University of Saskatchewan Geological Engineering GEOE 498.3 Introduction to Mineral Engineering Lecture 9 Mineral

2. Lecture 9 Comminution Methods Crushing Grinding Classification Methods Hydrocyclone Screening

4. Mineral Processing - Review

5. What is comminution?

6. Review This is an overhead photo of the surface operations of Kidd Mine and shows the key ventilation infrastructure for the mine. Greater detail is given in the following slides. Kidd Mine employs the use of a predominantly exhaust-only type of ventilation system whereby the primary flows through the mine are enabled through the operation of a surface exhaust fan system. The two exhaust shafts are as follows: NVS (North Vent Shaft): The primary exhaust route for the mine air. SVR (South Vent Raise): The secondary route of mine exhaust air. The fresh air is drawn into the mine through four main routes: #2 Shaft: Primary intake. Note that the air is heated during the winter to prevent freeze-up of the shaft conveyances. #1 Shaft: Main fresh air supply for Mine D. This air is refrigerated in the summer using the Refrigeration Plant and BAC (Bulk Air Cooler) Portal: Fresh air for upper part of the mine. Air is heated during winter to prevent icing of the ramp. Open Pit: Air is drawn through the bottom of the pit for the Cold Stope.This is an overhead photo of the surface operations of Kidd Mine and shows the key ventilation infrastructure for the mine. Greater detail is given in the following slides. Kidd Mine employs the use of a predominantly exhaust-only type of ventilation system whereby the primary flows through the mine are enabled through the operation of a surface exhaust fan system. The two exhaust shafts are as follows: NVS (North Vent Shaft): The primary exhaust route for the mine air. SVR (South Vent Raise): The secondary route of mine exhaust air. The fresh air is drawn into the mine through four main routes: #2 Shaft: Primary intake. Note that the air is heated during the winter to prevent freeze-up of the shaft conveyances. #1 Shaft: Main fresh air supply for Mine D. This air is refrigerated in the summer using the Refrigeration Plant and BAC (Bulk Air Cooler) Portal: Fresh air for upper part of the mine. Air is heated during winter to prevent icing of the ramp. Open Pit: Air is drawn through the bottom of the pit for the Cold Stope.

9. Liberation Particles can be: Fully liberated Partially liberated: middlings Gangue: waste Degree of liberation : percentage of a mineral occurring as free particles in relation to the total amount of that mineral present in ore

11. Liberation and Separation To recover and concentrate valuable minerals, product from the size reduction step is separated into two streams: valuable mineral (concentrate), and gangue (tailings). Middlings are often recycled internally. Flotation, magnetic separation, and gravity methods are typical separation processes. Ideally, concentrate should contain 100% valuable minerals and tailings should contain 100% gangue. In reality, concentrate contains some gangue and tailings contains some valuable minerals. Mineral processing combines liberation and separation to concentrate valuable minerals.

12. Terminology % Recovery: The fraction of valuable metal present in the ore that refers to the concentrate. Calculated by dividing the amount of metal in the concentrate by the amount of metal fed to the mill.

13. Terminology Grade: Purity of the product. It is the percentage (by mass) of a metal in the solids. Maximum achievable grade depends on chemical composition. Examples: Copper grade of a chalcopyrite concentrate is usually between 22% and 32%, vs. pure chalcopyrite contains 34.6% copper. Zinc concentrate is between 55% and 62% vs. pure sphalerite is about 67% zinc.

14. Liberation and Separation Grade and recovery are interdependent. In a well-run separation unit, there is a trade-off between grade and recovery. If the grade of a product increases, recovery usually drops. BUT Grade and recovery can both increase when liberation is improved.By improving liberation, we reduce the quantity of middlings in which grains of valuable mineral are locked with gangue. Grade and recovery targets are adjusted to maximize profits.

15. Liberation and Separation Particles can be described as: rounded slabby acicular flaky angular Particle shape affects processing. Usually particles that are not rounded are harder to grind, to classify, and to pump.

