Degradation of Glue in Lead Electrorefining

Abstract

Surface morphology of lead deposits from H2SiF6-PbSiF6 electrolyte containing an animal glue was investigated by means of the periodic galvanostatic electrolysis and SEM-EDS observation. The weight average molecular weight of the glue was also determined with the static light scattering measurement. The temporal changes of the overpotential and the weight average molecular weight were discussed in terms of the elapsed time after preparing the electrolyte which contained 100 g∙L−1 of H2SiF6 and 100 g∙L−1 of Pb(II) and in which the initial concentration of the glue (Cglue) was 0–10.0 g∙L−1. The dendritic growth of lead deposits was observed from the electrolyte of Cglue = 0 g∙L−1 in the galvanostatic electrolysis where the cathodic current density and the electrolyte temperature was −150 A∙m−2 and 40 ℃, respectively. Smooth and dense lead deposits were obtained in the electrolyte of Cglue = 0.5–5.0 g∙L−1 when the elapsed time τ = 0 d. The surface of the deposits gradually became rough with increasing τ in the electrolyte of Cglue = 0.1–1.0 g∙L−1. On the other hand, nodules or bearded deposits were observed on the deposits obtained in the electrolyte of Cglue = 5.0 g∙L−1 when τ = 1–2 d. After this, the surface became smooth with the further increase of τ. The change of the weight average molecular weight of the glue (Mr,w) with τ was described with the equation, 1/(Mr,w)τ = 1/(Mr,w)0 + k′τ, in the electrolyte containing 100 g∙L−1 of free H2SiF6 and 100 g∙L−1 of Pb(II) at 40 ℃. The k′ value was evaluated to be 5 × 10−9 min−1 in the electrolyte of Cglue = 1.0 g∙L−1 and 6 × 10−9 min−1 in the electrolyte of Cglue = 10.0 g∙L−1.

Keywords

• betts electrolytic process
• surface morphology
• polymer additives
• overpotential
• molecular weight