in various crushing and grinding, milling

minerals sepetration and equipments used in various crushing and grinding, milling

1. EQUIPMENT AND STEP INVOLVED IN LIBREATION AND COMMINUTION Presented By: Nishant Kumar mehta(nit jamshedpur)

2. INTRODUCTION “Run- of mine “ ore consists of valuable minerals and gangue Mineral processing sometimes also called ore dressing ,mineral dressing or milling and this process prepare the ore for extraction of valuable mineralsand produce a commercial end product History- Two fundamental operations in mineral processing are release or liberation of the valuable mineral are now known as concentration of ore A century ago- Ore concentration was done by simple gravity and hand sorting # Freshly mined material is easier to handle by scrapers, conveyors and ore carriers

3. Comminution - In this method particle size are progressively reduced by crushing and grinding until the the clean particles of the minerals can be separated # CRUSHING – Crushing is accomplished by compression of ore against rigid surface or by impact surfaces in rigidly constrained motion path It is dry process reduction ratio of crushing stage =( maximum particle size entering to crusher)/ (minimum particle size leave the crusher) # GRINDING – grinding is accomplished by impact ,attrition and abrasion of the ore by the free motion of unconnected media such as steel rods , steel, ceramic balls, or coarse ore pebbles It is usually performed “wet” to produce slurry feed to concentration process Primary autogenous or semi- autogenous mill are tumbling mills and capable of grinding very coarse feed

4. # It is greatest energy consumer and take up-to 50% of concentrator energy consumption # High grade – fine grind – high energy cost

5. NEW TECHNOLOGY – * HPGR – high pressure grinding rolls Intermediate between fine crushing and coarse tumbling mills It is dry crushing device utilize two rotating rolls creating compression breakage of a particle bed , in which inter particle breakage occurs HPGR reduces the comminution energy requirement needed by tumbling mills HPGR is 20% to 50% more efficient than conventional crushers and mills HPGR is generally used in cement industry

6. CRUSHERS – First mechanical process in the stage of comminution # Generally dry operations # Lump of run- of- mine ore about 1.5m - reduced in primary crushing stage to 10-20cm Secondary crushing includes all operations for reclaiming the primary crusher product from ore storage to disposal of final crusher product which is usually 0.5- 2cm diameter

7. PRIMARY CRUSHERS Primary crushers are heavy duty machines ,used to reduced the run –of –mine ore down to a size suitable for transport and for feeding the secondary crushers or AG (autogenous) ,SAG (semi – autogenous) They are always operated in open circuit Primary crusher – two type Jaw crusher Gyratory crusher

8. JAW CRUSHER It has two plates which open and shut like animal jaws The jaws are set at an acute angle to each other such that one jaw is pivoted so that it swings relative to the other fixed jaw Material fed into the jaws is alternatively nipped and released to fall further into the crushing chamber , eventually falls from the discharge aperature # feature of jaw crusher Simple structure easy maintenance Stable performance Even final particles and high crushing ratio # type of jaw crusher – (on the basis of method of pivoting the swing jaw ) Dodge crusher Blake crusher Universal crusher

9. BLAKE CRUSHER Blake crusher was patented by W.E Blake in 1858 Its basic form are found in most of the jaw crusher used today In blake crusher the jaw is pivoted at the top and thus has a fixed receiving area and a variable discharge opening The greatest amount of motion is at bottom which means it has little tendency to choke # two type of blake crusher Double toggle Single toggle

10. DODGE CRUSHER In the dodge crusher the jaw is pivoted at the buttom , giving it a variable feed area but fixed delivery area In the dodge jaw crusher the moving jaw is pivoted the buttom . As minimum movement is at the buttom it has a greater tendency to choke So it is not used in laboratory where close sizing is required and it is also never used to heavy duty crusher UNIVERSIAL CRUSHER The universal crusher is pivoted in an intermediate position and thus a variable delivery and receiving area

11. GYRATORY CRUSHER # a gyratory crusher includes a solid cone set on a revolving shaft placed with in a hollow body, which has a conical or vertical slopping sides