16. Liberation and Separation Density: calculate by dividing its mass by its volume – g/cc or kg/L or tonne/m3 Specific gravity: calculate by dividing the density of the material by the density of water.

17. Liberation and Separation When the particle does not have a regular shape, or when the sample contains particles of mixed sizes, diameter is not an adequate measure. Determine particle size by screen (sieve) analysis. Screen is made of a woven wire mesh. The size of the screen is given by the width of one opening, or by the number of openings in one linear inch. Particle size reported in mesh size or in microns: 1000 microns (µm) per millimeter. Example: A 65-mesh Tyler screen has 65 holes per inch, and each opening measures 212 µm,

18. Liberation and Separation Particle size measurements are used to determine the extent of liberation. They are also used to evaluate equipment performance. Screen analysis is the most common way of measuring particle size. Two common references for particle size measurements are 80% passing and % passing a specific size.

19. Liberation and Separation From grinding onward, ore is usually handled as a slurry. Slurry = mixture of water and ore particles, aka pulp. The two most important slurry properties are: Density Viscosity Strictly speaking, slurry density should refer to the mass of the slurry per unit volume. However, in the mineral processing industry, slurry density is usually reported as % solids by mass.

20. Liberation and Separation Slurry density can be measured with a Marcy pulp density scale. A Marcy scale is a spring balance fitted with a one-litre container Marcy scale converts this true slurry density (in kg / liter) to % solids, given the particle specific gravity.

21. Particle breakage Rate of breakage, also referred to as grinding kinetics, depends on the type of crushing or grinding equipment and on its operation  Small particles are more difficult to break because for the same mass many more particles must be broken a ten fold reduction in particle size requires 1000 times more breakage events to maintain the rate of breakage). Coarse - Impact breakage is a violent fracture that results from striking or compression. Fines - Attrition breakage is the result of abrasion and wear caused by rubbing and chipping.

22. Work index Ore hardness refers to the ability to withstand penetration and deformation. It requires more energy to crush or grind hard ores than soft ores. The work index is the most common measure of ore hardness. Ores that are difficult to grind have a high work index. The work index gives the electrical energy required to grind one tonne of ore to a specific size. The work index is given in kilowatt-hours per tonne of material ground. Competency is a qualitative term used to describe the structural integrity of ore. If ore is highly fractured or flawed, has poor cohesion, crumbles easily, it is said to not be competent.

23. Beneficiation Terminology Comminution: Reduction of particle size Starts at mine with blasting Two basic types of equipment used: Crushing Grinding

25. Comminution Primary crushing is the first stage in a circuit that may include additional stages of crushing, depending on the type of grinding mills being used.

26. Crushing

27. Comminution On the crushing stroke, a lump of ore is shattered and on the opening stroke those fragments that are larger than the discharge gap are retained for further crushing. Those that are small enough fall through and are no longer available to be broken. In this way a process of size classification takes place simultaneously with breakage in a crusher.

29. Comminution Crusher sizing specified by feed opening size Gape = largest feed size that a crusher will accept Set = largest product size it will discharge, aka Open Size Setting (OSS) Throw = OSS - CSS Typical reduction ratio is 6:1 to 8:1

30. Comminution Crushing action: When a large piece is broken the fragments fall downward in the crusher until they are again nipped and once more broken into smaller pieces throw of the mantle gets larger as the particle moves down in the chamber demand for power, becomes greater as the center of gravity of the rock mass moves downward in the crusher

31. Comminution Choke feed: Counteract slippage by maintaining a weight on top of the rock in the crusher. Promote crushing of particles by other particles. This reduces wear on mantle and concaves, produces more fines (finished product), and increases the effective reduction ratio. Even out the power demand. Maximize the machine's capacity. Choking: stoppage in the downward flow of rock in the crushing chamber.

