Energy & Fuels 1988,2, 157-163 burgh seam coal weathered for 45 days at 80 OC and for 268 days at 50 "C contained only 0.10 and 0.19 wt % of maf sulfatic sulfur, respectively. This indicates that the loss in flotation recovery a t the higher temperatures is strongly affected by organic matrix oxidation. The different behavior with temperature of pyrite and organic matrix oxidation underscores the importance of
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using realistic temperatures to study natural. weathering. Acknowledgment. We gratefully acknowledge the contributions of €3. B. Oblad and D. E. Lowenhaupt of Consolidation Coal Co. and D. H. Ralston of Conoco, Inc. This work was supported by Consolidation Coal Co. Registry No. Pyrite, 1309-36-0.
Correlation between Electrophoretic Mass Transport and Bulk Properties of Concentrated Coal Suspensionst E. Z. Casassa and E. W. Toor* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received July 13, 1987. Revised Manuscript Received December 28, 1987
The purpose of this study of six bituminous coals, some of which were available in both ROM and beneficiated forms, was to relate the surface chemistry of a coal to its beneficiation and oxidation history and to test the hypothesis that coal-water slurry properties depend upon slurry t potential. The particle size distribution, degree of oxidation, and density were determined for freshly ground samples of each of the dry coal powders. Slurry rheology and sedimentation rate and settled volume were examined as functions of pH and of coal concentration. The pH dependences of the electrophoretic mobility of both dilute suspensions and 50 w t % slurries of each coal were observed. Proximate, ultimate, and mineral ash analyses, surface analyses by XPS, and the concentrations of nine inorganic ions in slurry liquor were also obtained for each coal. Slurry rheology and stability toward sedimentation were in general related to the electrophoretic mobility measured in concentrated suspensions by the mass transport method, rather than to that determined in dilute suspensions by microelectrophoresis. Electrokinetic sonic amplitudes measured in concentrated slurries agreed with mass transport results. High soluble ash content of a coal powder decreased slurry electrophoretic mobility across the pH range, whereas oxidation of the carbonaceous surface produced slurry particles with high negative electrophoretic mobility at pH above 6. Low slurry electrophoretic mobility correlated with good stability toward sedimentation and high viscosity, while high electrophoretic mobility produced low viscosities and very poor stability toward sedimentation. These effects on slurry properties were more marked for powders of low median particle size.
Introduction The behavior of dilute dispersions of charged colloidal particles can be interpreted according to the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory of colloidal Stability.' Particles experience a van der Waals attraction toward each other that can be expressed as a negative potential energy diminishing with distance between the particles. Electrostatic charge on particle surfaces produces a positive repulsive potential energy that also decays to zero as interparticle separation increases. The stability of a dispersion depends upon the relative magnitudes of these opposing potential energies. High interparticle repulsion, arising from large electrostatic charges on particle surfaces, leads to stable dispersion of individual colloidal particles and to low viscosity. When the surface charge is lower, the balance between the repulsive potential and the attractive potential may allow either flocculation in a secondary minimum to form loosely bound aggregates or coagulation in a primary minimum to form tightly Presented at the Symposium on the Surface Chemistry of Coals, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987.
0887-0624/88/2502-Ol57$01.50/0
bound aggregates. Aggregation increases viscosity and decreases stability toward sedimentation. Increasing the ionic strength of the fluid compresses the electric double layer around the particles and changes the balance between attractive and repulsive potentials. The DLVO theory is strictly applicable only to dilute colloidal dispersions, but can be applied qualitatively to concentrated slurries, where many-body interactions occur. A second complication is that typical colloidal behavior is limited to dispersions for which the particle size and density relative to the fluid permit Brownian motion, which prevents settling of the unflocculated individual particles in the dilute dispersion. For coal-water suspensions this limit is on the order of 1-2 vm, considerably lower than the median particle size of most coal-water slurries. Thus high interparticle repulsion does not promote stability toward sedimentation in coal-water suspensions although it favors dispersion of the powder as single particles and promotes the most efficient packing, in which interparticle distances are maximized. (1) Verwey, E. J. W.; Overbeek, J. T. G . Theory of the Stability of Lyophobic Colloids, Elsevier: New York, 1948.
