Impurity removal of acid leachates from rare earth slag with sodium hydroxide and ammonium bicarbonate: Mechanism and efficiency optimization

The second recovery of rare earth elements from the bastnaesite-rich slags can effectively alleviate their industry demand and achieve cleaner production, but it still remains unclear and difficult to remove impurities from the leachate by hydrochloric acid. In this study, NaOH and NH4HCO3 as precipitants were used to remove impurities and explore the optimal precipitation conditions and the mechanism by the D-optimal mixture design. The results showed that among the factors of temperature, pH and concentration, pH was the key affecting factor in impurity removal by the two reagents. The synergistic effect between the temperature and concentration promoted the impurity removal for NaOH and NH4HCO3, and the antagonism of pH with temperature inhabited the removal in the NH4HCO3 group. At the optimal conditions of 35.7–70.0 °C, 2.0–6.0 solution pH and 2.0–6.0 mol L−1 concentrations, the removal rates of Fe3+, Al3+, and Cu2+ by the NaOH were about 100, 64.9, and 58.4%, respectively, and those were about 100, 36.1, and 26.6% under the optimal conditions of 25.9–60.0 °C, 3.0–5.0 solution pH and 0.5–2.5 mol L−1 concentrations by NH4HCO3, respectively. Therefore, NaOH and NH4HCO3 are two feasible reagents for impurity removal in rare earth leachate for cleaner production.