Pulping processes acid sulphite

In the sulphite pulping process wood cut into pieces is subjected to kiering in a calcium bisulphite solution under a pressure of 3-8 kg/cm2, at 120-145°C for 7-30 hr. The higher the temperature, the shorter the time of boiling. Substances accompanying cellulose dissolve in the liquor. Lignin is converted into a soluble lignin sulphonic acid. Cellulose is isolated from the solution, known as sulphite cellulose lye, and washed with water. 

Conventional sulphite pulping processes use aqueous solutions of sulphur dioxide at various pH levels. Sulphite solutions differ in their content of sulphur dioxide, bisulphite ions and sulphite ions, as shown in Figure 13.9. At a low pH of between 1 and 2 the sulphite liquor contains about 50% sulphurous acid and bisulphite ions respectively at a pH of 4 to 5 it contains approximately 100% bisulphite ions and at a pH of 8 to 10 it consists almost entirely of sulphite ions. When allowance is made for chemical charge, it is the pH and the relative amounts of bisulphite and sulphite ions that chiefly control the mode of pulping. In the original acid sulphite process... 

The first diflferentiation is used in various sectors of the wood-processing industry like the pulp and paper industry, as its chemical behaviour is defined by the polarity. Depending on the chemical pulping, certain extractives can show negative effects on the production, e.g. pinosylvin and its monomethyl ether during acid sulphite pulping. In contrast, other pine extractives (resin acids, terpenes, eic.) are useful by-products in Kraft pulping (see Section 9.4.3). 

Currently apphed processes for the chemical utilization of wood are almost exclusively geared towards chemical pulping. Although, to some extent, the utilization of by-products like vanillin, xylose, furfural, acetic acid or resins (tall oil, rosin and products thereof e.g. sitosterol) see Chapter 9.7) is also included, very often not more than 50% of material utilization is obtained e.g. sulphite mass balance in Figure 9.4.4). 

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