OJSC “Irkutsk Scientific-Research Institute of noble and rare metals and diamonds (IRGIREDMET)

Ye. V. Bogorodsky, Post Graduate Student, Leading Engineer, e-mail: e.bogor@mail.ru

V. G. Barankevich, Senior Researcher

S. V. Balikov, Chief Executive Officer of a Business Center

S. G. Rybkin, Senior Researcher

The paper describes the results of the study of the surfaces and cross-sections of the “uncovered” silver beads produced by a normal cupellation in comparison with the silver beads covered with a protective Pb–Sn film by thermal analysis, by wet chemical techniques and by WDS (Wavelength Dispersive X-Ray Spectroscopy) and EPMA (Electron Probe Microanalysis) techniques. A study of the nature of the constituents of the silver beads indicated that they normally contain both soluble and insoluble gold and platinum group metals (in the latter case pgm’s appear on the cupelled bead as grey/black particles which persist after the parting operation). Lead and other insoluble metals may come to the surface of the silver bead thus causing the loss of metals accompanied platinum by oxidation, by volatilization from the surface of the molten litharge or by absorption by the cupel. It was noted that an increased quantity of lead and tin or a pre-cupellation stage involving the addition of lead and tin can be used to ensure the formation of a film which can prevent the loss of gold and pgm’s by volatilization. It was found that the film on the top of the molten litharge and the silver bead is normally formed at 1153–1183 K. An investigation of the character of the constituents of the Pb–Sn containing film which covers the beads, the shape of the film and the state in which precious metals occurred in the silver bead was conducted. The EPMA showed no indications of the presence of insoluble gold, platinum or rhodium particles in the silver beads.

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