Size reduction

Size reductionParticles of solids are cut or broken into smaller pieces.Done by one or more of the following ways:CompressionImpactAttrition/RubbingCutting

Size reductionThe size of particle after reduction depends on the method used for size reduction.Only cutting is used to give shape and definite particle size.

Necessity of size reductionIncreases the reactivity of solidsEasier handling of solidsProducts may have specifications of size and shapeIntimate mixing may be achieved by small particles

Power required for size reductionThe amount of energy required for size reduction of a given material is given by following empirical laws:Rittingers law Kicks lawBonds law

Rittingers lawWork required in crushing is proportional to the new surface created

Dsa and Dsb are dia of feed and product particle respectivelyApplicable for fine grinding where increase in surface area per unit mass is large

Kicks law Work required for crushing a given mass of material is constant for the same reduction ratioReduction ratio is the ratio of initial particle size to the final particle size

Kicks lawIt holds good for coarse crushing.Case 1: Dsa/Dsb=200mm/20mm=10Case 2: Dsa/Dsb=100mm/10mm=10Case 3: Dsa/Dsb=1mm/0.1mm= 10Energy required will be nearly equal in case 1 and 2 But more in case 3 than in case 1 and 2 although the reduction ratio is same.

Bonds law Work required to form particles of size Dp from a very large size of feed is proportional to the square root of surface to volume ratio of the product.

Work index (Wi) Gross energy requirement in KW hr per tonn of feed needed to reduce a large feed to such a size that 80% of the product is passed through 0.1 mm screen.It is related to the Bonds law constant by following equation:KB=0.3162* Wi

Size-Reduction EquipmentsThe principal types of size-reduction machines are as follows:Crushers (coarse)

Jaw crushers2.Gyratory crushers3.Crushing rolls

Coarse mine material into lumps of 250 to 150 mm.

Size-Reduction Equipments

B. Grinders (intermediate and fine)1.Hammer mills, Impactors2.Attrition mills3.Tumbling millsa. Rod millsb. Ball millsThe principal types of size-reduction machines are as follows:The product from an intermediate grinder might pass a 40-mesh screen and product from fine grinders pass a 200-mesh screen with a 74 m opening.

Size-Reduction EquipmentsThe principal types of size-reduction machines are as follows:C. Ultrafine grinders1.Fluid-energy mills

D. Cutting machines1.Knife cutters, slitters

Feed < 6 mmProduct: 1- 50 micro m2 to 10 mm in length

CrushersCrusher are slow-speed machines for coarse reduction of large quantities of solids. The main types are Jaw crushersGyratory crushersSmooth -roll crushers and Toothed-roll crushers. The first three operate by compression and can break large lumps of very hard materials.Toothed-roll crushers tear the feed apart as well as crush it, they handle softer feeds like coal.

Jaw crusherFeed size: 6 feet in diameterProduct size: inch to 10 inchCapacity: 9 to 900 tons/hourUses: For reduction of hard rocks and coal

Jaw CrushersBased on human jaw, one fixed plate, one moving. One jaw, the fixed, is nearly vertical and does not move. Other jaw, the swinging jaw, reciprocates in a horizontal plane. It makes an angle of 20 to 30 degrees with the fixed jaw. As the material moves down the crushing action increases.

Product size is adjusted by adjusting the gap size. The jaws open and close 250 400 times per minute. Uncrushed rock may passes through.Blake Jaw Crushers

In Dodge jaw crusher, the movable jaw is pivoted at the bottom and the top end executes the reciprocating motion.Since the width of discharge opening is practically constant, a more uniform and closely sized product can be obtained.

The constant opening at the discharge end gives the crusher an annoying tendency to clog.Jaw Crushers

Gyratory crusherPrimarySecondary Cone crusher

Primary Gyratory crusherPrinciple: CompressionFeed size: 150-1900 mm Product size: 25.4 mm to 254 mmCapacity: 3000 tons/hourUses: For crushing of hard rocks

Gyratory CrusherPrimary Gyratory crusher

Gyratory crusher2. Secondary These have wider cone anglesAre smaller in sizeGives more finer product3. Cone crusherThese have still wider cone anglesGives still more finer productFeed opening is more restricted

Crushing rollsSmooth roll crusherToothed roll crusher

Smooth roll crusherPrinciple: CompressionFeed size: inch to 3 inch Product size: Upto 20 meshCapacity: 3 to 120 tons/hourUses: For secondary reduction of hard rocks and oresCrushing of oil seeds

Smooth Crushing RollsTwo or more heavy steel cylinders revolve towards each other.The distance between the surfaces of the rollers is termed as nipSize reduction ratios are normally below fiveCapacity is affected by speed, nip, diameter and length of the rollersRoll speeds range from 50 to 300 r/minSmooth roll crusher

