Saponified Gemfibrozil for Selective Recovery of Energy-Relevant Cobalt and Nickel from Magnesium-Rich Solutions

Abstract

Selective coextraction and recovery of Co and Ni from magnesium-rich sulfate solutions remain highly challenging due to the closely similar physicochemical properties of Co2+, Ni2+, and Mg2+. In this work, gemfibrozil (GEM) was employed as an extractant for the selective coextraction of Co2+ and Ni2+, enabling their efficient separation from Mg2+/Ca2+-containing solutions. The extraction behavior was systematically investigated, revealing that the separation performance was strongly influenced by the saponification degree, initial feed pH, extraction time, and Mg2+/(Co2+ + Ni2+) concentration ratio, with maximum separation factors of SFCo/Mg = 117.5 and SFNi/Mg = 172.7. The GEM system also exhibited excellent extraction-stripping cyclic stability, with Co2+ and Ni2+ loading capacities of 8.59 g/L and 8.99 g/L, respectively. Under optimized conditions, a counter-current extraction process consisting of three extraction stages, six scrubbing stages, and two stripping stages was established, producing a strip liquor with a total Co2+ + Ni2+ concentration of 18.5 g/L. The process increased the purity of Co2+ + Ni2+ from 12.5 wt % to 98.1 wt % and achieved an enrichment factor of more than nine compared with the initial feed solution. Mechanistic studies based on slope analysis and FT-IR spectroscopy revealed that the extraction of Co2+ and Ni2+ by saponified GEM proceeds primarily via a cation-exchange mechanism. This work provides an effective strategy for the selective recovery of cobalt and nickel from Mg-rich solutions and offers new insights into the design of separation systems for energy-critical metals in complex resources.