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SIZE REDUCTION SHUBHRAJIT MANTRY Asst.Prof Kottam Institute of Pharmacy A.P

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INTRODUCTION Size reduction is the process of reducing or breaking of larger particles into small particles. Size reduction is otherwise called as comminution or grinding or pulverization .

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Special forces Outer forces Mechanical comminution

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Chemical comminution biological acid Leaching and disssolution

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Ways of size reduction breaking attrition smashing splitting cutting crunching

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1)Size reduction increases the surface area of drugs that help in rapid solution formation in the case of chemical substance. 2) The extraction from animal glands such as liver and pancreas and from crude vegetable drugs is facilitated with an increase in surface area because solvent can easily penetrate into the tissues resulting in quick extraction of their active constituents. 3) To increase the therapeutic effectiveness of certain drugs by reducing the particle size e.g., the dose of griseofulvin is reduced to half than that of originally required. 4) The mixing of several solid ingredients is easier and more uniform if the ingredients are reduced to same particle size. Objectives of Size Reduction

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6) The physical appearance of ointments, pastes and creams can be improved by reducing its particle size. 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 rate of absorption of a drug depends on the dosage form, route of administration and particle size. The smaller the particle size, quicker and greater will be rate of absorption. 5) In the case of powdered pharmaceutical dosage forms the crystalline drugs are powdered before mixing them with other drugs in order to mix all the drugs uniformly and to avoid recognition of crystalline drugs by the patients.

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FACTORS AFFECTING OF SIZE REDUCTION 1) Hardness 2) Toughness 3) Abrasiveness 4) Stickiness 5) Softening temperature 6) Material structure 7) Moisture content 8) Physiological effect 9) Purity required 10) Ratio of feed size to product ratio 11) Bulk density

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LAWS GOVERNING ENERGY & POWER REQUIRMENT OF MILL A number of theories have been advanced to predict the energy requirements of a size reduction process, but none give accurate results. These theories depend upon the basic assumption that the energy required to produce a change d L in a particle of a typical size dimension L is a simple power function of L: d E / dL = KL n --------------------------------------------------- (1) Where d E is the differential energy required, dL is the change in a typical dimension; L is the magnitude of a typical length dimension and K , n , are constants.

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Kick assumed that the energy required to reduce a material in size was directly proportional to the size reduction ratio d L /L. This implies that n in equation. (1) is equal to -1. If K = K K f c Where K K is called Kick's constant and f c is called the crushing strength of the material, we have: d E / dL = K K f c L -1 Which, on integration gives: E = K K f c log e (L 1 /L 2 ) ------------------------------------ (2) Equation (2) is a statement of Kick's Law. Kick's Law Its states that the energy required for size reduction of a material is constant for the change in dimensions of the material, irrespective of the initial dimensions.

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Rittinger , states that the energy required for size reduction is directly proportional to the surface area. This leads to a value of -2 for n in equation. (1) as area is proportional to length squared. If we put: K = K Rfc and so d E / dL = K R f c L -2 Where K R is called Rittinger's constant, and integrate the resulting form of eqn. (1), we obtain: E = K R f c (1/L 2 – 1/L 1 ) ------------------------------- (3) Equation (3) is known as Rittinger's Law Rittinger's Law

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Cutting – here the material is cut by means of a sharp blade or blades. Compression – in this method, the material is crushed by application of pressure. Impact – impact occurs when the material is more or less stationary and is hit by an object moving at high speed or when the moving particle strikes a stationary surface. In either case, the material shatters to smaller pieces. Usually both will take place, since the substance is hit by a moving hammer and the particles formed are then thrown against the casing of the machine. Attrition – in attrition, the material is subjected to pressure as in compression, but the surfaces are moving relative to each other, resulting in shear forces which break the particles. Mechanism of Size Reduction

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Cutting and compression have limited uses in Pharmaceutical practice, impact and attrition are used much more widely, both separately and in combination, and there is a great variety in each type. The machines used for size reduction are often termed as Mills . Methods of size reduction

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Grinding equipment can be divided into two classes – crushers and grinders In the first class the major action is compressive, whereas grinders combine shear and impact with compressive forces. Grinding Equipment

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Equipments based on the mechanism of Cutting 1. Crushers Equipments based on the mechanism of Impact 1. Hammer mills 2. Plate mills Equipments based on the mechanism of Attrition 1. Roller mills Equipments based on the Combined Impact and Attrition 1. Colloid mill 2. Ball mill 3. Fluid energy mill

