Raw Materials

Abstract

Mullite composed of 72% Al2O3 and 28% SiO2 finds ever-increasing application in high-temperature technologies; mullite-based ceramics display low heat conductivity and high strength and are in widespread use as heat-insulating and refractory materials. For all its superior thermophysical properties and large natural mineral reserves, mullite production in Russia is still in the early stage of development. The demands of the domestic mullite market are at present mainly satisfied by the supply of ready-made mullite refractories purchased from foreign manufacturers [1, 2]. In Russia, a few manufacturers are concerned with the production of mullite-based refractory materials. Mullite materials are available from Ogneupory Joint-Stock Co. (Tula Region) and Semiluki Refractory Plant Joint-Stock Co. (Semiluki, Voronezh Region); however, they contain less than 62% Al2O3 and their refractoriness is much inferior to that of pure mullite materials. The best raw materials for production of mullite are quartz-topaz ores. In Siberia, the world’s largest deposits of quartz-topaz ores have been discovered with an overall topaz content of 200 million tons. The domestic market demand for topaz is about 300 thousand tons. The precursor concentrate obtained by ore flotation contains some 40% aluminum oxide, 45% silicon dioxide, and 10% hydroxyl fluoride compounds. To extract mullite, part of the silicon dioxide and fluorine should be removed. The processing of quartz-topaz ore includes two major stages: (i) flotation beneficiation in the topaz component and (ii) desiliconization and sintering of the desiliconized concentrate to a mullite condition. A schematic of the process and the averaged composition of initial, intermediate, and final products are shown in Fig. 1. Desiliconization and sintering (mullitization) of the desiliconized semi-finished product are critical and powerconsuming stages of the quartz-topaz-to-mullite conversion. For removing the excess SiO2, aluminum fluoride was used; this interacts with SiO2 and converts it to ammonium hexafluorosilicate [3]. An advantage of ammonium fluoride as a desiliconizing agent is that it can be completely recovered [4]. The cycle of fluoroammonium desiliconization is shown below: Refractories and Industrial Ceramics Vol. 47, No. 3, 2006