Selective Precipitation and Recovery of Rare Earth Elements from Acid Mine Drainage

Abstract

The ability to secure rare earth elements (REEs) in the United States is of concern due to the rapid growth in demand and the monopolistic supply chain. The demand for REEs has skyrocketed in recent years due to the development of many green technologies. The United States and many other countries are currently reliant on China for REEs, who currently control greater than 90% of the global supply. Due to the severe impact of previous Chinese export quotas, it is important to lessen the reliance on importation and explore methods of recycling REEs from secondary sources. A secondary REE source of interest is acid mine drainage (AMD), which has been shown to contain REE concentrations far above that of natural water sources. Focusing on AMD sources in northeastern West Virginia, the objectives of this study include 1) determining the viability of recovering REEs from AMD via selective precipitation, 2) assessing correlations between the raw water characteristics and the quality of the REE sludges produced, and 3) determining the effects of redox condition and flocculant usage on the precipitation procedure. To satisfy the first objective, acid mine drainage was collected and pH adjusted sequentially between pHs 3.0 and 8.0. The pH ranges of gangue metal and REE precipitation were analyzed to determine if efficient separation of REEs could occur. Based on the analysis, recovery of REEs was efficient due to the well defined separation of removal pH ranges. To achieve the second objective, AMD samples were collected from seven sites across northeastern WV. Each sample was adjusted sequentially to pH 4.0, 5.0, and 8.0. The composition of the precipitates and overall REE recovery were analyzed and compared for each sample. Linear regression was then used to assess correlations between raw water characteristics and the grade of the precipitates. It was determined that the precipitate grade was independent of the raw water REE concentration and could not be accurately predicted using raw water characteristics. The third objective was achieved by the repeating the precipitation procedure under varying redox conditions, which included complete oxidation, partial reduction with nitrogen sparging, and partial reduction without nitrogen sparging. The effects were assessed by comparing changes in precipitation patterns of gangue metals and REEs. The completely oxidized condition led to a greater separation of pH range between the metals and REEs when compared to the partially reduced conditions. The effects of flocculant usage were also examined for