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 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 obtained Although 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.
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 cone The 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.
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 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.
Pin-type Mill Two vertical steel plates Horizontal 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 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.
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 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.
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.
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 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 used large 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 Speed Low speeds of rotation, the balls simply roll over one another and little crushing At still higher speeds they are thrown greater distances At 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 time It is found that the optimum speed is between one- half and three-quarters of the 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 mill However, 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 pebbles The characteristics of the two mills are similar But 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 low Not suitable for very tough materials and Feed 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 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.
Ball or tube mill operate effectively only below its criticalPlanetary mill 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.
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
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 millhttp://www.sturtevantinc.com/demo/Micronizer.html
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 chamber Fine 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.
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