Membrane distillation of a waste stream from neodymium‑iron‑boron scrap recovery: Performance and scaling mitigation

Abstract

Neodymium‑iron‑boron (NdFeB) scraps contained 20–30 % rare earth elements such as neodymium, iron, boron, dysprosium, and praseodymium, which are valuable untapped rare earth resource. Hydrometallurgical extraction through hydrochloric acid treatment is often used to separate the rare earth metals and impurities in the NdFeB scraps, resulting in large amounts of acidic lixiviate and high-salinity brine, challenging to further treatment and detrimental to the environment if directly discarded. In this work, direct contact membrane distillation (DCMD) was explored to treat NdFeB scraps precipitation wastewater (NPW) by testing systematically three different hydrophobic commercial membranes. The impact of the membrane surface properties on the recovery performance and membrane scaling were investigated. The results showed that the membranes with strong hydrophobic properties and smaller pore size distributions possessed greater anti-scaling potential compared to the other materials. Additionally, the membranes with greater porosity offered longer stable operation at high flux, with the PTFE-0.22 membrane maintaining a flux over 20 kg/m2·h for up to 8 h. Membrane scaling caused by calcium carbonate during DCMD treatment was significantly mitigated by systematic chemical cleaning and after four cycles of testing, >75 % initial flux was restored via a simple acid cleaning. An average of 53.4 g/L sodium chloride with the purity of 99.3 % was obtained from the concentrated NPW by evaporation crystallization. This work demonstrated the potential of DCMD to act as a resource concentration and recovery tool on industrial effluents.