32. Comminution Oversize: Blocking: a single rock is too big to enter the crushing chamber Bridging: two or more rocks are small enough to enter, but straddle the opening to prevent each other from falling in Solutions: hydraulic rock breaker: jack hammers the rock into smaller pieces Rock hook lower the mantle with the hydroset

33. Comminution Jaw crusher is a much simpler piece of equipment. Its design utilizes box frame construction to allow it to handle tougher ore incorporates a flywheel to store energy

34. Comminution Gyratory crusher or a jaw crusher? Similarities: speeds are the same -- 100 to 200 revolutions per minute both break ore by compression accept rock of up to sixty inches across, discharge down to 7 inches Differences: gyratory crusher - can be fed from two sides, handle ore that tends to slab, more energy efficient jaw crusher – smaller makes it a logical choice for underground, can be used on tougher ores

35. Comminution Primary/Secondary Crusher Circuit: Standard installation in a truck haulage operation consists of the stone box, gyratory crusher, surge pocket, and feeder Grizzly screen: removes fines from the crusher feed. consists of heavy steel bars deal with coarse rock. Secondary crushing is usually done in a separate crushing and screening plant utilizing cone crushers and vibrating screens Typically used ahead of rod or ball grinding mills since these will not accept feed greater than 2.5 cm (1 inch) size range

36. Comminution Secondary and tertiary crushing circuits: purpose is to reduce the size of ore to a uniform size, usually +/- 1 cm ( 3/8" ). Cone crusher: The crushing head rotates in the bowl with an eccentric motion. As the head approaches the bowl, particles are nipped and broken between the mantle and the bowl liner. Rotates at 500-600RPM, causes hammering rather than squeezing like in gyratory very hard for ore to pass through this zone without being hit at least once greater angle of the cone crusher puts the pivot point below the distributor plate

37. Comminution feed inlet: funnels ore into crusher. feed distributing plate: spreads the feed uniformly around the cavity. bowl: forms the outside of the crusher cavity. It can be moved up or down to adjust gap between liner and mantle. bowl liner: protects bowl from wear. Mantle: protects cone from wear. Cone (or head): forms the inside of the crusher cavity. It is the moving part of the crusher that effects crushing. Spring release: protects the crusher from damage due to tramp metal or other non-crushable material. Crusher cavity (aka feed pocket): the space where the ore inside the crusher is located. Eccentric and Pinion: shaft that provides the circular movement to the cone. Bowl adjustment ring: acts as a giant nut into which the bowl is screwed. The bowl is raised or lowered by turning it (like a screw).

38. Comminution Other crushing equipment: High Pressure Grinding Rolls (HPGR) Newer technology Competes with SAG/AG mills More energy efficient

39. Comminution Other crushing equipment: Granulator (hammer mill) Minimize fines creation Typically used on salt, potash, coal Discharge screen determines product size

40. Comminution May be open or closed circuit Generally, the harder the ore, the more crushing stages Closed circuit ensures uniform size

41. Ancillary Equipment Dust collection – required! Crushing produces very fine dust, can be inhaled and presents a hazard. Dust Enclosures - confined where it is produced so it can be withdrawn. Ducting - conveys the dust laden air to the dust extraction equipment. Dry Cyclone - extracts coarse dust particles from the air by centrifugal force - effective when used to pre-treat Filter - bags that present a physical barrier to the dust particles. As air is passed through, particulates adhere to the cloth - periodic cleaning required. Wet Scrubber - passes contaminated air through water sprays. The dust particles adhere onto the fine water droplets, forming slurry with the dust. Electrostatic Precipitator - passes the contaminated air through an electric field. The particulates become electrically charged and are drawn to an electrode of opposite charge. The collector must be periodically cleaned. Water is typically used, forming slurry. Slurry Disposal - Slurried dust is collected in sumps and pumped to the wet grinding circuit. Fans - draws air into the collection system and through the dust extraction equipment. Cleaned air passes through fan and discharged.

42. Ancillary Equipment Stockpiles are used to store ore before further processing. Buffer feed rate variations from upstream operations can be on a pad, which is typical for coarse ores bins (silos), typical for fine ores.

