Modelling potentials, concentrations and current densities in porous electrodes for metal recovery from dilute aqueous effluents

One-dimensional steady-state models have been developed for the recovery of Pb(II) ions from lead-acid battery recycling plant effluent by simultaneous lead and lead dioxide deposition, including oxygen evolution/reduction and hydrogen evolution as loss reactions. Both monopolar and bipolar reactor with porous graphite electrodes were modelled, as a design aid for predicting spatial distributions of potentials, concentrations, current densities and efficiencies, as well as specific electrical energy consumptions and by-pass currents. Since the industrial effluent contains a large excess of supporting electrolyte (Na2SO4), the electrical migrational contribution to reactant transport rates was neglected and the current density-potential relationship was described by the Butler-Volmer equation, allowing for both kinetic and mass transport control. The models were implemented and the governing equations solved using commercial finite element software (FEMLAB). The effects were investigated of electrolyte velocity, applied cathode potential, dissolved oxygen concentration and inlet Pb(II) ion concentration on single-pass conversion, current efficiency and specific electrical energy consumptions. According to model predictions, de-oxygenation of the inlet process stream was found to be crucial to achieving acceptable (i.e. > 0.8) current efficiencies. Bipolar porous electrodes were also determined to be inappropriate for the recovery of Pb(II) from effluents, as the low concentration involved resulted in the predicted fraction of current lost as by-pass current, i.e. current not flowing in and out of the bipolar electrode, to be greater than 90% for the ranges of the variables studied