ELECTROWINNING OF NONNOBLE METALS WITH SIMULTANEOUS HYDROGEN EVOLUTION AT FLOW-THROUGH POROUS-ELECTRODES .1. THEORETICAL

A mathematical model is developed to simulate the electrowinning of no n-noble metals (e.g., Zn, Cr) within flow-through porous electrodes un der the conditions of simultaneous evolution of hydrogen gas bubbles. The results of the model are presented as a function of several dimens ionless groups representing kinetics, mass transfer, ohmic resistance, and gas bubbles. These coupled, nonlinear effects are investigated by examining the distributions,of the metal reduction and hydrogen evolu tion currents, coulombic efficiency of the metal electrowinning reacti on, and gas void fractions under a series of limiting conditions. The gas bubbles accentuate the nonuniform distribution of the potential an d the currents of both reactions by increasing the effective resistanc e of the gas-electrolyte dispersion filling the pore space. This resul ts in the underutilization of the internal surface area of the porous electrode and accelerates preferential localized plugging of the pores with the reduced metal. It can also instigate localized mass-transfer limitations, i.e., the polarization at some points within the pores b ecomes large enough to support the limiting current of the metal depos ition reaction (i.e., it becomes mass-transfer controlled) while at ot her points lower polarizations and hence smaller currents prevail. Con sequently, the optimum current which maximizes the removal rate of the metal is shown to be well below the theoretical Limiting current of t he electrode. This optimum current is significantly influenced by the evolving hydrogen gas bubbles. Neglecting this phenomenon leads to err oneous design and operational considerations.