In many industries the final product, or the raw material at somestage of the manufacturing process, is in powdered form and in consequence the rapid and cheap preparation of powdered materials is a matter of considerable economic importance.
In some cases the powdered material may be prepared directly; for example by precipitation from solution, a process which is used in the preparation of certain types of pigments and drugs, or by the vacuum drying of a fine spray of the material, a process which is widely adopted for the preparation of milk powder, soluble coffee extracts and similar products. Such processes are, however, of limited applicability and in by far the greatest number of industrial applications the powdered materials are prepared by the reduction, in some form of mill, of the grain size of the material having an initial size larger than that required in the final product. These processes for the reduction of the particle size of a granular material are known as milling or grinding and it appears that these names are used interchangeably, there being no accepted technical differentiation between the two.
Examples of the first two classes occur in mineral dressing, in which size reduction is used to liberate the desired ore from the gangue and also to reduce the ore to a form in which it presents a large surface to the leaching reagents.
Under the third heading may be classed many medicinal and pharmaceutical products, foodstuffs, fertilizers, insecticides, etc., and under the fourth heading falls the size reduction of mineral ores, etc.; these materials often being reduced to particles of moderate size for ease in handling, storing and loading into trucks and into the holds of ships.
The quantity of powder to be subjected to such processes of size reduction varies widely according to the industries involved, for example in the pharmaceutical industries the quantities involved per annum, can be measured in terms of a few tons, or in the case of certain drugs, possibly a few pounds; whereas in the cement industry the quantities involved run into tens of millions of tons; the British cement industry alone having produced, in round figures, 12 million tons of Portland Cement.
For the preparation of small quantities of powder many types of mill are available but, even so, the ball mill is frequently used. For the grinding of the largest quantities of material however, the ball, tube or rod mill is used almost exclusively, since these are the only types of mill which possess throughput capacity of the required magnitude.
The great range of sizes covered by industrial ball mills is well exemplified by Fig. 1.1 and Fig. 1.2. In the first illustration is shown a laboratory batch mill of about 1-litre capacity, whilst in Fig. 1.2 is shown a tube mill used in the cement industry the tube having a diameter of about 8 ft and length of about 45 ft.
In Fig. 1.3 is shown a large ball mill, designed for the dry grinding of limestone, dolomite, quartz, refractory and similar materials; this type of mill being made in a series of sizes having diameters ranging from about 26 in. to 108 in., with the corresponding lengths of drum ranging from about 15 in. to 55 in.
At this point it is perhaps of value to study the nomenclature used in connection with the mills under consideration, but it must be emphasized that the lines of demarcation between the types to which the names are applied are not very definite.
The term ball mill is usually applied to a mill in which the grinding media are bodies of spherical form (balls) and in which the length of the mill is of the same order as the diameter of the mill body; in rough figures the length is, say, one to three times the diameter of the mill.
The tube mill is a mill in which the grinding bodies are spherical but in which the length of the mill body is greater in proportion to the diameter than is the case of the ball mill; in fact the length to diameter ratio is often of the order of ten to one.
The rod mill is a mill in which the grinding bodies are circular rods instead of balls, and, in order to avoid tangling of the rods, the length to diameter ratio of such mills is usually within the range of about 15 to 1 and 5 to 1.
It will be noticed that the differentiation between ball mill and the tube mill arises only from the different length to diameter ratios involved, and not from any difference in fundamental principles. The rod mill, however, differs in principle in that the grinding bodies are rods instead of spheres whilst a pebble mill is a ball mill in which the grinding bodies are of natural stone or of ceramic material.
As the name implies, in the batch mills, Fig. 1.4a, the charge of powder to be ground is loaded into the mill in a batch and, after the grinding process is completed, is removed in a batch. Clearly such a mode of operation can only be applied to mills of small or moderate sizes; say to mills of up to about 7 ft diameter by about 7 ft long.
In the grate discharge mill, Fig. 1.4b, a diaphragm in the form of a grating confines the ball charge to one end of the mill and the space between the diaphragm and the other end of the mill houses a scoop for the removal of the ground material. The raw material is fed in through a hollow trunnion at the entrance end of the mill and during grinding traverses the ball charge; after which it passes through the grating and is picked up and removed by the discharge scoop or is discharged through peripheral ports. In this connection, it is relevant to mention that scoops are sometimes referred to as lifters in the literature. In the present work, the use of the term lifter will be confined to the description of a certain form of mill liner construction, fitted with lifter bars in order to promote the tumbling of the charge, which will be described in a later section.
In the trunnion overflow mill, Fig. 1.4c the raw material is fed in through a hollow trunnion at one end of the milland the ground product overflows at the other end. In this case, therefore, the grating and discharge scoop are eliminated.
A variant of the grate discharge mill is shown in Fig. 1.4d, in which the discharge scoop is eliminated by the provision of peripheral discharge ports, with a suitable dust hood, at the exit end of the mill.
Within the classes of mills enumerated above there are a number of variations; for example there occur in practice mills in which the shell is divided into a number of chambers by means of perforated diaphragms and it is arranged that the mean diameter of the balls in the various chambers shall decrease towards the discharge end of the mill; such an arrangement being shown in Fig. 1.6. The reason for this distribution of ball size is that, for optimum grinding conditions, the ratio of ball diameter to particle diameter should be approximately constant. In consequence smaller balls should be used for the later stages of the grinding process, where the powder is finer, and by the adoption of a number of chambers in each of which the mean ball diameter is suitably chosen an approximation is made towards the desired constancy in the ratio of the ball size to the particle size.
