About the Efficiency of the Most Common Bleaching Agents, 1999 Pulping Conference Proceedings

Dominique Lachenal

Ecole Française de Papeterie

Christine Chirat

Centre Technique du Papier

Bleaching of kraft pulp consists in delignification and removal of the residual chromophores. At industrial scale, delignification is performed by chlorine, chlorine dioxide, oxygen, hydrogen peroxide, ozone and peracetic acid. For all these chemicals, delignification can be described by one single global mechanism, i.e. ring opening of aromatic units, which necessitates the exchange of 4 electrons. This may explain why the same number of electrons is exchanged by chlorine, chlorine dioxide and ozone for the same kappa number drop. With oxygen, hydrogen peroxide and peracetic acid much more electrons have to be exchanged, which justifies the search for catalysts. This analysis gives useful information about the displacement ratio of one chemical by another one. It is also shown that at least 50% of the bleaching chemicals are wasted in secondary reactions which apparently do not contribute to lignin removal.