Smooth roll crusher

Cos = (r+d)/(r+R)R=Radius of feed particler= Radius of rolld=Radius of largest possible particle in the product A rBC R

Toothed Crushing RollsPrinciple: Compression, shearing Feed size: 20 inch Product size: 50 to 200 mmCapacity: 500 tons/hourUses: For coal crushing

Toothed-roll crushers may contain one or two rolls working against a stationary curved breaker plate.Here, the rolls are provided with corrugation or teeth.They are more versatile than smooth-roll crushers, within the limitation that they cannot handle very hard solids.Toothed Crushing RollsSingle-roll toothed crusher

The term grinder describes a variety of size-reduction machines for intermediate and fine duties. The product from a crusher is often fed to a grinder, in which it is reduced to powder.The chief types of commercial grinders are Hammer millAttrition mills and Tumbling mills.Grinders

Hammer millPrinciple: Impact, shearingFeed size: 10 inch Product size: 1 inch to 200 mmCapacity: 0.1 to 15 tons/hourUses: For grinding crystalline solids, fibrous materials, sticky materials

A high speed motor carries a number of hammers around its periphery, inside a close fitting case containing a toughened breaker plate.The individual hammers could weight as much as 100-150 kg.Reduction is mainly by impacts as the hammer drives the material against the breaker plate and crushing takes place.Hammer mill

Tumbling MillsBall millsTube millsRod mills

Tumbling MillsA rotating (tumbling) or vibrating chamber is filled with steel balls or rods.Feed material is subjected to impact and shear due to the movement of the balls or rods. Shear predominates at low speed while impact becomes more important at higher speeds

Ball millPrinciple: ImpactFeed size: 50 mm Product size: 200 meshCapacity: 50 tons/hourLength to diameter ratio of mill: 1

Ball MillTumbling Mills

Standard Chrome Plated Steel Grinding MediaTumbling MillsGrinding media

Factors influencing the size of productRate of feedWith high rate of feed, less size reduction is achieved since the material is retained in the mill for shorter time2. Properties of feed materialThe larger the size of the feed the larger will be the size of product.

Factors influencing the size of product3. Weight of ballsIf the mill is charged with heavy balls, the product will be more finer4. Diameter of the ballBalls of small diameter give only fine productBut do not work effectively on the larger particles

Factors influencing the size of product5. Speed of rotationsAt low speeds, the balls simply roll over one another and little crushing is obtained.At very high speeds, the balls are carried along the circumference of the mill and no grinding takes place. The minimum speed at which the balls are carried around in this manner is called the critical speed of the mill

Critical speed of ball mill It is given as:

R= radius of the ball millr= radius of ballBall mills run at 65 to 80 % of critical speed

Tube millLength to diameter ratio=3:1 or 4:1Mill is filled with pebblesThe material remains for a longer time compared to ball mill and hence a finer product is achieved.

Rod millHigh carbon steel rods of 50 mm diameter are usedNot suitable for very tough materialSuitable with sticky materialFeed size not more than 25 mm

Ultrafine grindersFluid energy millPrinciple: Impact and attrition between particlesFeed size: inch to 100mesh Product size: up to 10 mCapacity: 2 tons/hourUses: for grinding limestone, talc, silica gel, coal, graphite, etc

Ultrafine grindersFluid-energy millMany commercial powders must contain particles averaging 1 to 20 micro m in size with substantially all particles passing a standard 325-mesh screen that has opening 44 micro m wide. Mills that reduce solids to such fine particles are called ultrafine grinders.The fluid used is either superheated steam or compressed air (7atm). It produces essentially uncontaminated product, thus, these are used extensively in industries such as paints, cosmetics, drugs etc. It can handle safely heat sensitive and explosive materials.

In some problems the feed must be reduced to particles of fixed dimensions. These requirements are met by devices that cut, chop or tear into a product with the desired characteristics. Rotary knife cuttersThese devices are well adapted to size-reduction problems in the manufacturing of rubber and plastics.Cutting Machines

Applications The Cutter uniformly slices with precision at high capacities and is ideally suited for elongated products. Products effectively cut on the machine include potatoes, bread sticks, and many other applications. Cutting Machines

Size reduction operationIt can be:Open circuit operationClosed circuit operation

Open circuit operationIn many mills the feed is broken into satisfactory but not exact smaller size particles by passing it once through the mill.When no attempt is made to return oversize particles to the machine for further reduction,The mill is said to operating in open circuit.

Open circuit operationIf the particles are needed to be crushed to still smaller size and If for that the whole product is resend to the mill then,This may require excessive amount of power, since much energy is wasted in regrinding the particles that are already fine enough.

Closed Circuit GrindingHere, after grinding once Ground material is passed through a size screening device.The undersize becomes the product and oversize is recycled to the crushing device for re-crushingThis is called as closed circuit operation

CrusherScreensRoll crusherBall millClassifierfeedcoarseintermediatesfinesProduct