CHAPTER III

SIZE REDUCTION EQUIPMENTS

SIZE REDUCTION EQUIPMENTS

1. Coarse SIZE REDUCTION EQUIPMENTS

2. Intermediate SIZE REDUCTION EQUIPMENTS

3. Fine SIZE REDUCTION EQUIPMENTS

COARSE SIZE REDUCTION EQUIPMENTS

Machines for size reduction for feed having particles size 3 - 4 inches or larger are divided into two groups, namely:

1. Coarse crusher for hard materials, such as: jaw crusher, gyratory crusher or disk crusher

2. Coarse crusher for soft materials, such as : hammer mill or toothed roll crusher

Coarse Crusher for Hard Materials

1. Jaw Crusher

Consists of Blake type and Dodge type crushers, Blake type is easily encountered in industries, but not for the Dodge type

1. Blake Type Jaw Crusher (BTJC)

- Developed by E. W. Blake (1858)

- usually also called as Double Toggle Crusher

- suitable for reduction of hard materials having 2 - 96 inches in size

- the biggest size is 8 x 6 ft, cap.1250 ton/hr, requires 400hp (300KW), able to prepare 18 inches particle.

- made of cast steel lined with a tough abrasion resistant metal such as Manganese Steel

- operational cost: expensive

-advantages: able to crack hard materials, large capacity, seldom product discharge clogging

Working Principles:

- It works based on PRESSURE

- Two main parts of BTJC:

a. Frame: function as a fixed Jaw

b. Movable Jaws is pivoted at the top ad operated by eccentric, pitman, and toggles

Blake Type Jaw Crusher

The pitman is given a nearly Vertical motion by eccentric, and since one of the toggles is mounted in rigid journals at one end of the crusher frame, the reciprocating motion of the pitman causes the other toggle to move the jaw back and forth.

The jaw is held against the toggle by a tension link and spring. Crushing is accomplished if the movable jaw moves toward the fixed jaw. That means an intermittent power requirement. In order to equalize this, one or two heavy wheels are mounted on the main shaft of the crusher. The machine is driven by flat belts or V-belts

Operating variables: sharpness and angle between fixed jaw and movable jaw. This angle should be about 0.47 rad or 27o to retain the solid particles in the crusher by friction when the solid particles are inside the crusher, and also to prevent accumulation of particles in the crusher chamber

Reduction ratio can be adjusted by:

1. angle adjustment

2. height of crusher chamber

Max reduction ratio = 8

Discharging of product from crushing chamber is usually done by GRAVITATON, and is affected by rotation speed

At high rotation speed, movable jaw will be in the original position before particles inthe crushing chamber move down so that they can be pressed many times.

2. Dodge Crusher

- subject to uneven stresses inherent in its design

- only designed in SMALL size

- Movable jaw is pivoted at the bottom and the width of the discharge opening remain constant, therefore yielding a more closely sized product.

- No toggles are required.

- The jaw is operated through pitman by eccentric.

- If only one size reducing machine is employed, the uniformity of product size is an advantage, otherwise it is of limited use.

-The power is applied through along lever, if the crusher becomes clogged enormous stress are set up in the members which become excessive in machines with gape above 11 inch.

- The constant opening of the jaws at the discharge end gives this equipment an annoying tendency of CLOG….Not happen in BTJC

Working Principles:

- Fixed jaw is placed INCLINE

- Movable jaw is pivoted at the bottom and is operated by eccentric and pitman

- The eccentric drive the pitman to move nearly vertical. When pitman moves down, movable jaw frame will be pulled causing movable jaw to move toward he fixed jaw, and vice versa. Crushing is accomplished when movable jaw moves toward the fixed jaw. This movement takes place repeatedly and the product can be drawn from the bottom.

Dodge Crusher

3. Gyratory Crusher

- designated for greater capacity and continuous operation, suitable for hard materials having 6 – 60 inches to obtain particles of 0.2 – 4 inches in size.

- It has large feed opening and smaller product opening, so that more product can be generated in one time.

- If one considers to crush large size material in large capacity, gyratory crusher is more economical than BTJC with respect to ENERGY requirement. If large size but less amount of feed is to be reduced, BTJC is more economical in terms of OPERATING COST.

