Kyanite, Sillimanite and Andalusite all these three minerals convert to Mullite and Silica when they are calcined to temperatures between 1250 and 1500 degree C. The inversion of kyanite to mullite and silica (glass) begins at the periphery of grains and this rate of conversion depends the following:
Kyanite is one of the Alumino-Silicate group of minerals comprising Sillimanite, Kyanite, Andalusite, Dulmortierite, Topaz and Mullite all with similar chemical composition but different physical behavior and hence, uses. Kyanite, raw as well as calcined, have separate applications as refractory raw materials because of their distinct characters.
Hence, to be suitable for refractory raw material the kyanite should have very negligible amount of impurities which include free silica, alkali, iron oxides, calcium and magnesium.
(Raw Kyanite is used for making insulating refractory bricks and other insulating refractories, why?)
The heating of raw kyanite is accompanied with its volume expansion which ranges from 6 - 20 percent along with a decrease in its specific gravity from 3.6 to 3.06, which takes place over a small range of temperature around up to 1350 degree C. Because of this property -
In order to make it a volume stable refractory material, kyanite is pre-calcined at 1420 degree C to mullite and cristobalite before use. Sometimes the lumps of calcined kyanite are very hard to crush. To avoid this, after calcination the kyanite lumps are, sometimes, quenched in water to make them crumble easily. Thereafter, it is ground, graded into various fractions as per requirement. These grains of calcined kyanite being volume stable are used with other raw materials for making refractory bricks and castables.
Properly calcined kyanite is a very good refractory raw material because of its high alumina percentage and low iron contents. As compared to other refractory raw materials kyanite can be sometimes, very handy for boosting alumina content and other refractory properties of the product at the same time maintaining its cost effectiveness.
Refractory bricks made from calcined kyanite possess: