Kraft pulping with sulfide pretreatment Part 1. Delignification and carbohydrate degradation

This paper discusses the influence of [HS-]/[OH-]-ratio and sulfide sorptio n as well as the influence of the pretreatment conditions, hydroxide ion co ncentration and temperature, on the delignification and carbohydrate degrad ation in kraft pulping with sulfide pretreatment. The cooking experiments w ere conducted as two-stage kraft cooks with a pretreatment and cooking stag e. The hydrosulfide ion concentration in the pretreatment liquor was kept o n a high level (greater than or equal to 0.3 mol/L) characteristic of indus trial displacement kraft pulping. The liquor-to-wood ratio in the cooking s tage was 10:1. The results show that with a high hydrosulfide ion concentration in the pre treatment liquor, the [HS-]/[OH-]-ratio had a slight positive influence on the delignification rate in the cooking stage. However, at cooking times lo nger than 60 minutes, the lignin content in pulp and the amount of residual lignin were found to be unaffected by the [HS-]/[OH-]-ratio in the pretrea tment liquor. Among the studied pretreatment conditions, only an increase i n the pretreatment temperature from 130 to 150 degrees C had a small positi ve effect on the delignification in the cooking stage. The faster delignification and the lower amount of residual lignin observed in displacement kraft pulping of the RDH or SuperBatch type is mainly a re sult of the high hydroxide ion concentration in the cooking stage. In addit ion, the high pretreatment temperature further improves the delignification in the cooking stage. The hydrosulfide ion concentration in the cooking st age has only a minor effect on the delignification within the investigated experimental domain, provided that the hydrosulfide ion concentration in th e pretreatment liquor is high (greater than or equal to 0.3 mol/L) and that the pretreatment is conducted at a high temperature (150 degrees C). The h igh pretreatment temperature in combination with the low hydroxide ion conc entration in the pretreatment liquor favoured the pulping selectivity, dete rmined as viscosity at a given kappa number, at a constant cooking liquor c omposition in the cooking stage.