Abstract:Bauxite residue is the solid waste generated in the alumina production process from bauxite ores, and it is also a low-grade alumina resource. Alumina is difficult to be directly extracted through alkali digestion process due to the high content of silica and low alumina-to-silica ratio. Phosphoric acid was introduced in this study to achieve the selective dissolution of silica and separation of alumina against silica in the non-magnetic material derived from reductive roasting and magnetic separation process of bauxite residue. Under the optimum conditions including phosphoric acid concentration of 1.0 mol/L, liquid to solid ratio of 15 mL/g, leaching temperature of 50 ℃, and leaching time of 60 min, the leaching rate of silica is approximately 82%, and the alumina-to-silica ratio of the leached residue is 4.4, enriched about 2 times compared with the non-magnetic material. Subsequently the acidic leached residue was further subject to alkali digestion process with NaOH solution at 35% at leaching temperature of 260 ℃. The leaching ratio of Al2O3 is 93%, which is only 69% in the non-magnetic material before desiliconization. The selective separation of silica in non-magnetic material through phosphoric acid leaching process is owing to the dissolution of silica in aluminosilicate minerals in the acid solution whereas the aluminum-contained components is transformed into the insoluble "AlPO4" precipitate after reacting with phosphoric acid.