Particle Size reduction

Size reduction is a process of reducing large solid unit masses - vegetables or chemical substances into small unit masses, coarse particles, or fine particles. Size reduction is commonly employed in pharmaceutical industries. The size reduction process is also referred to as Comminution and Grinding.

1. SIZE REDUCTION Shivanee Vyas Assistant Professor School of Pharmaceutical Technology & Management, NMIMS

2. Content Size reduction –Introduction Objectives and factors affecting size reduction M ethods of size reduction Equipment's used for Size reduction

3. Introduction Size reduction is a process of reducing large solid unit masses - vegetables or chemical substances into small unit masses, coarse particles or fine particles. Size reduction is commonly employed in pharmaceutical industries. Size reduction process is also referred to as Comminution and Grinding.

4. Objectives of Understanding Size Reduction 1. Size reduction leads to increase of surface area. 2. To increase the therapeutic effectiveness of certain drugs by reducing the particle size. 3. To get the uniform powder because particle size reduction helps the uniform mixing of drugs for the preparation of dosage forms. 4. To increase the rate of absorption of a drug. The smaller the particle size, the greater the rate of absorption . 5. Pharmaceutical suspensions require finer particle size. It reduces rate of sedimentation. 6 . The stability of emulsions is increased by decreasing the size of the oil globules. 7 . All the ophthalmic preparations and preparations meant for external application to the skin must be free from gritty particles to avoid irritation of the area to which they are applied. 8. The physical appearance of ointments, pastes and creams can be improved by reducing its particle size

5. Factors affecting size reduction 1 Hardness: The hardness of the material affects the process of size reduction .It is easier to break soft material to a small size than hard material. 2 Toughness: The crude drugs of fibrous nature or those having higher moisture content ,are generally tough in nature . A soft but tough material may present more problem in size reduction , than a hard but brittle substance. 3 Stickiness: Stickiness causes a lot of difficulty in size reduction. This is due to the fact that material adheres to the grinding surfaces or sieve surface of the mill. It is difficult to power a drugs of having gummy or resinous nature, if the method used for size reduction generates heat. Complete dryness of material may help to overcome this difficulty. 4 Material structure: Materials which show some special structure may cause problem during size reduction e.g. vegetable drugs which have cellular structure, generally produce long fibrous particles on its size reduction . Similarly a mineral substances having lines of weakness, produces flake like particles on its size reduction.

6. 5. Moisture content : The presence of moisture in the material influences a number of its properties such as hardness, toughness or stickiness which in its turn affects the particle size reduction. The material should be either dry or wet . It should not be damp . The material having 5% moisture in case of dry grinding and 50% moisture in wet grinding does not create any problem. 6. Softening temperature: Waxy substances such as stearic acid or drugs containing oils or fats ,become softened during the size reduction processes if heat is generated. This can be avoided by cooling the mill. 7. Purity required: Various mills used for size reduction often cause the grinding surfaces to wear off and thus impurities come in the powder .If a high degree of purity is required, such mills must be avoided. Moreover, the mills should be thoroughly cleansed between batches of different material in order to maintain purity. 8. Physiological effect : Some drugs are very potent . During their particle size reduction in a mill dust is produced which may have an effect on the operator .In such cases , the enclosed mills may be used to avoid dust. 9. Ratio of feed size to product size : To get a fine powder in a mill , it is required that a fairly small feed size should be used. Hence it is necessary to carry out the size reduction process in several stages , using different equipment e.g. preliminary crushing followed by coarse powder and then fine grinding. 10. Bulk density : The output of the size reduction of material in a machine , depends upon the bulk density of the substance.

7. Methods of size reduction The following are the methods of size reduction, in which different mechanism are involved; CUTTING: The material is cut on a small scale by means of sharp blade, knives etc. On a large scale, a cutter mill is used. CUTTER MILL Principle: The size reduction is done by cutting with the help of sharp knives. Construction: there are two types of knives which are fitted in this mill. These are stationary knives and rotating knives. The stationary knives are fixed in the casting whereas the rotating knives are attached to the rotor. The upper part of casting also contains hopper inlet, whereas the lower part has a screen of desire aperture size.

8. Working: the material to be cut into the hopper of the mill. The rotor rotates at a high speed. Due to the rotation of the rotor, the material comes very close between stationary knives and rotating knives. It cut down the material into small pieces. Screen retains the material in the mill, until the desire degree of size reduction has been affected. The product is collected which comes out of the sieves. Uses: T his mill is used for the size reduction of soft materials such as roots, peels or wood.

9. 2. COMPRESSION: In this method, the material is crushed by the application of pressure. Example is Roller Mill. ROLLER MILL Principle: The material is crushed by the application of pressure. The mill works on the principle of compression of material by applying a pressure on it . Construction: It consists of two cylindrical rollers made of stone or metal, which are mounted horizontally. These rollers are capable of rotation on their longitudinal axis. These rollers may be from a few centimeters up to a meter in diameter. Generally, one of the roller is driven directly, while the second one runs freely . The gap between the rollers can be controlled to obtain the desired particle size. Working: The rollers are allowed to rotate. The material is fed from the hopper into the gap between the two rollers. The material is crushed while passing through the rollers can be adjusted to control the degree of size reduction. The product is collected into a receiver. Uses: The roller mill is used for crushing and cracking of seeds before extraction of fixed oil. It is also used to crush the soft tissues, to help in the penetration of solvent during the extraction process .

