Kraft pulp delignification with peroxy compounds

Peroxy compounds, such as peracids, dioxiranes and polyoxometalates, are effective delignification agents and can be used in chemical pulp bleaching. These compounds are electrophilic and according to the literature their reactions with lignin model compounds are similar. In this study, the delignification and residual lignin activation efficiencies and the treatment selectivities of peroxyformic acid, peroxyacetic acid, peroxyplopionic acid, peroxosulphuric acid (Caro's acid), dimethyldioxirane and polyoxomolybdate were compared. For a constant chemical charge, calculated as active oxygen, peracids and dimethyldioxirane were effective in removing lignin in both hardwood and oxygen-delignified softwood kraft pulps, whereas polyoxomolybdate reduced the lignin content only slightly. Increasing the pH of the peracid treatment from the natural pH Co about 4.5 had little effect on delignifcation, but significantly increased the removal of hexenuronic acid. Pulp treatment with peroxy compounds at 50 degrees C and at pH above 3 did not significantly affect pulp viscosity. On the other hand, below pH 3 the viscosity reduction became significant. Viscosity reduction was caused by acid hydrolysis, since the reduction was nearly the same with the reference treatment without any oxidant, Peracid-treated pulps were more easily bleached with alkaline hydrogen peroxide than those treated with. dimethyldioxirane or polyoxomolybdate. Despite the lower carbonyl group content in the residual lignin, pulps treated with peracid at pH about 4.5 brightened with lower peroxide consumption than pulps treated with the same peracid at pH 1-3. However, the pulp treated with peroxyformic acid in strong formic acid solution was easier to bleach than the other peracid-treated pulps.