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Fluoride control and flowsheet development for the hydrometallurgical processing of bastnaesite concentrates

Rich, Austin J.
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2024-04-22
Abstract
Rare Earth Elements (REEs) are materials which are critical to the landscape of modern and evolving technology. In the United States, these elements often occur in bastnaesite ore, which has been historically mined and processed at the Mountain Pass mine. The most recent leaching process used on the Mountain Pass bastnaesite ore achieved a REE recovery of 55-60%. As a result of the poor recovery values, the Critical Materials Institute (CMI) commissioned an investigation on leaching recovery improvement. Colligan et al. showed that a decreased slurry density could yield leaching REE recovery values greater than 80%, and as high as 97%. However, this method still required improvement, as the leaching of such a low percentage of solids left large concentrations of hydrochloric acid in the residual leach liquor. This acid could be recycled for further leaching, which would result in much better process economics than using a new batch of acid per leach. It was ultimately found that any recycling of the solution would cause the fluoride from previous batch leaches to react with fresh bastnaesite, which would form insoluble Rare Earth Fluoride and decrease process REE recovery. Therefore, this study was conducted to develop a process which could remove the fluoride from the process solution, thus mitigating its detrimental effects. To achieve this goal, analytical procedures were developed to quantify fluoride in both solid and liquid REE-bearing matrices. As a byproduct of the testing of a fluoride removal process, a continuous bastnaesite leaching process flowsheet was developed throughout the test work. The test work evaluated data for processes including bastnaesite leaching, solvent extractant, stripping, precipitation, fluoride removal, and activated alumina regeneration. Overall, it was determined that a continuous, single-stage hydrometallurgical process was metallurgically viable, as it achieved a leaching REE recovery of 91% and an effective recovery (all processes combined through product precipitation) of 80%. Additionally, it was proven that fluoride could be successfully removed from the solution, so that the solution could be recirculated for subsequent leaching cycles. Finally, an economic analysis was performed, where the pre-tax NPV equated to $29 million USD at a 10% discount rate, the IRR was 31%, and the payback period was 3 years. From all data gathered, it can be concluded that the process can be viable on an industrial scale, and scale-up of the process is recommended.
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