Effect of sulfide concentration in the final part of a kraft cook on the selectivity and delignification rate

The effect of of sulfide concentration in the final part of a kraft cook on selectivity and delignification rate was studied. After a specified first stage, which was interrupted at Kappa number 50, the sulfide concentration in the second part of a kraft cook was varied by varying sulfidity (at a co nstant effective alkali charge) in the fresh liquor charged to the second p art of a kraft cook. An increase in the sulfidity from 0 to 100% was found to have no significant effect on selectivity,either with respect to pulp vi scosity and yield or on the delignification rate. An increase in the sulfid ity however, involved an increase in the ionic strength (sodium ion concent ration) and this impairs selectivity. The results must therefore be interpr eted as indicating that improved selectivity resulting from an increase in the sulfide concentration (through higher sulfidity) in the present case, i s counterbalanced, however, by impairment, due to increased ionic strength.