Residual lignin is removed in later stages with expensive, but highly selective bleaching agents like chlorine dioxide, hypochlorite, and hydrogen peroxide. Due to its importance for many years, however, lots of what we know about bleaching chemistry is related to the reactions of lignin with chlorine. Elemental chlorine has been used since shortly after its discovery in 1774 by Scheele. Chlorine is not overly specific to lignin, and much carbohydrate degradation occurs through its use. Oxidation of the carbohydrates leads to a decreases cellulose viscosity and decreased pulp strength.
Chlorination produces chlorinated organic materials including a very small amount of dioxins.
Since these compounds are recognized as powerful toxins and carcinogens, their detection in bleach plant effluents prompted a flurry of investigative activity in Europe and North America to identify point sources and corrective measures. Specified load limits for chlorinated material, measured as AOX, have either already been introduced or will be valid in the middle of 1990s for pulp mills in many pulp-producing countries including Scandinavia, Germany, Canada and Australia. The CD or CD is a modification of C stage bleaching, where some of the chlorine is replaced with ClO 2 . Chemistry of alkaline extraction Removal of chlorinated lignin. A portion is insoluble even at severe alkaline conditions.