43. Ancillary Equipment Feeders: introduce material at a controlled rate. Apron feeder - most common for run-of mine / crushed ore Pan feeder – aka hydra-stroke Typically located under stockpiles

44. Ancillary Equipment Screw or vibratory feeder for finer material Usually fines stored in bins

45. Ancillary Equipment Belt conveyor: moves bulk solids from one area to another. Keep away from a running conveyor!

46. Comminution Tramp metal is a serious problem in crushing circuits. One of the most serious is crusher plugging Can damage machinery Common practice to put a belt magnet over the crusher feed conveyor belt Also may place a metal detector after the primary crusher.

47. Tumbling mills

48. Comminution AG/SAG mills accept a coarser feed than do rod/ball mills. Typical AG/SAG feed particle sizes range up to 30 cm (12 inches) which corresponds with the product size of primary crushing. They do require primary crushing so that the randomly sized run-of-mine ore is reduced to a uniformly distributed feed size acceptable to the AG/SAG mill. AG/SAG circuits do not require secondary and tertiary crushing stages between primary crushing and grinding.

49. Comminution Motion in a tumbling mill  Cascading: produces attrition breakage which leads to fine particle grinding. Cataracting: produces impact breakage which leads to coarse particle grinding. As ore particles become smaller they become less susceptible to breakage by impact; this means that ore must be reduced by attrition Speed: critical speed is when the grinding media are pinned to the shell by centrifugal force Normally, mill speed is between 55% and 80% of critical speed. Mill speed is usually fixed, but some mills have variable speed drives.

50. Comminution Feed chute: introduces ore into the mill. A seal between the stationary feed chute and the rotating mill prevents leaking. Lifters: promote the tumbling action of grinding media. Liners: protect the shell from wear. Shell: holds liners and lifters. Trunnions: provide entry and discharge points for slurry. Usually lined with spiral flights. Normally support the mill. Trommel screen: prevents large rocks, tramp metal, or grinding media from leaving with the ground product. Grinding media: loose objects that move freely inside the mill.

51. Comminution Drive assembly:  bull gear/ring gear: transmits the motion of the pinion to the mill. The mill rotates as the pinion gear meshes with the bull gear Trunnion bearings: support the mill at either end. Pinion bearings: support the pinion and motor shaft. Electric motor: supplies energy to rotate the mill. Motor shaft: transfers the energy supplied by the motor to the pinion. Air clutch connects the motor shaft to the pinion. The air clutch protects the motor from overload during startup: the motor is brought up to full speed before the clutch is engaged Pinion gear: transfers the motion of the shaft to the bull gear.

52. Comminution Lifters:  High profile promotes cataracting and impact grinding, low profile or beveled lifters promote cascading and attrition grinding. Lifter wear: grinding efficiency is affected rubber liner is typically used in ball and SAG mills and is light, long wearing, easily replaced and quiet steel liner is typically used in rod mills where abrasion and impact factors are high. As liners wear out, the lifting portion of the liner will be reduced

53. Comminution Grinding media wear down. Steel consumption is somewhere between 0.2 kg and 1 kg of steel per tonne of mill feed. To maintain grinding efficiency, new grinding media must be added periodically. Mill power and other factors are used to determine when to add new grinding media. The rate of breakage inside a mill is directly affected by the size of the grinding media. Grinding efficiency is poor if the grinding media are too large or too small for the ore. small media offers more, but lower energy, collisions per unit of time than larger media.

54. Comminution Wet vs. dry grind Because of the dust problems associated with grinding solids (health, explosion, and fire hazard, mechanical losses, etc), grinding is usually carried out in water. Presence of water in the product does not harm subsequent separation processes, since most of these operations are carried out in water. Wet grinding advantageous - requires less power per ton of material ground than dry grinding. Dry grinding consumes more energy because the fine particles adhere to the balls, forming a layer that causes the solids to occasionally slide between the balls without fracture. The disadvantage of wet grinding, however, is that there is more wear.

