Membrane pre-concentration as an efficient strategy to enhance the enrichment of rare earth by MgO precipitation

Abstract

Taking advantage of the environmentally-friendly properties of a magnesium salt leaching agent, magnesium oxide (MgO) was used as a precipitant to enrich rare earth (RE) from leaching liquor in the magnesium salt system. This approach not only tackles the issue of ammonia–nitrogen pollution but also enables efficient magnesium recycling. However, due to the high concentration of magnesium ions and the low concentration of RE ions in the leaching solution, the precipitation kinetics of RE concentrates are sluggish, leading to low RE purity in the resulting precipitated product. In this work, we proposed a novel strategy that combines complexation-ultrafiltration (UF) pre-concentration with subsequent MgO precipitation to enhance the hydrolysis of MgO, promoting the precipitation of RE. We further improved the RE enrichment efficiency by optimizing membrane type, complexing agent selection, concentration levels, precipitation temperature, and MgO dosage. Under the conditions of concentrating the leaching solution using optimal UF membrane and employing glyphosate as the complexing agent, the RE ions concentration increased from 101 to 3940 mg/L, and we successfully separated magnesium ions with a separation factor of 36.4. A comparison between direct MgO precipitation and the pre-concentration approach revealed an 80 % reduction in precipitation time and an 80 % decrease in MgO usage with pre-concentration. Furthermore, this combined process led to an 8 % increase in RE recovery and a 2.8-fold enhancement in RE purity, and similar results were obtained when using actual mine leaching solutions. Additionally, the glyphosate present in the UF permeate and the precipitated mother liquor can be efficiently reclaimed through resin adsorption. This proposed process holds significant potential for engineering applications aimed at enriching RE.