Smelting of bauxite residue to form a soluble sodium aluminium silicate phase to recover alumina and soda

Optimum fluxing, smelting and leaching practices have been determined for maximising the recovery of alumina and soda, but not iron, from a slag produced by oxidative smelting of bauxite residue at 1400°C. The rate and extent of dissolution of alumina and soda increased in a water leach as the CaO/SiO₂ ratio in the slag was increased from 0·1 to 2 by the addition of calcium carbonate to the dried bauxite residue. A water leach at 60°C, extracted 55% of the Al and 90% of the Na at 20 wt-% solids and 50 and 75% respectively, at 50 wt-% solids. Liquors containing ∼43 g L^–1 Al and Na were obtained from leaches at 50 wt-% solids. By recycling this liquor in two further leaches on fresh slag at 50 wt-% solids, levels of 130 g L^–1 Al and 125 g L^–1 Na in solution were achieved. At these concentrations of alumina and soda, it is expected that the leach liquor should be suitable for recycling to the Bayer process. The dissolution of the alumina and soda from the slag was shown to be due to the leaching of a sodium aluminium silicate phase considered to be a member of the sodium aluminate–carnegieite solid solution series with a composition of (Na, Ca)_2–x(Al, Fe³⁺)_2–xSi_xO₄ (0·2≤x≤0·5). Quantitative analyses in an environmental SEM and the leaching results indicated that the solubility of this phase increased with a decrease in the silicon content of the phase as a result of addition of calcium carbonate to increase the CaO/SiO₂ ratio and form larnite (Ca₂SiO₄).