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- 1 DoChE, GEC TCR Breaking large particles to smaller ones by the utilization of energy Compression, impact, attrition(rubbing), shear, cutting/tearing Impact Attrition Shear Compression
- DoChE, GEC TCR 2 ENERGY SPEND DURING SIZE REDUCTION: o Elastic deformation before fracture occurs o inelastic deformation of material leading to breakage o Causing elastic distortion of the equipment o Heat generation, noise and vibration o Friction between material particles and between particle and equipment surface o Friction between equipment parts in movement
- DoChE, GEC TCR 3 Crushing efficiency Crushing efficiency is defined as the ratio of the surface energy created by crushing to the energy absorbed by the solid where, ηc = crushing efficiency Wn = energy absorbed by the material, J/Kg es = surface energy per unit area
- DoChE, GEC TCR 4 Ab = area of the product, m2 Aa = area of feed, m2 Crushing efficiencies are low as surface energy created is small ; ranges from 0.06 to 1 % Mechanical efficiency, ηm Ratio of energy absorbed to the energy input ηm = Wn /W W= energy input, J/Kg W = Wn /ηm = es(Ab-Aa)/ ηmηc
- DoChE, GEC TCR 5 OPERATIONS: Crushing vs. Grinding Crushing impact or compression only Coarse product with high irregularity from large lumps Dry feed Max. Reduction ratio –> 6-8 Heavy duty slow speed machines Low energy consumption Grinding compression and attrition Crushed feed to Fine product Dry or wet As high as 100 Light duty, high speed machines High energy consumption Equipments: Jaw & gyratory crushers, crushing rolls etc. Equipments: Hammer mill, attrition mill, tumbling mill (ball, rod, tube)
- DoChE, GEC TCR 6 Ultra fine grinders • Fluid- energy mill • Agitated mill Cutting machines • Knife cutter • Dicer • slitter Feed size < 6mm • Product : 1- 50µm Product with definite size and shape • 2-10 mm in length
- DoChE, GEC TCR 7 FACTORS INFLUENCING COMMINUTION Nature of material • Hardness • Toughness (impact resistance) • Crystallinity and Cleavage • structure (fibrous, flake, granular) Moisture content • Dry grinding m.c.< 3% • Wet grinding m.c.> 50% • For 3% - 50% m.c. , grinding is difficult – material tends to form sticky or pasty mass •
- DoChE, GEC TCR 8 Crushing strength. The power required for crushing is almost directly proportional to the crushing strength of the material. Friability. The friability of the material is its tendency to fracture during normal handling. In general, a crystalline material will break along well- defined planes and the power required for crushing will increase as the particle size is reduced. Stickiness. A sticky material will tend to clog the grinding equipment and it should therefore be ground in a plant that can be cleaned easily. Soapiness. In general, this is a measure of the coefficient of friction of the surface of the material. If the coefficient of friction is low, the crushing may be more difficult. Explosive materials must be ground wet or in the presence of an inert atmosphere. Materials yielding dusts that are harmful to the health must be ground under conditions where the dust is not allowed to escape.
- DoChE, GEC TCR 9 Dry grinding Vs Wet grinding Grinding may be carried out either wet or dry, although wet grinding is generally applicable only with low speed mills. The advantages of wet grinding are: (a) The power consumption is reduced by about 20–30 per cent. (b) The capacity of the plant is increased. (c) The removal of the product is facilitated and the amount of fines is reduced. (d) Dust formation is eliminated. (e) The solids are more easily handled. Against this, the wear on the grinding medium ~ 20 per cent greater, and It may be necessary to dry the product.
- DoChE, GEC TCR 10 OPEN CIRCUIT AND CLOSED CIRCUIT GRINDING Open circuit: material passed only once through the crusher FREE CRUSHING – Product removed continuously as soon as it is formed. • Large capacity. Very less fines CHOKE FEEDING - crusher equipped with feed hopper and kept filled – does not freely discharge the product Feed Intermediate Product Product crusher grinder
- DoChE, GEC TCR 11 Closed circuit grinding: Over size from product separated & returned to crusher
- DoChE, GEC TCR 12 ENERGY AND POWER REQUIREMENTS
- DoChE, GEC TCR 13 n D c dD dE ENERGY AND POWER REQUIREMENTS E - Energy required per unit mass D – Particle size c, n – parameters For n=1; sb sa K D D K m P E ln . Kick’s law ‘Work required in crushing is constant for a given reduction ratio irrespective of original size’ - Applicable to coarse crushing --- (1) --- (2)
- DoChE, GEC TCR 14 ‘Work required in crushing α new surface created’ -applicable to fine grinding -particle diameter is volume-surface dia. For n=2 ; sa sb R D D K m P E 1 1 . Rittinger’s law --- (3)
- DoChE, GEC TCR 15 sa sb B D D K m P E 1 1 . For n=1.5 --- (5) --- (4) If Dsa >>> large size, sb B D K m P E . BOND’S LAW States that work required to form particle of size Dsb is proportional to the square root of the surface to volume ratio of the product, sp/vp sb s p p D v s 6
- DoChE, GEC TCR 16 q D E E sb i 1 1 100 Ei – work index amount of energy required to reduce unit mass of material from an infinite size to a size of 100 µm. Size of material = size of mesh through which 80% of particles pass through To use eqn (4) work index is required, defined by; i i B E E K 3162 . 0 10 100 3
- DoChE, GEC TCR 17
- DoChE, GEC TCR 18 Solution: a) Rittinger’s law This is given by: Thus: 13 = KR(1/10 - 1/50) and: KR = 13 x 504/4 = 162.5 kW/(kg.mm) sa sb R D D K m P E 1 1 .
