Medium Consistency Oxygen Delignification Design Using a Two Phase Concept. Part 2. High Solids Carry-Over, 1998 Pulping Conference Proceedings

This paper explores the effect of pulp washing and high levels of carry-over on oxygen delignification. In this study mixed northern hardwood chips were used. Pulping experiments were performed in a rocking digester at a four to one (4 to 1) liquor to wood ratio. Black liquor carry-over was varied by washing the pulp from the digester with a given amount of wash water in a single stage washer. Using this technique, carry-over solids levels were varied between 0 and 1100 kg solids/ metric ton pulp. Data are presented for the kappa number reduction, pH, COD, viscosity and wet zero span tensile strength after exposure to oxygen delignification. These data are correlated to the black liquor carry-over solids (TDS).

Carry-over of black liquor solids decreased the rate of delignification in the first rate period but had little effect on the second rate period. Inorganic black liquor solids contribute residual alkali to the delignification reaction while organic black liquor solids consume alkali and oxygen. In the carry-over experiments, the COD was found to decrease, while in the well-washed experiments, the COD was found to increase with delignification. Also, there was a large decrease in alkali content and drop in pH with heavy black liquor carry-over to the oxygen reactor.

There is a reduction in delignification rate at high levels of carry-over because the oxygen and caustic are free to react with either the pulp or the black liquor solids in competing parallel reactions. The change in COD during oxygen delignification is due to two effects, the solids emanating from the fiber which raises the level of COD, and loss of CO2 from oxidation of the dissolved organic material lowering the level of COD. These two effects acting together cause the observed COD level. These results reinforce the importance of good pulp washing prior to oxygen delignification. A two-stage oxygen design concept is demonstrated as a method to control COD.