A Novel Experimental Study of Temperature Enhanced CohesiveInterparticle Forces

摘要

Interactions at elevated temperatures between solid particles occur in a wide range ofindustrial processes, for instance, in the filtering of hot gases, in the drying of pharmaceuticalgranules, in the curing of ceramics, in the combustion of solid fuels and regeneration ofnuclear waste.Often these interactions can cause major problems in the operation of such processes. Forexample, it is well established that the fluidisation behaviour of certain powders issignificantly affected by the presence of strong interparticle forces that, in turn, are the causeof both agglomerate formation and possible operative problems within the reactor. On theother hand, not much is known about the mechanisms of agglomeration, other than that it ismainly due to interfacial phenomena. High temperature adhesion forces arise from theformation of material bridges, usually due to the particle surfaces changing phase througheither 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 dryingprocesses, may also enhance strong interparticle bonds leading to solidification.It is obvious that for the reliable operation of high-temperature processes a goodunderstanding of the fundamental mechanisms of adhesion and cohesion between particles atelevated temperatures is required. Here, adhesion is meant as the force that holds the particlestogether, after which they exhibit cohesive behaviour. Unfortunately, the level ofunderstanding has been hampered by the lack of techniques available to observe and measuresuch interactions.In order to fulfil this technology gap, a novel device, termed a High Temperature Micro-ForceBalance, has been designed and developed which combines force and direct observationmeasurements operated through an adapted micromanipulator technique. The flexibility ofuse for the HTMFB represents its strongest design advantage, permitting experimentalinvestigations over different types of particle interactions at a small scale (e.g. crystallizationof liquid binders, reactive coatings, sintering and composite material interactions).In this work a new approach is presented in order to determine the fundamental mechanismsbehind the formation of agglomerates of uranium oxide in the thermal denitration processduring nuclear fuel reprocessing. A new diagram shows results where the dependency of therupture force for a material bridge (here Magnesium Nitrate Hexahydrate is used as a simulantof Uranyl Nitrate) is plotted when the initial liquid bridge volume and temperature are varied.The results provide an original contribution towards academic and industrial understanding ofthe micro scale mechanisms of agglomeration and material properties at different operativeconditions.