Oxalic acid recovery from high iron oxalate waste solution by a combination of ultrasound-assisted conversion and cooling crystallization

In order to achieve the global goals related to renewable energy and responsible production, technologies that ensure the circular economy of metals and chemicals in recycling processes are a necessity. The recycling of spent Nd-Fe-B magnets typically results in rare earth elements (REEs) free wastewaters that have a high ferric ion concentration as well as oxalate groups and for which, there are only a few economically viable methods for disposal or reuse. The current research provides a new approach for the effective recovery of oxalic acid and the results suggest that during the initial oxalate groups separation stage, > 99% of oxalate ions can be precipitated as ferrous oxalate  (FeC₂O₄⋅2H₂O) by an ultrasound-assisted iron powder replacement method (Fe/Fe(III) = 2, t_u/s = 5 min, T = 50 ºC). Subsequently, almost all of the FeC₂O₄⋅2H₂O was dissolved using 6 mol/L HCl (T = 65 ºC, t = 5 min) and the dissolved oxalates were found to mainly exist in form of H2C2O4. Furthermore, over 80% of the oxalic acid was recovered via crystallization by cooling the oxalate containing HCl solution to 5 ºC. After oxalic acid crystallization, the residual raffinate acid solution can then be recirculated back to the ferrous oxalate leaching stage, in order to decrease any oxalic acid losses. This treatment protocol for high iron REEs-free solution not only avoids the potential harm to the environment due to waste water but also significantly improves the circular economy of metals in the typically utilized permanent magnet recycling processes.