Mining Engineeering Department

Transcription

1 Instructor Engr. Muhammad Shahzad (Assistant Professor) Mining Engineeering Department University of Engineering & Technology Lahore

2 The practice of minerals processing is as old as human civilization. Minerals and products derived from minerals have formed our development cultures from the flints of the Stone Age man to the uranium ores of Atomic Age. METSO 3

3 Section 1: CRUSHING Contents Exp. # 1) Machine Study of Laboratory Jaw Crusher and to perform a crushing test on the given sample, and to analyze the product for reduction ratio Exp. # 2) Machine Study of Laboratory Roll Crusher and to perform a crushing test on the given sample, and to analyze size distribution in the product by sieve analysis Exp. # 3) Machine Study of Laboratory Hammer Mill and to perform a crushing test on the given sample, and to analyze size distribution in the product by sieve analysis Section 2: GRINDING Exp. # 4) Machine Study of Laboratory Disc Mill and to perform a grinding test on the given sample, and to analyze size distribution in the product by sieve analysis Exp. # 5) Machine Study of Laboratory Rod Mill and to perform a grinding test on the given sample, and to analyze size distribution in the product by sieve analysis Exp. # 6) Machine Study of Denver Laboratory Ball Mill and to perform a grinding test on the given sample, and to analyze size distribution in the product by sieve analysis Section 3: SIZING Exp. # 7) Machine study of a double-deck vibratory screen & hummer electromagnetic screen and determining their performance by a screening test on the given sample. Exp. # 8) Machine study of a Laboratory centrifugal hydro classifier and to find its cut size under the given conditions. Section 4: GRAVITY CONCENTRATION Exp. # 9) Machine Study of a Laboratory type Mineral/Coal Jig and to perform a gravity separation test on the given sample Exp. # 10) Machine Study of a Concentrating Shaking Table and to perform a gravity separation test on the given sample Exp. # 11) Machine Study of a Humphrey s Spirals and to perform a gravity separation test on the given sample (Coal/Metallic Minerals) Section 5: FROTH FLOTATION Exp. # 12) To study the principle and operation of Laboratory Flotation Machine, and to perform a froth flotation test on a coal sample and to determine its separation efficiency 4

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6 Experiment No. 1: 1.1. Objectives The main objectives of this experiment are to study the various parts of Laboratory Jaw crusher with special emphasis on their functions, to perform a crushing test on a given sample, to analyze the product by sieve analysis and to calculate its reduction ratio by feed size and product size measurement Apparatus/Materials Laboratory Jaw Crusher Vernier Caliper Rock sample for crushing Sieve set (26.67, 18.85, 13.33, 6.70, 2.36 and 0.85 mm) Ro-Tap Sieve Shaker Torsion Balance 1.3. Procedure Study each part of the machine and know the function of every component Switch on the machine and study the movement of the moving jaw and the variation of set with motion. Measure the side of the gape and adjust the set with the help of a lead lump and meter rod or coarse sieve and record measurements. Examine the feed, measure the largest lump either with a meter rod or coarse sieve as the case may be and record it. Feed the machine and crush the entire sample. Perform sieve analysis on the product by using coarse sieve set as given in 1.2. Calculate the reduction ratio of the machine Specifications of Laboratory Jaw Crusher The specifications of laboratory jaw crusher are given in Table 1.1. Table 1.1: Specifications of Laboratory Jaw Crusher Name Denver Blake Jaw Crusher Motor 5hp Motor RPM 1440 Crusher RPM Face Of Flywheel 3 ¼ Movable Jaw Depth 14 Fix Jaw Depth 12 Width Of Jaw Plate 6 7

7 Crusher Designed Capacity 600 lbs/hr Flywheel Diameter 18 Max: Feed Size (Gape) 5 6 Size Of Set close ½ʺ open 1 ¼ ʺ 1.5. Observations and Calculations Feed Size Sr. No. Length (mm) Width (mm) Height (mm) Product Sieve Analysis Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Reduction ratio Determine the reduction ratio by the following relation: 80% Passing Feed Size Reduction Ratio 80 = 80% Passing Product Size 1.6. Graphs 1. Draw graph of cumulative passing and retaining mass percentage against aperture size (geometric mean) and determine cut size, d10, d25, d50, and d Draw log-normal plot between aperture size (geometric mean) and cumulative passing mass percentage and determine the standard deviation. 3. Express Gaudin-Schuhmann distribution on graph and determine the constants involved. 4. Express Rosin-Rammler distribution on graph and determine the constants involved. *Note: Read Topic 2.2. Particle Size Distribution in Mineral Processing Design and Operation by A. Gupta and D. S. Yan 1.7. Discussions Discuss the results and the information deducted from sieve analysis in details Conclusions Give concluding remarks about the experiment and its results. 8

8 1.9. Related Theory [References: Mineral Processing Technology by B. A. Wills (7 th Edition-2005) & Mineral Processing Design and Operation by A. Gupta and D. S. Yan (1 st Edition-2006)] Introduction Jaw crushers are used as primary crushers, or the first step in the process of reducing rock. They crush primarily by using compression. The distinctive feature of this class of crusher is the two plates which open and shut like animal jaws. The jaws are set at an acute angle to each other and one jaw is pivoted so that it swings relative to the other fixed jaw. Material fed into the jaws is alternately nipped and released to fall further into the crushing chamber. Eventually it falls from the discharge aperture Types There are three basic types of jaw crusher as shown in Figure Blake Type Jaw Crusher Figure 1.1:1 Types of Jaw Crushers The Blake crusher was patented by Eli Whitney Blake in The swing jaw is fixed at the upper position. The Blake type jaw crusher has a fixed feed area and a variable discharge area. A. Double Toggle Blake Crusher In the double toggle jaw crushers, the oscillating motion of the swing jaw is caused by the vertical motion of the pitman. The pitman moves up and down. The swing jaw closes, i.e., it moves towards the fixed jaw when the pitman moves upward and opens during the downward motion of the pitman. This type is commonly used in mines due to its ability to crush tough and abrasive materials. 9

9 B. Single Toggle Blake Crusher In the single toggle jaw crushers, the swing jaw is suspended on the eccentric shaft which leads to a much more compact design. The swing jaw, suspended on the eccentric, undergoes two types of motion- swing motion towards the fixed jaw due to the action of toggle plate and vertical movement due the rotation of the eccentric. These two motions, when combined, lead to an elliptical jaw motion. This motion is useful as it assists in pushing the particles through the crushing chamber. This phenomena leads to higher capacity of the single toggle jaw crushers but it also results in higher wear of the crushing jaws. These type of jaw crushers are preferred for the crushing of softer particles. A comparison between two can also be seen in Figure 1.2. It shows the difference in the movement and the method of operation. Figure 1.2: Comparison between Single and Double Toggle Blake Type Dodge Type Jaw Crusher In the Dodge type jaw crushers, the jaws are farther apart at the top than at the bottom, forming a tapered chute so that the material is crushed progressively smaller and smaller as it travels downward until it is small enough to escape from the bottom opening. The Dodge jaw crusher has a variable feed area and a fixed discharge area which leads to choking of the crusher and hence is used only for laboratory purposes and not for heavy duty operations Universal Jaw Crusher This jaw crusher continuously reduces material as it passes through the crushing chamber with its aggressive force feed action as the moveable jaw compresses inward and downward. The sharp primary blow at the top of the chamber reduces material instantly, while a secondary crushing action at the bottom further reduces material to the predetermined output size. Universal Jaw Crushers offer a compressive stroke that is nearly equal at both the top and bottom of the chamber, producing more spec material at a lower cost per ton. 10

10 Construction Main Frame The main frame is often made from cast iron or steel, connected with tie-bolts. It is often made in sections so that it can be transported underground for installation. Fully stress relieved after fabrication. Jaws The jaws are usually constructed from cast steel and are fitted with replaceable manganese steel liners, which are bolted in sections on to the jaws so that they can be removed easily and reversed periodically to equalize wear. One jaw is move able and other one is fixed. Cheek Plates These are fitted to the sides of the crushing chamber to protect the side main frame from wear. These are also made from manganese steel and have the similar life to the jaw plates. Flywheel Rotational energy is fed into the jaw crusher eccentric shaft by means of a sheave pulley which usually has multiple v-belt grooves. Heavy fly wheel attached to the drive which is necessary to store energy on the idling half of the stroke and deliver it on the crushing half. And maintain inertia. Toggle The toggle rolls across the flat pressure face of the toggle seat. No rubbing or scuffing takes place and friction is kept to a minimum. This type of toggle system has the following advantages over the socket end type toggle and seat: No lubrication whatsoever is required The system can handle far greater crushing pressures. The life factor of toggle and seats is many times greater. Jaw-holder & Main Bearing Housings Can be removed from the frame as an assembly. The jaw-holder is a robust box construction with a fully machined face to support the moving jaw. Spring Load It is a safety device and in the event of an uncrushable lump entering the gape. The movable jaw is pushed to the limit by compressing the spring and the un-crushable lump is passed out 11

