Crystal Engineering in Antisolvent Crystallization of Rare Earth Elements (REEs)

Antisolvent crystallization is a separation technology that separates the solute from the solvent by the addition of another solvent, in which the solute is sparingly soluble. High yields are achieved by using higher antisolvent-to-aqueous ratios, but this generates higher supersaturation, which causes excessive nucleation. This results in the production of smaller particles, which are difficult to handle in downstream processes. In this work, the effect of varying the organic (antisolvent)-to-aqueous (O/A) ratio and seed loading on the yield, particle size distribution, and morphology of neodymium sulphate product, during its recovery from an aqueous leach solution using antisolvent crystallization, was investigated. A batch crystallizer was used for the experiments, while ethanol was used as an antisolvent. Neodymium sulphate octahydrate [Nd2(SO4)3.8H2O] seeds were used to investigate the effect of seed loading. It was found that particle sizes increased as the O/A ratio increased. This was attributed to the agglomeration of smaller particles that formed at high supersaturation. An O/A ratio of 1.4 resulted in higher yields and particles with a plate-like morphology. The increase in yield was attributed to the increased interaction of ethanol molecules with the solvent, which reduced the solubility of neodymium sulphate. Increasing the seed loading resulted in smaller particle sizes with narrow particle size distribution and improved filtration performance. This was attributed to the promotion of crystal growth and suppression of agglomeration in the presence of seeds.