Particle size analysis: past, present and future

Abstract

Although the Particle Size Analysis Group of the Society for Analytical Chemistry has been in existence for only 8 per cent. of its parent's life, the methods used for the analysis of particle size can be traced back into antiquity. For example, the Egyptians were mining precious metals in the Sinai peninsula at about 4000 B.C. and there is documentary evidence that sieving was utilised by these early technologists.

The behaviour of particulate solids is influenced by size and shape; the analysis of particle size is therefore of vital concern to many different modern technologies ranging from the manufacture of paint to the formulation of drugs and phytochemicals. A feature of particle size analysis today is that it is not strictly a discipline in its own right but cuts across many scientific areas. The techniques of analysis are applied by many different types of scientist from chemists and physicists to pharmacists and chemical engineers. Indeed, this cross-fertilisation of disciplines has been one of the most attractive features of the Particle Size Analysis Group, and this is illustrated by reference to some of the personalities associated with the Group since its formation.

Methods of analysis vary from the direct microscope analysis to the indirect methods that depend on inertial or optical properties of the particles themselves. In addition there are other indirect methods that depend on measuring properties of packed powder beds, such as gas permeability or adsorption. Many of these methods have been developed and improved in recent years, but each method yields a unique estimate of size for materials that are other than spheres. Interpretation of the results obtained by methods of size analysis may therefore present a different type of problem from, say, the interpretation of a relatively simple chemical analysis. At present there is an urgent need to reconsider established methods of analysis by using modern ideas of mathematics, statistics and, in some situations, hydrodynamics. The criticism has been made that the instrument manufacturer has outstripped the theoretician, and it seems that now is the time in this field to sit down and think.

The techniques of size analysis do not exist in their own right and have no use in isolation without their applications. For this reason there have been genuine advances made in very recent years in areas where there is a need, for example, for on-line analysis in the control of a continuous production line. The main problem here is the sampling process used, as the vast majority of sizing procedures require considerable dilution before they can be applied. Sampling and analysis can be carried out automatically, but the final choice of the method of sizing has to be made on the ultimate application or use of the end product.

Some recent advances in instrumentation are described, including the use of scanning electron microscopy to characterise and identify particulate contamination of the environment. In addition, the improvement brought about in the application of laser light to applications in which monochromatic light was previously used is also described.

Modern developments in the subject have been kept continually under review by the Group since its foundation, including the organisation of two International Conferences, with a third scheduled for 1977. The child spawned by a healthy and interested parent is continuing to be blessed with a lusty and vigorous growth.