Treatment of effluents in a chlorine based pulp bleaching plant

Abstract

An improved method for purifying dark colored chlorine containing effluents from a chlorine based pulp mill bleaching plant is disclosed, wherein the purification process involves treating the effluent in a bed containing a particulate weakly basic anion exchange and/or absorbent resin which takes up the colored lignin degradation products in the effluent until the resin has been saturated with pollutants to a given degree, whereupon the resin bed is eluted by means of an alkali solution. At least a fraction of the eluate which is released from the resin bed during the elution is acidified to a pH of less than 2, so that the majority of the total solids of the acidified effluent is precipitated, the precipitate is separated from the residual acidic liquor which contains the majority of the inorganic chlorine of the acidified eluate, and the precipitate then may be suitably transferred to a recovery plant of the pulp mill for eventual disposal. The purification process permits the chlorine content of the eluate to be reduced to such an extent that the precipitate can be transferred to the recovery plant without substantial risk of corrosion damage. Thus pulp mills which have undesirably high chlorine concentrations in their bleach plant effluents may use the ion exchange and/or absorbent resin purification method without substantial corrosion damage to the recovery plant. Furthermore, this permits the use of hydrochloric acid solutions as activating agents for the resin bed, which permits the most economic selection of activation agents.