How Pb-Ca-Sn Anodes Are Used in the Electrowinning of Copper?

 How Pb-Ca-Sn Anodes Are Used in the Electrowinning of Copper?

The classical anode used in copper electrowinning is lead alloyed with 6-10% antimony (Pb-Sb anode). Antimony in lead alloys offers advantages of improved mechanical properties, uniform grain structures, and ease in conventional casting. However, the inadequate electrochemical and corrosion properties are a disadvantage of Pb-Sb anodes. A reduction in the antimony content of the alloys decreases the mechanical properties markedly. On the other hand, increasing the concentration of antimony increases the corrosion rate. The overvoltage for oxygen evolution is similar to that of pure lead anodes.

New alloys of lead with calcium and/or strontium were developed to replace the conventional Pb-Sb anodes. The addition of calcium or strontium to the lead improves the hardness.

The Pb-Ca-Sn anodes are the most widely used anodes in the electrowinning of copper. Tin is normally added to improve both the conductivity of the anodic film and the mechanical properties. The addition of calcium with tin produces an anode with a high mechanical strength to prevent distortion, warping, and short circuits during electrolysis. Tin also reacts with calcium to form fine uniform Sn3Ca particles within the grain structure, preventing corrosion from penetrating into the anode and improving the stability of the anode The optimum calcium content is between 0.05 and 0.08%, while the concentration of tin in the alloy should be between 1.0 to 2.2%.

The techniques used in the manufacture of anodes for electrowinning have changed over the years in order to provide a more corrosion resistant and hence longer lasting anode. Rolled anodes are preferable to cast anodes due to their superior performance since the rolling process has been shown to produce a more uniform cross section and grain structure and also less porosity and lower corrosion rates. The production of cast anodes results in an oriented, dendritic grain structure and the formation of numerous holes or voids in the anode surface that can initiate internal corrosion in localized areas.

Therefore, the anode sheets are generally rolled to the desired thickness from a cast billet to break up the original cast grain structure to produce a smoother surface with less corrosion and easier removal of the PbO2-MnO2 scale. The rolling process is normally performed at elevated temperatures to produce a fine uniform grain structure both at the surface and throughout the rolled sheet. It has been shown that the corrosion behavior improved with increasing rolling temperature.