12. SECONDARY CRUSHER Secondary crusher is immediately follow by primary crusher Secondary crushers take primary crushed ore as feed , the maximum feed size will normally be less than 15cm in diameter It is used to get our required size of product It is much easier to handle because most of harmful constituents in the ore ,such as tramp metal , wood, clays and slimes have already removed Secondary crushers also operate with dry feeds Tertiary crushers have all intents and purposes of the same design as secondaries , except that they have a closer set

13. Secondary Crushers

14. Introduction Secondary crushers are much lighter than the heavy-duty, rugged primary machines. Since they take the primary crushed ore as feed, the maximum feed size will normally be less than 15 cm in diameter and, because most of the harmful constituents in the ore, such as tramp metal, wood, clays, and slimes have already been removed, it is much easier to handle. Similarly, the transportation and feeding arrangements serving the crushers do not need to be as rugged as in the primary stage. They can operate on dry as well as wet feed.

15. Types of secondary crushers Cone crusher Roll crusher Impact crusher

16. Cone crushers The cone crusher is a modified gyratory crusher. Cone crusher and gyratory crusher work on the same principle. Both have the same operation. If cone crusher differs then it is only from crushing chamber. Cone crusher has a less steep crushing chamber and more parallel zone between crushing zones. Unlike a gyratory crusher, which is identified by the dimensions of the feed opening and the mantle diameter, a cone crusher is rated by the diameter of the cone lining. Cone crushers range in size from 559mm to 3.1 m and have capacities up to 1100 t/h with a discharge setting of 19 mm. The high-speed action allows particles to flow freely through the crusher, and the wide travel of the head creates a large opening between it and the bowl when in the fully open position. This permits the crushed fines to be rapidly discharged, making room for additional feed.

17. Working Principal Of Cone Crushers It breaks the rocks by squeezing it between the gyrating spindles. These spindles are fully covered with resistant mantle and a manganese bowl liner covers the hopper. Rocks get squeezed at the same moment when it enters in between the bowl liner and mantle. Only one time breaking is carryout of larger pieces of rocks from ore. Broken pieces of rocks fall down to the next position where it is broken again. Same process continues until the broken pieces become small enough so that it can pass through the narrow opening that is at the bottom of the cone crusher.

18. Features of cone crushers A cone crusher is suitable for crushing a variety of mid-hard and above mid-hard ores and rocks. It has the advantage of reliable construction, high productivity, easy adjustment and lower operational costs. The spring release system of a cone crusher acts an overload protection that allows tramp to pass through the crushing chamber without damage to the crusher.

19. Types of cone crushers Symons cone crusher Compound cone crushers Single cylinder hydraulic cone crusher Multi-cylinder hydraulic cone crusher The gyradisc crusher The Rhodax crusher

20. Symons cone crusher Symons cone crusher (spring cone crusher) can crush materials of above medium hardness. It is widely used in metallurgy, building, hydropower, transportation, chemical industry, etc. The Symons cone crusher is the most widely used type of cone crusher. It is produced in two forms: the Standard for normal secondary crushing and the Short-head for fine, or tertiary duty. They differ mainly in the shape of their crushing chambers. The Standard cone has "stepped" liners which allow a coarser feed than in the Short-head. They deliver a product varying from 0.5 to 6cm. The Short-head has a steeper head angle than the Standard, which helps to prevent choking from the much finer material being handled. It also has a narrower feed opening and a longer parallel section at the discharge, and delivers a product of 0.3-2.0 cm.

21. Standard Symons Cone Crusher Short-Head Symons Cone Crusher

22. Modern Standard Symons Cone Crusher Modern Short-Head Symons Cone Crusher

23. The parallel section between the liners at the discharge is a feature of all cone crushers and is incorporated to maintain a close control on product size. Material passing through the parallel zone receives more than one impact from the crushing members. The set on the cone crusher is thus the minimum discharge opening. The distributing plate on the top of the cone helps to centralise the feed, distributing it at a uniform rate to all of the crushing chamber.

24. Compound cone crusher Compound cone crusher (VSC series cone crusher) can crush materials of over medium hardness. It is mainly used in mining, chemical industry, road and bridge construction, building, etc. VSC series cone crusher’s enhanced laminating crushing effect on material particles makes the cubic shape of crushed materials better, which increases the selling point.

