Removal of Residual Ammonium from Weathered Crust Elution-Deposited Rare Earth Ore Tailings by Magnesium Chloride: Kinetics and Mass Transfer

After in situ leaching of weathered crust elution-deposited rare earth ore (WREO), large amounts of residual ammonium (RA) salts remain in the ore body and slowly release, causing persistent ammonia-nitrogen pollution in surrounding waters. This study proposes using magnesium chloride for in situ elution remediation of closed mines. Column experiments were conducted to evaluate the effects of eluent concentration, liquid–solid ratio, flow rate, pH, and temperature on residual ammonium removal, and a kinetic model was established based on the shrinking unreacted-core model. The results show that increasing Mg2+ concentration, temperature, or flow rate accelerates the eluting rate, with temperature being the most influential. A higher liquid–solid ratio in the tested range could enhance the elution efficiency of residual ammonium, but it will substantially raise the production cost. Weakly acidic pH 4–6 favors the reaction, while alkaline conditions inhibit it. Kinetic analysis indicates inner particle diffusion control, with an activation energy of 6.03 kJ/mol and a reaction order of 0.3009. Under optimal conditions of 0.1 mol/L Mg2+, 2:1 liquid–solid ratio, 0.6 mL/min, pH 4–6 and room temperature, elution efficiency reaches 95.45%. This work provides theoretical and technical support for green remediation of historical ammonium contamination in WREO.