Chemical pulp is routinely delignified to remove the 5-10 percent of lignin remaining after the pulping process. In the past, chlorine gas was used for this, followed by several stages of treatment with chlorine dioxide or hyperchlorite to further whiten the pulp.
Today, we can avoid discharging harmful chloro-organic pollutants into the environment when bleaching pulp. The dioxin scares of the mid-1980s drove forward bleaching technology towards Elemental Chlorine Free (ECF) and Totally Chlorine Free (TCF) processes and the use of elemental chlorine for pulp bleaching has been totally abandoned in Europe, North America and several other parts of the world. Chlorine dioxide has replaced gaseous chlorine in many of these cases, and the expectation is that chlorine elimination will soon be worldwide.
While chlorine dioxide has chlorine in its name, its chemistry is radically different than that of chlorine. Chlorine dioxide is the most effective and, at the same time, gentlest agent for removing resin and lignin from cellulose fiber. Chlorine dioxide is more effective than chlorine gas and produces less than one-fifth as much chlorinated organic material in the effluent stream. Most importantly, however, is that the waste products are low-chlorinated organic compounds. Naturally decomposible, they do not constitute any threat to the environment.
Bleaching with chlorine dioxide causes the production of a certain amount of chlorate, which although toxic, is an easily decomposed vegetable toxin. At the mill, chlorate in the effluent is normally reduced to naturally occurring chloride through a simple stage of treatment.