Title: Crushers & Grinder.
· Introduction :
Crushers & grinding mills are types of grinding equipment used to transform or reduce a coarse material such as stone, coal, or slag into a smaller, finer material. Grinding equipment can be classified into to two basic types, crushers and grinders.
Outcomes
Jaw Crushers Types
Jaw crushers compress the feed between a stationary and a movable jaw. There are 2 types of Jaw Crushers.
1. Blake Jaw crusher.
2. Dodge jaw crusher.
1.Blake Jaw Crusher Pricipal
The Blake Jaw crusher is it works on the Principal of compression.
Construction of Jaw Crusher
Ø A schematic diagram of the Blake jaw Crusher is showing in fig. It has a fixed jaw and a movable jaw, which is pivoted at the top. The jaws are not set to form a V open at the top. The swinging jaw which reciprocates in a horizontal plane usually makes an angle of 20 to 30° with the fixed jaw.
Ø The jaws are usually made of manganese steel or some other material that will withstand abrasion. The faces of the crushing jaws are usually corrugated for concentrating the pressure on relatively small areas.
Ø In addition to the jaws, crusher consists of a Pitman, toggles, flywheel, eccentric shafts, drawback rod and spring and frame. In this vertical movement is communicated horizontally to the movable jaw by the toggles.
Ø The speed of operation should not be high or otherwise a large quantity of fines is produced as the material cannot escape quickly and gets repeatedly crushed. As the crushing action is intermittent, the loading on the machine is uneven. Hence, crusher incorporates a heavy flywheel.
Protection of Jaw Crusher
Ø The machine is usually protected so that it is not damaged if accidental pieces of iron such as hammer heads, stray bolts, etc. enter the crusher, by making one of the toggles in the driving mechanism relatively weak. That is, one particular toggle is made into two pieces which are held together with bolts that are purposely made the weakest part in the crusher.
Ø Thus, the failure is made at a point that can be easily and quickly repaired, instead of breaking some vital part of themachine.
Jaw Crusher Working Principle
Ø The material to be crushed is admitted between two jaws from the top. The material caught between the upper part of the jaws is crushed to a smaller size during forward motion by compression. The crushed material then drops/falls into the narrower space bellow during the backward motion and is recrushed as the jaws close next time. After sufficient reduction, the material drops out the bottom of the machine.
Ø The jaws open and close 250 to 400 times per minute.
2.Gyratory Crusher
Construction :
Ø It consists of a funnel shaped casing, open at the top. A conical crushing head, in the from of a truncated cone, gyratory inside a casing. The crushing head is mounted on a heavy shaft pivoted at the top of the machine.
Ø The upper end of the shaft is held in a flexible bearing and the lower end of the shaft is driven by an eccentric so as to describe a circle.
Ø Thus,at any point on the periphery of the casing, therefore, the bottom end of the crushing head moves towards, and then away from the stationary wall. The crushing action takes places around the whole of the cone.
Working
Ø The material to be crushed is changed from the top. The conical head gyrates inside the casing. At any point on the periphery of the casing, the bottom of the crushing head moves towards, and then away, from the stationary wall.
Ø The solids caught in the V-shaped space between the head and the casing are broken and rebroken until they are discharged from the bottom. The speed of the crushing head is usually 125 to 425 gyrations per minute.
Ø Sinceas the some part of the crushing head is working at all times, the discharge from this crusher is continuously instead of intermittent as in a Blake crusher.
Features:
Ø 1.continuous in action 2. fluctuations in stresses are smaller 3. load on the motor is nearly uniform 4. power consumption per ton of material crushed is smaller and 5. requires less maintenance than a jaw crusher.
Ø Since the capital cost of this crusher is high, it is suitable only where large quantities of materials are to be handled.
Crushing Rolls/ Roll Crushers
Smooth Roll Crushers:
Principle:
Sizereduction is achieved by compression (i.e. it employs compressive force for size reduction).
Construction
Ø Smooth Roll crusher consists of two heavy metal rolls of the same diameter placed side by side each other in the horizontal position. The rolls are rotated towards each other at the speed. One of the shafts moves in fixed bearings while others moves in movable bearings.
Ø The clearance between the rolls can be adjusted accordingly to the size of feed and the required size of the product. One of the rolls driven directly, the other by friction with the solids. The rolls have relatively narrow faces and are large in diameter, therefore they can nip moderately large lumps.
Ø The machine is protected by spring loading (i.e. by mounting the bearings of one of the roll shafts against coiled spring) against vibration damage due to tramp and very hard material.
Ø The speed of rolls varies from 50 to 300 rev/min. Crushing rolls are secondary crushers accepting feed 12 to 75 mm in size and yielding products -12 mm to about 20 mesh.
Working:
The material to be crushed is fed from the top. As the rolls rotate, the material gets caught between them and gets reduced in size by compression and discharged from the bottom.