25. Single cylinder hydraulic cone crusher Single cylinder hydraulic cone crusher is mainly composed of main frame, transmission device, eccentric shaft, bowl-shaped bearing, crushing cone, mantle, bowl liner, adjusting device, adjusting sleeve, hydraulic control system, hydraulic safety system, dust-proof ring, feed plate, etc.

26. Multi-cylinder hydraulic cone crusher Multi-cylinder hydraulic cone crusher is mainly composed of main frame, eccentric shaft, crushing cone, mantle, bowl liner, adjusting device, dust ring, transmission device, bowl-shaped bearing, adjusting sleeve, hydraulic control system, hydraulic safety system, etc. The electric motor of the cone crusher drives the eccentric shaft to make periodic swing movement under the shaft axis, and consequently surface of mantle approaches and leaves the surface of bowl liner now and then, so that the material is crushed due to squeezing and grinding inside the crushing chamber. The safety cylinder of the machine can ensure safety as well as lift supporting sleeve and static cone by a hydraulic system and automatically remove the blocks in the crushing chamber when the machine is suddenly stuffy. Thus the maintenance rate is greatly reduced and production efficiency is greatly improved as it can remove blocks without disassembling the machine.

27. The gyradisc crusher Specialised form of cone crusher. Used for producing very fine material. Application in the quarrying industry for the production of large quantities of sand at economic cost. Working principal Crushing is by interparticle comminution by the impact and attrition of a multi-layered mass of particles. Transfer through the crushing zone is by movement of the head. Each time the lower liner moves away from the upper liner, material enters the attrition chamber from the surge load above. When reduction begins, material is picked up by the lower liner and is moved outward. Due to the slope of the liner it is carried to an advanced position and caught between the crushing members. After the initial stroke the lower liner is withdrawn faster than the previously crushed material falls by gravity. This permits the lower liner to recede and return to strike the previously crushed mass as it is falling.

28. Comparison of gyradisc and conventional cone crusher

29. The Rhodax crusher Specialised form of a cone crusher, referred to as an inertial cone crusher. Developed by the FCB Research Centre (now Fives fcb) in France. It is based on inter- particle compression crushing.

30. Schematic of Rodax cone crusher

31. Construction of Rodax cone crusher It consists of a frame supporting a cone and a mobile ring, and a set of rigid links forming a set of ties between the two parts. The frame is supported on elastic suspensions isolating the environment from dynamic stresses created by the crushing action. It contains a central shaft fixed on a structure. A grinding cone is mounted on this shaft and is free to rotate. A sliding sleeve on this shaft is used to adjust the vertical position of the cone and therefore the gap, making it simple to compensate for wear. The ring structure is connected to the frame by a set of tie rods. The ring and the cone are made of wear resistant steel.

32. Working of Rodax crusher One set of synchronised unbalanced masses transmits a known and controlled crushing force to the ring when they rotate. The relative positions of the unbalanced masses can be changed if required, so the value of the crushing force can thus be remotely controlled. As feed particles enter the fragmentation chamber, they slowly advance between the cone and the moving ring. These parts are subjected to horizontal circular translation movements and move towards and away from each other at a given point. During the approach phase, materials are subjected to compression. During the separation phase, fragmented materials pass further down in the chamber until the next compression cycle. The number of cycles is typically 4-5. During these cycles the cone rolls on a bed of stressed material a few millimetres thick, with a rotation speed of a 10-20rpm. The unbalanced masses rotate at 100-300rpm.

33. The following three parameters can be adjusted on the Rhodax crusher: • the gap between the cone and the ring • the total static moment of unbalanced masses • the rotation speed of these unbalanced masses. Operating principal of Rodax crusher

34. Courtesy of “fives” Working of Rodax crusher - Animation

35. Roll crushers Replaced in most installations by cone crushers. Still used in some mills. Still have a useful application in handling friable, sticky, frozen, and less abrasive feeds, such as limestone, coal, chalk, gypsum, phosphate, and soft iron ores. These are starvation fed.