0 1988 American Chemical Society
158 Energy & Fuels, Vol. 2, No. 2, 1988
In suspensions it is possible to measure quantities, like the electrophoretic mobility, which are proportional to the { potential at the shear plane around the particle at a small distance from the particle surface. The { potential depends upon the potential at the particle surface as it is modified by counterions and other materials present within the immobile fluid layer. High slurry 1potential reflects strong interparticle repulsion leading to low viscosity, high maximum solids content, and poor stability toward sedimentation. Sediments are hard-packed and stratified by size because of size-dependent settling. Mass subsidence, in which all particles fall at the same rate, may not occur at all. On the other hand, low values of the { potential indicate low interparticle repulsion favoring flocculation, which increases the effective particle size in the suspension but decreases the effective particle density and the effective void space. Smaller effective void space increases viscosity, and shear-thinning may also occur when flocs are broken apart under shear. For small flocs, larger effective particle size may increase both sedimentation rate and the volume of the settled layer. With more extensive flocculation, lower density and effective void space become dominant, increasing stability toward sedimentation. Thus maximum viscosity and good sedimentation stability should occur at very low { potential in concentrated dispersions of powders larger than colloidal size. That flocculation is necessary for sedimentation stability of such dispersions of coal in oil was pointed out by Rowell et d2 Since coal powders are heterogeneous, nonspherical, and polydisperse, coal is not an ideal material for the study of concentrated dispersion behavior. However from a practical viewpoint it is of interest to determine whether coal-water slurry rheology and sedimentation stability can be predicted from the electrokinetic properties of coal dispersions. Both electrophoretic mobility, the velocity of a suspended particle in an applied electric field, and streaming potential, the potential arising from the motion of fluid past an immobilized solid surface, can be related to the { potential. Microelectrophoresis, the microscopic observation of electrophoresis in very dilute suspensions, and streaming potential measurements have been used in a number of studies of coal powders, usually in reference to beneficiation processes. Sun and Campbell used streaming potential measurements to determine the dependence of { potential on pH for anthracite c ~ a l sand ~ * a~bituminous cod5 finding isoelectric points between pH 2.5 and 4.5 for anthracites and at pH 4.6 for the bituminous coal. They concluded that hydronium and hydroxide ions were potential determining for both the bituminous and anthracite coals. Jessop and Strettod also used the streaming potential method to observe the electrokinetic properties of a group of British coals, finding no apparent correlation with coal rank. Prasad' observed the electrokinetic properties of four coals by the microelectrophoretic technique, finding for an anthracite and one bituminous coal { potential versus pH curves similar to a number of those obtained by the streaming potential method. Two other bituminous coals with larger iron contents showed some(2) Rowell, R. L.; Vasconcellos, S. R.; Sala,R. J.; Farinato, R. S. Ind. Eng. Chem. Process Des. Deu. 1981,20, 283-288. (3) Sun, S. C.; Campbell, J. A. L. Coal Science; Given, P., Ed.; Advances in Chemistry 55; American Chemical Society: Washington, - -DC,1 s 6 ; pp 363-375. (4) Campbell, J. A. L.; Sun, S. C. Trans. SOC.Min. Eng. AIME, 1970, 247, 120-122. (5) Campbell, J. A. L.; Sun, S. C. Trans. SOC.Min. Ena. - AIME, 1970. 247, i i i - i i 4 . (6) Jessop, R. R.; Stretton, J. L. Fuel 1969,48, 317-320. (7) Prasad, N. J . Inst. Fuel 1974, 9, 174-177.
Casassa and Toor what different behavior and had higher isoelectric points at pH 5.2 and 6.3. Wen and Sune also used microelectrophoresis to study the effects of coal rank, of oxidation, and of soluble oxidation products. They observed that the { potential of the coal becomes more negative with decreasing rank and increasing pH, and that the isoelectric point occurs at lower pH for lower rank coals. Oxidation also produces a more negative {potential, particularly below pH 7. They concluded that in both cases the { potential and the isoelectric point depended upon the oxygen-containing functional groups on the surface of the carbonaceous portion of the coal and that the range of isoelectric points found for coals of the same rank might be due to differences in surface oxidation. Other authors>1° employing microelectrophoresis, have also found that oxidation lowers the isoelectric point of coals. Coca et al." report that the {potential of coal particles, measured by microelectrophoresis, was constant for coal concentrations between 0.001 and 0.1 wt % but became less negative for solids concentrations from 0.1 to 1wt %, a concentration range where specific conductivity increased markedly. A more detailed discussion of the electrokinetics of coal-water suspensions can be found in a review by Laskowski and Parfitt.12 Inorganic minerals dissolved from the coal surface may also affect { potential. Wen and Sun8 demonstrated that lo4 M Fe2+or Fe3+or M A13+ led to two charge reversals in the { potential versus pH curve of an oxidized bituminous vitrain. The charge reversal from negative to positive, between about pH 3 and 5, was attributed to precipitation of the metal hydroxide on the coal surface, while the charge reversal