11 without causing mechanical damage to the machine. In industry we use greening screen to prevent sticky and wet material Working Figure 1.3: Labelled Diagram of Jaw Crusher A Toggle and Pitman is the popular mechanism, driving the Blake jaw crusher. It normally comprises of a pitman working on an eccentric on the crusher shaft and two toggles travelling and traversing like a birds wings, one between the main frame and the pitman and the other between the pitman and the moveable jaw, so that with the rotation of the shaft the pitman is translated up and down whereby the distance between the two jaws is increased intermittently and crushing effected. But our laboratory model, which is typically a Blake jaw crusher, has a modified drive mechanism called single toggle mechanism ; it has no pitman separately but the purpose of pitman is served by the moveable jaw itself. Instead of the two long toggles it has one small toggle which rests on steel bearings, at one end on back body of the moveable jaw and at the other end on a vertically slid able wedge block beside the main frame. It can be moved back & forward depending upon product required (e.g. if moved back then the product will be coarser). The feed opening of the jaw crusher is called the gape and the discharge opening is called set. The moveable jaw is spring loaded and connected to a screw mechanism which helps in adjusting the set. Spring loading is a safety device and in the event of an uncrushable lump entering the gape. The moveable jaw is pushed to the limit by compressing the spring and the uncrushable lump is passed out without causing mechanical damage to the machine. 12

12 Related Terminology Figure 1.4: Jaw Crusher present in Mineral Processing Lab Gape: The feed opening of the jaw crusher is called gape, which is the distance between the jaws at the feed opening & which is given as 5'' 6'' (Max size of feed) Set: The maximum opening of the jaws at the discharge end is called set. The discharge size of the material from the crusher is controlled by set. This can be adjusted by using toggle plates of the required length. Throw: The jaw is pivoted from above; it moves a minimum distance at the entry point and a maximum distance at the delivery. This maximum distance is called the throw of the crusher. Free Crushing: In free crushing, no accumulation of the material takes place in the crusher. Choke Crushing: In choke crushing, accumulation of the material takes place in the crusher. Reduction Ratio: Ratio of Size of Feed to Size of Output. Feed Material: Such a material which is introduced in the crusher for crushing purpose is called the feed material. It should be 80% to 90% of the gape and it should also be a uniform size. Product Material: Such material which is discharge from the set after crushing is called product. The size of the product can be adjusted by adjusting the size of the set Parameters Capacity The capacity of the jaw crusher available in the laboratory is 725 t/h. Jaw crushers range in size up to 1680 mm gape by 2130 mm width. This size machine will handle ore with a maximum size of 1.22 m at a crushing rate of approximately 725th -~ with a 203mm set. However, at crushing rates above 545th -1 the economic advantage of the jaw crusher over the gyratory diminishes; and above 725th -1 jaw crushers cannot compete with gyratory crushers. 13

13 Speed The speed of jaw crushers varies inversely with the size, and usually lies in the range of revmin -1. The main criterion in determining the optimum speed is that particles must be given sufficient time to move down the crusher throat into a new position before being nipped again Amplitude The maximum amplitude of swing of the jaw, or "throw", is determined by the type of material being crushed and is usually adjusted by changing the eccentric. It varies from 1 to 7 cm depending on the machine size, and is highest for tough, plastic material and lowest for hard, brittle ore. The greater the throw, the less danger is there of chokage, as material is removed more quickly Applications Jaw crusher perform better on clayey, plastic material: due to their greater throw. Jaw Crusher is used in Cement Raw Crushing. It can also be used in Mining industry along with recycling of the concrete Limitations Jaw crusher is applicable to feed size up to 1m & giving a product of about cm in size. Jaw crusher is not suitable for hard and abrasive material. It is an 180 o machine which means that it works 90 o and rests 90 o. Some of the lumps can pass without being crushed. Sticky material cannot be crushed in this crusher Specifications of Industrial Models The industrial model of jaw crushers are more powerful and of larger dimensions than the laboratory scale and therefore Table 1.2 expresses the specifications of the industrial model of Jaw Crushers. 14

14 Table 1.2: Yuhong Jaw Crusher Specification 15

15 Experiment No. 2: 2.1. Objectives The main objectives of this experiment are to study the various parts of Laboratory Roll crusher with special emphasis on their functions, to perform a crushing test on a given sample, to analyze the product by sieve analysis and to calculate its reduction ratio by feed size and product size measurement Apparatus/Materials Laboratory Roll Crusher Rock sample for crushing Sieve set (9.42, 6.70, 4.75, 2.36, 0.85 and 0.30 mm) Ro-Tap Sieve Shaker Torsion Balance 2.3. Procedure Study each part of the machine and know the function of every component Switch on the machine and study the movement of the moving rolls. Adjust the set to 10 mm by measuring distance between the rolls at the line joining their centers. Examine the feed, measure the feed size by using a set of sieve and determine 80% passing feed size by plotting graph between cumulative passing and geometric mean of passing and retaining size. Feed the machine and crush the entire sample. Perform sieve analysis on the product by using sieve set as mentioned in 2.2. Calculate the reduction ratio of the machine Specifications of Laboratory Roll Crusher Table 2.1: Specifications of Laboratory Roll Crusher Name of Machine Denver Roll Crusher Diameter of roll 10 inch Roll RPM Motor Power 8 hp Set Max. 30 mm, Min. 1 mm Material of rolls Cast steel Material of sheet Hard Manganese steel Face 6 inch Capacity of roll crusher 2 ton/hr 16

16 2.5. Observations and Calculations Feed Size Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Product Sieve Analysis Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Reduction ratio Determine the reduction ratio by the following relation: 80% Passing Feed Size Reduction Ratio 80 = 80% Passing Product Size 2.6. Graph 1. Draw graph of cumulative passing and retaining mass percentage against aperture size (geometric mean) and determine cut size, d10, d25, d50, and d Draw log-normal plot between aperture size (geometric mean) and cumulative passing mass percentage and determine the standard deviation. 3. Express Gaudin-Schuhmann distribution on graph and determine the constants involved. 4. Express Rosin-Rammler distribution on graph and determine the constants involved. *Note: Read Topic 2.2. Particle Size Distribution in Mineral Processing Design and Operation by A. Gupta and D. S. Yan 2.7. Discussions Discuss the results and the information deducted from sieve analysis in details Conclusions Give concluding remarks about the experiment and its results. 17

17 2.9. Related Theory [References: Mineral Processing Technology by B. A. Wills (7 th Edition-2005) & Mineral Processing Design and Operation by A. Gupta and D. S. Yan (1 st Edition-2006)] Introduction Roll crushers are secondary crushers machine. This type of crusher are characterized by production of smaller properties of fines and also by smaller reduction ratios. Roll crushers are operated dry or wet. Roll crushers are still used in mills, although they have been replaced in most installations by cone crushers Types of Roll Crushers There are four basic types of roll crushers as shown in Figure 2.1. Figure 2.1: Types of Roll Crushers Single Roll Crushers Single Roll Crushers are primary crushers that provide a crushing ratio of up to 6:1. They reduce materials such as ROM coal, mine refuse, shale, slate, gypsum, bauxite, salt, soft shale, etc., from large size particles to a medium size, while producing minimal fines. Designed with an interrupted opening between the roll teeth and corresponding grooves in the crushing plate liners, they are also extremely effective in reducing slabby materials. Figure 2.2 will illustrate how the Single Roll Crusher looks like. 18

18 Figure 2.2: Single Roll Crusher Double Roll Crushers Double Roll Crushers provide a 4:1 reduction ratio. They are typically used as a secondary or tertiary crusher for materials such as ROM coal with refuse, limestone, gypsum, trona, shale, bauxite, oil shale, clean coal, coke, salt, quicklime, burnt lime, glass, kaolin, brick, shale, and wet, sticky feeds. Each machine is custom engineered with roll elements and tooth patterns selected depending on each unique application to produce a cubical product with minimal fines. Figure 2.3: Double Roll Crusher Triple Roll Crushers Triple Roll Crushers are ideal for producers who want to accomplish two stages of reduction in one pass. They can be used in coal, salt, coke, glass, and trona operations, among others. Triple Roll Crushers combine a Single Roll Crusher with a Double Roll Crusher to form a crusher that is capable of achieving a 6:1 reduction ratio in the primary stage and a 4:1 reduction in the secondary stage while producing a cubicle product at high capacity. 19

