A study of steric stability of coal—water dispersions by ultrasonic absorption and velocity measurements

Abstract

The temperature dependence of the steric stability of concentrated coal-water dispersions was studied by ultrasonic absorption and velocity measurements. The dispersions were stabilized by a thin layer of the random block copolymer poly(vinyl alcohol)-poly(vinyl acetate) (PVA) with two different degrees of hydrolysis and in a manner known to result in adsorption of the PVA in a loop/train conformation. There is a destabilization process which leads to flocculation of the dispersion with a rise in temperature. This process appears to be reversible for dispersions stabilized with adsorbed PVA whereas uncoated dispersions show irreversible flocculation. Flocculation is market by a sudden rise of the ultrasound velocity, and this property was used to determine the temperature and reversibility of flocculation, which was found to occur at the Θ-temperature of the PVA solution (also detected by ultrasonic velocity measurements) or at higher temperature for a coal—water—PVA dispersion with “enhanced steric stability.” The stabilized coal—water dispersions showed nearly shear-rate-independent flow behavior, indicating an insignificant viscous interaction between the adsorbed PVA and the supernatant at low shear rates. The depth of penetration of shear waves at ultrasonic frequencies is comparable to the thickness of the adsorbed PVA layer. Ultrasonic absorption measurements indicated that there are energy dissipating mechanisms, which are interpreted to be due to the elastic and viscous response of the adsorbed PVA layer to the shear flow of the supernatant in the viscinity near the interface.