Raffi M. Turian

Characterization, settling, and rheology of concentrated fine particulate mineral slurries

Abstract The properties, settling behavior and the rheology of fine particulate slurries were investigated. The test slurries used in these studies included laterite, gypsum, titanium dioxide and silica flour. All were industrial slurries. The property characterization experiments included measurements of the particle size distributions, particle surface areas, pore size distributions and particle densities. Slurries containing different concentrations of the test solids were made up, and their pH and zeta potentials were measured. The rates of settling of the slurries as a function of concentration were determined. The rheological data included directly measured yield stresses using the vane apparatus, and the shear stress-shear rate dependence over ranges of shear rate from less than 1 to more than 10 4 s −1 using both rotational and capillary rheometers. The settling rate data were analyzed using hindered settling correlations. The yield stress-concentration data were correlated using relationships based on interparticle spacing and the maximum packing volume fraction of solids. Both two-parameter as well as three-parameter empirical rheological models were used to fit the shear stress-shear rate data. The three-parameter Sisko model, which combines low- and intermediate-shear power-law with high-shear Newtonian limiting behavior, was found to provide the best overall description of the flow curves for all slurries, at all solids loadings, over the entire measured range of shear rates. This finding is significant because this model evidently works for most non-Newtonian solid-liquid suspensions, and also because it is applicable over precisely the range of shear rates relevant to pipeline transport of the suspensions.

Estimation of the critical velocity in pipeline flow of slurries

Abstract A detailed examination of the critical velocity in pipeline flow of non-colloidal slurries was carried out. Published critical velocity correlations were collected and were recast into a standard form so that they could be compared with each other, and they were also tested against a broad collection of experimental critical velocity data. Altogether, a total collection of 864 experimental critical velocity data, representing a broad variety of solid materials and pertaining to wide ranges of the variables involved, were used in these tests. This rather substantial body of experimental data was also used as the basis for developing a set of improved critical velocity correlations, which were established by fitting the data to various forms of the standard equation. Among the principal results of this work are the following: The dependence of the critical velocity on pipe diameter is very nearly equal to D 1 2 , while its dependence on particle size, for slurries of non-colloidal particles, is very nearly equal to d0. The analytical result due to Oroskar and Turian indicates that υc depends on pipe diameter as D0.6, and on particle size as d0, while the best empirical fits to the data suggest that the dependence on pipe diameter is approximately D0.5, and that on particle size is at most d0.06. In addition, both the newly established empirical correlations and the analytical correlation due to Oroskar and Turian predict a maximum in the υc vs. C curve, the maximum occurring at solids volume concentration of 0.25 to 0.30. The comparisons with experimental data further established that the analytical result by Oroskar and Turian, and the empirical correlations developed in this work, do a superior overall job of predicting the data than other published correlations.

Yield stress of laterite suspensions

The yield stresses of aqueous laterite suspensions as functions of solids concentration and suspension pH, or ζ potential, were measured using the direct vane method. In addition, capillary rheometer shear stress—shear rate data were taken for the most concentrated test suspension, and these were used to estimate the yield stress by various extrapolations, and to compare them with the directly measured value using the vane geometry. Detailed characterizations of all test suspensions were carried out. The yield stress was found to depend strongly on suspension pH, attaining a minimum at the isoelectric point occurring at high pH. It was found to follow approximately a straight-line relationship with the square of the ζ potential, and its dependence on solids concentration is described by a two-parameter nonlinear relationship which gives the appropriate behavior in both the dilute limit and the concentrated limit near maximum packing.In situ particle size measurements by sedimentation suggest that the degree of flocculation is high under acidic conditions. The capillary rheometer data reflect strongly non-Newtonian, shear thinning behavior.