19 Figure 2.4: Working Mechanism Triple Roll Crushers Quad Roll Crushers Quad Roll Crushers are ideal for producers, including those with preparation plants, who want to accomplish two stages of reduction in one pass. They can be used in coal, salt, lime, pet coke and potash operations, among others. Quad Roll Crushers are capable of achieving a 4:1 reduction ratio before dropping crushed material to the secondary stage crusher for an additional 4:1 reduction to make the final product. These are utilized on various scales on the industry level according to the need of the output. Figure 2.5:2 Quad Roll Crusher There are other types of roll crushers based on the type of roll surface and design. These are given in Figure 2.6. Figure 3 Types of Rolls 20

20 Smooth Surface Rolls A. These rolls are usually used for fine crushing. B. Wear on the roll surface is very high, and they often have manganese steel tire, which can be replaced when worn Slugger Rolls Coarse crushing is often performed in rolls having corrugated surfaces, or with stub teeth arranged to present a cheered surface pattern High Pressure Grinding Rolls A. HPGR consists of a pair of counter rotating rolls, one fixed and the other floating. B. Material is fed between the rolls with the floating roll pressing against the material flow by means of hydraulic pressure in excess of 50 M Pa. C. The resulting force causes the material to compact by inter-particle breakage. D. Pressures and roll speeds are adjusted to obtain optimum grinding conditions. E. The roll faces are typically studded because of improved wear characteristics Construction A rolls crusher essentially consists of two steel rolls revolving towards each other, both driven, with their very robust bearings mounted in a strong steel frame. The bearings of one of the rolls are spring loaded while those of the other connected to a screw mechanism. Spring loading serves as a safety device against uncrushable lumps whereas the screw mechanism provides Set adjustment for different product sizes. The rolls are made of cast steel but a shell of Hard Manganese Steel is shrunk over them which resist powerful wear during operation. Figure 2.7: Labelled Diagram of Roller Crusher 21

21 Crushing Rolls: The rolls are made up of cast steel. A roll crusher essentially consists of steel rolls revolving towards each other. The bearings of one of the rolls are spring loaded while those of other connected to a screw mechanism. Spring loading serves as a safety device. Set: The set is determined by shims which causes the spring-loaded roll to be hold back from the solidly mounted roll. Figure 2.8: Roll Crusher Present in Lab Working Mechanism A. The particles are drawn into the gap between the rolls by their rotating motion and a friction angle formed between the rolls and the particle, called the nip angle. B. The two rolls force the particle between their rotating surface into the ever smaller gap area, and it fractures from the compressive forces presented by the rotating rolls. C. Some major advantages of roll crushers are they give a very fine product size distribution and they produce very little dust or fines Related Terminology Angle of Nip: The largest angle that will just grip a lump between the jaws, rolls, or mantle and ring of a crusher. Reduction Ratio: Ratio of Size of Feed to Size of Output Parameters Effecting the Performance Angle of Nip: Consider a spherical particle of radius r being crushed by a pair of rolls of radius R then for a particle to be just gripped by rolls equating: C sin (θ/2) = µ C cos (θ/2) µ = tan (θ/2) The coefficient of friction b/w steel and most ore particles is in the range of so that the value of range θ should never exceed about 30 degrees, or the particle will slip. In the Figure 8, θ represents the Nip Angle. 22

22 Figure 2.9: Representation of Nip Angle Speed of Roll: Speed of rolls depends on the angle of nip and the type of material being crushed. The larger the angle of nip, the slower the peripheral speed to be allow the particle to be nipped and vice versa. Peripheral speeds vary b/w about 1m/s for small rolls upto about 15m/s for 1800mm dia rolls. The value of µ b/w a particle and roll is given by: µk = ν / 1+6 ν Size of Roll: Cos (θ/2) = R+a/R+r Above equation is used to determine the maximum size of rock gripped in relation to roll diameter and reduction ratio (r/a) is required. Capacity: The capacity of rolls can be calculated in terms of ribbon of material that will pass the space b/w rolls. Thus Capacity = NDWsd (Kg/h) Applications Roll crusher is used in Mining industry to crush the coal or other rocks. It is utilized on large scale industry too. This is a secondary crushing tool. Roll Crushers are best suited for controlled reduction of friable materials to granules where fines are undesirable in applications such as aggregates, ceramics, chemicals, minerals and sintered metals Limitations The machine unit mass production capacity is low. Moreover, it covers a large area. The roll surface grinding damage uneven, need often repair. The great disadvantage of roll crushers is that, in order for reasonable reduction ratios to be achieved, very large rolls are required in relation to the size of the feed particles. They therefore have the highest capital cost of all crushers. 23

23 Specifications of Industrial Models Table 2.2: Sturtevant Roll Crusher Specifications Table 1: Kurimoto Roll Crusher Specifications 24

24 Experiment No. 3: 3.1. Objectives The main objectives of this experiment are to study the various parts of Laboratory Impact crusher with special emphasis on their functions, to perform a crushing test on a given sample, to analyze the product by sieve analysis and to calculate its reduction ratio by feed size and product size measurement Apparatus/Materials Laboratory Impact Crusher Rock sample for crushing Sieve set (4.75, 3.35, 2.00, 1.00, 0.50, and 0.25 mm) Ro-Tap Sieve Shaker Torsion Balance 3.3. Procedure Study each part of the machine and know the function of every component Switch on the machine and study the movement of the moving hammers. Examine the feed, measure the feed size by using a set of sieve and determine 80% passing feed size by plotting graph between cumulative passing and geometric mean of passing and retaining size. Feed the machine and crush the entire sample. Perform sieve analysis on the product by using sieve set as mentioned in 3.2. Calculate the reduction ratio of the machine Specifications of Laboratory Impact Crusher Table 3.1: Specifications of Laboratory Impact Crusher Name of Machine Hammer Mill Motor Power 5 hp Motor RPM 1420 Mill RPM 2130 Motor Pulley 18 cm Mill Pulley 12 cm Grate opening 1 cm Full swing diameter of shaft and hammer 35 cm Number of hammers 8 x 4 = 32 Capacity 200 t/h 25

25 3.5. Observations and Calculations Feed Size Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Product Sieve Analysis Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Reduction ratio Determine the reduction ratio by the following relation: 80% Passing Feed Size Reduction Ratio 80 = 80% Passing Product Size 3.6. Graph 1. Draw graph of cumulative passing and retaining mass percentage against aperture size (geometric mean) and determine cut size, d10, d25, d50, and d Draw log-normal plot between aperture size (geometric mean) and cumulative passing mass percentage and determine the standard deviation. 3. Express Gaudin-Schuhmann distribution on graph and determine the constants involved. 4. Express Rosin-Rammler distribution on graph and determine the constants involved. *Note: Read Topic 2.2. Particle Size Distribution in Mineral Processing Design and Operation by A. Gupta and D. S. Yan 3.7. Discussions Discuss the results and the information deducted from sieve analysis in details Conclusions Give concluding remarks of the experiment and its results. 26

26 3.9. Related Theory [References: Mineral Processing Technology by B. A. Wills (7 th Edition-2005)] Introduction & Mode of Operation In this class of crusher, comminution is by impact rather than compression, by sharp blows applied at high speed to free-falling rock. The moving parts are beaters, which transfer some of their kinetic energy to the ore particles on contacting them. The internal stresses created in the particles are often large enough to cause them to shatter. These forces are increased by causing the particles to impact upon an anvil or breaker plate Applications There is an important difference between the states of materials crushed by pressure and by impact. There are internal stresses in material broken by pressure which can later cause cracking. Impact causes immediate fracture with no residual stresses. This stress-free condition is particularly valuable in stone used for brick-making, building, and roadmaking, in which binding agents, such as bitumen, are subsequently added to the surface. Impact crushers, therefore, have a wider use in the quarrying industry than in the metal-mining industry. They may give trouble-free crushing on ores that tend to be plastic and pack when the crushing forces are applied slowly, as is the case in jaw and gyratory crushers. These types of ore tend to be brittle when the crushing force is applied instantaneously by impact crushers. Impact crushers are also favoured in the quarry industry because of the improved product shape. Cone crushers tend to produce more elongated particles because of their high reduction ratios and ability of such particles to pass through the chamber unbroken. In an impact crusher, all particles are subjected to impact and the elongated particles, having a lower strength due to their thinner cross section, would be broken Construction & Working Figure 3.1 shows a cross-section through a typical hammer mill. The hammers are made from manganese steel or, more recently, nodular cast iron, containing chromium carbide, which is extremely abrasion resistant. The breaker plates are made of the same material. The hammers are pivoted so that they can move out of the path of oversize material, or tramp metal, entering the crushing chamber. Pivoted hammers exert less force than they would if rigidly attached, so they tend to be used on smaller impact crushers or for crushing soft material. The exit from the mill is perforated, so that material 27