- Maintenance cost of GC is less than BTJC

Working principles:

- Crushing action similar to JAW CRUSHER, moving crushing element approaches to and recedes from a fixed crushing plate

- It has an outer frame carrying an inverted conical surface (concaves) and inner gyrating crushing head

- Lower end of the spindle is a circular shaft free to rotate in an eccentric sleeve (driven by a rotating main shaft through a set of bevel gears and rotates within a fixed cylindrical housing.

- Feed is introduced from the top opening and moving down into a space between concave and crushing head.

- The crushing spindle is free to rotate, but as soon as feeding of the machine starts, rotation ceases and gyration is only motion, causing the head to approach and recede from the concave surfaces, breaking the feed by a CRUSHING PRESSURE as it passes down through the crusher.

- The crushed materials are getting smaller in size while they move down as the space between crushing head and cone is getting narrower and the crushing pressure give by crushing head is more .

Working principles:

- This action will continue until the crushed materials have a certain size to pass through the gap and exit the discharge opening.

- Material breaking is accomplished by rotating crushing head

- The rotation of crushing head is driven by free revolving circular shaft on eccentric shaft.

- Circular shaft moves vertically as drive shaft move horizontally. The conversion of movement from horizontal to vertical is adjusted by a gear designated to handle this rotation.

Suspended-spindle Gyratory crusher

Fixed Spindle Gyratory Pinch Crusher

-Eccentric sleeve is inserted between the fixed vertical shaft and movable vertical cone

-By rotating of this part, the axis of the cone is given a cylindrical motion with a PARALLEL PINCHING action on the material being crushed

-Gyratory crusher has large capacity (equal to jaw crusher having the same GAPE and a LENGTH equal to the perimeter of gyratory) because the ACTION is Continuous.

-Capacity

T = 0.6 LS (Taggart formula)

T = capacity, ton/hr, L = length of opening = gape (BTJC) or average diameter of two cones (GC), (in), S = greatest width of discharge opening, (in)

Coarse Crusher for Soft Materials

Soft materials coal, gypsum, limestone, ice, fire clay and shale (< 4 Mosh Scale

1. Bradford Breaker

- It combines BREAKING and SCREENING.

- Breaking is accomplished by ROTATION of the cylinder

(Material is lifted on the interior shelves and broken by falling and striking the material below as the cylinder is rotated

- Its periphery is a reinforced screen which allow the coal when sufficiently reduced in size to pass through

- Harder material such as iron are not broken and gradually pass out form the open end of the breaker

Bradford Breaker

2. Toothed Roll Crusher

- Applicable for coal, gypsum, ice and other soft material.

- BREAKING is accomplished by PRESURE of teeth against the larger lumps of the material, DISINTEGRATING it in the same manner, ice is broken manually using ice pick

(breaking, tearing and pressing)

- The teeth SHAPES are depending on their FUNCTIONs

exp. : pyramidal teeth for coarse feed catching and toothed dish for crushing.

- Based on the number of roll, Roll Crusher is divided into: Single Roll and Multiple Roll.

-Crushing power depends on the number of roll and construction

- Single roll is suitable for PRIMARY crushing, while double roll is suitable for SCONDARY crushing

-Multiple rolls is suitable for successive primary and secondary crushings. This could receive continuous feeding to produce 1 cm product in one time.

Working Principles:

- Feed enters the mounting crusher, caught and driven into grinding space by pyramidal teeth. In this space, crushing and tearing take place. Materials are pressed into the rolls teeth held by adjustable plates through the gaps.

- Its periphery is a reinforced screen which allow the coal when sufficiently reduced in size to pass through

- Excessive production of fines is PREVENTED

Toothed Roll Crusher

3. Hammer Mill

- Applicable for brittle and easy to crack (coal and limestone) or even fibrous material (wood and peels).

- It works as combination of CRUSHING and GRINDING

- its is the most versatile crushing device

- Heavy blocks of steel are attached by pins to a disk or disk revolving at high speed within a sturdy housing.

- The hammer delivers heavy blows to the feed materials while it is in suspension, driving it against a breaker plate until it is fine enough to pass through the opening in the cage bars at the bottom of the mill constituting the screen

Cross Sectional of Hammer Mill

-The cylindrical screen covers the whole part of rotor. The rotor has a number of disks positioned on the horizontal axis. Between the disks hanging hammers are held on.

