Design and optimization of an integrated UF-NF membrane process for treating rare earth refining wastewater

As the rare earth industry develops and energy efficiency improves, high-salinity rare earth smelting wastewater poses a significant environmental concern. This study applied integrated ultrafiltration (UF) - nanofiltration (NF) membrane technology to treat high-salt rare earth smelting wastewater (Ca²⁺: 10.75 g/L, COD: 3.5 g/L, turbidity: 1.36 NTU) for salt removal and recovery. The effects of UF pretreatment were explored, and the operating parameters for NF were optimized, with the performance of various commercial NF membranes being evaluated. Membrane fouling conditions were also investigated to determine appropriate cleaning methods. The experimental results demonstrate that the UF-NF integrated process is effective in actual wastewater treatment. The calcium ion content in the permeate was reduced from 10.75 g/L in the feed solution to 0.96 g/L, achieving a removal rate of 91.1 %, which meets reuse water standards. Trace amounts of organic molecules in the water, such as 2-Ethylhexyl phosphonic acid mono-2-ethylhexyl ester (P507), were also effectively removed. The removal rates for COD and TOC were notably high at 97.7 % and 98.3 %, respectively. Acid washing, alkaline washing, and water washing were employed to clean the membranes, all of which achieved flux recovery rates exceeding 95.0 %. In summary, the integrated UF-NF process demonstrated outstanding performance in salt recovery/separation of different valence salts from rare earth smelting wastewater. Its excellent operational stability and high flux recovery rates highlight its strong industrial applicability. The exceptional treatment efficacy it displays provides a feasible technological pathway for resource recovery and water recycling in practical applications.