55. Comminution % Solids Optimization  Trade-off between: increasing % solids to maximize the number of particles in the slurry thereby increasing breakage events decreasing % solids to ensure flow through the mill and grinding media collide with high energy Operating at the optimum % solids can have a large impact on grinding efficiency. AG and SAG mills - 65% to 75% range provides enough water flowing through the mill to remove ground ore.

56. Comminution To design a circuit there are some factors that have to be known: the hardness of the ore the tonnage to be processed how fine the ore has to be ground Work index: determined by the electrical output (measured in Kilowatt Hours consumed) required to reduce one short ton (2000 lb.) of ore to the point that 80% will pass through a 100 micron screen.

57. Comminution Energy input:  Comminution is generally most energy intensive circuit Exponentially higher energy input as grind becomes finer

58. Comminution Principles of operation of a tumbling mill:  motion of material inside a mill is described in terms of the changes in center of gravity. These changes affect torque and the power required to keep the mill turning. Torque: The distance, or arm, is measured from the point where the force is applied, in this case, the center of gravity of the load, to the mill center line. Power is torque multiplied by angular velocity

59. Comminution Autogenous and Semi-Autogenous Grinding mills fed directly into the mill from either the primary crusher or the mine itself In a (semi-)autogenous mill the diameter is greater than the length. The diameter can be as much as 11 m (36')

60. AG vs. SAG Autogenous – self-breaking AG mill – fully autogenous SAG mill – semi-autogenous AG and SAG mills, coarse particles (ideally about 20 % of 10 cm to 25 cm) are very important since they are part of the grinding media  In SAG mills large balls (10 cm to 15 cm) are added (typically 6 to 12 % volume loading) to enhance the grinding action, especially for critical sized material. Other common option is to combine AG mill with screens and cone crushers to break critical size in the circuit

61. Comminution AG/SAG: grate discharge assembly serves two purposes: prevent coarse material from leaving the mill "pump" slurry out of the mill.  Grate sections: prevent coarse material from leaving the mill while letting fine slurry pass through Pulp lifters: carry slurry from the grate to the cone as the mill turns. Slurry leaves the mill by this pumping action.

62. Comminution Inside a SAG/AG mill

63. Comminution AG and SAG Mill Overload  Potential for material to accumulate in the mill due to the grate discharge. If there is too much material in the mill, the mill will not grind properly, the load will increase further To avoid this situation mill power and mill load are monitored. Stopping the feed for a few minutes is a quick method to check if the mill is an overload condition. If power increases, then the mill had been overloaded.

64. Comminution Critical Size In a grate discharge mill there can be a buildup of what is known as critical size material (typically 2.5 cm to 7.5 cm). The rate of breakage of critical size is not fast enough and causes buildup. In ball mills this occurs for material that is too large for the balls. In AG and SAG mills it occurs for material that is too small to act as grinding media but is too large to be ground at a sufficient rate.

65. Comminution Rod mill: used to grind the product from a crushing circuit (typical particle size of 3 cm) and grinds it to a size fine enough for a ball mill to handle (0.5 cm). grinding takes place preferentially on coarser particles produce less fines than ball mills Media: steel rods almost as long as the mill and can weigh 400- 600 lbs

66. Comminution Ball mill : Takes the product from a rod mill or AG/SAG circuit (typical particle size of 0.5 cm and finer) grinds it to a finer size (0.1 cm or finer) – usually final size.  usually connected to a classifier to form a closed circuit - coarse particles are recycled to the mill for further grinding

67. Comminution Other grinding equipment: Ultra fine grinding – uses internal stirrer Tower mill – vertical cylinder, small media  Isamill – ultra high intensity mixing

68. Comminution Operating a grinding circuit the most important areas to monitor are ... the tonnage coming into the circuit the grind leaving the circuit

69. Beneficiation Terminology Classification : Separation based on particle size Behavior affected by size, shape, and density of the particles Two common types of classifiers: Screens - mechanical sorting Generally for larger particles Stationary or vibratory Wet or dry feed Hydrocyclones – centrifugal force Generally smaller particles (final sizing) Slurry feed

70. Classification Classification is the process of separating a mixture containing particles of different sizes into two streams: coarse and fine particles. Perfect classifier: all coarse particles report to the coarse stream, and all fine particles report to the fine stream. The line that separates the two is called the cut size. In practice, classification efficiency is poorer. Some fine particles leave with the coarse stream and some coarse particles leave with the fine stream.

