Bleaching paper with ozone
Åse Dragland
Around the world, paper pulp manufacturers have started using ozone in their bleaching processes. The Norwegian Peterson Group has already introduced ozone bleaching in one of their plants located in Sweden, and may also utilize it in one of their Norwegian mills.
NTH's Professor Per Koch Christensen
Ozone is produced by passing oxygen through a high-voltage electric field. This is a relatively expensive process, but the bleaching process itself has no harmful by-products and gives a bright paper if combined with oxygen og hydrogen peroxide bleaching.
NTH scientists have been doing research on oxygen and ozone for a long time and has been involved in the Peterson Group bleach developments.
The use of ozone makes it easier to totally avoid the use of chlorine in the bleach processes. Bleach plants in Austria, Finland and the USA are already using the new ozone technology.
Only a handful paper-making plants in Scandinavia still use chlorine in the bleaching process. Most of them have gone over to chlorine dioxide combined with non-chlorine containing bleaching chemicals. With the small quantities of chlorine dioxide that are used nowadays, no toxins have so far been identified in emissions from the process.
"The old emission levels for chlorine in chlorinated organic compounds were 7 - 9 kg/ton paper pulp. The set limits for the Norwegian mills today are about 2- 2.5 kg/ton, but in fact most mills are well below this level. The largest bleach plant in Norway has effluents with less than 0.25 kg/ton, their limit being 2,1 kg/t," says Professor Per Koch Christensen of Dept. of Chemical Engineering.
"Emissions of chlorinated compounds are now so low that biologists and biochemists agree that they have no toxic effects. Modern industry would probably prefer to hang on to this "low-chlorine" bleaching technology, but I believe that it is on the way out" says Christensen. "The environmental movements and public opinion will not accept chlorine-based technologies at all, and they will force it out from the manufacturing process."
What we are left with are solutions based on bleaching with only non-chlorine containing chemicals, so-called TCF (Total Chlorine Free) bleaching with oxygen, hydrogen peroxide and ozone. Hydrogen peroxide is very expensive, and it may not brighten the pulp sufficiently. Ozone will, together with oxygen and reduced amounts of hydrogen peroxide, give acceptable brightnesses. The plants must invest in an ozone facility to produce their own ozone at the mill.
"The most likely scenario is that paper manufacturers will not choose either the one or the other solution, but will use all these non-chlorine chemicals in their bleaching process," says Christensen.