36. Working of roll crushers The standard spring rolls consisting of two horizontal cylinders which revolve towards each other. Rolls Crushers consists of two parallel rotating rolls turning together (in opposite directions), with feed being directed through the moving gap between them. One roll is fixed, and the other moveable, using some spring mechanism or hydraulic pressure. As the ore moves through the gap, the force behind the moveable roll acts to crush the particles as they are forced together into a crushed particle “bed”. Each roll is equipped with its own motor. Operating with high pressures normally results in a product that contains a significant amount of ultra-fines (crushed particles less than 1mm), which provides a favourable ore size distribution for ball mill feed. Unlike jaw and gyratory crushers, where reduction is progressive by repeated pressure as the material passes down to the discharge point, the crushing process in rolls is one of single pressure. Smooth-surfaced rolls are usually used for fine crushing, whereas coarse crushing is often performed in rolls having corrugated surfaces.

37. "Sledging" or "slugger" rolls have a series of intermeshing teeth, or slugs, protruding from the roll surfaces. These dig into the rock so that the action is a combination of compression and ripping. Their main application is in the coarse crushing of soft or sticky iron ores, friable limestone, coal, etc., rolls of 1 m diameter being used to crush material of top size 400 mm. Since there is no provision for the swelling of broken ore in the crushing chamber, roll crushers must be "starvation fed" if they are to be prevented from choking. Choked crushing causes so much pressure that the springs are continually "on the work" during crushing, and some oversize particles escape. Rolls should therefore be used in closed circuit with screens. Wear on the roll surfaces is very high and they often have a manganese steel tyre, which can be replaced when worn. The feed must be spread uniformly over the whole width of the rolls in order to give even wear. One simple method is to use a flat feed belt of the same width as the rolls.

38. Calculation of feed size for roll crushers Let r : radius of spherical particle being crushed by a pair of rolls R : radius of rolls 2a : gap between the rolls µ : coefficient of friction 𝜃 : angle of nip (angle formed by the tangents to the roll surfaces at their points of contact with the particle) C : compressive force exerted by the roll

39. For a particle to be just gripped by the rolls, equating vertically 𝐶 𝑠𝑖𝑛(𝜃/2) = µ𝐶 𝑐𝑜𝑠(𝜃/2) Therefore, 𝜇 = 𝑡𝑎𝑛(𝜃/2) The value of the coefficient of friction between a particle and moving rolls can be calculated from the equation 𝜇 𝑘 = ((1+1.2𝑣 )/(1+6𝑣))* 𝜇 Where 𝜇 𝑘 is the kinetic coefficient of friction and, 𝑣 is the peripheral velocity of the rolls (m/s). Equation below can be used to determine the maximum size of rock gripped in relation to roll diameter and the reduction ratio (r/a) required. 𝑐𝑜𝑠(𝜃/2) = ((R+a)/(R+r))

40. Different types of roll crushers Roll crusher are mainly distinguished on the number of crushing rolls they have. Roll crushers are also manufactured with only one rotating cylinder, which revolves towards a fixed plate. Other roll crushers use three, four, or six cylinders. They also differ on the type of crushing rolls they have. For example “toothed” or “smooth”. Single toothed roll crusher Double toothed roll crusher

41. Reasons for roll crushers being replaced by cone crushers Better control over product size in cone crushers . More energy is consumed in roll crushers. Generally the capacity of cone crushers ore more as they are choke fed.

42. Impact crushers Comminution is by impact rather than compression. Sharp blows applied at high speed to free-falling rock. The impact crushers are used to process from 200 t/h up to 1900 t/h and feed sizes of up to 350 mm in the largest model. Impact crushers are generally used in nonabrasive applications and where the production of fines is not a problem. 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.

43. Types of impact crushers Hammer mill Impact mill VSI

44. Horizontal shaft impactor (HSI) / Hammermill Construction The hammers are made from manganese steel or 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 as to move out of the path of oversize material (or tramp metal) entering the crushing chamber. The exit from the mill is perforated, so that material that is not broken to the required size is retained and swept up again by the rotor for further impacting.