27 which is not broken to the required size is retained and swept up again by the rotor for further impacting. Figure 3.1: Hammer mill This type of machine is designed to give the particles velocities of the order of that of the hammers. Fracture is either due to the severity of impact with the hammers or to the subsequent impact with the casing or grid. Since the particles are given very high velocities, much of the size reduction is by attrition, i.e. breaking of particle on particle, and this leads to little control on product size and a much higher proportion of fines than with compressive crushers Parameters of Impact Crushers The hammers can weigh over 100 kg and can work on feed up to 20 cm. The speed of the rotor varies between 500 and 3000 rev min -1. Due to the high rate of wear on these machines (wear can be taken up by moving the hammers on the pins) they are limited in use to relatively non-abrasive materials. They have extensive use in limestone quarrying and in the crushing of coal. A great advantage in quarrying is in the fact that they produce a very good cubic product. Large impact crushers will reduce 1.5 m top size run-of-mine ore to 20 cm, at capacities of around 1500 th -1, although crushers with capacities of 3000 th -1 have been manufactured. Since they depend on high velocities for crushing, wear is greater than for jaw or gyratory crushers. Hence impact crushers should not be used on ores 28

28 containing over 15% silica (Lewis et al., 1976). However, they are a good choice for primary crushing when high reduction ratios are required (the ratio can be as high as 40:1) and a high percentage of fines, and the ore is relatively non- abrasive Types of Hammer Mills Fixed Hammer Mill: - For much coarser crushing, the fixed hammer impact mill is often used (Figure 6.26). In these machines the material falls tangentially on to a rotor, running at revmin -1, receiving a glancing impulse, which sends it spinning towards the impact plates. The velocity imparted is deliberately restricted to a fraction of the velocity of the rotor to avoid enormous stress and probable failure of the rotor bearings. The fractured pieces which can pass between the clearances of the rotor and breaker plate enter a second chamber created by another breaker plate, where the clearance is smaller, and then into a third smaller chamber. This is the grinding path which is designed to reduce flakiness and gives very good cubic particles. Figure 3.2: Impact mill Rotary Hammer Mill: - The rotary impact mill gives a much better control of product size than does the hammer mill, since there is less attrition. The product shape is much more easily controlled and energy is saved by the removal of particles once they have reached the size required. The blow bars are reversible to even out wear, and can easily be removed and replaced. 29

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30 Experiment No. 4: 4.1. Objectives The main objectives of this experiment are to study the various parts of Laboratory Disc mill with special emphasis on their functions, to perform a crushing test on a given sample, to analyze the product by sieve analysis and to calculate its reduction ratio by feed size and product size measurement Apparatus/Materials Laboratory Disc mill Rock sample for crushing Sieve set (0.85, 0.60, 0.425, 0.30, and 0.15 mm) Ro-Tap Sieve Shaker Torsion Balance 4.3. Procedure Identify each part of the machine. Switch on the machine and study the working of each part. Note the rpm of machine with the help of a tachometer. Examine the feed for its size range and record the average maximum size in the feed. Adjust the sat for fine crushing. Feed the material slowly and check the size of the product. Make the adjustment of set, if necessary. Switch off the machine and recover the product. Transfer the ground material to a sieve set and sieve for 10 minutes. Switch off the sieve shaker and recover the retained weights for each sieve. Calculated the reduction ratio of the machine for the test performed. Tabulate the sieve test and plot a graph on a suitable graph paper Specifications of Laboratory Disc Mill Table 4.1: Specifications of Laboratory Disc Mill Grinding Mill Motor power Motor r.p.m Disc r.p.m Size of Disc Max: feed size Capacity Disc Mill 5 hp 1800 r.p.m 275 r.p.m 9 ½ inch (241.3 mm) ¼ inch (6.35 mm) 2 lbs/min No. of grooves on moving disc 6 No. of grooves on stationary disc 5 31

31 4.5. Observations and Calculations Feed Size Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Product Sieve Analysis Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Reduction ratio Determine the reduction ratio by the following relation: 80% Passing Feed Size Reduction Ratio 80 = 80% Passing Product Size 4.6. Graph 1. Draw graph of cumulative passing and retaining mass percentage against aperture size (geometric mean) and determine cut size, d10, d25, d50, and d Draw log-normal plot between aperture size (geometric mean) and cumulative passing mass percentage and determine the standard deviation. 3. Express Gaudin-Schuhmann distribution on graph and determine the constants involved. 4. Express Rosin-Rammler distribution on graph and determine the constants involved. *Note: Read Topic 2.2. Particle Size Distribution in Mineral Processing Design and Operation by A. Gupta and D. S. Yan 4.7. Discussions Discuss the results and the information deducted from sieve analysis in details Conclusions Give concluding remarks of the experiment and its results. 32

32 4.9. Related Theory Introduction A disc mill, is a type of crusher that can be used to grind, cut, shear, shred, fiberize, pulverize, granulate, crack, rub, curl, fluff, twist, hull, blend, or refine. It works in a similar manner to the ancient Buhrstone mill in that the feedstock is fed between opposing discs or plates. The discs may be grooved, serrated, or spiked. Disc mill can be used for secondary or fine crushing and also grinding. The disc mill idea originated from hand flour mill. Figure 4.1: A Disc Mill Types of Disc Mill Disc Mill consists of three types. Those are also shown in the Figure 4.2. Figure 4.2: Types of Disc Mill Single Wheel Disc Mill This disc mill utilizes only one disc which spins along the base in order to crush the feed Double Wheel Disc Mill This disc mill utilizes two interconnect discs in order to grind the feed. 33

33 Vibrating Disc Mill This disc mill utilizes high speed vibration to separate the items after they have been crushed or ground Construction Disc mill consists of the following parts and components. Figure 4.3: Basic Parts of Disc Mill Feed Hopper At this part of the disc mill, the material to be grinded is inserted into the machine. It has a specific size and feed should be processed at least through the primary crushers. The feed size is not large Grinding Discs A disc mill consists of two saucer shaped discs with their surface having specially shaped grooves the depth of which reduced towards the circumference. The discs are face to face mounted vertically or horizontally and revolve at the different speeds and in the opposite directions. In most designs one of the discs is driven while the other is not. Also one of the discs is rather strongly fixed while the other flutters or gyrates during revolving. Our laboratory model has heat treated meehanite metal discs mounted vertically, one revolving in the planetary manner always having a proper curvature with relation to the other which is stationary. Like other crushing machines, a disc mill has not of the discs spring loaded through a screw mechanism that helps in adjusting the set and also provides safety against uncrushable lumps. 34

34 Discharge Vessel At this point of the machine, the feed which was inserted comes out after being grinded Working The material is fed through a hopper at the top and falls into the axial conic between the discs during revolving. Due to the centrifugal force, the feed is pushed through the tapper grooves. Towards the periphery and gets grind progressively. The product is finally discharged peripherally and collected in a peripheral receptacle. Like other crushing machines a disc mill has not of the discs spring-loaded through a screw mechanism that helps in adjusting the set and also provides safety against un-crushable lumps. Disc mill is a type of attrition mill in which two surfaces rotate pass each other at high speeds with close tolerance Related Terminology A) Centrifugal Force: This reaction force is sometimes described as a centrifugal inertial reaction, that is, a force that is centrifugally directed, which is a reactive force equal and opposite to the centripetal force that is curving the path of the mass. B) Periphery: The outer limits or edge of an area or object. C) Feed Material: Such a material which is introduced in the grinder for crushing purpose is called the feed material Parameters Affecting Performance Feed Material The type of the feed introduced in the disc mill will affect the performance of the mill. In case of hard and abrasive material the disc will undergo severe abrasive forces and may distort. So this factor plays important role in controlling performance of disc mill Distance between Two Discs The distance between two disc plates will also be an important parameter in determining the performance of the disc mill. If the distance is large, then large, coarse material will be output of the mill while the feed size will be same. So efficiency can increase as mill will grind feed more quickly and in less time Applications Disc Mills are popular tools for agricultural applications, where they are used for milling corn and grains after harvest. Disc mills are also used in food and chemical processing, and to crush stone and metal products. Disc mills may be used in mining operations to separate minerals and other valuable elements from the surrounding rock. They are also widely used in recycling plants for grinding paper, plastics and other reusable materials Limitations Disc mills are relatively expensive to run and maintain however, and tend to require frequent maintenance. Discs may experience wear over time as they grind various materials, which can reduce performance. The machines also produce a large amount of dust, and must be carefully ventilated when used in an indoor workspace. 35

