The High Yield Chemical Pulping Process by the Use of Polysulfides Part 2.

Two-Stage Cooking Process by the Use of Polysulfides and Alkaline Cooking Liquor.

Abstract

To obtain as high an yield as possible in the manufacture of chemical pulp by the use of polysulfides (Na₂S_x) the presence of caustic soda (hydroxyl-ions) seems to be undesirable because the carbohydrates in woods will be hydrolyzed partly during they are stabilized by the action of Na₂S_x.
In order to improve the effectiveness of Na₂S_x, therefore, the digestion should be carried out into two stages. In the first stage, Japanese red pine (Pinus densiflora) wood chips are treated with Na₂S_x solution, and in the second stage, the pretreated chips are cooked by the use of alkaline aqueous solutions such as caustic soda or kraft white liquor. We call this process P.S.A.
The following results were obtained from the cooking experiments.
(1) Upon the type of Na₂S_x used in the first stage cook, Na₂S₄ was more profitable than others in the increase of pulp yield.
(2) The optimum pretreatment conditions were found to be 5.0% Na₂S₄ (as Na₂O) based on wood chip, 130°C cooking temperature and 60 min. treatment time.
(3) The second stage cook with alkaline liquor was carried out under the cooking condition of conventional kraft process. And no difference could be observed essentially between the two different alkaline cooking liquors (caustic soda and kraft white liquor) employed.
Compared with the kraft process an increase in yield of pulp 56% on wood resulted from this process. (P.S.A.) This is also approximately twice as much as if the addition of polysulfides was made directly to the cooking liquor.