Size reduction of solids Flashcards
-usually carried out in order to increase the surface because, in most reactions involving solid particles, the rate is directly proportional to the area of contact with a second phase
Size Reduction
- applicable mainly to that part of the process where new surface is being created and holds most accurately for fine grinding where the increase in surface per unit mass of material is large
- energy required for size reduction is directly proportional to the increase in surface
Rittinger’s Law
- more closely relates to the energy required to effect elastic deformation before fracture occurs and is more accurate than Rittinger’s Law for coarse crushing where the amount of surface produced is considerably less
- energy required is directly related to the reduction ratio L1/L2
Kick’s Law
*2 DISTINCT METHODS OF FEEDING MATERIAL TO A CRUSHER
- Free Crushing
2. Choke Feeding
-involves feeding the material at a comparatively law rate so that the product can readily escape; its time of residence in the machine is therefore short and the production of appreciable quantities of undersize material is avoided
FREE CRUSHING
- in this case, the machine is kept full of material and discharge of the product is impeded so that the material remains in the crusher for a longer period.
- this method is therefore used only when a comparatively small amount of materials is to be crushed and when it is desired to complete the whole of the size reduction in one operation
CHOKE FEEDING
if the plant is operated, as in “choke feeding”, so that the material is passed only once through the equipment, the process is known as
Open Circuit Grinding
when the product contains material which is insufficiently crushed, it may be necessary to separate the product and return the oversize material for a second crushing. This system is generally to be preferred and is known as
Closed Circuit Grinding
*MOHR SCALE OF HARDNESS
- Tale
- Rock Salt or Gypsum
- Calcite
- Fluorspar
- Apatite
- Felspar
- Quartz
- Topaz
- Carborundum
- Diamond
*NATURE OF THE MATERIAL TO BE CRUSHED / PROPERTIES OF THE FEED
- Hardness
- Structure
- Moisture Content
- Crushing Strength
- Friability
- Stickiness
- Soapiness
affects the power consumption and the wear on the machine
HARDNESS
normal granular materials such as coal, ores and rocks can be effectively crushed employing the normal forces of compression, impact, etc. with fibrous materials it is necessary to effect a tearing action
STRUCTURE
it is found that materials do not flow well if they contain between about 5 and 50 per cent of moisture; under these conditions the material tends to cake together in the form of balls
MOISTURE CONTENT
The power required for crushing is directly proportional to the crushing strength of the material
CRUSHING STRENGTH
material’s tendency to fracture during normal handling
FRIABILITY
a sticky material will tend to clog the grinding equipment and therefore it should be ground in plant that can be readily changed
STICKINESS
a measure of the coefficient of friction of the surface of the material.
SOAPINESS
must be ground wet or in the presence of an inert atmosphere
EXPLOSIVE MATERIALS 💣
*TYPE OF CRUSHING EQUIPMENT
Coarse Crushers
- Blake Jaw Crusher
- Dodge Jaw Crusher
- Gyratory Jaw Crusher
*TYPE OF CRUSHING EQUIPMENT
Intermediate Crushers
- Crushing rolls
- Disc crusher
- Edge runner mill
- Conical crusher
- Stamp battery
- Hammer mill
- Single roll crusher
- Pin mill
- Squirrel cage disintegrator
- End runner mill
*TYPE OF CRUSHING EQUIPMENT
Fine Crushers
- Buhrstone mill
- Roller mill
- Raymond mill
- Griffin mill
- Centrifugal ball mill
- Ring roller mill
- Ball mill
- Tube mill
- Hardinge mill
- Lopulco mill
has a fixed jaw and a moving jaw pivoted at the top. The crushing faces themselves are formed either of manganese steel or of chilled cast iron and must be carefully fitted because they re brittle
Blake 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, but the crusher is less widely used because of its tendency to choke
- this crusher is usually made in smaller sizes than the Blake crusher, because of the high fluctuating stresses that are produced in the members of the machine
Dodge Jaw Crusher
-employs a crushing head, in the form of a truncated cone, mounted on a shaft, the upper end of which is held in a flexible bearing, whilst the lower end is driven eccentrically so as to describe a circle
Gyratory Crusher
a common form of _______________ consists of a large hollow cylinder with perforated walls.
Coal Breaker
- has been used extensively for moderately fine crushing in the past but it has now been very largely superseded by more efficient equipment.
