Effect of various ligands on the selective precipitation of critical and rare earth elements from acid mine drainage

Acid mine drainage (AMD) has been of environmental concern for decades but recently found to be a viable source of critical elements including rare earth elements (REEs). Recovery of these elements while treating AMD for environmental compliance improves the sustainability of the treatment process. The precipitation behavior of the REEs and other cations during the AMD neutralization process depends strongly on the solution chemistry, available ligands, and concentration of elements. Several chemicals were used to study the effect of various ions/ligands (i.e., OH⁻, SO₄²⁻, NH₄⁺, CO₃²⁻, and PO₄³⁻) on precipitation behavior of REEs and other elements from AMD as a function of pH. It was found that only up to 70% of total REEs can be recovered using NaOH at circumneutral pH. (NH₄)OH suppressed the precipitation of REEs up to pH 8. The presence of phosphate and carbonate ions in the solution increased the precipitation yield of REEs at lower pH values. Both Na₂HPO₄ and Na₂CO₃ were found to increase the precipitation of REEs at pH below 7, as over 85% of REEs were recovered. Calculated saturation indices and speciation diagrams for selected REEs confirmed the experimental data. Considering the elemental recovery values, environmental effects, as well as chemical consumption and cost, a two-step AMD treatment process using Na₂CO₃ was formulated. Through the proposed process, 90% of the aluminum was recovered in the first step (at pH 5), while 85% of REEs was recovered in the second step (at pH 7) with a significantly high concentration of 1.6%.