45. Working The HSI crushers break rock by impacting the rock with hammers that are fixed upon the outer edge of a spinning rotor. The hammer mill is designed to give the particles velocities of the order of that of the hammers. Fracture is either due to impact with the hammers or to the subsequent impact with the casing or grid. Since the particles are given high velocities, much of the size reduction is by attrition (i.e., particle on particle breakage), and this leads to little control on product size and a much higher proportion of fines than with compressive 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 3000rev/min. 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 nonabrasive materials. They have extensive use in limestone quarrying and in the crushing of coal. Normally horizontal shaft impact crusher is used for soft materials and materials like gypsum, phosphate, limestone and weathered shales.

46. Working of hammer mill in slow motion

47. Impact mill The fixed hammer impact mill is often used for much coarser crushing. The material falls tangentially on to a rotor, running at 250-500rev/min , receiving a glancing impulse, which sends it spinning towards the impact plates. 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. 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. 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 containing over 15% silica.

48. Working of impact mill Courtesy of “mesto”

49. Vertical shaft impactor (VSI) VSI crushers use a different approach involving a high speed rotor with wear resistant tips and a crushing chamber designed to 'throw' the rock against the anvil. The VSI crushers utilize velocity rather than surface force as the predominant force to break rock. Applying surface force (pressure) results in unpredictable and typically noncubical resulting particles. Utilizing velocity rather than surface force allows the breaking force to be applied evenly both across the surface of the rock as well as through the mass of the rock. Rock, regardless of size, has natural fissures (faults) throughout its structure. As rock is 'thrown' by a VSI rotor against a solid anvil, it fractures and breaks along these fissures. Final particle size can be controlled by (a) the velocity at which the rock is thrown against the anvil and (b) the distance between the end of the rotor and the impact point on the anvil. Using this method also allows materials with much higher abrasiveness to be crushed than is capable with an HSI and most other crushing methods.

50. Working of VSI VSI crushers generally utilize a high speed spinning rotor at the centre of the crushing chamber and an outer impact surface of either abrasive resistant metal anvils or crushed rock. The feed falls in the centre of the rotating chamber and due to the rotation is thrown outwards with a very high velocity towards the outer impact surface. On colliding with the impact surface the particles are broken into smaller fragments. These fragments fall down out of the crushing chamber. Utilizing cast metal surfaces 'anvils' is traditionally referred to as a "shoe and anvil VSI". Utilizing crushed rock on the outer walls of the crusher for new rock to be crushed against is traditionally referred to as "rock on rock VSI".

51. Working of VSI Courtesy of “KPI-JCI” “ASTEC”

52. Types of VSI Barmac Vertical Shaft Impact Crusher Canica Vertical Shaft Impact Crusher

53. Barmac Vertical Shaft Impact Crusher Developed in New Zealand in the late 1960s. The crusher is finding application in the concrete industry. The mill combines impact crushing, high-intensity grinding, and multi-particle pulverizing, and as such, is best suited in the tertiary crushing. Producing products in the 0.06-12 mm size range. It can handle feeds of up to 650 t/ h at a top size of over 50 mm.

54. Working of Barmac VSI Courtesy of “mesto”

55. Canica Vertical Shaft Impact Crusher This crusher developed by Jaques (now Terexs Mineral Processing Solutions) . It has several internal chamber configurations available depending on the abrasiveness of the ore. Examples include the Rock on Rock, Rock on Anvil and Shoe and Anvil configurations. These units typically operate with 5 to 6 steel impellers or hammers, with a ring of thin anvils. Rock is hit or accelerated to impact on the anvils, after which the broken fragments freefall into the discharge chute and onto a product conveyor belt.

56. Working of Canica VSI Courtesy of “TEREX MPS”

57. GRINDERS Grinding is the last stage in the process of comminution ; in this stage the particles are reduced in size by a combination of impact and abrasion, either dry or in suspension in water. It is performed in rotating cylindrical steel vessels which contain a charge of loose crushing bodies - the grinding medium- which is free to move inside the mill, thus comminuting the ore particles. According to the ways by which motion is imparted to the charge, grinding mills are generally classified into two types: tumbling mills and stirred mills. As opposed to crushing, which takes place between relatively rigid surfaces, grinding is a more random process, and is subject to the laws of probability. The degree of grinding of an ore particle depends on the probability of the ore entering a zone between the medium shapes and the probability of some occurrence taking place after entry. Grinding can be done by several mechanisms, including impact or compression, due to forces applied almost normally to the particle surface; chipping due to oblique forces; and abrasion due to forces acting parallel to the surfaces .These mechanisms distort the particles and change their shape beyond certain limits deter- mined by their degree of elasticity, which causes them to break.

