Desilication Reactions at Digestion Conditions: An in Situ X-ray Diffraction Study

The reactions of silica minerals at high temperature Bayer digestion conditions have been investigated using an in situ X-ray diffraction technique. Rapid precipitation of sodalite was observed following the addition of kaolin or quartz to Bayer liquor at 250 degrees C. The addition of calcium with kaolin/quartz resulted in the formation of calcium cancrinite. A reduction in the rate of both sodalite and calcium cancrinite formation was observed on decreasing the temperature in the range 180-250 degrees C. Seeding with sodalite/calcium cancrinite solids did not noticeably influence the reaction of quartz at 250 degrees C, but a slight increase in the formation of calcium cancrinite in the presence of calcium cancrinite seed was observed at 220 degrees C. The direct transformation of sodalite to calcium cancrinite was also investigated in the range 200-250 degrees C. The reaction proceeds quickly in the presence of calcium and hydrogarnet and is unaffected by the presence of titanium.