The main objective of size reduction is to produce smaller particles from larger ones. Smaller particles are the desired product either because of their large surface area or because of their shape, size, and number. The energy efficiency of the operation can be related to the new surface formed by the reduction in size. The shape features of particles, both alone and in mixtures, are important for product evaluation after size reduction. In actual processing, using particular equipment does not produce a uniform product, whether the feed is uniformly sized or not. The product normally consists of a mixture of particles, which may contain a wide variety of sizes and even shapes. Some types of equipment are designed to control the magnitude of the largest particles in their products, but the fine sizes are not under such control. In some equipment, fines are minimized, but they cannot be eliminated.
In comminuted products,
the term “diameter” is generally used to describe the characteristic dimension
related to particle size. The shape of an individual particle is conveniently
expressed in terms of the sphericity (s), which is independent of particle
size. For spherical particles ‘s’ equals unity, while for many crushed
materials its value lies between 0.6 and 0.7. There are different types of
particle size distributions and no single distribution applies equally well to
all comminuted products, particularly in the range of coarser particle sizes.
For finer particles, however, the most commonly found distribution follows a
log-normal function, which is the most useful among the different types of
functions.
Thus, in pharmaceutical practice the objective of this operation is to:
- Increase the surface area to enhance the rate of a physical or chemical process.
- Perform separation of two constituents in cases where one is dispersed in small isolated pockets.
- Meet stringent specifications regarding the sizes of commercial products.
- Accomplish intimate mixing of solids in a solid-solid operation since the mixing is more complete if the particle size is small.
- Improve dissolution rate, solubility, binding strength, and dispersion properties.
- Increase the therapeutic effectiveness of certain drugs by reducing the particle size, for example, the dose of griseofulvin is reduced to half than that of originally required.
- Improve mixing of several solid ingredients.
- Improve the physical appearance of products.
- Enhance flowability, improve compression and dose uniformity.
- Enhance the stability of the dispersed system, for example, stability of emulsions is increased by decreasing the size of the oil globules.
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