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Size reduction equipmentsClassificationJaw crushersCompress feed between stationary & movable jawBlake jaw crusher movable jaw is pivoted at top, maximum movement at the bottom, no tendency to choke, does not give uniform productDodge jaw crusher movable jaw is pivoted at bottom, maximum movement at the top, tendency to choke, gives uniform productBlake jaw crusher Fixed jaw called anvil jaw nearly vertical Swinging jaw reciprocates in a horizontal plane

Jaw crushersMakes an angle of 20-30 degree with anvil jawJaw may carry groovesLarge lumps are broken down between upper part of the jaw and moves downwards where it again gets crushedJaws open and close 250-400 times per minute

Gyratory crusherPrinciple compressionFunnel shaped casing, open at the topA conical crushing head in the form of a truncated cone gyrates inside the casingCrushing head mounted on a shaft pivoted at top on a flexible bearingLower end of the shaft driven by an eccentric so as to describe a circle

125 to 425 gyrations per minute

Smooth roll crusherSecondary crusher feed12 to 75mm prod. 12 to 1mmTwo heavy smooth-faced metal rolls turning on parallel horizontal axes are the working elements of the smooth-roll crusherPrinciple compression between the rollsRolls turns towards each other at the same speedTypical rolls are 600mm in dia with 300 mm face to 2000mm in dia with 914mm faceRoll speed range from 50 to 300 r/min

Limiting size for nippingConsider a spherical particle of diameter D2 to be crushedIt is positioned between a pair of crushing rolls of diameter D1 Let D3 be the minimum spacing between the rolls which is also the maximum dimension of the crushed particlesAN be the angle between the two common tangents to the particles and each of the rolls

Let FT and FN be the tangential and normal forces acting on the particle respectively at a point of contact with the rollsIf the resultant force is directed downwards, then the particle will be nipped and crushedIn other words vertical component of tangential force, FTv should be greater than vertical component of normal force FNv

Where is the coefficient of friction given by tangential force/ normal forceUnder the limiting conditionThe angle AN is called the angle of Nip

For all practical purposes AN is taken as 32

Toothed roll crusher

Principle - compression, impact and attritionNot limited by problem of nip, can handle large feed sizeDisintegrators two corrugated rolls turning at diff. speeds

Grinders hammer mill Grinding means subdividing the solids to a finer product than crushingPrinciple impact and attritionSwing hammers are pinned to a rotor diskHammers may be straight bars of metal with plain or enlarged endsProduct fall through a grate or screen which forms the lower portion of the casingSeveral rotor disks each carrying 4 to 8 swing hammers are often mounted on a single shaftThe rotor disk diameter ranges from 150mm to 250 mm

Intermediate hammer mill yield a product 25 mm to 20 mesh sizeFine hammer mill product size less than 200 mesh, limited to soft materialsIMPACTORAn impactor is a heavy duty hammer mill with no screen or grateParticles are broken by impact aloneImpactor is a primary size reduction equipment

IMPACTORTumbling millsA cylindrical shell slowly rotating on a horizontal axis and charged with a grinding medium to about half its volume forms a revolving millThe shell is made of steel and lined with abrasion resistant materials such as manganese steelGrinding medium is usually made of metalBall, tube and rod mill are the various type of revolving millsBall mill length approx. Equals diameter, grinding medium usually steel ballsConical Ball Mill

The component of gravitational force opposing the centrifugal force is mg cosWhen these forces balances each other ball falls down

Disk Attrition Mill

Particles of soft solids are rubbed between the flat faces of rotating circular disksAxis of the disk is usually horizontal, sometimes verticalIn a single runner mill, one disk is stationary and one rotatesIn a double runner mill, both disk are driven at high speeds in opposite directionFeed enters through an opening in the hub of one of the disksPasses outward through the narrow gap between the disks and discharges from the periphery into a stationary casingMetal/buhrstone disk are usedAir is drawn through the mill to prevent chokingDisks are colledDisk of double runner mill turn faster and produces fine products compared to single runner millUltrafine grindersMicro Atomizer / Classifying Hammer millPrinciple impact and attritionSet of swing hammers is held between two rotor disksRotor shaft also carries two fans, which draw air through the mill and discharges into duct leading to collectors for the productShort radial vanes on the rotor disk separate oversize particlesAcceptable particles are carried away by air stream while coarse particles are left in the grinding zoneProduct size 1-20m

Fluid Energy MillsParticles are suspended in a high velocity gas streamJets are placed which oppose one anotherReduction occurs when particle rub against the walls of the confining chamber and due to inter particle attritionInternal classification keeps the larger particle in the mill until they are reduced to desired sizeSuspending gas is usually air / superheated steam admitted at a pressure of 7 atm through energizing nozzlesFeed enters the bottom of the loop through a venturi injectorClassification of the ground particles takes place at the upper bend of the loop

Oval loop of pipe 25 200 mm in dia and 1.2 to 2.4 m highAt the top bend coarse particles are thrown outward against the wall while fines keep closer to the inner wallA discharge opening in the inner wall at this point leads to a cyclone seperator and bag collector for the productCan accept particles as large as 12 mm, but effective with particles no larger than 100 mesh

Agitated MillsVertical vessel 4-1200 l in capacity filled with liquid mediaGrinding medium such as ball, pellet or sand grains are suspended in the mediaIn some designs the charge is agitated with a multiarmed impellerOr a reciprocating central column vibrates the vessel at 20 HzConcentrated feed slurry admitted at top and product is withdrawn through a screen at the bottomProduct size 1m or less

Colloid millsIntense fluid shear in a high velocity stream is used to disperse particles or liquid droplets to form a stable suspension or emulsionOften there is little actual size reduction in the millThe principal action is disruption of lightly bonded clusters or aglomeratesSyrups, ointments, paints and greases are typical products processed in this wayIn most colloid mills feed liquid is pumped between closely spaced surfaces one of which is moving relative to the other at speed of 50m/s or moreCapacity is low

Cutting Machines Knife CuttersA rotary Knife cutter consists of a horizontal rotor turning at 200 to 900 rpm in a cylindrical chamber2 12 flying knives on the rotorFlying knives pass over 1 to 7 stationary bed knivesApplication - recycling paper and plastic materialsSize reduction in rubber and plastics

Equipment operationFeed should be of suitable size and enters at a uniform rateThe product should be removed as soon as possible after the particles are of desired sizeUnbreakable material is kept out of the machineIn the reduction of low melting or heat sensitive products, the heat generated in the mill is to be removed

Open circuit and Closed Circuit operationOpen circuit 1 pass through the mill, oversize particles are not returned to the millEnergy is wasted in regrinding particles that are fine enoughIn closed circuit the mill is connected to a separator, where the oversize and undersize are separated, and oversize sent back to the millThe separation device can be inside the mill as in some ultrafine grindersFor coarse particle the separation device is a screen/grizzlyFor fine particle it can be some form of classifiers

Mill DischargeChoke feeding crushed particles stay at the grinding zone and are regrindedFree feeding crushed particles are removed fastEnergy consumptionMost inefficient of all unit operationsCreating fine particles requires more energyA ballmill consumes more energy than a crusher