Refining of Platinum by Solvent Extraction
Solvent extraction may be included in the refining operation of PGM (Platinum Group Metals) as an alternative to the conventional precipitation technology. Platinum concentrates, and by-products materials are treated separately. Each sample is sampled on receipt employing a mechanical and sub sampling procedure. The sampling and subsequent analytical and agreement procedures are statistically designed to detect sampling and analytical bias. This ensures a high degree of confidence in values determination to benefit of both transfer parties. Whilst the concentrate can be fully and directly dissolved in a single operation, the gravimetric concentrate must be subjected to an oxidizing roast to remove sulphur, followed by and acid leach to remove base metals. These products are together with any internally recycled materials, dissolved conventionally and blended prior to feed to the solvent extraction circuit.
It is a feature of the dissolving circuits installed that high efficiencies above 98% can be obtained employing chlorine/hydrochloric acid media. This is clearly significant in terms of overall achievement of first pass targets. Following dissolution of the platinum group metals, the first process stage is the extraction of gold with Methyl-Iso-Butyl Ketone. Gold is stripped employing a direct reduction process and as a result the Methyl-Iso-Butyl Ketone is recirculated in a closed loop. Ketone losses are confined to solubility and relatively small degradation losses. Platinum can be efficiently removed by amine extraction, in the absence of gold and palladium, provided iridium in the Ir 3+ oxidation state. Hence the next stage is the removal of palladium using an oxime based system. It is a feature of oxime systems that they suffer from slow reaction kinetics. Fortunately, this problem has been overcome employing accelerating additives to the extent that allows system operation in a conventionally designed mixer settler system.
Following palladium removal and confirmation that Iridium is present, the platinum is extracted employing an amine reagent. This sequence of extraction not only ensures the early extraction of platinum with its effect on economy of the process. This also, has influence on its effective removal from all Rhodium, Iridium and Ruthenium processing due to the extremely high extraction efficiencies achieved (more than 99%). This has a significant bearing on the potential health situation in those areas. Ruthenium and any Osmium not distilled during dissolution is removed employing conventional oxidative distillation. This leaves the remaining separation of iridium and rhodium. Ruthenium removal that also oxidizes the iridium to Ir 4+, and a further oxidative treatment to ensure complete conversion to the Ir 4+ species. Rhodium is separated employing an ion-exchange process from nickel and the sodium introduced during Ruthenium distillation operations to prepare a suitable base metal free feed for the final stages.
The above stages describe the primary core separation of the individual platinum group metals and some further treatment may be required. The basic process philosophy employed in the construction of the process is to ensure maximum removal efficiency of the individual element in the operation, then to employ sidestream to take the final step to purity. Due to the enhanced intermediate purities achieved in the solvent extraction process is possible to exploit simplified sidestream routings giving high yields, whilst maintaining the quality of the product. Effluents of the process are treated with lime to precipitate base metals and discharge to solar evaporation dams. No effluents, excepting gaseous discharge, leave the site. Gaseous effluents are subjected to a two stage treatment. Firstly, each is treated either at source or in a localized collection system, tailored to its requirement. Secondly, these are passed via the main site system, which although operating in back-up mode are capable of bulk removal in the event of a primary system failure.