Inhibition expansion mechanism of compound eluent during ammonium removal from weathered crust elution-deposited rare earth tailings

In-situ leaching with (NH 4 ) 2 SO 4 is widely employed for rare earth the separation and extraction, yet residual ammonium salts readily adsorb onto clay minerals and are prone to elution, threatening aquatic systems. Conventional ammonium removal induces clay swelling upon water absorption, resulting in poor ore permeability and slope instability. This study investigates a K/Mg/Na ternary compound eluent designed to suppress tailings expansion during ammonium removal and clarifies its inhibitory mechanism. Compared with single or binary eluents, the ternary formulation exhibited superior ammonium removal, and the optimized composition (0.1 mol/L, ratio 5:6:4) markedly improved ore body stability. The eluent reduced the zeta potential of clay minerals from −13.2 mV to nearly 0 mV, thereby diminishing electrostatic repulsion. Under identical conditions, tailings treated with the compound exhibited 61.3 % less expansion than untreated controls or samples exposed to conventional leaching agents. Mechanistic analysis indicates that the ternary eluent neutralizes surface charges, compresses the electric double layer, and mitigates inter-crystalline repulsion within clay minerals, stabilizing hydration and suppressing expansion. This approach improves the ammonium leaching efficiency, mitigates environmental risks, and strengthens mining safety.