The additive is extracted from the pulping liquid by the procedures of purification, evaporation, chemical treatment, and drying. It can be distributed in a liquid form or as a powder.
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 (SO3 2−) or bisulfites (HSO3−), depending on the pH. In this case, the counterion is Ammonium (NH4+). The cooking liquor is prepared by adding the counter ions as hydroxides or carbonates. The relative amounts of each species present in the liquid depend largely on the relative amounts of sulfurous used.
For Monovalent (M+) hydroxides, MOH:
SO2 + MOH
MHSO3 + MOH → M2SO3 + H2O
Sulfite pulping is carried out between pH 1.5 and 5, depending on the counter ion to sulfite (bisulfite) and the ratio of base to sulfurous acid. The pulp is in contact with the pulping chemicals for 4 to 14 hours and at temperatures ranging from 130 to 160 °C (266 to 320 °F), again depending on the chemicals used.
Chief among sulfite process byproducts are lignosulfonates, which find a wide variety of industrial uses. Ammonium Lignosulphonate is recovered from the spent sulfite by decanting (to remove insoluble material) and
evaporation, the latter to increase the solids content to around 50%. (The solids content of the original material recovered from the spent sulfite liquor is less than 10%.
Further processing may involve adjusting the pH and spray drying, depending on the target industrial application.
Read more about the different industries lignin is used in here
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