The Bayer digestion behaviour of transition aluminas formed from roasted gibbsite

Bauxite with reactive silica content greater than ~ 8% cannot be economically processed by the Bayer process. Roast-leach technology (sometimes called thermo-chemical processing) is potentially one of the options for removal of silica from bauxite using a thermal pre-treatment and leach step. However, the thermal pre-treatment transforms aluminium oxides in both kaolinite and valuable phases (gibbsite and boehmite) to transition aluminas which are difficult to extract using normal Bayer digestion. To better understand and solve this problem, we simplified the roast-leach process by roasting pure gibbsite and digesting the resultant transition aluminas. We investigated the relationship between roast temperature, dehydration extent, transition aluminas and Bayer extraction behaviour. We found that transition aluminas with thermodynamically relatively stable structures and better crystallinity were the cause of extraction difficulty. Extraction can be enhanced using higher caustic concentration and digestion temperature although these measures are not preferred in reality. The practical and effective method is perhaps the counter current double digestion which can extract all transition aluminas and produce pregnant liquor with desired aluminate concentration.