Speciation and separation of rhodium(III) from chloride solutions: A critical review

The recovery and purification of rhodium has always been difficult because of its complex aqueous chemistry in chloride solutions. A review of this chemistry, with special emphasis on speciation, is presented. Using available kinetic data, a new speciation diagram is proposed and it is shown that, even in relatively strong chloride solutions, there are at least two different rhodium aquo/chloro anionic species. In fact, it is shown that aquation may be even more prevalent than previously thought. This has serious implications on the behaviour of such solutions towards modern recovery processes such as solvent extraction. In addition, published solvent extraction and ion exchange separation schemes are reviewed, demonstrating the difficulties that have been encountered in devising a suitable process, due to the aqueous chemistry of rhodium.