Evaluation of chipping and impregnation of Scots pine heartwood with sulfite cooking liquor

Heartwood and sapwood of Scots pine were procured and chipped using a newly developed pilot drum chipper, which for the heartwood resulted in a combined fraction of pin chips and fines of similar to 3%. Heartwood wood chips were processed using a set of 15 different reaction conditions that differed with respect to impregnation and cooking procedures. The result was evaluated with regard to absorption of impregnation liquid, pulp yield, fraction of reject, viscosity, kappa number, brightness, fiber properties, and chemical composition measured using two different techniques (compositional analysis using two-step hydrolysis with sulfuric acid and Py-GC/MS). The chemical analyses provided detailed information about how all main organic constituents of the wood, cellulose, hemicelluloses, and lignin, were affected by operational parameters. Inclusion of a pressurized (9 bar) impregnation step resulted in a more efficient cook, but the duration of the impregnation step ( five minutes and four hours were compared) was not decisive for the outcome. Omission of the impregnation step or using low-pressure impregnation resulted in high fractions of reject, poor delignification, and, with a cooking time of two hours, no advantages with regard to fiber length and fraction of fines. The results indicate that the conditions used during impregnation, such as pressure, temperature, and acidity of impregnation liquid, warrant further attention in future studies.