Substantially pure alumina is recovered from kaolinitic clay by leaching calcined kaolinitic clay with at least 36-wt pct hydrochloric acid to dissolve the impurities and convert the aluminum to insoluble aluminum chloride while maintaining the hydrochloric acid concentration in the leach liquor at at least 36 weight percent by the injection of hcl gas. The insoluble aluminum chloride is separated from the dissolved impurities and subjected to a second leaching step to solubilize and separate it from the silica gangue. In an alternate embodiment, the process may be practiced on uncalcined clay and includes the addition of a source of fluoride ions, such as h2sif6 to the at least 36-wt pct hydrochloric acid leach liquor. An optional preleaching step utilizing 2n to 6n mineral acid is also disclosed. Thus, 50 g of calcined kaolinitic clay was leached with 150 ml of 36-wt pct hcl for 2 h at 90 deg c, during which time hcl gas was injected into the leach solution to maintain the concentration of hcl at at least 36 pct. The slurry was cooled to room temperature while injection of hcl gas was continued and then was filtered and the filter cake was washed with 36-pct hcl. The filter cake was washed with 200 ml of hot water for 1 h and filtered again. The resulting solution contained most of the aluminum in the clay but little of the iron, phosphorus, potassium, calcium, and magnesium impurities, while the final filter cake was mainly silica with only traces of aluminum.
U.S. Pat. 4,239,735; Dec. 16, 1980; Chem. Abstr. Not Found