Granulation

GRANULATION

Granulation is a term given to a number of processes used to produce materials in the form of coarse particles. In pharmacy, it is closely associated with the preparation of compressed tablets. Here discussion is limited to a general account of the process.

Ideally, granulation yields coarse isodiametric particles with a very nar-row size distribution. The several advantages of this form can be inferred from the discussion above. Granules flow well. They will feed evenly from chutes and hoppers and will pack into small volumes without great variation of weight. Segregation in a mixture of powders is prevented if the mixture of powders is granulated. Each granule contains the correct proportions of the components so that segregation of granules cannot cause inhomogeneity in the mixture. The hazards of dust are eliminated, and granules are less susceptible to lumping and caking. Finally, granular materials fluidize well and a material may be granu-lated to gain the advantages of this process.

The starting materials for granulation vary from fine powders to solutions. Methods can be classified as either wet or dry granulation. In the latter, a very coarse material is comminuted and classified. If the basic material is a fine powder, it is first aggregated by pressure with punches and dies to give tablets or briquettes, or by passage through rollers to give a sheet that is then broken.

In wet methods, a liquid binder is added to a fine powder. If the pro-portion added converts the powder to a crumbly, adhesive mass, it can be granulated by forcing it through a screen with an impeller. The wet granules are then dried and classified. If a wetter mass is made, it can be granulated by extrusion. Alternatively, the powder can be rotated in a pan and granulating fluid is added until agglomeration occurs. Granule growth depends critically on the amount of fluid added, and other variables, such as particle size, pan speed, and the surface tension of the granulating fluid, must be closely controlled.

Granular materials are also prepared by spray drying and by crystallization.