Sillimanite as a natural and untreated mineral is a very important refractory raw material for high alumina refractories which are extensively used in Iron and steel, Petrochemical, Electrical, Cement, Zinc and Glass industries.
Sillimanite when heated above 1545 degree Celsius converts to Mullite and the excess silica as glass, crystoballite or tridyamite. The formation of the glassy phase can be reduced by addition of a small percentage of technical or calcined pure alumina fines which reacts with this excess silica to form mullite, which in turn help in enhancing the quality of the product.
Another benefit of sillimanite is because of its very low expansion or contraction on heating, sillimanite does not need any pre-calcination to be used as raw material in refractories. Sillimanite from most of the sources in the World can be used as-it-is without any pre-treatment. However, few like the Rewa sillimanite (found in Madhya Pradesh, India), because of its impurities, should not be used as such. Sillimanites containing loose impurities
Particularly in Glass industry sillimanite refractories have got many applications, such as in glass melting tank furnaces (to be discussed in detail in a separate article) in all parts open to the products of combustion like combustion chambers, flues, door pillars which may have to support heavy load at high temperatures, recuperators and such other parts which are liable to be subjected to fluctuations of temperature. Many years ago pure sillimanite for example, Assam sillimanite of India once available in good quality and quantity, even for export, were used to be cut into blocks of various sizes from solid rock at the site of deposits itself which were then sent to the user’s site for their direct use in construction of Glass Melting Tank Furnace bottom. But now it is a forgotten past!
Sillimanite refractories are characterized of high refractoriness, very low coefficient of thermal expansion, high refractoriness under load (RUL) and mechanical strength with great resistance to thermal shock (spalling resistance), abrasion and slag corrosion. Due to their exceptionally high resistance to spalling and corrosive actions of molten glass, chemical attacks of soda, borax and other frits, they are most suitable for Glass Melting Furnace (GMT); Oil Fired Furnace, Cement Rotary Kiln, Blast Furnace, Electric Arc Furnace (EAF) roofs, Hot Metal Mixer, Combustion Chambers and Metallurgical operations done in Zinc Furnace, Gold Refining Furnace etc.
are always better to wash these lumps once in the raw material yard itself and then after crushing, grinding it should be passed through magnetic separator to eliminate the free iron impurities.
The ideal firing temperature of green bricks made of sillimanite grains as a major raw material is 1450 - 1500OC, to be fired either in a batch type or in a tunnel kiln. The soaking time will vary depending upon the volume, shape, stacking and other constituents (particularly raw clay or any sintering aid, if used at all) of the refractory bricks.
However, there are quite a few refractory suppliers in India who manufacture these GT blocks, mostly using certain percentage of sillimanite sand or even raw sillimanite lumps after crushing and grinding. So far the properties like density (BD), porosity, mechanical strength, slag corrosion resistance and consistency of performance etc. are concerned, high capacity machine pressed refractory bricks are far too superior to those made by pneumatic ramming. Although it is a general practice to give some ‘patching/finishing’ to particularly big and complicated shaped refractory bricks manually before their inspection-dispatch but from the customers’ point of view it is most important that during inspection it must be ensured that except for the ‘look’ only, the refractory bricks (shapes) do not depend much on the ‘finishing’, if at all, done on it. Patching is a wrong practice as it is done to camouflage some flaws in the refractory bricks which could be detected even at the first stage of inspection.
For GT blocks (Glass Tank Blocks) machine finish of the surface is very important. It must be evenly polished or ground to ensure that the warpage is negligible. One of the main criteria for acceptance of these refractory blocks should be that in the assembly there should not be any open joint (the specification could be from 0.2mm - 0.3mm filler gauge up to a maximum 20 - 25mm depth from the top). This is a must to avoid the penetration and subsequent crystallization of glass and alkali vapours in these joints. To meet this criterion the manufacturer of these refractory blocks should have facilities for grinding of these blocks minimum in 4 faces and in some bricks up to 6 faces and then marked accordingly.