THE IMPACT OF HIGH-YIELD PULPING PRETREATMENT CONDITIONS ON SPENT LIQUOR TOXICITY AND OXYGEN-DEMAND

In order to optimize the chemical pretreatment of a high yield sulphit e pulping operation, thirty-six pilot scale pulping runs have been con ducted according to a central composite design. Second order polynomia l models for digester yield, chip sulphonation as well as for spent li quor oxygen demand and toxicity have been obtained with adjusted R(2) fits above 75 %. Optimization results indicate that high chip sulphona tion can be attained most efficiently at low initial pH with low SO2 c oncentrations. Increases in spent liquor toxicity associated with SO2 concentrations in excess of 3.5% can be offset by performing chip sulp honation at high pH.