Recovery of critical and strategic metals from secondary resources: Solvent extraction using the radiotracer technique

Securing economically and ecologically viable sources of certain strategic elements, such as chromium and vanadium, represents an important challenge facing Europe currently.[1] The CHROMIC project seeks to address this issue by developing novel processes for the procurement of such metals from secondary resources. Solvent extraction (SX) is a well-known method which can separate and concentrate such chemical species, and which can provide numerous advantages over more established pyrometallurgical processes for metal purification.[2] This present work aims to construct a solvent extraction process applicable to highly alkaline slag-leach-solutions for the selective recovery of chromium, vanadium, and eventually niobium.
The extractant used in this study is Aliquat 336, an ionic liquid composed of quaternary ammonium salt, dissolved in diluents such as kerosene and methyl isobutyl ketone.[3] Model feed solutions have been created based on the most probable leach solutions for this process, with pH in the range of 12 – 13 and chromate as the major species with an approximate chromium concentration of 1 g/L. Radiolabelling samples with the isotopes chromium-51 (half-life 27.7 days) and vanadium-48 (half-life 16.0 days) allows determination of the metal content in each phase, without the need for any sample preparation of the sort which is required for other analytical techniques such as ICP-OES or ICP-MS. To the best of our knowledge, this work represents the first use of ⁴⁸V radiotracer individually (and the first use of ⁵¹Cr radiotracer in combination) analysis for a solvent extraction study.[4]
Thus far experiments have principally been carried out on single-element solutions containing only one of the desired target elements. In this way we have elucidated the influence of various factors on the efficiency of extraction. The pH of extraction medium plays an important role, not just on the extractability where increasing pH causes a decrease in metal extraction, but also on the stability of the extracted solution. Similarly, the concentration of competing anions such as chloride, sulphate, or nitrate has a strong negative influence on the amount of metal extracted. For example, addition of 0.5 M sodium sulphate will in general reduce extraction of chromium to half the salt-free value. Kinetics of mass transport were determined to be rapid, a significant factor when it comes to scaling up the process from laboratory to industrial scale (mixer-settler unit). Extraction isotherms have been constructed for different possible sets of extraction conditions, and used to guide our continuing research into these systems. Preliminary experiments on mixed-element solutions seem to show coextraction of chromium and vanadium together; in future experiments a suitable scrubbing technique will be employed to attempt to fully separate and purify these elements.
The results obtained thus far indicate the viability of Aliquat 336 as extractant for these key strategic metals from expected leach solutions of industrial slags. The continuation of the project will focus on optimising the process under realistic industrial conditions.
1. Communication from the European commission to the European Parliament on Critical Raw Materials, 13.09.2017
http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:52017DC0490&from=EN
2. Weinhardt et al, Industrial and Engineering Chemistry,43(7), 1676-1684, (1951)
3. Wionczyk et al, Hydrometallurgy, 78, 116-128, (2005)
4. Katsuta et al, Journal of Radioanalytical and Nuclear Chemistry, 222(1-2), 45-50, (1997)