71. Classification Partition Curve When classification is not perfect, the cut size represents the size at which particles have an equal chance of going to either the underflow or overflow. Sharpness of separation: indicated by the slope of the curve. For a perfect classifier, the line is vertical. When sharpness of separation is poor, the line is closer to horizontal. Bypass: the percentage of fine particles brought into the underflow by water.

73. Screens Separates the feed into two or more streams, each containing a different size range of particles. Separation takes place by letting fine particles fall through openings in the screen deck. Screen shape: Rectangular or slotted openings offer more open area and less blinding for most ores. However, square and round openings produce a higher sharpness of separation.

74. Grizzly Screen Scalping: removing any material that may slow down production. May be rock that is too big for the equipment to effectively handle May be fine material that is taking up valuable space and will consume precious energy if it is handled further some grizzlies are placed on an incline, others flat Slabby rock may sit on top

75. Vibratory Screen The screen deck has openings to let the smaller material flow through it. Screen vibration keeps coarse material moving on the deck. A screen can have several decks, each with a different size opening. feed box: (aka feed pan) distributes the feed across the width of the screen. Counter weight: balances the screen weight to control vibration more easily. Springs: isolate the floor from screen vibration. Discharge lip: directs the flow out of the screen. Tensioning plates: keep the deck secure.

76. Classification Design considerations: Vibration: amplitude and frequency - promotes stratification Screen load: bed of an overloaded screen is too thick to allow fine particles through Screen slope: must be steep enough to ensure that the oversize solids will flow across the deck Area: capacity is proportional to the screen width, while efficiency is proportional to screen length Water sprays: used to clean coarse particles and prevent agglomerations of particles

77. Classification Other common screen types: Derrick screen: Fine vibratory screen, down to 200 mesh Alternative to cyclones Urethane construction Beware holes in screening!

78. Classification Other common screen types: Sieve bend (DSM): Wedge wire screen Static or vibratory Typically used for scalping trash from cyclone or gravity concentrator feed 1-5mm opening typical

79. Hydrocyclone - Principles Large particles settle faster than small particles of the same ore or mineral. Dense particles settle faster than light particles of the same size and shape - allows us to separate individual particles. To speed up settling, we can create an artificial gravitational force, called centrifugal force. a cyclone uses a rotating motion to create a centrifugal force.

80. Classification Size Separation. The tangential inlet shape of the cyclone forces feed to travel in a rapid circular path. The circular motion of the slurry creates the centrifugal force necessary for particle settling. Larger and heavier particles, shown in blue, which have a higher settling rate, are thrown against the cyclone wall and flow down towards the apex. Because of the cyclone design, the conical bottom in the vortex finder being larger than the apex, most water moves to the outflow stream, dragging the lighter particles, shown in yellow, with it. These fines and water form an inner spiral, which leaves through the vortex finder.During normal operation, an air core at the center of the cyclone extends from the apex to the vortex.

81. Classification Inlet: directs the feed into the cyclone - creates a circular motion. Vortex finder: collects fine material near the top of the cyclone - called overflow. Most of the water in the feed leaves with the overflow. The vortex finder extends into the cylindrical section to prevent the feed from short-circuiting to the overflow. Cylindrical section: where classification takes place. Conical section: guides coarse material towards the bottom of the cyclone. Apex: at the bottom, discharges coarse or heavy material, called underflow. On some cyclones, the size of the apex can be adjusted.

