Process for the preparation of pure aluminum oxide

Process for the preparation of pure aluminum oxide from clay or alumina containing minerals which comprises the steps of:

• mechanically pretreating the clay by applying sufficient shearing stress thereto to provide clay having a particle size of 2-15 mm;
• drying the mechanically pretreated clay at a temperature of 130.degree.-250.degree. C. to a residual moisture content not exceeding 3%;
• thermally actuating the mechanically pretreated clay at a temperature of 600.degree.-800.degree. C.;
• subjecting the thermally activated clay to leaching with hydrochloric acid having a concentration of 15-25%, wherein the hydrochloric acid is used in an amount of 80-120% of the stoichiometric amount calculated on Al.sub.2 O.sub.3 and the temperature for the leaching is 105.degree.-110.degree. C.;
• separating the SiO.sub.2 containing residue from the AlCl.sub.3 solution formed in the leaching;
• subjecting the SiO.sub.2 containing residue to washing to separately recover the washed SiO.sub.2 ;
• extracting the AlCl.sub.3 solution with an extraction agent consisting of a higher paraffin hydrocarbon for removing iron therefrom, said extraction agent being used in a ratio with respect to the AlCl.sub.3 containing solution of 1:1 to 1:5;
• subjecting the organic phase containing iron to extraction with water;
• subjecting the now iron free aluminum chloride solution to evaporation to form a solution containing about 30 wt % of AlCl.sub.3 ;
• thereafter subjecting the thusly evaporated solution to crystallization and recovering the crystals thus formed;
• subjecting the recovered crystals to a two stage thermal decomposition, the first stage being carried out at 150.degree.-200.degree. C. and the second stage at 1200.degree.-1300.degree. C.;
• recovering the pure .alpha. alumina thereby formed.