Figure 1. Metal concentration for Al and Fe as a function of time in the aqueous phase for (a) 0.50 M and 0.75 M OA at T = 100 °C, S/L = 15 g/L and N s = 600 rpm (b) 0.50 M and 0.75 M KHO at T = 100 °C, S/L = 15 g/L andN s = 600 rpm (c) 0.25 M KTO at T = 100 °C, S/L = 15 g/L and 20 g/L and N s = 600 rpm (d) 0.50 M OA, 0.50 M KHO and 0.25 M KTO at T = 100 °C, S/L = 15 g/L andN s = 600 rpm
Separation of Al and Fe from the Aqueous Phase. The oxalate-based acids (OA, KHO, and KTO) provide an efficient extraction (greater than 95 wt%) for Al and Fe into the aqueous phase and an easy separation for the remaining solid residue which is primarily made of SiO2. The next step is to efficiently recover the Al from the aqueous phase and minimize any co-precipitation of Fe or other impurities.
The Al and Fe in the NIST SRM 600 bauxite ore are both present in the +3 oxidation state. The Al and Fe can be hydrolyzed to precipitate as metal hydroxides. To selectively precipitate these metals the pH must be optimized. The “Atlas of metal-ligand equilibria in aqueous solutions” by J. Kragten provides the pH range required for precipitation of Fe(OH)3 and Al(OH)3 using 0.1 M\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\)solution.28 The Al(OH)3 is amphoteric in nature and can act as a Lewis acid that binds with OH- ions to form a water-soluble\(\mathrm{Al(O}\mathrm{H)}_{\mathrm{4}}^{\mathrm{-}}\)ion.29 Hence, the concentration of Al3+ metal in the aqueous phase decreases during hydrolysis up to about a pH = 9, but starts increasing as precipitate dissolves at higher pH. The Fe(OH)3 precipitation begins before the Al(OH)3 but is not amphoteric and can be precipitated efficiently at high pH.
To precipitate Al and Fe effectively without any impurities, the basicity of the aqueous phase was increased to a pH of about 14 using KOH. To avoid the precipitation of insoluble metal oxalates, the base KOH is preferred versus NaOH. The high solubility of K2C2O4 (Table 1) enables the efficient precipitation of Al and Fe. Greater than 99% of the Fe precipitates at a pH of about 14, while the majority of the Al remains in solution. Any remaining Al can be separated by lowering the pH to about 10.5 using H2SO4 or H2C2O4. The approach is described in Table 3 for filtrate recovered after refining bauxite ore using OA, KTO, and KHO. Under these concentrations, a pH = 10.5 was found to be optimum for Al precipitation. Using this approach, Al can be precipitated without any Fe impurity. The elemental composition of Al and Fe precipitates recovered from the aqueous filtrate of 0.25 M KTO extraction are shown in Table 4. A similar elemental composition was observed for the precipitates recovered from the aqueous filtrate after extracting Al and Fe using 0.50 M OA and KHO. It should be noted that around 10% of Al precipitates with the Fe. To recover this additional Al, the Fe precipitate can be dissolved in an acidic oxalate solution (similar to the oxalate reagent initially used), and the approach described in Table 4 can be repeated to separate the remaining Al without any Fe impurity.