YIELD STRESS OF COAL-WATER MIXTURES

The yield stresses of coal-water mixtures as functions of solids volume fraction, suspended particle size, and suspension pH were measured using the vane method. The suspension yield stress-solids volume fraction data were used to estimate the asymptotic values of solids concentration in the limit of infinite yield stress. The limiting solids volume fractions, obtained by such an extrapolation, were found to be approximately the same as those estimated independently from the irreducible sediment volumes obtained by centrifugation. The yield stress-concentration dependence was found to depend on particle size as well as pH, or the corresponding zeta potential which was also measured. However, for a given particle size mix the yield stress-concentration plots for different pH appeared to converge, as the yield stress became large, and to approach a common asymptotic value of solids concentration in the limit of infinite yield stress. A number of different extrapolation procedures were tested to establish the type of plot capable of providing a consistent, unambiguous value of these limiting concentrations. The procedures for estimating the maximum packing solids concentrations from centrifugal sedimentation were also tested by varying the speed and the duration of centrifugation. The solids concentration dependence of the yield stress of CWM was found to be described by two-parameter non-linear relationships based upon the mean interparticle spacing, and incorporating the maximum packing volume of. solids. Furthermore, it is established that the yield stress is proportional to the reciprocal of the mean particle size.