Post on 29-Sep-2015
description
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