25 Years of Particle Size Conferences

B. Scarlett

DEPARTMENT OF CHEMICAL ENGINEERING, TECHNICAL UNIVERSITY, P.O. BOX 5045, DELFT, 2600 GA, THE NETHERLANDS

1 1966 AND ALL THAT

It is almost precisely twenty five years since the first of these conferences, organised then by the Particle Size Sub-Committee of the Society of Analytical Chemistry which has now grown into the Particle Size Characterisation Group of the Analytical Division of the Royal Society of Chemistry. The sub-committee had recently completed a review of 74 different methods of particle size analysis and these efforts were the inspiration both for the first conference and for the formation of the permanent subject group. This is the seventh conference in the series and it is entirely appropriate that it should be held in the same place, at Loughborough University. The proceedings of each of the conferences is permanently documented by a bound book of conference proceedings. Several authors feature in all the seven conferences.

The closing address at the first conference was given by Professor Harold Heywood, an address which was based on many years of experience and which could be given, almost without modification, at the end of this conference and still be entirely appropriate. Thus, some truths do not change and although they must be learnt anew by each succeeding generation, they remain essentially the same.

The developments which he foresaw have largely occurred over the intervening twenty five years but it may be that now those basic messages have even more practical importance and that the rate of development grows exponentially. In this paper, my intention is to re-state some of the message of Heywood and to illustrate it in the circumstances of today.

2 WHY OH WHY?

The first quotation I would like to take concerns why particle size measurement is important, why it commands the attention of professional groups, companies, journals and conferences specifically devoted to its development. Heywood wrote:-

"However, it must be realised that particle size analysis is not an objective in itself but is a means to an end, the end being the correlation of powder properties with some process of manufacture, usage or preparation."

A particle size measurement does not have a meaning unless the objective of the measurement is also specified. Thus, the techniques which should be used depend entirely on the accuracy which is required and the circumstances of place and time in which the measurements must be made. There is no such thing as the "best" particle size technique unless the circumstances are also specified. A particle analysis laboratory cannot operate on the same basis as some normal analytical laboratories, samples come in and numbers go out. In a particle analysis, the question must always first be addressed. "What do we need to know?" Another way of saying this is that, eventually, particle analysis is an engineering tool, not a basic science. Engineers must use all the known laws of science to solve the particular problem with which they are confronted in the most elegant manner.

In this respect, it may be that the circumstances are more difficult than they were twenty five years ago. At that time, the number of instruments was more restricted than now as was certainly the number of companies who supplied those instruments. Current instrument manufacturers must try to provide a package of hardware and software which will handle a large number of powders and applications. The commercially available instruments are thus, inevitably, a compromise between what is desirable and what is possible, the same instrument being sometimes too sophisticated and sometimes inadequate. To be a manufacturer of particle measuring instruments is not an easy life and it can only get worse because the tendency will be for the applications to become more diverse and sophisticated. Why should the applications become more diverse? That comes partly from the development of new technologies, of course, but it also comes from a gradually increasing sophistication in the use of the measurements. (Fig. 1).

At the simplest level we use particle size measurements to monitor their concentration or to control the reproducibility of a product. Thus, we compare what we find with what we expect and if the two do not coincide we reject the product. The science of powder technology, however, is concerned to use the microscopic properties of the system, for example the particle size distribution, to interpret the bulk behaviour of the powder. If it is to be used in dilute circumstances, then the bulk behaviour can be derived by integrating the behaviour of the individual particles but usually this is not so and the relationship between the microscopic and macroscopic properties must take account of the particle interactions. By observing the difference in particle size distribution of samples which exhibit a different bulk behaviour, we begin to make a "correlation" between the two which, whether empirical or theoretical, quantitative or qualitative, involves interpretation of the mechanisms involved. Somewhere between these two purposes usually lies the purpose of a particle size measurement. There is, however, a far more ambitious level at which powder technology must eventually operate and, as yet, rarely does. That is to design the particles and the particle mixture to produce required properties, to use the relationships between microscopic and macroscopic properties in a predictive manner. It is the more rigorous use of particle size measurements which introduces the real diversity and which requires the measurements to be carefully matched to the problem. The increased diversity does not alter the basic needs which Heywood described. He wrote:-

"Finally, we should examine the problems awaiting solution in the study of particle sizing and applications to industrial production".

He then went on to classify the problems into three groups. The first was:-

Standards through Standards

"First there is the development of some standard method of analysis which gives an absolute measure of particle size distribution. This may already exist but, if so, the validity needs establishing beyond question. Time of operation is not a vital factor for such a procedure, as it would mainly be used for fundamental research".

Of course, the fundamental method has always existed and is indeed terribly tedious. In a review of the 1981 conference, Leschonski wrote:-

"It would, therefore, be most desirable to describe size more often, based on the volume of the particle, e. by its volume diameter".

Thus, if it were possible to measure the volume of each individual particle, for example by weighing and if the density is known then the result can be presented either as a distribution by number, or a distribution by mass, of the equivalent volume diameter. Thus, in order to carry out an "absolute" method of particle size measurement, all that is required is an accurate balance. Of course, the method is impractical and, as the particles become smaller, impossible but it is the ideal against which other methods can be assessed and can be calibrated. Over the past twenty five years, the view has increasingly prevailed that we should regard the equivalent volume diameter of the particle as the basic size and that other equivalents are dependent upon both the size and shape of...