Über die Kieselsäure im Bayer-Prozess

Abstract

It could be shown that within the extreme alkaline Bayer conditions with temperatures of at least 110 °C, the combined silica in bauxite always reacts to a fairly constant sodium-alumosilicate composition. This silicate has a sodalite structure which, with increased soda content (Na₂CO₃) of the aluminate liquor, expands and finally distorts into the so- called "basic" cancrinite configuration. The formation of this silicate which influences the economical aspect of the Bayer-Process always takes place with the digestion of hydrargillite. As in the case of bauxite, the main silica content is combined as kaolinite; the insoluble binding of alumina and alkali on this silica, in consequence of the Bayer-Process, is thus unavoidable and correspondingly the production costs of alumina increases. In accordance with digestion conditions, an alkali loss of 0.6 to 0.7 g Na₂O and about 1 g Al₂O₃ of non-digestible alumina must be reckoned within each gram of combined silica in bauxite. Cutting out these losses, in other words, the formation of the silicate, is only possible under conditions which are no longer economically justified.