Sodium sulphite pulping of Scots pine under neutral and mildly alkaline conditions (NS pulping)

Abstract

Abstract Sodium sulphite pulping of Scots pine chips in the initial pH range of 7.5–9 was investigated at 180°C and 165°C (for 120–240 min), using chemical charges between 25 and 50% and anthraquinone (AQ) charges 0–0.2%. As a result, pulps with yields of 52–73% and kappa numbers of 35–106 were produced. Delignification was accelerated by higher Na2SO3 charges and temperatures, and by AQ addition, whereas the effect of initial pH was negligible. The high pulp yields at a given kappa number were attributable to high retention of hemicelluloses, especially that of galactoglucomannan. The relatively low viscosities of pulps were apparently caused by depolymerisation of cellulose by sulphite. The sulphonic acid groups in the pulps correlated well with the residual lignin content, although some sulphonation of polysaccharides may also be possible. The results indicate that pulping of softwood in buffer-free (without Na2CO3 and/or NaOH) sodium sulphite liquor may result in pulps with tailored characteristics.