Recovery of platinum group elements from waste material
Ngcephe, Ayanda Maria
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The aim of this study was to recover the platinum group elements (PGE) from recycled or waste material using hydrometallurgical techniques. The waste material that was investigated for the possible isolation of PGE is a spent automotive catalytic converter sample, ERM®-EBS504 which is a certified reference material for Pt, Pd and Rh. Surface analysis on the catalyst sample was performed using the scanning electron microscope coupled with energy dispersive X-ray spectroscopy (SEM-EDS) in order to identify the main components of the sample. However, the identification and quantification of the PGE using this technique was ineffective due to the concentrations of PGE in the catalyst sample which were below the detection limits of the SEM-EDS. Two types of dissolution processes, namely aqua-regia open-beaker dissolution and sodium peroxide (Na2O2) fusion were employed to try and achieve the total dissolution of the automotive catalytic converter sample for the complete and accurate characterisation of PGE using the inductively coupled plasma optical emission spectroscopy (ICP-OES). Aqua-regia dissolution offered partial digestion of the sample and percentage recoveries of 66.9(4) %, 63.9(8) % and 41(1) % were obtained for Pt, Pd and Rh respectively at 80 °C and after a reaction time of 180 minutes. On the other hand, sodium peroxide was able to oxidise all the metals in the catalyst sample to easily soluble oxidation states which allowed for the further dissolution of the sample in aqua-regia. However, challenges in PGE quantification with the ICP-OES were experienced due to unacceptable and unsatisfactory PGE recoveries which were attributed to spectral interferences caused by the excess Na ions introduced from the Na2O2 flux. The introduction of Sc (361.363 nm) as an internal standard compensated effectively for the spectral interference and excellent PGE recoveries were obtained, namely 100(1) % for Pt, 100(3) % for Pd and 103(2) % for Rh. These results were successfully validated in accordance with the criteria of the Internal Standards Organisation (ISO 17025). This method was found suitable and reliable for the quantification of PGE in the automotive catalyst sample at different separation stages. The complete digestion and characterisation of the automotive catalyst sample enabled separation studies of PGE in various aqueous solutions. The separation methods were firstly studied on artificial samples which emulated to a degree of the original composition of the dissolved automotive catalyst sample. Both solvent extraction and selective precipitation methods were investigated for the possible separation and purification of PGE. Trioctylphisphine oxide (TOPO) and 2- mercaptopyridide N-oxide sodium salt (NaPT) were employed as extractants while NH4OH and 8-hydroxyquinoline (oxine) were used as precipitants. Oxine was found to be highly selective towards the Pd in the solution precipitation in acidic solutions, resulting in the isolation of this element as a highly stable compound which was characterised using XRD, FT-IR, NMR and CHNS micro-elemental analysis. The selective precipitation of the non-precious elements in the presence of PGE using NH4OH was found time-consuming and inconclusive results were obtained for the PGE. Solvent extraction of PGE using TOPO was found very suitable for the extraction of PGE from chloride solutions. Various parameters which included HCl concentration, ligand concentration, type of diluent and type of stripping reagent were investigated in order to optimise the extraction and selectivity for PGE. An increase in HCl concentration suppressed the extraction of PGE by TOPO, while the degree of Pt extraction was unaffected by this variation. Maximum extraction of all the PGE was observed at 4 M HCl and at high TOPO concentrations. These results improved remarkably when kerosene and hexane were used as diluents. The presence of the non-precious elements interfered to some extent with the extraction of PGE with TOPO. However, various stripping reagents such as NH4SCN proved to be highly selective in the stripping of Pt from Pd, Rh and the non-precious elements, while 2 M HCl proved to be selective towards Rh. Solvent extraction with NaPT proved to be more selective and effective towards Pd. This allowed for the selective isolation of Pd from the PGE and from the non-precious elements. The Pd-mercaptopyridine complex was isolated and successfully characterised with XRD, FT-IR, NMR and CHNS micro-elemental analysis. These isolation methods were successfully applied to a dissolved and pre- concentrated catalyst sample, and Pt and Pd were successfully recovered with percentage recoveries of 87(4) % and 99(3) % respectively. However, the recovery of Rh using TOPO was unsuccessful, and this was attributed to the formation of the highly stable [RhCl6]3- in the presence of high chloride concentrations. The high chloride content in the solution were as a result of the formation and isolation of NaCl during the dissolution process which complicated the sample matrix, and thus the extraction process of PGE using TOPO. This method was also evaluated on a highly- concentrated rhodium waste solution which had been accumulated in the department, but proved to be ineffective. The recovery of Rh from this waste solution was achieved by the co-precipitation of the non-precious elements using oxine, resulting in Rh recovery of 80(4) % in the filtrate.