Sillimanite (Al2 SiO2) forms mullite (3Al2O3. 2SiO2), on heating and the conversion to mullite is without any appreciable volumetric changes; the maximum expansion by volume is 4%. Mullite is a stable product up to a temperature of 1810° C. One outstanding feature of sillimanite is that the volume of porosity of grains hardly exceeds 6% after heating. It has many other valuable properties that distinguish it as a super duty refractory material. Sillimanite is used in refractory industry alone. The user industries prescribe the following two graders for specifications:
The above specifications were for lumpy sillimanite. The sub-committee on refractory raw material of the development panel on refractories appointed by DGTD in their report dated August 1985 has suggested the following specifications.
** IBEL, however, strive to produce sillimanite concentrate of 60% Al2O3. Physical Characteristics: a) Lumpy Ore: Mineral should not be below 10mm in size. It should be free from micaceous and other impurities such as garnet, tourmaline, etc. b) (i) Sand grains should be free from any coating material, as bricks made from coated material are prone to spelling. (ii) Size fraction in microns may represent to the following percentage:
Future trends: Beach sands contain appreciable amount of sillimanite grains. It is, however, known that some refractory plants in India have been using beach sand sillimanite produced by IRLE in the manufacture of high-alumina bricks. The performance of these bricks vis-à-vis with those made out of lumpy sillimanite is not known. Recent survey made by IBM reflected that Indian refractory makers could utilize beach sand sillimanite to the tune of 15,000 to 20,000 tpa. it regular supplier are assured. With the gradual depletion of massive sillimanite deposits, the use of granular sillimanite is likely to increase in future. Beneficiation of low-grade quartz. Sillimanite rock may also receive attention. The following end-use classification of reserves has been reported in the mineral inventory as on 01.01.1985.
The following end use grades for the purpose of classification of reserves of sillimanite are prevalent. Refractory Grade:
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