When the Sivola process was applied for the manufacturing of dissolving grade pulp from hardwoods, it was found by the present authors (unpublished) yield of the pulp was also higher than that by conventional process, as similar as found for the softwood. In this study, it was tried to clarify characteristics of the hardwoods in the Sivola pulping process and the reasons why the process gave higheryields. Conclusions of this study are as follows ;
(1) The principal reasons of the yield gain are due to pulping of screenings from the first stage sulfite cooking with the second stage alkaline digestion (Fig. 3), and alkaline digestion with a solution of sodium carbonate and sodium sulfite instead of a caustic solution in the second stage (Fig. 4).These are the same as in the case of softwood.
(2) Contrary to the case of soft wood, sulfite spent liquor presenting in the second stage had no effect on the yield gain in the case of hardwood (Fig. .4). This difference seems to be due to some special nature of the sulfite spent liquor from hardwoods and are under further investigation.
(3) Hardwoods are, in general, denser and more resistant to the penetration of cooking liquor thansoftwoods. Sodium-base in the first stage of the Sivola process contributes to a part of the yield gain, especially at the range of higher viscosity level of produced pulp (Fig. 1 and 2).