The problem of the optimum distribution of ball size within a mill will be dealt with more fully in a later chapter, but at this point it is relevant to mention a mill in which the segregation of the balls is brought about by an ingenious method; especially as the mill carries a distinctive name, even though no principles which place it outside the classification given previously are involved.
The Hardinge mill, Fig. 1.7, uses spheres as a grinding agent but the body is of cylindro-conical form and usually has a length to diameter ratio intermediate between those associated with the ball mill and the tube mill. The reason for this form of construction is that it is found that, during, the operation of the mill, the largest balls accumulate at the large end of the cone and the smallest balls at the small end; there being a continuous gradation of size along the cone. If then the raw material is fed in at the large end of the mill and the ground product removed at the smaller end, the powder in its progression through the mill is ground by progressively small balls and in consequence the theoretical ideal of a constant ratio between ball size and particle size during grinding is, to some extent, attained.
The type of ball mill illustrated in Fig. 1.3, incorporates a peripheral discharge through line screens lining the cylindrical part of the mill. Heavy perforated plates protect the screens from injury and act as a lining for the tumbling charge; sometimes also the fine screen is further protected by coarse screens mounted directly inside it. This type of mill, which is often known as the Krupp mill, is of interest since it represents a very early type of mill which, with modifications, has retained its popularity. The Krupp mill is particularly suited to the grinding of soft materials since the rate of wear of the perforated liners is then not excessive. At this point it will perhaps be useful to discussthe factors upon which the choice between a ball a tube or a rod mill depends.
When a mill is used as a batch mill, the capacity of the mill is clearly limited to the quantity which can be handled manually; furthermore the mill is, as far as useful work is concerned, idle during the time required for loading and unloading the machine: the load factor thus being adversely affected. Clearly then, there will be a considerable gain in throughput, a saving in handling costs and improved load factor, if the mill operation is made continuous by feeding the material into the mill through one trunnion and withdrawing it either through the other trunnion or through discharge ports at the exit end of the mill body.
Since, however, the flow of powder through the mill is now continuous, it is necessary that the mill body is of such a length that the powder is in the mill for a time sufficiently long for the grinding to be carried to the required degree of fineness. This, in general, demands a mill body of considerable length, or continuous circulation with a classifier, and it is increased length which gives rise to the tube mill.
In the metallurgical industries very large tonnages have to be handled and, furthermore, an excess of fine material is undesirable since it often complicates subsequent treatment processes. In such applications a single-stage tube mill in circuit with a product classifier, by means of which the material which has reached optimum fineness is removed for transport to the subsequent processing and the oversize is returned to the mill for further grinding, is an obvious solution. Once continuous feed and a long mill body have been accepted, however, the overall grinding efficiency of the mill may be improved by fairly simple modifications.
As has already been mentioned; for optimum grinding conditions there is a fairly definite ratio of ball size to particle size and so the most efficient grinding process cannot be attained when a product with a large size range is present in the mill. If, however, a tube mill is divided into a number of compartments and the mean ball size of the grinding media decreases in each succeeding compartment; then the optimum ratio between ball size and particle size is more nearly maintained, and a better overall performance of the mill is achieved; this giving rise to the compartment mill shown in Fig. 1.6. The tube mill has the further advantage that, to some extent, the grinding characteristics of the mill are under control; for example, an increase in the size of the balls in the final chamber will reduce the rate of grinding of the finer fractions but will leave the rate of grinding of the coarser fractions sensibly unchanged and so the amount of coarse material in the final product will be reduced without any excessive overall increase in fineness.
The principal field of application of the rod mill is probably as an intermediate stage between the crushing plant and the ball mills, in the metallurgical industries. Thus, material between about 1-in. and 2-in. size may be reduced to about 6 mesh for feeding to the ball mills. Rod mills are, however, being used in closed circuit with a classifier to produce a product of less than about 48-mesh size, but such applications are unusual.
A machine that reduces the size of particles of raw material fed into it. The size reduction may be to facilitate removal of valuable constituents from an ore or to prepare the material for industrial use, as in preparing clay for pottery making or coal for furnace firing. Coarse material is first crushed.
Grinding mills are of three principal types, as shown in the illustration. In ring-roller pulverizers, the material is fed past spring-loaded rollers. The rolling surfaces apply a slow large force to the material as the bowl or other container revolves. The fine particles may be swept by an air stream up out of the mill. In tumbling mills the material is fed into a shell or drum that rotates about its horizontal axis. The attrition or abrasion between particles grinds the material. The grinding bodies may be flint pebbles, steel balls, metal rods lying parallel to the axis of the drum, or simply larger pieces of the material itself. In hammer mills, driven swinging hammers reduce the material by sudden impacts. See Crushing and pulverizing, Pebble mill, Tumbling mill
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Henan LIMING Heavy Industry Science and Technology Co. LTD which mainly manufacture large and medium-sized crushing and grinding equipments was founded in 1987. It is a modern joint-stock corporation with research, manufacturing and sales together. The Headquarter is located in HI-TECH Industry Development Zone of Zhengzhou and covers 80000 m . Another workshop in Shangjie Industry Park covers 67000 m . Over the past 30 years, our company adheres to modern scientific management system, precision manufacturing, pioneering and innovation. Now LIMING has become the leader in domestic and oversea machinery manufacturing industry.
