Purification of corundum by calcination under controlled reducing conditions

The alkaline impurities detract from the mechanical properties of corundum materials, specially at high temperatures. So it is important that they are minimized. This paper describes a new purification method of aluminas by calcination at high temperatures (>l200°C) in controlled reducing atmospheres (PO2∼10−15 atm). Under these conditions, alkaline impurities are removed by reduction to volatile species. Over 95% of Na2O existing in the alumina can be removed in this way. SiO2 and Fe2O3 can also be removed with these treatments.