35 Specifications of Industrial Models Below (Table 28) we can see the various specifications of an industrial model disc Table 4.2: JXCS Disc Mill Specifications Figure 4.4: Disk Mill Present in Laboratory 36

36 Experiment No. 5: 5.1. Objectives The main objectives of this experiment are to study the various parts of Laboratory Rod mill with special emphasis on their functions, to perform a crushing test on a given sample, to analyze the product by sieve analysis and to calculate its reduction ratio by feed size and product size measurement Apparatus/Materials Laboratory Rod mill Rock sample for crushing Sieve set (0.85, 0.60, 0.425, 0.30, and 0.15 mm) Ro-Tap Sieve Shaker Torsion Balance 5.3. Procedure Switch of the machine and study each part of machine. Note the RPM of machine with tachometer Examine the feed for its size range and record the average size of largest lump in the feed. Note the total weight of feed. Load the mill cylinder with its feed sample and its rod load. Switch on and run the machine for 30 minutes and then recover the ground product. Transfer the ground material to the set of sieve by consolation and sieve for 20 minutes. Switch off the sieve shaker and record the retained weight of each sieve. Note the weights of the individual sieve and of the base pan. Calculate the reduction of the machine for the test performed. Tabulate the sieve test and plot a graph on a suitable graph sheet Specifications of Laboratory Rod Mill Table 5.1: Specifications of Laboratory Rod Mill Name Rod Mill R.P.M Of Mill 160 Motor Power ½ Hp Motor R.P.M 710 Cylinder Depth Inches Rod Material Hard Carbon Steel Weight Of Each Rod 1220 Grams Size Of Rod 12 1 No. Of Rods 3 37

37 5.5. Observations and Calculations Feed Size Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Product Sieve Analysis Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Reduction ratio Determine the reduction ratio by the following relation: 80% Passing Feed Size Reduction Ratio 80 = 80% Passing Product Size 5.6. Graph 5. Draw graph of cumulative passing and retaining mass percentage against aperture size (geometric mean) and determine cut size, d10, d25, d50, and d Draw log-normal plot between aperture size (geometric mean) and cumulative passing mass percentage and determine the standard deviation. 7. Express Gaudin-Schuhmann distribution on graph and determine the constants involved. 8. Express Rosin-Rammler distribution on graph and determine the constants involved. *Note: Read Topic 2.2. Particle Size Distribution in Mineral Processing Design and Operation by A. Gupta and D. S. Yan 5.7. Discussions Discuss the results and the information deducted from sieve analysis in details Conclusions Give concluding remarks of the experiment and its results. 38

38 5.9. Related Theory [References: Mineral Processing Technology by B. A. Wills (7 th Edition-2005) & Mineral Processing Design and Operation by A. Gupta and D. S. Yan (1 st Edition-2006)] Introduction Grinding is the second step of mineral processing and the last stage of the comminution process. The product from a crushing unit is fed to a mill in order to decrease the particle size (sometimes even to 10 microns) for subsequent processing. The purpose of grinding differs with the material being ground Introduction of Grinding Mills In materials processing a grinder is a machine for producing fine particle size reduction through attrition and compressive forces at the grain size level. A mill is a device that breaks solid materials into smaller pieces by grinding, crushing, or cutting. Such comminution is an important unit operation in many processes. The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcoming of the interior bonding forces. After the grinding the state of the solid is changed: the grain size, the grain size disposition and the grain shape Types of Mills According to the ways by which motion is imparted to the charge, grinding mills are generally classified into three types: tumbling mills, stirred mills, and vibrating mills. Types of Mill Tumbling Mill Stirred Mill Vibrating Mill Rod Mill Ball Mill Horizontal Mill Vertical Mill Figure 5.1: Types of Mills Tumbling Mill In this mill, the mill shell is rotated and motion is imparted to the charge via the mill shell. The grinding medium may be steel rods, balls, or rock itself. Tumbling mills are typically employed in the mineral industry for coarse-grinding processes, in which particles between 5 and 250 mm are reduced in size to between 40 and 300 microns. 39

39 Figure 5.2: Types of Mills Introduction of Rod Mill Rod Mills are considered as either fine crushers or coarse grinding machines. They are capable of taking maximum size of 50mm and produce a product of 300µm. Its distinctive feature is that the length of the cylindrical shell is between the 1.5 and 2.5 times its diameter. Reduction ratio is in between the range of 15-20: Types of Rod Mill Rod mill are classified on the basis of their discharge. The types of Rod Mills are as follows: Centre Peripheral Discharge Mill Rod Mill End Peripheral Discharge Overflow Rod Mill Figure 5.3: Types of Rod Mill Centre Peripheral Discharge Mill These are feed at both ends through the trunions and discharge the ground product through circumferential ports at the center of the shell. Short path and steep gradient gives a coarse grind with minimum of fines, but the reduction ratio is limited End Peripheral Discharge Mill Material is fed from one end and discharge ground product from 2 nd end by means of several peripheral apertures into a close fitting circumferential chute. This is used for dry and damp grinding where moderately coarse products are involved. 40

40 Figure 5.4: Centre Peripheral Discharge Mill Figure 5.5: End Peripheral Discharge Mill Overflow Rod Mill This is most widely used rod mill in the mining industry, in which feed is introduced through one trunions and discharge from other. This is only used for wet grinding. A flow gradient is provided by making the overflow Trunion diameter cm larger than that of feed opening. Figure 5.6: Overflow Rod Mill Construction From the trunnion liner out wards first we will come to the face plate. It is slightly concave to create the pooling area for the rock to collect in before entry to the rod -load. On the outside attached to the face plate is the bull gear. This gear completely circles the mill and provides the interface between the motor and the mil. The bull gear and drive line may be the other end of the mill instead. The face plate, attached to the other side of the face plate is the shell. The shell is the body of the mill. On the inside of the mill there are two layers of material. The first layer is the backing for the liners. This is customarily constructed from rubber but wood may be used as well. The purpose of this backing is two-fold. One to absorb the shock that is transmitted through normal running. And to provide the shell with a protective covering to eliminate the abrasion that is produced by the finely ground rock and water. Without this rubber or wood backing, the life of the mill is drastically reduced due to metal fatigue and simply being warn away. On top of this backing is the liners themselves. There are many different patterns and types of liners depending upon the job they are doing and the design of the mill. The trunnion liner may also be referred to as the throat liner. Next to this liner is the end liners. The filler ring which is next is not standard in all mills, some mills have them, and some don't. Their job is to fill the corner of the mill up so the shell will not wear at that point. They don't provide any lift to the media, in fact quite often the media will not come into contact with them at all, but 41

41 what they do is make changing liners that much easier. With different liner designs the replacement of a single liner may be quite difficult and to change one could become a lengthy project. The liner that butts into the filler liner is known as a shell liner. These liners and/or lifters give the media its cascading action and also receive the most wear. They cover the complete body of the mill and have the largest selection of types to choose from. As the two ends of the mill are the same there isn't any reason to go over the other face plate. The discharge trunnion assembly is very much like the feed trunnion except that, it won't have a worm as part of the liner. Instead of a feed seal bolted to it, it may have a screen. This is called a trummel screen and its purpose is to screen out any rock that didn't get ground. Figure 5.7: Labelled Diagram of Rod Mill Working When the mill is rotated without feed or with very fine feed, the rods are in parallel alignment and in contact with one another for their full length. New feed entering at one end of the mill causes the rod charge to spread at that end. This produces a series of wedge shaped slots tapering toward the discharge end. The tumbling and rolling rods expend most of their crushing force on the coarse fractions of the feed material and only to a lesser degree on the finer material filling the interstices in the rod charge. The horizontal progression of material through the mill is not rapid compared to the movement of the rods and material resulting from rotation of the mill. The average particle is subjected to an action similar to many sets of rolls in series, before it is discharged. 42

