Sulfite Treatments in RDH and Kraft Pulping

Abstract

This research is divided into two parts. First was sodium sulfite as a treatment agent in RDH and kraft pulping. Second was sodium sulfide as a treatment agent in modified kraft pulping.

RDH cooks were done using air dried Southern Pine chips with the addition of sodium sulfite in different stages. Sodium sulfite charge was kept at 2% and 4% on OD chips. Improvement in yield and bleachability were not as expected, so to better understand the behaviour of sulfite, it was used as a pre-treatment chemical in modified kraft pulping. Sodium sulfite charge was varied upto 9% on OD chips in modified kraft cooks. Sodium sulfite was added in the pre-cooking stage, cooking and post-cooking stage. The sulfite charge was even split up between pre-treatment and post-treatment. Split addition of sodium sulfite gave good yield. Bleachability, however, didn't show the expected results. Results of this study indicated that RDH cooks with 4% sulfite charge in the Hot Black Liquor stage did not show any improvement on yield and bleachability. Split addition (pre-cook and post cook stages) of sulfite charge in the modified kraft cook increased the yield but no improvement in bleachability was seen. 0.1% AQ addition in the modified kraft cook improved the yield, but again no improvement in bleachability was achieved.

Sulfide concentration is a crucial factor in kraft pulping. This study looks into the effects of sulfidity on the yield and kappa number in kraft pulping. Sulfidity was varied from 0-100% in all the stages in a 3-stage modified kraft pulping process. Pre-treatment of chips with sodium sulfide resulted in lower kappa number. Yield was higher for cooks at lower H-Factor. Higher sulfidity in the cooking stage gave higher yield at high H-Factor. The results obtained from the experimental and model data indicated that sodium sulfide treatment in the pre-treatment (pre-cook) stage is the most affective with regards to yield and kappa number. The sulfide concentration in the post-treatment (post-cook) stage had no substantial effect on kappa number and yield.