THERMAL CONDUCTIVITY OF GRANULAR COALS, COAL-WATER MIXTURES AND MULTI-SOLID/LIQUID SUSPENSIONS

Abstract The thermal conductivities of suspensions of particulate coals in water, in fuel oil and in other liquids, and of mixtures of different particulate solids in liquids were measured. The binary suspension data were used to determine the solid thermal conductivities of the particulate inclusions by various extrapolations, and to test the different extrapolation schemes. A general, second-order, effective conductivity formula for binary mixtures, valid for small differences between the conductivities of dispersed and continuous phases, was derived. For suspensions containing a mixture of N different solids, of solid to liquid conductivity ratios β t = k i k 0 and volume fractions φ i , the formulae ( k ∗ k 0 ) = β 1 φ1 β 2 φ2 … β N φN and ( k ∗ k 0 ) = [1 + ∑(β i 1 3 − 1)φ i ] 3 were derived for the suspension thermal conductivity k ∗ . The former was found to do a good job of prediction for binary and multi-solid suspensions. As the volume fraction weighted geometric mean of the constituent conductivities, this formula is known to lie between the harmonic and arithmetic means as lower and upper bounds, respectively. Furthermore, over the range of conductivity ratios β i where it works well, it either lies between tighter bounds based on variational principles, or when it falls outside it differs indistinguishably from these bounds, which in this case turn out to be quite close.

Yield stress of coal-water mixtures

Abstract 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.

Chemically enhanced filtration and dewatering of narrow-sized coal particles

Abstract An experimental study of the filtration and dewatering of fine coal with and without additives was carried out. Filtration and dewatering behavior were determined using a vacuum filtration cell. The variables studied included the driving force, solids concentration, filter area, particle size and slurry pH. The effects of several flocculants, surfactants and hydrophobing additives (oils) on final moisture content of the filter cakes were determined. The investigations included determination of the rates of filtration and dewatering, assessment of the effects of additive type (anionic, cationic and nonionic), and establishment of optimum dosage of additives. Overall, it was found that the filtration and dewatering behavior were enhanced by increased driving force, higher initial slurry concentration, larger filtration area, larger particle size and lower slurry pH. Among surfactants and flocculating additives the anionic types were found to do the best job of filtration enhancement. At optimum dosages, the final cake moisture contents were reduced by 3.6% and 5.8% by weight using the appropriate surfactant and flocculant, respectively. However, the best filtration enhancements were achieved by addition of oils, which yielded cake moisture reductions of approximately 10% by weight.

FRICTION LOSSES FOR FLOW OF SLURRIES IN PIPELINE BENDS, FITTINGS, AND VALVES

ABSTRACT Resistance coefficients for flow of suspensions of well defined glass beads of narrow size fractions in 1-inch and 2-inch straight pipes, in standard 45°, 90° and 180° bends, in 90° smooth bends of various curvature radii, and also in gate and globe valves were measured. The measurements were made for two sizes of fine glass beads, -325 mesh and -200-325 mesh, covering wide ranges of turbulent Reynolds number and solids concentrations from 0 to 50 weight percent. These friction loss data were analyzed with regard to the effects of Reynolds number and suspended solids concentration, and the calculated resistance coefficients were compared with those estimated from available design procedures recommended for turbulent single-phase Newtonian flow. Within the range of particle sizes examined in this work no particle size effects could be discerned. The effects of Reynolds number and suspended solids concentration on the friction loss measurement were calculated.

Capillary Flow of Power-Law Non-Newtonian Liquids in Circular Tubes

The penetration rate of power-law non-Newtonian liquids in a circular tube driven by the capillary pressure is derived. The dimensionless fractional height of liquid in the tube relative to the equilibrium height, χ = h(t)/h(∞), is calculated as a function of an appropriately chosen dimensionless time, permitting expression of the height-time dependence as a function of the reciprocal of the exponent in the power-law model, s = 1/n. It is found that the rate of penetration and, therefore, also withdrawal of liquid is dramatically retarded as the liquid becomes more strongly shear-thinning, i.e., as n becomes small relative to unity.