Behavior of sulfate in preparation of single light rare earth carbonate by Mg(HCO3)2 precipitation method

Abstract

The precipitation of the water-leaching solution of Baotou mixed rare earth (RE) concentrate roasted with sulfuric acid using ammonium bicarbonate for producing RE carbonate produces a mass of ammonia-nitrogen wastewater because of the relatively low solubility of rare earth sulfate. To solve the serious problem of ammonia-nitrogen pollution, new precipitators need to be developed urgently so as to meet the requirements of environmental protection and impurities content of the product (SO42−<1.8 wt% in RE carbonates products). In this paper, we studied the effects of feeding modes on the behavior of SO42− during the preparation of light RE carbonate (RE = La, Ce, Pr, Nd) from their sulfate solutions using Mg(HCO3)2 as a precipitant. The results indicate that the contents of SO42− in the La and Ce precipitates using positive feeding mode exceed 16 wt% because of the formation of La2(CO3)2.15(SO4)0.85·4H2O and Ce2(CO3)2.15(SO4)0.85·3H2O, while those of the Pr and Nd precipitates are 4 wt%–5 wt% since they exist in the form of n-carbonate. The precipitates prepared using synchronous feeding mode are all RE carbonate with only 4 wt%–5 wt% of SO42− enclosed in the precipitation. The content of SO42− in the RE carbonate obtained using reverse feeding mode is the lowest. Among them, the content of SO42− in La precipitate is only 1.40 wt%. Both synchronous and reverse feeding modes can effectively reduce the content of SO42− in RE carbonate, which provides theoretical guidance for the preparation of qualified light RE carbonate products by Mg(HCO3)2 precipitation method.