Machinery for Crushing and Grinding

Stag Jaw Crusher

The Stag jaw crusher has a fixed jaw and a moving jaw pivoted at the top

Crushing faces are formed of manganese steel. The max movement of the jaw is at the bottom; so there is

little tendency for the machine to clog, though some uncrushed material may fall through and have to be returned to the crusher.

The maximum pressure is exerted on the large material which is introduced at the top.

One of the toggle plates in the driving mechanism is made relatively weak so that, if any large stresses are set up, this is the first part to fail.

Easy renewal of the damaged part is then possible.

Stag Jaw Crusher

Stag Jaw Crusher

Stag Jaw Crusher

The speed of operation should not be so high that a large quantity of fines is produced

The angle of nip, the angle between the jaws, is usually

about 30◦.Because the crushing action is intermittent, the

loading on the machine is uneven and the crusher therefore incorporates a heavy flywheel.

The power requirements of the crusher depend upon size and capacity and vary from 7 to about 70 kW,

the latter figure corresponding to a feed rate of 10 kg/s.

Dodge Jaw Crusher

The moving jaw is pivoted at the bottom.The minimum movement is thus at the bottom and

a more uniform product is obtainedAlthough the crusher is less widely used because of

its tendency to choke. The large opening at the top enables it to take very

large feed and to effect a large size reduction.This crusher is usually made in smaller sizes than

the Stag crusher, because of the high fluctuating stresses that are produced in the members of the machine.

Dodge Jaw Crusher

Dodge Jaw Crusher

Both Crushers employs a compressive force for size reduction

Gyratory Crusher

Gyratory Crusher

Crushing head is in the form of a truncated cone, mounted on a shaft

The crushing action takes place round the whole of the coneThe crusher is continuous in action, thus fluctuations in the

stresses are smaller than in jaw crushers and the power consumption is lower.

It also does not take such a large size of feed as a jaw crusher, although it gives a rather finer and more uniform product.

Because the capital cost is high, the crusher is suitable only where large quantities of material are to be handled.

Employs a compressive force for size reduction

Other Coarse Crushers – Coal Breaker

Consists of a large hollow cylinder with perforated walls.

Feed is introduced at the top. The cylinder is rotated and the coal is lifted by

means of arms attached to the inner surface and then falls against the cylindrical surface.

The coal breaks by impact and passes through the perforations as soon as the size has been sufficiently reduced.

This type of equipment is less expensive and has a higher throughput than the jaw or gyratory crusher.

Rotary Coal Breaker

Intermediate Crushers

Edge Runner Mill

Edge Runner Mill

Edge Runner Mill

It has heavy cast iron or granite wheel, or muller mounted on a horizontal shaft which is rotated in a horizontal plane in a heavy pan.

Alternatively, the muller remains stationary and the pan is rotated

Material is fed to the centre of the pan and is worked outwards by the action of the muller, whilst a scraper continuously removes material that has adhered to the sides of the pan, and returns it to the crushing zone.

Edge Runner Mill

In many models the outer rim of the bottom of the pan is perforated, so that the product may be removed continuously as soon as its size has been sufficiently reduced.

The mill may be operated wet or dry and it is used extensively for the grinding of paints, clays and sticky materials.

Hammer Mill

Hammer Mill

Type of Impact mill with high speed rotating disc, to which are fixed a number of hammers

Hammers are swung outwards by centrifugal force. Material is fed in, either at the top or at the centre,

and it is thrown out centrifugally and crushed by hammer bars, or against breaker plates fixed around the periphery of the cylindrical casing.

The material is beaten until it is small enough to fall through the screen at the lower portion of the casing.

Hinged hammers: the presence of any hard material does not damage to equipment.

Hammer Mill

The bars are replaced when they are worn out.

Suitable for the crushing of both brittle and fibrous materials

For fibrous materials it employs a screen with cutting edges.

Suitable for hard materials Since a large amount of fines is produced,

pressure lubrication is recommended for bearings.