10. 3. IMPACT: This involves the operation of hammers at high speed. When a lump of material strikes the rotating hammers, the material splits apart. This action continue until particle of desired size are obtained. Example is hammer mill. HAMMER MILL Principle: The hammer mill operate on the principle of impact between rapidly moving hammers mounted on a rotor and the powder material. Construction: The hammer can be either horizontal or vertical shaft type. Hammers are usually made of hardened steel, stainless steel with impact surface. This unit is enclosed with a chamber containing a removable screen through which the material must pass. Screen are prepared using metal sheet of varying thickness with perforated holes.

11. Working : The hammers are allowed to be in continuous motion (8000-15000RPM) the feed material is placed into the hopper, which flows vertically down and then horizontally, while hammers are in continuous motion. These rotating hammers beat the material to yield smaller particle. Then, these pass through the screen. Due to the high speed of operation, heat is generated which may affect thermolabile drugs or materials. Moreover, high speed of operation also causes damage to the mill if foreign objects such as stone or metal is present in the feed. Uses The hammer mill is used for producing intermediate grades of powder from almost all types of substances except sticky materials that choke the screen . The expected particle size may vary from 10 to 400mm

12. THE DISINTEGRATOR Principle: The size reduction in disintegrator is done by impact . Construction: It consists of a steel drum having a shaft in the center. The shaft contains a disc, on which four beaters are fixed. The shaft rotates with a speed of 5000-7000 RPM. The side and upper inner surface of the drum is rough. The lower part of the drum has a detachable screen or sieve. The sieve has a definite pore size . Working: The beaters are mainly responsible for grinding but are helped by the undulation of the inner surface and roughness of drum. The material is fed to beaters, through hopper which is fitted to the drum. The material is broken into small particles by impact of the beaters. Due to high velocity of beaters the air velocity inside the chamber is increased. The air is allowed to pass through an outlet on which the dust bag is tied, which retains the fine particles of powder.  Uses: The mill is used to powder all types of drugs including very hard drugs. The drug should be dried before feeding into the disintegrator to get a fine powder. To avoid the jamming of the beaters of the disintegrator, use moderately small pieces.

13. 4. ATTRITION: This process involves breaking down of the material by rubbing between two surfaces, Example is ball mill. BALL MILL These are also known as tumbling mills. Principle: It works on the principle of impact and attrition both for the size reduction. Construction: It consists of a hollow cylinder which is mounted on a metallic frame in such a way, that it can be rotated on its longitudinal axis. The length of the cylinder is slightly greater than its diameter. The cylinder and balls are made of metal and are usually lined with chrome. In the pharmaceutical industry, sometimes the cylinder of the ball mill is lined with rubber or porcelain. The balls used in these mills are also made of rubber or porcelain.

14. Working: The drug to be ground is put into the cylinder of the mill in such a quantity that it is filled to about 60% of the volume. A fixed number of balls are introduced and the cylinder is closed. The mill is allowed to rotate on its longitudinal axis. The speed of rotation is very important. At a low speed, the mass of balls will slide or roll over each other and only a negligible amount of size reduction will occur. At a high speed, the balls will be thrown out to the walls by centrifugal force and no grinding will occur. But at correct speed , the centrifugal force just occurs with the result that the balls are carried almost to the top of the mill and then fall in. By this way the maximum size reduction is effected by the impact of particles between the balls and by attrition between the balls. After a suitable time, the material is taken out and passed through a sieve to get powder of the required size. Uses: The mill is used to grind brittle drugs to fine powder. High speed Low speed Correct speed

15. Advantages: It can produce very fine particles. It can be used for continuous operation. Ball mill is suitable for both wet and dry grinding process. Toxic substance can be ground, as the cylinder is a closed system. Since the mill is closed system, sterility can be achieved. In the ball mill, installation, operation and labor costs are low. It is capable of grinding a large variety of materials of different character and of different degree of hardness. Disadvantages: The ball mill is a very noisy machine. Soft, sticky, fibrous material cannot be milled by ball mill.

16. Fluid Energy Mill Principle: fluid energy mill works on the principle of impact and attrition. Construction: It consist of a loop of pipe, which has a height about 2 meter and diameter may be ranging from 20-200 millimeter. The mill surface may be made of stainless steel. There is an inlet for the feed and a series of nozzles for the inlet of air or inert gas. An outlet with a classifier (cyclone separator or bag filter) is fitted to allow the escape of air.

17. Working: The air or inert gas is introduced with a very high pressure through the nozzles. Solids are introduced into air stream through inlet. Due to high degree of turbulence, impact and attrition forces occurs between the particles the fine particles are collected through a classifier. Fluid energy mill reduces the particles to 1-20 micron. The powder remains in the mill, until its size is reduced sufficiently. Uses: fluid energy mill is used to reduce the particle size of most of the drugs such as antibiotics and vitamins. Ultra fine grinding can be achieved. Advantages: It has no moving parts, hence heat is not produced so it is useful for heat sensitive substance such as vitamins, antibiotics and sulphonamides. The particle size of powder can be controlled due to the use of a classifier. It is rapid and an efficient method for reducing powder to 30mm or less. Disadvantages: It is not suitable for milling of soft, sticky and fibrous material. The equipment is expensive.