Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use

CHLORINE DIOXIDE (as a biocide)

Composition: ClO2. This is a reactive gas formed from chlorine and oxygen. It is a strong oxidizing agent with a tendency to open up unsaturated compounds and aromatic rings. In water solution it may be present as an ion with a single negative charge.

Function: Papermakers who use bleached kraft pulp are likely to have heard about chlorine dioxide even if they have never set foot in a pulp mill. That's partly because it is very often present in the very last bleaching stage (D-stage), increasing the likelihood that some of it is carries over with the pulp into the paper machine system. Another reason is that it is one of the most popular biocide treatments for alkaline papermaking. Its use in that role is pretty much restricted to bleached grades, since any unbleached fibers will rapidly consume the chemical.

Strategies for Use: Chlorine dioxide has the desirable attribute of "quick kill" of bacteria and fungi, and it also decomposes relatively rapidly. It is conventional practice to treat the pulp furnish and then monitor the residual activity at a later point in the process. Typically the addition rate is controlled to give a residual chlorine content of about 1 ppm. The best residual level can be found by trial and error. Too much chlorine dioxide may adversely affect dyes, starch, and some other additives. Too little chlorine dioxide might result in a worsening of slime problems, bacterial counts, slippery surfaces, and odors. Excellent overall control of slime growth often can be achieved by combining chlorine dioxide treatment with the addition of organic chemical biocides.

Cautions: Generation of chlorine dioxide is hazardous and the equipment needs to be maintained by qualified people. The product is a very strong gaseous oxidizing agent.

Chlorine dioxide   Resonance structure proposed for chlorine dioxide gas


