KTO is a double salt of OA and KHO and comprises two moles of\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) per
mole of KTO. Hence, 0.25 M KTO (0.50 M\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\)) was
used to perform the Al and Fe extraction from NIST SRM 600 bauxite at a
15 g/L S/L ratio. In the KHO study, no significant difference was
observed in the rate of metal extraction at higher\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\)concentration due to essentially the same pH; therefore, only 0.25 M KTO
was studied in detail. In Figure 1a and 1b, the 0.50 M\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\)concentration led to an efficient extraction of Fe and Al using 15 g/L
S/L ratio of ore; therefore, a higher S/L ratio of 20 g/L with 0.25 M
KTO was attempted, as shown in Figure 1c. The Al and Fe extraction
kinetics for 20 g/L was slower in comparison to 15 g/L experiments
because of the decreased concentration of\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) per
metal ion. However, greater than 95% of Al and Fe was extracted in 2.5
h at 20 g/L S/L ratio, which makes this process energy-efficient. An
important observation was the rates of Al extraction for OA, KTO, and
KHO at the 0.50 M\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\)concentration were similar (Figure 1d), which confirms the negligible
effect of pH on Al extraction kinetics. To understand this phenomenon in
more detail, ore was digested in 0.50 M
K2C2O4, and no Al
extraction was observed, confirming the importance of acidity to
initiate the extraction. The elemental composition shown in Table 2
after the extraction experiments indicates less than 10% of Al and 2%
of Fe remain in the solid residue. The typical mass of solid residue
recovered was 2.5 g from 15 g of ore digested in 1 L of acid. From the
elemental composition of the ore and remaining solid residue shown in
Table 2, it can be seen that in addition to Al and Fe, some other metals
such as Zr and Mg are also leached into the aqueous phase. The residues
recovered in the experiments shown in Figure 1a-1d were also confirmed
using PXRD to be primarily quartz (SiO2). The PXRD
patterns for the NIST SRM 600 and the solid residues from the bauxite
digestion using 0.50 M OA, 0.50 M KHO, and 0.25 M KTO are shown in
Figures S1 and S2, respectively.
Table 2. Elemental composition of NIST SRM 600 and the solid
residues remaining after refining of NIST SRM 600.