Efficient separation of rhenium and molybdenum in phosphoric-sulfuric mixed-acid medium via tertiary amine extraction

Abstract

The selective recovery of rhenium (Re) from molybdenum (Mo)-rich hydrometallurgical solutions is a persistent challenge, hindered by the chemical similarity of the two elements and the poor performance of conventional extraction systems. This study elucidates a highly efficient separation strategy using the tertiary amine extractant N235, uniquely enabled by a synergistic phosphoric-sulfuric mixed-acid medium. We reveal a novel dual-pathway separation mechanism termed “acidity regulation and ion competition.” Specifically, sulfuric acid modulates the solution acidity to promote the dissociation of extractable molybdate anions (PMo₁₂O₄₀³⁻) into non-extractable MoO₂²⁺. Concurrently, phosphoric acid introduces a high concentration of competitive phosphate anions that saturate the extractant's active sites, effectively suppressing the co-extraction of any residual anionic Mo species. This synergistic effect, corroborated by FTIR and DFT calculations, leads to exceptional selectivity. Under optimized conditions, a single-stage Re extraction of 98.6% was achieved with only 1.4% Mo co-extraction, yielding an outstanding separation factor (βRe/Mo) of 4183. Furthermore, the loaded Re was efficiently stripped (>98.2%) using 1 mol/L aqueous ammonia, and the organic system demonstrated excellent stability, maintaining over 96.3% extraction efficiency after six cycles. This work presents not only a robust and industrially viable process but also a new mechanistic paradigm for separating chemically analogous metals by synergistically manipulating solution speciation and competitive equilibria.

Keywords

rhenium / molybdenum / separation / solvent extraction / tertiary amine / mixed-acid media

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Central South University