Abstract: Spent sulphite liquor is a lignocellulosic residue obtained in the acid sulphite pulping process after the digestion of hardwood chips, and composed mainly by lignosulphonates and hemicelluloses. The aim of this work is to study the main process variables at different digestion conditions: maximum temperature, heating rate, and total SO2 content, which affect the delignification process of hardwood chips to obtain dissolving pulp and to increase the total amount of fermentable sugars and lignosulphonates presented in this lignocellulosic waste. The best results are 210.5 g/L of lignosulphonates and 47.26 g/L of total monosaccharides obtained at higher temperatures of 1.058T and 1.072T, low heating rates of 0.334R and 0.285R, and total SO2 content of 6.20%. Furthermore, concentrate-acid hydrolysis of the liquor with sulphuric acid was performed in order to study the evolution of the sugar content and the release of the inhibitors. Temperature, acid concentration, acid/liquor ratio and time were modified, showing that acid/liquid ratio is the most influential variable. Although increase in the content of sugars in neither case, a notable furfural and acetic acid concentration decrease is registered, 97.88% and 36.57% respectively, at 20 °C, 60% (w/w) sulphuric acid and acid/sample ratio of 1/0.1 (v/v).

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