Figure 4. Flowsheet for the proposed closed-loop bauxite refining process using ion-exchange resins.
The alternative method for regenerating the oxalate-based reagents involves the acidification of the post precipitation filtrate using H2SO4. Similar to the ion-exchange resin approach, the final pH is critical in achieving the regenerated oxalate reagent. In this approach, the low solubility of potassium containing oxalate reagents is beneficial for precipitation, and the precipitated reagents can be efficiently filtered and separated. For the operating conditions described in this work, the aqueous phase will be oversaturated with KHO and KTO and undersaturated for OA; therefore, only KHO and KTO will precipitate. In our preliminary studies to acidify the post precipitate filtrate to a pH = 2.5 using H2SO4, approximately 65% of the initial\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) ion was recovered in the form of KHO. Decreasing the pH to 1.5 led to 80% of the initial\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) ion recovery in the form of KTO. The precipitates were confirmed as KHO and KTO using PXRD, as shown in Figure S4 and Figure S5, respectively. The closed-loop process is summarized in Figure 5. Decreasing the pH below 1.5 led to precipitate dissolving due to the higher solubility of OA compared to KTO. The approach shown in Figure 5 has the advantage because no ion exchange resin is required, but the regeneration of OA is a drawback.