December, 2016: LIMING launched MB5X158Pendulum Suspension Grinding Mill. Besides, the company was rated as 2016 annual innovative enterprise of the sand and gravel industry and 2016 annual excellent entrepreneur of the sand and gravel industry.
October, 2016: LIMING passed the certification of intellectual property management system. There are 106 national patents including 4 invention patents, 12 design patents and 90 patents for utility models.
March,2015: The model line of VU aggregate optimization system was completed at Zhengzhou headquarters base, 26 meters high, including six core modules, showing the superiority of dry sand making technology
April, 2015: MTW European Trapezium Mill was jointly awarded Industrial Energy Saving Products by The Ministry of Industry And Information Office of Henan Province, and Science And Technology Department Of Henan Province; Liming Heavy Industry was assessed as Intellectual Property Advantage Cultivate Enterprise of 2014.
January, 2015: Liming Heavy Industry was selected to The Fifth Member Of The National Mining Machinery Standards Committee; VSI5X Vertical Shaft Impact Crusher was identified as Brand-Name Products in Henan Province.
November, 2014: Customer Center was fully upgraded to Customer Security Center, and sublimated the service concept. HPT Series High-Efficiency Hydraulic Cone Crusher and LUM Series superfine vertical roller grinding mill both got the provincial Science and Technology Achievement, which reached worlds advanced level.
July, 2014: Liming Heavy Industry had the honor to be on the list of "Henan province Quality credit AAA grade enterprise brand", and became a member unit of "China quality integrity enterprise association".
June, 2014: Tired Mobile Crushing Plant Wheeled Mobile Crusher won the second prize for Scientific and Technological Achievements of Henan province. And the world largest Crawler Mobile Vibrating Screen was successfully delivered in Shangjie Industrial Park.
May, 2014: 2014 First international trade e-commerce services base of Google in China, as well as first Google Adwords experience center, both settled in Liming Heavy Industry. With their powerful combination, e-commerce of Henan was well boomed. MTW European Type Trapezium Mill won the first prize for Science and Technology progress award in Zhengzhou. Liming Heavy Industry had the honor to be on the list of top 100 enterprises for the employment of college students, which is the only involved enterprise in mining and crushing field.
March, 2014: Liming Heavy Industry got financial support from " high growth " program of government, and was recognized as " Outstanding Innovation Enterprise". Sales volume of the first quarter got a 30% year-on-year contrarian growth, and sales volume of March renovated the historic records of monthly sales volume.
May, 2013: Liming Heavy Industry had the honor to be on the list of top 100 enterprises in Zhengzhou. And Liming Heavy Industry would like to make greater contribution to the economic development of Central China.
March, 2013: Liming Heavy Industry was awarded the Title of" National high-tech enterprises" which highlights the important value of technology in mining machinery industry. At the same time, Liming Heavy Industry was awarded the Fast growing Company and became the demonstration enterprise in regional economy and the development of the industry.
October 2012: Hydraulic Cone Crusher and VSI Vertical Shaft Impact Crusher were rated as the leading domestic level, European Jaw Crusher was rated as the domestic advanced level by Science and Technology Department of Henan Province.
June, 2012 HPC efficient hydraulic cone crusher and VSI sand making machine are rated as the leading domestic level by the Science and Technology Department of Henan; European type jaw crusher has been assessed as a national advanced level.
March, 2012 Crushing and milling series of more than 100 models passed the Russian GOST quality certification, becoming the second international quality certification following the European CE certification.
JApril, 2011 A base for the new headquarters completed in a smooth relocation of various functional departments. Of building a base for the new headquarters will be completed by the end of the year, when the company will become the domestic mining machinery industry, the most superior hardware capabilities, production and processing of the strongest, most advanced scientific research strength enterprise.
December, 2010 The group company crossed the line of 1,000,000,000 RMB in annual sales./p> March, 2010 Henan Liming Heavy Industry Science & Technology Co.,Ltd. is evaluated as "The Famous Brand" in Henan Province. February, 2010The Employee Award Congress for 2009 close with great success. Annual Work Conference for 2010 is held successfully.
June, 2009YSY Mobile Cone Crusher Crushing Plant for medium reduction is delivered to Azerbaijan. It marks the production of mobile crushing plants in complete seriality and maturity after YGE and YSF series. Meanwhile, the newly introduced VSI5X sand making machine is set up in Russia. The upgrading of new products kicks off.
June, 2009YSY Mobile Cone Crusher Crushing Plant for medium reduction is delivered to Azerbaijan. It marks the production of mobile crushing plants in complete seriality and maturity after YGE and YSF series. Meanwhile, the newly introduced VSI5X sand making machine is set up in Russia. The upgrading of new products kicks off.
December, 2008the sales of the company is nearly 600,000,000RMB. Meanwhile, MTW Series European Technology Trapezium Mill and large LM Series Vertical Mill are launched. Additionally, the researching and development of large Jaw Crusher achieve many breakthroughs. The models of jaw crusher become more complete because of the introduction of PE10001200.
