A. Mishrra, B.C. Hedin, H. Hsu-Kim
Duke University,
United States
Keywords: acid mine drainage, critical minerals, iron oxidation, staged precipitation
Summary:
Acid mine drainage (AMD) is a potential secondary source of critical minerals such as rare earth elements (REEs) and cobalt (Co). Although typically low in REE content, AMD can be treated via pH-controlled precipitation to selectively recover these elements. As pH increases, REEs and Co are removed through adsorption or co-precipitation with Al- and Mn-bearing solids. The composition of these precipitates depends on the feedstock chemistry and pH. Iron (Fe), particularly its oxidation state, plays a key role in determining the grade of recovered REEs and Co. While Fe does not directly bind REEs, its redox state affects precipitate composition and can dilute REE concentrations in the solids. To assess this, batch experiments with real and simulated AMD with either Fe(II) or Fe(III) dominant, were carried out to evaluate how Fe redox chemistry influences the formation structure, and enrichment potential of the resulting solids. Precipitates were collected at the onset of turbidity and recovered by sequential vacuum filtration (0.7 μm followed by 0.22 μm). Samples were immediately frozen and freeze-dried at –85 °C. Filtrate and solids were analyzed by ICP-MS, a fraction of solids digested in aqua regia for elemental quantification. The elemental composition of both the leachate and the resulting solids was determined using inductively coupled plasma mass spectrometry (ICP-MS) and complementary solid-phase characterization techniques. A total recovery of 95–97% for rare earth elements (REEs) and 99% for cobalt (Co) was observed in Fe(III)- and Fe(II)-containing feedstocks as the pH was increased from 2 to 10. However, the grade of REE and Co was threefold higher in Fe(III)-containing AMD compared to Fe(II), indicating the importance of iron oxidation state in enrichment. Through precipitation and filtration, 28 mg/g of total REEs and 33 mg/g of Co were obtained at pH 8.95. The purity of REE and Co at this pH was high due to the prior removal of nearly all Fe and Al fractions at pH 4.3 and 5.3, resulting in a relatively low mass of solids formed at pH 9, comprised predominantly of Mn (22 wt%). In contrast, the presence of Fe(II) in AMD reduced Fe removal in early pH stages, leading to lower recovery: only 9 mg/g of total REEs at pH 6–7 and 14 mg/g of Co at pH 9. Due to the slow oxidation of Fe(II) to Fe(III), Fe was gradually removed as pH increased. Therefore, the presence of Fe(III) in AMD is critical for achieving higher enrichment of critical minerals. In addition, Fe speciation influenced REE sequestration in the treatment solids. When Fe solids were allowed to form but not removed from solution—i.e., precipitation occurred without filtration—these suspended Fe-bearing solids sequestered up to 80% of REEs and 99% of Al at pH 5.3. The presence of Fe(III) solids was found to be essential for the co-precipitation of REEs with Al as pH increased. These results underscore the role of Fe(III) in enhancing REE retention and offer valuable guidance for optimizing AMD treatment processes for critical mineral recovery.