Separation of REs from Ca and Mg Ions by Ammonium Bicarbonate Precipitation and the Influence of Fe and Al Ions

The presence of impurities such as Ca, Mg and Al directly affects the purity of rare earth products precipitated by ammonium bicarbonate (ABC). Therefore, precipitation method and optimum conditions are the key to improving the separation efficiency of rare earths (REs) from impurities. In this paper, rare earth chloride (RECl3) was precipitated by ABC solution, and the co-precipitation rules and separation efficiency of Al, Fe, Ca and Mg ions from REs were investigated with or without the addition of triammonium citrate (TAC). The results show that as long as the REs precipitation percentage of RECl3 is controlled below 94%s, Ca and Mg ions will not enter the precipitate from their solution only containing them, and the purity of rare earth oxide (RE2O3) is up to 100%. However, the presence of Al and Fe impurities can lower the separation efficiency of REs from Ca and Mg. To obtain high purity of RE2O3, it is necessary to separate Al and Fe in advance. Firstly, Fe is precipitated and filtered 100% by adjusting pH to 4.12. Then, 90% of Al is precipitated only with 6% loss percentage of REs when adjusting pH at 4.6. When REs are precipitated at pH=6.43, the purity of the obtained RE2O3 is 97.83% with 1.05% Al and 0.21% Mg, without Ca and Fe. Which proves that Al has not been completely removed and party of Mg still enters the product. When REs are precipitated 60% by fractional precipitation method after removing Al and Fe, the purity of the obtained RE2O3 is 98.83% with 0.73% of Al and 0.06% of Mg. Furthermore, a small amount of TAC can assist the completely removing of Al, letting Ca and Mg remaining in solution until the rare earth precipitation percentage up to 99%. The purity of RE2O3 is 99.66% with only 0.09% of Al at the 94% precipitation percentage of REs. With a continuous precipitation crystallization method, the purity of RE2O3 reaches 99.87% with only 0.03% of Al.