March, 2006Our company is awarded as "The Advanced Company in Sand-Stone Association". The jaw crusher, impact crusher and vertical shaft impact crusher is listed as the recommended products in the association.
November, 2001 The first machine exported is delivered to Kazakhstan and put into use. In the same year, our products are exported to Malaysia, Australia, South Korea, Indonesia, Russia and other countries and get good reputation there.
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In recently year, because of the energy-saving advantage in cement industry by vertical roller mill, the comprehensive benefit become more and more obvious, widely application and popularize in power, metallurgy, chemical industry and nonmetal etc.
Vertical roller mill is mainly used for grinding raw meal, clinker, Ground Granulated Blast Furnace Slag, iron ore, limestone, coal powder, coke powder, coal gangue, fly ash, volcanic ash, gypsum, calcite, pyrophyllite, quartz, clay, sandstone, bauxite and other processing industry related to metal and non-metal mine.
1Powerconsumptionslag25kWh/tcement18kWh/trawmeal9kWh/t 2Moisturecontentslag15%cement3%rawmeal7% 3Ffinishedproductfinenessspecificsurfaceareaofslag4200~112500px/gspecificsurfaceareaofcement3300~380cm/grawmealfineness-80mR12~15% 4Moisturecontentofrefinedpowderslag0.5~1.0%cement0.5~1.0%rawmeal0.5~1.0% 5Thespecificinstalledcapacityandparameterscanbeadjustedproperlyaccordingtoactualmaterialnature. 6Theaboveparametersareapplicableforgranularblastfurnaceslag,cementandrawmealandthemodelselectionisdeterminedbymaterialproperty.
Grinding circuits are fed at a controlled rate from the stockpile or bins holding the crusher plant product. There may be a number of grinding circuits in parallel, each circuit taking a definite fraction of the feed. An example is the Highland Valley Cu/Mo plant with five parallel grinding lines (Chapter 12). Parallel mill circuits increase circuit flexibility, since individual units can be shut down or the feed rate can be changed, with a manageable effect on production. Fewer mills are, however, easier to control and capital and installation costs are lower, so the number of mills must be decided at the design stage.
The high unit capacity SAG mill/ball mill circuit is dominant today and has contributed toward substantial savings in capital and operating costs, which has in turn made many low-grade, high-tonnage operations such as copper and gold ores feasible. Future circuits may see increasing use of high pressure grinding rolls (Rosas et al., 2012).
Autogenous grinding or semi-autogenous grinding mills can be operated in open or closed circuit. However, even in open circuit, a coarse classifier such as a trommel attached to the mill, or a vibrating screen can be used. The oversize material is recycled either externally or internally. In internal recycling, the coarse material is conveyed by a reverse spiral or water jet back down the center of the trommel into the mill. External recycling can be continuous, achieved by conveyor belt, or is batch where the material is stockpiled and periodically fed back into the mill by front-end loader.
In Figure 7.35 shows the SAG mill closed with a crusher (recycle or pebble crusher). In SAG mill operation, the grinding rate passes through a minimum at a critical size (Chapter 5), which represents material too large to be broken by the steel grinding media, but has a low self-breakage rate. If the critical size material, typically 2550mm, is accumulated the mill energy efficiency will deteriorate, and the mill feed rate decreases. As a solution, additional large holes, or pebble ports (e.g., 40100mm), are cut into the mill grate, allowing coarse material to exit the mill. The crusher in closed circuit is then used to reduce the size of the critical size material and return it to the mill. As the pebble ports also allow steel balls to exit, a steel removal system (such as a guard magnet, Chapters 2 and 13Chapter 2Chapter 13) must be installed to prevent them from entering the crusher. (Because of this requirement, closing a SAG mill with a crusher is not used in magnetic iron ore grinding circuits.) This circuit configuration is common as it usually produces a significant increase in throughput and energy efficiency due to the removal of the critical size material.
An example SABC-A circuit is the Cadia Hill Gold Mine, New South Wales, Australia (Dunne et al., 2001). The project economics study indicated a single grinding line. The circuit comprises a SAG mill, 12m diameter by 6.1m length (belly inside liners, the effective grinding volume), two pebble crushers, and two ball mills in parallel closed with cyclones. The SAG mill is fitted with a 20MW gearless drive motor with bi-directional rotational capacity. (Reversing direction evens out wear on liners with symmetrical profile and prolongs operating time.) The SAG mill was designed to treat 2,065t h1 of ore at a ball charge of 8% volume, total filling of 25% volume, and an operating mill speed of 74% of critical. The mill is fitted with 80mm grates with total grate open area of 7.66m2 (Hart et al., 2001). A 4.5m diameter by 5.2m long trommel screens the discharge product at a cut size of ca. 12mm. Material less than 12mm falls into a cyclone feed sump, where it is combined with discharge from the ball mills. Oversize pebbles from the trommel are conveyed to a surge bin of 735t capacity, adjacent to the pebble crushers. Two cone crushers with a closed side set of 1216mm are used to crush the pebbles with a designed product P80 of 12mm and an expected total recycle pebble rate of 725t h1. The crushed pebbles fall directly onto the SAG mill feed belt and return to the SAG mill.
