Advances in Understanding of the Application of Unit Operations in Metallurgy of Rare Earth Elements

Unit operations (UO) are mostly used in non-ferrous extractive metallurgy (NFEM) and usually separated into three categories: hydrometallurgy, pyrometallurgy, and electrometallurgy. The high demand for critical metals, such as rare earth elements (REE), indium, scandium, and gallium raises the need for an advance in understanding of the UO in NFEM. The aimed metal is first transferred from ores and concentrates to a solution using a selective dissolution under atmospheric and high pressure conditions in an agitating glass reactor and an autoclave or tubular reactor. The purification of the obtained solution was performed using neutralization agents or selective precipitation agents. Separation of metals is achievable through solvent extraction and solid-liquid filtration, with crystallization converting the liquid compound into a crystalline state. The final metal production follows different methods, including aqueous and molten salt electrolysis. Advanced processes, such as ultrasonic spray pyrolysis and microwave-assisted leaching, can enhance metal powder production. This review outlines the critical advancements and methodologies in the application of UO in metallurgy concerning REE and highlights novel approaches to optimize metal extraction and production processes.