Mm. Saleh et al., ELECTROWINNING OF NONNOBLE METALS WITH SIMULTANEOUS HYDROGEN EVOLUTION AT FLOW-THROUGH POROUS-ELECTRODES .1. THEORETICAL, Journal of the Electrochemical Society, 142(12), 1995, pp. 4113-4121
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.