MILL CLOSURE WITH HIGH-KAPPA PULPING AND EXTENDED OXYGEN DELIGNIFICATION

Oxygen delignification and bleaching are integral parts of the closed mill concept. However, total closure can result in a chemical imbalanc e in the system as well as overloading of critical recovery equipment, such as evaporators, the recovery furnace, and the lime kiln. The che mical imbalance may be alleviated by purging some bleach effluent, by purging some recovery system streams (such as precipitator catch), or by using oxidized white liquor in the bleach plant. With the higher se lectivity of oxygen vs. kraft chemicals, multiple oxygen stages may be used to reduce the impact on the recovery system load. Pulp produced at a kraft kappa number of 40-50 followed by multiple oxygen stages an d TCF bleaching, when compared to a conventional kraft (kappa no. 30) pulp followed by one oxygen stage and ECF bleaching, showed a similar or lower cost and a lower boiler throughput at constant production. Un der constraints of limited digester capacity or limited boiler thermal capacity, production can be increased with a multiple oxygen/TCF clos ed system relative to a conventional kraft-oxygen system.