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.