Bauxite versus diaspore clay in high alumina (70%) refractory brick

The last twenty five years have seen a remarkable change in the character and types of refractories for furnace construction. Not much more than a few years ago, the production of refractories consisted almost solely of fireclay and silica shapes. A few high alumina bricks were made from Missouri diaspore clays or from bauxite and fire clays. At a later date, although little used, high alumina refractories were available which were made from alumina of a higher degree of purity, by the fusion process. Today refractories covering the entire range of composition from about 98% SiO2 to 99 1/2% Al2 03 , except for a short interval from about 4 to 12% Al2 03 , are finding wide commercial applications. High alumina refractories are those having an alumina content of 50% or higher with the remainder being essentially silica, and are arbitrarily divided into three groups depending upon alumina content. They are 50%, 60%, and 70% Al2 03 refractory bricks. Fused alumina refractories are grouped separately. To manufacture such high alumina refractories, raw materials high in alumina content such as diaspore clay and bauxite are mainly used. Naturally occurring alumina-silicate minerals like sillimanite, kyanite, and andalusite are also used for manufacturing high-alumina refractories or mullite refractories. In this country, diaspore clay, since its discovery has remained the chief source of raw material of high alumina content. The deposits of diaspore clay of economically workable size are located only in the South Central Missouri district. This area being the only commercial source of diaspore clay, the refractory industries are alive to the fact that, at the present rate of consumption of diaspore clay, this source will be exhausted within a comparative short time. Greaves Walker had pointed out this situation back in 1928, and suggested that other raw materials such as bauxite, kyanite, Georgia Kaolin may be substituted for diaspore clay. As a result of this situation, the use of bauxite for manufacturing high alumina refractories is being seriously considered by the refractories industry. In the last few years the importation of gibbsite from Dutch, French or British Guiana, has supplied a very high grade of refractory material to make high alumina refractories in place of diaspore clay. Much effort is made now days to utilize this material. Very recently some refractory manufacturers who have used Dutch Guiana gibbsite for making high alumina refractories, have experienced strange behaviors of such refractories toward reheat. They observe that their refractories show appreciable expansion when subjected to reheat tests. This is quite contrary to the general belief at present that bauxite refractories show continuous shrinkage on repeated firings. In light of such development this investigation was undertaken. This investigation is a comparative study of bauxite and diaspore clay when used to manufacture high alumina (70%) refractories. High alumina grog is prepared by calcining these raw materials at various temperatures, and bricks of 70% Al2 03 were made using this grog with plastic fireclay as the bond. These bricks are then subjected to various reheat temperatures and all changes in volume stability of the bricks are recorded. Finally an effort is made to explain the cause or causes of such expansion changes by x-ray diffraction investigation, and by other methods --Introduction, pages 1-3