- Hammers are made of metal, and can be in rectangular form or as metal plates with knifelike ends.

- In the cylindrical housing, there are breaker plates to hold the reduced materials. While in the inner side of bottom parts of this, a rectangular screen equipped with vertical metal plate is constructed.

- The screens are held above the cover or bottom casing shield, which is hanged to be drop to gain the product and separate the unprocessed material. Rectangular screens area applicable for cal, limestone and other brittle materials.

- The PRODUCT SIZE is adjusted by increasing the rotor speed, feed or distance between hammer and the plate. It can also be done by selecting suitable hammers and discharge diameter

Screen Bar on Hammer Mill

Working principles:

- High speed central axis is rotated causing centrifugal force to be transferred to hammers which are hanging on the disk.

- This movement will crush the materials entering from the top opening along the mill peripheral section by hammers held by breaker plate.

- The reduced materials will enter the screen bars for further separation based on specific desirable size

Capacity:

- It can handle feed having 0.75 inches depend on the feed neck size to obtain 200 mesh particles. However, feed should not be rough or having roughness les than 1.50.

- Medium size hammer mill can produce 1 inch to 20 mesh particle size

- Combination of hammer mill and air separator (angular velocity of hammer about 2000 ft/min) can reduce 0.5 – 1.5 ton/hr feed to obtain 200 mesh particles

3. Disintegrator

- Applicable for TEARING of FIBROUS materials, such as wood blocks and asbestos.

- It has two or more concentric CAGES rotated in opposite directions.

- The feed is introduced into the INNER CAGE

- Centrifugal force drives the material into the spaces between the rotating cages where it is torn apart, and thence into the outer casing from which it is DISCHARGE to a conveyor or storage BIN.

Squirrel Disintegrator

INTERMEDIATE SIZE REDUCTION EQUIPMENT

1. Cone Crusher

Cone crusher can be used for coarse feed of finer feed (short head).

Reduction ratio is 2-3 times of Roll crusher.

Less maintenance is needed.

Feed must be dry and rather uniform in size

Cone crusher is best operated in closed circuit with screens

Telsmith Gyrasphere is a modified cone crusher with spherical contour of crushing head (facilitates discharge of crushed product) and the crushing plate is held in position by spring under compression instead of tension.

Working Principles:

- Similar to gyratory crusher.

- The inner cone (crushing head) is supported by the tapered eccentric journal which is rotated by the bevel gears driven by the main shaft.

- The entire weight of crushing head and spindle is supported on a bearing plate supplied with OIL under pressure

- Differences with gyratory crusher:The outer stationary crushing plate flares outward to provide an

increasing area of discharge so that the machine can quickly CLEAR itself of the reduced product.

This stationary crushing plate is held in position by a nest of heavy helical tension springs so that when tramp iron or other uncrushable objects enter the crushing zone the plate is lifted, preventing fracture of the plate and injury to the machine

Cross Sectional of Cone Crusher

Cutway of Cone Crusher

Telsmith Gyrasphere

2. Crusher Rolls

- It consists of two heavy cylinders revolving toward each other, the feed being nipped and pulled downward through the rolls by FRICTION.

- Modern crusher drives both rolls positively, breakage being prevented by mounting bearings of one of the rolls against nest of a heavy compression ring.

- There is a considerable amount of wear on the rolls, the crushing surface consist of a tough steel sleeve which is shrunk on the main cylindrical casting, making possible replacement of worn crushing surface.

- The size reduction by rolls is relatively small, average diameter of the product is about ¼ of feed diameter.

- Cone crusher may replace rolls for intermediate size reduction of ores (reduction ration 2 to times of rolls, les maintenance)

Crushing Rolls

Theoretical Capacity: weight of a ribbon of feed having a width equal o the width of the rolls, a thickness equal to the distance between the rolls, and a length to peripheral velocity of the rolls in linear units per interval time.

- T = capacity (ton/hr); v = peripheral velocity, for rolls up to 72 inch, v = 300+84Dr, L = width of rolls (ft), Dp = distance between rolls (ft), = density of material (lb/ cu ft)

Actual Capacity: 0.1 to 0.3 of Theoretical Capacity.

