Size reduction f copy
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Size reduction is the process of reducing the particle size of a substance through mechanical means like grinding or milling. It has several objectives like increasing surface area, improving mixability and compressibility. Factors like hardness, toughness, abrasiveness affect size reduction. Common mechanisms are cutting, compression, impact and attrition. Equipment used include colloid mill, hammer mill, ball mill and jet mill which work on different principles to produce fine particles.
Size reduction f copy
- 1. SIZE REDUCTION Phr.Nirmal Raj Marasine (B. Pharm.TU, M. Pharm clinical pharmacy PU) WHSA
- 2. SIZE REDUCTION Definition Size reduction is the process of reducing the particle size of a substance to a finer state of subdivision to smaller pieces, to coarse particles or to powder. Synonyms: Comminution, Grinding. When the particle size of solids is reduced by mechanical means it is called milling . 2
- 3. OBJECTIVES OF SIZE REDUCTION a) Size reduction increases the surface area of drugs which helps in rapid solution formation in the case of chemical substances b) With increase in surface area,the extraction from animal glands such as liver and pancreas from crude vegetables drug is facilitated. c) The therapeutic effectiveness of certain drugs is increased by reducing the particle size d) The mixing of several solid ingredients is easier and more uniform if the ingredients are reduced to same particle size
- 4. e)Particle size is important in tablet manufacturing during compression f) The stability of emulsion is increased by decreasing the size of the oil globules g) The suspended particle size in suspension is important for the stability of suspension h)Appropriate size of particle is important for inhalation product such as aerosol . i)The elegance of creams, ointments and pastes can be improved by reducing the particle size j)The smaller the particle size ,quicker and greater will be rate of absorption.
- 5. FACTORS AFFECTING SIZE REDUCTION a. Hardness Harder the materials; more difficult to reduce the size. b. Toughness Toughness is the ability of a material to absorb energy and resist impact. Soft but tough material is more difficult to reduce size than hard but brittle. Increase toughness, decrease in reduction of sizes For example it is difficult to break rubber than a stick of blackboard chalk
- 6. c. Abrasiveness Abrasiveness is a property of hard materials (particularly those of mineral origin) and may limit the type of machinery that can be used. Increase abrasiveness, Increase particle size reduction rate but increase contamination (worn out). d. Softening temperature During size reduction process sometimes heat is generated which may cause some substances to soften, and the temperature at which this occurs can be important. Waxy substances, such as stearic acid, or drugs containing oils or fats are examples that may be affected and choke or block the machine.
- 7. e. Stickiness & Slipperiness pharmaceutical substances that are gummy or resinous may be troublesome to the size reduction process If the method used for size reduction produces heat. f.Moisture content Moisture content of a material can affect its hardness, toughness or stickiness. Generally materials should be dry and contain less than 5% moisture
- 8. g. Physiological effect Some substances are very potent and small amounts of dust have an effect on the operators. To avoid this dust, enclose mills must be used, systems that can extract air are also desirable, and wet grinding also, if possible, as it eliminates the problem entirely. h. Purity required Certain types of size reduction apparatus cause the grinding surfaces to wear, and such methods must be avoided if a high degree of purity of product is needed. Similarly, some machines will be unsuitable if cleaning between batches of different materials is difficult.
- 9. j. Ratio of feed size to product size Machines that produce a fine product require a small feed size. k. Bulk density The capacity of most batch mills depend on volume, whereas processes usually demand solid materials by weight. Hence, all other factors being equal, the output of the machine is related to the bulk density of the substance.
- 10. MECHANISM OF SIZE REDUCTION a. Cutting b. Compression/Shearing c. Impact d. Attrition Cutting Compression Impact Attrition Approx. increase in fineness of product
- 11. 11 Mechanism of size reduction 1.Cutting: During this process the material is cut into small pieces by means of a sharp blade or any other sharp instruments. 2.Compression: During this process the material is crushed by application of pressure with a suitable device. 3.Impact: During impact the material which is more or less stationary, is hit by an object moving at a high speed or the moving particles strike a stationary surface and the material is broken into smaller particles. 4.Attrition: During attrition the pressure is applied to the material as in the case of compression but the surfaces are in motion in relation to each-other, resulting in shear force which breaks the particles into still smaller particles.
