Effects of softwood morphology and chip thickness on pulping with a displacement kraft batch process

This study examines the effect of softwood raw-material properties on pulp quality. Logs of Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and loblolly pine (Pinus taeda) were the raw materials. Wood from Scots pine and Norway spruce was further classified by age as (a) pulpwood (mature wood and older thinnings) or (b) young wood (first thinnings and top wood). Pulpwood from loblolly pine served as the reference. Logs in each category were chipped and screened into discrete chip fractions. Chips were cooked in the laboratory using a displacement batch kraft process, and the pulps were bleached to 89% ISO. Results for cooking characteristics, bleachability, pulp strength, and optical properties are presented. Younger wood gave lower yields and strength but superior optical properties. Scots pine showed the best bleachability, while Norway spruce had the best reinforcing ability. Loblolly pine was more difficult to cook and bleach. Delignification was retarded with increasing chip thickness, but rejects occurred only with the thickest chips studied (over 8 mm). Application : demonstrates the effects of wood properties (species, age) and chip thickness on pulp yield, quality, and uniformity. The results emphasize the potential benefits of segregating incoming wood by species and age.