High-yield Pulping of Scots Pine under Strongly Alkaline Conditions

The high price of raw material and the global and competitive pulp market is a combination that creates an additional challenge for the Nordic softwood pulping industry. During conventional kraft pulping, less than half of the raw material ends up in the pulp. In order to reduce carbohydrate losses while producing a bleachable-grade pulp, this work investigated different aspects that effect the degradation and dissolution behavior of Scots pine polysaccharides, as well as delignification. Emphasis was put on the effect of liquor alkali concentration and pulping temperature. Under alkaline conditions, considerable polysaccharide losses occurred during the initial pulping or heating-up phase. Increases in the liquor alkali concentration at constant sulfidity provided substantially faster delignification. Pulp yield, on the other hand, was comparable regardless of alkali concentration due to the higher galactoglucomannan (GGM) and slightly lower arabinoxylan and cellulose content in the pulp.The effect of polysulfide (PS) and anthraquinone (AQ) on carbohydrate preservation was investigated, with a special emphasis on the influence of liquor alkalinity and temperature. In the presence of PS, the influence of liquor alkali concentration on overall polysaccharide preservation was rather insignificant. However, the rate of delignification was considerably faster at higher liquor alkali concentrations, thus ensuring effective delignification even at 130°C. During high-alkali low-temperature polysulfide pulping (HALT-PS), carbohydrate losses were substantially reduced compared to conventional kraft processes at 160°C. In particular, the preservation of GGM and cellulose was improved, thus pulp yield in HALT-PS concept pulping was up to 7.2% higher. The viscosity of HALT-PS pulp was well preserved or even improved compared to reference kraft pulp due to the lower pulping temperature. Despite the high brownstock pulp kappa number of 50, the high-yield HALT-PS pulp was effectively delignified before bleaching. Therefore, pulp yield advantage and fiber strength were well-maintained in the fully bleached pulp. Pulping according to the HALT-PS concept would require additional black liquor (BL) treatment due to large amounts of unused alkali. In order to recycle and re-use the alkali-rich liquor fraction and thus support conventional chemical recovery, membrane filtration was effectively used to purify the BL from dissolved hemicelluloses and fragmented lignin.