Process Development for Rare Earth Elements Recovery and Struvite Production from Biocrudes

Phytomining emerges as an innovative technique for extracting rare earth elements (REEs) from soil by employing hyperaccumulators. REE hyperaccumulators were treated using microwave-assisted hydrothermal carbonization (MHTC) in acid-mediated systems to efficiently transfer REEs and other elements into biocrudes and produce high purity and value-added hydrochar. However, the subsequent treatment of biocrudes to recover valuable elements still presents a significant challenge. In this study, a process that combines solvent extraction and struvite precipitation was first developed to address this challenge. In the extraction step, 95.6% of REEs were extracted using 0.05 mol/L di(2-ethylhexyl)phosphoric acid (D2EHPA) with an aqueous to organic (A/O) ratio of 1:1 at pH 3.0. However, 75.1% of Al, 81.2% of Ca, 54.5% of Fe, 61.5% of Mn, and 81.3% of Zn were co-extracted into the organic phase with the REEs. To solve this issue, a subsequent scrubbing step using deionized water was applied, with the removal of over 98% of these impurities, while incurring negligible loss of REEs. After the scrubbing step, over 97% of REEs were ultimately stripped out from the organic phase as REE oxalates using 0.01 mol/L oxalic acid as the stripping agent. Furthermore, phosphorous (P) was found to be retained in the raffinate after the solvent extraction process. 94.4% of the P was recovered by forming struvite precipitate at pH 9.0 and a Mg/P molar ratio of 1.5. In general, high purity and value-added REE products and struvite precipitate were eventually achieved from biocrudes in environmentally friendly and economically viable ways.