Effect of ion water washing of neutralized sludge contaminated by high concentrations of chloride

The effects of NO₃^- and SO₄^2- ions on chloride ion removal from neutralized sludge were investigated to determine if removal of chloride ions enabled the use of sludge for cement production. An artificial sludge prepared from iron, calcium and chloride that primarily consisted of two-line ferrihydrite was prepared. Chloride ions in the sludge were easily washed out by distilled water, NO₃^- water or SO₄^2- water to levels below those specified in the standard for Eco-cement production (1,000mg/kg), but the washed sludges did not meet the standard for ordinary Portland cement (350mg/kg). Conversely, artificial sludge prepared from aluminum, calcium and chloride mainly consisted of low crystalline boehmite with other minor components. Chloride ions in this sludge were only reduced to below those specified in the standard for the ordinary cement production if SO₄^2- water was used to wash the sludge. The filtration rate during washing using SO₄ ^2- water was faster than when distilled water or NO₃ ^- water was used because SO₄^2- ions were adsorbed onto the sludge particles and the absolute value of the zeta potential of the sludge particles decreased. Overall, sludge washing using SO ₄^2- water was the best process for chloride ion removal and efficient filtration.