Optimizing the Effects of Interstage Peroxymonosulfate Treatment on Two-Stage Oxygen Delignification, 1996 International Pulp Bleaching Conference Proceedings

Laboratory studies were performed to optimise the effects of interstage peroxymonosulfate (Ps) treatments on improving the overall selectivity of two-stage oxygen delignification. To be viable, OPsO delignification requires maximum selectivity improvements at minimum peroxymonosulfate charges, due to the high cost and sulfur content of peroxymonosulfate. Both additives to, and pretreatments before, the Ps stage in OPsO delignification were investigated for their ability to minimise peroxymonosulfate demand. Dimethyldioxirane (DMD), a selective pulp delignifier, is formed in the presence of acetone and peroxymonosulfate. However, when acetone was added to the Ps stage of OPsO delignification, selectivity improvements were only observed at high charges of peroxymonosulfate, ie, 5% HSO5 -. This option was not favoured due to the cost and complexity of acetone addition as well as the need for high Ps charges. The selectivity of OPsO delignification was, however, substantially improved by the use of enzymes in conjunction with chelation pretreatments prior to the Ps stage. Enzyme use increased subsequent delignification while incurring little pulp viscosity loss. The increase in overall OPsO selectivity allowed decreases of up to 25% of the peroxymonosulfate charge required to achieve OPsO pulp of given kappa number and viscosity. Enzyme use also improved final bleaching with chlorine dioxide. OPsO pulp of a given kappa number required 10-15% less active chlorine to reach high brightness when enzymes were incorporated into chelation pretreatments prior to the Ps stage.