Abstract
The development and recycling of valuable metals, especially rare earths, has recently become a major economic and industrial issue. Extraction and recovery of rare earths are strategic for many industries such as automotive, aviation, electronics and renewable energies. To remediate with this dependence, many countries are developing research programs concerning the recycling of rare earths present in the mining waste, the urban ores. In this work, cerium is the rare earth studied due to its abundance (66 pbm in the earth's crust), its use for the permanent magnets production, cathode ray tubes or fluorescent lamps. Cerium extraction by hydrometallurgy/pyrometallurgy processes generally use conventional organic solvents, harmful for the environment. In order to reduce the amount of organic effluents, supercritical CO2 (SC-CO2) is a cheap with low-environmental impact. It is an accessible solvent due to its low critical point (TC = 31 ° C, PC = 7.38 MPa). Moreover, CO2 is recyclable and chemically stable. Since CO2 is an apolar molecule and will not be able to extract the metal directly. It is essential to use an apolar ligand sufficiently soluble in SC-CO2, with high affinity towards cerium. Generally, organophosphorus compounds are used for cerium extraction in SC-CO2 medium, in particular tributylphosphate-nitric acid. In this study, the molecules used are extractants with an amidophosphonate backbone that have a potential extraction towards rare earths, in particular cerium, not yet been investigated. The solubility of such ligands and the influence of the length and branching of the alkyl chains present was studied. Moreover, the addition of ethanol/isopropanol allowed a gain in solubility, profitable for the extraction of cerium deposited on a solid cotton support. Extraction results showed that when cerium forms aggregates, extraction is facilitated whereas when cerium forms multilayers, extraction becomes difficult. Futhermore, by adding a co-solvent, the collection and extraction yields are multiplied by 3 in the absence of water and by 6 in the presence of water. In order to explain the gains in terms of extraction, the diffusion of small angle X-rays (SAXS) coupled to a high-pressure cell made it possible to determine the potential presence of structures formed in a SC-CO2 medium. This made it possible to establish a link between the presumed structure of the extractant system (microscopic scale) and the cerium extraction (macroscopic scale). The potential structures retained are spheres consisting of a mixture of ligand and CO2 molecules. In the presence of co-solvent the structure contains one more ligand relative to the observed structure without co-solvent (resp. 6 and 5 ligands), which could explain the higher extraction yields in the presence of co-solvent. First prospective extraction runs on rare earth have been carried out and seems to prove selectivity towards rare earths with atomic number between 58 and 64. These results have to be confirmed by extraction runs on real ore or Waste Electronic and Electrical Equipment (WEEE).