Periodic Reporting for period 3 - SOLCRIMET (Solvometallurgy for critical metals)
Reporting period: 2019-09-01 to 2021-02-28
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.
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.