Size reduction (Communition)
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The document discusses various methods of size reduction or comminution. It describes different types of mills used for size reduction including hammer mill, ball mill, colloid mill, and fluid energy mill. The objectives of size reduction are to reduce particle size, increase surface area, improve handling and flow of powders. Factors affecting size reduction include hardness, toughness, moisture content and specific mills are described such as how a cutter mill uses cutting blades and a ball mill uses grinding balls to reduce particle size.
Size reduction (Communition)
- 1. COMMINUTION The process of reducing the particle size of a substance to a finer state or powder is known as comminution. It is done with the help of mortar and pestle or mills. Different mills used for the reduction of particle size are hammer mill, ball mill, colloid mill, fliud energy mill, etc.
- 2. Objectives/Advantages of Size Reduction 1. To produce smaller particles. 2. To increase surface area. 3. To achieve proper mixing. 4. To improve handling of powders. 5. To prepare pharmaceutical dispersions. 6. To improve the flow of powder. 7. To improve physical stability. 8. To improve dissolution rate of powders. 9. To improve rate of absorption. 10. To improve the extraction process.
- 3. Principals of size reduction 1. Cutting The material is cut into small pieces by means of shape blade. e.g: cutter mill 2. Compression The material is crushed by the application of pressure. e.g: mortar and pestle, colloid mill 3. Impact The material is stationery and hit by a moving object at high speed or vice versa. e.g: hammer mill 4. Attrition The material is in motion with the surface and the pressure is applied to the material. e.g: roller mill, fluid energy mill
- 5. Factors affecting size reduction 1. Hardness Hardness is the resistance of a material to deformation. It is measured using Moh’s scale (1-10) from graphite to diamond. 2.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. e.g: toughness-rubber & hardness-chalk 3. Abrasiveness It is the property of hard materials especially minerals that causes scraping or wearing of machine. Abrasive substance when reduced in size may contain 0.1% metal from machine.
- 6. 4. Stickiness It is the property of a material to stick to the machine parts during size reduction and block the parts. Drying or addition of inert substance may help in sticky materials. e.g: gum, resins 5. Softening temperature Temperature required to soften or melt materials is important in size reduction. So, temperature should be kept low to avoid softening of materials and reduce their size. e.g: waxy substance can melt at high temperature 6. 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.
- 7. 7. Bulk density Bulk density of powder is the defined as the mass per unit volume. Higher density means less volume occupied and more batch capacity. 8. Physiological effects Some substances are very potent (podophyllum, hormones, griseofulvin) and dust may effect person. So, such substances should be grinded on closed mills. 9. Ratio of feed size to product size The amount of material feed affects the product size. So, size reduction may be done in several stages with different equipments. e.g: coarse grinding followed by fine grinding
- 8. Cutter mill It works on the principle of cutting. Stationery and rotating blades are attached to the mill and screen of desired size is present at the lower part. The material is feed through hopper and size of material is reduced by blades moving at high speed. Coarse powder is obtained through this mill.
- 10. 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.
- 12. Advantages of colloid mill: 1. Products with particle size less than 1 µ can be obtained. 2. Size reduction is carried out in the presence of liquid. 3. Useful for preparing suspensions, emulsions, lotions, ointments and creams.
- 13. 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.
- 16. Advantages of hammer mill: 1. They are simple to install and operation is continuous. 2. They are fast in action and can reduce the size of different types of materials. 3. There is no chances of contamination due to abrasion of metal. 4. The particle size of the product can be controlled by changing speed, hammer or screen.
- 17. Roller mill It works on the principle of attrition. Roller mill consists of two metal rolls with adjustable gap between them. The speed of the rollers and gap between rollers determines the size of particles. The material is feed to hopper and the size of the particles is reduced by the moving rollers.
- 19. 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.
- 22. Advantages of ball mill 1. It produces very fine powders. 2. It can grind different types of materials from wet to dry. 3. It can be used for continuous operation. 4. They are simple to operate and clean. 5. It is used in completely closed form so suitable for toxic materials.
- 23. Fluid energy 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.
- 27. Advantages of fluid energy mill: 1. It can produce very fine particles. 2. It is suitable for heat sensitive products. 3. No contamination due to abrasion. 4. It does not contain any moving parts.