Dynamic elution of residual ion-exchangeable ammonium from weathered crust elution-deposited rare earth tailings

Ion-exchangeable ammonium (IEA), as one of the major existing forms of the residual ammonium in weathered crust elution-deposited rare earth tailings, has great potential to be released under natural conditions, which may lead to serious environmental pollution. Herein IEA distribution feature was determined, and five common ions (Al3+, Ca2+, Mg2+, Na+, and K+) were used as eluent to remove it from the tailings. Elution process and mechanism were studied in details. The results showed that IEA was mainly concentrated on illite which accounts for the largest proportion of the rare earth clay minerals (>65%). During the elution process, the metal ions initial concentration, flow rate, and particle size affected IEA removal efficiency while pH exhibited only a small effect, and the elution rate was controlled by internal diffusion. Among the selected eluents, the elution efficiency decreased in the order of K+>Mg2+>Al3+>Ca2+>Na+. The highest elution efficiency of IEA can reach 95.9% at an initial K+ concentration of 0.1 mol/L. Characterization results demonstrated that IEA could be effectively eluted by the metal ions through ion-exchange reaction. The results are valuable for choosing an efficient eluent and controlling the environmental pollution caused by the release of IEA.