Properties of lignins as CWM dispersing agents

Three kinds of lignin were added to CWMs of Taiheiyou and Horonai coals and their dispersing abilities were evaluated from the change in viscosity of the CWMs. All three lignins markedly decreased the viscosity of both CWMs. Their addition at ca. 6.0 mg/g-coal was found to be optimal, because the viscosity reached minimum at this value. The maximum coal concentration in the practically applicable CWMs, whose viscosity has to be below 1, 000 mPa . s, increased from 58.25 to 65.71 wt% for Taiheiyou coal and from 59.20 to 67.31 wt% for Horonai coal. Irrespective of the kind of lignin, the amount adsorbed on coal particles reached saturation at 6.0 mg/g-coal, which agreed with the optimal amount for addition to the CWMs. The zeta-potential of coal particles was influenced only by sulfonated lignin among the three lignins. From these results, lignin was considered to disperse coal particles mainly through its steric repulsion effect. Co-addition of lignin with the commercial dispersant PSSNa was examined from an economic point of view, but it was found not to be effective, because lignin might be adsorbed preferentially on coal particles and hinder the adsorption of PSSNa.