SAG mill product feeds two parallel ball mills of 6.6m11.1m (internal diameterlength), each with a 9.7MW twin pinion drive. The ball mills are operated at a ball charge volume of 3032% and 78.5% critical speed. The SAG mill trommel undersize is combined with the ball mills discharge and pumped to two parallel packs (clusters) of twelve 660mm diameter cyclones. The cyclone underflow from each line reports to a ball mill, while the cyclone overflow is directed to the flotation circuit. The designed ball milling circuit product is 80% passing 150m.
Several large tonnage copper porphyry plants in Chile use an open-circuit SAG configuration where the pebble crusher product is directed to the ball mills (SABC-B circuit). The original grinding circuit at Los Bronces is an example: the pebbles generated in the two SAG mills are crushed in a satellite pebble crushing plant, and then are conveyed to the three ball mills (Mogla and Grunwald, 2008).
Pulverizer systems, which integrate drying, grinding, classification, and transport of the ground fuel to the burners, can present the greatest problems when switching coals/fuels (Carpenter, 1998). Low quality fuels may have grinding properties that are markedly different from the pulverizer design coal (Kitto and Stultz, 2005; Vuthaluru et al., 2003). Consequently, problems are experienced with pulverizer capacity, drying capacity, explosions, abrasive wear of the pulverizer grinding elements, erosion of the coal classifiers and/or distributors, coal-air pipes, and burners.
Whenever there is a loss of a pulverizer, the operator should light oil burner/s to help the operating group of pulverizers to stabilize the flame. At the same time, the operator should bring down the load matching to the capability of the running puverizer/s. Effort should be made to cut in standby pulverizer/s depending on draft fan group capability. Faults in electric supply, if there are any, can then be inspected and rectified. In the case of jamming in the pulverizer internals, the affected pulverizer should be cooled and cleaned and prepared for the next operation.
a pulverizer that is tripped under load will be inerted as established by equipment manufacturer, and maintained under an inert atmosphere until confirmation that no burning or smouldering fuel exists in the pulverizer or the fuel is removed. Inerting media may be any one of CO2, Steam or N2. For pulverizers that are tripped and inerted while containing a charge of fuel, following procedure will be used to clear fuel from the pulverizer:1.Start one of the pulverizers2.Isolate from the furnace all shut-down or tripped pulverizers3.Continue to operate the pulverizer until empty4.When the operating pulverizer is empty, proceed to another tripped and inerted pulverizer and repeat the procedure until all are cleared of fuel
NFPA 85 recommends the pulverizer system arrangement should be such as to provide only one direction of flow, i.e., from the points of entrance of fuel and air to the points of discharge. The system should be designed to resist the passage of air and gas from the pulverizer through the coal feeder into the coal bunker. To withstand pulverizer-operating pressures and to resist percolation of hot air/gas, a vertical or cylindrical column of fuel at least the size of three coal-pipe diameters should be provided between the coal-bunker outlet and the coal-feeder inlet as well as between coal-feeder outlet and the pulverizer inlet. Within these cylindrical columns there will be accumulation of coal that will resist percolation of hot air/gas from the pulverizer to the coal bunker. All components of the pulverized coal system should be designed to withstand an internal explosion gauge pressure of 344kPa .
Number of Spare Pulverizers: To overcome forced outage and consequent availability of a number of operating pulverizers it is generally considered that while firing the worst coal one spare pulverizer should be provided under the TMCR (Turbine Maximum Continuous Rating) operating condition. In certain utilities one spare pulverizer is also provided even while firing design coal, but under the BMCR (Boiler Maximum Continuous Rating) operating condition. Practice followed in the United States generally is to provide one spare pulverizer for firing design coal, in larger units two spare pulverizers are provided. However, provision of any spare pulverizer is not considered in current European design .
Pulverizer Design Coal: The pulverizer system should be designed to accommodate the fuel with the worst combination of properties that will still allow the steam generator to achieve the design steam flow. Three fuel properties that affect pulverizer-processing capacity are moisture, heating value, and HGI, as discussed earlier.
Unit Turndown: The design of a pulverizer system determines the turndown capability of the steam generator. The minimum stable load for an individual pulverizer firing coal is 50% of the rated pulverizer capacity. Normally in utility boilers, the operating procedure is to operate at least two pulverizers to sustain a self-supported minimum boiler load. Thus, the minimum steam generator load when firing coal without supporting fuel is equal to the full capacity of one pulverizer. Therefore, a loss of one of the two running pulverizers will not trip the steam generator because of loss of fuel and/or loss of flame.
Pulverizer Wear Allowance: A final factor affecting pulverizer system design is a capacity margin that would compensate for loss of grinding capacity as a result of wear between overhauls of the pulverizer (Figure 4.6). A typical pulverizer-sizing criterion is 10% capacity loss due to wear.
The grinder consists of a body with a conical inner surface in which is arranged an internal moving milling cone. The two cones form the milling chamber. On the axle of the internal moving milling cone a debal-ancing vibrator is fitted, which is driven through a flexible transmission. During vibrator rotation, the centrifugal force is generated, leading the internal cone to roll along the inner cone surface of the grinder body without clearance, if material is absent in the milling chamber or across a material layer. Such innercone movement difference is possible owing to the absence in these machines of kinematic limitation of inner cone amplitude. Thus, KID does not have a discharging gap as for eccentric crushers, therefore, the diametric annular between cones is received by coincidence of their axes.
