Kraftpulp

Kraft pulp, also known as sulfate pulp, is a type of wood pulp produced by the kraft process, an alkaline chemical pulping method that uses sodium hydroxide and sodium sulfide to break down lignin and separate cellulose fibers. It is the dominant pulp for many papers due to its strength and versatility.

In the kraft process, wood chips are cooked under pressure in a white liquor solution of sodium hydroxide and sodium sulfide. Lignin is dissolved, while most cellulose remains with the fibers. The resulting slurry is washed to yield pulp, and the spent chemicals are recovered from the black liquor in a recovery boiler, which also generates energy.

Kraft pulp is typically stronger than sulfite pulps, particularly when derived from softwoods, and is widely used for packaging, corrugated cardboard, and many printed papers. The fiber properties can be adjusted by refining and by selecting different wood species; softwoods provide long fibers, while hardwoods contribute shorter fibers for different applications. Bleaching can increase brightness.

Compared with sulfite processes, kraft pulping offers higher strength fibers and more efficient chemical recovery, and it accommodates a wider range of wood species. However, the process may require bleaching to achieve the desired whiteness for printing papers.

Environmental aspects include energy and chemical recovery from the black liquor, which reduces waste and emissions. Modern mills use chlorine-free bleaching (ECF or TCF) to minimize chlorine-based pollutants.

The kraft process was developed in the 19th century and became commercially dominant in the late 19th and early 20th centuries, eventually becoming the standard method for producing high-strength printing and packaging pulps.