Low Sulfidity Kraft PulPing of Domestic Hardwoods

Abstract

A study was made on the kraft pulping of domestic mixed hardwoods at low sulfidities ranging from 5 to 10% in search of the means to improve the odor problem of Kraft mills.Compared with the normal kraft pulping at 25% sulfidity, a dramatic reduction of the amount of malodorants in the exhaust gas from the digester was observed at the low sulfidity pulping, methyl mercaptan concentration at 10% sulfidity and 5% sulfidity being one-fifth and one-tenth of that at 25% sulfidity respectively. The pulping rate, the yield of pulp, the amount of screening residues, and K-value and pentosan content of the unbleached pulp in the low sulfidity pulping were not substantially different from those in the usual pulping, while the pulp viscosity was found to be slightly lower in the former case. It was also noticed that the low sulfidity pulping could produce pulps of sufficient bleachability. Although the mechanical properties of papers, prepared on a laboratory test machine according to the condition specified for a writing and printing-grade paper, showed a slight tendency to deteriorate with lowering of sulfidity, it was judged that they were still practically good enough for most applications even when the pulping was carried out at 5% sulfidity.It was concluded from the results abovementioned that to carry out the pulping of hardwoods in the low sulfidity range of 5 to 10% could provide an effective way to alleviate the odor problem of kraft mills. For softwoods such as red pine, larch, etc., however, the kraft pulping in this low sulfidity range did not prove to be suitable because of difficulty of delignification during pulping and the subsequent poor bleachability of pulp, even though more detailed studies might be needed before coming to a conclusion.