Separation of transition metal oxoanions from complex feed solutions by solvent extraction

Certain metals, which can be defined as “strategic” due to a predicted high future demand and/or unstable supply, can currently be found trapped in the slag by-products of European steel production.[1-3] Notably these metals include chromium and vanadium. The H2020 project CHROMIC seeks to develop a process for the separation and purification of these metals from the slag matrix host, leaving the slag as a valorisable product for use in e.g. construction. This process will involve selective high-alkaline oxidative leaching of the metal value, followed by solvent extraction (SX) on the alkaline leach solutions thus-produced.
The alkaline and oxidising conditions mean the target metals are present in the form of oxoanions in the leach liquor; therefore a cationic extraction agent, namely Aliquat 336, is employed for the SX method. This extractant has proven capable of selectively removing chromium(VI) and vanadium(V) from complex mixtures representative of leach solutions from industrial slags, with a strong influence of the pH value and the presence of competing anions. To optimise the procedure we use the radiotracer technique, labelling the metal concentration with the produced-in-house isotopes ⁵¹Cr and ⁴⁸V respectively.[4,5] This technique offers many advantages over conventional options, including high precision measurement at extremely low concentrations, no effect of the solvent medium, and no need to further workup the solution after phase separation. Our results indicate that chromium(VI), which is the major species in the leachate feed, is preferentially extracted by the Aliquat 336 system over all other species present under most conditions. Studies are underway to examine the behaviour of Cr(III) in these systems, as this ion may be present in place of Cr(VI).
References
[1] Report by Grand View Research, Chromium Market Outlook Report By Application (Metallurgy, Chemicals, Refractory), By Region (North America, Europe, Asia Pacific, Central & South America, Middle East & Africa), And Segment Forecasts, 2018 – 2025, Report ID GVR-2-68038-305-8, (2018).
[2] Communication of the European Commission, COMMUNICATION FROM THE COMMISSION TO THE EUROPEAN PARLIAMENT, THE COUNCIL, THE EUROPEAN ECONOMIC AND SOCIAL COMMITTEE AND THE COMMITTEE OF THE REGIONS on the 2017 list of Critical Raw Materials for the EU, (2017).
[3] Bio Intelligence Service, Study on data for a raw material system analysis: Roadmap and test of the fully operational MSA for raw materials – Final report, for the EC DG GROW, (2015).
[4] Katsuta et al, Journal of Radioanalytical and Nuclear Chemistry, 222(1-2), 45-50, (1997).
[5] Bonardi et al, Journal of Radioanalytical and Nuclear Chemistry, 263(1), 23-28, (2005).