Nitrogen recovery from a palladium leachate via membrane distillation: System performance and ammonium chloride crystallization

Ammonia leaching process was often used to separate precious and rare-earth metals during their recovery via hydrometallurgy, resulting in the generation of nitrogen-rich leachate. Therefore, it is significantly important to recover the nitrogen from the leachate for reuse. In this work, we explored the treatment of palladium leachate (PL) via direct contact membrane distillation (DCMD) for nitrogen recovery. Effects of temperature and pH on system performance and nitrogen recovery mainly in the form of ammonium chloride (NH4Cl) were evaluated. Results showed that DCMD performance decline caused by membrane scaling was primarily impacted by feed temperature rather than pH. The higher temperature, the more severe membrane scaling occurrence. However, there was little to no effect of initial pH on distillate flux decline. Nitrogen in the form of NH4Cl was successfully recovered from PL, which was significantly influenced by initial pH. The weight of NH4Cl obtained ranged from 17.4 to 21.5 g/L under various operating conditions, and the purity of the NH4Cl was up to 97.4%, near the industrial grade, at feed temperature of 40°C and initial pH of 8.5. Additionally, results of gas chromatography-mass spectrometry (GC–MS) revealed that the primary dissolved organic matter (DOM) in the virgin PL were tetradecanoic acid and isosorbide. Our results suggest that DCMD can be used as a promising and feasible solution for nitrogen recovery from the PL.