A method to concentrate boehmite in bauxite by dissolution of gibbsite and iron oxides

Some bauxites contain boehmite which is not dissolved by lower temperature Bayer extraction and which may cause precipitation of additional boehmite and an associated reduced recovery of alumina. For the mineralogical and morphological characterisation of boehmite in bauxite, it is necessary to concentrate boehmite by dissolving gibbsite and iron oxides while preserving boehmite intact. There is no standard method to concentrate boehmite from bauxite without affecting the properties of boehmite. Therefore we developed a laboratory method to concentrate the boehmite in diverse bauxites using “one standard procedure”. Five molar NaOH containing dissolved Al(III) was used to dissolve gibbsite and kaolin in bauxite by shaking at 80 °C for 48 h. A mass balance calculation and mineralogical analysis by XRD showed that gibbsite and kaolin were completely dissolved and boehmite and some other minerals (anatase, quartz) were unaffected. Boehmite was further concentrated by removing iron oxides from the caustic digest residues by the reduction–complexation dithionite–citrate–bicarbonate method. Sand size quartz was removed by sedimentation. After these pre-treatments boehmite was sufficiently concentrated for detailed analysis by XRD and electron microscopy. Results for bauxites from Guinea, Jamaica, Queensland (Australia) and Western Australia are presented to illustrate the effectiveness of this procedure.