A 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 precipit ation of the solids. The precipitating agent, a calcium salt dissolved in a n alcohol, has been shown to be very effective. The procedure facilitates s olids separation, permits alcohol recovery and produces a liquid effluent w hose residual pollutant load depends on the precipitation conditions. In th is work, the effect of precipitation variables was studied to minimise effl uent pollutant load. Results showed that by adjusting the calcium-alcohol r atio, the precipitation produces a major reduction in some pollution parame ters and gives an effluent, from which most of the biodegradable resistant material has been removed, which could be treated in a conventional treatme nt process. The precipitation method could be integrated with solids combus tion, or oxidation of solids into saleable phenolic derivatives, and is app licable to chemical or semichemical residual black liquors where the high l ignin content limits the treatment effectiveness.