Chemical speciation of RhIII complexes in acidic, halide-rich media by means of 103Rh NMR spectroscopy : the importance of speciation in the selective separation and recovery of rhodium

[摘要] ENGLISH ABSTRACT: In this thesis, the recovery of RhIII from both synthetically prepared and authenticindustrial PGM-containing solutions was systematically investigated via organic precipitationmethods using several commercially available, N-containing organic receptors including(amongst others) diethylenetriamine (Deta), triethylenetetramine (Teta),tetraethylenepentamine (Tepa) and tris(2-aminoethyl)amine (Tren). These organic receptorsact as precipitating agents in the presence of an appropriate protonating agent (HCl) bylowering the solubility of the PGM chlorido-anions through an ion-pairing mechanism. Therecovery of RhIII from synthetically prepared PGM (RhIII and PtIV) containing solutions usingthese precipitants was excellent, while poor Rh recovery from authentic industrial processsolutions was achieved. The poor Rh recovery from these process solutions was ascribed tothe species distribution of the [RhCln(H2O)6-n]3-n complexes. In order to validate theproposition that RhIII speciation effects are responsible for the poor Rh recovery observedduring the precipitation studies, attempt were made to describe the species distribution of the[RhCln(H2O)6-n]3-n (n=3-6) by means of high-resolution 103Rh NMR spectroscopy.A detailed high-resolution 103Rh NMR spectroscopic study of the series of[RhCln(H2O)6-n]3-n (n=3-6) complexes was conducted. During this study, all six RhIII aquachlorido-complexes have unambiguously been characterized by means of high-resolution103Rh NMR spectroscopy, proving the powerful analytical capability of this technique.Characterization of these complexes is based on the detailed analysis of the 35Cl/37Cl isotopeeffects which is observed in the 19.11 MHz 103Rh NMR resonances of the [RhCln(H2O)6-n]3-n(n=3-6) complexes in aqueous HCl solutions at 292 K. These resonances show that the 'finestructureof each of the 103Rh resonances may be understood in terms of its uniqueisotopologue, and in certain cases, the isotopomer distribution of each complex, whichmanifests as a result of its statistically expected 35Cl/37Cl isotopologue and isotopomerdistributions. As a result, the 103Rh NMR resonance structure serves as a unique 'NMRfingerprint,which allows for the unambiguous assignment of [RhCln(H2O)6-n]3-n (n=3-6)complexes, without the reliance on accurate δ(103Rh) chemical shifts.Furthermore, this study reports the first direct species distribution diagram for the[RhCln(H2O)6-n]3-n (n=3-6) series of complexes (in aqueous HCl solutions at 292 K) as a function of the 'free (unbound) chloride concentration, constructed from 103Rh NMRmeasurements. The need for a revised speciation diagram of [RhCln(H2O)6-n]3-n (n=3-6)complexes is clearly reflected by the vast differences observed in the literature reportedspecies distribution diagrams, which makes it difficult to decide which set of experimentalconditions (if any) is required for the quantitative and 'selective recovery of RhIII fromaqueous HCl solutions containing associated PGMs (Pt, Pd, Ir, Ru) as well as other transitionmetals. The documented species distribution diagrams for RhIII have been generallyconstructed via data from indirect (kinetic and spectrophotometric) measurements usingdilute RhIII solutions at relatively high HCl concentrations, which implies that the RhIII:Clmoleratio is higher than what may be expected in authentic process solutions – an importantaspect to consider when optimizing RhIII recovery methods. In addition, RhIII kineticinvestigations reported in this study shows that ionic strength and temperature effects areimportant factors that dramatically influences the rate of RhIII ligand exchange (i.e. RhIIIaquation reactions) which, in turn, have contributing effects on the species distribution of[RhCln(H2O)6-n]3-n complexes. Notable differences exist between the speciation diagramreported in this study and those documented in literature, especially at a 'free chlorideconcentration of 1.0 M. At this 'free chloride concentration, the [RhCl5(H2O)]2- complexanion was found to have an abundance of 34%, while literature reports an abundance of 80%.In order to ascertain its practical relevance, the proposed 103Rh NMR speciationmethod was extended, for the first time, to authentic industrial Rh feed solutions (AngloPlatinum PLC). Each of the 103Rh resonances was unambiguously assigned, and each speciesquantified. Moreover, the RhIII species distribution of the industrial Rh feed solution wasaccurately predicted by the 'direct speciation diagram constructed form 103Rh NMRmeasurements.After careful optimization of the Heraeus industrial feed solutions (optimal chlorideconcentration followed by thermal treatment for enhancing RhIII chloride anation reactions),the recovery of Rh via precipitation was repeated. In this instance, Rh recovery improveddramatically, with up to 95% of Rh removed from solution. This improvement is ascribedprimarily to the increased 'free (unbound) chloride concentration. The presence ofassociated PGMs as well as other transition metals would lower the effective 'free chlorideconcentration, since these metals would act as 'chloride binders. By adjusting the totalchloride concentration, RhIII chloride anation reactions is enhanced which leads to the [RhCln(H2O)6-n]3-n (n=5,6) complex anions being the dominant species in solution, thereforeleading to improved Rh recovery. Moreover, it was shown that, under carefully controlledconditions, 'selective recovery of Rh is achieved using tris(2-aminoethyl)amine (Tren).Considering the fact that Rh is the last precious metal recovered in all PGM refineries, thiscan possibly provide a cost-effective route for the 'upfront (early stage) recovery of Rh fromindustrial PGM feed solutions.

[发布机构] Stellenbosch University