Size Reduction(Unit Objective: Student able to understandMechanism of Size Reduction)Mr. S.C. AtramVidyabharati College of Pharmacy,Amravati
Size Reduction Size refers to physical dimension of an object, Reduction refers to decrement or the process ofdecreasing the size Size reduction: An operation carried out for reducingthe size of bigger particles into smaller one of desiredsize and shape with the help of external forces. Size reduction is the process of reducing large solidunits or substance into smaller unit mass, coarseparticles or fine particle. Comminution/Diminution/Pulverisation is anotherterm used for size reduction.
Objectives of Size Reduction:In the materials processing industry, size reduction orcomminution is usually carried out to, Increase the surface area because, in mostreactions involving solid particles, the rate ofreactions is directly proportional to the area ofcontact with the second phase. Break a material into very small particles in orderto separate the valuable amongst the twoconstituents. Achieve intimate mixing. To dispose solid wastes easily. To improve the handling characteristics. To mix solid particle more intimately.
Advantages/ Applications of Size Reduction: Content Uniformity Uniform Flow Effective Extraction of Drugs Effective Drying Improved Physical Stability Improved Dissolution Rate Improved Rate of AbsorptionDisadvantages of Size Reduction: Drug Degradation Contamination
Factors Affecting on Size Reduction Hardness Toughness Abrasiveness Stickiness Softening temperature Material structure Moisture content Physiological effect Purity required Ratio of feed size to product ratio Bulk density
Principles of Size Reductiono Cutting: Material cut by sharp blades. E.g. cutter millo Compression : Material crushed b/w roller bypressure. E.g. roller millo Impact: Material breaks by hammer or bars at highspeed. E.g. hammer mill,Moving particles strike against a stationarysurface. E.g. fluid energy millo Attrition: Break down of material by rubbing actionb/w two surfaces. e.g. fluid energy mill
Classification of Size Reduction Equipments
Classification of Size Reduction EquipmentsSr. No. Name of theMillMechanism ofactionUses1Hammer millsImpactUsed for almostall the drugs2Roller millsAttrition andCompressionSoft materials3Colloid millsImpact andAttritionUsed for almostall the drugs,Brittle drugs4Ball millsImpact andAttritionUsed for almostall the drugs,Brittle drugs5Fluid energymillsImpact andAttritionModerately hardand friablematerials
Theories of Size Reduction/ MillingA number of theories have been proposed to establish arelationship between energy input and the degree ofsize reduction produced.Rittinger’s theoryBonds’ s theoryKick’s theoryWalker’s theory
Advantages Disadvantages It is rapid in action, and is Heat buildup duringcapable of grinding differenttypes of materials. They are easy to install andoperate , the operation iscontinuous. There is littlecontamination of theproduct with metalabraded from the mill as nosurface move against eachother. The particle size of thematerial to be reduced canbe easily controlled bychanging the speed of themotor, hammer, type, shapeand size of the screen.milling is more, thereforeproduct degradation ispositive. Hammer mills cannot beemployed to mill sticky,fibrous and hardmaterials. The screens may getclogged. Wearing of themill and screen is movewith abrasive materials.
Advantages It can produce very finepowder. Ball mill is used for bothwet and dry grindingprocesses. Toxic substances can beground, as the cylinder isclosed system. Rods or bars can also beused as grinding media. In ball mill installation ,operation and labour costsare low. Disadvantages The ball mill is a verynoisy machine. Ball mill is a slowprocess. Soft, tacky, fibrousmaterial cannot be milledby ball mill.
Roller MillPrincipal: Material is crushed by theapplication of stress. Stress is applied byrotating heavy wheels, mullers or rollers.Uses: For crushing seeds before extractionof fixed oil. Also used to crush soft tissue to helppenetration of solvents.