Redox Processes in the Organic Phase during Cobalt Extraction with the bis(2,4,4-trimethylpentyl)dithiophosphinic Acid and Trioctyl Phosphine Oxide Mixtures

Abstract

The redox processes taking place in the organic phase during cobalt extraction with the mixtures of bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HR, CYANEX 301) and trioctyl phosphine oxide (TOPO, CYANEX 921) have been studied. It was shown that CoR₂ stabilization by the phosphine oxide prevents cobalt oxidation with air oxygen as well as with the disulfide of bis(2,4,4-trimethylpentyl)dithiophosphinic acid (R–R). The introduction of TOPO into the extract containing Co(III) dithiophosphinate results in an increase in the rate of cobalt reduction to Co(II). Analysis of the interphase cobalt distribution as well as of the IR and electronic absorption spectra of the extracts showed that cobalt forms a pentacoordinated tetragonal-pyramidal high-spin complex with the dithiophosphinate ions and TOPO with a tetragonal-pyramidal geometry. The base of the pyramid is made up of sulfur atoms while an oxygen atom is on the top. The composition of the complex was found to be CoR₂·TOPO. Examples of the possible use of the CYANEX 301 and trialkyl phosphine oxide (TAPO, CYANEX 923) mixtures in cobalt extraction technology are given.

Keywords:

• liquid-liquid extraction
• cobalt
• bis(2,4,4-trimethylpentyl)dithiophosphinic acid
• trioctyl phosphine oxide
• redox processes

ACKNOWLEDGMENTS

Our special thanks go to CYTEC INDUSTRIES INC. Canada for free samples of CYANEX 301. The authors are grateful to the leading researcher N. I. Pavlenko of the ICCT SB RAS for help in recording the IR spectra. Appreciation for valuable remarks is extended to Prof. D. S. Flett.