Document Type

Article

Subject Area(s)

Chemical Engineering

Abstract

A mathematical model is developed to simulate the electrowinning of non-noble metals (e.g., Zn, Cr) within flow-through porous electrodes under the conditions of simultaneous evolution of hydrogen gas bubbles. The results of the model are presented as a function of several dimensionless 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 evolution currents, coulombic efficiency of the metal electrowinning reaction, and gas void fractions under a series of limiting conditions. The gas bubbles accentuate the nonuniform distribution of the potential and the currents of both reactions by increasing the effective resistance of the gas-electrolyte dispersion filling the pore space. This results 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 becomes large enough to support the limiting current of the metal deposition reaction (i.e., it becomes mass-transfer controlled) while at other points lower polarizations and hence smaller currents prevail. Consequently, the optimum current which maximizes the removal rate of the metal is shown to be well below the theoretical limiting current of the electrode. This optimum current is significantly influenced by the evolving hydrogen gas bubbles. Neglecting this phenomenon leads to erroneous design and operational considerations.

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