The idea of using the vibrator drive of the cone crusher appeared as long ago as 1925 (US Patent 1 553 333) and then its later versions (German Patent 679 800, 1952; Austrian Patent 200 598, 1957; and Japanese Patent 1256, 1972) were published. In the Soviet Union, the first experimental KID specimens had been created by the early 1950s. Now, in the various branches of industry in the Commonwealth of Independent States, KIDs with capacity from 1 to 300 t/h are produced.
The basic KID feature absence of rigid kinematic bondings between the cones allows the inner moving cone to change its amplitude depending on the variation of grindable material resistance or to stop if a large non-grindable body is encountered; but this is not detrimental and does not lead to plugging. Another KID feature is the nature of the crushing force. In KID, the crushing force is the sum of the centrifugal force of debalance of the inner cone by its gyrating movement. Such force is determined by mechanics and does not depend on the properties of the processed material. The crushing force acts as well on idle running as the result of gapless running in of cones. Therefore, the stability of the inner cone on its spherical support during idle running is ensured.
The grinder consists of a body with a conical inner surface in which is arranged an internal moving milling cone. The two cones form the milling chamber. On the axle of the internal moving milling cone, an unbalanced vibrator is fitted, driven through a flexible transmission. During vibrator rotation, centrifugal force is generated, leading the internal cone to roll along the inner cone surface of the grinder body without clearance if a material that is being grinded is absent in the milling chamber or on this material layer. Such inner cone varying movement is possible owing to the absence in these machines of kinematic limitation of inner cone amplitude. KID does not have a discharging gap as do ordinary cone crushers; therefore, the diametric annular between cones is received by coincidence of their axes.
The idea of using the vibrator drive of the cone crusher appeared as long ago as 1925 (US Patent 1,553,333) and then its later versionsGerman Patent 679,800 (1952), Austrian Patent 200,598 (1957), and Japanese Patent 1256 (1972)were published. The first experimental KID specimens were created in Russia in the early 1950s. Subsequently, in the various branches of industry in the Soviet Union, KIDs with a capacity from 1 to 300t/h were produced. The manufacture of KIDs under license from Soviet Union was developed in Japan in 1981.
The basic KID featurethe absence of rigid kinematic bonding between the conesallows the inner moving cone to change its amplitude, depending on the variation of grindable material resistance, or to stop if a large nongrindable body is encountered. This is not detrimental and does not lead to stopping the debalance. Another KID feature is the nature of the crushing force. In KID, the crushing force is the sum of the centrifugal force of debalance and the inner cone by its gyrating movement. Such force is determined by mechanics and does not depend on the properties of the processed material. This characteristic in combination with the resilient isolation of KID from the foundation allows a two-fold increase in the inner cone vibration frequency.
The principle objective for controlling grinding mill operation is to produce a product having an acceptable and constant size distribution at optimum cost. To achieve this objective an attempt is made to stabilize the operation by principally controlling the process variables. The main disturbances in a grinding circuit are:
The mill control strategy has to compensate for these variations and minimize any disturbances to the hydrocyclone that is usually in closed circuit. The simplest arrangement is to setup several control loops starting from the control of water/solid ratio in the feed slurry, sump level control, density control of pulp streams at various stages and control of circulating load. Presently most mills use centrifugal pumps for discharging from the sump. This helps to counter surges and other problems related to pumping. For feed control the most likely option is to use a feed forward control while for controlling the hopper level and mill speed and other loops the PI or PID controller is used. The control action should be fast enough to prevent the sump from overflowing or drying out. This can be attained by a cascade control system. The set point of the controller is determined from the level control loop. This type of control promotes stability.
Each of these is controlled by specific controlled inputs, i.e., feed rate, feed water and discharge water flows. The overflow solids fraction is controlled by monitoring the ratio of total water addition (WTOT) to the solid feed rate. The ratio being fixed by the target set point of the overflow solid fraction.
Usually the charge volume of SAG mills occupy between 30-40% of its internal volume at which the grinding rate is maximized. When the charge volume is more, then the throughput suffers. The fill level is monitored by mill weight measurement as most modem mills are invariably mounted on load-cells.
During the operation of SAG mills, it is sometimes observed that the sump levels fall sharply and so does the power draft. This phenomenon is attributed to flow restrictions against the grate. When this occurs it is necessary to control, (or in extreme circumstances), stop the incoming feed.
The power draft is the result of the torque produced by the mill charge density, lift angle of the charge within the mill and fill level. The relationship between these parameters is complex and difficult. Therefore to control mill operation by power draft alone is difficult.
For the purpose of stabilization of the circuit, the basis is to counteract the disturbances. Also the set points must be held. The set points are attributed by dynamic mass balances at each stage of the circuit.
In modern practice the structure and instrumentation of the control systems of tubular grinding mills are designed to operate in three levels or in some cases four levels. The control loops and sensors for a SAG-mill and the levels of control are illustrated in Fig. 18.22. According to Elber [10,17], the levels are:
The function of Level 2 is to stabilize the circuit and to provide the basis of optimizing function in Level 3. Three cascade loops operating in level 2 controls that function in conjunction with level 1 controllers. The cascade loops are:
The set points are supplied by level 3 controllers for all the cascade loops. The mill load and percent solids in the two streams are calculated from signals received by sensors in the water flow stream, the sump discharge flow rate and the density readings from density meters in the pulp streams. The mill load cells supply the charge mass. The load cell signals are compensated for pinion up thrusts .
