Process for extracting alumina from alumna-containing ores

Abstract

In a process for extracting alumina from alumina-containing ores by individually preheating a slurry of alumina-containing ores in alkali solution, and an alkali solution, and feeding the preheated slurry and the preheated alkali solution to a digester maintained at a temperature of about 200.degree. to about 300.degree. C, thereby extracting alumina from the alumina-containing ore. The alumina extraction is carried out without any formation of scales on the inside surface of the preheater with a considerably prolonged operating duration by preparing a slurry having a molar ratio of Na.sub.2 O/Al.sub.2 O.sub.3 of not more than 1 from the alumina-containing ores and an alkali solution, subjecting the slurry to preliminary desilication treatment, if necessary, then adding an alkali material to the slurry while preheating the slurry before the slurry reaches a temperature of forming scales originating from deposition of boehmite-type alumina from the slurry, thereby obtaining the slurry having an adjusted molar ratio of Na.sub.2 O/Al.sub.2 O.sub.3 of more than 1, further preheating the slurry having the adjusted molar ratio to a temperature higher than that of forming scales originating from deposition of boehmite-type alumina from the slurry before the adjustment of the molar ratio, and then extracting the alumina from the alumina-containing ores in the slurry admixed with a separately preheated alkali solution in the digester.