- 12. 12 Size reduction equipment 1.Cutting: I. Rotary cutter mill 2.Compression: I. Roller mill II. Colloid mill III. Mortar & Pestle : a. End runner mill , b. Edge runner mill 3.Impact: I. Hammer mill II. Vibration mill 4.Attrition: I. Roller mill 5. Combined Impact & Attrition I. Ball/Tube/Rod/Hardinge mill II. Fluid energy/jet mill III. Cage mill IV. Pin mill
- 13. COLLOID MILL It works on the principle of shearing. Colloid mill consists of a conical rotor and stator. The distance between rotor and stator is adjusted between 0.005 to 0.075 cm. The rotor is connected to a high speed motor which can revolve at a speed of 3000 to 20000 rpm. The material is feed to hopper and size of material is reduced as it passes through moving rotor and stator.
- 16. Uses Used for the preparing colloidal dispersions, suspension, emulsion ointments. Advantages Extremely fine particle distribution through optimal shear force. High capacity with minimal space requirements. Rapid handling and easy cleaning. Virtually unlimited application due to highly flexible homogenization system. Disadvantages Air might be incorporated within the product. Thus the product should be allowed to deaeration. Heat generation occur during milling so that water should be circulated around the milling chamber. Not suitable for the dry milling
- 17. HAMMER MILL It works on the principle of impact. Hammer mill consists of central shaft attached with number of hammers in a steel case. When shaft is rotated hammers swing in radial position. Screen of desired size is fitted on the lower portion. The material is feed to hopper and size of material is reduced by the impact of hammers.
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- 19. Advantage They are simple to install and operate, the operation is continuous. They are rapid in action and many different types of materials can be ground with them. There is no chance of contamination due to abrasion of metal from the mill because no surfaces of the mill move against each other. The particle size of the material to be reduced can be easily controlled by changing the speed of the rotor, hammer type, shape & size of the screen. They are easy to clean and may be operated as a closed system to reduce dust and explosion hazards.
- 20. Disadvantage A hammer mill must be operated with internal or external classification to produce ultra fine particles. Cannot be used for abrasive material. Use In pharmaceutical industry they are used for grinding dry materials, wet filter cakes, ointments and slurries. Can be used for granulation and close control of the particle size of powders.
- 21. BALL MILL This mill works on the principle of impact and attrition. It consists of cylindrical container filled with number of balls made up of steels. The balls acts as grinder. The size of the balls determines the particle size. The cylindrical container is rotated at slow speed. The material is feed to the hopper and the moving balls reduces the size of the material.
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- 25. 25 Use Used for time grinding of hard, abrasive materials. They can be easily sterilized therefore can be used for grinding the materials to be used in parenteral and ophthalmic preparation. Used for batch or continuous operation. Advantage They are economical and simple to operate. They can be used for wet and dry grinding. They afford a large grinding surface within a compact space A wide variety of materials can be ground with it. They are easy to clean as compared to other mills. Disadvantage Not applicable for soft and sticky materials.
- 26. FLUID ENERGY/JET MILL It is also known as jet mill, ultrafine grinding mill or micronizer. It works on principle of impact and attrition. A fluid (usually air) is passed through the nozzle at high pressure which will move the materials at high velocity with turbulence. Coarse particle are feed into the mill and turbulence causes impact and attrition to produce fine particles.
- 29. Advantages The particle size of the product is smaller than that produced by any other method of size reduction. Expansion of gases at the nozzles leads to cooling, counteracting the usual frictional heat which can affect heat-sensitive materials. Since the size reduction is by inter-particulate attrition there is little or no abrasion of the mill and so virtually no contamination of the product. For special cases with very sensitive materials it is possible to use inert gases. Having a classifier as an integral part of the system permits close control of particle size and of particle size distribution.