Figure 3. Effect of visible light on the concentration of Fe in the aqueous phase.
Recycling of Oxalic Acid and its Derivatives. A critical step for improving the economics of a metal extraction process is the recycling of the acids used for leaching. For the regeneration of KTO, KHO and OA, an ion-exchange resin was used to regenerate the acid and recycle it for future metal extraction. The precipitation of Al and Fe using KOH and H2C2O4produces K+ ions as the major cation in the aqueous phase. A strong acid cation exchange resin like Amberlyst 15 H-form was used to exchange the K+ with H3O+ ions for regeneration of the oxalate reagents used in this study. The overall closed-loop process is shown in Figure 4. The amount of activated H-form resin added to the aqueous phase (containing K+) determines the final pH. The KHO was regenerated to a pH of about 2.50 and KTO is produced at a pH of about 1.50. The regeneration of OA required a pH of about 1.0 and utilized the most resin. The ideal pH after the ion-exchange treatment should be close to the initial pH needed for the metal extraction process. A small amount of H2C2O4 or K2C2O4 is likely required to achieve the same starting oxalate concentration, but overall a significant amount of oxalate can be recycled; therefore, reducing the amount of fresh acid required and closing the loop on acid recovery and reuse. In the preliminary studies, around 30 g of Amberlyst-15 H-form (dry) was used to regenerate OA (pH = 1.2) from 1-L of filtrate recovered after Al precipitation at a pH of about 10.9. The recovery process requires an excess of ion-exchange resin and water; however, the preliminary results are promising, and the work to optimize this process is ongoing.