Nano-milling of the sillimanite mineral, kyanite, and its reactions with alumina to form mullite

The sillimanite group of minerals, kyanite, andalusite and sillimanite, the three polymorphs of Al₂SiO₄, is a natural raw material for the production of mullite in many traditional ceramics. However, the sillimanites form at high pressures and are alumina-deficient relative to mullite. This paper addresses the effects of attrition milling kyanite, the most common of the three to the nanoscale. Additions of aluminum metal, that is then oxidized to alumina and an independent addition of pure alumina to yield a phase-pure mullite after firing, are also described. The attrition milling size reduction of natural kyanite to the nanoscale has several pronounced beneficial effects. It reduces the phase decomposition temperature, it eliminates the large volume expansion of the decomposition and it produces a highly reactive form of the rejected silica. When fired with alumina additions adjusted to the mullite stoichiometry, the resulting phase-pure mullite is a high density, fine grain size ceramic body.