Among methods proposed for separating lignosulfonate from spent pulping liquor, ultrafiltration seems to be industrially applicable. Kraft lignin may be extracted by means of technology from black liquor.
Among the methods proposed to increase the degree of sulfonation of lignin, sulfonation with sodium sulfite was relatively easy to implement, but sulfomethylation was more industrially attractive because it increased the degree of sulfonation and molecular weight of lignin to some degree.
To improve the sulfomethylation efficiency of lignin, various pretreatment methods were proposed, but alkaline oxidation was reported to be very effective in activating lignin for the sulfomethylation reaction. Alkaline pretreatment with or without oxygen was also reported as a method to desulfonate lignosulfonate. Sulfonated lignin-based products have been extensively used as dispersants in cement admixtures and dye solutions.
It was reported that the molecular weight and sulfonate groups had significant effects on the efficiency of lignosulfonate as a dispersant. Although kraft lignin was not a suitable dispersant because of its low solubility in water, a high degree of sulfonation was not recommended for dispersant production for cement admixtures. For battery use, lignosulfonates with a low degree of solubility were recommended; this could be achieved by desulfonating lignosulfonates.
For flocculant applications, sulfonated products with a high molecular weight were recommended; this could be achieved through cross-linking lignin of lignosulfonates by phenolation or hydroxymethylation, for instance. Sulfonated lignin-based products could also be used as dust suppressants, but the main barrier was their water solubility. In composite applications, the compatibility and hydrophilicity of sulfonated lignin-based products was reported to be important.
In polystyrene composites, lignin-based sulfonated products improved their hydrophilicity, whereas in PF resin they were hydroxymethylated initially, so that they would not hamper the hydrophobicity of resin when applied. Sulfonated lignin-based products have also been proposed for use as ion-exchange resins and antioxidants, but more research is required to examine these new applications.