Process for Clarifying Sulfate Pulp Waste Water

Abstract

In a sulfate pulp mill, the waste liquors from alkali treatment in the bleaching of pulp and of the washing of unbleached pulp are highly colored and muddy, and these greatly affect the coloring of the waste water of the sulfate pulp mill.Clarifying the waste water of the mill by decoloring these waste liquors with iron or aluminium scrap under special treatment was studied .The process is as follows : Iron or aluminium scrap is dipped into chlorine waste liquor of bleaching treatment while aerating, thus metallic ion is dissolved into the liquor .The liquor is added to the colored water containing alkali waste liquor of the bleaching treatment and waste liquor of washing of unbleached pulp in suitable quantity, and mixed together to produce ferric or aluminium hydroxide, whereby the organic colloidal substance contained in the colored liquor is absorbed, bringing about cohesion and then precipitated together.In this process where is used iron scrap, chlorine waste liquor is to be fed with flue gas to strengthen the acidity, and when mixed with the colored waste liquor slaked lime is added.The clarifyiug results by these processesareasfollows : Color degree of the treated liquid…r/3-r/7 of the original liquid (mixture of colored waste liquor and chlorine waste liquor).B.O.D.of treated liquid…reduced by about 70% compared with the original liquid.Sulfide and metallic content in treated liquid…practically undetectable.