Sulfite pulps, lignin content

Figure 6.1 The acid group content of pulps expressed as an ion exchange capacity as a function of kappa number (measure of lignin content) for Kraft and sulfite pulps.

Since the prices of petroleum products have soared, valorization of natural resources has intensively attracted the attention of researchers in different fields. To fully utilize wood, which is a renewable raw material, one must make better use of its 25-30% lignin content which forms the principal dissolved material in the spent liquor of chemical pulping processes. The annual production of sulfite pulp in Canada is approximately 2.6 million tons (data based on 1979), thus, at least one million tons of lignosulfonate is produced, representing a potential resource of raw material for other uses. However, among 35 sulfite mills in Canada, only two or three of them possess a recovery system for spent liquor, that means a very small portion of lignosulfonate has been recovered. The recovered lignin has been... 

Methods of determining the sulfonate content of lignin fall into two main classes those typified by conductometric titration, in which sulfonate content is measured directly and those, as represented by combustion/ion chromatography, which measure the sulfur content and assume that all the sulfur is present as sulfonate groups is approximate in many ultra-high-yield sulfite pulps (Katz et al. 1984) but may lead to large errors (Samuelson and Westlin 1947, 1948, Regestad and Samuelson 1958, Schoon 1961, Janson and Sjostrom 1966, Katz et al. 1984). 

Conductometric titrimetry can be applied to a wide range of pulp types with a wide range of sulfonate contents. The method has been applied to softwood chips treated with sulfite, softwood sulfite mechanical pulps, high-yield bisulfite pulps, low-yield acid sulfite pulp (Katz et al. 1984, Gummerus 1985, Heitner and Hattula 1988) and sulfite treated aspen wood (Beatson et al. 1985) with sulfonate contents ranging from 20 to 300 mmol kg-1. In addition, the method has been adapted to measure sulfonate content of sulfonated kraft lignin (Oster et al. 1988). 

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