A Hybrid Experimental and Theoretical Approach to Optimize Recovery of Rare Earth Elements from Acid Mine Drainage Precipitates by Oxalic Acid Precipitation

Yan Wang,Aaron Noble,Paul Ziemkiewicz

doi:10.3390/min12020236

Abstract

The development of processing techniques for the extraction of rare earth elements and critical minerals (REE/CM) from acid mine drainage precipitates (AMDp) has attracted increased interest in recent years. Processes under development often utilize a standard hydrometallurgical approach that includes leaching and solvent extraction followed by oxalic acid precipitation and calcination to produce a final rare earth oxide product. Impurities such as Ca, Al, Mn, Fe and Mg can be detrimental in the oxalate precipitation step and a survey of the literature showed limited data pertaining to the REE precipitation efficiency in solutions with high impurity concentrations. As such, a systematic laboratory-scale precipitation study was performed on a strip solution generated by the acid leaching and solvent extraction of an AMDp feedstock to identify the optimal processing conditions that maximize REE precipitation efficiency and product purity while minimizing the oxalic acid dosage. Given the unique chemical characteristics of AMDp, the feed solution utilized in this study contained a moderate concentration of REEs (440 mg/L) as well a significant concentration (>7000 mg/L total) of non-REE contaminants such as Ca, Al, Mn, Fe and Mg. Initially, a theoretical basis for the required oxalic acid dose, optimal pH and predicted precipitation efficiency was established by solution equilibrium calculations. Following the solution chemistry calculations, bench-scale precipitation experiments were conducted and these test results indicate that a pH of 1.5 to 2, a reaction time of more than 2 h and an oxalic acid dosage of 30 to 40 g/L optimized the REEs recovery of at ~95% to nearly 100% for individual REE species. The test results validated the optimal pH predicted by the solution chemistry calculations (1.5 to 5); however, the predicted dosage needed for complete REE recovery (10 g/L) was significantly lower than the experimentally-determined dosage of 30 to 40 g/L. The reason for this discrepancy was determined to be due to the large concentration of impurities and large number of potential metal complexes that cause inaccuracies in the solution equilibrium calculations. Based on these findings, a hybrid experimental and theoretical approach is proposed for future oxalic acid precipitation optimization studies.

Highlights

Rare earth elements (REEs) are considered critical raw materials by the USA Department of Energy, USA Department of the Interior and other federal agencies, for their use in numerous green energy, defense and technology applications [1,2,3]
The acid mine drainage precipitate sample utilized in this study was collected from an abandoned coarse coal refuse disposal facility in Greenbrier County, West Virginia in the Central Appalachian Basin, USA
Initial results from the solution equilibrium reaction modeling showed that a pH of 1.5 to 2 and an oxalic acid dose of 10 g/L would be suitable for optimization studies

Summary

Introduction

Since 2014, China has restricted the export of REE metals and mineral products, owing to concerns of protecting nonrenewable resources [5]. The sourcing of metals and minerals from unconventional secondary sources such as end-of-life products or mine and industrial waste has been of increased interest in recent years [6]. This increased interest has in turn prompted significant public and private investment to delineate these unconventional resources and identify suitable processing technologies to extract and recover these materials. In the past few years, coal, coal wastes and combustion byproducts, as well as acid mine drainage and acid mine drainage precipitates have been tested as potential secondary resources for REEs extraction and recovery [7,8,9,10]

Methods

Results

Conclusion