Comminution is the reduction of materials to smaller pieces to course particles or to powder Objective of size reduction:In the preparation of suspension or to facilitate mixing of powdersTo increase the surface area to the action of solvents thus facilitating solution in the case if chemical substances Reduce the bulk of the material to facilitate shipping.Materials are crushed to expose cells to extractionFactors affecting size reduction:The methods of size reduction are numerous, and selection of the appropriate method involves consideration of the material properties that may influence the process. The properties that can affect size reduction include:Hardness: hardness is a surface property of the material Mohs scale of hardness was devised, where a series of mineral substances given hardness number between one (1) and ten (10) ranging from graphite to diamond, up to 3 are known as soft and can be marked with finger nail. Above 7 are hard and cant be marked with a good penknife blade, while those between are described as intermediate. In general, the harder the material the more difficult it is to reduce in size.Toughness: Toughness is often more important than hardness, so that a soft but tough material may present more problems in size reduction than a hard but brittle substance; e.g compare the ease which a stick of blackboard chalk can be broken and the difficulty of attempting to break a rubber. Toughness is encountered in many pharmaceutical materials, particularly in fibrous drugs, and is often related to moisture content compare the toughness of a green herb (twig) with the brittleness of a one. For special cases toughness can be reduced by treating the material with a liquefied gas, liquid nitrogen by this means substances can cooled to temperature lower than 100 to 150 C when even rubber becomes brittle and will break like glass. The method has advantages in reducing the decomposition of thermolabile materials, in the loss of volatile materials. In practice it is little used due to the cost.Stickiness: the property may cause considerable difficulty in size reduction for material may adhere to the grinding surfaces, or the meshes of screens may become blocked. Pharmaceutical substances that are gummy or resinous may be troublesome particularly if the method used for size generate heat. Complete dryness may help and manufacturers may employ the addition of inert substances e.g. Kaolin to sulphur and to D.D.TAbrasiveness: is a property of hard materials of mineral origin and may limit the type of machinery that can be used. It has been reported that during the grinding of some very abrasive substances the final powder has become contaminated with more than 0.1% of metal from the grinding mill.Softening Temperature: Many of the size reduction processes result in the generation of heat, which may cause some substances such as stearic acid or drugs contains oils or fats are examples that may be affected. With some methods, it may be possible to cool the mill either by a water jacket or by passing a steam of air through the equipment.Material structure: some substances are homogenous in character, but the majority show some special structure e.g. mineral substances may have lines of weakness along which the mineral splits, while vegetable drugs have a cellular structure often leading to long fibrous particles.Moisture content: in general, materials should be dry or wet and not merely damp. Usually, less than 5% of moisture is suitable if the substance is to be ground dry, or more than 50% if it is being subjected to wet grinding.Physiological Effect: some substances are very potent (e.g. Podophllum resin or hormone drugs) and small amounts of dust may have an effect on the operators. in such cases, enclosed mills must be used to avoid dust.Purity Required: some machines will be unsuitable if cleaning between batches of different materials is difficult.Ration of feed size to product size: generally, machines that produce a fine product require a fairly small feed size, this it may be necessary to carry out the size reduction process in several stages with different equipment e.g. preliminary crushing followed by coarse grinding and then fine grinding

Mechanisms of Size Reduction:There are four main methods affecting size reduction, involving different mechanisms:Cutting: the materials are cut by means of a sharp blade or blades.Compression: the material is crushed by application of pressureImpact: occurs when material is stationary and is hit by an object moving at high speed or the moving particles strikes a stationary surface the materials shatters to smaller piecesAttrition: the material is subjected to pressure as in compression but the surfaces are moving relative to each other, resulting shear forces which break the particles.