- DoChE, GEC TCR 19 a) Kick’s law This is given by: Thus: 13 = KK ln(50/10) and: KK = 13 /1.609 = 8.08 kW/(kg.mm) sb sa K D D K m P E ln .
- DoChE, GEC TCR 20 What is the power required to crush 100 TPH of limestone if 80% of the feed passes a 2-in. screen and 80% of the product a 1/8 – in. screen? Work index for limestone = 12.74 kWhr/ton Solution: Dpa= 2 x 25.4 =50.8 mm Dpb = 1/8 x 25.4 = 3.175 mm Power, P = 100 ton/hr x 0.3162 x 12.74 kWhr (1/√3.175 – 1/√50.8) = 169.6 kW
- DoChE, GEC TCR 21 Jaw crushers Works on impact Two jaws 1. Fixed 2. movable/swing Angle between jaws : 20o – 30o Impact speed : 250 – 400 times/minute feed size : upto 1.8 m Product size: 250 mm capacity = 1200 ton/hr Applications : Mining, metallurgical industries, Road, railways etc. 2 types : Blake & Dodge Equipments:
- DoChE, GEC TCR 22
- DoChE, GEC TCR 23 Single toggled Blake jaw crusher
- DoChE, GEC TCR 24 Toggles
- DoChE, GEC TCR 25 Tie rod
- DoChE, GEC TCR 26 Blake jaw crusher Movable jaw is pivoted at the top Max. movement at bottom No tendency to choke/clog Suitable for high production rate Large reduction ratio not possible Low maintenance Large sized equipment Does not give uniform product Dodge Jaw crusher Movable jaw is pivoted at the bottom Max. movement at top tendency to choke/clog Low production rate Large reduction ratio possible High maintenance Comparatively small size Gives uniform product
- DoChE, GEC TCR 27 • Jaw crusher with circular jaw. • continuous • less power required • less maintenance GYRATORY CRUSHER
- DoChE, GEC TCR 28 Crushing Rolls Reduction by Compression Heavy cylindrical rolls rotating at low speeds in opposite directions Speed: 50 – 300 rpm Feed size: 12 – 75 mm Product size: 12 – 1 mm Reduction ratio < 5
- DoChE, GEC TCR 29 Angle of nip, α
- DoChE, GEC TCR 30 T cosα N sinα T N For a particle to be crushed by roll, vertical component of radial force (N) by roll should be less than vertical component of tangential frictional force (T) ie., N sinα ≤ T cosα Or, T/N ≥ tan α >> µ ≥ tan α since, T/N = µ , coefft. of friction For a typical roll α ≈ 16o
- DoChE, GEC TCR 31 Limiting size of particle that can be nipped Depends on coefficient of friction Esimated using Dp, max = 0.04 r1 + b r1 = roll radius b= half the width of the gap between rolls Max. size of product ≈ 2b Theoritical capacity, Qth (kg/h) = 60πD1D3 L N ρ D1 = diameter of roll = 2r1 D3 = Distance between rolls = 2b L = length of roll face,m N =speed in rpm ρ = Density of particles
- DoChE, GEC TCR 32 TUMBLING MILLS Ball Mill, conical ball mill (uses balls of diff. dia.) , tube mill (long shell), rod mill (uses short rods instead of balls), Pebble mill R = Radius of shell r = Radius of ball mu2/(R-r) mg mg cosθ θ
- DoChE, GEC TCR 33 Critical speed (speed at which centrifuging occurs): Nc mu2/ (R-r) = mg cosθ --- (1) u = [(R-r)g cosθ ]½ Also, u = (R-r)ω = (R-r) 2πN (R-r) 2πN=[(R-r)g cosθ ]½ 2 / 1 2 / 1 ) ( cos 2 1 ) ( 2 ] cos ) [( r R g r R g r R N 1/2 c r) (R g 2π 1 N At critical speed, θ = 0, cosθ = 1, N= Nc Operating speed 65-80% of Nc
- DoChE, GEC TCR 34 Ultrafine grinders FLUID ENERGY MILL Dry: High speed hammer mill, Fluid-energy / Jet mill Wet: Agitated mill Colloid mill
- DoChE, GEC TCR 35 Hammer mill with classifier Mikro-Atomizer Colloid mill

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