Selection of Crushing Rolls [ Derivation of the angle of nip ]
Ø In selecting the rolls for a certain duty, it is necessary to know the size of the feed and the size of the product.
Ø Consider a system as shown in Fig. Wherein the spherical particles B of a material is just being caught between the rolls. A and A' are centres of two crushing rolls of radius r.
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Ø The line AB passes through the centre of the left-hand roll, through the centre of particle, and through point C where the particle is in contact with the roll. Let the angle between line AB and the horizontal i. e. line joining two centres of rolls be α. The line OD is tangent to the roll at point C. As the line OD is perpendicular to line AB, it makes the same angle α with the vertical. Neglecting gravity, the two forces are acting at a point C, the tangential frictional force T having vertical component T cos α, and the radial force N, having vertical component N sin α. Force T is related to force N through the coefficient of friction μ, so T = μN.
Ø The vertical component of forces T and N are opposed. Force N sin α tends to expel the particle from the rolls and force T cos α tends to draw the particle between the rolls. If the particle is to be drawn between the rolls and crushed,
T cos α ≥ N sin α
T and N are related through
T = μN
Therefore, μNcos α ≥ N sin α
μ ≥ tan α
Ø In other words, the tangent of angle α must be less than the coefficient of friction. The value of μ varies from material to material, but for all practical purposes, the value of the angle α is usually taken about 16°. The angle DOE, Which is twice the angle α is called angle of nip.
Grinders
Grinding means sub-dividing the solids to a finer product than crushing. The size reduction machines employed for an intermediate duty are referred to as grinders. A grinder is often charged with the product from a crusher which it reduces to powder. The commercial grinders described in this section are hammer mills and revolving mills.
1.Hammer Mill :
Principle:
Size reduction is achieved by impact and attrition.
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Construction :
Ø The hammer mill consists essentially of a high speed rotor turning inside a cylindrical casing. The rotor is mounted on a shaft which is usually horizontal. The swing hammers are pinned to a rotor disk. The hammers are rectangular bars of metal with plain or enlarged ends. In this mill, the particles are broken by the sets of swing hammers. The product falls through a grate or screen which forms the lower portion of the casing
Ø Several disks each carrying four to eight swing hammers are often mounted on a single shaft. The rotor disk diameter ranges from 150 mm to 250 mm. As the hammers are hinged, the presence of any hard material does not cause damage to the equipment. The hammers can be readily replaced when they worn out.
Working :
Ø The material to be crushed is fed from the top of the casing. The shaft is rotated at a high speed and centrifugal force causes the hammers to swing out radially. The material is beaten by the hammers around inside of the casing and by impact against the breaker plates (located on inside of the casing) or the screen is crushed until it is small enough to fall through the screen.
Ø Hammer mills are employed to tough fibrous solids like bark or leather, steel turnings, hard rock, sticky clay, etc.
2. Ball Mill :
Principle:
It works on the principle of impact, i.e., size reduction is done by impact as the balls drop from near the top of the shell.
Construction :
Ø A ball mill consists of a hollow cylindrical shell rotating about its axis. The axis of the shell may be either horizontal or at a small angle to the horizontal. It is partially filled with balls. The grinding medium is the balls which may be made of steel, stainless steel or rubber.
Ø
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The inner surface of the cylindrical shell is usually lined with an abrasion-resistant material such as manganese steel or rubber. The length of the mill is approximately equal to its diameter.
Ø The balls occupy about 30 to 50 percent of the volume of the mill. The diameter of ball used is/lies in between 12 mm and 125 mm. The optimum diameter is approximately proportional to the square root of the size of the feed. The shell is rotated at low speed through a drive gear (60-100 rpm) and in a large ball mill, the shell might be 3 m in diameter and 4.25 m in length.
Ø The ball mill may be operated in a batch or continuous fashion, wet or dry. In: continuously operated mill as shown in Fig. 2.7, the outlet is normally covered with coarse screen to prevent the escape of the balls.
Working :
Ø In case of continuously operated ball mill, the material to be ground is fed from the left through a 60° cone and the product is discharged through a 30° cone to the right. As the shell rotates, the balls are lifted up on the rising side of the shell and then they cascade down (or drop down on to the feed), from near the top of the shell. In doing so, the solid particles in between the balls are ground and reduced in size by impact.
Ø The mill contains balls of various ages and sizes since the balls continually wear by attrition and are replaced by new ones. As the shell rotates, the large balls segregate near the feed end and small balls segregate near the product end/discharge. The initial breaking of the feed particles is done by the largest balls dropping from the largest distance and small particles are ground by small balls dropping from a much smaller distance. If the rate of feed is increased, a coarser product will be obtained and if the speed of rotation is increased (less than critical speed), the fineness for a given capacity increases.
Ø During grinding, balls themselves wear and are constantly replaced by new ones So that mill contains balls of various ages and thus of various sizes.
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