42 Related Terminology Critical Velocity: The "Critical Speed" for a grinding mill is defined as the rotational speed where centrifugal forces equal gravitational forces at the mill shells inside surface. Cascading: It is the rolling down the surface of the load. Cataracting: It is the parabolic free fall above the mass. Feed Material: Such a material which is introduced in the crusher for crushing purpose is called the feed material Parameters Affecting Performance Media Size Effect Finer media were found to be more efficient for fine particle grinding. Several media sizes were tested in the Pilot Tower mill. By decreasing the media size from 12 to 6.8 mm the size reduction achieved in the mill was greatly increased. Further decreasing the media size to 4.8 mm caused mill efficiency to deteriorate Stirrer Speed Effect In addition to specific energy input, and the grinding media size, the stirrer tip speed can affect the grinding results. It can be seen that in both cases higher stirrer speed had a positive effect on grinding efficiency. For the coarser media this effect was stronger in the coarser product size range. For the finer media the stirrer speed effect was small over the range of speeds tested Slurry Density Effects The results from the tests performed to investigate the slurry density effect have shown that the mill grinding efficiency increased with slurry % solids over the range tested. The increase in grinding efficiency at higher % solids can be explained by a drop in power draw due to buoyancy effects Applications The rod mill, a tumbling mill characterized by the use of rods as grinding media, grinds ores, coal/coke, and other materials for both wet and dry applications. The rod mill accepts feed ore as coarse as 1 1/2 top size although better performance is obtained by restricting ore feed size to 3/4. Product sizes range from 4 mesh to 16 mesh operating in open circuit, or as fine as 35 mesh operating in closed circuit with a screen or other sizing device. The steel rod takes regular movement in mill. It is convenient to install and maintain. It rapidly discharges. Figure 5.8: Rod Mill Present in Lab 43

43 Specifications of Industrial Models Table 5.2: Specifications of Industrial ACX Rod Mill 44

44 Experiment No. 6: 6.1. Objectives The main objectives of this experiment are to study the various parts of Laboratory Ball mill with special emphasis on their functions, to perform a crushing test on a given sample, to analyze the product by sieve analysis and to calculate its reduction ratio by feed size and product size measurement Apparatus/Materials Laboratory Ball mill Rock sample for crushing Sieve set (0.85, 0.60, 0.425, 0.30, and 0.15 mm) Ro-Tap Sieve Shaker Torsion Balance 6.3. Procedure Study the machine and its each part. Switch on the machine and study the function of each part. Examine the feed its size range and recover the average maximum size in it. Feed the material slowly. Switch off the machine and recover the product and weight it. Transfer the product to a set of sieves and sieve for 20 minutes. Switch off sieve-shaker and recover the retained weight on each sieve. Record the retained weight for each sieve. Calculate the reduction ratio of the machine for the test performed. Tabulate the sieve results and plot graph on a suitable graph paper 6.4. Specifications of Laboratory Ball Mill Table 6.1: Specifications of Laboratory Ball Mill Name of the Machine Ball Mill R.P.M of Mill 42 Motor Power ¼ hp (Gear Reducer) Motor R.P.M 1425 Capacity Material of Ball Drum Material Total Weight of Balls 4 kg/hr Alloy Steel Cast Iron 3550 gram 45

45 6.5. Observations and Calculations Feed Size Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Product Sieve Analysis Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Reduction ratio Determine the reduction ratio by the following relation: 80% Passing Feed Size Reduction Ratio 80 = 80% Passing Product Size 6.6. Graph 9. Draw graph of cumulative passing and retaining mass percentage against aperture size (geometric mean) and determine cut size, d10, d25, d50, and d Draw log-normal plot between aperture size (geometric mean) and cumulative passing mass percentage and determine the standard deviation. 11. Express Gaudin-Schuhmann distribution on graph and determine the constants involved. 12. Express Rosin-Rammler distribution on graph and determine the constants involved. *Note: Read Topic 2.2. Particle Size Distribution in Mineral Processing Design and Operation by A. Gupta and D. S. Yan 6.7. Discussions Discuss the results and the information deducted from sieve analysis in details Conclusions Give concluding remarks of the experiment and its results. 46

46 6.9. Related Theory [References: Mineral Processing Technology by B. A. Wills (7 th Edition-2005) & Mineral Processing Design and Operation by A. Gupta and D. S. Yan (1 st Edition-2006)] Introduction of Ball Mill The final stages of comminution are performed in tumbling mills using steel balls as the grinding medium and so designated "ball mills." Since balls have a greater surface area per unit weight than rods, they are better suited for fine finishing. The term ball is restricted to those having a length to diameter ratio of 1.5 to 1 and less. They are often used dry to grind cement, gypsum and phosphate. Figure 6.1: Working Mechanism of Ball Mill Types of Ball Mill Ball Mills are classified to by the nature of discharge. We have three types of Ball Mills. Overflow Type Ball Mills Grate-Discharge Type Figure 6.2: Types of Ball Mills Compartment Type Overflow Type The overflow type of Ball Mill is designed to overflow and discharge materials from the trunnion on the outlet side. By combining it with a mechanical classifier or wet-processing cyclone, you are able to extensively use this type for grinding in closed circuit or for special applications such as re-grinding in open circuit. Generally, it is best suited to fine-grind materials up to the particle sizes ranging from 150 to 200 mesh Grate-Discharge Type The grate-discharge type of Ball Mill has a grate at the outlet of the shell and causes less excessive grinding, compared to the overflow type. Therefore, generally, it is best suited to grind materials up to the particle sizes ranging from 60 to 100 mesh. 47

47 Figure 6.3: Labelled Diagram of Overflow Type Figure 6.4: Labelled Diagram of Grate-Discharge Type Compartment Type The compartment type of Ball Mill has a longer shell, inside of which is comparted into 2 to 3 chambers with grates and is best suited to produce products grinding from coarse particles of some 25 mm to fine particles of some 200 mesh. Figure 6.5: Labelled Diagram of Compartment Type 48

48 Construction Shell Mill shells are design to sustain impact and having loading and are constructed from rolled mild steel plates but welded together Mill Ends The mill ends may be of nodular or grey cast iron for diameter less than about 1m, larger heads are constructed from cast steel which is relatively light and can be welded Trunnions & Bearings Trunnions are of same type as that of rod mills. They are highly polished to reduce bearing frictions. Similarly, oil lubricant bearings is favored in large mils, via motor driven oil pumps Liners Ball mill ends usually have ribs to lift the charge with mill rotation. These prevent excessive slipping and increase liner life. They can made from white cast iron, alloyed with Ni- hard and rubber Drum Feeders The entire mill feed enters the drum via a chute a spout and an internal spiral carries it into the trunnion liner. The drum also provides a convenient methods of adding grinding balls to mill. Figure 4: Labelled Diagram of Bill Mill Working The steps involved in the working process are as follows: 49

49 Figure 6.7: Flowchart of Working in Ball Mill Initial Stage The powder particles are get flattened by the collision of the balls. It leads it changes in the shapes of individual particles or cluster of particles being impacted repeatedly by the milling balls with high kinetic energy Intermediate Stage Significant changes occur in comparison with those in the initial stage Final stage Reduction in particle size takes place. The microstructure of the particle also appears to be more homogenous in microscopic scale than those at the initial and intermediate stages Completion stage The powder particles possess an extremely deformed metastable structure. The speed of the rotation is more important. By this way the maximum size reduction is effected by the impact of particles between the balls and by attrition between the balls. After the suitable time the material is taken out and passed through a sieve to get powder of the required size. Figure 6.8: Ball Mill present in Lab Related Terminology Critical Velocity: The "Critical Speed" for a grinding mill is defined as the rotational speed where centrifugal forces equal gravitational forces at the mill shells inside surface. 50

50 Cascading: It is the rolling down the surface of the load. Cataracting: It is the parabolic free fall above the mass. Height attained: The maximum height up to which the particles go along the mill shell and then get thrown off and follow a parabolic path. Feed Material: Such a material which is introduced in the crusher for crushing purpose is called the feed material Parameters Affecting Performance Pulp Density The pulp density of the feed should be as high as possible. It is essential that the bals are coated with layer force; to dilute a pulp increase metal-to-metal contact, giving increase steel consumption and reduced efficiency. Ball mill should operate b/w 65-80% solids by weight, depending on the ore Surface Area of Medium Balls should be as small as possible and the charge should be graded such that the largest balls are just heavy enough to grind the largest and hardest particle in feed. The correct ratio of ball size to ore size is often determined by trial and error, primary grinding usually requiring a grade charge of cm diameter balls, while secondary grinding requires 5-2cm Charge Volume The charge volume is about 40-50% of the internal volume of mill about 40% of this being void space. The energy input to the mil increases with the ball charge and reaches a maximum at a charge volume of approx.50%, but for a number of reasons, 40-50% is rarely exceeded. The efficiency curve is in any case quite flat about maximum Speed The optimum mill speed increases with the charge volume, as increased weight of charge reduces the amount of cataracting taking place. Ball mills are often operated at higher speed than rod mills, so that the larger balls cataract and impact on the ore particles. The work input to a mill increases in proportion to the speed and the ball mills are run at as high a speed as is possible Applications Ball mills are used extensively in the mechanical alloying process. The ball mill is a key equipment to grind the crushed materials and cement, silicate, new type building material, refractory material, fertilizer, ore dressing of ferrous metal and non-ferrous metal, glass ceramics, etc. In mineral, cement, refractory, chemical industry the ball mill is mainly used to grind materials etc. The ball mill is used for grinding materials such as coal, pigments, and feldspar for pottery. Ball Mill is widely used in metal and nonmetal mines, building materials and other industrial sector Limitations The ball mill is a very noisy machine. Wear occurs from the balls as well as from the casing which may result in contamination of the product. It has low working efficiency. Large total weight is of hundreds of tons, so one must be very great investment. Too much large sized particles requires greater investment in the reduction process 51

