Physico-Chemical Features of the Synthesis of Mullite in Mixtures of Quartz-Pyrophyllite Rock with a Fluoride-Forming Component

It is established that the use of an ammonium fluoride treatment on quartz-pyrophyllite rock (45% pyrophyllite and 53% quartz) at 650°C changes the traditional mechanism by which mullite is synthesized; it is formed not from metapyrophyllite but from fluoridated pyrophyllite, with the formation of aluminum trifluoride and topaz being intermediate stages of the synthesis process. The optimum ratio of quartzpyrophyllite rock to ammonium hydrodifluoride is 1:1.6, which at 1300°C increases the yield of acicular mullite (with particles 20 – 30 μm long and 3 – 5 μm in cross section) from 12 to 68 wt.%. The content of residual quartz in the mullite is no greater than 15 – 18 wt.%.