Solvent extraction (SX) can play a central role in the development of circular hydrometallurgical processes if a solution can be found for the consumption of bases in the extraction step and acid in the stripping step of solvent extraction processes with acidic extractants. The CIRMET project develops several innovative approaches to reduce the consumption of acids and bases. The first approach is to leach with concentrated acids (so the proton activity is high), while recovering the excess acid after leaching by SX. It is important that the extracted acid is stripped from the organic phase by (hot) water, and not a base, since this would destroy the acid. The recovered acid can be sent directly to the leaching concentration, or can be concentrated further, step-by-step, to a very high concentration in a sequence of extraction and stripping steps with well-chosen phase ratios. This has been demonstrated for the recovery of methanesulfonic acid (MSA). Second, the concept of “dual SX” is being elaborated. Here, two SX circuits are coupled: one for metal extraction and one for acid removal. Between every extraction step, the protons released are removed from the aqueous solution by SX instead of adding a base. The proof-of-principle of this approach has been demonstrated for SX of iron(III). Third, acidic extractants are preloaded with Mg so that extraction involves exchange of Mg ions rather than protons. The exchanged Mg ions are recovered downstream as MgCO3, which can be reused for preloading the organic phase.
Metal carbonates are very useful to remove metal ions from a hydrometallurgical system because it will not remove the anions that are required for the regeneration of the acid. Rather than precipitating by addition of soda ash, CIRMET is developing processes based on SX-assisted precipitation of metal carbonates, using CO2 gas. The protons formed by reaction of CO2 with water are removed by acid extraction with a basic solvent and the acid can be recovered from the loaded organic phase by stripping with hot water. The proof of principle has been given for the precipitation of Li2CO3, CaCO3 and transition metal carbonates, and the process is being further optimized.
CIRMET is developing novel approaches to remove iron from hydrometallurgical systems, such as SX-assisted precipitation of FeCO3, removal of iron from the loaded organic phase by hydrolysis at high temperatures, and direct hydrogen reduction of iron in organic solutions by hydrogen gas. The proof of principle for the production of metallic iron has been given.
In CIRMET, it has been shown that the extraction mechanism at conditions observed in hydrometallurgy is different from what is currently accepted and written in textbooks; the the cobalt/nickel selectivity of acidic extractants has been explained, and it is rationalized why commercial extractants of technical grade purity perform often better than the purified extractants.
