Metal Extraction using Oxalate Reagents. To understand the
extraction of metals from NIST SRM 600 bauxite ore using aqueous
H2C2O4 (OA),
KHC2O4 (KHO) and
KHC2O4·H2C2O4(KTO), the temperature, agitation speed, and S/L ratio were kept
constant at T = 100 °C, N s = 600 rpm, and
15 g/L respectively, for all the experiments in this section unless
specified otherwise. The S/L ratio can be optimized after understanding
the mechanism. NIST SRM 600 bauxite contains Al in the form of gibbsite
(Al2O3·3H2O), along with
hematite (Fe2O3) and quartz
(SiO2). A significant difference between the three
oxalate reagents is the reaction pH. The OA is the most acidic, followed
by the KTO and then KHO. The acidity of the reagent is critical for
breaking the metal-oxygen bonds via hydrogen bonding that initiates the
extraction of metals from metal oxides.27 Figure 1a
summarizes the Al and Fe extraction kinetics for NIST SRM 600 bauxite
using OA.
The Fe extraction rate was significantly improved by increasing the OA
concentration from 0.50 M to 0.75 M, whereas the Al extraction rate
increased slightly. The Al extraction rate was faster than Fe, at both
0.50 M and 0.75 M OA, indicating a preference towards Al in the acidic
pH range of 0.75-1.2. Greater than 90% of the Al was extracted in 1 h
for at both 0.50 M and 0.75 M OA, while about 1.5 h was required for
extracting 90% of the Fe. However, when similar extractions were
performed using 0.50 M KHO and 0.75 M KHO (Figure 1b), the rate of Fe
extraction was faster than Al. For both 0.50 M and 0.75 M KHO, less than
1 h was required to extract greater than 90% of the Fe. This
observation indicates the importance of the\(\mathrm{H}\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{\mathrm{-}}\)in Fe extraction. The KHO behaves as a pH buffer; therefore, when
increasing the concentration of KHO, the pH remains the same, and no
significant changes in metal extraction kinetics were observed. OA is
the most acidic and effective at Al extraction from bauxite ore;
however, the difficulty is regenerating the acid after precipitation of
the Al and Fe. For this reason, the moderately acidic KTO is the
preferred acid, which can be easily regenerated after Al and Fe
precipitation. The stoichiometric reactions for the extraction of
trivalent metal (M3+ like Al3+,
Fe3+) from its oxide
(M2O3) using OA, KHO and KTO are shown
in eqs 3-5.