cross flow supplemented by a large amount of water continuously supplied to the concentrate chamber. Jig efficiency improves with relatively coarse feed material having wide variation in specific gravity. Jigs are widely used as efficient and economic coal cleaning device.

Pinched sluice and cones is an inclined trough made of wood, aluminum, steel and fiberglass, 60-90cm long. The channel tapers from about 25cm in width at the feed end to 3cm at the discharge end. Feed consisting of 50-65% solids enters the sluice and stratifies as the particles flow through the sluice. The materials squeeze into the narrow discharge area. The piling causes the bed to dilate and allows heavy minerals to migrate and move along the bottom. The lighter particles are forced to the top. The resulting mineral strata are separated by a splitter at the discharge end (Fig. 12.30). Pinched sluices are simple and inexpensive device. It is mainly used for separation of heavy mineral sands. A large number of basic units and recirculation pumps are required for an industrial application. The system is improved by development and adoption of the Reichert cone. The complete device is comprised of several cones stacked vertically in circular frames and integrated.

Spiral concentrator is a modern high-capacity and a low-cost device. It is developed for concentration of LGOs and industrial minerals in slurry form. It works on a combination of the solid particle density and its hydrodynamic dragging properties. Spirals consist of a single or double helical conduit or sluice wrapped around a central collection column. It has a wash water channel and a series of concentrate removal ports placed at regular intervals along the spiral. Separation is achieved by stratification of material caused by a complex combined effect of centrifugal force, differential settling and heavy particle migration through the bed to the inner part of the conduit (Fig. 12.31). The most extensive application is treatment of heavy mineral beach sand consisting of monazite, ilmenite, rutile, zircon, garnet etc. It is widely used to upgrade chromite concentrate. Two or more spirals are constructed around one central column to increase the amount of material that can be processed by a single integrated unit.

Shaking table consists of a sloping deck with a rifled surface. A motor drives a small arm that shakes the table along its length, parallel to the rifle pattern. This longitudinal shaking motion drives at a slow forward stroke followed by rapid return strike. The rifles are arranged in such a manner that heavy material is trapped and conveyed parallel to the direction of the oscillation (Fig. 12.32). Water is added to the top of the table and perpendicular to the table motion. The heaviest and coarsest particles move to one end of the table. The lightest and finest particles tend to wash over the rifles and to the bottom edge. Intermediate points between these extremes provide recovery of the middling (intermediate size and density) particles.

Shaking tables find extensive use in concentrating gold. It is also used in the recovery of tin and tungsten minerals. These devices are often used downstream of other gravity concentration equipments such as spirals, Reicherts cone, jigs and centrifugal gravity concentrators for final cleaning prior to refining or sale of product.

Multi-gravity separator (MGS) is a new development in flowing film concentration expertise which utilizes combined effect of centrifugal force and shaking (Fig. 12.33). Centrifugal force enhances the gravitational force and obtains better metallurgical performance by recovering particles down to 1m in diameter. It would otherwise escape into tailing stream if other conventional wet gravity separators like jigs, spiral, table etc. are used. The principle of the system consists essentially in wrapping the horizontal concentrating surface of a conventional shaking table into a cylindrical drum and then rotates. A force, many times greater than the normal gravitational pull, is exerted by this means on particles in the film flowing across the surface. This enhances the separation process to a great extent. MGS in close circuit with lead rougher cells of graphite schist-hosted sulfide ore improves the lead concentrate metallurgy from 20 to +40% Pb. Graphitic carbon content reduces simultaneously from >10 to less than 3%. Presence of graphitic carbon interferes with the flotation of sulfide ore resulting in low metal recovery and unclean concentrate. MGS improves the metallurgical recovery and quality of concentrate for graphite carbon-bearing sulfide ore and high alumina-bearing fine iron ore. MGS technique is working successfully at Rajpura-Dariba zinc-lead plant and all iron ore plant in India by decreasing graphitic carbon and alumina respectively. MGS improves 42.9% Cr2O3 with 73.5% recovery from the magnetic tailings of Guleman-Sori beneficiation plant in Turkey.

