Kraft pulp delignification with peroxy compounds

Peroxy compounds, such as peracids, dioxiranes and polyoxometalates, are ef fective delignification agents and can be used in chemical pulp bleaching. These compounds are electrophilic and according to the literature their rea ctions with lignin model compounds are similar. In this study, the delignif ication and residual lignin activation efficiencies and the treatment selec tivities of peroxyformic acid, peroxyacetic acid, peroxyplopionic acid, per oxosulphuric acid (Caro's acid), dimethyldioxirane and polyoxomolybdate wer e compared. For a constant chemical charge, calculated as active oxygen, peracids and d imethyldioxirane were effective in removing lignin in both hardwood and oxy gen-delignified softwood kraft pulps, whereas polyoxomolybdate reduced the lignin content only slightly. Increasing the pH of the peracid treatment fr om the natural pH Co about 4.5 had little effect on delignifcation, but sig nificantly increased the removal of hexenuronic acid. Pulp treatment with p eroxy compounds at 50 degrees C and at pH above 3 did not significantly aff ect pulp viscosity. On the other hand, below pH 3 the viscosity reduction b ecame 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 hyd rogen peroxide than those treated with. dimethyldioxirane or polyoxomolybda te. Despite the lower carbonyl group content in the residual lignin, pulps treated with peracid at pH about 4.5 brightened with lower peroxide consump tion 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.