Wood is a natural composite material and is far more complex than any man-made composite. Wood consists of fibers with lignin as a glue in between, and is thus a “fiber composite material”. The purpose of the pulping process is to separate the wood into individual fibers and to make the fibers suitable for papermaking. For that the lignin and, to a large extent also the hemicellulose, must be either dis¬solved and degraded in the chemical pulping process or removed, as is done in thermochemical pulping, or more or less softened as in thermomechanical pulping.
For chemical delignification and defibration of wood several basic chemicals are used, depending on the process (see Volume 1). In kraft pulping, which is the dominant alkaline process, the main active compounds are sodium hydroxide (NaOH) and sodium sulfide (Na2S). In addition, soda (Na2CO3), sodium sulfate
3.3 Bleaching Chemicals
(Na2SO4), sodium chloride (NaCl), sodium chlorate (NaClO3) and sulfuric acid (H2SO4) necessary for chlorine dioxide manufacture, and calcium carbonate (CaCO3) are used. In a modified kraft process polysulfide is used by adding ele¬mental sulfur to the cooking liquor. Some pulp mills use unmodified anthraqui¬none (AQ) to stabilize carbohydrates against alkaline degradation by slowing down the peeling reaction and hereby gain a few % higher yield. In industrial sulfite cooking of the wood chips, calcium, ammonium, sodium and magnesium hydro¬gensulfite – Ca(HSO3)2, NH4(HSO3), NaHSO3, Mg(HSO3)2 – as well as sulfur dioxide (SO2) are applied. Neutral sulfite semi-chemical (NSSC) pulping uses a solution of sodium sulfite and carbonate or bicarbonate to impregnate the wood chips.
Solvent-based pulping (Organosolv) is still in a very early stage of industrial development. In this chemical pulping method the delignification of the biomass (usually wood) is done in an organic solvent or solvent plus water system. There are a variety of processes, which use different solvents e. g. ethanol, methanol, acetic acid, formic acid, often in combination with sodium hydroxide, alkaline sulfite, and/or anthraquinone.
In the production of mechanical pulps (stone groundwood, thermomechanical pulp/TMP) sodiumsulfite or sodium hydrogensulfite is sometimes added to sup¬port softening of lignin and the defibration. These chemicals are generally used to produce chemimechanical pulp (CMP) or chemithermomechanical pulp (CTMP). Sodium hydroxide or soda is very often applied to control the pH during or at the end of the production process.
Recovered paper is disintegrated in water at a consistency of 4–30 %, depending on the machine type. For the production of packaging paper no or only a few chemicals e. g. dispersant, wetting agent (nonfoaming detergent) are used in the disintegration. Using recovered paper to produce a white paper stock a relatively high amount of different chemicals has to be added, to separate the printing ink and adhesives from the recovered paper. By volume the most important chemicals in this so-called “deinking process” are sodium hydroxide, hydrogen peroxide, so¬dium silicate (water glass) and fatty acid