Friction ratio =

ratio of the force tangent to the surface to the force normal to the surface = FT/FN

2000

60 pLDT

Force exerted by crushing roll for a SPHERE particle

Force exerted by crushing roll for a SLAB particle

3. Gravity Stamp Hill

- Based = rock crushing device involving a weight to be lifted and dropped on the material to be broken

- It is still used in size reduction for intermediate and fine size material

- Also used in gold ores crushing (when gold is amalgamated with mercury)

- Capacity is low and expensive costs

- Modern design=

The stamps are vertical shafts raised by CAMs operating under collars fastened to the upper art of the shafts

The lower end of each shafts is equipped with a heavy metal block which strikes on a stationary hard metal slab.

Operation is carried out on suspensions of solid in water, which pass slowly through the crushing zone

- Reduction ration, max 150

Gravity Stamp Hill

FINE SIZE REDUCTION EQUIPMENT

Usually is also called GRINDING.

- Common parts in size reduction equipments:

stationary surface and a surface rubbed against the stationary surface

- Upper and nether millstone is used for grinding flour from grain

- Disintegration machine uses application of SHEAR loads

- Most size reduction machines depend on IMPACT rather than SHEAR

- In Ball Mill and Rod Mills, the material is reduced in size between ROLLERs, or HEAVY BALLs, rolling against a crushing ring.

1. Bowl Mill = horizontal axes of the rolls are stationary, and the flat pan carrying the crushing ring revolves.

Bowl Mill With Air Separator

2. Raymond Roller Mill

Rollers suspended on balanced journals from a rapidly rotating spider mounted on the upper end of the main shaft.

The revolving part exerts PRESSURE on a stationary confining ring by centrifugal force

A plow mounted on the apron or sleeve revolves with the shaft to throw the material into the crushing zone

The mill is usually provide with a sizing feature whereby the material cannot leave the machine until it is fine enough to pass through a screen of given mesh or be lifted by a stream of air at constant velocity.

Raymond Mill with Air Classifier Separator

3. Ball Mills

They are rotating cylindrical or conical steel chambers, approx. half full (20 -50% volume) of steel or iron balls or flint stones

They can process coal, peels or feldspar.

Size reduction is accomplished by the IMPACT of these balls as they fall back after being lifted by the ROTATING Chamber.

The LENGTH of the chamber is usually equal to the DIAMETER

Mostly are continuously operated, feed entering at one end and discharging through the opposite end or through the periphery. It can be operated in either WET or DRY

In cylindrical ball mills, the product can be discharged by overflow through a hollow trunnion. The smaller particles are suspended and carried out by the circulating fluid, such as AIR or WATER

3. Ball Mills

They are rotating cylindrical or conical steel chambers, approx. half full (20 -50% volume) of steel or iron balls or flint stones

They can process coal, peels or feldspar.

Cylindrical or conical shell can be smooth or waved with baffle plates. Cylindrical shell rotates on horizontal axis (slightly angled/tilted). Grinding media is usually metal, wood or rubber balls filling about 20-50% volume of void space. Material to void space ratio (M/V) is about 1-5. The surface of the void space is covered by anti abrasive materials such as rubber, stone ware or manganese steel. Rubber balls have larger friction coefficient than metal balls so that they have to be placed far inside from the cylindrical shell to enable them to fall on the feed surface from higher position.

In cylindrical ball mills, the feed enters one end of the trunnion and the product can be discharged by overflow through another end of the hollow trunnion. Usually a rough screen is placed in the discharge point to prevent the balls to exit the cylindrical shell.

Vibrated Mill can be used to replace rotating mill and feed flow is adjusted by mill INCLINATION. Efficiency increases while time goes by from the initial until the void space between balls is filled.

Large Ball Mill having 10 – 14 ft shell diameter may use 1 – 5 inches balls. Large balls can only crush the larger material without affecting finer particle as vice versa.

Cylindrical or conical shell can be smooth or waved with baffle plates. Cylindrical shell rotates on horizontal axis (slightly angled/tilted). Grinding media is usually metal, wood or rubber balls filling about 20-50% volume of void space. Material to void space ratio (M/V) is about 1-5. The surface of the void space is covered by anti abrasive materials such as rubber, stone ware or manganese steel. Rubber balls have larger friction coefficient than metal balls so that they have to be placed far inside from the cylindrical shell to enable them to fall on the feed surface from higher position.

