A novel experimental approach to high temperature particle-particle interactions
Interactions at elevated temperatures between solid particles occur in a wide range of industrial processes, for instance, in the filtering of hot gases, in the drying of pharmaceutical granules, in the curing of ceramics, in the combustion of solid fuels and regeneration of nuclear waste. Often these interactions can cause major problems in the operation of such processes. For example, it is well established that the fluidisation behaviour of certain powders is significantly affected by the presence of strong interparticle forces that, in turn, are the cause of both agglomerate formation and possible operative problems within the reactor. On the other hand, not much is known about the mechanisms of agglomeration, other than that it is mainly due to interfacial phenomena. High temperature adhesion forces arise from the formation of material bridges, usually due to the particle surfaces changing phase through either chemical reaction or simply melting. Moreover, thin liquid layers of sticky material, which may be present on particle surfaces, such as during reactive coating or drying processes, may also enhance strong interparticle bonds leading to solidification. It is obvious that for the reliable operation of high-temperature processes a good under standing of the fundamental mechanisms of adhesion and cohesion between particles at elevated temperatures is required. Here, adhesion is meant as the force that holds the particles together, after which they exhibit cohesive behaviour. Unfortunately, the level of understanding has been hampered by the lack of techniques available to observe and measure such interactions. However, recent developments in microscopic analysis techniques now mean that high temperature particle interactions can be studied directly, which will lead to the development of new predictive models. The main aim of the work described in this thesis is to provide experimental evidence and justification of particle-particle interactions at high temperature and to deliver original insights of the underlying mechanisms, which play a fundamental role in particulate cohesion in relation to fluidisation process. In order to fulfill this goal, a novel device, termed a High Temperature Micro-Force Balance, has been designed and developed which combines force and direct observation measurements operated through an adapted micro manipulator technique. The flexibility of use for the HTMFB represents its strongest design advantage, permit ting experimental investigations over different types of particle interactions at a small scale (crystallization of liquid binders, reactive coating, sintering and composite materials interactions). Results reported in this work provide an original contribution towards academic and industrial understanding of the micro-scale mechanisms of agglomeration and material properties at different operative conditions.