Crushers and Grinders

Coarse size reduction : feed from 2 to 96 or more

Intermediate size reduction : feed from 1 to 3

Fine size reduction: feed from 0.25 to 0.5

Size Reduction types based on feed size

Reduction ratio = Avg. diameter of feed /Avg. diameter of product

Coarse crushers : 3 to 7

Fine grinders : as high as 100

Free Crushing

Crushed product with fines removed quickly relatively after short sojourn

in crusher

by flow out due to gravity blow out by compressed airwashed with water thrown by centrifugal acton

Important Terms

Methods of Size Reduction

Compression particle-particle, or/and particle equipment interaction Low speed: 0.01 10 m/s Crushing and shear

Impact particle-particle, or/and particle equipment interaction High speed: 10 2000 m/s Impact and shear

Attrition Shear applied by medium

Cutting

Jaw Crusher Blake

Blake crusher

Operate by crushing pressure Cast steel frame- supports fixed and movable jaw Jaws- cast steel lined with tough abrasive resistant

metal- Manganese steel

Movable jaw pivoted at top Operated by eccentric and toggles

Crushing when movable jaw moves towards the fixed jaw Intermittent power requirement

Driven by flat V belts for low capacityLower fixed and maintenance cost

Jaw Crusher Dodge

Only in small sizesMovable jaw pivoted at the bottomWidth of the opening remains practically constant-

uniform products

Gape : Greatest distance between two jaws

Dodge crusher

Jaw crushers frequently used for first coarse breakage operation, followed by gyratories

Gyratory Crusher

Developed for greater capacity crushing action similar to jaw crusher moving crushing element approaches to and recedes from a fixed

crushing plate

Outer frame carrying inverted conical surface concaves

inner gyrating crushing head hangs from topLower end of spindle circular shaft free to rotate in an eccentric sleeve

Continuous power requirement

Suspended spindle gyratory crusher

Taggart guideline: Applicable for GCs with gapes 4 to 2

T (tons/h) = 0.6 LS

L- length of feed opening, inches

in jaw crushers, L is normal to gape

in gyratories, L is the perimeter of a circle whose diameter is the arithmetic avg. of the two cones

S greatest width of the discharge opening

Power requirements of jaw and gyratory crushers are same. But the gyratory load is uniform as it is crushing

continuously, whereas JC intermittently

Choosing between JC and GC

based on capacity criteria

small capacity JC b/c lower original cost and upkeep

higher capacity GC b/c continuous operation

(hourly tonnage / square of gape in inches) < 0.115 then use a jaw crusher otherwise gyratory crusher

Jaw and Gyratory Crusher (GC) Capacity

Rotary Coal Breaker

Crushing action upon transference of kinetic energy from the

hammers to material, repeated dropping of material

High speed M/Cs 600 rpm hammer tip velocity* 40 m/s

Less expensive

Higher throughput than Jaw and gyratory crushers

*Tip Velocity (TV) - speed of the hammer at it's tip or edge furthest away from

the rotor, and is calculated by multiplying the rotational speed of the drive

source (shaft rpm) by the circumference of the hammer tip arc. i.e.,

TV = SD * shaft rpm

Intermediate Crushers

Edge Runner Mill

Hammer Mill

Pin type Mill

Single roll crusher

Crushing rolls

Symons disc crusher

Edge Runner Mill

Crushing shearing between roller and casing, crushing below rollers

Muller Heavy cast iron or granite wheel mounted on horizontal shaft

Heavy pan horizontal plane on which muller rotates, outer rim perforated

Alternatively Pan can rotate, muller being stationary

Feeding- at centre worked outside by the mullers action

Scraper removes material adhered to the sides of pan, returns to crushing zone

Wet or dry operation

Used extensively grinding of paints, clays and sticky materials

Speed : 15-25 rpm

Muller

Hammer Mills

Swing claws

Hammers

Crushing Impact

Design and placement of hammers is determined by rotor speed, motor horsepower, and open area in the screen

Common range of tip speed : 83 120 m/s Advantages:

Operational Flexibility - are able to produce a wide range of particle sizeswork with any friable material and fibre ease of use lower initial investment when compared with a roller mill minimal maintenance needed particles produced using a hammermill will generally be spherical, with a

surface that appears polished.

