Physicochemical investigation of anodic processes involved in silver electrowinning in refining technology

During silver electrowinning in refining technology, irrespectively of the nature of electrolyte and anode material, an anode deposit is formed due to oxidation of singly charged silver ions to higher degrees of oxidation (+2 and +3) under polarization. Absorption spectra of Ag(II) have been obtained in solutions with various concentrations of silver ions and nitric acid using anodic polarization in combination with electronic absorption spectroscopy; silver ions of high oxidation degrees were found to be unstable in solutions. At anode potentials φ_a ≥ 1.15–1.22 V, Ag(I) ions in liquid phase are oxidized to Ag(II), which paint the anolyte; their content depends on silver and nitric acid concentrations in the electrolyte. The deposit crystallizes on anode at potential φ_a ≈ 1.5 V. Chemical analysis, X-ray powder diffraction, and a thermogravimetric study of the anode deposit have made it possible to identify its initial composition as Ag₇O₈NO₃. The deposit composition varies in time as a result of reduction of highly oxidized silver.