Treatment for residual sulfate black liquors The effect of operational variables on effluent pollutant load

A procedure is proposed to treat sulfate pulp mill black liquor by precipitation of the solids. The precipitating agent, a calcium salt dissolved in an alcohol, has been shown to be very effective. The procedure facilitates solids separation, permits alcohol recovery and produces a liquid effluent whose residual pollutant load depends on the precipitation conditions. In this work, the effect of precipitation variables was studied to minimize effluent pollutant load. Results showed that by adjusting the calcium-alcohol ratio, the precipitation produces a major reduction in some pollution parameters and gives an effluent, from which most of the biodegradable resistant material has been removed, which could be treated in a conventional treatment process. The precipitation method could be integrated with solids combustion, or oxidation of solids into saleable phenolic derivatives, and is applicable to chemical or semichemical residual black liquors where the high lignin content limits the treatment effectiveness.