Increasingly stringent demands have been placed by the authorities on the pulp and paper industry to decrease the use of chlorine gas in bleaching. Permitted discharges of organic chlorine compounds (AOX), together with the effluent water from the bleaching plant have gradually been lowered and are now at such a low level that, in most cases, pulp mills have stopped using chlorine gas. Instead, chlorine dioxide is used as the main bleaching agent.
For the same bleaching effect, chlorine dioxide forms lower quantities of AOX than does chlorine gas. However, the use of chlorine dioxide was also called into question. The pulp and paper industry was therefore searching for new, and even more environmentally friendly bleaching methods.
One promising solution has been developed by Eka Chemicals in cooperation with the Munksjö Aspa mill in Sweden. The bleaching method, which is termed Lignox, involves the unbleached pulp first being delignified with oxygen and then, after washing, treated with EDTA or another suitable chelating agent in order to remove metal ions that tend to decompose hydrogen peroxide. After the EDTA stage follows an intensive bleaching stage, during which relatively high charges of hydrogen peroxide are used.
Lignox is now patented in several countries and forms the basis of most TCF (Totally Chlorine Free) bleaching methods.