Mechanochemistry and the Bayer process of alumina production

The concept of mechanical activation has been applied to the traditional Bayer process of alumina production. The basic idea has been to achieve moderation in process conditions, reduce alumina and soda losses in the red mud, and alter the rheological character of red mud to improve its settling behavior. The research pursued has primarily focused on: (a) the particle breakage and structural changes in bauxite due to attrition milling; and (b) alumina recovery in simultaneous milling and leaching experiments. Particle breakage dominates during the initial stage (3-5 min) of the milling. Both XRD and TEM studies have indicated that the gibbsite present in the bauxite undergoes structural changes due to the milling. In typical 'simultaneous milling and leaching' experiments, it is found that almost all the gibbsitic alumina can be dissolved in alkali at 90oC after 15 min. Unlike the plant red mud, that was found to contain gibbsite as the dominant mineral, the solid residue produced in the laboratory experiments contained hematite as the dominant mineral.