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Solvometallurgy for critical metals

Periodic Reporting for period 4 - SOLCRIMET (Solvometallurgy for critical metals)

Reporting period: 2021-03-01 to 2021-08-31

SOLCRIMET targets solvometallurgical solutions for the selective recovery of critical metals from end-of-life waste streams, using environmentally-friendly solvents. Solvometallurgy differs from hydrometallurgy by the absence of a discrete water phase. Most of the unit processes in solvometallurgy are very similar to those in hydrometallurgy, with the main difference that the water is replaced by a non-aqueous solvent. This new approach offers several advantages: (1) the consumption of water is very limited offering a major advantage in regions where there is a shortage of water; (2) the leaching and solvent extraction can be combined in a single step, which leads to simplified process flow sheets; (3) solvent leaching can be more selective than leaching with acidic aqueous solutions, leading to reduced acid consumption and less purification steps; (4) solvometallurgy is useful for the treatment of ores that are rich in soluble silica (such as eudialyte) as no silica gel is formed.

The overall objective is to develop a new, metallurgical approach – i.e. solvometallurgy – that can be applied to different types of pre- and post-consumer waste in order to recover the following critical metals: the rare earths, tantalum, niobium, gallium, indium, germanium and antimony. This processing must be environmentally friendly, economically viable and produce metals of acceptable purity. The achievement of the main objective is built on four sub-objectives:
• New routes for dissolving metals and alloys in organic solvents;
• New ways to separate metal chlorides based on differential solubility in organic solvents;
• New paths to purify critical metals using two mutually immiscible organic solvents;
• New techniques for refining critical metals in an electrolytic cell using organic solvents.
SOLCRIMET is focusing on trichloride ionic liquids as reactive solvents for metals. Ionic liquids are special solvents that consist entirely of ions. These ionic liquids can safely store chlorine gas at atmospheric pressure, yet they are reactive towards metals. A series of trichloride ionic liquids with different cations have been synthesised and characterised. Iron, copper, indium, zinc, gallium, antimony, gold, germanium, samarium and dysprosium dissolved fast in the ionic liquid, but platinum and tantalum were not soluble. This selective dissolution can be used for separation of metals. The differences in the solubility of metal chloride salts in water are not large enough to design efficient separation processes. However, there exist remarkable differences in the solubility of chloride salts in organic solvents. GaCl3 has a remarkably high solubility in aliphatic organic solvents, whereas other chloride salts are not. A separation process was developed to separate GaCl3 and AlCl3.

Solvent extraction is in industry for the separation and purification of metals, such as the rare earths, platinum-group metals, cobalt, nickel, copper and uranium. In conventional solvent extraction, metal ions are distributed between an aqueous phase and a water-immiscible organic phase. At equilibrium, the ratios of the concentrations of the metal ion in the organic phase and the aqueous phase are different for different metal ions. This is the basis for the separation of mixtures. SOLCRIMET extends solvent extraction to non-aqueous solvent extraction in which the metals are distributed between two immiscible organic phases. The extraction mechanisms are often different in aqueous and non-aqueous solvent-extraction systems. These differences in the mechanisms can be exploited to develop new, highly selective separation processes.
Different non-aqueous solvent extraction systems have been developed based on extractions from ethylene glycol with Cyanex 923 and Aliquat 336 as extractants. Knowledge of the speciation of the metal complexes in the two phases is required for an understanding of the extraction mechanism. Speciation studies give information about the composition (stoichiometry) and the structure of metal complexes in solution. For the speciation studies, vibrational spectroscopy (FTIR and Raman) is an invaluable tool. Information gained by this method is about the nature of the coordinating atoms and whether donor molecules are coordinating or not. Extended X-ray absorption fine structure (EXAFS) spectroscopy, which makes use of synchrotron radiation and which can be applied to different metal ions, provides information about the coordination number, the nature of the coordinating atoms and the interatomic distances between the metal and the first shell. The combination of extraction data and the structural information of the metal complexes in the two immiscible phases gives an insight into the solvent-extraction mechanism and the change in speciation upon extraction. Continuous solvent extraction tests are being carried out in a battery of mixer-settlers for the most promising non-aqueous solvent extraction systems. These include yttrium/europium separations and purification of indium.

Novel electroactive compounds and novel non-aqueous electrolytes are being developed for the electrodeposition of rare earths, gallium, indium and other critical metals. Liquid indium metal was electrodeposited from the ionic liquid trihexyltetradecylphosphonium chloride (Cyphos IL 101) at 180 degrees centigrade. The deposition of liquid indium allows for the easy separation of the indium and the possibility to design a continuous electrowinning process. Novel non-aqueous electrolytes for the electrodeposition of indium have been obtained by dissolving equimolar mixtures of In(Tf2N)3 and InCl3 in the solvents 1,2-dimethoxyethane (DME) and PEG400, and In(CH3SO3)3 in DMSO. Indium metal could be deposited below and above the melting point of indium. Gallium metal could be electrodeposited from GaCl3 in DME. Successful electrodeposition of rare earths was possible by using novel rare-earth borohydride complexes in DME or 2-methyltetrahydrofuran.
From a science/technology point of view, SOLCRIMET challenges existing paradigms and explores uncharted territory for the recycling of critical metals. SOLCRIMET creates new metallurgical systems for the recovery of critical metals from urban mines. The work can inspire other researchers to enter the field of solvometallurgy and invent other, complementary, solvometallurgical methods. Chemical engineers can further develop the processes to continuous closed-loop processes and work on the recycling of the solvents. SOLCRIMET explores the field of non-aqueous solvent extraction, i.e. solvent extraction with two mutually immiscible organic phases. This offers opportunities for designing new separation and purification methods for many metals, not only the critical metals targeted in this study. Besides non-aqueous solvent extraction, SOLCRIMET is developing other methodological approaches such reactive ionic liquids, electrorefining in organic solvents, the use of liquid electrodes, as well as high-pressure electrolysis cells. The success of the project will open up new horizons for the recycling of critical metals. By focusing on these critical metals – which are also essential for the transition to a low-carbon economy – SOLCRIMET addresses the “Climate action, environment, resource efficiency and raw materials” Grand Societal Challenge of the EU.
SOLCRIMET workpackages