Neutral sulphite pulping of softwood as a potential biorefinery was investigated, by producing pulps at two main yield levels (55–65% and 75–85%) under various process conditions and by analysing the composition of the resulting fibre fractions and pulping spent liquors. It was found that both delignification rate and selectivity were enhanced by addition of anthraquinone (AQ 0.1% on o.d. wood) whereas delignification was accelerated by higher temperature (180 °C vs. 170 °C) and initial liquor pH (10 vs. 7). The established conditions preserved significant amount of carbohydrates, especially galactoglucomannan, in the fibres at the selected yield levels. At the yield level of 65%, still over 60% of galactoglucomannan was detected in the pulps whereas 50–70% of lignin was removed. In the spent liquors, lignosulphonates as the principal component represented the possible source of by-product in addition to acetic and formic acids.
- Neutral sulfite pulping
- Sulfite spent liquor