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.