THE EFFECT OF LEAD ON THE CEMENTATION OF GOLD BY ZINC

Rotating disc electrodes were used to evaluate the effect of lead on the two half-cells involved in the cementation process: gold reduction and zinc dissolution. Evans diagrams constructed from the current-potential curves show that the cementation process is controlled by the diffusion of gold cyanide ions to the zinc surface (activation energy of 2.55 kcal/mol). The initial rates of cementation predicted from the Evans diagrams were not affected by lead addition. These rates were also confirmed from experiments in which zinc discs were used to cement gold from different solutions. At 10-degrees-C the addition of lead (at 3 ppm) would improve the cementation process by extending the diffusion zone for the reaction. At 40-degrees-C and in the presence of lead the hydrogen overpotential was increased, leading to a decrease in the zinc corrosion. The deposit morphology also changed as a result of lead addition which in some cases affect the cementation kinetics.