Citation: Sibanda, J.; Chivavava, J.; Lewis, A.E. Crystal Engineering in Antisolvent Crystallization of Rare Earth Elements (REEs). Minerals 2022, 12, 1554. https://doi.org/10.3390/ min12121554 Academic Editor: Chiharu Tokoro Received: 31 October 2022 Accepted: 28 November 2022 Published: 1 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article Crystal Engineering in Antisolvent Crystallization of Rare Earth Elements (REEs) Jonathan Sibanda , Jemitias Chivavava and Alison Emslie Lewis * Crystallization and Precipitation Research Unit, University of Cape Town, Cape Town 7700, South Africa * Correspondence: alison.lewis@uct.ac.za Abstract: 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 supersatura- tion, 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 [Nd 2 (SO 4 ) 3 .8H 2 O] 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 perfor- mance. This was attributed to the promotion of crystal growth and suppression of agglomeration in the presence of seeds. Keywords: precipitation; seeding; agglomeration; neodymium; supersaturation 1. Introduction Rare Earth Elements (REEs) consist of 15 lanthanides, from lanthanum (La) to lutetium (Lu), including yttrium (Y) and scandium (Sc) [1]. REEs, such as neodymium (Nd), praseodymium (Pr), and dysprosium (Dy), are used in applications, such as Hybrid Electric Vehicles (HEVs), magnets for wind turbines, and fuel cells [2–5]. The use of REES in these applications has increased their demand significantly. There has been an impetus to develop strategies and ways of recycling REEs due to concerns about the future availability of these elements. One such strategy is to recover REEs from spent Nickel Metal Hydride (NiMH) batteries. As of 2016, the production of NiMH batteries was one billion cells per year due to their use in HEVs [6], which therefore presents a major source for recycling REEs at the end of life of the NiMH batteries. To recover the REEs from the NiMH battery, the battery is dismantled, and the anode material is leached using a strong acid. The residue is then separated from the leach solution using filtration [7]. Crystallization and precipitation methods have been employed to recover the REEs from the leach solution. Antisolvent crystallization is a potentially novel method in metallurgy that could be used. It involves the addition of a solvent, in which the solute is sparingly soluble, to induce the crystallization or precipitation of the solute [8]. This method has the advantage that the antisolvent can be recovered and recycled. The process can be carried out at ambient temperatures, which, aside from convenience and economic considerations, is important for Minerals 2022, 12, 1554. https://doi.org/10.3390/min12121554 https://www.mdpi.com/journal/minerals