Model of local potentials and reactions at electrode/electrolyte/air interfaces in zinc electrowinning reactors

摘要

The work reported addresses the problem of severe localised corrosion in the meniscus region of electrolytes at aluminium alloy cathodes in zinc electrowinning reactors. The distribution of electrical potentials has been modelled for such cathodes, using the finite element method to solve Laplace's equation for a typical industrial reactor geometry and operating conditions, though neglecting concentration gradients. At zero contact angle and surface tension of 72 mN m~(-1), the potential difference between the bottom and top of the meniscus was computed to be ca. 0.13 V, but, as expected from geometric considerations, this value was predicted to decrease with increasing contact angle, suggesting possible solutions to the corrosion problem. As zinc cathodes are operated typically at 500 A m~(-2), corresponding to a (mean) overpotential estimated to be <0.1 V, the potential drop in the meniscus parallel to the cathode results in electrode potentials and current densities that decay with increasing height, leaving some of the aluminium in the meniscus unprotected by any zinc deposit. Such surfaces are predicted to be at potentials at which aluminium corrosion will occur by a net current process involving proton and oxygen reduction. Restricted diffusion of Al~(3+) species, parallel to the cathode surface, out of the meniscus and protons into the meniscus region probably results in local electrolyte compositions that eventually cause precipitation of phases such as Al_2(SO_4)_3 centre dot 6H_2O, as observed experimentally.