58. Grinding is usually performed wet, although in certain applications dry grinding is used. When the mill is rotated, the mixture of medium, ore, and water, known as the mill charge, is intimately mixed, the medium comminuting the particles by any of the above methods depending on the speed of rotation of the mill and the shell liner structure. Most of the kinetic energy of the tumbling load is dissipated as heat, noise, and other losses, only a small fraction being expended in actually breaking the particles. Grinding may serve the following purpose: • Correct degree of liberation i.e. manufacturing of a solid with desired grain size. • Increasing the surface area of solid • Pulping of resources

59. Tumbling Mill Tumbling mills are of three basic types: rod, ball, and autogenous. Structurally, each type of mill consists of a horizontal cylindrical shell, provided with renewable wearing liners and a charge of grinding medium. The drum is supported so as to rotate on its axis on hollow trunnions attached to the end walls. The diameter of the mill determines the pressure that can be exerted by the medium on the ore particles and, in general, the larger the feed size the larger needs to be the mill diameter. The length of the mill, in conjunction with the diameter, determines the volume, and hence the capacity of the mill. The feed material is usually fed to the mill continuously through one end trunnion , the ground product leaving via the other trunnion , although in certain applications the product may leave the mill through a number of ports spaced around the periphery of the shell. All types of mill can be used for wet or dry grinding by modification of feed and discharge equipment. In tumbling mills 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 coarsegrinding processes, in which particles between 5 and 250 mm are reduced in size to between 40 and 300 µm. Grinding within a tumbling mill is influenced by the size, quantity, the type of motion, and the spaces between the individual pieces of the medium in the mill.

60. Motion of charge in tumbling mill The distinctive feature of tumbling mills is the use of loose crushing bodies, which are large, hard, and heavy in relation to the ore particles, but small in relation to the volume of the mill, and which occupy slightly less than half the volume of the mill. Due to the rotation and friction of the mill shell, the grinding medium is lifted along the rising side of the mill until a position of dynamic equilibrium is reached, when the bodies cascade and cataract down the free surface of the other bodies, about a dead zone where little movement occurs, down to the toe of the mill charge. The speed at which a mill is run is important, since it governs the nature of the product and the amount of wear on the shell liners. For instance, a practical knowledge of the trajectories followed by the steel balls in a mill determines the speed at which it must be run in order that the descending balls shall fall on to the toe of the charge, and not on to the liner, which could lead to rapid liner wear.

61. The driving force of the mill is transmitted via the liner to the charge. At relatively low speeds, or with smooth liners, the medium tends to roll down to the toe of the mill and essentially abra - sive comminution occurs. This cascading leads to finer grinding, with increased slimes production and increased liner wear. At higher speeds the medium is projected clear of the charge to describe a series of parabolas before landing on the toe of the charge. This cataracting leads to comminution by impact and a coarser end product with reduced liner wear. At the critical speed of the mill the theoretical trajectory of the medium is such that it would fall outside the shell. In practice, centrifuging occurs and the medium is carried around in an essentially fixed position against the shell. In travelling around inside the mill the medium (and the large lumps of ore) follows a path which has two parts. The lifting section near to the shell liners is circular while the drop back to the toe of the mill charge is parabolic.

62. MECHANICAL CONSTRUCTION Mill shells:-  Mill shells are designed to sustain impact and heavy loading and are constructed from rolled mild steel plates welded together. Mill ends : The mill ends or trunnion heads may be of nodular or grey cast iron for diameters less than about 1m. Trunnions and bearings : The trunnions are made from cast iron or steel and are spigoted and bolted to the end plates, although in small mills they may be integral with the end plates. Drive Tumbling mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. Mill feeders:-  The function of the feeder is to transport pulp from some receiving point outside the mill into the mill barrel smoothly and with sufficient driving force to overcome any tendency for the pulp to move in the opposite direction. Three types of feeder are in use in wet-grinding mills. Spout feeder Drum feeder Combination drum - scoop feeder Liners:-  Liners are the materials, which are used on the inner surface of the grinding shell to provide the necessary strength and resistance. Shell lining acts as the final link in the transmission of energy to the tumbling load i.e. the charge. It usually differs in type according to the size of feed, whether the feed is coarse or fine.

