Optimizing the effects of interstage peroxymonosulphate treatment on two-stage oxygen delignification

Laboratory studies were performed to optimize the effects of interstage peroxymonosulphate (Ps) treatments on improving the overall selectivity of two-stage oxygen delignification. Dimethyldioxirane (DMD), a selective pulp delignifier, is formed in the presence of acetone and peroxymonosulphate. However when acetone was added to the Ps stage of OPsO delignification, selectivity improvements were observed only at high charges of peroxymonosulphate, i.e. 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 improved substantially 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 peroxymonosulphate charge required to achieve OPsO pulp of given kappa number and viscosity. Enzyme use also improved final bleaching with chlorine dioxide.