Abstract
The modes of action of the commercial solvent extractants used in extractive hydrometallurgy are classified according to whether the recovery process involves the transport of metal cations, M(n+), metalate anions, MXx(n-), or metal salts, MXx into a water-immiscible solvent. Well-established principles of coordination chemistry provide an explanation for the remarkable strengths and selectivities shown by most of these extractants. Reagents which achieve high selectivity when transporting metal cations or metal salts into a water-immiscible solvent usually operate in the inner coordination sphere of the metal and provide donor atom types or dispositions which favour the formation of particularly stable neutral complexes that have high solubility in the hydrocarbons commonly used in recovery processes. In the extraction of metalates, the structures of the neutral assemblies formed in the water-immiscible phase are usually not well defined and the cationic reagents can be assumed to operate in the outer coordination spheres. The formation of secondary bonds in the outer sphere using, for example, electrostatic or H-bonding interactions are favoured by the low polarity of the water-immiscible solvents.
Highlights
(2) Extractive metallurgy is an efficient method for metal refining and is underpinned by coordination chemistry
(5) An understanding of the fundamental coordination chemistry allied to solvent extraction processes can form the basis of new reagent designs for more efficient and sustainable metal refining
The Hofmeister bias underpins the efficient recovery of gold as its monoanion, AuCl4À, from mixed-metal chloride streams and several of the reagents listed in Table 5 have been used in gold recovery, including the triether dibutylcarbitol (DBC) which has been exploited by a number of Platinum Group Metal (PGM) refiners
Summary
Reagents which achieve high selectivity when transporting metal cations or metal salts into a water-immiscible solvent usually operate in the inner coordination sphere of the metal and provide donor atom types or dispositions which favour the formation of stable neutral complexes that have high solubility in the hydrocarbons commonly used in recovery processes. In the extraction of metalates, the structures of the neutral assemblies formed in the water-immiscible phase are usually not well defined and the cationic reagents can be assumed to operate in the outer coordination spheres. (3) Reagents used commercially in the selective separation of metals by solvent extraction can be classified according to their modes of operation. (5) An understanding of the fundamental coordination chemistry allied to solvent extraction processes can form the basis of new reagent designs for more efficient and sustainable metal refining
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