Effect of additives on interfacial interactions for viscosity reduction of carbonaceous solid-water slurries

The interfacial interactions in carbonaceous solid-water slurries (CSWS) influence slurry viscosity. Additives impact the slurry viscosities by modifying the surface chemistry of the carbonaceous solids, thereby affecting the interfacial interactions in CSWS. The focus of this study was to determine the effect of the additives on the interfacial interactions which results in viscosity reduction in CSWS. Since additives are extremely specific to surface chemistry of solids in the slurry, this understanding would help in the selection of appropriate additive for viscosity reduction for specific carbonaceous water slurries. The study was conducted using three carbonaceous solids with different O/C ratios and two additives-one anionic and the other non-ionic. The adsorption of the additives on the carbonaceous solids, the change in the zeta potential and hydrophobicity/hydrophilicity of the solids and the change in the free water content of the slurries were experimentally determined. It was observed that adsorption of the additives increased with an increase in the mineral matter content of the carbonaceous solids. There was also an increase in the zeta potential of the carbonaceous solids in water upon addition of the anionic additive. However, the calculated resultant electrostatic repulsion energy upon the addition of the anionic additive was 5-6 orders of magnitude lower than the polar interaction energy of the carbonaceous solids in water. Contact angle measurements indicated that both additives changed the hydrophobicity/hydrophilicity of the solid surface. This resulted in the release of bound water into the bulk slurries, resulting in greater fluidity. The increase in free water content of the slurries with additives was confirmed by thermogravimetric analysis (TGA). A correlation predicting slurry viscosity on the basis of weight fraction of free water in the slurries was developed.