Improving kraft pulp mill sustainability by lignosulfonates production from processes residues

Abstract

Kraft lignin is considered residue of process, but the integration of the biorefinery concept is important to achieve a more sustainable development of kraft pulp mills. Biorefinery is a promising approach to promote a wider use of kraft lignin, which is currently incinerated in kraft recovery boilers for power and steam generation. The production of lignosulfonates is one of possible uses of technical lignin for the production of lignin derivative bioproduct. This study evaluates the production of lignosulfonates from eucalypt kraft lignin by two processes: sulfomethylation and acid sulfonation. For sulfomethylation process, three temperatures (100, 130 and 160 °C) and 3 M ratios of sodium hydroxymethylsulfonate/lignin (0.8; 1.2 and 1.6) were evaluated. For acid sulfonation process, the temperatures of 80, 100 and 120 °C and molar ratio H2SO4/lignin of 10, 20 and 30 were considered. The lignosulfonate produced by the sulfomethylation process (160 °C and molar ratio of 1.6) presented a sulfur content (5.23%) similar to that found for commercial lignosulfonate (5.28%). A reduction of acid-insoluble lignin content and increase of acid-soluble lignin content was also observed compared to unmodified kraft lignin. In addition, it was confirmed that the insertion of sulfonate groups verified through FTIR analysis. Some lignosulfonates produced by sulfomethylation obtained solubility values similar to the commercial lignosulfonate, showing that the chemical modification of kraft lignin was achieved. On the other hand, the acid sulfonation was not efficient for the production of lignosulfonates from kraft lignin. Based on the studied conditions, the best results were obtained with the molar ratio H2SO4/lignin of 20 at 80 °C. The lignosulfonate produced in this condition presented a sulfur content of 3.28%, solubility in water of 15.0% and FTIR analysis similar to those found in unmodified kraft lignin, showing the inefficiency of the process.