Combination of Fire Assay and Modern Instrumental Techniques for Precious Metals Analysis

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Myint Myint Sein, Wolfgang van Leeuwen, Peter Glörfeld

The extraction of the precious metals as pure metal or alloys from the primary raw materials (ores, concentrates) and recycling materials is an important issue for ecological perspective. The recycling of materials containing precious metals include e-scraps, automotive catalysts, chemical catalysts, production waste from the electronic and galvanic industries and other numerous waste materials. The production and recycling of non-ferrous metals requires very precise knowledge about the chemical processes to get the composition of the materials. Due to their high value, the precision and accuracy of the analytical methods for the determination of precious metals is important. The analysis of gold and silver has a tradition, which is based on the miniaturisation of the smelting process and was described by Agricola around 1550 A.D in detail. This century-old traditional technique is still in use in precious metals analysis due to its accuracy. Nowadays, the determination of platinum group metals have to be carried out in addition to gold and silver so that modifications of this existing method are required. In order to get statistically proved results, samples of heterogeneous materials such as e-scrap and complex composites are prepared by means of combinations of mechanical and thermal procedures to get homogeneous samples for analyse. The samples that have been prepared for analysis are suitable for the combination of fire assay and subsequent measurement either directly or after wet chemical preparatory treatment. In traditional fire assay, the precious metals are separated and accumulated by using lead flux. The sample-lead flux mixture is fused in a furnace at > 1000 °C and finally a lead regulus is obtained. After then lead is removed by means of oxidation in order to get pure precious metal. Nickel sulphide flux is also a useful collector especially for platinum group metals. The precious metals are recovered in concentrated form by a wet chemical separation and then transferred into solutions forICP-OES measurement. The use of copper flux is another mean. Ion exchange separation could be made for the precious metal-containing copper, but a relatively complex procedure is included. The more efficient technique is the homogenization of the copper reguli in an induction oven for direct spectroscopic measurement of the precious metals. The combination of the fire assay technology with instrumental methods such as ICP-OES, spark OES, XRF and GDMS assures a significant lowering of the detection limits of precious metals down to the ppb level. Elements which interfere in the measurement of the precious metals are excluded or minimised by the fire assay process.

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