Hammer Mill

The size of the product is regulated by the size of the screen and the speed of rotation.

In some cases the hammer bars are rigidly fixed in position.

Since a large current of air is produced, the dust must be separated in a cyclone separator or a bag filter.

Laboratory Hammer Mill

Pin-type Mill

Pin-type Mill

Two vertical steel platesHorizontal projections at their near faces One disc may be stationary whilst the other

disc is rotated at high speed; or the two may be rotated in opposite directions

The material is gravity fed in through a hopper or air and is thrown outwards by centrifugal action and

broken against of the projections before it is discharged to the outer body of the mill

Pin type Mill

Discharged material falls under gravity The mill gives a fairly uniform fine product

with little dust Used with chemicals, fertilisers and other

materials that are non-abrasive, brittle or crystalline.

Control of the size of the product is effected by means of the speed and the spacing of the projections

and a product size of 20 μm is readily attainable.

Single Roll Crusher

Single Roll Crusher

Consists of a toothed crushing roll Roll rotates close to a breaker plate. The material is crushed by compression and

shearing between the two surfaces. It is used extensively for crushing coal.

Single Roll Crusher

Crushing Rolls

Crushing Rolls

Two rolls, one in adjustable bearings, rotate in opposite directions

The clearance between them can be adjusted according to the size of feed and the required size of product.

Protected, by spring loading, against damage from very hard material.

Both rolls may be driven, or one directly and the other by friction with the solids.

Effect a small size reduction ratio, 4 : 1 in a single operation

commonly a no. of pairs of rolls are employed in series one above the other

Crushing Rolls

Roll shells with either smooth or ridged surfaces are held in place

See Ex. 2.2

Symons Disc Crusher

Symons Disc Crusher

Has 2 saucer-shaped discs mounted on horizontal shafts of which one is rotated

The two crushing faces continuously approach and recede.

Material is fed in the centre between the two discs

The product is discharged by centrifugal action as soon as it is fine enough to escape through the opening between the faces.

Fine Crushers

Buhrstone mill

Buhrstone mill

Grinding takes place between two heavy horizontal wheels, one of which is stationary and the other is driven.

The surface of the stones is carefully dressed so that the material is continuously worked outwards from the centre of the circumference of the wheels.

Size reduction takes place by a shearing action between the edges of the grooves on the two grinding stones.

Used for the grinding of grain, pigments, harmaceuticals, cosmetics and printer’s ink,

Although used where the quantity of material is very small.

Roller Mill

The roller mill consists of a pair of rollers that rotate at different speeds in opposite directions.

one of the rollers is held in a fixed bearing whereas the other has an adjustable spring-loaded bearing

since the rollers rotate at different speeds, size reduction is effected by a combination of compressive and shear forces.

The roller mill is extensively used in the flour milling industry and for the manufacture of pigments for paints.

Roller Mill

Centrifugal Attrition Mills

Babcock MillThe Lopulco mill or ring-roll pulveriser.The NEI pendulum mill.

Szego grinding mill

Szego grinding mill

A planetary ring-roller mill stationary, cylindrical grinding surface with a

no. of grooved rollers rotate. Connected to the central drive shaft; they are

pushed outward by centrifugal force and roll on the grinding surface.

The material is fed by gravity is discharged at the bottom of the mill.

Szego grinding mill

The particles, upon entering the grinding section, are repeatedly

crushed between the rollers and the stationary grinding surface.

Crushing and shearing force caused by rotational motion of the rollers.

Ball Mill

Consists of a rotating hollow cylinder, partially filled with balls

Horizontal or at a small angle to the horizontal.

The outlet is normally covered with a coarse screen to prevent the escape of the balls

Balls fall on the grinding medium from a height

Size reduction mainly by Impact, Compression and Attrition

Ball Mill

Ball Mill

Inner surface of the cylinder is lined with an abrasion-resistant material

such as manganese steel, stoneware or rubber.

