Electrically enhanced washing of ionic species from fine particle filter cakes

A technique to enhance the removal of solutes and unwanted liquors from fine particle filter cakes by the application of an electric field during washing has been developed. For aqueous feeds of titania (rutile), the effects of d.c. electric field strength as well as cake and suspension properties such as pH and ionic strength were investigated experimentally. Cake washing experiments showed how the rate of removal of cations (Na+ and Al3+) and wash liquors could be increased substantially at raised electric field when the downstream electrode worked as the cathode and the suspension forming the cake was at a pH well removed from its iso-electric point (IEP). In some circumstances, cations remaining from the initial production process of rutile could be removed from the particle surfaces by the electric field. For anions (SO42-, Cl- and NO3-) under the same experimental conditions, the removal rate also varied with the electric field but in a manner generally contrary to that observed for the cations. When the polarity of the electrodes was reversed, reduced effects were observed; however, a modest increase in the removal rate of anions was detected at longer wash times. With suspensions adjusted to their IEP, filter cakes formed with more open structures and larger values of porosity. For cake washing under these circumstances an increase in the cation removal rate was noted. Liquor flow rates tended to reduce when the electric field strength was increased and cake porosity decreased with time during the course of washing.