82. Classification Cyclones are grouped together in a compact arrangement to increase capacity. The number of cyclones on-stream can be changed to adjust capacity. The valves also allow cyclones to be switched for maintenance. The central feed distributor directs the feed to each cyclone. The cyclone inlet valves are used to isolate the cyclones. The cyclones separate fine or light particles from coarse or heavy particles. The common underflow launder collects the underflow from individual cyclones. The common overflow launder collects overflow from individual cyclones.

83. Classification Cyclone variables: feed % solids is the most important operating variable When water is added, the cut size becomes smaller. A higher feed rate produces a slightly finer overflow. particles with high specific gravity have finer cut size than particles of the same size but with a lower specific gravity. A smaller vortex produces finer overflow If the apex capacity is exceeded, the air core inside the apex collapses and the spiraling motion is almost lost. The discharge looks like a rope. If there is a surge in the feed rate, quite often coarse material incorrectly reports to the overflow.

84. Classification Other types of classifiers: Rake classifier Spiral classifier Both convey free-settling solids from the bottom Allow fines to overflow the launder

85. Ancillary Equipment Pumps: transfer slurry from one point to another. increases the pressure of a fluid to give it the driving force required for flow. In a grinding circuit, usually centrifugal pumps pump box provides the pump with surge capacity

86. Circuits Comminution and Classification Circuits Building blocks of grinding circuits: Tumbling mills cyclones or other classification devices pump boxes, pumps conveyors, Stockpiles and feeders The three basic types of grinding circuits are: Open circuit Closed circuit Reversed closed circuit The circuits differ in the way their components are put together.

87. Circuits Open circuit: the simplest circuit. An open circuit has: a feeder a conveyor a mill Dry ore is fed to the mill by the feed conveyor. Water is added to the feed at the mill inlet to form slurry. The mill discharge flows out of the mill to the next process operation.

88. Circuits Closed circuit: some of the ground product is recycled to the mill. Water is added to the mill discharge pump box. The cyclone feed is pumped from the pump box to the cyclone. The cyclone underflow returns to the mill. The cyclone overflow goes to the next process operation. Because of this recycle loop returning coarse particles to the mill, closed loop has higher circuit capacity and a more uniform product size distribution.

89. Circuits Reversed closed circuit: classification takes place before grinding has a higher capacity than a normal closed circuit because fines are removed before grinding Often used when the circuit feed comes from a first grinding stage (rod mill or a SAG mill for example) to remove the fines before they reach the ball mill.

90. Circuits Circulating load: The quantity of coarse material recycled to the mill in a closed circuit. A circulating load of 300% means that for every tonne of solids fed to the circuit, three tonnes are recycled from the cyclone to the mill. Roughly indicates how many passes particles make through the mill. Calculate by dividing the recycle rate by the circuit feed (or discharge) rate: Circulating load = Recycle solids rate / Fresh feed solids rate Given as a percentage. Circulating loads for ball mill circuits usually range from 100% to 400% and from 10% to 60% for AG/SAG mill circuits.

91. Circuits Typical SAG mill/ball mill circuit configuration Feed to an AG or SAG circuit is stockpiled from the mine or from a coarse crushing stage with a typical top size of between 15 cm and 25 cm The grate discharge acts both to retain the grinding media and to effect classification to the desired size.

92. Circuits Typical AG mill / ball mill circuit Screen oversize is mainly composed of critical sized material that does not grind readily. The crusher prevents a build-up of critical sized material in the AG or SAG circuit, thereby increasing circuit capacity. splitter is used to regulate the flow of ore to the crusher 

93. Circuits Typical rod mill / ball mill circuit Feed is nomally from secondary/tertiary crushing. The rod mill is open circuit, ball mill is closed circuit for size control

94. Circuits In general, the most common low-level control loops for grinding circuits are: Tonnage Water flows Mill % solids Pump box level

95. Assignment / Tutorial #9 Tutorial / Assignment Complete Review questions on Edumine: The Mill Operating Resource - 1: Ore Preparation Part 3 - Secondary and Tertiary Crushing, Review #3 Part 4 - Storage and Grinding, Review #4 Equipment Nomenclature

96. Gold

97. Oil Sands