Comminution

Comminution, or size reduction, is the operation carried out for reducing the size of bigger particles into smaller ones of the desired size and shape with the help of external forces. Moreover, the function of size reduction may be to aid efficient processing of solid particles. Additionally, it facilitates powder mixing or the production of suspensions. There are also some special functions of size reduction, such as exposing cells in plant tissue prior to the extraction of the active principles. Another function is reducing the bulk volume of a material to improve transportation efficiency.

Advantages of comminution:

  1. Content uniformity.
  2. Uniform flow.
  3. Effective drying.
  4. Increases surface area or viscosity.
  5. Uniform mixing and drying.
  6. Improve rate of absorption. Smaller the particles greater is absorption.
  7. Improve dissolution rate.

Disadvantages of comminution:

  1. Drug degradation.
  2. Contamination.

Theory Of Comminution

In milling, particle fracture occurs randomly. A single particle impacted suddenly will break into a few large and many fine particles, with few intermediate sizes. Higher impact energy results in smaller and more numerous large particles, while the fine particles increase in number but not significantly in size. The size of fine particles is related to the material’s internal structure, whereas the size of larger particles is more closely related the milling process itself.

Size reduction begins with the opening of any small cracks that were initially present. Thus, larger particles with numerous cracks fracture more readily than smaller particles with fewer cracks. In general, fine grinding requires more energy, not only because of the increased new surface, but also because more energy is needed to initiate cracks.

Crystalline materials fracture along crystal cleavage planes, and non-crystalline materials fracture at random. If an ideal crystal were pressed with an increasing force, the force would be distributed uniformly throughout its structure until the crystal disintegrated into its individual units. In contrast, a real crystal fractures under much less force into a few relatively large particles and several fine particles. Consequently, there are relatively few particles of intermediate size. Crystals of pure substances have internal weaknesses due to missing atoms or ions in their lattice structures and flaws arising from mechanical or thermal stress.

Energy for Comminution

The energy required to reduce the size of particles is inversely proportional to the size raised to some power. This general differential equation may be expressed mathematically as:

$$ \frac{dE}{dD}=-\frac C{D^n} $$

where, dE is the amount of energy required to produce a change in size, dD, of unit mass of material, and where C and n are constants.

Kick’s Law

Kick’s theory states that the energy used in deforming or fracturing a set of particles of equivalent shape is proportional to the ratio of the change in size. or:

$$ E=C\;\ln(\frac{D_1}{D_2}) $$

where C is Kick’s constant of energy per unit mass, D1 is the initial particle diameter and D2 is the new particle diameter.

Rittinger’s Law

Rittinger’s hypothesis relates the energy, E, used in a size reduction process to the new surface area produced, S2, or

$$ E=k\;(S_2-S_1) $$

where S1 is the initial surface area and k is Rittinger’s constant, expressing energy per unit area.

Bond’s Law

Bond’s theory states that the energy used in crack propagation is proportional to the new crack length produced, which is often related to the change in particle dimensions according to the following equation:

$$ E=2k_b\;(\frac1{D_2}-\frac1{D_2}) $$

Here kb is known as Bond’s work index and represents the variation in material properties and size reduction methods, with dimensions of energy per unit mass.

Mechanisms of Comminution

Mechanisms of Comminution
Fig. 1: the mechanisms of comminution

Mills are equipments designed to impart energy to the material and cause its size reduction. There are four main methods of effecting size reduction, involving different mechanisms:

  1. Cutting: It involves application of force over a very narrow area of material using a sharp edge of a cutting device.
  2. Compression: In compression, the material is gripped between the two surfaces and crushed by application of pressure.
  3. Impact: It involves the contact of material with a fast moving part which imparts some of its kinetic energy to the material. This causes creation of internal stresses in the particle, there by breaking it.
  4. 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.

Reference:

  • Aulton, M. (2018). Aulton’s pharmaceutics, the design and manufacture of medicines. Edinburgh. : Elsevier
  • Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd