Solvent extraction of rhodium using amines and amides Andrew Carrick 1 , Dr Euan Doidge 1 , Alexander Bouch 1 , Dr Gary Nichol 1 , Dr Jane Patrick 2 ,
Dr Emma Schofield 2 , Prof. Carole Morrison 1 , Prof. Jason Love 1 1 University of Edinburgh, UK, 2 Johnson Matthey Technology Centre, UK
The recovery of rhodium during platinum group metal (PGM) refining is particularly challenging due to its speciation in hydrochloric acid. 1 In this work, we present a new synergistic system which exploits a combination of amines and amides to recover rhodium by solvent extraction (SX). 2 In a typical refining flowsheet, PGMs are dissolved in HCl and the resulting chloridometalates separated sequentially by SX, distillation, or precipitation steps. 3 Rhodium is usually recovered last in this process with single-use precipitants as the formation of aquated complexes [RhCl n (H 2 O) 6-n ] (n-3)− in hydrochloric acid means there are multiple target complexes to consider when designing a reagent for SX. An effective SX process for rhodium could offer significant environmental and economic benefits over single-use precipitants. For the first time we show that the synergistic combination of 2-ethylhexylamine (L A ) and 3,5,5-trimethylhexanamide (L 1 ) adapts to the rhodium speciation in the aqueous phase, extracting two different rhodium complexes depending on the conditions. The ion-pair [HL A ] 3 [RhCl 6 ] is predominately extracted from high HCl concentrations, while the amide complex [HL A ] 2 [RhCl 5 (L 1 )], in which the amide is tautomerised to the enol form and coordinated via the nitrogen atom, is recovered from lower HCl concentrations. Additional L A HCl molecules are found to cluster around these complexes in the organic phase to form hydrophobic supramolecular assemblies.
By adapting to the conditions, the synergistic components maximise the overall rhodium recovery. The metal is readily stripped from the loaded organic phase using a dilute base such as aq. NaOH, recycling the extractants in the organic phase for further extraction cycles. This process demonstrates the potential of using synergistic reagents to target different metal complexes when designing future extractants. References 1. Narita, H.; Kasuya, R.; Suzuki, T.; Motokawa, R.; Tanaka, M., Precious Metal Separations. Encycl. Inorg. Bioinorg. Chem. 2020 , 1-28. 2. Carrick, A. I.; Doidge, E. D.; Bouch, A.; Nichol, G. S.; Patrick, J.; Schofield, E. R.; Morrison, C. A.; Love, J. B., Simple Amides and Amines for the Synergistic Recovery of Rhodium from Hydrochloric Acid by Solvent Extraction. Chem. Eur. J. 2021, 27 (34), 8714-8722. 3. Crundwell, F. K.; Moats, M. S.; Ramachandran, V.; Robinson, T. G.; Davenport, W. G., Chapter 37 - Refining of the Platinum- Group Metals. In Extractive Metallurgy of Nickel, Cobalt and Platinum Group Metals , Crundwell, F. K.; Moats, M. S.; Ramachandran, V.; Robinson, T. G.; Davenport, W. G., Eds. Elsevier: Oxford, 2011; pp 489-534.
P29
© The Author(s), 2022
Made with FlippingBook Learn more on our blog