Coal-water slurry has received considerable research nowadays due to its ability in substituting energy sources. The present work reports the formulation of highly concentrated coal-water slurry using a natural occurring surface active compound, saponin, extracted from the fruits of plant Sapindous laurifolia. The isolation of saponin from the plant and its surface activity has been discussed. The rhelogical characteristics of coal-water slurry have been investigated as a function of coal loading, ash content of coal, pH, temperature, and amount of saponin. The viscosity of the slurry and zeta potential are substantially decreased with concomitant shift of the isoelectric point of coal on adsorption of saponin to it. In the presence of 0.8% of saponin, coal-water slurry containing 64% weight fraction of coal could be achieved. The slurry is stable for a period of as long as 1 month in contrast to 4-5 h in the case of bare coal-water slurry. The results confirm the use of saponin as a suitable additive for coal-water slurry similar to the commercially available additive such as sodium dodecyl sulfate. Basing on the effect of pH on the zeta potential and viscosity of slurry, a suitable mechanism for saponin-coal interaction and orientation of saponin at the coal-water interface has been proposed.

The rheological characteristics of three varieties of noncoking coals have been modulated by three starchbased additives, starch xanthate, starch xanthide, and starch phosphate. The apparent viscosity of the modified coal-water slurry has been measured by varying coal loading, additive percentage, pH, temperature, etc. Considerably stable concentrated coal-water slurry containing high weight fraction of coal could be achieved in each case, and slurry stays in a nonsettling form for 20-25 days with viscosity in a permissible low range. The effectiveness in reducing the apparent viscosity decreases in the order, starch xanthide > starch phosphate > starch xanthate. The plausible mechanism of slurry stabilization has been suggested considering the structural basis of an effective additive for the stabilization of coal-water slurry.

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