51 Specifications of Industrial Models Table 6.22: Ball Mill of Industrial Level Specifications Table 6.3: DAVE Ball Mill Specifications 52

52 53

53 Experiment No. 7: 7.1. Objectives The objective of this experiment is to study Denver Dillon Vibrating Screen (double deck & Junior Hummer Vibrating Screen (single deck) and to perform a screening test on the given sample by using these screens. It also includes the determination of the performance of both screens by two product and three product formulas and the comparison of their efficiencies Apparatus/Materials Laboratory Denver Dillon Vibrating Screen (Double Deck) Laboratory Junior Hummer Screen (Single Deck) Meter rod Feed sample Torsion / Electrical balance Sieve Shaker Set of Sieves 7.3. Procedure Identify each part of the both machines Make 2 representative samples of the given feed by the method of coning and quartering or the rifflers. Use sieves of same aperture size as that of screens for each feed and shake them for 30 minutes in sieve shaker. Collect the product from each sieve and measure their respective weights. Determine the percentage of oversize, middling and undersize material. Properly mix the products of each shaking operation separately and then feed it to the respective vibrating screen while it is running. Switch off the machine and collect the overflow and the underflow products. Weigh the products and calculate weigh percentages. Perform sieve analysis on each product separately by using the same sieves as used for the feed and determine the percentage of oversize and undersize material in each product Specifications of Laboratory Vibrating Screens Table 7.1: Specifications of Laboratory Screens Feed Size Angle of screen 4 mesh to 48 mesh 15 o Screen Size Junior Hummer Screen Denver Vibrating Screen 10 mesh 10 mesh and 20 mesh 54

54 7.5. Observations and Calculations Sieve Analysis of Feed Sieve Size 10 mesh 20 mesh Pan Screening test results Aperture Size 10 mesh 20 mesh Pan Sieve analysis of Products Product Name Overflow Middling Underflow Sieve Size 10 mesh 20 mesh Pan 10 mesh 20 mesh Pan 10 mesh 20 mesh Pan Denver Vibrating Screen Hummer Vibrating Screen Measured Wt. Percentage Measured Wt. Percentage Denver Vibrating Screen Hummer Vibrating Screen Measured Wt. Percentage Measured Wt. Percentage Denver Vibrating Screen Hummer Vibrating Screen Measured Wt. Percentage Measured Wt. Percentage Determination of Screen Efficiency Derive an expression for determination of performance of each screen by using mass balancing techniques and determine the efficiency of each screen Discussions Compare the results of both screening tests and discuss them in detail Conclusions Give concluding remarks of the experiment and its results Related Theory [References: Mineral Processing Technology by B. A. Wills (7 th Edition-2005) & Mineral Processing Design and Operation by A. Gupta and D. S. Yan (1 st Edition-2006)] Introduction of Sizing With size control we understand the process of separating solids into two or more products on basis of their size. This can be done dry or wet Types of Sizing In mineral processing practices we have two methods dominating size control processes as shown in Figure 1. 55

55 Figure 7.1: Types of Sizing Processes Screening Screening is done by using a geometrical pattern for size control. It is the practice of taking granulated ore material and separating it into multiple grades by particle size Classification Classification is done by using particle motion for size control. It is the method of separating mixtures of minerals into two or more products on the basis of the velocity with which the grains fall through a fluid medium Screening Introduction Mechanical screening, often just called screening, is the practice of taking granulated ore material and separating it into multiple grades by particle size. Industrial sizing is extensively used for size separations from 300 mm down to around 40 µm, although the efficiency decreases rapidly with fineness. We utilize screening process for number of the reasons. Some of them are mentioned below. Sizing or Classifying: This is to separate particles by size, usually to provide a downstream unit process with the particle size range suited to that unit operation. Scalping: This is to remove the coarsest size fractions in the feed material, usually so that they can be crushed or removed from the process. Grading: This is to prepare a number of products within specified size ranges. This is important in quarrying and iron ore, where the final product size is an important part of the specification. Media Recovery: Media recovery is for washing magnetic media from ore in dense medium circuits. Dewatering: This is to drain free moisture from a wet sand slurry. Desliming: This is to remove fine material, generally below 0.5 mm from a wet or dry feed. Trash removal: This is usually to remove wood fibers from a fine slurry stream Types of Screening processes There are two types of screening processes as shown in Figure 2. 56

56 Figure 7.2: Types of Screening Wet Screening: The addition of water to a screen to increase its capacity and improve its sizing efficiency. Wet screening down to around 250 µm is common. Wet screening is shown in Figure 7.3(A). Dry Screening: The screening of solid materials of different sizes without the aid of water. Dry screening is generally limited to material above about 5 mm in size. Dry screening is shown in Figure 7.3(B). Figure 7.3: (A) Wet screening; (B) Dry screening Types of Screening There are a number of types of mechanical screening equipment that cause segregation. These types are based on the motion of the machine through its motor drive as shown in Figure 7.4. Figure 7.4: Types of Screening 57

57 Vibrating Equipment: This type of equipment has an eccentric shaft that causes the frame of the shaker to lurch at a given angle. This lurching action literally throws the material forward and up. As the machine returns to its base state the material falls by gravity to physically lower level. This type of screening is used also in mining operations for large material with sizes that range from six inches to +20 mesh. High Frequency Vibrating Equipment: This type of equipment drives the screen cloth only. Unlike above the frame of the equipment is fixed and only the screen vibrates. However, this equipment is similar to the above such that it still throws material off of it and allows the particles to cascade down the screen cloth. These screens are for sizes smaller than 1/8 of an inch to +150 mesh. Gyratory Equipment: This type of equipment differs from the above two such that the machine gyrates in a circular motion at a near level plane at low angles. The drive is an eccentric gear box or eccentric weights. Trommel Screens: This screen do not require vibrations, instead, material is fed into a horizontal rotating drum with screen panels around the diameter of the drum Construction The standard unit is a single-shaft, double-bearing unit constructed with a sieving box, mesh, vibration exciter and damper spring. The screen framing is steel side plates and crossmembers that brace static and dynamic forces. At the center of the side plates, two roller bearings with counterweights are connected to run the drive. Four sets of springs are fixed on the base of the unit to overcome the lengthwise or crosswise tension from sieves and panels and to dampen movement. An external vibration exciter (motor) is mounted on the lateral (side) plate of the screen box with a cylindrical eccentric shaft and stroke adjustment unit. At the screen outlet, the flows are changed in direction, usually to 90 degrees or alternate directions, which reduces the exiting stream speed. Strong, ring-grooved lock bolts connect components. Variations in this design regard the positioning of the vibration components. One alternative is top mounted vibration, in which the vibrators are attached to the top of the unit frame and produce an elliptical stroke. This decreases efficiency in favor of increased capacity by increasing the rotational speed, which is required for rough screening procedures where a high flow rate must be maintained. A refinement adds a counter-flow top mounting vibration, in which the sieving is more efficient because the material bed is deeper and the material stays on the screen for a longer time. It is employed in processes where higher separation efficiency per pass is required. A dust hood or enclosure can be added to handle particularly loose particles. Water sprays may be attached above the top deck and the separation can be converted into a wet screening process. 58

