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SILLMANITE MINERALS
G. DIXON and H. JAMERSON, Kyanite Mining Corp.
The sillimanite-group of minerals include andalusite, kyanite and sillimanite. They are naturally occurring polymers of Al2O^sub 3^ SiO^sub 2^. These aluminum silicates are primarily used as raw materials in the manufacture of refractory products used by the steel and aluminum industries.
Each polymer, however, has a unique crystalline structure that yields slightly varying physical properties. A high, hot-strength phase mullite, 3Al2O^sub 3^ 2SiO^sub 2^ can be produced by calcining naturally occurring sillimanitegroup minerals, especially kyanite.
Uses and production
Sillimanite-group minerals are efficient materials for refractory use because they share the property of conversion (at high temperatures) to mullite and free quartz. The main performance criterion for alumina silicates in refractory applications is the percentage of mullite that is formed in the material during firing. These minerals, therefore, are used for higher performance refractory applications than fire clays or silicas. They offer the best tradeoff available in terms of price paid for a given amount of alumina content.
Refractory uses account for about 90% of world consumption of sillimanite-group minerals. Coarser mesh products are used in high-temperature motors and cements, ramming mixes, castables, kiln furniture, insulating brick and firebrick.
Finer material has greater use in ceramic applications, including sanitary ware, tiles and investment casting moulds.
Andalusite is used largely as a refractory aggregate in its uncalcined form. It has a low coefficient for thermal expansion. Kyanite, on the...