Assessment of intensive grinding effects on alumina as refractory compound: Acceleration of gamma to alpha phase transformation mechanism

Abstract

In this study, the feasibility of alumina phase transition enhancement by mechanical activation was conducted. It was showed that the milling environment plays an important role on the physical, chemical and thermal behavior of the alumina powder utilized as refractory component material. The aim of the investigation was to increase the reactivity of the starting gamma-Al2O3 by mechanical treatment in two types of high-energy activators - vibratory disc mill and vibratory ball mill. In continuation, the decrease of the subsequent sintering temperature as well as the treatment duration would be induced by making the transition into final thermo-stable alpha-Al2O3 modification easier and faster. Full factorial experiment was conducted and the results were analyzed by the proposed mathematical model in order to understand the effects of the activation process variables on the amount and physical characteristics of the synthesized (activated and subsequently thermally treated) product and to establish the optimal activation period. As the result of the analysis, operation parameters of the activator and activation period were found to be the most important factors. The initial gamma-Al2O3 and synthesized alpha-Al2O3 were thoroughly analyzed by DTA, XRD, IR and SEM methods. Thermal behavior of gamma and alpha-modification were studied by differential thermal analysis conducted in the same environment, under same heating rates. X-ray diffraction analysis gave reliable identification of the crystal phases and changes in crystallinity of treated alumina. Based on XRD peak intensity measurements, the gamma-Al2O3 almost completely passed (95%) into alpha-Al2O3 after 180 min of activation in vibratory ball mill and subsequent thermal treatment (2 h/1200 degrees C). SEM microphotographs with accompanying image analysis PC program highlighted changes in size and shape of particles of initial and synthesized Al2O3. Synthesized Al2O3 exquisite thermal behavior characteristic for refractory compounds, demonstrated that it is possible to obtain alpha-alumina at lower transformation temperatures in shorter time intervals by applying mechanical activation.