Production of High-Grade Mixed Rare Earth Oxides from Acid Mine Drainage via Solvent Extraction: Laboratory-Scale Process Development

Several recent studies have shown that acid mine drainage (AMD) may be a promising source of rare earth elements (REEs), which are essential feedstocks for many high tech applications and defense products. AMD is a longstanding environmental challenge and is currently the primary pollutant of water in the Appalachian coal mining region. Acid generated during the coal mining process tends to leach several transition metals from the surrounding rock strata. While iron, aluminum, and manganese have traditionally been noted as the predominant metals in AMD, recent studies have also shown that REEs are also present, albeit in trace concentrations, often less than 5 μg/L. The recovery of REEs from AMD can be both an economic and environmental advantage; however, the low REE concentrations and high contamination from other metals makes the concentration and purification of REEs quite difficult.

This research seeks to develop and optimize a process capable of producing mixed rare earth concentrates with purities exceeding 90% from an AMD feedstock. Parallel efforts by other members of the research team showed that a solid preconcentrate, nominally 0.1 to 2% REE, can be readily produced from AMD; however, that pre-concentration process cannot provide the further enrichment needed to generate high purity oxides suitable for downstream markets. In this project, solvent extraction was investigated as secondary process used to further enrich the low grade preconcentrate to a purity exceeding 90%. Initially, laboratory-scale batch solvent extraction tests were performed on synthetic REE solutions to determine the influence of various process parameters (e.g. pH, extractant dosage, diluent type, and feedstock concentration). Next, the separation of REEs from major AMD gangue elements was investigated using synthetic leachate solutions with concentrations similar to those expected from the pre-concentrate samples. This process showed that the grade targets could easily be met when combining optimal parameters from each step. From this preliminary work with synthetic solutions, an optimal SX process was developed and validated using a real leachate generated from a pre-concentrate sample. By integrating leachate preparation, solvent extraction, scrubbing, stripping, and oxalic acid precipitation, an oxide containing 90.5% rare earth oxides was generated. Details on the process development, experimental optimization, and opportunities for process improvement are described.