The objective of the study reported here was to investigate experimentally the effect of the major Kraft pulping variables on the response of brownstock pulps to medium consistency oxygen delignification. Kraft pulping was performed on mixed Northeastern hardwood chips in a laboratory rocking digester. Kraft pulps of kappa number 16 and 20 mL KMnO4 were produced by pulping under a variety of conditions. The effective alkali (EA) charged to the digester was varied over the range of 12 to 21% while the sodium sulfide content of the pulping liquor was varied between 0 to 30% sulfidity. Cooking temperatures of 160 to 180°C were investigated by varying the time at temperature as measured by the Hfactor. Anthraquinone was used as a pulping catalyst at levels between 0 to 4%. Conditions of low EA charge, low temperature, high sulfidity and high levels of anthraquinone led to pulps with high pulp yield. When the effective alkali charge in the digester was low, cooking at low temperature gave rise to pulps with high viscosity and improved selectivity in the Krafi pulping process. The response variables following oxygen delignification were found to depend upon the cooking conditions applied in the digester. The kappa number versus time data could be fit to a classical power law model, indicative of lignin fiagmentation occurring by an infinite number of parallel first order reactions (Schoon, 1982). There was a systematic increase in the oxygen delignification rate, as measured by a reduction in the kappa number and an increase in the Schoon rate constant, with decreasing brownstock pulp yield in the digester. The pulp selectivity, when measured as the change in pulp viscosity with changes in kappa number, increased with increasing pulp yield until a maximum was reached. Chemical analysis of the carbohydrates in the pulps indicated that the loss in pulp yield in the digester was caused primarily by a reduction in the hemicellulose polymers as additional effective alkali was added or by cooking at higher temperature in the digester. The loss in pulp yield could be roughly correlated to the COD content of the residual spent liquor
