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

Geswindt, Theodor Earl (2013-12)

Thesis (PhD)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: In this thesis, the recovery of RhIII from both synthetically prepared and authentic industrial PGM-containing solutions was systematically investigated via organic precipitation methods 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 receptors act as precipitating agents in the presence of an appropriate protonating agent (HCl) by lowering the solubility of the PGM chlorido-anions through an ion-pairing mechanism. The recovery of RhIII from synthetically prepared PGM (RhIII and PtIV) containing solutions using these precipitants was excellent, while poor Rh recovery from authentic industrial process solutions was achieved. The poor Rh recovery from these process solutions was ascribed to the species distribution of the [RhCln(H2O)6-n]3-n complexes. In order to validate the proposition that RhIII speciation effects are responsible for the poor Rh recovery observed during 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 aqua chlorido-complexes have unambiguously been characterized by means of high-resolution 103Rh NMR spectroscopy, proving the powerful analytical capability of this technique. Characterization of these complexes is based on the detailed analysis of the 35Cl/37Cl isotope effects 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 “finestructure” of each of the 103Rh resonances may be understood in terms of its unique isotopologue, and in certain cases, the isotopomer distribution of each complex, which manifests as a result of its statistically expected 35Cl/37Cl isotopologue and isotopomer distributions. 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 NMR measurements. 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 reported species distribution diagrams, which makes it difficult to decide which set of experimental conditions (if any) is required for the quantitative and “selective” recovery of RhIII from aqueous HCl solutions containing associated PGMs (Pt, Pd, Ir, Ru) as well as other transition metals. The documented species distribution diagrams for RhIII have been generally constructed via data from indirect (kinetic and spectrophotometric) measurements using dilute RhIII solutions at relatively high HCl concentrations, which implies that the RhIII:Clmole ratio is higher than what may be expected in authentic process solutions – an important aspect to consider when optimizing RhIII recovery methods. In addition, RhIII kinetic investigations reported in this study shows that ionic strength and temperature effects are important factors that dramatically influences the rate of RhIII ligand exchange (i.e. RhIII aquation 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 diagram reported in this study and those documented in literature, especially at a “free” chloride concentration of 1.0 M. At this “free” chloride concentration, the [RhCl5(H2O)]2- complex anion 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 speciation method was extended, for the first time, to authentic industrial Rh feed solutions (Anglo Platinum PLC). Each of the 103Rh resonances was unambiguously assigned, and each species quantified. Moreover, the RhIII species distribution of the industrial Rh feed solution was accurately predicted by the “direct” speciation diagram constructed form 103Rh NMR measurements. After careful optimization of the Heraeus industrial feed solutions (optimal chloride concentration followed by thermal treatment for enhancing RhIII chloride anation reactions), the recovery of Rh via precipitation was repeated. In this instance, Rh recovery improved dramatically, with up to 95% of Rh removed from solution. This improvement is ascribed primarily to the increased “free” (unbound) chloride concentration. The presence of associated PGMs as well as other transition metals would lower the effective “free” chloride concentration, since these metals would act as “chloride binders”. By adjusting the total chloride 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, therefore leading to improved Rh recovery. Moreover, it was shown that, under carefully controlled conditions, “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, this can possibly provide a cost-effective route for the “upfront” (early stage) recovery of Rh from industrial PGM feed solutions.

