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Carbohydrate content of black liquor and precipitated lignin at different ionic strengths in flow-through kraft cooking

Dang, Binh T.T., Brelid, Harald, Theliander, Hans
Holzforschung 2018 v.72 no.7 pp. 539-547
Pinus sylvestris, arabinans, arabinogalactans, arabinose, beta-glucans, carbohydrate content, chemical bonding, cooking, galactose, glucose, ionic strength, lignin, mannose, molecular weight, pH, sodium, sodium carbonate, sodium chloride, waste liquors, wood flour, xylose
The influence of sodium ion concentration [Na⁺] on the dissolution of carbohydrates in black liquor (BL) during flow-through kraft cooking of Scots pine wood meal (Pinus sylvestris) was studied. Fractions of BL were collected at different times and the carbohydrate content of the various fractions was analysed. Lignin was precipitated from the BL by lowering the pH, and the carbohydrate content of the precipitated lignins (Lₚᵣₑc) was also examined. The molecular weight distribution (MWD) of the Lₚᵣₑc samples was analysed. Xylose (Xyl) was found to be the most predominant sugar in BL aside from arabinose (Ara) and galactose (Gal), while the amounts of these sugars decreased with increasing levels of [Na⁺] in the cooking liquor. The minor amounts of mannose (Man) found in BL was not influenced by the [Na⁺]. The effects of NaCl and Na₂CO₃ on the carbohydrate dissolution were similar, but slightly lower concentrations of Ara and Xyl were found in the case of NaCl application. All of the Lₚᵣₑc samples contained some carbohydrate residues, the contents of which increased with increasing cooking time and decreased with higher [Na⁺]. It can be concluded that arabinoglucuronoxylan (AGX) along with arabinogalactans (AG) and arabinan, are covalently linked to lignin. The glucose (Glc) residue detected in Lₚᵣₑc may originate from 1,3-β-glucan linked to lignin.