Solvometallurgy: An Emerging Branch of Extractive Metallurgy

Abstract

This position paper introduces the reader to the concept of solvometallurgy, the term used to describe the extraction of metals from ores, industrial process residues, production scrap, and urban waste using non-aqueous solutions. Here, non-aqueous is not used to imply anhydrous, but rather a low water content. The unit operations are as follows: solvent leaching; separation of the residue; purification of the leach solution by non-aqueous solvent extraction or non-aqueous ion exchange; and metal recovery by precipitation or electrolysis in non-aqueous electrolytes. Solvometallurgy is similar to hydrometallurgy in that both the branches of extractive metallurgy use lowtemperature processes, but with solvometallurgy, there is no discrete water phase. Both branches use organic or inorganic solvents (excluding water in the case of solvometallurgy). However, for solvometallurgical processes to be sustainable, they must be based on green solvents, which means that all toxic or environmentally harmful solvents must be avoided. Solvometallurgy is complementary to pyrometallurgy and hydrometallurgy, but this new approach offers several advantages. First, the consumption of water is very limited, and so the generation of wastewater can be avoided. Second, the leaching and solvent extraction can be combined to form a single step, resulting in more simplified process flow sheets. Third, solvent leaching is often more selective than leaching with acidic aqueous solutions, leading to reduced acid consumption and fewer purification steps. Fourth, solvometallurgy is useful for the treatment of ores rich in soluble silica (e.g., eudialyte) because no silica gel is formed. In short, solvometallurgy is in a position to help develop the near-zero-waste metallurgical processes, and with levels of energy consumption that are much lower than those with the high-temperature processes. The Technology Readiness Level (TRL) of this emerging branch of extractive metallurgy is still low (TRL = 3-4), which is a disadvantage for short-term implementation, but offers a great opportunity for further research, development, and innovation.

Removal of metallic coatings from rare-earth permanent magnets by solutions of bromine in organic solvents

“…No solvometallurgical treatments, in which the lixiviant is dissolved in an organic solvent, have been tested yet to dissolve metallic coating from REE PMs. 17,18 In this paper, a selective solvometallurgical method is described to remove the nickel-copper-nickel coating from Sm-Co and Nd-Fe-B PMs, using a solution of elemental bromine in an organic solvent. In the literature, bromine in organic solvents such as ethyl acetate or methanol has been reported to be able to dissolve metals such as uranium, thorium, zirconium, titanium, iron, chromium and nickel.…”

Scandium and Titanium Recovery from Bauxite Residue by Direct Leaching with a Brønsted Acidic Ionic Liquid

“…For these reasons and thanks to the vast number of combinations of the cation and the anion during synthesis, ILs have potential for many applications, such as solvent extraction [26,27], catalytic reactions [28,29], and electrodeposition of metals [30,31]. In the past few decades, ILs have been used also as lixiviants for metals dissolution [25,[32][33][34][35]. Applying ionic liquid leaching on secondary raw material resources eventually improves efficiency yields, reduce waste effluent, and increases selectivity.The aim of this work is to investigate the direct leaching of bauxite residue by using a Brønsted acidic ionic liquid, achieving high Ti and Sc recovery yields.…”

Using Life Cycle Thinking to Assess the Sustainability Benefits of Complex Valorization Pathways for Bauxite Residue

“…Approaches to reduce silica gel formation, including dry digestion with mineral acids and the addition of hydrogen peroxide are also being investigated [26,29]. The use of ionic liquids, molten salts which can act as highly selective solvents [30], is another important line of inquiry. Direct leaching of BR using ionic liquids is being investigated as a route for the extraction of valuable metals [31].…”