Investigating non-disclosing precious metal components in the UG2 reef

McDowell, Anna Cecilia Magdalene
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It is well understood that quality assurance is fundamental to the commercial evaluation of data, and the chemistry discipline in the mining industry is experiencing exciting challenges as it seeks to supply comprehensively verified analysis in support of the commercially rewarding fields of geology, mining and metallurgical extraction processes. Lead Fire Assay (Pb-FA) and Nickel Sulphide Fire Assay (NiS-FA) have for what seems like an eternity been the pre-eminent techniques for the analysis of Platinum Group Elements (PGEs) in Merensky and Upper Group 2, (UG2) ores. Simultaneously reliable and robust, they are at the same time expensive, time consuming and labour intensive. An alternate method was being investigated for these analyses which dissolved the siliceous component of the ores, but insoluble dark green chromitite crystals became apparent under a light microscope. This unique study was established to extract such crystals from a UG2 tailing stream (which is the waste stream from UG2 Concentrator operations) and to investigate whether non-liberated PGEs existed within such crystals. A representative UG2 tailings composite sample was prepared which was fused using sodium peroxide as a flux and dissolved in nitric acid prior to successful base metal (BM) characterisation. Pb-FA and NiS-FA collection techniques were successfully used for PGE characterisation of the sample at mg kg-1 levels. Residual chromitite crystals (RCCs) were successfully extracted through fusion of the sample with a basic flux mixture of potassium hydroxide and sodium carbonate. This removed 98% of the siliceous matrix and 18% by weight of RCCs were collected for further investigation. Methods were developed for the dissolution of the RCCs by hotplate and microwave digestion using an acidic mixture of 1:1 H2SO4:H3PO4. The RCCs were successfully characterised for base metals using inductively coupled plasma optical emission spectroscopy (ICP-OES). PGEs in solution were pre-concentrated as tellurites through tellurium co-precipitation and dissolved using aqua regia prior to analysis by inductively coupled mass spectroscopy (ICP-MS). The precious metal analysis of the RCCs using microwave digestion (MW) indicated predominance of Pt and to a lesser extent Pd and Ru which is consistent with the elemental distribution in UG2 ores. The Pt concentration for the 195Pt isotope was determined at around 27 μg kg-1 at an relative standard deviation (RSD) of less than 10%. As an appropriate certified reference material (CRM) was not available, the applicability of the Te co-precipitation method was assessed by recovery tests involving the recovery of precious metals from acidic solutions and from acidic solutions containing significant base metals. Recoveries of greater than 96% of microgram quantities of PGEs from spiked acidic solutions in the absence of base metals were achieved for Pt, Pd, Rh, Au and Ir. In the presence of those major base metals as determined in the RCCs, which included Fe and Cr, recoveries of Pt, Pd, Rh, Au and Ir were seen to fall to levels of just above 90% at a concentration level of 250 μg L-1 PGEs as opposed to recoveries of above 95% for these metals at a concentration level of 750 μg L-1 PGEs in the same base metal matrix. This trend was even more pronounced for Ru where recoveries dropped to 70% at a concentration level of 250 μg L-1. In all likelihood therefore the PGE analysis of the RCCs was understated in particular Ru, due to an under-recovery of PGEs using Te co-precipitation from the BM loaded acidic solution. Unfortunately for various reasons the PGEs results determined by Te co-precipitation and ICP-MS could not be verified by two other techniques, Te co-precipitation with graphite furnace atomic absorption spectrometer (GFAAS) and NiS-FA by ICP-MS finish. In the absence of such verification it was decided to reconfirm the presence of PGEs in the RCCs through mineralogy studies which would also attempt to confirm whether the PGEs might be present as inclusions or in solid solution. Fourteen Laurite minerals were identified in the RCCs plus one mineral of the form PtRuAsS thereby confirming the presence of Pt and Ru within the RCCs. The identification of varying quantities of Os and Ir in the Laurite minerals which could substitute for Ru to some extent, confirmed that some of the PGEs in the minerals contained within the RCCs were in solid solution.