On the efficiency of the bleaching chemicals for kraft pulp

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 the 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. However, this number is higher than what would be required for ring opening alone, indicating that at least 50% of the bleaching chemicals are wasted in secondary reactions, which apparently do not contribute to lignin removal. The situation is even worse with oxygen, hydrogen peroxide and peracetic acid, which justifies the search for catalysts.