AFRIKAANSE OPSOMMING: In hierdie tesis word die herwinning van RhIII uit laboratorium voorbereide sowel as ware industriële PGM-bevattende oplossings sistematies ondersoek deur middel van organiese neerslag metodes, deur gebruik te maak van verskeie kommersieël beskikbare, Nbevattende organiese reseptore insluitende dietileentriamien (Deta), tri-etileentetramien (Teta), tetra-etileenpentamien (Tepa) en tris(2-aminoetiel)amien (Tren). Hierdie organiese reseptore tree op as neerslag-agente in die teenwoordigheid van 'n geskikte protoneringsagent (in hierdie geval HCl) deur 'n verlaging van die oplosbaarheid van die PGM chloriedanione deur middel van 'n ioon parings meganisme. Die herwinning van RhIII vanuit laboratorium voorbereide PGM (RhIII en PtIV) oplossing met behulp van hierdie organiese neerslag-agente was uitstekend, terwyl Rh herwinning vanuit ware industriële oplossings swak was. Die onvolledige Rh herwinning uit hierdie industriële oplossings word toegeskryf aan die spesie-verspreiding van die [RhCln(H2O)6-n]3-n komplekse. Ten einde die effek van RhIII spesie-verspreiding op die herwinning van Rh te bestudeer, is gepoog om die spesieverspreiding van [RhCln(H2O)6-n]3-n (n=3-6) komplekse, deur middel van hoë resolusie 103Rh KMR spektroskopie, te beskryf. 'n Gedetailleerde hoë resolusie 103Rh KMR spektroskopiese studie van die reeks van [RhCln(H2O)6-n]3-n (n=3-6) komplekse was uitgevoer. Tydens hierdie studie was al ses RhIII aqua chlorido-komplekse ondubbelsinnig gekarakteriseer deur middel van hoë resolusie 103Rh KMR spektroskopie, wat bewys lewer van die kragtige analitiese vermoë van hierdie tegniek. Karakterisering van hierdie komplekse is gebaseer op die gedetailleerde analise van die 35Cl/37Cl isotoop effekte wat waargeneem word in die 19.11 MHz 103Rh KMR resonansies van die [RhCln(H2O)6-n]3-n (n=3-6) komplekse in HCl oplossings by 292 K. Hierdie resonansies toon dat die "fyn struktuur" van elk van die 103Rh resonansies verstaan kan word in terme van die unieke isotopoloog, en in sekere gevalle, die isotopomeer verspreiding van elke kompleks, wat manifesteer as 'n gevolg van die die statisties verwagte 35Cl/37Cl isotopoloog en isotopomeer verspreiding. Die 103Rh KMR resonansie-struktuur kan sodoende dien as 'n unieke "KMR-vingerafdruk", wat voorsiening maak vir die ondubbelsinnige karakterisering van [RhCln(H2O)6-n]3-n (n=3-6) komplekse, sonder om vertroue op akkurate δ(103Rh) chemiese verskuiwings te plaas. Hierdie studie rapporteer verder die eerste direkte spesie-verspreiding-diagram vir die [RhCln(H2O)6-n]3-n (n=3-6) reeks komplekse (in HCl oplossings by 292 K) as 'n funksie van die "vrye"(ongebonde) chloried-konsentrasie, verkry van 103Rh KMR metings. Die behoefte vir 'n aangepasde spesiasie-diagram vir die [RhCln(H2O)6-n]3-n (n=3-6) komplekse word duidelik weerspieël deur die groot verskille waargeneem in die literatuur gerapporteerde verspreidings diagramme, wat dit moeilik maak om te besluit watter stel eksperimentele toestande (indien enige) benodig word vir die kwantitatiewe en “selektiewe” herwinning van RhIII in HCl oplossings in die teenwoordigheid van gepaardgaande PGM (Pt, Pd, Ir, Ru) sowel as ander oorgangsmetale. Die gedokumenteerde spesie-verspreiding-diagramme vir RhIII is oor die algemeen verkry via data vanaf indirekte (kinetiese en spektrofotometriese metings) deur gebruik te maak van verdunde RhIII oplossings in relatiewe hoë HCl konsentrasies, wat impliseer dat die RhIII:Cl mol verhouding hoër is as wat verwag kan word in ware industriële proses oplossings - 'n belangrike aspek om te oorweeg gedurende die optimalisering van RhIII herwinning-metodes. Verder, die RhIII kinetiese ondersoeke gerapporteer in hierdie studie toon dat ioniese sterkte sowel as temperatuur effekte belangrike faktore is wat die tempo van RhIII ligand uitruiling (d.w.s. RhIII “aquation” reaksies), wat ‘n betekenisvolle invloed hê op die spesie-verspreiding van [RhCln(H2O)6-n]3-n komplekse. Aansienlike verskille bestaan tussen die spesiasie-diagram gerapporteer in hierdie studie en dit gedokumenteer in die literatuur, veral by 'n "vrye" chloried-konsentrasie van 1.0 M. By hierdie "vrye" chloried-konsentrasie was die [RhCl5(H2O)]2- komplekse anioon gevind om in 34% teenwoordig te wees (hierdie studie), terwyl die publiseerde verslae 80% rapporteer. Ten einde die praktiese toepaslikheid van die voorgestelde 103Rh KMR spesiasiemetode te bepaal, was (vir die eerste keer) ware industriële Rh oplossings (Anglo Platinum PLC) gebruik. Elk van die 103Rh resonansies was ondubbelsinnig gekarakteriseer, en elke Rh spesie teenwoordig gekwantifiseer. Daarbenewens is die RhIII spesie-verspreiding van die industriële Rh oplossing deur die "direkte" spesiasie-diagram saamgestel vanuit 103Rh KMR metings akkuraat voorspel. Die berekende RhIII spesie-verspreiding van die industriële Rh oplossings was akkuraat voorspel deur die voorgestelde “direkte” spesiasie-diagram soos saamgestel vanuit die 103Rh KMR metings. Na deeglike optimalisering van Heraeus industriële oplossings (optimale chloriedkonsentrasie gevolg deur termiese behandeling vir effektiewe RhIII chloried anasie reaksies), is die herwinning van Rh via neerslag metodes herhaal. In hierdie geval, het die Rh herwinning dramaties verbeter, met tot 95% van die Rh uit oplossing verwyder. Hierdie verbetering is hoofsaaklik toegeskryf aan die verhoogde "vry" (ongebonde) chloriedkonsentrasie. Die teenwoordigheid van geassosieerde PGM's sowel as ander oorgangsmetale sal die effektiewe "vrye" chloried-konsentrasie verlaag, aangesien hierdie metale sou optree as "chloried-binders". Deur die aanpassing van die totale chloried-konsentrasie, word RhIII chloried anasie reaksies verbeter, wat daartoe lei dat [RhCln(H2O)6-n]3-n (n = 5,6) komplekse anione die dominante spesies in oplossing word, en dus lei tot verbeterde Rh herwinning. Daarbenewens word verder aangetoon dat, onder noukeurig gekontroleerde voorwaardes, "selektiewe" herwinning van Rh bereik word deur gebruik te maak van tris(2-aminoetiel)amien (Tren). Met inagneming van die feit dat Rh die laaste edelmetaal is wat verhaal word in alle PGM-raffinaderye, kan dit 'n koste-effektiewe roete word vir die "vooraf" (vroeë-stadium) herwinning van Rh vanuit industriële PGM bevattende oplossings.

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