63. Types of mills Rod mills These may be considered as either fine crushers or coarse grinding machines. They are capable of taking feed as large as 50mm and making a product as fine as 300txm, reduction ratios normally being in the range 15-20:1. They are often preferred to fine crushing machines when the ore is "clayey" or damp, thus tending to choke crushers. The distinctive feature of a rod mill is that the length of the cylindrical shell is between 1.5 and 2.5 times its diameter. This ratio is important because the rods, which are only a few centimetres shorter than the length of the shell, must be prevented from turning so that they become wedged across the diameter of the cylinder. The ratio must not, however, be so large for the maximum diameter of the shell in use that the rods deform and break. Since rods longer than about 6 m will bend, this establishes the maximum length of the mill. Thus, with a mill 6.4 m long the diameter should not be over 4.57 m. Rod mills of up to 4.57 m in diameter by 6.4 m in length are in use, run by 1640 kW motors (Lewis et al., 1976). Rod and other grinding mills are rated by power rather than capacity, since the capacity is determined by many factors, such as the grindability , determined by laboratory testing,, and the reduction in size required. The power required for a certain required capacity may be estimated by the use of Bond's equation: 𝑊 = (10𝑊𝑖̇ /√𝑃 )− (10𝑊𝑖/ √𝐹)

64. Centre peripheral discharge mills These are fed at both ends through the trunnions and discharge the ground product through circumferential ports at the centre of the shell. The short path and steep gradient give a coarse grind with a minimum of fines, but the reduction ratio is limited. This mill can be used for wet or dry grinding and has found its greatest use in the preparation of specification sands, where high tonnage rates and an extremely coarse product are required. Rod mills are classed according to the nature of the discharge. A general statement can be made that the closer the discharge is to the periphery of the shell, the quicker the material will pass through and less overgrinding will take place.

65. End peripheral discharge mills These are fed at one end through the trunnion , discharging the ground product from the other end of the mill by means of several peripheral apertures into a close-fitting circumferential chute. This type of mill is used mainly for dry and damp grinding, where moderately coarse products are involved.

66. Trunnion overflow mill The most widely used type of rod mill in the mining industry is the trunnion overflow, in which the feed is introduced through one trunnion and discharges through the other. This type of mill is used only for wet grinding and its principal function is to convert crushing- plant product into ball-mill feed. A flow gradient is provided by making the overflow trunnion diameter 10-20 cm larger than that of the feed opening. The discharge trunnion is often fitted with a spiral screen to remove tramp material.

67. Ball mill:

68. 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.

69. Tube Mills: The term ball mill is restricted to those having a length to diameter ratio of 1.5 to 1 and less. Ball mills in which the length to diameter ratio is between 3 and 5 are designated tube mills. These are sometimes divided into several longitudinal compartments, each having a different charge composition; the charges can be steel balls or rods, or pebbles, and they are often used dry to grind cement clinker, gypsum, and phosphate.

70. Pebble Mill: Tube mills having only one compartment and a charge of hard, screened ore particles as the grinding medium are known as pebble mills. They are widely used in the South African gold mines. Since the weight of pebbles per unit volume is 35-55% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower.

71. Ball mills are also classified by the nature of the discharge. They may be: Simple trunnion over- flow mills (operated in open or closed circuit, or Grate discharge (low-level discharge) mills

72. Grate discharge mills: Grate discharge mills are fitted with discharge grates between the cylindrical mill body and the discharge trunnion. The pulp can flow freely through the openings in the grate and is then lifted up to the level of the discharge trunnion. Very little overgrinding takes place and the product contains a large fraction of coarse material, which is returned to the mill by some form of classifying device. Closed-circuit grinding, with high circulating loads, produces a closely sized end product and a high output per unit volume compared with open circuit grinding.

