CARBON-PASTE ELECTRODES IN MINERAL PROCESSING - AN ELECTROCHEMICAL STUDY OF SPHALERITE

The electrochemical behaviour of two sphalerite samples with 0.8% and 8.2% iron, respectively, have been studied with the carbon paste electrode technique. Potentiodynamic sweeps with rotating disc electrodes were performed in acidic perchlorate, chloride and sulphate solutions in the temperature range 23-85 degrees C. In order to investigate the nature of the overall anodic dissolution reaction of the sphalerite samples, chronocoulometry, in combination with atomic absorption spectrometry was used. The dissolution of sphalerite with 0.8% iron was found to be independent of potential. The dissolution of sphalerite with 8.2% iron was found to follow a two-electron process at low overpotentials, while a total of 6 electrons were needed at high overpotentials. The activation energy was determined as a function of potential. For chloride solutions the activation energy was found to be 58 +/- 3 kJ/mol at 0.25-0.8 V. At 1.0 V the activation energy was found to be 43 kJ/mol, indicating a transition to diffusion-controlled reactions. In perchlorate solutions the activation energy was 60 +/- 2 kJ/mol in the entire potential region studied: 0.25-1 V. In sulphate solutions the activation energy was found to be 43 +/- 2 kJ/mol in the region 0.25-0.5 V and 34 +/- 1 kJ/mol at potentials up to 1 V. The results indicate that the rate-determining process changes with the potential and solution composition. A comparison with the result of leaching experiments on the same minerals is given.