Mixing of particulate materials is widely encountered in the process and related industries. So is segregation. They are competing processes; the one a deliberate act intended to increase the ho- mogeneity of a mixture of different components or sizes, and the other an involuntary process, occurring as a result of the fact that the various forces that may act on the individual components of a particulate mixture may cause them to move in different directions, or to different positions in a bulk, due to their different characteristics. Generally, the most important of these is particle size, although density, shape, surface roughness, resilience, electrostatic properties etc. all can play a role, as can process conditions.
The mechanisms that are mobilised in order to effect mixing are basically three: diffusion (or preferably dispersion), convection and shear. Diffusion with particulate materials only occurs over very short distances and, in order to mobilise the mechanism, the different components need to be brought into the vicinity of one another. This is generally accomplished by convection or shear, the latter being considered by many to be an idealised form of convection. Modern processes of mixing such as hybridisation using mechanical forces etc. are not covered in this review.
Twelve mechanisms have been identified which lead to segregation. Some of these can act within the mixer itself, while others are only encountered in subsequent handling operations. Wherever they are encountered, they reduce the homogeneity of mixtures. Since the three mix- ing mechanisms and several segregation mechanisms can act simultaneously, doubt has been expressed whether a mixing process can be modelled in a deterministic manner. The weight of opinion appears to be that heuristic experimentation must be accompanied by the development of algorithms to create an expert system in order to improve on the state of the art. Suggestions have been made that chaos theory may be applicable. Chemometrics may be another option.
The review includes a description of the classical approaches to characterising mixture quality and concludes that these have inadequate relevance to modern day needs. For the continuous monitoring of mixers and mixtures it is suggested that both the intensity of segregation (as indi- cated by the variance) AND the scale of segregation (as indicated by an autocorrelogram) be used. Such an approach would allow the continuous monitoring of mixing processes. Whatever method is chosen to monitor or evaluate the quality of a mixture, the sample size is critical. It will vary with the purpose for which the mixture is being created, and the demands made a on a mixer will very much depend on the homogeneity it can achieve in a sample of the chosen size. Optical probes offer the possibility of continuously monitoring mixtures, as do combinations of acoustic probes and chemometrics, but these may not be sufficient to determine the quality of a mixture at the microscope level.
The review concludes with a description of models proposed to describe mixing and segregation processes.
Research plans in these two areas at Telemark College and Telemark Technological R & D Centre are described in an appendix.