Process Development and Techno-Economic Analysis for the Recovery of Rare Earth Elements and Critical Materials from Acid Mine Drainage

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Virginia Tech


Rare earth elements (REE) exhibit particular and unique properties that render them essential to technological applications. Of particular interest is their involvement in the transition toward global sustainability and their military applications. The magnetic properties of the rare earth elements is of primordial importance to sustainable development. More specifically, terbium and dysprosium are two elements with no known substitutes in critical applications and with no domestic or allied sourcing available. These elements are currently mined by in-situ leaching of ion-absorbed clays, mostly from illegal operations in Myanmar financed by Chinese companies. The demand from both elements, and for the other magnet rare earths is projected to growth at very high rates through 2035 while the world undergoes a transition toward sustainability, and a drastic reduction in greenhouse gases emissions. Our team has been evaluating the potential of acid mine drainage (AMD) as a source of rare earth elements and critical materials (CM). Acid mine drainage is the result of in-situ generation of sulfuric acid due to the weathering of sulfide ores. It is a significant legacy environmental issue and one of the largest pollutants in many mining districts throughout the world. The objective of the present work is to provides a roadmap for the utilization of AMD as a critical material feedstock to preserve the independence of the United States of America with regards to these materials. To that effect, a fundamental economic assessment of REE/CM recovery from AMD using a network sourcing strategy in addition to a robust, flexible feedstock separations and refining facility was undertaken. A techno-economic analysis of the extraction, refining, separation and reduction to metal is presented along with a sensitivity analysis.The results of this analysis show that, with the exception of the minimum price scenario, all operational configurations have positive economic indicators with rates of return varying from 25% to 32% for the contemporary price scenario. This is primarily due to the very high enrichment in terbium and dysprosium of AMD. The optimal configuration was determined to be production of Co, Mn, and all REEs except for mischmetal, which is not recovered. Sensitivity analysis and Monte Carlo Simulation show that capital cost and HCl consumption are the two major factors influencing rate of return, thus indicating opportunities for future technology development and cost optimization. In order to reduce both the capital and operation cost of the facility, alternative ionic liquids extractants based on conventional acidic extractants where synthesized and investigated. The results show that the ionic liquids varied in performance, with [c101][D2EHP] and [c101][EHEHP] performing poorer than their conventional counterparts and [c101][c572] performing better. The performance of [c101][c572] was 13% superior to Cyanex 572, 20% superior to EHEHPA and 27% superior to D2EHPA the current commercially used extractants. Recommendations for further study on [c101][c572] include stripping tests, continuous pilot testing, and techno-economic analysis. The test work revealed that zinc and to a lesser extent calcium were significant deleterious elements in the solvent extraction circuit, and that selective removal would significantly reduce the acid-base consumption of the separation circuit. A process was developed to selectively remove calcium and zinc from AMD-derived feedstock and from REE products. The ammonium chloride leach process offer many advantages, including the possibility of closing the cycle by using carbon dioxide sequestration as a step to regenerate the ammonium chloride in a zero-discharge process.



Acid Mine Drainage, Rare Earth Elements, Critical Materials, Ionic Liquids