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.