The spiral concentrator applies differential density separation between particles to separate the valuable minerals from the gangue minerals. They have been widely utilized in coal washing plants worldwide to treat material in the particle size range 1mm150 m (other reports show that spirals are capable of treating material down to 45um). Material of such size range is too coarse to be treated using froth flotation and too fine to be treated in large diameter heavy medium cyclones (Atasoy and Spottiswood, 1995; Holland-Batt, 1995; Honaker etal., 2007; Mohanty etal., 2014; Shi etal., 2018). The major factors that makes this concentrator attractable for its application is low capital and operating cost, higher recoveries and no reagents used.

The drawback of spirals is that they are density separators that are unable to obtain a D50 lower than 1.65g/cm3 but tend to have a D50 between 1.7 and 2.1g/cm3 and misplaces a significant amount of ash fines into the clean coal. Even the application of wider diameter spirals couldnt solve this problem. The D50 was further reduced by either reducing the feed rate, but that is uneconomic or by utilizing two stage spirals in a series platform, but such a setup is still not sufficient to obtain a fine coal product of 10% ash value (Barry etal., 2015; de Korte, 2016; Shi etal., 2018; Ye etal., 2018). With the development of LC3 spiral model, lower separation cut point densities (1.41.55) were achieved (de Korte, 2016; Palmer, 2016). Palmer (2016) investigated the ash level in both the middling stream and clean coal product using LC3 spiral model and the results are shown in Fig.10. The results show that LC3 spiral model has high separation efficiency at both low and high D50.

Limited work has been done on the structural parameter of spirals and too much attention is given to the particle size distribution, feed rate, solid concentration and splitter position(Gulsoy and Kademli, 2006). Trough profile of the spirals is important factor to consider on separation performance of fine coal. Spirals come in three forms of trough profiles (ellipse, cubic parabola and synthetic curves) as illustrated in Fig.11 (Atasoy and Spottiswood, 1995; Kapur and Meloy, 1998; Kwon etal., 2017; Ye etal., 2018). It was observed that coal tends to move to the peripheral end of the spiral as the feed rate and solid concentration increases. The D50 also increases with feed rate and concentration of solids. Spiral concentrator with the elliptical profile is preferable to collect the light particles in outer place, while spiral concentrator with the trough profile of cubic parabola is effective for the accumulation of heavy particles in the inner region of the trough. The spiral concentrator with the trough profile of cubic parabola in inner place, together with the elliptical profile in outer place, is more desirable to separate the coal fines and ultrafines.

Run-of-mine (ROM) chromite (mined ore, prior to beneficiation) is usually beneficiated with relatively simple processes. The most commonly applied processes include primary and secondary crushing, screening, milling, dense media separation and gravity separation methods (Murthy etal., 2011). More sophisticated processes such as flotation can also be used (Wesseldijk etal., 1999), but are usually not economically feasible.

In order to generate beneficiated lumpy, chip and/or pebble chromite ore (the coarser fractions, typically 6150mm) crushing, screening and dense media separation would be applied. The finer fraction (typically<6mm) of ROM chromite would normally be milled to approximately<1mm and then upgraded with a series of hydrocyclones and spiral concentrators to generate metallurgical and/or chemical grade chromite concentrate (Murthy etal., 2011). Fig.5 presents a process flow diagram for the beneficiation of chromite concentrate (<1mm) adapted from Murthy etal. (2011), who reviewed chromite beneficiation. The shaking tables (slime and scavenger tables) in this diagram would probably not be used in large-scale operations and the single spiral concentrators would probably consist of numerous banks of spiral concentrators operating in parallel.

