Alumina monohydrate is produced by leaching calcined clay and subjecting the leach liquor to pressure hydrolysis wherein the vapor and heating resulting from the pressure hydrolysis are directly utilized for leaching of calcined clay. The aluminum monohydrate may be roasted to yield alumina. Thus, two autoclaves, a-1 (the pressure hydrolysis vessel) and a-2 (the leaching vessel) were connected by a valved transfer line. 128 G of al(no3)3.9H2o in 222 g of h2o were heated in a-1 for 3 h at 310 deg c. Calcined clay and water were placed in a-2. With the valve between the autoclaves closed, the pressue in a-1 rose to 1,500 psi. After heating a-2 to 100 deg c and 10 psi pressure, the valve was opened, whereupon the volatile components in a-1, primarily hno3, flashed into a-2. The crystalline material removed from a-1 was identified as alooh, alumina monohydrate. Essentially complete recovery was obtained and no nitrogen was detected in the product. The slurry in a-2 was filtered and the residue washed. 86 pct of the aluminum in the calcined clay was recovered as al(no3)3.9H2o. The pressure hydrolysis stage may be conducted at temperatures ranging from 250 deg to 350 deg c and pressures from approximately 500 to 2,500 psi. The time will vary with temperature, pressure, and solution concentration, but generally will be in the range of 1-6 h.