Pulp

Abstract

AbstractGlobal virgin pulp production in 2017 was 196 million tonnes. Combined with recycled fibers, the global paper and paperboard production was 413 million tonnes with packaging paper being dominant. The two major chemical processes, kraft and sulfite, cook wood chips at high temperature and pressure to liberate fibers after about 90% of the lignin is dissolved to allow the release of pulp fibers without mechanical force. The mass yield of chemical pulp is 45–55% because most hemicelluloses and some cellulose are also dissolved. Kraft pulp represented 83% of a total chemical pulp production of 123 million tonnes in 2018. Mechanical pulp fibers are obtained by pure mechanical action such as grinding and refining and are called groundwood pulp (GWP) and thermomechanical pulp (TMP), respectively. However, because of its global dominance, this article focuses on kraft chemical pulp. The wood anatomy and chemical composition and fiber dimensions and properties of different species are described. The ultrastructure of pulp fibers and cell walls is interpreted in terms of their relevance for lignin removal during pulping and the structural properties of the final chemical pulp, respectively. Flow sheets for the production of softwood and hardwood bleached kraft and sulfite pulps are presented. Different unit operations of chemical pulping and the governing physical and chemical phenomena are described. The article reviews the mechanisms and kinetics of delignification and carbohydrate degradation and retention that occur inside the wood chips during impregnation with and subsequent pulping in cooking liquor. Oxygen delignification and pulp bleaching conclude this article.