IMPROVED PROCESS AND APPARATUS FOR DIGESTION OF BAUXITE

An improved process for extracting monohydrate and trihydrate alumina from bauxite containing both forms of alumina. The slurry of bauxite is subjected to a first digestion in a first portion of spent liquor at conditions sufficient to extract the trihydrate. The liquor containing the dissolved trihydrate is separated from the slurry under conditions of temperature and pressure that are similar to those existing in the trihydrate digestion. The slurry of undissolved solids is then subjected to a second digestion in a second portion of spent liquor under conditions sufficient to dissolve the monohydrate. To retain the free caustic content of the liquor passing to the high temperature second digestion at a level below a point at which it will severely corrode the high temperature heaters, optionally a portion of suitable liquor or slurry of low free caustic concentration is added to this liquor to reduce its free caustic concentration just before it is passed through the high temperature heaters. The invention makes it possible to simply and easily retrofit a double digestion process into an existing single digest circuit.