Balls occupy a volume b/n 30 and 50 per cent of total volume of the mill

Balls wear out during grinding and are replaced Different sized balls may be usedlarge balls deal effectively with the feed and the

small ones are responsible for giving a fine product.

For very fine grinding in small mills pebbles are often used instead of balls.

Compound Ball Mill

Cylinder is divided into compartments by vertical perforated plates.

Material flows axially along the mill Can pass from one compartment to the next only

when its size has been reduced to less than the perforations in the plate.

Each compartment is supplied with balls of a different size.

large balls are at the entry while the small balls before discharge.

Results in economical operation and the formation of a uniform product.

Wet grinding in Ball Mill

Power consumption is generally about 30 per cent lower than that for dry grinding

Continuous removal of product as it is formed is facilitated

Factors affecting the size of the product

The rate of feed. With high rates of feed, less size reduction is effected

The properties of the feed material. The larger the feed the larger is the product A smaller size reduction is obtained with a hard material.

Weight of balls. A heavy charge of balls produces a fine product. The weight of the charge can be increased, either by

increasing the number of balls, or by using a material of higher density

Factors affecting the size of the product

Diameter of the balls. Small balls facilitate the production of fine material But do not deal so effectively with the larger particles in

the feed. The For an economical operation, the smallest possible balls

should be used.

The slope of the mill. An increase in the slope of the mill increases the capacity But a coarser product is obtained.

Discharge freedom. same effect as increasing the slope.

Factors affecting the size of the product

The speed of rotation of the mill– Critical SpeedLow speeds of rotation, the balls simply roll over one

another and little crushing At still higher speeds they are thrown greater distancesAt very high speeds, the balls are carried right round in

contact with the sides of the mill and little relative movement or grinding takes place again.

The minimum speed at which the balls are carried round in this manner is called the critical speed of the mill

Hence at Critical speed there will be no resultant force acting on the ball when in the uppermost position

Factors affecting the size of the product

At critical speed the centrifugal force will be exactly equal to the weight of the ball.

If the mill is rotating at the critical angular velocity ω

rω2 = g

Nc is the no. of revolutions per unit timeIt is found that the optimum speed is between one-

half and three-quarters of the critical speed.

Ball mill at correct Critical speed

Factors affecting the size of the product

The level of material in the mill. Power consumption is reduced by

maintaining a low level of material can be done by providing suitable discharge

opening for the product. If the level of material is raised, the

cushioning action is increased and power is wasted by the production of an excessive quantity of undersize material.

Advantages of the Ball Mill

The mill may be used wet or dry The costs of installation and power are low.The ball mill may be used with an inert

atmosphere and therefore can be used for the grinding of explosive materials.

The grinding medium is cheap.The mill is suitable for materials of all degrees of

hardness.It may be used for batch or continuous operation.It may be used for open or closed circuit grinding.

Tube Mill

Similar to ball millHowever, length to the diameter is usually 3

or 4 : 1, as compared with 1 or 1.5 : 1 for the ball mill.

The mill is filled with pebblesThe characteristics of the two mills are

similarBut material remains longer in the tube mill

because of its greater length, and a finer product is therefore obtained.

Rod Mill

High carbon steel rods about 50 mm diameter and extending the whole length of the mill are used.

This mill gives a very uniform fine product Power consumption is lowNot suitable for very tough materials andFeed should not exceed about 25 mm in size. It is particularly useful with sticky materials which would

hold the balls together in aggregates, because the greater weight

of the rods causes them to pull apart again. Worn rods must be removed from time to time and

replaced by new ones, which are rather cheaper than balls.

Hardinge Mill

A ball mill in which the balls segregate themselves according to size.

The main portion of the mill is cylindrical as in the ball mill, although the outlet end is conical and tapers towards the discharge point

large balls collect in the cylindrical portion while the smaller balls, in order of decreasing size,

Like a Compound Ball Mill

Hardinge Mill

Hardinge Mill

The mill has an advantage over the compound ball mill in that the large balls are raised to the greatest height and therefore are able to exert the maximum force on the feed.

