Electroosmotic dewatering of clays. II. Influence of salt, acid and flocculants

Abstract Electroosmotic dewatering of sodium kaolinite, a kaolinitic clay slime from sandwashing, and a coal froth-flotation tailing, was studied at various concentrations of salt, acid, and polymeric flocculants. Salt or acid up to about 10−2 M usually allowed dewatering to proceed at lower voltages or better dewatering at the same voltages, compared with suspensions in pure water, so that the energy requirement (mW h/ml) was lower. The current efficiency (ml/mA h) was not inversely proportional to the concentration of ions present and actually increased in some cases, particularly for acidified samples. At high electrolyte concentrations (0.1 M) strong electrolytic polarisation occurred and electroosmotic dewatering was limited. Flocculants (non-ionic) usually had an adverse effect on current efficiencies and energy requirements, but even at high dosages did not unduly impair the electroosmosis. It is proposed that the direct-current conductivity depends on the discharge of ions at the electrodes, and not directly on the concentration of ions. Added electrolytes do not increase the d.c. conductivity in the same manner as they increase the a.c. conductivity, and any extra direct current usually does contribute to the electroosmotic dewatering to some extent. The improved dewatering due to the presence of acid is interpreted as an enhancement of the cathode electrolysis reaction 2H2O+ + 2e → H2 + 2H2O. This also explains the reduced pH of the electroosmosis water. The energy consumed (expressed as kilowatt-hours per tonne) in all the experiments was reasonably low, particularly for attaining a spadeable consistency rather than a relatively dry cake. It is concluded, contrary to traditional expectations, that reasonable concentrations of electrolytes are not necessarily detrimental to electroosmotic dewatering and may well be beneficial.