The Behavior of Ruthenium in Copper Electrowinning

The recycling of material containing precious metals can lead to the entry of ruthenium into the copper electrowinning process, by so far unknown effects. There, ruthenium is oxidized to highly volatile ruthenium tetroxide. In order to avoid ruthenium losses during electrolysis, the oxidation behavior of ruthenium in copper electrowinning was investigated by testing different oxygen overvoltages using lead alloy and diamond anodes. Furthermore, the temperature and the current density were varied to investigate a possible chemical or electrochemical reaction. The results of the study show that ruthenium is not directly electrochemically oxidized to ruthenium tetroxide at the anode. Especially at anodes with high oxygen overvoltage, the formation of other oxidants occurs parallel to the oxygen evolution in the electrolyte. These oxidants oxidize ruthenium compounds to highly volatile ruthenium tetroxide by chemical reactions. These reactions depend mainly on temperature; the formation of the active oxidants depends on the anodic potential. To avoid ruthenium losses in the copper electrowinning process, anodes with a low anodic potential should be used at low electrolyte temperatures

Refining of Precious Metal Bearing Materials from Secondary Sources-Methanesulfonic Acid Leaching of Raw Silver Granules as a Promising Approach towards a Green Way of Silver Refining

The state-of-the-art technology of raw silver refining in a silver nitrate-based electrorefining process (Moebius-electrolysis) is accompanied by several disadvantages, both from a technological and from an ecological point of view. In addition, increasing concentrations of critical impurities from secondary sources, like palladium, in raw silver are a further challenge for the future of silver refining. Thus, there is strong motivation for the development of an adequate, alternative process of raw silver refining to substitute the existing Moebius-electrolysis. Due to its less environmentally toxic character and the high aqueous solubility of its silver salt, methanesulfonic acid (MSA) is a possible base chemical for the design of an efficient refining method based on leaching of raw silver followed by electrowinning, with less ecological and technological complications. In this paper the results of some fundamental investigations on the leaching of raw silver granules, containing approx. 94% silver, with methanesulfonic acid and hydrogen peroxide as an oxidation agent are presented. Agitation leaching experiments were conducted on a laboratory scale and the effects of the solid concentration, the hydrogen peroxide dosage and the temperature as leaching parameters were studied. The obtained results indicate that silver leaching yields of more than 90% are achievable with leaching at elevated temperatures of 65 °C or 80 °C, solid concentrations of 500 g/L and at a stoichiometric H2O2:Ag-ratio of 3:1. Increased solid concentrations greater than 500 g/L and elevated temperatures of 65 °C or 80 °C additionally improved the selectivity of the process regarding the leaching of Pd