Preparation of crystalline mixed rare earth carbonates by Mg(HCO_3)_2 precipitation method

In acid treatment technology of Baotou mixed rare earth ore, large quantities of ammonia-nitrogen wastewater are produced in the step of ammonium bicarbonate precipitation to transform rare earth sulfate. In this paper, we adopted a green precipitant magnesium bicarbonate (Mg(HCO₃)₂) to substitute ammonium bicarbonate to eliminate ammonia-nitrogen pollution. The effects of n(HCO₃⁻):n(RE³⁺), aging temperature and aging time on the crystallization using Mg(HCO₃)₂ precipitation method were investigated. The results indicate that the rare earths could be completely recovered when n(HCO₃⁻):n(RE³⁺) is higher than 3.15:1. The crystal water content of rare earth carbonates is affected by the aging temperature. The precipitate has a bad filterability when the aging temperature is over 40 °C. This can be attributed to the less crystallized water molecules of the hydrated rare earth carbonate precipitation. The mixed rare earth carbonates are prone to be crystalline, and have a good filterability at aging temperatures below 40 °C. Meanwhile, the evolution mechanism of crystalline mixed rare earth carbonates is reasonably deduced, the amorphous rare earth carbonates are first dissolute and then recrystallized. Under the optimized aging conditions, the purity of the crystalline precipitate meets the requirements of the fine product standard (GB/T 16479-2008). The filtrated could be used to produce Mg(HCO₃)₂, thus to realize the recycling of magnesium sulfate.