Precipitation of rare earth elements from acid mine drainage by CO2 mineralization process

Abstract

Acid Mine Drainage (AMD) has been of environmental concern for decades but found to be a viable source of critical Elements (CEs), including Rare Earth Elements (REEs). The current AMD treatment methods typically use hydroxide base chemicals for neutralization which results in precipitation of ions from the solution. In these methods, up to 70% of REEs precipitate at the target treatment pH of AMD, and the rest is discharged with the treated water. To address this issue, a novel staged precipitation process was investigated for efficient, low-cost, and environmentally friendly recovery of REEs and other CEs while treating AMD for environmental compliance. This process incorporates CO2 mineralization in staged precipitation to achieve a high recovery of these elements at a relatively narrow pH range and within the target pH of AMD treatment. Staged precipitation experiments were performed using NaOH, as the beeline, and the CO2 mineralization process. The corresponding precipitation patterns of the REE-hydroxides and REE-carbonates were then studied. The experiments were conducted using a sample collected from an active AMD treatment facility in Pennsylvania. Through the CO2 mineralization process, 90% of Al and over 85% of REEs were recovered at pH values below 7, while suppressing the precipitation of most of the Fe content (i.e., 65%). It was found that the precipitation pattern of REE-hydroxides follows the tetrad classification of the coordination number of the corresponding elements. In contrast, the precipitation pattern of REE-carbonates in the CO2 mineralization process followed the hydration energy trend of these elements. The findings were used to recommend a two-step staged precipitation process for the recovery of CEs during the AMD treatment process.