Hardness
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.
Toughness
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
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.
Stickiness
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
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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