It was previously reported by the present authors that, when the Sivola cooking process was applied for hardwoods, chlorine number of the pulp increased during the second alkaline stage of the processand the spent liquor of the first sulfite cooking stage had no effect on the yield gain of the produced pulp, contrary to the results in the case of softwoods. Some investigations were carried out on the above speciality of the sulfite spent liquor from hardwoods, and the obtained results are summarized as follows ;
(1) More sodium carbonate is needed for neutralization of the sulfite spent liquorof the hardwoods than the softwoods at the same level of residual sulfur dioxide content. This is due to some volatile (organic) acids in the spent liquor from the hardwoods because of less sodium carbonate in neutralization of the concentrated one. (Fig. 1, 2 and 3)
(2) If the unbleached sulfite pulp of the hardwoods is digested with an alkaline solution containing sulfite spent liquor of low sulfur dioxide content, the chlorine number of the digested pulp becomes higher (Tab. 1, Fig. 6 and 7) and refining effect on cellulose is also low (Fig. 8). Much residual sulfur dioxide content is necessary for the second alkaline digestion of hardwoodsin the Sivola process.
(3) In order to clarify the above increasing of chlorine number, it was investigated how lignosulfonates of the first stage spent liquors at different residual SO₂ contents changed duringthe second alkaline stage. As seen from Tab. 2 and Fig. 4, there are no differences one another in the chemical and physical properties except their colors.
(4) The high chlorine number of pulp after digestion seems to be due to adsorptionof reacticn droducts from lignosulfonates and modified hemicelluloses in the sulfite spent liquor (Tab. 5).