Notes on the Meaning of Size Reduction Dispensing Practice

In dispensing practice drugs and excipients are used in the preparation of dosage forms in specified size for effective result or action. Size reduction is necessary to some extent. ‘Grinding’, ‘comminution’ and ‘milling’ are the terms commonly used to signify size reduction of solids.

Size reduction of a drug may include one or more of the operations e.g. cutting, slicing, chopping, rasping or grating, contusion, grinding, pulverizing, milling, micronizing etc. Selection of the processes and equipment depends on type of material, desired degree of size reduction, moisture content etc. in the material, mechanism of size reduction, stability of final product and economic and fast method of size reduction.

Some manual processes employed in the size reduction of substance used in the dispensing of prescription are explained below.

Trituration

The term trituration refers to the process of size reduction of substances to a fine powder but it is also applied for the process whereby a mixture of fine powders is intimately mixed in a mortar. The circular mixing motion causes blending and also breaks up soft aggregates of powders. Crushing or grinding can also be effected by means of application of force on the pestle.

Pulverization by Intervention

This is the process of powdering a substance with the help of another substance, which can be removed easily after the pulverization has been completed. Substances such as camphor, which are gummy and tend to re-agglomerate or which resist grinding, can be powdered by this method. Camphor cannot be pulverized by trituration because the particles tend to cohere as quickly as they are powdered.

This difficulty is overcome by addition of a small amount of alcohol or other volatile solvent during trituration. The solvent can be removed after pulverization process is complete, by spraying the powdered camphor in a thin layer.

Spermaceti wax may also be powdered in this manner. Iodine crystals may be pulverized with the help of a small quantity of ether. Picric acid is often mixed with 10 to 20% of water for safety in transportation. The water content serves a dual purpose. When picric acid is to be powdered for use in ointments, it prevents dangerous explosion during comminution, and aids in the grinding process by intervention.

Levigation

A pharmacist often uses this process to incorporate solids into dermatological and ophthalmic ointments and suspensions. The term refers to the reduction of a substance to an extremely fine state of subdivision by rubbing it, in a glass mortar or on a slab with the aid of an insoluble liquid called the levigating agent.

The process is one of wet grinding. Levigating agent should be selected on the basis of its ability to form a paste with the substance to be levigated, and its compatibility with other ingredients in the product. For instance, water cannot be used as levigating agent with zinc oxide, intended to be incorporated into an oleaginous ointment base. Liquid paraffin is the levigating agent of choice in this case.

Principle of Size Reduction

Size reduction involves four different mechanisms.

1. Cutting:

This implies cutting of materials by means of sharp blade(s).

2. Compression:

This is accomplished by application of force by a suitable device.

3. Impact:

Impact implies hitting of a more or less stationary material by an object moving at high speed or striking the moving particles at a stationary surface. In either case, the material is shattered to small pieces.

4. Attrition:

Attrition occurs when the material is subjected to pressure as in compression but the surfaces are mobile in relation to each other, resulting in shear forces, which break the particles. The mechanisms of impact and attrition may be combined as in ball mill and fluid energy mill.

Equipment in Small Scale:

Mortars and pestles (China and Glass), Household mixer, Grinder, Hand mill and Laboratory mill.

Large Scale Equipment

Following mill are commonly used in industry for size reduction. Selection of a mill is based on types of material, size required, cleaning, and mechanism of milling.

a. Cutter mill

b. Hammer mill

c. Ball mill

d. Rod mill

e. Fluid-energy mill

f. Roller mill

g. Colloid mill

h. Disc mill

i. Edge runner mill

j. Vibration ball mill