CHEMICAL AND BIOLOGICAL LEACHING OF ALUMINUM FROM RED MUD

Chemical and biological leaching of aluminum (Al) from red mud, the ma jor waste product of the alkaline extraction of Al from bauxite, was e xamined. Sulfuric, citric, and oxalic acids were employed individually or as mixtures in chemical leaching experiments. The highest concentr ation extracted was 13 530 mg of Al/L (96 % solubilization) using a 2: 1 ratio of citric and oxalic acids and subsequent H2SO4 addition to lo wer the pH to 1.5. Despite a lower concentration of extracted Al obtai ned with H2SO4 alone at pH 1.0 (12 140 mg/L), it may be more economica l due to the high price of organic acids. Biological leaching was carr ied out using sewage sludge bacteria (adapted indigenous thiobacilli) and pure strains of fungi: Aspergillus niger, Penicillum notatum, Peni cillum simplicissimum, and Trichoderma viride. All microorganisms were tested for acid-producing and -leaching capabilities in the,presence of increasing amounts of red mud. In thiobacilli cultures, 6265 mg of Al/L was solubilized in the presence of 5 % v/v red mud and 1 % w/v su lfur by recycling the leachate. P. simplicissimum was the most efficie nt of the fungal cultures; 1880 mg of Al/L (56 % solubilization) was s olubilized at 3 % v/v red mud initial concentration. These concentrati ons are not high enough to be applicable on an industrial scale. Howev er, 75 % of the Al (10585 mg of Al/L) of a 10 % v/v initial red mud co ncentration was solubilized using the acids produced by P. simplicissi mum. The high affinity of the acids produced by this fungi to leach Al from red mud is quite unusual when compared with pure citric acid. Le aching of Al from red mud appears feasible on an industrial scale usin g the addition of either sulfuric acid (chemical extraction) or biolog ically produced acids (microorganism-aided extraction).