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.