58 Figure 7.55: Labelled Diagram of Vibrating Screen Working A screening machine consist of a drive that induces vibration, a screen media that causes particle separation, and a deck which holds the screen media and the drive and is the mode of transport for the vibration. There are physical factors that makes screening practical. For example, vibration, g force, bed density, and material shape all facilitate the rate or cut. Electrostatic forces can also hinder screening efficiency in way of water attraction causing sticking or plugging, or very dry material generate a charge that causes it to attract to the screen itself. As with any industrial process there is a group of terms that identify and define what screening is. Terms like blinding, contamination, frequency, amplitude, and others describe the basic characteristics of screening, and those characteristics in turn shape the overall method of dry or wet screening. In addition, the way a deck is vibrated differentiates screens Related Terminology Blinding: When material plugs into the open slots of the screen cloth and inhibits overflowing material from falling through. Brushing: This procedure is performed by an operator who uses a brush to brush over the screen cloth to dislodged blinded opening. Contamination: This is unwanted material in a given grade. This occurs when there is oversize or fine size material relative to the cut or grade. Another type of contamination is foreign body contamination. Deck: A deck is frame or apparatus that holds the screen cloth in place. Screen Media: It is the material defined by mesh size, which can be made of any type of material such steel, stainless steel, rubber compounds, polyurethane, brass, etc. 59

59 Shaker: A generic term that refers to the whole assembly of any type mechanical screening machine. Mesh: Mesh refers to the number of open slots per linear inch Parameters Affecting Performance The parameters which affect the performance are as follows: Feed Rate: The principle of sieve sizing analysis is to use a low feed rate and a very long screening time to effect an almost complete separation. In industrial screening practice, economics dictate that relatively high feed rates and short particle dwell times on the screen should be used. At these high feed rates, a thick bed of material is presented to the screen, and fines must travel to the bottom of the particle bed before they have an opportunity to pass through the screen surface. The net effect is reduced efficiency. High capacity and high efficiency are often opposing requirements for any given separation, and a compromise is necessary to achieve the optimum result. Screen Angle: If a particle approaches the aperture at a shallow angle, it will "see" a narrower effective aperture dimension and near mesh particles are less likely to pass. The slope of the screening surface affects the angle at which particles are presented to the screen apertures. Some screens utilize this effect to achieve separations significantly finer than the screen aperture. Where screening efficiency is important, horizontal screens are selected. The screen angle also affects the speed at which particles are conveyed along the screen, and therefore the dwell time on the screen and the number of opportunities particles have of passing the screen surface. Particle Shape: Most granular materials processed on screens are non-spherical. While spherical particles pass with equal probability in any orientation, irregular-shaped near-mesh particles must orient themselves in an attitude that permits them to pass. Elongated and slabby particles will present a small cross-section for passage in some orientations and a large cross-section in others. The extreme particle shapes therefore have a low screening efficiency. Open Area: The chance of passing through the aperture is proportional to the percentage of open area in the screen material, which is defined as the ratio of the net area of the apertures to the whole area of the screening surface. The smaller the area occupied by the screen deck construction material, the greater the chance of a particle reaching an aperture. Open area generally decreases with the fineness of the screen aperture. In order to increase the open area of a fine screen, very thin and fragile wires or deck construction must be used. This fragility and the low throughput capacity are the main reasons for classifiers replacing screens at fine aperture sizes. Vibration Screens: Vibration Screens are vibrated in order to throw particles off the screening surface so that they can again be presented to the screen, and to convey the particles along the screen. The fight type of vibration also induces stratification of the feed material (Figure 7), which allows the fines to work through the layer of particles to the screen surface while causing larger particles to rise to the top. Stratification tends to increase the rate of passage in the middle section of the screen. 60

60 Moisture: The amount of surface moisture present in the feed has a marked effect on screening efficiency, as does the presence of clays and other sticky materials. Damp feeds screen very poorly as they tend to agglomerate and "blind" the screen apertures. As a rule of thumb, screening at less than around 5 mm aperture size must be performed on perfectly dry or wet material, unless special measures are taken to prevent blinding. These measures may include using heated decks to break the surface tension of water between the screen wire and particles, ball-decks (a wire cage containing balls directly below the screening surface) to impart additional vibration to the underside of the screen cloth, or the use of non-blinding screen cloth weaves. Wet screening allows finer sizes to be processed efficiently down to 250µm and finer. Adherent fines are washed off large particles, and the screen is cleaned by the flow of pulp and additional water sprays. Figure 7.6: Stratification of Particles on Screen Applications The high frequency vibrating screens achieves a high efficiency of separation and differs from its counterparts since it breaks down the surface tension between particles. Also the high level of RPMs contributes to increasing the stratification of material so they separate at a much higher rate. Separation cannot take place without stratification. Furthermore, since the screen vibrates vertically, there is a popcorn effect whereby the coarser particles are lifted higher and finer particles stay closer to the screen, thus increases the probability of separation. In some high frequency vibrating screens the flow rate of the feed can be controlled, this is proportional to the popcorn effect ; if the flow rate lowers, the effect is also decreased Limitations Limitations of the high frequency vibrating screen are that the fine screens are very fragile and are susceptible to becoming blocked very easily. Over time the separation efficiency will drop and the screen will need to be replaced Specifications of Industrial Screens The industrial model screens have following specifications as shown in Table 61

61 Table 7.2: Keller's Industrial Screen Table 7.3: Jeeves Industrial Screen 62

62 8.1. Objectives The objective of this experiment is to study hydroclassifier and to perform a classification test on the given sample. It also includes the determination of the cut size of hydroclassifier and determine its separation performance Apparatus/Materials Laboratory hydroclassifier Feed sample Torsion / Electrical balance Sieve Shaker Set of Sieves (0.71, 0.50, 0.35, 0.21, 0.15, 0.105, 0.075, 0.044) 8.3. Procedure Identify each part of the machine and find out its function. Take a feed sample and perform sieve analysis by using the sieve set as given in 8.2. After measuring the weights retained on each sieve, mix the sample properly. Close the spigot valve at the bottom of the hydroclassifier and the fill the tank with water by opening the valve of water supply connected to the hydroclassifier. When the tank is filled, open the spigot valve and adjust it to maintain a uniform flowrate at overflow and underflow ends. Put collection lauders beneath the overflow and underflow tanks for collection of test products. Pour the mixed sample at an appropriate uniform feed rate from the feed end at the center position of the classifier. Collect the underflow and overflow products into the launders and kept them there until all the solid particles settle down at the bottom. Decant the clear water from each launder very slowly so that no solid particle may go away from the launder. Collect dewatered overflow and underflow products in separate trays and dry them in oven at 110 C. Use sieves of same aperture size as that of screens for feed and perform sieve analysis on each product separately. Determine the cumulative weights and draw graph between geometric mean of passing and retained size of each fraction versus cumulative passing size. Determine the cut size and performance of classifier Specifications of Laboratory Hydroclassifier Table 8.1: Specifications of Laboratory hydroclassifier Name of Machine Centrifugal Hydroclassifier Motor Power 0.25 HP Motor RPM Size of classifier 9 inch 63

63 8.5. Observations and Calculations Feed Size Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Product Sieve Analysis Sr. No Cumulative Mass Sieve Aperture Size Individual Mass Percentage Passing Retaining Geometric mean Measured Percentage Passing Retaining Cut size determination and performance measurement 8.6. Discussions Compare the results of both screening tests and discuss them in detail Conclusions Give concluding remarks of the experiment and its results Related Theory [References: Mineral Processing Technology by B. A. Wills (7 th Edition-2005)] Introduction of Classification Classification is a method of separating mixtures of minerals into two or more products on the basis of the velocity with which the grains fall through a fluid medium. In mineral processing, this is usually water, and wet classification is generally applied to mineral particles which are considered too fine to be sorted efficiently by screening. Since the velocity of particles in a fluid medium is dependent not only on the size, but also on the specific gravity and shape of the particles, the principles of classification are important in mineral separations utilizing gravity concentrators. Classifiers also strongly influence the performance of grinding circuits. 64

64 Working Principle The Laboratory Centrifugal Classifier embodies a new principle in classification. The feed is introduced into a center well where it falls on a rotating impeller which forces the material outward and upward. The sands settle through the upward current and are discharged at the bottom spigot. The slimes rise and overflow around the rim of the classifier, while a portion of the slimes is recirculated with the primary feed to help in washing the sand particles. The velocity imparted to the pulp by the rotating impeller supplies the necessary rising current without an excess of water. There are several adjustments for regulating the size of the particles in the overflow and sand discharge. Recommended where sand grains are coated with colloids and a very fine and uniform overflow product is desired without excessive dilution, this unit also made in commercial sizes. Figure 8.1: Laboratory Centrifugal Classifier Types of classifier Many different types of classifier have been designed and built. They may be grouped, however, into two broad classes depending on the direction of flow of the carrying current. Horizontal current classifiers such as mechanical classifiers are essentially of the free-settling type and accentuate the sizing function; vertical current or hydraulic classifiers are usually hindered-settling types and so increase the effect of density on the separation. 65