Process for extracting alumina from aluminous ores

In a process for extracting alumina from aluminous ores containing mixtures of alumina trihydrate and alumina monohydrate which comprises the steps of:

• (a) a first-stage digestion by introducing a slurry of said ores and a portion of caustic solution into a tubular preheater, preheating individually the slurry and the other portion of caustic solution each in preheaters according to a two-stream system, mixing the preheated slurry with the preheated caustic solution, digesting alumina trihydrate from the ores and separating the slurry resulting after digestion of trihydrate into a caustic aluminate solution and an insoluble mud containing alumina monohydrate, and (b) a second-stage digestion by preparing a slurry of said insoluble mud containing alumina monohydrate separated in the first-stage digestion and a caustic solution, preheating the resulting slurry and digesting alumina monohydrate from the preheated slurry, the improvement wherein a desilication reaction zone is established in the first-stage digestion at a location following the inlet portion of the tubular preheater into which the slurry of said ores and a portion of caustic solution is introduced and before the step of separating the said slurry into caustic aluminate solution and insoluble mud, and carrying out the desilication reaction in said desilication reaction zone at a temperature of from about 130.degree. C. to about 180.degree. C. at a rentention time in said desilication reaction zone of from 20 to 120 minutes so that at least 50 weight % of reactive silica in the ores is converted into or deposited as a high-temperature desilication reaction product.