Chemical Composition of Spent Liquors from Acidic Magnesium-Based Sulphite Pulping of Eucalyptus globulus

The major components of industrial thin (SSL) and thick (THSL) liquors from acidic magnesium-based sulphite pulping of Eucalyptus globulus wood have been characterized. The analysis of sugars in SSL revealed the predominance of xylose, which was present in the form of sugar monomer (ca. 70%) and as xylo-oligosaccharides (ca. 30%). Lignosulphonates (LS) were the most abundant organic fraction of spent liquor (ca. 50% of liquor dry matter) consisting of sulphonated oligomers (SO3H approximate to 20% w/w) of low molecular weight (Mw approximate to 1000-1300 Da) and constituted mainly by syringyl units (syringyl:guaiacyl ratio = 81:19). The major phenolic extractives of THSL were pyrogallol (93 g/ton) and gallic acid (1020 g/ton), and the most abundant fatty acid and sterol were tetracosanoic acid (6 g/ton) and -sitosterol (45 g/ton), respectively. The structural changes of macromolecular components during the liquor evaporation were discussed.