Milling is the only process step applied during chromite beneficiation that has been implicated in the possible generation of Cr(VI). However, only dry milling of chromite has been proven to generate Cr(VI) (Beukes and Guest, 2001; Glastonbury etal., 2010). Extreme grinding (i.e. pulverization), which is not a typical comminution technique, was applied in both the afore-mentioned referenced studies and it could therefore be argued that Cr(VI) is less likely to be formed by industrial dry milling. However, Beukes and Guest (2001) also report relatively high levels of Cr(VI) in samples gathered from a dry ball mill circuit at a FeCr producer. In contrast, wet milling does not seem to generate Cr(VI) (Beukes and Guest, 2001). Wet milling would also be the obvious choice during chromite concentrate beneficiation, since hydrocyclones and spiral concentrators are wet processes. Also, during chromite concentrate beneficiation the milling step would be aimed only at liberating the chromite crystals from the gangue minerals. This is in contrast to the dry milling tests conducted by Beukes and Guest (2001) and Glastonbury etal. (2010), during which Cr(VI) was generated, where the intent was to obtain particle sizes fine enough for pelletization (which will be discussed in Section 3.2.2 and 3.2.3).

Rare earth minerals are good candidates for gravity separation as they have relatively large specific gravities (47) and are typically associated with gangue material (primarily silicates) that is significantly less dense (Ferron et al., 1991). The most commonly utilized application of gravity separation is in monazite beneficiation from heavy mineral sands. Beach sand material is typically initially concentrated using a cone concentrator to produce a heavy mineral pre-concentrate (2030% heavy minerals) before a more selective gravity separation step, often employing a spiral concentrator, is used to achieve concentrations of 8090% heavy minerals (Gupta and Krishnamurthy, 1992). At this point, a series of magnetic, electrostatic and further gravity separation operations must be applied, according to each individual deposits mineralogy (Ferron et al., 1991).

An example of a flowsheet designed to concentrate monazite from Egyptian beach sands containing approximately 30wt.% valuable heavy minerals can be seen in Fig. 3 (Moustafa and Abdelfattah, 2010). In this flowsheet, low specific gravity gangue is discarded via wet gravity concentration (the authors employed a Wifley shaking table for this purpose), then low intensity magnetic separation is used to discard any ferromagnetic minerals without removing paramagnetic monazite (Moustafa and Abdelfattah, 2010). The non-magnetic stream that remains contains most of the valuable monazite, zircon and rutile as well as a portion of the gangue minerals which were not removed in the first two steps. A series of gravity, magnetic and electrostatic separations are then applied to exploit the different properties of the monazite, zircon and rutile minerals and produce the final concentrate streams. Rutile is removed as it reports to the conductor fraction after electrostatic separation (monazite and zircon are non-conductive) and then diamagnetic zircon may be removed from the paramagnetic monazite via further magnetic separation (Moustafa and Abdelfattah, 2010).

Fig. 3. Flowsheet for concentrating monazite from Egyptian beach sand. For each stream, the first and second percentages represent total weight recovery and monazite grade respectively. Reproduced with permission from (Moustafa and Abdelfattah, 2010).

In addition to the processing of beach sands, gravity separation, (shaking tables, spiral concentrators, and conical separators) is used in combination with froth flotation at many rare earth mineral processing operations throughout China (Chi et al., 2001). An example of this is at Bayan Obo, where gravity separation has been employed between the rougher and cleaner flotation circuits to efficiently separate monazite and bastnsite from the iron-bearing and silicate gangue material (Chi et al., 2001; Jiake and Xiangyong, 1984). Some challenges associated with gravity separation of the Bayan Obo ore are that gangue minerals (e.g. barite) have similar specific gravities to the desired rare earth minerals and report to the concentrate stream. In addition, gravity separation is ineffective at separating very fine particles resulting in large losses of rare earths (Ming, 1993). Some separation of very fine particles can be achieved for minerals with very large differences in specific gravity, such as gold from silicate gangue, by employing centrifugal gravity separators such as the Knelson, Falcon and Mozley Multi-Gravity Separators (Falconer, 2003; Gee et al., 2005). Most of these fine particle separators are designed for semi-continuous operation where the valuable dense material is present in low concentrations (<0.1wt.%) which may limit their suitability to REE mineral separation (Fullam and Grewal, 2001). The ongoing development of centrifugal separators capable of continuous operation (e.g. Knelson CVD) may address this issue as the manufacturers claim to be able to process feed materials with valuable heavy mineral contents of up to 50wt.% (Fullam and Grewal, 2001).

