Development of green supercritical fluid extraction processes for the selective recovery of strategic and critical metals from e-waste products

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1. Introduction and research opportunity Waste electrical and electronic equipment (e-waste) are full of valuable materials, including strategic and critical metals which can be recovered favoring sustainable development and circular economy. Reviewing metal recovery processes using hydrometallurgical methods, researchers emphasized the need for new selective leaching and extraction approaches for most of the valuable metals. These processes also need to be greener, avoiding the use of strong inorganic acids and toxic organic solvents frequently used today. To face these challenges, an original research program is proposed including the development of a green and selective supercritical fluid extraction process using promising metal ligands. 2. Summary of the scientific approach Biodegradable and mild organic acids will be used to solubilize metals from selected e-waste (LCDs, capacitors, and fluorescent tubes). Different ligands (N,N-di(alkyl)acetamide, aminopolycarboxylates, crown ethers) will then be evaluated to selectively extract targeted metals (indium, tantalum or rare-earths) from the reaction medium to a supercritical carbon dioxide (ScCO2) phase which will act as green solvent. The CO2 decompression will allow the recovery of the metal complex which will be treated to recover the ligand and the metal of interest. 3. Requested equipment To accomplish this research, an experimental setup is needed and should include a: high-pressure stirred reactor equipped with an electric heater for temperature control and windows for phases observations, CO2 pump, co-solvent pump (ligands or additives introduction), back-pressure regulator (pressure regulation and continuous mode of operation), product recovery vessel, liquid autosampler, and control module (computer communication and data acquisition). 4. Enabled research activities (i) This setup will be used for the development and optimization of the ScCO2 extraction processes from continuous/semi-batch/batch experiments. The impact of ScCO2 on the leaching efficiency will also be studied. (ii) Solubility studies (ligands, metal salts and chelates, and organic acids) in ScCO2 will be performed and are essential to evaluate and predict the species phase distribution and extraction performances. Solubility will be measured either by liquid sampling (GC or HPLC analysis) or by cloud point visual observation. 5. Importance of this research Many aspects of this research program are original and deserve to be investigated. The combination of ScCO2 extraction with novel ligands to selectively separate valuable metals from e-waste will bring vital experimental and fundamental knowledge to many scientific disciplines (process engineering, mining, metallurgy) while being industrially highly relevant. This research is environmentally friendly and will enable saving natural resources and energy, reducing pollution, and make Canada less dependent to international suppliers of strategic and critical metals.