73. Trunnion over- flow mills: The trunnion overflow mill is the simplest to operate and is used for most ballmill applications, especially for fine grinding and regrinding. Energy consumption is said to be about 15% less than that of a grate discharge mill of the same size, although the grinding efficiencies of the two mills are the same

74. Overflow mills vs Grate discharge mills: Grate discharge mills have a lower pulp level than overflow mills, thus reducing the dwell time of particles in the mill. These mills usually take a coarser feed than overflow mills and are not required to grind so finely, the main reason being that with many small balls forming the charge the grate open area plugs very quickly.

75. Segregation of the ball charge within the mill is achieved in the Hardinge mill. The conventional drum shape is modified by fitting a conical section, the angle of the cone being about 30⁰. Due to the centrifugal force generated, the balls are segregated so that the largest are at the feed end of the cone, i.e. the largest diameter and greatest centrifugal force, and the smallest are at the discharge. By this means, a regular gradation of ball size and of size reduction is produced.

76. Autogenous Mills operate, mechanically, similar to the ball mill. They differ in the media they use to break or grind the ore. Autogenous Mills use large particles of ore instead of steel or other balls for grinding media. Autogenous mills use large pieces of ore as grinding media. The grinding is facilitated in autogenous mills by attrition with limited grinding by impact. For an ore to successfully grind autogenously, the ore must be competent, and it must break along grain boundaries at the desired product size. Another requirement is that the finer sizes should break easily and should be removed from the mill, otherwise, there will be a critical size buildup. Autogenous Mill:

77. Autogenous grinding has two advantages:  (1)  it reduces metal wear and  (2)  eliminates secondary and tertiary crushing stages. Thus it offers a savings in capital and operating costs. Autogenous mills are available for both wet and dry grinding. The diameter of autogenous mills is normally two to three times the length. The ore charge is usually 25 to 35% of the mill volume. Autogenous mills have grate discharges to retain the coarse grinding media in the mill.

78. Vibratory Mill: A vibration mill is a size reduction equipment that applies the process of continuous impaction in carrying out its size reduction function. The grinding container is made up of a tube that is held in a frame that is supported by means of springs which is filled to approximately 80% total volume with porcelain or stainless steel balls. During milling, the entire body of the mill undergoes a small but frequent vibration that is generated by an eccentric motor and size reduction occurs by repeated impact. This vibration is usually, but not necessarily, in a vertical plane.

79. Centrifugal Mill: The concept of centrifugal grinding is an old one, and although a patent of 1896 describes this process, it has so far not gained full-scale industrial application. In centrifugal milling, the forces on the charge inside the mill are increased by operating the mill in a centrifugal, rather than a gravitational, field. Comminution is more rapid, and the size of machine needed for a given grinding duty is thus reduced.

80. Tower Mill: For fine and ultra-fine grinding tower mills and stirred mills are used. In a tower mill, steel balls or pebbles are placed in a vertical grinding chamber in which an internal screw flight provides medium agitation. The feed enters at the top, with mill water, and is reduced in size by attribution and abrasion as it falls, the finely ground particles being carried upwards by pumped liquid and overflowing to a classifier. Oversize particles are returned to the bottom of the chamber, and final classification is by hydrocyclones , which return underflow to the mill sump for further grinding. Advantages are a small installation area, low noise levels, efficient energy usage, minimal overgrinding, and low capital and operating costs.

81. Stirred Mills: The stirred milling is simple and robust, and has excellent energy efficiency through a wide operational range of process parameters. As the energy efficiency is consistent, it is possible to control the specific grinding energy and product particle size by adjusting the shaft speed using the variable speed drive. The machine configuration allows for internal classification, whereby the coarser particles move towards the chamber walls while finer particles move faster upwards through the disc openings, reducing the potential for over grinding, allowing excellent energy efficiency. The stirred milling consumes less energy compared to conventional tumbling mills. The small bead sizes used in the grinding chamber result in a large amount of contacts and high grinding efficiency in the fine product size range.

82. Roller Mills: These mills are often used for the dry grinding of medium soft materials of up to 4-5 mohs hardness. Above this hardness, excessive wear offsets the advantage of lower energy consumption compared with conventional mills.