A novel, clean, closed-loop process for directional recovery of rare earth elements, fluorine, and phosphorus from mixed rare earth concentrate

Abstract

The recycling of Bayan Obo rare earth has attracted widespread attention due to its potential environmental risks and the importance of key metal supplies. As a traditional technology, concentrated sulfuric acid roasting has led to the continuous deterioration of the local environment because of severe fluoride and sulfur dioxide emissions. Although it is difficult to achieve efficient recovery of rare-earth elements and their associated resources with this process, it is possible to eliminate pollutant emissions, and achieve a high recovery ratio of rare-earth elements using this technique. Fluorine and phosphorus from wastewater have been well controlled. Here, a novel, clean, closed-loop process is introduced to treat rare earth concentrates, which not only does not pollute the environment, but also generate more economic values. Through the oxidative roasting-hydrochloric acid leaching step, more than 98% of fluorine and phosphorus can be stably retained in the leaching residue. The leaching residue is then converted into rare earth hydroxides, sodium fluoride, and sodium phosphate in the sodium hydroxide sub-molten salt system. Moreover, most of the concentrated lye can be reused in the sub-molten salt decomposition step by filtration. The washed filter residue is then dissolved in the hydrochloric acid leachate to produce a rare earth chloride solution. Sodium fluoride and sodium phosphate can be recovered in the washing solution. In summary, this process can realize the internal circulation of concentrated lye and water while producing a rare earth chloride solution, sodium fluoride, and sodium phosphate. Finally, the rare earth concentration in the rare earth chloride solution reached 280 g L−1, 96.40% of fluorine was converted to sodium fluoride, and 99.82% of phosphorus was converted to sodium phosphate from the washing liquid. An environmentally-friendly, easy-to-implement strategy is proposed: the entire recycling process is highly selective, closed-loop and simplified, and can be used as an alternative method to recover different resources from the current Bayan Obo concentrate.