A color removal process for kraft pulp and paper mill effluents

An optimized co-precipitation process for color removal from Kraft pulp and paper mill effluents is described. The process involves sequential addition of alum and lime under controlled conditions, as well as the influence of selected polyelectrolytes on the degree of color removal. Results obtained were in the range of 86 to 95% removal for caustic extract effluent and whole mill effluent. An Al⁺³/Ca⁺² ratio of 0.23 gave optimal results with a final pH in the 4 to 5 range. The chemicals used in the suggested process are commonly available in most Kraft mills. As well, significant reductions in chemical oxygen demand (COD) were observed during the color removal process. An economic analysis indicates that the process is financially attractive.