To determine the set point for the optimum mill load, a relation between load, consisting of different feed blends and performance (the maximum achievable throughput) is established. Similar observations are made for mill discharge density and mill discharge flow.
The primary function at Level 3 is optimisation of the SAG mill operation. That is, control of the product at optimum level. In an integrated situation where ball mill and cyclone is in the circuit, the optimisation must take place keeping in mind the restraints imposed by down stream requirements. This optimisation can best be achieved by developing a software for computer use. Usually a large database is required to cover infrequent control actions.
Ore samples were taken from a grinding mill operating as a batch process. The feed size distribution, breakage functions and size analysis of samples taken at intervals of 10minutes up to 30minutes are given. Determine:
An alkaline slurry from a bauxite grinding mill was scheduled to be classified using a spiral classifier at the underflow rate of 1100t/day. The width of the classifier flight was 1.3m and the outside diameter of the spiral flights was 1.2m. Estimate the pitch of the spirals if the spiral speed is 5rev/min and the bulk density of the underflow solids is 2000kg/m3.
The diameter of a typical hydrocyclone was 30.5cm. The apex was fitted with a rubber sleeve 12cm in length and 8.0cm in internal diameter. A quartz suspension at a density of 1.33 was fed to the cyclone at the rate of 1000L/min. The underflow measured 75% solids. The apex diameter was reduced by 10% twice. Estimate:
The volume flow rate of pulp fed to a hydrocyclone was 129L/min. Its solid content was held at 32% by volume. Samples of the feed, under flow and over flow streams were taken simultaneously, dried and a size analysis carried out. The results obtained were:
An hydrocyclone is to be installed in a closed circuit grinding circuit with a mill discharge containing 30% solids by volume. The solid density is 2800kg/m3 and the density of water is 1000kg/m3. Given that the maximum pressure deferential between the inlet and overflow was 50kPa and the throughput from the mill was 800t/h, estimate:
A hydrocyclone classifier is fed with quartz slurry at the rate of 20.8t/h from a grinding mill. The underflow is recirculated. The screen analysis of each stream were determined with the following results:
The input and output streams of an operating cyclone were sampled simultaneously for the same period of time. The dried samples were analysed for size distribution and the mass per cent retained on each size fraction was determined with the following results:
After a steady state operation the solid content of feed slurry was increased by 20% while all other conditions remained the same. Determine the size distribution of each stream under the altered condition.
If a second cyclone is added in series to the cyclone in problem 12.8, what is the effect of the overall efficiency of the classification. What will be the size distribution of the cyclone U/F of the second stage? The partition coefficient of the second stage cyclone is given as:
A crushing plant delivered ore to a wet grinding mill for further size reduction. The size of crushed ore (F80) was. 4.0mm and the S.G. 2.8t/m3. The work index of the ore was determined as 12.2kWh/t. A wet ball mill 1m1m was chosen to grind the ore down to 200microns. A 30% pulp was made and charged to the mill, which was then rotated at 60% of the critical speed. Estimate:
Everell  believed that the mechanism of breakage of particles in a grinding mill was analogous to the slow compression loading of irregular particles and that the specific rate of breakage for a particular size of fragment is an inverse function of the average failure load of the particles. Everell et al  developed a model to describe the relationship between the grinding selection function (breakage rate) and the physio-mechanical properties of the rocks. The advantage in such a relationship lies in the wealth of rock strength data determined on drill core during mine development being available to predict energy demands in the comminution circuits.
Briggs  measured the tensile strength, using the Brazil tensile test, and the point load compressive strength of four rock types of different grindabilities. These results were compared to the Bond Work Index of the ores as measured by the Magdalinovic method . The results in Fig. 3.8 show that there is a good correlation between the Bond Work Index and the tensile strength and the Equivalent Uniaxial Compressive Strength (EUCS). Some of the scatter in the graphs are due to the structure of the rock. For example one rock type was a banded iron, heavily mineralised with sulphides with numerous planes of weakness on a macro scale. This affected the mechanical properties when tested on large specimens. However when the grinding tests were carried out at relatively small particle sizes, the planes of weakness were no longer present and the ore became more competent.
The correlation between Bond Work Index and tensile strength is an indication that the grinding mechanism in the work index test favours abrasion type breakage given that tensile strength is a fair indication of the abrasiveness of a rock.
Briggs also measured the breakage rate and breakage distribution function of the ores and compared the breakage rate and Bond Work Index. There was a good correlation between the rock strength data and the breakage rate with higher strength rocks having a lower breakage rate. However the data set for these tests was small and further work needs to be done to confirm the relationship.
Bearman et al  measured a wide range of rock strength properties and correlated these to the JKMRC drop weight test data. Conclusions were that this technique will enable the data required for comminution plant design to be obtained from mechanical tests on drill core samples.
During normal operation the mill speed tends to vary with mill charge. According to available literature, the operating speeds of AG mills are much higher than conventional tumbling mills and are in the range of 8085% of the critical speed. SAG mills of comparable size but containing say 10% ball charge (in addition to the rocks), normally operate between 70 to 75% of the critical speed. Dry Aerofall mills are run at about 85% of the critical speed.
