An investigation of blending and mixing in bulk materials handling and process systems

Research Doctorate - Doctor of Philosophy (PhD)The mixing and blending of bulk materials can be undertaken using several techniques. Fluidised mixers, mechanical mixers, and multi-tube type blenders, with internal mechanical recirculation or external recirculation, are commonly employed for this purpose. However, the use of well designed mass-flow silos can often achieve good mixing and blending performance, usually by a recycling process, provided that the silo geometry is matched to the bulk material properties. Of direct benefit in the use of mass-flow silos in the mixing and blending of bulk materials is the reduction, over that of a mechanical mixer, of particle attrition and degradation during the process, a minimisation in the contamination of the bulk material by the mixer's lining, and a lower input energy requirement to the system. The mass-flow silo also allows the mixing and blending of both cohesive and free-flowing bulk materials whereas the multi-tube type blenders are limited in their application to free-flowing materials. If particle attrition and material contamination is not a problem in the mixing and blending of the materials then mechanical mixers may be employed. However, well designed blending silos and feeders can still be employed in the process to ensure the segregation of the materials will be avoided in the subsequent downstream handling of the mechanical mixer. Previous studies have examined the mixing and blending phenomena in a mass-flow silo, or 'in-bin blending' of bulk materials as it is commonly referred. However, these studies have all been based upon assumptions, or experimental data obtained from pilot scale test rigs, rather than being based upon analytical predictions derived from bulk material flow properties and silo geometry. The purpose of this thesis, therefore, has been to propose a theoretical prediction of the flow patters which occur in a mass-flow silo, with no flow restricting device at the outlet, and expand this concept to enable the determination of the blending efficiency of a given silo by utilising linear systems theory. With a knowledge of the blending efficiency of a mass-flow silo known, the theory was then extended, by the application of feedback response transfer functions (in systems terminology), to allow the determination of the mixing and blending of bulk materials in mass-flow silos with hopper-in-hopper type inserts/with a belt feeder interface. With the importance of supporting inserts in silos, the current study has examined normal flow stresses existing in mass-slow silos, with and without hopper-in-hopper type inserts. With a knowledge of these stresses, and by nothing the differing flow patters in silos sue to eccentric type material discharge, basic design constraints of rectangular supports have been shown with reference to their structural integrity in supporting various insert types in material flow channels