Oxidation process concept to produce lignin dispersants at a kraft pulp mill

Abstract

This paper presents the concept of an alkali-O2 oxidation process for producing lignin dispersants in a feasible manner at a kraft pulp mill. The oxidation process uses O2 gas to increase the anionic charge of lignin and the final oxidised lignin can serve as a concrete plasticiser or versatile dispersant. The integrated process provides several benefits for lignin recovery, the following lignin oxidation, and recycling of chemicals. Firstly, CO2 precipitated kraft lignin can be used without a sulphuric acid washing step. This simplifies the lignin recovery and no excess sulphur is introduced into mill cycle. As the unwashed lignin is alkaline in nature, the need for fresh alkali in the lignin oxidation is reduced. Moreover, oxidised white liquor, readily available in the mill, can be applied as a partial source of alkali. Finally, membrane filtration of the oxidised lignin solution enables the recirculation of sodium and sulphur back to the chemical cycle and provides a concentrated lignin product. Based on a simulation study, this novel concept significantly decreased the need for purging fly ash at the mill when compared to conventional lignin recovery. It was also demonstrated that the novel lignin dispersants can compete with the tested commercial oil-based plasticisers.