In cylindrical ball mills, the feed enters one end of the trunnion and the product can be discharged by overflow through another end of the hollow trunnion. Usually a rough screen is placed in the discharge point to prevent the balls to exit the cylindrical shell.

Working principles:

- Ball mill can process dry or wet material under batch or continuous operation. Conical ball mill has internal crushing SEGREGATION characteristic.

- Feed enters through 60o from the left side into main crushing zone (Shell diameter is Max.) and product exit from the shell at 30o the right side.

- Ball mill may have different size of filling balls. Bigger balls can be added periodically into the shell while mill is operating. Bigger balls will move toward main crushing zone, while smaller ball will move toward secondary crushing zone. Initially, crushing is carried out by downward moving bigger ball and followed by smaller balls grinding the short distance downward moving smaller particles produced by bigger balls

Ball mill showing feeder and hollow trunnion

Hardinge Mill is a typical of cylindro-conical ball mill.

The larger balls and larger particles of feed are supposed to SEGREGATE to a certain extent in the cylindrical portion of the mill with the GREATEST DIAMETER.

- There is a relationship between size of particles and size of Balls

In any cases, the LIFTING EFFECT on the ball is greatest at the greatest diameter, and the larger balls will be most effective in size reduction of this point.

Cutway of ideal operation of Conical Hardinge Ball Mill

Outside view of grate showing radial plate which raise the product and cause it to be discharge through hollow trunnion

Grate Mills, the product passes through the openings in a vertical grate or diaphragm

In the trunnion mills, the product may be raised by radial plates or scoops on the outside of the grate, pushed away the grate by helical vanes on the inner periphery of the cylinder, and discharge from the hollow trunnion by which the mill is supported.

Grate Mill with open end discharge

If the mills are supported by peripheral tires riding on rollers, the material simply flows out through the grate and through the open end of the mill

Compound Ball Mills

- Consists of two or four cylindrical compartments separated by grates.

- Each successive compartment is of smaller diameter and contains balls of smaller sizes for finer grinding.

Tube Mills

- It is a long cylindrical mill (usually about 22 ft length) utilising pebbles of FLINT or CERAMIC LININGs and operated intermittently on a batch of material.

- It is usually replaced by ball mill, except in case where iron in the product cannot be tolerated

Rod Mills

- It is similar to ball mills except that the grinding media are STEEL RODS rather than balls.

- The rods are always longer than the DIAMETER of the mills and therefore lie in the mill PARALLEL to the axis. The impact of the rods is received mainly by he larger particles causing preferential reduction on the coarsest particles and giving a more CLOSELY SIZED product.

- It is more expensive to operate than ball mills, the use is when LESS fine particles are desired in the product.

Interior View Rod Mill

OPERATING CONDITIONS

- The rate of ROTATION of balls mills should be LESS than the speed at which the charge is held against the inside surface by centrifugal force.

(no size reduction will take place unless, the balls fall upon the materials to be crushed)

- Critical maximum speed may be determined I the same manner as described for TROMMEL Screen.

dDN

65.76

N = revolutions per minutes; D = diameter of the mill (ft); d = diameter of the balls (ft)

- At low speed, the contents are simply tumbled and rolled over, the power required to drive the mill varies directly with the rotation speed

- At higher speed slippage occurs between the content and the lining, and the power requirements increase more slowly with rotation speed.

- Increasing the load (balls and materials) in a ball mill will increase the power requirements until the max value is reached, after which the power requirements decreases with increasing load as the centre of gravity of the load approaches the axis of the rotation.

- For wet grinding, max power req. when the weight fraction of solids in the feed ia about .60 – 0.75

How to increase the LOAD:

Increase of Weight of balls introduced into the mill

Operating material of high density (wet pulp)

Operating at a higher pulp level

(a). Sectional diagram of overflow ball mill. (b). Sectional diagram of ball mill equipped with diaphragm or grate allowing lower pulp level

Relation between Circulating Load and Production of fines in a Ball Mill

Single Roll Crusher

Hammer Mill