Outside of China, lab-scale gravity separations have been successfully completed on Turkish and Australian deposits with very fine-grained mineralizations (Guy et al., 2000; Ozbayoglu and Umit Atalay, 2000). In both of these cases, one of the key findings was that the rare-earth minerals were concentrated into the very fine (<5m) particle size range (Guy et al., 2000; Ozbayoglu and Umit Atalay, 2000). This was dealt with by either modifying the grinding steps to prevent excess fine generation or by employing a Multi-Gravity Separator, specifically designed to recover ultrafine particles via gravity separation (Guy et al., 2000; Ozbayoglu and Umit Atalay, 2000). The modified grinding procedure employed an attrition scrubbing step prior to further grinding to produce a product that was 100% 300m (the size which was identified as the maximum limit for downstream flotation), while reducing the slime losses to the 5m size fraction by an average of 7.8% (Guy et al., 2000). The results from Guy et al. (2000) can be seen in Fig. 4. The importance of adequately liberating rare earth minerals without excessive fine production has also been shown by Fangji and Xinglan (2003) who employed screening and secondary grinding steps after gravity and magnetic separations at a mine in Maoniuping, China to produce a bastnsite flotation concentrate with a grade of 62% REO and a recovery of 8085%.

A final interesting application of gravity separation to rare earth mineral concentration is the use of roasting operations prior to gravity separation as outlined in a 1956 patent (Kasey, 1956). The idea presented involved roasting a rare earth carbonate ore at temperatures in excess of 1000C to convert the carbonates into oxides, thereby increasing the mineral density and susceptibility to gravity separation (Kasey, 1956). The process proposed by Kasey (1956) included an industrial application involving quenching the roasted ore particles from high temperatures; a process that would likely significantly decrease the energy required for crushing and grinding operations as detailed by Fitzgibbon (1990) in their research into thermally assisted liberation. To the best of authors knowledge, this process was never successfully applied on an industrial or pilot scale.

ll spiral chute - shanghai zenith company

ll spiral chute - shanghai zenith company

LL Spiral Chute is a type of new gravity separation equipment, which is applicable to separate metallic minerals whose granularity is between 4-0.02, such as iron as well as other nonferrous metals, rare metals and nonmetallic minerals with adequate gravity difference.

Location:Saham, Oman Material:Limestone Input Size:Below 720mm Output Size:0-5mm, 5-10mm, 10-20mm, (Oman standard) Capacity:300t/h

Location:Russia Material:Plagiogranite Input Size:Below 700mm Output Size:0-5mm, 40-70mm (0-5mm, 5-10mm, 10-20mm) Capacity:300-350t/h

Location:Mecca Material:Granite Input Size:Below 1000mm Output Size:0-10mm, 10-13mm, 13-20mm, 20-25mm Capacity:400-500TPH (12 hours per day)

silica sand purifying process and equipment for the silica sand processing plant - xinhai

silica sand purifying process and equipment for the silica sand processing plant - xinhai

In general, the main mineral of natural silica sand and quartz sandstone is quartz, and some other impurity minerals are often associated with it, such as mica minerals, feldspar minerals and iron and aluminum oxide minerals. However, the mixing of these impurities greatly reduces the value of quartz sand and affects the quality of products. Therefore, it is necessary to carry out the impurity removal for the silica sand. Here are the silica sand purifying process and equipment for the silica sand processing plant.

The impurity resolution process of silica sand means to adopt the crushing, grinding and classifying to break the natural quartz sandstone ore and make the impurities and minerals reach the state of monomer dissociation, and then use the classifying operation to obtain the raw silica sand meeting the requirements of grain size, so as to prepare for the subsequent impurity resolution of silica sand.

