Charged ion transport in electrodialyzers with ion mixing spacers was numerically investigated solving NavierStokes and Nernst-Planck equations in a three-dimensional Cartesian coordinate system. An electrically nonconductive spacer placed in the membrane channels in the electrodialyzer enhances three-dimensional hydrodynamic and ionic mechanical dispersion, resulting in a substantial increase in the effective mass transfer coefficient over the membrane surfaces. This mechanical dispersion retards depletion of ions and increases the value of the limiting current density, which is of great importance in practical applications of electrodialyzers. Full detailed three-dimensional numerical computations were conducted, for the first time, to investigate the effect of the ion mixing spacers on the limiting current density, using a voxel method based on Cartesian staggered grids along with no-slip velocity and harmonic mean averaging boundary treatments. The results obtained from the three-dimensional numerical simulation were found in good agreement with the experimental data available in the literature.