In chemical pulping, the wood lignin is made water soluble by the action of bisulphite ions (sulphite
pulping) or hydroxide ions in combination with hydrosulphide ions (kraft pulping). Although
other processes such as soda pulping with aqueous sodium hydroxide with or without
the presence of anthraquinone are commercially used, they play a minor role for the pulping of
wood. For the pulping of annual plants, soda pulping is the preferred process, however. The sulphite
process was invented by Tilghman (Great Britain) in 1866 and the kraft process by Dahl
(Germany) in 1879. The latter type of pulp was, however, only used in unbleached and semibleached
paper products where fibre strength was an important parameter and sulphite pulping
remained the predominant process for making chemical pulp until around the second World
war. At that time, an efficient way of bleaching the dark brown kraft pulp to full brightness by
employing chlorine dioxide was found. Since the kraft process gave stronger pulps, was less dependent
on the wood raw material and had an efficient chemical recovery process, a successive
replacement of small inefficient sulphite mills with large and energy efficient kraft mills started
and, today, the kraft process is totally dominant for the production of chemical pulps for a large
variety of end uses.