Size reduction operations in Chemical Engineering

Unit operation -Size reduction in Chemical Engineering

1. Size reduction

2. Size Reduction Size reduction operation is a mechanical process that breaks down large solid materials into smaller particles.  Used in many industries, including chemical engineering, pharmaceuticals and food processing. To make particles more usable To improve separations To increase reactivity – (Synthetic chemical are ground into powder To make materials easier to handle – Chunks of ore to workable size To produce a finer, more marketable product To expose valuable minerals so they can be extracted from ore To increase the surface area available for processing.

3. Methods for Size Reduction

4. Size Reduction Methods - Application Compression  Used to reduce the size of large particles Used for coarse reduction of hard solids Impact Used to break materials into coarse, medium, or fine products  Used in pulverization where an object with strong force hits suddenly and repeatedly to break the material  Attrition  Used to produce very fine products from soft, nonabrasive materials Also known as rubbing Cutting Used to give a definite particle size or a definite shape  Used in cutter mills to reduce the size of soft, medium-hard, fibrous and tough materials.

5. Characteristics of Comminuted product Objective: Small particles from larger one Small particles are desired either because of their large surface Or because of their shape, size & number Non uniform product – mixture of different sizes ranging from max. particle size (coarse) to min. particle size Ratio of diameters of largest and smallest particle is of the order of 10 4 Comminuted products are smoothed by abrasion and their size is specified

6. Crushing laws and work Index Laws of Size Reduction: To express energy required for size reduction of a solid material in terms of the sizes of feed and products. Energy required depends on: Amount of material Composition and structure Nature of equipment Size of particle For same reduction ratio, the energy required for reduction of unit mass of solid material in a given machine is higher for smaller particles.

7. Crushing laws and work Index Rittinger Law (1867): “Work required to crush a given amount of material is proportional to new surface created / increase in specific surface” Hypothesis Energy requirement is usually times that required for creation of new surfaces. Kick’s Law (1885) “ Work required for crushing a given amount of material is constant for same reduction ratio” Unrealistic - Same energy to convert 10 μm to 1 μm and 1 m to 10 cm!! Bond’s Law (1952) “ Work required to form particle of size Dp from very large feed is proportional to square root of the surface to volume ratio of the product”

8. Work Index Gross energy required in kWh per ton of feed needed to reduce very large feed to such a size that 80% of the product passes a 100 μm screen If 80% of feed passes from mesh size Dpa and 80% of product of mesh size Dpb P = ( Wi * (10/√ Dpb - 10/√DPaF80)) * Q  P:  Power required (kW)  Wi :  Work index (kWh/ tonne )  Dpb :  Particle size at which 80% of the product is finer (microns)  DPa :  Particle size at which 80% of the feed is finer (microns)  Q:  Mass flow rate of the material ( tonnes /hour) 

9. Equipment

10. Crushers These are low speed machines. Main types are: Jaw Crushers – Primary Type Gyratory crushers - Primary Type Smooth-roll crushers – Secondary Type Toothed –roll crushers - Secondary Type Used mainly in Mining, Cement manufacture etc.

11. Jaw Crusher Feed is admitted between two jaws, set to form a V shape at top. One jaw is stationary, other driven by an eccentric, reciprocates in a horizontal plane and crushed lumps caught between the jaws.

12. Gyratory Crusher Consists of a conical crushing head gyrates inside a funnel-shaped casing, open at the top. An eccentric drives the shaft carrying the crushing head. Solids caught between the head and casing are broken and re-broken until they pass out the bottom.

13. Smooth Roll Crusher Limited by the particle size that can be nipped by rolls to feeds that range between 12 to 75mm

14. Toothed Roll Crushers They are not limited by the problem of nip inherent with smooth rolls and operated by compression, impact and shear. They handle soft material like coal.

15. Grinders For Intermediate duty. Product from a crusher is often fed to a grinder for further reduction. Main types are: Hammer mill Impactor Rolling Compression Machines Attrition mills Tumbling Mills

16. Hammer mill Hammers are pinned to a rotor disk, and this disk is mounted on a single horizontal shaft. The shaft is rotated at high speed in continuous motion between 6000 to 15000 rpm. The material is fed from the top of the casing vertically through the hopper and is passed into the milling chamber while the hammers are in continuous motion. The rotating hammers beat the material to yield smaller particles. These particles then pass through the screen holes. Since the particles exit the holes tangentially, the size of the particles is considerably smaller than the holes (Refer to the figure below). A Hammer mill operating under lower speed generates larger-sized particles than the mill operating at high speed. Similarly, the screen thickness influences the size of the particles exiting the screen apertures. Under constant milling speed, the size of particles exiting the thicker screen is smaller when compared to the ones from the thin screen.

17. Impactor Resembles a heavy duty hammer mill, except that it contains no grate or screen. Particles are broken by impact alone. Rotor may be run in either direction to prolong the life of hammers.

18. Attrition mill For soft solids Corrugated or flat faces of rotating circular disk Horizontal axis , sometimes vertical Single runner mill – one disk is stationary and one rotates Double runner mill– both disks are driven – high speeds Product – very fine / powders Air is drawn – to get product cooling is essential Speed – 350 to 700 rpm ( single )to 7000 rpm (Double) Capacity 0.5 to 8 ton/hr , - < 200 mesh 8 – 80 kWh per ton

19. Tumbling Mill Cylindrical shell Slowly turning about its horizontal axis Half filled by solid grinding medium Grinding medium metal rods in Rod mill. Balls of metal, rubber or wood in Ball mill Pebble, porcelain or zircon spheres in pebble mill Operation - Continuous or Batch Impact + attrition Product – fines

20. Tumbling Mill Amount of Energy supplied depends upon Mass of grinding media speed of rotation Max. distance of fall Best predicted by computer simulation based on experimentation Rod mill – 5 to 200 ton/h of 10-mesh product – 4 kWh/t Ball mill – 1 to 50 ton/h of 70 to 90% of 200-mesh – 16 kWh/t

21. Cutting Machines Feed – sticky , hard and elastic size reduction with fixed dimensions cut, chop or tear the feed Rotary knife cutters or Granulator Rubber and plastics

22. Equipment Operation Size reduction equipment performs satisfactorily: When Feed is of suitable size entering at uniform rate and Product is removed as soon as possible as the particles are of desired size Unbreakable material is kept out of machine Heat generated is removed Open-circuit – Material is passed only once through machine Closed-circuit Oversize material is recycled for second cycle of crushing

23. Challenges Size reduction is energy intensive, with more energy required for finer particles  Most inefficient of all unit operations 99% energy goes to operating equipment, heat, sound and vibration It can be difficult to achieve uniformity in the size and shape of the final product 

24. Heat Removal Temperature rise during size reduction occurs, which may melt or decompose solids. Cooling methods are: Cooling water Refrigerated brine Air is blown Solid carbon dioxide Liquid nitrogen Tough materials become brittle to be broken easily

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