How does the kraft process separate cellulose fibres from wood?

The kraft process separates cellulose fibres from wood by cooking wood chips in an alkaline solution known as "white liquor," which consists of caustic soda (sodium hydroxide) and sodium sulfide.12

During the cooking operation—conducted in either batch or continuous digesters—this chemical mixture dissolves the lignin that binds the wood fibres together. The alkaline nature of the liquor is particularly effective at dissolving resinous components, such as those found in pine, which are then removed from the pulp and recovered as valuable by-products like tall oil.12

Once the cooking is complete, the spent "black liquor" is washed from the pulp. The resulting fibres are stronger than those produced by other chemical methods, such as the soda process or the sulfite process, because the kraft method is less damaging to the cellulose structure. The spent chemicals are subsequently sent to a recovery plant to be reconstituted for future use, an essential step for the economic viability of the process.12