Review on micro-mechanism of forming emulsification during rare earth extraction by acidic extractants

Solvent extraction is the main method used to separate and purify rare earth elements. In the process of rare earths extraction, emulsification often generated due to the instability of the aqueous and organic phases or improper operating conditions. Once emulsification occurs, it would not only lead to low rare earths recovery efficiency, small product quantities, high production costs and the losing of extractant and rare earth resources, but also result in serious environmental pollution. Therefore, it is very important to study the micro-mechanisms of emulsification and establish new methods to prevent emulsification at the source. In this paper, possible factors resulting in emulsification, such as the compositions and properties of the organic and aqueous phases, the operating conditions of the rare earths extraction are reviewed. The micro-mechanisms of emulsification are summarized basing on the microscopic structures in the bulk phase, aggregations of the extractants at the organic-aqueous interface, spectral characterizations and computational simulations. On this basis, new formation mechanisms are proposed for emulsification. Preliminary explorations are employed to verify the correctness of these new viewpoints. Finally, future directions for studies of the emulsification micro-mechanism are proposed. This study provides a theoretical basis for further understanding the micro-mechanisms of interfacial instability resulting in emulsification in the process of rare earths extraction.