The significance and application of shape factors in particle size analysis

Abstract

By use of a new method for measuring the thicknesses of individual sub-sieve particles with the optical microscope and being able to measure the dimensions of their projected images as well, the surface and volume shape factors of the particles can be calculated. An expression can be derived for the ratio of the particle's projected diameter to its Stokes' diameter in terms of these factors. Results of the examination of coal dust particles revealed a large variation in the shape factors between particles, which should also be found for particles of other materials. It is shown that in the application of shape factors to the correlation of results obtained by different methods for particle size analysis, the variation of these factors should be taken into account. The method of application has been devised for use when comparing results of the microscope method with those of sedimentation methods, both gravimetric and light extinction. This method has been compared with the normal way in which only the average shape factor is applied. Differences occurred which could account for some of the past difficulties when the correlation of the sizing methods was attempted. The largest particles of a sub-sieve fraction of a powder seen under the microscope have shape factors different from those of the smaller sizes.