Factors Affecting Size Reduction


Factors Affecting Size Reduction


Hardness is a surface property of the material and is frequently confused with strength. Material may be very hard posing a size reduction difficult. If the material is brittle then size reduction may present no special problems. An arbitrary scale of hardness has been devised known as Moh’s Scale. A series of mineral substances have been given hardness numbers between 1 and 10, ranging from graphite to diamond. Up to 3 are known as soft and can be marked with the fingernail. Hardness above 7 is designated as hard and cannot be marked with a good penknife blade, while those between 3 and 7 are described as intermediate. In general, the harder the material the more difficult it is to reduce in size.


The toughness of a material is generally much more important than the hardness. Soft but tough material may present more problems in size reduction than hard material. For example, tough material like rubber is difficult to break than brittle substances, a stick of blackboard chalk. Toughness is encountered in fibrous drugs and is often related to moisture content. Sometimes material toughness can be reduced by treating them with liquefied nitrogen at a temperature lower than −100 to −150 °C. The method has additional advantages in that there is a reduction in the decomposition of thermolabile materials, in the loss of volatile materials, in the oxidation of constituents, and the risk of explosion.


Abrasiveness is a property of hard materials (minerals) that limits the mill to be used for size reduction. During the grinding of some very abrasive substances, the final powder may be contaminated with more than 0.1 % of metal worn from the grinding mill.


The stickiness of material causes considerable difficulty in size reduction. This type of material may adhere to the grinding surfaces, or choke the meshes of the sieve. Usually, the size reduction equipments produce heat. Gummy or resinous substances may be troublesome to reduce in size as their hardness changes with the generation of heat and becomes sticky. Sometimes the addition of inert substances such as kaolin to sulfur may reduce stickiness.


Slipperiness is the reverse of stickiness. This property also gives rise to size reduction difficulties, since the material acts as a lubricant and lowers the efficiency of the grinding surfaces. While size reduction material slips create problems in milling.

Softening Temperature

During the size reduction process sometimes heat is generated which may cause some substances to soften, and thus the temperature at which this occurs is important. For example, waxy substances such as stearic acid or drugs containing oils or fats may find difficulties in size reduction with a reduction in their functionalities. This can be overcome by cooling the mill, either by a water jacket or by passing a stream of cold air through the equipment. Another alternative is to use liquid nitrogen.

Material Structure

Materials used in pharmaceuticals are of wide variety with some being homogeneous but the majority show some special structures.

For example, mineral substances have lines of weakness. Along the lines of weakness, these materials split into forms like flakes, while vegetable drugs have a cellular structure that often leads to long fibrous particles. Thus, the resulting product at particular operating conditions may vary in their size. The energy required to perform this operation may vary.

Moisture Content

The moisture content of substances influences several properties that can affect size reduction. These properties include hardness, toughness or stickiness, etc. In general, for size reduction materials should be dry or wet and not entirely damp. Usually, less than 5 % 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 and small amounts of fines generated affect the operator’s health. To avoid these fines, mills must be enclosed; in addition, exhaust systems should be provided. If possible wet grinding is performed to eliminate the problem.

Purity Required

Some of the size reduction equipments cause wear and tear of the grinding surfaces. Use of these equipments must be avoided whenever a high degree of purity of the product is needed. Similarly, some of those equipments are so complex that they are unsuitable for cleaning between batches of different materials.

Ratio of Feed Size to Product Ratio

Machines that produce a fine may need to carry out the size reduction in several stages with different equipments. For example, preliminary crushing followed by coarse grinding and then fine grinding. In such cases feed size is needed to be controlled to perform reduction efficiently.

Bulk Density

The capacities of most batch mills depend on volume. These mills usually demand solid materials by weight rather than volume. The output of the mill is related to the bulk density of the substance. The higher the bulk density more is the product.

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