Abstract: A method for removing color from the caustic effluent produced during kraft pulping and bleaching is characterized by heating the caustic effluent to a temperature and under a pressure sufficient to cause an alteration in the chemical structure of the lignin chromophores in the effluent. The cooked effluent is cooled and has its pressure reduce to near atmospheric pressure. An acid material, such as chlorine extract from the bleaching process, is added to the effluent to lower the pH of the effluent to between 2.6 and 3.8 to initiate flocculation of the altered chromophores of the effluent. During flocculation, the chromophores are continuously separated from the effluent in order to produce a relatively clean and color-free liquid which may be deposited in the sewer system of the pulp mill.

Abstract: A method for removing color from the caustic effluent produced during kraft pulping and bleaching is characterized by heating the caustic effluent to a temperature and under a pressure sufficient to cause an alteration in the chemical structure of the lignin chromophores in the effluent. The cooked effluent is cooled and has its pressure reduced to near atmospheric pressure. An acid material, such as chlorine extract from the bleaching process, is added to the effluent to lower the pH of the effluent to between 2.6 and 3.8 to initiate flocculation of the altered chromophores of the effluent. During flocculation, the chromophores are continuously separated from the effluent in order to produce a relatively clean and color-free liquid which may be deposited in the sewer system of the pulp mill.