Methods of Size ReductionThe term mill is used normally for machines for size reduction.Cutting: on small scale, size reduction by cutting can be affected by a knife. On large scall a cutter mill is usedCutter mill: as illustrated in the figure, knives are attached to a rotator and act against stationary knives in the casing. The method is used to obtain a coarse degree of size reduction of soft materials.The commonest application is the treatment of drugs such as roots, peels or woods prior to exctraction.Compression: size reduction by compression on the small scale is usually carried out in a pestle and mortar. The commonest method in the pharmaceutical industry is the roller mill.Edge and End Runner Mills: the edge runner mil consists of one of two heavy granite or case iron wheals or mullers mounted on a horizontal shaft and standing in a heavy pan. Either the muller or the pan is driven. The materials is fed into the center of the pan and is worked outwards by the action of the muller. Whilst in the zone traversed by the muller, comminution will occur by compression, due to the weight of the muller, and by shear. The end runner mill is similar principle and consists of a rotating pan or mortar made of case iron or porcelain. A heavy pestle is mounted vertically within the pan. The mechanism of size reduction is compression due to the weight of the pestle, and shear. A scraper is used to redirect the material into the grinding zone. Both mills operate at slow speeds and produce moderately fine powders. Wet grinding with very viscous materials, such as ointments and pastes, is possible.Single-Drive Roller Mill: the roller mill has two cylindrical rolls of stone or metal, mounted horizontally, which are capable of rotation on their longitudinal axes. Usually, one of the rolls is driven directly while the second runs free, so that when material is placed above the rolls it is drawn in through the nip and the second roll is rotated by friction. The rolls may be from a few centimeters up to a meter or more in diameter and the gap between the rolls can be adjusted to control the degree of size reduction. This roller mill is used as cracking seeds prior to extraction of fixed oils.Impact: there is no hand method affecting size reduction by impact, apart from the shattering of brittle substances with a hammer or with a pestle and mortar.Hammer mill: the hammer mill consists of a stout metal casing enclosing a central shaft to which four or more hammers swing out to a radial position when the shaft is rotated. The lower part casing consists of a screen through which material can escape, when sufficiently size reduced, and be collected in a suitable receiver. The screens can be changed according to the particle size required.Advantages: Its rapid in action, and is capable of grinding many different types of materials.The produce can be controlled by variation of rotor speed, hammer type, and size and shape of mesh.Operation is continuous.No surfaces move against each, so that there is little contamination of the product with metal abraded from the mill.Disadvantages:The high speed of operation causes generation of heat which may affect thermolabile materials or drugs containing gum, fat or resin.Because of the high speed of operations, the hummer mill is susceptible to damage by foreign objects such as stones or metals in the feed.Attrition: size reduction by attrition can be effected in the laboratory by using pestle and mortar or, if a small scale mechanical method is required, the roller mill can be used.Multiple drive Roller Mill: this type of roller mills uses the principles of attrition for the size reduction of solids in suspensions, pastes or ointments. Two or three rolls, usually in metal but possible in porcelain, are mounted horizontally with a very small, but adjustable, gap between the rolls rotate at different spaces, so that the material is sheared as it passes through the gap and is transferred from the slower to the faster roll, from which it is removed by means of scraper. The method is very effective for size reduction and dispersing solids in semisolid media.Combined impact and attrition: the mechanisms of impact and attrition can be combined in bal mill, the particles receive impacts from balls and are subjected to attrition as the balls slide over each other.Ball Mill:The ball mill consists of a hollow cylinder mounted in such a way that it can be rotated on horizontal longitudinal axis. Some models are very large with cylinders 3 m in diameter, but about 1 m is more common in pharmaceutical practice. The cylinder contains balls that occupy 30 to 50 % of the mill volume. Usually a mill 1 m in diameter will use balls of approximately 75 mm. the cylinder be of meta, porcelain or of rubber, to reduce abrasion. The balls may be of metal or porcelain. The factor of greatest importance in the operation of the ball is the speed of rotation.At low speeds, the mass of balls will slide or roll over each other and only a negligible amount of size reduction will occur. At high speeds, the balls will be thrown out to the wall by centrifugal force and no grinding will occur. At about two thirds of the speed at which centrifuging just occurs, it is found that movement takes place, the balls are carried almost to the top of the mill and then fall in a cascade across the diameter of the mill. By this means, the maximum size reduction is effected by the impact of the particles between the balls and by attrition between the balls.Advantages:It is capable of grinding a wide variety of material of different degrees of hardness.It can be used in a completely enclosed form makes it suitable for use with toxic materials.It can produce very fine powders.It can be used for continuous operationIt is equally suitable for wet or dry grinding processesDisadvantages:Wear occurs, principally from the balls, but partially from the casing and this may result in the product being contaminated.Soft or stick materials may casus problems by caking on the sides of the mill or by holding the balls in aggregate.The ball mill is very noisy machine, particularly of the casing of metal, but much less so if rubber is used