Use of Fully Oxidized White Liquor as Alkali Source in Oxygen Based Delignification and Bleaching Stages, 2001 Pulping Conference Proceedings

H. Parpala, O. Dahl, S. Pohjanvesi, J. Nyman, E. -L. Mustonen, J. Jakara, J. Niinimaki--The lack of sodium hydroxide has forced pulp mills to consider alternative ways to produce alkali for bleaching purposes. One very interesting way is produce expedient alkali using white liquor.

The problem with the use of white liquor in bleaching stages is sodium sulfide which consumes main reagent in delignification and bleaching thus decreasing the pulp quality. An additional problem is the formation of hydrogen sulfide, if the liquor pH drops below 10. For these reasons, sodium sulfide has to be oxidized to sodium sulfate before white liquor can be used as an alkali source in such places.

In this study the effects of four different alkali sources in oxygen delignification (O), alkaline extraction (E) and peroxide bleaching (P) were examined in a laboratory scale. The used alkali sources were pure sodium hydroxide, white liquor oxidized with oxygen in a laboratory scale and in a kraft pulp mill and white liquor oxidized with oxygen and hydrogen peroxide in a laboratory scale.

The results showed that all oxidized white liquors (fully and partially oxidized) performed much better than NaOH as an alkali source in oxygen delignification. In alkaline extraction, there were no significant differences between oxidized white liquors and NaOH as alkali sources. In peroxide bleaching, white liquor oxidized with oxygen and peroxide gave the best brightness, although the results obtained with NaOH and white liquor oxidized with oxygen in a kraft pulp mill did not differ significantly from it.