Sulfite process

The sulfite process produces wood pulp which is almost pure cellulose fibers by using various salts of sulfurous acid to extract the lignin from wood chips in large pressure vessels called digesters. The salts used in the pulping process are either sulfites (SO₃²⁻), or bisulfites (HSO₃⁻), depending on the pH. The counter ion can be sodium (Na⁺), calcium (Ca²⁺), potassium (K⁺), magnesium (Mg²⁺) or ammonium (NH₄⁺).

The use of wood to make pulp for paper began with the development of mechanical pulping in Germany by F. G. Keller in the 1840s. Chemical processes quickly followed, first with Julius Roth's use of sulfurous acid to treat wood in 1857, followed by Benjamin Chew Tilghman's US patent on the use of calcium bisulfite, Ca(HSO₃)₂, to pulp wood in 1867. Almost a decade later in 1874 the first commercial sulfite pulp mill was built in Sweden. It used magnesium as the counter ion and was based on work by Carl Daniel Ekman.

By 1900 sulfite pulping had become the dominant means of producing wood pulp, surpassing mechanical pulping methods. The competing chemical pulping process, the sulfate or kraft process was developed by Carl F. Dahl in 1879 and the first kraft mill started (in Sweden) in 1890. The first sulphite mill in the United States was the Richmond Paper Company in Rumford, Rhode Island in the mid-1880s. The invention of the recovery boiler by G. H. Tomlinson in the early 1930s allowed kraft mills to recycle almost all of their pulping chemicals. This, along with the ability of the kraft process to accept a wider variety of types of wood and produce stronger fibers made the kraft process the dominant pulping process starting in the 1940s. Sulfite pulps now account for less than 10% of the total chemical pulp production and the number of sulfite mills continues to decrease.

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