Adopting the jaw crusher to break the natural quartz sandstone ore into small pieces, and then using the grinding mill to grind the small pieces of quartz ore into finer silica sand through the wet grinding way. The wet grinding process can not only eliminate the dust generated in the grinding process, but also make the particles of silica sand rub against each other, and prompt the impurities attached to the surface of silica sand (such as iron oxide and hydroxide) to dissociate from the silica sand particles. Subsequently, the grinding products enter into the classification operation, the silica sand meeting the particle size requirements can be used as raw silica sand to carry out the impurity removal of silica sand, the coarse grains are returned to grinding mill for re-grinding.

The impurity removal process of silica sand is to remove the mineral impurities from the raw silica sand by scrubbing and desliming, magnetic separation, chute gravity separation, flotation, pickling or several combined methods, so as to obtain the high-purity silica sand with required particle size and impurity content.

Adopting the mechanical force of the washing machine and the grinding force among the sand particles to remove the thin film iron outside the silica sand, the binding and muddy impurity mineral deposits.

Magnetic separation method is one of the effective methods to remove iron impurities from raw materials. The hematite, limonite, biotite and other minerals contained in the raw materials are weakly magnetic, which can be selected by the wet type strong magnetic separator with the magnetic field strength of 8105A/m. and using the weak magnetic separator or the medium magnetic separator can be used when the raw material contains the strong magnetic mineral (such as magnetite). In order to further remove other small amounts of weakly magnetic minerals (such as amphibole, pyroxene and the continuous growth of magnetic minerals and quartz), a high gradient magnetic separator with a magnetic field strength greater than 12,000 Gauss can be used for secondary magnetic separation.

When the raw material contains a very small amount of heavy mineral impurities (such as zircon), the spiral chute gravity separation method can be used to effectively remove such heavy mineral impurities because they don't have the magnetism and has a greater weight than silica sand.

When the raw mortar rotates along the spiral chute surface, the various minerals will be separated according to their respective specific gravity. Under the dual action of gravity and centrifugal force, the greater the difference in mineral specific gravity, the higher the separation degree will be. It is worth noting that the mineral morphology also has a certain influence on the separation of spiral chute. The flake minerals are easy to be separated from the granular minerals. Therefore, the separation of quartz mortar body can be divided into three parts when the spiral chute is used to separate the silica sand slurry: granular heavy mineral area, granular silica sand area, flaky and light mineral area, which can not only remove the heavy mineral in the silica sand, but also remove some sheets mica minerals.

If there are many mica minerals in the raw materials, it is impossible to completely remove the impurities only by using the spiral chute. In addition, if the raw material also contains a certain proportion of feldspar minerals who has a similar specific gravity to silica sand, the flotation process can be used to remove these impurities minerals. At the same time, it is necessary to increase the grinding fineness when the silica sand and gangue minerals do not reach a certain degree of monomer dissociation.

Adopting the three-stage flotation process to remove the mica and feldspar minerals from silica sand respectively. In the first stage flotation, the iron-containing argillite is extracted from the slurry by the corresponding flotation reagent system under a neutral or weak acid environment. In the secondary flotation, the mica minerals and their symbionts with silica sand are extracted from the slurry by the corresponding flotation reagent system under neutral or weak acid conditions. In the third stage flotation, the feldspar minerals and their symbionts with silica sand from slurry by the corresponding flotation reagent system under neutral or weak acid environment.

The pickling method can be adopted if the color of the finished sand or the product sand after the separation operation is red and the high content of iron and titanium does not meet the quality requirements of the product.

The above are process and the equipment used in the impurity removal of silica sand. Carrying out the impurity removal when the impurity mineral and silica sand reach the state of monomer dissociation can remove almost all the impurity mineral in the raw material, and greatly improve the purity of silica sand, so as to obtain the corresponding silica sand concentrate.

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