The breakage of particles depends on the speed of rotation. Working with a 7.32m diameter and 3.66m long mill Napier-Munn et al  observed that the breakage rate for the finer size fractions of ore (say 0.1mm) at lower speeds (eg. 55% of the critical speed), were higher than that observed at higher speeds (eg. 70% of the critical speed). For larger sizes of ore, (in excess of 10mm), the breakage rate was lower for mills rotating at 55% of the critical speed than for mills running at 70% of the critical speed. For a particular intermediate particle size range, indications are that the breakage rate was independent of speed. The breakage ratesize relation at two different speeds is reproduced in Fig. 9.7.
Ultrafine grinding (UFG) has continued to evolve in terms of equipment development. A number of specialist machines are commercially available including Xstrata's IsaMill, Metso's Vertimill, Outotec's High Intensity Grinding (HIG) mill, and the Metprotech mill. UFG equipment has been developed with installed powers of up to 5MW.
Compared with conventional ball or pebble milling, the specialist machines are significantly more energy efficient and can economically grind to 10m or lower, whereas the economical limit on conventional regrind mills was generally considered to be around 30m. Coupled with improvements in downstream flotation and oxidation processes, the rise of UFG has enabled treatment of more finely grained refractory ores due to a higher degree of liberation in the case of flotation or enhanced oxidation due to the generation of higher surface areas.
In 1993, the Salsigne Gold Mine was reopened. Salsigne treated a gold-bearing pyrite/arsenopyrite ore by flotation, with the flotation tails treated in a CIL circuit and the concentrate reground in a conventional mill to approximately 2530m. The oxygen demand for reground concentrate was high and the rate of oxidation was slow. The concentrate was initially oxidized for approximately 6h using oxygen injection via a Filblast aerator before cyanidation. Additional oxygen was added in in the second CIL stage and hydrogen peroxide was added into the fourth unit to maintain dissolved oxygen concentrations of >10ppm.
Goldcorp have commenced operations at the Elenore Gold Project in Quebec, Canada. The mineralogy of the ore and hence the circuit selection show similarities to those at Salsigne. The main sulfides are arsenopyrite, pyrite, and pyrhottite. The ore is floated, with the flotation tails passing to a tails CIL circuit and the flotation concentrate reground before passing to the concentrate CIL circuit via preaeration tanks designed to achieve 18-h contact with oxygen. The main difference between the Salsigne and Elenore projects is that the Elenore concentrate is ground to 10m and oxidation of the sulfides is substantially complete before cyanidation.
Large projects are typically associated with more complex contracting strategies but not necessarily greater flow sheet complexity. The complexity of the contracting strategy and the increased focus on key items of equipment, such as large grinding mills, elevate the manning requirements. Higher throughput, and associated larger equipment, does lead to increased complexity in service equipment such as lubrication, cooling, and control systems. Gearless motor drives on large semi-autogenous grinding (SAG) and ball mills require significant installation testing and commissioning effort. The 20 MW drive for a large Australian gold/copper project with a capital cost of approximately A$295M in 1998, took three technicians over 6weeks to test and commission with the total vendor cost (installation and commissioning) for the 20MW ring motor alone exceeding A$1M.
The first step in slag processing is size reduction to liberate metallic iron and iron-bearing minerals. This is done by crushers or by autogenous grinding, that is, the slag is ground on its own in the grinding mill without any balls. The latter process yields higher quality product as the iron product discharged from grinding mill contains as high as 80% Fe (Shen and Forssberg, 2003). Metallic iron and iron minerals are separated by magnetic separation. The phosphorus-bearing minerals occurring in steel slag are removed in the tailings of high gradient magnetic separation. The flow diagram is shown in Figure 8.2.
Reduction of iron oxide at high temperature has been shown to be an attractive low energy cost process (Olginskij and Prokhorenko, 1994). The iron-free mineral residue is suitable for applications in construction industry. An alternative route applies microwave heating with carbon and the recovery of iron by magnetic separation (Hatton and Pickles, 1994).
Low-competency ores such as oxides are unlikely to have a problem with generation of pebbles. They are more likely to have a problem with slurry viscosity. For smaller plant in the 1980s, these ores were treated through a single-stage SAG mill grinding to 75 or 106m. This type of circuit is still straightforward in its design concept. High-competency ores or ores requiring a finer product size frequently require two stages of grinding and a number of design issues become important.
Mill orientation. For smaller plant, the mills can be arranged in parallel if the products feed the same discharge hopper. This is more difficult for larger mills as the diameter of the mills drives the height required to maintain the discharge launder slope. In this situation the mills are often located at right angles. Alternatively, a transfer hopper can be used. This decision is usually based on the difference in cost and operability of each option.
Transfer size from the SAG to the ball mill. Transfer size prediction is somewhat uncertain but has an important impact on the balance of power between the two stages of the milling circuit. For feasibility studies, a combination of modeling and benchmarking is generally used.
Mill discharge arrangement. If the SAG and ball mills feed separate pump systems this is not an issue. If the two stages feed the same pump system, the SAG mill usually drives the discharge arrangement. This is because the SAG mill is usually of larger diameter and may have a requirement for screening of pebbles to protect the mill discharge pumps and cyclones, and to facilitate pebble crushing.