Effect of chromate additive on the kinetics and mechanism of skeletal copper formation

The addition of chromate to the leach liquor for the preparation of skeleta l copper increases and stabilizes the copper surface area and slows the lea ching rate. The kinetics have been studied using a rotating disc electrode (RDE) at 269-293 K in 2-8 M NaOH and 0-0.1 M Na2CrO4. The rate of leaching was found to be constant with time, with an activation energy of 74 +/- 7 k J mol(-1). By monitoring the kinetics and free (mixed) corrosion potential, it was possible to elucidate the mechanistic effect of chromate causing th e increased surface area. Chromate was found to deposit on the copper surfa ce as chromium(III) oxide, hindering the leaching reaction as well as the d issolution/redeposition of copper, the main mechanism of structural formati on/rearrangement for skeletal copper. This blocking of the surface resulted in a finer structure, with a corresponding larger surface area. It also st abilised the surface area by minimizing the rearrangement. The effect of ch romate was found to reach a limit at around 0.03 M Na2CrO4.