Process for extracting alumina from aluminous ores

Abstract

In the production of alumina from aluminous ores containing alumina trihydrate and alumina monohydrate comprising a first-stage digestion by digesting alumina trihydrate from a slurry of the ores and a caustic solution and separating the resulting slurry into a caustic aluminate solution and an insoluble residue, and a second-stage digestion by digesting alumina monohydrate from a slurry of the insoluble residue separated in the first-stage digestion and a caustic solution, the present invention provides an improvement wherein scale formation onto a preheater for the slurry in the second-stage digestion is completely or substantially prevented and reduction in heat transfer coefficient of heat transfer surface is remarkably lowered. The improvement is attained by setting up a desilication reaction zone in the course of the first-stage digestion, whereby the slurry introduced into said zone is subjected to desilication reaction treatment.