As the size of the material is reduced, smaller forces are needed to cause fracture and it is therefore unnecessary to raise the smaller balls as high.

The capacity of the Hardinge mill is higher than that of a ball mill of similar size and it gives a finer and more uniform product with a lower consumption of power.

It is difficult to select an optimum speed, however, because of the variation in shell diameter.

Hardinge Ball Mill

Planetary mill

Ball or tube mill operate effectively only below its critical speed

Planetary mill obviate this constraint by rotating the mill simultaneously about its own axis and about an axis of gyration.

In practice, several cylinders are incorporated in the machine, all rotating about the same axis of gyration.

Planetary Ball Mill

Smaller than common ball mills Mainly used in laboratories for grinding sample

material down to very small sizes. Consist of at least one grinding jar which is

arranged eccentrically on a so-called sun wheel. The difference in speeds between the balls and

grinding jars produces an interaction between frictional and impact forces, which releases high dynamic energies.

The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill.

Vibration Mill

By imparting a vibrating motion to a mill either by the rotation of out-of-balance weights or by the use of electro-mechanical devices,

accelerations many times the gravitational acceleration may be imparted to the machine.

The body of the machine is generally supported on powerful springs and caused to vibrate in a vertical direction.

Grinding may take place in two stages, the material falling from an upper to a lower chamber when its size has been reduced below a certain value.

Vibration Mill

Has much higher capacity than a conventional mill of the same size

So either smaller equipment may be used or a much greater throughput is obtained.

Well suited for incorporation in continuous grinding systems.

Vibration Mill

Colloid Mill

Colloidal suspensions, emulsions and solid dispersions are produced by means of colloid mills or dispersion mills.

Droplets or particles of sizes less than 1 μm may be formed

Feed material of approximately 100-mesh or 50 μm in size is used

Rotor and stator of a Colloid Mill

Colloid Mill

Clearances could be from virtually zero to 1.25 mm, although in practice the maximum clearance used is about 0.3 mm

The gap setting between rotor and stator is not necessarily in direct proportion to the droplet size or particle size of the end product.

The thin film of material continually passing between the working surfaces is subjected to a high degree of shear, and consequently the energy absorbed within this film is frequently sufficient to reduce the dispersed phase to a particle size far smaller than the gap setting used.

Colloid Mill

The rotor speed varies with the physical size of the mill and the clearance necessary to achieve the desired result

The required operating conditions and size of mill can only be found by experiment.

In all colloid mills, the power consumption is very high, and the material should therefore be ground as finely as possible before it is fed to the mill.

Fluid energy mill

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Fluid energy mill

Fluid energy mill

solid is pulverised in jets of high pressure superheated steam or compressed air, supplied up to 3.5 MN/m2 (35 bar).

The pulverising takes place in a shallow cylindrical chamber with a number of jets arranged tangentially at equal intervals around the circumference.

The solid is thrown to the outside walls of the chamberFine particles are formed by the shearing action

resulting from the differential velocities within the fluid streams.

The jet pulveriser will give a product with a particle size of 1–10 μm.

Specialised applications

Electrohydraulic crushing an underwater discharge is generated by the release

of energy from a high-voltage capacitor The spark length depends on the nature of the

material to be crushed

Ultrasonic grinding fed between a drive roll and a curved plate, both of

which are ultrasonically activated.

Specialised grinding--Cryogenic grinding

Materials like plastics, rubber, waxes etc. tend to distort rather than to fracture when subjected to compressive forces.

However can be done by subjecting it to very low temperatures.

Material is cooled with liquid nitrogen at a temperature of about −196◦C (77K) to render it brittle before it enters the grinder

cooling causes the crystal lattice to shrink and to give rise to microscopic cracks which act as nuclei and then grow

thereby reducing the energy input required to cause fracturing to occur.

Application in market for frozen foods

Specialised grinding--- Explosive Shattering

energy is transmitted to particles as shock waves

suddenly releasing steam from an explosion chamber containing the solid to be compressed.

still at the development stage.