Use of Green Liquor and its Derivatives in Improving Kraft Pulping, 1997 Pulping Conference Proceedings

Extended kraft delignification by modifying cooking chemistry has been a major topic for a wide variety of studies in both past and present. As well as rules concerning concentrations of hydroxyl ions and dissolved matter, the role of sulfide has shown its significant effect on improving kraft cooking selectivity. It has been possible to upscale the former modifications into industrial techniques, whereas sulfide enrichment has been limited to reusing sulfur-containing black liquors. This is due to the fact that sulfide enters the contemporary cooking plants only in the white liquor. However, there is one sulfide-rich liquor in every pulp mill: the green liquor which consists essentially of sodium sulfide and carbonate. The objective of our study was to use green liquor as a new source of extra sulfide in kraft cooking. The effect of the green liquor charge and the optimal method to introduce it were studied using a modern displacement batch cooking pilot digester. In addition to cooking, the bleaching and paper technical properties of the pulps were considered. Green liquor showed clearly positive results when applied in impregnation and the early stages of the cook. The most important results are improved bleachability and pulp strength.

In addition to the use of green liquor in improving kraft cooking we also used mother liquor from green liquor carbonate crystallization which has been proposed for on-site production of pure caustic for bleaching. This liquor is extremely rich in sulfide as most of its carbonate has been removed and water has been evaporated. The advantage thereof is a much lower increase in the inorganic load on the recovery cycle when compared to using green liquor as such. Repeated pulping and bleaching studies show equal results to using green liquor.