Limitations: less energy efficient when compared to a roller millmay generate heat (source of energy loss) produce greater particle size variability (less uniform)hammer mills are noisy

Crushing rolls

Crushing compression, friction

Two rolls one on adjustable bearing rotate in opposite directions Clearance b/w rolls adjusted based on feed size rolls few cms to 1.2 m in diameter small size reduction ratio of 4: 1 in a single operationLet

r2 radius of the cylindrical or spherical particle fed r1 radius of the crushing rolls 2D -angle of nip angle between the two common tangents to the particle and each of the rolls ; for steel rolls angle of nip is 32q 2b is the distance between the rollsthen

Actual capacity : 0.1 0.3 of theoretical capacity T Extensively used for crushing oil seeds and in the gunpowder industry Suitable for abrasive materials. Simple in construction No large percentage of nes.

Theoretical cap. T = (60vLDpU/2000)WhereT in Tons/hV- peripheral velocity (fpm). For rolls up to 72 dia, visapproximated as (300+84*roller dia)L width of rolls (ft)Dp- Distance b/w rollers (ft)U- Density of material (lb/Cu.ft

Crushing Impact, friction

Rotating hollow cylinder-partially filled with balls-axis either horizontal or at small angle to horizontal

Inner surface of cylinder lined with abrasive materials-eg. Manganese steel, stoneware, rubber

Rubber lining Coeff. of Friction-high- high impact Feed, Delivery Hollow trunnion Exit covered with coarse screens prevent balls escapeWide range Coal, pigment, Feldspar Feed size: up to 50 mm Grinding balls

Flint or steel 30- 50 % of vol. mill ( ~ 5Mg/m3) 12-125 mm Optimum Diameter D (Dfeed) i.e., = K Dfeed K proportionality constant = fn. (Nature of material) Large balls Effective with feed (large size), Small balls -

fines

Steel balls abrasion - dry grinding: 0.3 kg/Mg of material- Wet grinding : 1-1.5 kg/Mg of material

Hollow trunnion (feed)

Ball Mill

Compound ball mill no. of compartments- vertical perforated plate Compartments with different size balls Economical and uniform product

(Power required)dry grinding ~ 0.7 * (power required)wet grinding Factors influencing size of production

Rate of feed high rate decreases size reduction Properties of feed Large feed larger product Diameter of balls Small balls facilitate production of fine product

Free grinding limit : Limiting size reduction obtained with a given size of balls Slope of mill increase in slope increase in capacity Discharge freedom same effect as slope of mill Speed of rotation

Low speeds balls simply roll over one another - Little crushing Slightly high speed balls are projected short distances across mill Still higher speed thrown greater distances wear increases Very high speed balls carried right round in contact with the sides of mill critical speed

minimum speed at which the balls are carried round along the inner surface

Ball Mill Contd

Ball Mill Contd Advantages

wet or dry although wet grinding facilitates the removal of the product Low costs of installation and power can be used for the grinding of explosive materials using inert atmosphere grinding medium is cheap. suitable for materials of all degrees of hardness used for batch or continuous operation used for open or closed circuit grindingWith open circuit grinding, wide range of particle sizes is obtained in the productWith closed circuit grinding, the use of an external separator can be obviated by continuous removal

of the product by means of a current of air or through a screen

Limitations/Advantages Bulky size Noisy operationWork efficiency is low, the effective energy utilization rate of only about 2%, most of the remaining

energy converted to heat loss

very heavy requires strong foundation Low speed device 15-20 RPM Inner lining consumption

Attrition Mills

Attrition : - rubbing action in opposite llel planes between two particles or particle and surface Can be designed to Shred, Curl, Granulate, Grind, Shear, Twist, Blend, Rub, Fiberize, Pulverize,

Crack, Cut, Fluff, Hull, and Refine

S.No. Crushing Grinding

1 Mechanism : Compression Impact and attrition

2 Large particles to small lumps Crushed feed to powder

3 Mostly operated in open circuit mode

Mostly operated in closed circuit mode

4 Dry screens are used as size separation unit

Classifiers are used as size separation unit

5 Mostly dry operation Both wet and dry operation

6 Reduction Ratio seldom exceeds 6 to 8

Reduction Ratio as high as 100 possible

7 Residence time in crusher is large and throughput is high

Residence time in crusher is small and throughput is relatively low

8 Two types: Primary and secondary crushers

Two types: Fine and ultrafine grinders

9 Heavy duty low speed M/cs Relatively light duty high speed M/cs

10 Energy consumption per unit mass of product is low due to coarse particle production

Energy consumption per unit mass of product is high due to fine particle production

Size Reduction Equip. Feed Size, mm Product size, mm

Blake Jaw crusher 1500 -40 50- 5

Gyratory crusher 1500 -40 50- 5

Crushing rolls 50 5 5-0.1

Hammer mills 50-5 5-0.1

Ball mills 5-2 0.1