SETTLING AND RHEOLOGY OF SUSPENSIONS OF NARROW-SIZED COAL PARTICLES

Settling rates, yield stresses, and shear-stress/shear-rate dependences were determined as functions of solids concentration for suspensions in water of coal particles of narrow size fractions. Particles and suspensions are characterized by coal analyses, determination of heating value, solid heat capacity and thermal conductivity, particle size analyses, and determination of densities and maximum packing concentrations for each particle size range. Analysis of the settling rate data establishes that the usually strong retardation of (hindered) settling due to concentration is compounded by particle shape effects. In addition, observed particle shapes are subsumed within particle shape classes inferred from the settling data. Shear-stress/shear-rate data for concentrated suspensions of the three larger and relatively narrower sized particles over the broad range of shear rates from about 1 to 10(4) s-1 suggest the behavior to be essentially Newtonian, albeit with a marked particle concentration augmentation. Suspensions of the smallest coal particles of broader size distribution were strongly non-Newtonian, possessing a yield stress at low shear, essentially power-law shear-thinning behavior over the intermediate range and approaching high-shear Newtonian limiting behavior above about 10(3) s-1. Relations developed include viscosity-and yield stress-concentration correlations incorporating the maximum packing volume fraction of solids. These are useful generally for suspensions and particularly as the basis for guiding the formulation of coal-water fuels.