Redox processes during cobalt extraction with bis(2,4,4-trimethylpentyl)dithiophosphinic acid

The paper presents data on cobalt extraction from sulfate solutions with bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HR, trade mark CYANEX 301) diluted with nonane and toluene. Using the cobalt distribution data, electronic spectroscopy and IR-spectroscopy methods, it was concluded that cobalt extraction with bis(2,4,4-trimethylpentyl)dithiophosphinic acid is characterized by a fast reaction resulting in the formation of the tetrahedral СoR2 complex in the organic phase followed by Co(II) oxidation with air oxygen and the formation of the octahedral СoR3 complex.Using a spectrophotometric method, it was shown that the oxidation rate of cobalt with air oxygen essentially increases in the presence of dithiophosphinic acid in the organic phase. In the absence of HR, there is no evidence of cobalt oxidation in the extracts over a long time period (up to 24 h). The cobalt oxidation reaction is first order with regard to both the metal and bis(2,4,4-trimethylpentyl)dithiophosphinic acid concentrations.Oxidation of cobalt(II) dithiophosphinate to cobalt(III) dithiophosphinate by the disulfide of dithiophosphinic acid (R–R) has been studied. The cobalt oxidation reaction is second order with regard to the metal concentration and first order with regard to the disulfide concentration. The redox process was shown to be reversible. The stoichiometry of the reaction has been determined.Suggestions for prevention of cobalt oxidation with air oxygen during extraction with bis(2,4,4-trimethylpentyl)dithiophosphinic acid are given.

► Redox processes during cobalt extraction with CYANEX 301 (HR) were studied.
► In the absence of HR, there is no evidence of cobalt oxidation in the extracts.
► Oxidation of CoR2 to CoR3 by the disulfide of HR has been studied.
► The redox process was shown to be reversible.
► The stoichiometry of the reaction has been determined.