Desulfurization of kraft lignin

Kraft lignin is a product of the sulfate pulping process. It usually contains 2–3% total sulfur. Sulfur content acts as deterrent to lignin use in some areas. Sulfur-free lignins are more versatile and can be heat-treated without the release of foul-smelling sulfur compounds. These lignins are used to produce various low- and high-molecular products, as well as fuel. The existing soft desulfurization methods of kraft lignin allow reducing the total sulfur content, for example, fivefold by a method combining extraction of elemental sulfur, oxidation and reduction in sulfur compounds. Deeper desulfurization has earlier been achieved by catalytic hydrogenation, which is accompanied by depolymerization of lignin. In this paper, it was established that sequential treatment of sulfate lignin with benzene, silver in dimethylformamide, aqueous sodium sulfite solution and perchloric acid + acetic anhydride mixture can reduce the total sulfur content a hundredfold.