The effects of NO3- and SO42- 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 SO42- 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 SO42- 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 SO42- 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 42- water was the best process for chloride ion removal and efficient filtration.
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