The use of ozone to decolorize residual direct paper dyes in kraft paper machine whitewater

Production of colored kraft papers results in strongly colored paper machine spent Whitewaters (ww). The residual dyes prevent efficient recycle of the ww and necessitate periodic system cleanings. Many of the dyes are not adsorbed or degraded by biological effluent treatment and therefore discolor receiving waters. Ozone was tested as a bleaching agent for 15 direct paper dyes added at typical residual levels to kraft mill ww. These dyes included azo, polyazo, stilbene, benzothiazole and phthalocyanine direct dyes commonly used in Canadian dyed paper production. Ozone charges of 15 ‐100 mg/L rapidly bleached most of the dyes, although a few of the yellow stilbene and azo dyes were quite resistant. The efficiency of ozone‐mediated residual dye bleaching was altered substantially by the pH, the ww concentration, and the target dye's chemical structure. Preozonation of residual dye‐containing ww greatly decreased the color conferred to bleached kraft pulp fibers added to the ww. Thus, ozonation should permit much more ww recycle (less effluent) when changing colors on the paper machine. Depending on the availability of oxygen or ozone in a mill, and local ozone economics, ozone may be the most efficient, cost‐effective way to improve ww quality, increase ww recycle and minimize effluent color problems and environmental impact.