from positive to negative at pH 7-8 occurs at the isoelectric point of the metal hydroxide. Ca2+and Mg2+at a concentration of lo4 mol dm-3caused the { potential of the vitrain to become less negative above pH 5, while M Na+ had no effect. Additionally the concentration of iron dissolved from the coal surface was shown to increase 40-fold after oxidation at 125 "C for 48 h, while calcium, magnesium and aluminum concentrations increased only slightly. Studies of the relationship between slurry properties and {potential include the observation of Baker and Miller13 that increasing the { potential of the coal particles decreased flocculation in slurries of coal fines. The work of Funk14demonstrated that high { potential, as measured by microelectrophoresis, led to good dispersion, efficient packing, and minimum viscosity, particularly when the particle size distribution was very broad with at least 5 wt % of the solids in the colloidal size range. Parftt et d.15studied four bituminous coals, finding that coal-water slurry rheology, stability toward sedimentation and maximum solids content were more closely related to the electrophoretic mobility of concentrated coal suspensions, measured by the mass transport method, than to (8) Wen, W. W.; Sun,S. C. Trans. Soc. Min. Eng. AIME 1977,262, 174-180. (9) Kelebek, S.; Salman, T.; Smith, G. W. Can. Metall. Q. 1982,21, 205-209. (10) Mori, S.; Hara, T.; Aso, K.; Okamoto, H. Powder Technol. 1984, 40, 161-165. (11) Coca, J.; Bueno, J. L.; Sastre, H. J. Chen. Technol. Biotechnol. 1982,2, 637-642. (12) Laskowski, J. S.; Parfitt, G. D., Interfacial Phenomena in Coal Technology; Botaaris, G. D., Glazman, Y., Eds.; Marcel Dekker: New York, 1987. (13) Baker, A. F.; Miller, K. J. Min. Congr. J. 1968, 54, 43-44. (14) Funk, J. E. United States Patent 4282006, 1981. (15)Parfitt, G. D.; et al. 'A Program of Basic Research on the Utilization of Coal-Water Mixture Fuels"; Final Report to the U S . Department of Energy, DOE/PC/40285; Pittsburgh Energy Technology Center: Pittsburgh, PA, 1984; pp 1-213.
Concentrated Coal Suspensions electrophoretic mobility measurements made on very dilute Suspensions with a zeta meter. In the mass transport method, developed by Sennett and Olivier,lBan average electrophoretic mobility is determined for a concentrated suspension from the change in weight of the contents of a measuring cell placed a t one electrode in a larger reservoir. Particles with an electrophoretic mobility u, (cm2 s-l V-l) move into the measuring cell in time t (s) under an applied potential displacing an equal volume of fluid. Then
where Aw (g) is the weight change of the measuring cell, pp and pf are the densities (g cmS) of particles and fluid respectively, 4 is the volume fraction of particles in the suspension, I is the current (A), and X is the specific conductance (W cm-l). Thus both conductivity and the cell weight change in a set time at a set current are required to determine the electrophoretic mobility of a slurry of known concentration and particle density. James1' has criticized use of the mass transport method on several grounds, chief among them that spurious results due to sedimentation of flocculated colloidal suspensions occurred near isoelectric points. As pointed out above, flocculation improves stability toward sedimentation for suspensions with median particle size greater than colloidal dimensions, so that interference from sedimentation can be expected only at high { potentials for the coal-water suspensions studied here. The other complications discussed, Joule heating, gassing at electrodes, and contamination through electrode dissolution, can be minimized or avoided by choosing optimum experimental conditions. There are a number of reasons that electrophoretic mobility in very dilute suspensions might differ from that in concentrated slurries. First, if ionic materials dissolve from the coal surface, the ionic strength in the concentrated suspension will be greater than that in the very dilute suspension, even though the compositions of the particle surfaces are identical in both cases. Second, a sparingly soluble material may be completely dissolved from the surface in the dilute case, but only partially dissolved in the concentrated case, so that the surfaces become different. Third, the extent of redeposition of dissolved materials by precipitation or adsorption accompanying changes in pH is likely to vary in the two cases. Fourth, particle-particle interactions may alter the { potential at high solids concentrations. The experimental conditions used may also account for differences between mobilities measured by the two methods. Particles of all sizes contribute to the mass transport average mobility, but in microelectrophoresis, the smaller particles may be observed preferentially if large particles settle rapidly from the dilute suspension. If the distribution of materials on the heterogeneous surface varies with particle size, the mobilities observed by the two methods could well be different. Secondly, while ionic strength can be kept constant between about pH 3 and 11 in microelectrophoresisby adding a mol dms solution of indifferent electrolyte, a much higher concentration would be required to keep ionic strength constant over the same pH range in the mass transport method. For comparison to bulk properties of slurries containing no added indifferent electrolyte, mass transport mobilities should also be measured without added electrolyte. Thus conditions are not comparable for the two methods. Mass (16) Sennett, P.; Olivier, J. P. Ind. Eng. Chem. 1965,57, 32-50. (17) James, S. D. J. Colloid Interface Sci. 1978, 63, 577-582.