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General characteristics of various types of mills

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Size reduction CRUSHING dry, + 50 mm GRINDING wet, - 50 mm

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A. Crushers (coarse and fine) 1. Jaw crushers 2. Gyratory crushers 3. Crushing rolls B. Grinders (intermediate and fine) 1. Hammer mills, Impactors 2. Attrition mills 3. Tumbling mills a. Rod mills b. Ball mills C. Ultrafine grinders 1. Fluid-energy mills D. Cutting machines 1. Knife cutters, slitters Coarse mine material into lumps of 250 to 150 mm. Again these lumps are broken into particlse of 6 mm in size. The product from an intermediate grinder might pass a 40-mesh screen and product from fine grinders pass a 200-mesh screen with a 74 m opening. Feed < 6 mm Product: 1- 50 m 2 to 10 mm in length

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Crusher are slow-speed machines for coarse reduction of large quantities of solids. The main types are jaw crushers, gyratory crushers, smooth-roll crushers and toothed-roll crushers. The first three operate by compression and can break large lumps of very hard materials, as in the primary and secondary reduction of rocks and ores. Toothed-roll crushers tear the feed apart as well as crush it, they handle softer feeds like coal. Crusher

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The term grinder describes a variety of size-reduction machines for intermediate and fine duties. The product from a crusher is often fed to a grinder, in which it is reduced to powder. The chief types of commercial grinders are hammer mill, attrition mills and tumbling mills . Grinders

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Hammer Mill Principle: The hammer mill operates in the principle of impact between rapidly moving hammers mounted on a rotor and the powder material.

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Hammer Mill

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Hammer Mill

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It is rapid in action, and is capable of grinding many different types of materials. 2. They are easy to install and operate, the operation is continuous. 3. There is little contamination of the product with metal abraded from the mill as no surface move against each other. 4. The particle size of the material to be reduced can be easily controlled by changing the speed of the rotor, hammer type, shape and size of the screen Advantages:

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Disadvantages: Heat buildup during milling is more, therefore, product degradation is possible. Hammer mills cannot be employed to mill sticky, fibrous and hard materials. The screens may get clogged. Wearing of mill and screen is more with abrasive materials.

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Ball Mill These are also knows as tumbling mills. Principle: The ball mill works on the principle of impact between the rapidly moving balls and the powder material, both enclosed in a hollow cylinder. At low speeds, the ball roll over each other and attrition (rubbing action) will be the predominate mode of action. Thus, in the ball mill, impact or attrition or both are responsible for the size reduction.

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Ball Mill

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Ball Mill

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Advantages: It can produce very fine powder. Ball mill is used for both wet and dry grinding processes. Toxic substances can be ground, as the cylinder is closed system. Rod or bars can also be used as grinding media. (example: Sticky material are size reduced) In ball mill, installation, operation and labour costs are low.

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Disadvantages: The ball mill is a very noisy machine. Ball mill is a slow process. Soft, tacky, fibrous material cannot be milled by ball mill.

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FLUID ENERGY MILL Principle: Fluid energy mill operates on the principle of impact and attrition. In this equipment , the feedstock is suspended within a high velocity air stream . Milling takes place because of high velocity collisions between the suspended particles.

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Fluid energy mill It consists of a loop of a pipe, which has a diameter of 20 to 200 mm, depending on the overall height of the loop which may be up to about 2 meters, a fluid, usually air, is injected at high pressure through nozzles at the bottom of the loop, giving rise to a high velocity circulation in a very turbulent condition, Solids are introduced into the stream and, as a result of the high degree of turbulence, impact and attritional forces occur between the particles. A classifier is incorporated in the system, so that particles are retained until sufficiently fine. The feed to the mill needs to be pre-treated to reduce the particles size to the order of 100 mesh, enabling the process to yield a product as small as 5 micrometers or less. Despite this, mills are available which are capable of outputs up to 4 mg per hour.

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Fluid energy mill

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Advantages: Upto 6000 kg of feed is milled per hour. 2) Feed particles of size 12 mm are easily size reduced. 3) Since there is no wear of the mill, contamination is not possible.

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Disadvantages: Fluid energy mill is not suitable for milling of soft, tacky and fibrous material. The equipment is expensive, because it needs additional accessories particularly fluid energy source and dust collection equipment.

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