Lignin is the largest reservoir of aromatic compounds on earth and has great potential to be used in many industrial applications.
Alternative methods to produce lignosulfonates from spent sulfite pulping liquors and kraft lignin from black liquor of kraft pulping process are critically reviewed herein. Furthermore, options to increase the sulfonate contents of lignin-based products are outlined and the industrial attractiveness of them is evaluated. This evaluation includes sulfonation and sulfomethylation of lignin.
To increase the sulfomethylation efficiency of lignin, various scenarios, including hydrolysis, oxidation, and hydroxymethylation, were compared. The application of sulfonated lignin-based products is assessed and the impact of the properties of these products on the characteristics of their end-use application is critically evaluated.
Sulfonated lignin-based products have been used as dispersants in cement admixtures and dye solutions more than other applications, and their molecular weight and degree of sulfonation were crucial in determining their efficiency. The use of lignin-based sulfonated products in composites may result in an increase in the hydrophilicity of some composites, but the sulfonated products may need to be desulfonated with an alkali and/or oxygen prior to their use in composites. To be used as a flocculant, sulfonated lignin-based products may need to be cross-linked to increase their molecular weight. The challenges associated with the use of lignin-based products in these applications are comprehensively discussed herein.
An increase in the molecular weight, phenolic hydroxyl content, and sulfonation as a result of modifying lignosulfonates with hydroxybenzyl alcohol allowed for better dye dispersion capabilities. The adsorption of lignosulfonates onto dye was increased due to increased molecular weight and hydroxyl content, whereas the sulfonate groups caused stabilization through strong repulsive charge.
By increasing the available functional groups through oxidation, the dispersing capabilities of lignosulfonates were increased dramatically. As mud dispersants, the oxidized lignosulfonates outperformed commercial products by at least 90 %based on gel point determination. For gypsum dispersion,dispersion efficiency was improved by 50 % following oxidation of lignosulfonates.Increasing the thermal stability of lignosulfonates as a dispersant for drilling mud through hydroxymethylation allowed for values similar to those of the commercial additive ferric chromium lignosulfonate.