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I.

INTRODUCTION

S I L V E R nitrate is the common starting material for nearly all chemical products involving silver. The classic method for manufacturing silver nitrate consists of dissolving 99.9 pct pure silver in a hot nitric acid solution containing equal volumes of H20 and 64 pct nitric acid by weight in a closed jacketed vessel, m The separation of silver-gold alloys by dissolving the silver in nitric acid is common in the precious metals industry. Alloys with a high silver percentage by weight can be treated directly with HNO3 to dissolve it, leaving a practically pure gold residue. This method is used preferably in small-scale operations and for refining alloys when their composition prevents them from dissolving in aqua regia. Alloys containing less than 15 pct silver by weight dissolve in aqua regia, precipitating the silver as chloride. L21 When the silver concentration is more than 15 pct, silver does not dissolve even with aqua regia because the AgC1 film that forms is too compact and prevents the reaction from proceeding to completion. The maximum silver content in the alloy that will allow complete dissolution of gold in aqua regia and the maximum silver content that can be totally dissolved in nitric acid have not yet been precisely defined, t3.41 The selective dissolution of silver from its gold alloys is also the basis for the usual method of analyzing alloys, such as commercial gold. The sample to be analyzed first undergoes cupellation, melting the alloy with lead in a cupel at 1050 ~ The lead is oxidized by atmospheric oxygen and absorbed by the magnesite in the cupel, leaving only the precious metals in a button which L.L. MARTINEZ, M. SEGARRA, and M, FERN,g, NDEZ are former Graduate Students, Department of Metallurgy, University of Barcelona. F. ESPIELL, Professor of Metallurgy, is with the Department of Chemical Engineering and Metallurgy, University of Barcelona, 08028 Barcelona, Spain. Manuscript submitted August 4, 1992. METALLURGICAL TRANSACTIONS B

can be weighed to determine the total precious metal content (gold plus silver plus metals of the platinum group). This alloy usually contains too much gold to be separated by nitric acid. At this point, the common procedure is to melt the button after adding pure silver to give an alloy from which the silver can be dissolved and the residual gold weighed. The same procedure is used to separate silver-gold alloys, although the bullion is usually treated by electrolytic refining of the silver and the gold is left in the anode sludge. Detailed knowledge of the mechanism for dissolving silver from these alloys with nitric acid may very well be the basis for determining the acceptable working limits in electrolytic refining as well. In order to treat the alloy with nitric acid, the percentage of silver in the alloy must be such that it meets two objectives: On the one hand, all the silver must be soluble, while, on the other, the gold should be compact enough so that it can be weighed after slight annealing with none being lost during handlin