Removal of Na by an “active” AlF3 filter seemed to be so efficient that all the sodium was removed in the present experiments. The removal of Na is considered to be limited by the resistance in the melt boundary layer only. However this is not the case for calcium. For calcium to be transfered into the AlF3, resistance in the filter grains has to be taken into account in addition to the resistance in the melt boundary layer. This was indicated by the total mass transfer coefficient of only 4x10-5 m/s. A kinetic model was derived that can describe the removal of Ca from molten aluminum in an “active” AlF3 filter by deep bed filtration following first order kinetics.