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 1400uC. The rate and extent of dissolution of alumina and soda increased in a water leach as the CaO/SiO 2 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 60uC, 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 y43 g L 21 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 21 Al and 125 g L 21 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) 22x (Al,Fe 3z ) 22x Si x O 4 (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/SiO 2 ratio and form larnite (Ca 2 SiO 4 ).
It
is shown that two water-soluble redox catalysts, namely methyl
viologen dichloride and diquat dibromide, can accelerate the rate
of the oxygen reduction reaction on metallic iron in aqueous solutions.
It is also shown that the same catalysts can speed the removal of
metallic iron from reduced ilmenite in the aeration step of the Becher
process. Under industrial conditions (2% ammonium chloride and 70
°C) aeration times can be decreased from 10 h to less than 3
h without irreversible adsorption of the catalysts. In all cases where
catalysts are present, the dissolved iron precipitates as magnetite
(Fe3O4). This environmentally preferred product
is easier to separate and faster to dewater than conventional mixtures
of iron(II,III) oxide hydroxides.
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