A method of converting wood chips into paper pulp for use in papermaking accomplished by chemical cooking of the chips, as opposed to mechanical pulping. The purpose of pulping is to reduce wood (or other fibrous raw material) to individual cellulose fibers. A non-fibrous constituent of wood, lignin, binds cellulose fibers together, and is primarily responsible for reducing paper quality and permanence.
Chemical pulping methods produce high-quality papers as the chemical cooking dissolves most of the lignin and hemicelluloses present in the wood, resulting in better separation of the cellulose fibers. There are two primary means of chemical pulping. The sulfite process cooks wood chips in sulfurous acid combined with limestone to produce calcium bisulfite. The combination of sulfurous acid and calcium bisulfite dissolves the lignin in the wood and liberates the cellulose fibers. Sulfite pulp is soft and flexible, is moderately strong, and is used to supplement mechanical pulps (most typically in newsprint). Problems with the process (including limitations on the types of trees for which it is suitable, strict pollution laws, and the inability to recover some of the chemicals ejected by the system) have resulted in new chemicals being used in the process, and the wholesale adoption of new processes.
The sulfate process is now the most widely used chemical pulping system. It evolved from the soda processes developed in the nineteenth century, which used strong bases (alkaline solutions) such as lye to digest wood. Pulpers began adding sodium sulfate to the soda process, and a significantly stronger pulp was produced. Incorrectly termed the "sulfate" process (it was much later that chemists discovered that the active ingredient was actually sodium sulfide, it is perhaps more accurately known as the kraft process ("kraft" is the German and Swedish word for "strength"). The advantages of kraft pulping include not only increased pulp strength, but also a better heat- and chemical-recovery system which reduces processing costs, its effectiveness in digesting nearly every known species of tree, and the insertion in the process of bleaching processes which increase pulp brightness. The pulp, as the name "kraft" indicates, is also much stronger than pulp produced via other methods and the paper generated from the process runs well on high-speed presses.
To increase pulp whiteness and brightness (unbleached kraft pulp is usually a dark brown color), and to remove residual lignin, chemical pulps are bleached. It is at this point that additional non-fibrous materials called fillers are added to the pulp—a process called loading—and the resulting furnish—the mixture of pulp and fillers—is ready to begin the refining process.