Recent Works on Powder Mixing and Powder Coating Using an Optical Measuring Method

During the last few decades a great amount of effort has been addressed to the description, qualitative in most of the cases, of the basic mechanisms operating in the mixing of particulate solids. Given a particular process, the specific mechanism by which mixing proceeds depends fundamentally on the characteristics of the motion of the powder within the mixer, which in tum depend both on the physical properties of the powders and the characteristics of the mixing equipment. Another aspect of the powder mixing process is that concerning the quality of the resulting mixture and the time required for the particular mixer in consideration to yield the mixture of the desired quality. To evaluate the quality of the mixture it is necessary to know its composition at different points, task that has traditionally been done by stopping the mixer at a prescribed time to withdraw a number of samples from as many places within the mixture and analyzing the composition of each one of the samples. This procedure is, however, impractical if the performance evaluation of different types of mixer in order to select the most suitable for a given duty is to be accomplished. Both the flow pattern of powder within the mixer and the temporal variation of the mixture quality can be followed up by a recently developed on-line type optical measuring method. The method has already been applied to the evaluation of the mixing characteristics of a number of mixers. The results of these works are reviewed in the first part of this paper. The second part of the review is devoted to the recent works of the authors on powder