Process intensification by resonant vibratory mixing for samarium–cobalt magnet leaching with deep eutectic solvents

Abstract

Due to limited primary resources and to fulfil the demand for renewable energy in the automobile industry, defence applications, and other high-tech applications, recycling rare earth elements (REEs) and other critical elements from their secondary resources is imperative. Rigorous research and development for viable extraction and separation of REEs and other critical elements has resulted from this need. Previously, Sm–Co recycling has been done using pyrometallurgy, physical separation, and hydrometallurgy, all of which have energy, cost, time, and environmental limitations. Chemical leaching has successfully recovered and separated Sm–Co but has limitations associated with slow mass transfer and leaching kinetics, especially using viscous lixiviants, and there is a need to intensify the process for efficiency. Additionally, environmental impact is high due to extensive use of toxic, corrosive, non-selective, and expensive reagents. In this research, chemical leaching of Sm–Co was performed using Deep Eutectic Solvents (DESs), a new class of solvents under solvometallurgy. Four different DESs were studied, which are green, non-toxic, biodegradable, cheap, and selective. To enhance the mass transfer and reduce leaching time, resonant vibratory mixing (RVM) was tested. RVM intensifies mixing by establishing low energy conditions through vessel resonance, improving kinetics. Four DES combinations were prepared: choline chloride and tetrabutylammonium chloride, with oxalic acid, urea, and ethylene glycol at 80 °C. Chemical leaching of Sm–Co was performed using factors of time (hours), temperature (°C), and combination of DESs. Post-leaching, the samples were process intensified using RVM (time, intensity, and four DESs). Filtered aliquots were analysed with ICP-OES. Oxaline was the most selective DES, achieving 82% Co conversion after 2 hours of leaching and 30 minutes of RVM at 80 °C. A preliminary kinetic study determined the activation energy barrier and showed an increase in conversion (%) for both Co and Sm. 93.4% Co and 5.3% Sm conversions were achieved with only 5 hours of leaching at 90 °C, and after process intensification with RVM, conversions of 97% Co and 9.3% Sm were achieved.