Energy & Fuels, Vol. 2, No. 2, 1988 159 transport data can only be considered as a qualitative measure of the electrokinetic behavior of concentrated slurries. However, for the reasons indicated above, mass transport electrophoretic mobilities may be a better guide to the properties of concentrated slurries than mobilities determined by microelectrophoresis as was found by Parfitt et al.16for bituminous coals. Indeed, Sennett and Olivierle demonstrated that for a 47.5 vol % kaolinite suspension mass transport relative electrophoretic mobilities could be correlated to slurry viscosity. The work reported here was a study of the effects of beneficiation and oxidation upon coal slurry electrophoretic mobility, rheology and sedimentation stability, employing both microelectrophoresis and the mass transport method. For a few cases { potential versus pH in concentrated suspensions was investigated with a recently developed instrument based on the inverse Debye effect.ls The relative motions of a charged particle and its accompanying ion cloud in an alternating electric field generate an ultrasonic wave, the amplitude of which depends upon the particle {potential. Since the suspension can be stirred during the measurement of electrokinetic sonic amplitude (ESA), settling of particles can be prevented, and the method can be used over a wide concentration range.
Experimental Section The six bituminous coals chosen included Splash Dam, Lower Kittanning, and Pittsburgh Seam No. 8, for all of which both run-of-mine (ROM) and physically beneficiated samples were available, and Upper Freeport, Black Creek, and Illinois No. 6. The coals were obtained as pieces with a diameter of l / * in. or larger and portions were pulverized to 100% < 100 mesh with a hammer mill immediately before use, with the exceptions of the Illinois No. 6 coal, which had been ground approximately 18 months earlier and constituted an example of a coal aged under air, and the 6.3% ash Splash Dam coal, which was all ground at one time. For the latter coal, portions were used for various experiments from 1 week to 3 months after grinding. Deionized water was used to prepare all suspensions, which were made by stirring weighed quantities of coal and water in a Waring laboratory blender at high speed for 4 min. Slurries were allowed to cool to room temperature after blending and then remixed for 10 s immediately before measurements were made. Sufficient HCl or NaOH to achieve the desired pH was added to the water before weighing. To obtain a solution of slurry supernatant, 20 wt % slurries were filtered under suction. The filter cake could be further rinsed and then air-dried to obtain a washed coal sample with a lower surface concentration of water-soluble minerals. For each coal the filtrate of a 10 wt % slurry, made without added acid or base, was analyzed by a commercial laboratory,to determine the concentrationsof sodium, potassium, calcium, magnesium, iron, aluminum, silicon, chloride, and sulfate by atomic absorption spectrometry or turbidimetry. The particle size distribution of each coal was determined by dry sieving with a Model L3P sonic sifter, using sieves with openings ranging between 7 and 150 pm. For some samples, the particle size distribution was also measured with a Coulter electronic particle counter, Model TA 11, using aperture tubes providing a size range from 1to 160 pm. For a nonspherical coal particle, the electronic particle counter measures its envelope volume and counts the particle as one with the diameter of a sphere of equivalent volume. Provided particle density does not vary with particle size, the size distribution obtained from sieving should be almost equivalent, and indeed agreement between particle size distributions obtained by the two methods was good. Median size (the diameter of a sphere of equal volume) and the standard geometric deviation, a measure of distribution width, were determined from log normal plots of the cumulative distributions. Proximate, ultimate, and mineral ash analyses were performed for each coal by a commercial laboratory. For most of the coals, (18) Debye, P. J. Chem. Phys. 1933, I , 13-16.
160 Energy & Fuels, Vol. 2, No. 2, 1988 surface analyses by X-ray photoelectron spectroscopy (XPS) were carried out by Gulf Research and Development Corp., and for Illinois No. 6 surface analysis was also obtained for a sample that had been made into a 50 wt % slurry and then filtered and dried under air. The density of each coal powder in water was determined by pycnometry. Degree of oxidation was determined for each coal powder by the US. Steel alkali extraction test.l8 A transmission of 100% at 520 nm for the sodium hydroxide solution in which a gram of coal powder has been heated indicates that the carbonaceous portion of the surface has not undergone sufficient oxidation to produce any alkali-solubleoxidation prcducta such as humic acids. In addition, 25 wt % suspensions of each coal were titrated with HC1 and NaOH solutions, pH measurements being made in the slurry rather than in supernatant fluid. The microelectrophoretic mobility versus pH for each coal sample was measured with a Pen-Kem Lazer Z Model 501 instrument in lo9 mol dmW3KNOB,a concentration of indifferent electrolyte sufficientto keep ionic strength approximatelyconstant between pH 3 and pH 11. Previously a zeta meter, with which individual particles are observed under a microscope, had been mol dm" KNO, used to compare electrophoretic mobility in lom3 of a coal with a median size of 41 pm as ground to that of fractions of the same coal sieved to be
