Enhanced pH adaptability in vanadium solvent extraction: Synergistic coordination of octanohydroxamate and D2EHPA for efficient extraction at low pH

Di-(2-ethylhexyl) phosphoric acid (D2EHPA) is the predominant extractant for vanadium recovery from acidic leachates. However, its narrow pH extraction window necessitates pH adjustment prior to extraction, generating significant neutralization residues and causing vanadium loss. Therefore, enhancing the pH adaptability of D2EHPA to enable direct vanadium extraction at low pH is of significant importance. This study proposes an octanohydroxamate (OHA)-D2EHPA extraction system, which broadens the extraction pH range through intermolecular cooperative coordination, providing an innovative solution for the efficient separation of vanadium from impurity ions under low-pH conditions. Under optimized conditions (OHA:V molar ratio of 2:1, pH of 0.6, D2EHPA concentration of 20 vol%, organic-to-aqueous volumetric phase ratio(O/A) (phase ratio(O/A)) of 1:2, 20 min extraction at room temperature), a three-stage countercurrent extraction process achieved a vanadium extraction efficiency of 99.56 %. Using a mixed solution of sulfuric acid and oxalic acid as the stripping agent under optimal conditions (0.4 mol/L H 2 C 2 O 4 +2 mol/L H 2 SO 4 , phase ratio(O/A) of 5:1, 20 min stripping at 25°C), three-stage stripping reached a vanadium stripping efficiency of 99.21 %, enabling highly efficient separation of vanadium from impurities. Studies confirm that the extraction process follows a H⁺-desorbing cation-exchange mechanism, where synergistic dehydrogenation of N-OH groups in OHA and P-OH groups in D2EHPA facilitates vanadium coordination. Vanadium complexes in the organic phase exist as [C 8 H 16 NO 2 ]VO, [C 8 H 16 NO 2 ]VOA, and [C 8 H 16 NO 2 ]VOA(HA). Among these, [C 8 H 16 NO 2 ]VOA(HA) exhibits the lowest binding energy for vanadium and is preferentially formed during extraction. Consequently, the OHA-D2EHPA system achieves highly efficient vanadium extraction at low pH.