They are calcined to sufficiently high temperature in order to convert them to into volume-stable materials. Calcination of these raw materials prevents the refractory bricks and monolithics made out of these calcined refractory raw materials, from much volume change, whether shrinkage or expansion during final firing of these bricks and refractory shapes which in turn helps achieving better properties like - higher density, low porosity, minimum variation in dimension and so on.
The calcination refractory raw materials can be done in an intermittent Down Draft (DD) or Chamber kiln or Shaft kiln or Rotary kiln. Calcination of such raw materials in small quantities can be done in any other batch type kiln also.
'Calcination’ is the heat treatment below the melting point to which a material is subjected, preparatory to further processing or use, for the purpose of eliminating chemically combined volatile constituents and to ensure stability through high temperature volume changes. In refractory, calcination is done with an intention to produce physical and chemical changes in certain refractory raw materials e.g. Bauxite, Kyanite, Magnesite, Fireclay, etc..
The best calcination in terms of quality consistency and time efficiency is possible in a rotary kiln. But calcination in a rotary kiln is the most expensive and also the investment for its installation is quite high. However, for the materials like Kyanite, which require long soaking at calcination temperature for achieving full conversion, the down draft kilns, chamber or other batch type kilns may be the most suitable one. Almost all refractory raw materials can be calcined in a rotary kiln although the loss of material for a softer raw material like fireclay will be higher in that kiln. In case of high temperature calcination as required for magnesite, which is also done in shaft kilns, but again rotary kiln is the best. The calcination kyanite and bauxite can be carried out up to their highest grain-density, as required for the best quality of refractory bricks and monolithics, in a rotary kiln due to the high temperature at which it can operate. Shaft kiln is most suitable for non-plastic flint type clays although it may be used for other refractory raw materials too.
Fig: Rotary Kiln of a Lime Calcination Plant