Electrically enhanced removal of solutes from filter cakes - interpretation of peak mass transfer rates

Some results from an experimental and theoretical investigation of cake washing assisted by d.c. electrical fields are reported. Electric fields are shown to increase the rate of removal of cations (Na+) from rutile filter cakes when the downstream electrode was the cathode. For anions (NO3 -) under the same experimental conditions, the removal rate also varied with the electric field but the effect was to slow the rate of mass transfer. To give initial insight into the observed phenomena, the effects are explained through a first order model. The basic assumptions of the model are that: (1) there are two external forces driving the transport of ions: (i) a pressure difference that causes a mean fluid flow in which the ions are embedded, and (ii) the DC electric field applied across the cake; and (2) there are two pools of ions: (i) those trapped in the pores, and (ii) those that move with either the main fluid flow or the electrically generated ionic current. The model demonstrates the same qualitative effects as seen in the experiments, with the magnitude of the effects dependent on the magnitudes of lateral and axial ion flux component constitutive forms.