- it consists of a number of heavy stamps-up to 500-1000kg in weight-which are raised mechanically and allowed to fall under gravity on to the material to be crushed
STAMP BATTERY
In this type, a heavy cast iron or granite wheel (muller), is 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
Edge Runner Mill
usually made in small laboratory sizes and consists of a cast iron or porcelain mortar which is rotated so that grinding takes place against a cylindrical pestle mounted with its axis vertical
End Runner Mill
an impact mill employing a high speed rotating disc, to which are fixed a number of hammer bars which are swung outwards by centrifugal force
Hammer Mill
a form of pin mill and consists of two horizontal steel plates with vertical projections on their near faces
Kek Mill
- the upper disc is stationary whilst the lower disc is rotated at high speed.
- the mill gives a fairly uniform fine product with little dust and is extensively used with chemicals, fertilisers and other materials that are non-abrasive and easily broken
Pin-type Mill
similar in action but employs vertical discs with horizontal bars. It is used with friable materials, such as coal and limestone, and with fibrous materials
Squirrel Cage Disintegrator or Bar Mill
consists of a toothed crushing roll which rotates close to a breaker plate. It is used extensively in crushing coal.
Single Roll Crusher
they are similar in construction to the gyratory crusher, though they will not take such a coarse feed and give a very much finer product; they operate at rather higher speeds
Conical Crushers
employs two saucer-shaped discs mounted on horizontal shafts, one of which is rotated and the other is mounted in an eccentric bearing so that the two crushing faces continuously approach and recede
Disc Crusher
- one of the oldest forms of fine crushing equipment though it has, very largely, been superseded now by roller mills
- this equipment has been used in the past for the grinding of grain, pigments for paints, pharmaceuticals, cosmetics and printer’s ink but is now used only where the quantity of materials is very small
Buhrstone Mill
- consists of a pair of rollers that rotate at different speeds (e.g. 3:1 ratio) in opposite directions
- now extensively used in the flour milling industry and for the manufacture of pigments for paints
Roller Mill
-This mill consists of a series of pushers which cause heavy cast iron balls to rotate against a bull ring like a ball race, and the pressure of the balls on the bull ring is produced by pressure applied from above
Babcock Mill
-these machines are being manufactured in large numbers at the present time for the production of industrial minerals such as limestone and gypsum
Lopulco Mill or Ring Roll Pulveriser
slightly less economical in operation than the Lopulco mill, but will give a rather finer and more uniform product
Raymond Mill
similar to the Raymond mill except that it employs only one grinding head and the separation of the product is effected using a screen
Griffin Mill
In its simplest form, it consists of a rotating hollow cylinder, partially filled with balls, with its axis either horizontal or at a small angle to the horizontal
Ball Mill
*FACTORS INFLUENCING THE SIZE OF THE PRODUCT
- Rate of feed
- Properties of the feed material
- Weight of balls
- Diameter of the balls
- Slope of the mill
- Discharge freedom
- Speed of rotation of mill
- Level of material in mill
The minimum speed at which the balls are carried around in this manner
Critical Speed of the Mill
-similar to the ball mill in construction and operation, but the ratio of length to the diameter is usually 3 or 4:1, as compared with 1 or 1.5 for the ball mill
Tube Mill
- High carbon steel rods about 50 mm diameter and extending the whole length of the mill are used in place of balls
- this mill gives a very uniform fine product and power consumption is low, but it is not suitable for very tough materials and the feed should not exceed about 25 mm in size
Rod Mill
- the body of the machine is generally supported on powerful springs and caused to vibrate in a vertical direction; vibration frequencies of between 6 and 60 Hz are common.
- has a very much higher capacity than a conventional mil of the same size and consequently either smaller equipment can be used or a much greater throughput obtained
Vibration Mill
Colloidal suspensions, emulsions, and solid dispersions are produced by means of ________________
Colloid Mills or Suspension Mills
- another form of mill which does not give quite such a fine product.
- the solid is pulverised in jets of high pressure superheated steam or compressed air, supplied at pressures up to 3.5 MN/m^2
Jet Pulveriser
In this pulveriser, reduction is by bombardment of the particles against each other and classification is effected by arranging for the fluid to leave the circulating gas stream through vanes which are situated just downstream of the top of the loop and on the inner face of the loop, i.e. against the centrifugal force
Wheeler Fluid Energy Mill
