Effect of Zinc Ions on Copper Electrodeposition in the Context of Metal Recovery from Waste Printed Circuit Boards

This paper evaluates the effect of zinc ions on copper electrodeposition. Although zinc is not found at significant concentrations in conventional industrial copper electrolytes, in copper-concentrated sulfate leach liquors produced in the treatment of waste printed circuit boards (WPCB) its concentration may reach high levels. In this context, the effect of zinc on physical-chemical properties of copper sulfate electrolyte, performance indicators of copper electrowinning, and morphology and crystal structure of copper deposits were investigated. Copper electrowinning tests indicated that the current efficiency was negligibly affected by adding zinc (up to 75 g L-1) to synthetic copper sulfate electrolyte (30 g L-1 Cu2+ and 110 g L-1 H2SO4) at two levels of current density (250 A m-2 and 450 A m-2) and fixed temperature (35 °C). The energy consumption slightly increased: up to 4% at 250 A m-2 and up to 3% at 450 A m-2, both effects at 75 g L-1 Zn2+. This behavior was ascribed to decreases in the electrical conductivity of the electrolyte by adding zinc (up to 35% at 75 g L-1) and consequent increases in the ohmic drop and cell voltage. Electrochemical measurements by cyclic voltammetry and linear sweep voltammetry (LSV) indicated no cathode polarization by adding this impurity to the electrolyte, even at the highest level. In turn, the addition of zinc caused significant drops in the limiting current for copper deposition (e.g., the values recorded at 100 min-1 decreased about 60% when 75 g L-1 Zn2+ was added to the electrolyte). This effect was explained by the measured increases of electrolyte viscosity and calculated decreases of copper diffusion coefficient. The addition of 75 g L-1 Zn2+ to the electrolyte increased its viscosity in the order of 76%, and decreased the diffusion coefficient in the order of 45%. Regarding the effect of zinc on product quality, SEM analysis revealed negative effects on the morphology of the deposits (i.e., higher roughness and nodulation), mainly at the highest concentration of zinc and highest current density. XRD analysis showed modifications in the growth pattern of copper crystals under some conditions. For example, the addition of 30-75 g L-1 Zn2+ to the electrolyte led to the formation of (220) textured deposits at 250 A m-2. The findings of this work may be helpful to define proper operating conditions for copper electrowinning in order to produce high quality products, thus maximizing the recycling of copper from the selected secondary source