Electro-dewatering of drilling sludge with liming and electrode heating

The present work considers optimisation of processing parameters during dewatering of bentonitic drilling sludge by combination of flocculation and electro-filtration. The quantity of lime used for the sludge flocculation was varied within the range from 0 g/l to 10 g/l. The two-sided laboratory filter cell with constant distance between the electrodes was used for electro-filtration. The experiments were carried out at the constant pressure of 5 × 105 Pa with application of direct current with constant current densities of 0, 40 and 80 A/m2. The impact of lime concentration and current density on electrokinetic properties and permeability of the initial sludge as well as on electro-filtration rate, filtrate quality, final dryness of the filter cake and energy consumption was investigated. The role of different effects (structuring of bentonite particles, electroosmotic transfer, electrophoresis, ohmic heating, electrolysis) in the electro-filtration of limed sludge was analysed.It was shown that increase of lime concentration improved permeability, however it impaired electrokinetic properties of the sludge and notably decreased the final dryness of the filter cake as compared to the non-limed sludge. Application of direct current during filtration of the sludge flocculated with low quantity of lime (⩽1 g/l) also improved filtration rate, however it had no negative effect on the final filter cake dryness. The direct current application allowed efficient dewatering of the sludge (short dewatering time, high dryness of the filter cake) at optimal quantity of lime (0.3 g/l) that was noticeably lower as compared with those usually used in simple filtration (10 g/l).In the two-sided filter cell the asymmetric ohmic heating of the filter cake with notable heating at the anode side, and moderate heating at the cathode side was observed. This asymmetry was explained by asymmetry of different processes (electroosmosis, electrophoresis, cake formation) at the cathode and anode sides. Supplementary thermal heating of the cathode side was demonstrated to be effective for improvement of electro-filtration rate with preservation of the final dryness of the filter cake (above 67 wt.%) at the optimal quantity of lime (0.3 g/l).

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► Overliming accelerates ordinary filtration but decreases the final cake dryness.
► Moderate liming accelerates electro-filtration and increases final cake dryness.
► Ohmic heating of filter cell is asymmetric (high at anode and low at cathode).
► Additional thermal heating of cathode significantly improves electrofiltration rate.