Industrial waste silica-alumina gel recycling: Low-temperature synthesis of mullite whiskers for mass production

Abstract

The increasing demand for mullite whisker-reinforced, toughened ceramic materials and mullite raw materials that meet industrial requirements has prompted the search for new and alternative sources, as well as effective technologies to obtain the target products. In this work, mullite whiskers of high purity were synthesized by a vapor-liquid–solid (V-L-S) process using industrial waste silica-alumina gel and Al2(SO4)3·18H2O as raw materials, with AlF3·3H2O and Na2SO4 as additives. The effects of sintering temperatures on the mullitization reactions and mullite morphology were investigated by XRD, TG-DTA, SEM and so forth. The results suggest that the introduction of AlF3·3H2O and Na2SO4 alters the mullitization reaction path, which leads to an initial mullitization reaction temperature of 720 °C. The SEM results demonstrate that mullite whiskers transformed from secondary growth to anisotropic growth when the sintering temperature was increased from 720 °C to 825 °C. By analyzing the experimental results, the mechanism of AlF3·3H2O-assisted growth of mullite whiskers with Na2SO4 as the liquid phase template is proposed based on the “dissolution-precipitation” process. Herein, a novel and feasible solution for the recycling of silica-rich industrial waste is proposed, which offers new and simple insights into the high value-added recycling of industrial waste, which provides new ideas for the actual mass production of mullite whiskers.