Coal-water Suspensions Research Articles

96/04814 Rheological and thermophysical properties of water-coal suspensions

96/04814 Rheological and thermophysical properties of water-coal suspensions

The effect of adsorbed and non-adsorbed additives on the stability of coal-water suspensions

The effect of adsorbed and non-adsorbed additives on the stability of coal-water suspensions

The effect of adsorbed and non-adsorbed additives on the stability of coal-water suspensions

Modified coal-water suspensions were stabilized using two additives: an adsorbed non-ionic surfactant, dinonylphenol poly(oxyethylene) with 150 mol of ethylene oxide per dinonylphenol group (DNP 150), and a non-adsorbed water-soluble polymer, poly(oxyethylene) (POE). Settling rate, sediment concentration profile and compressional resistance and other properties were measured to assess these stabilized suspensions and to discriminate between the effects that these additives have on the strength of the flocculated networks formed in the coal suspension and their sediments. Improvement of the flocculated structure of the suspension resulted in sediments that had increased resistance to consolidation and were relatively non-settling. Yield stress measurements on the sediment allowed the strength of the flocculated network structure to be quantified and the mechanisms leading to particle interactions to be explored and related to the adsorption density of the additives on the coal surface. Full coverage of surfactant, together with soluble polymer, gave a suspension that resisted settling for 2 weeks. After this time the suspension exhibited minimal settling without large increases in suspension viscosity. There was a significant difference in ease of redispersion between suspensions flocculated with DNP 150, where ‘bridging’ network structures were formed, and those flocculated with ‘free’ POE.

Surface-charge density and potential of coal-liquid mixtures and control of their stability and fluidity

The preparation conditions of concentrated (>70%) aqueous suspensions of coal were optimized, as were the surface charge density and the surface potential of the coal-water suspensions. The theoretical study showed that (1–3) or (2–3) electrolytes such as Na2SO4 or Na5P3O10, greatly increase the surface charge density and the surface potential of the coal particles in water. This study has led to the interesting result that thinners and stabilizing agents of concentrated clay suspensions, such as lignosulphonates, tripolyphosphates, polynaphtalene sulphonates, etc., must also be good thinners and stabilizing agents for ground coal-water suspensions. Two parameters were controlled: the stability and the fluidity of the suspensions. A penetrometry technique revealed the importance of the resistance coefficient,C r that can be used as a criterion for the evaluation of coal-water mixture stability and fluidity. Concentrated coal suspensions (about 75% of solid by weight) were prepared in water by using different additives, for instance sodium tripolyphosphate (NaTPP) and surfactants like lignosulphonates (by coupling of the dispersion and stabilizing effects) with the optimum concentration ratio of these two products equal to 1.9. The stability of the suspensions was then greater than 2 months and the viscosity was 1900 cP.

95/04417 Concentrated coal-water suspensions containing nonionic surfactants and polyelectrolytes. Asdorption of nonyl phenyl propylene oxide-ethylene

95/04417 Concentrated coal-water suspensions containing nonionic surfactants and polyelectrolytes. Asdorption of nonyl phenyl propylene oxide-ethylene

Concentrated coal-water suspensions containing nonionic surfactants and polyelectrolytes 2. Adsorption of nonyl phenyl propylene oxide-ethylene oxide on coal and the rheology of the resulting suspension

Adsorption isotherms of a series of nonionic surfactants (Synperonic NPE) on coal showed a systematic decrease in the amount of adsorbed surfactant (Γ in μmol g −1) with increase in polyethylene oxide (PEO) chain length. A plot of Γ vs. n 1 2 (where n is the number of EO units) was linear indicating that the adsorbed amount is determined by the size of the PEO chain. Viscoelastic measurements on coal dispersions stabilized by these nonionic surfactants showed a rapid increase in the complex and storage moduli ( G ∗ and G′ respectively) at a critical volume fraction of coal, Φ cr. The latter was highest for the surfactant containing 48 EO units. The lower Φ cr obtained with the surfactant containing the shorter PEO chain (27 units) was due to the weak flocculation obtained with these dispersions. At longer PEO chains (79 and 174) desorption of the chains may occur and this results in greater flocculation. It was concluded that an optimum PEO chain length was required for maximum stability.

Theoretical and experimental study of the surface charge density and the surface potential of coal—water suspensions in dissymmetrical electrolytes

To estimate the physicochemical interactions between dispersed particles and the stability of a suspension it is necessary to know the surface charge and surface potential of the particles. In practice, the surface potential can reach very high values (±100 to ± 400 mV). In that case, the solution of the Debye—Hückel equation becomes inaccurate. The same holds if the particle radius a becomes very small (500-10 Å) and if κ a < 100 where κ is the Debye—Hückel reciprocal length. In the latter case, the Poisson—Boltzmann equation (implying parity between a spherical particle and a plane surface) becomes unsuitable. A more suitable and precise solution of the non-linear Poisson—Boltzmann equation for charged spherical particles in the presence of dissymmetrical electrolytes was worked out. The relationship between the electrostatic potential Ψ( x) and the surface charge σ 0 was established. It was found that electrolytes with trivalent anions (1–3 and 2–3 electrolytes) give higher surface densities. These theoretical results were confirmed by studying the surface charge and potential of coal-in-water suspensions in the presence of various dissymmetrical electrolytes such as Na 2SO 4, Na 5P 3O 10, CaCl 2, Na 3PO 4 and NACl. Using our theoretical model and experimental results we were also able to obtain the changes in electrostatic potential as a function of the distance x (from the particle surface) in various electrolytes and for different pH values. Our experimental results were in good agreement with the theoretical calculations. It was demonstrated (in order of decreasing surface charge density) that σ 0(NaTPP) > σ 0(Na 3PO 4) > σ 0(Na 2SO 4) > σ 0(NaCl) > σ 0(CaCl 2) or σ 0(1–5) > σ 0(1–3) > σ 0(1–2) > σ 0(1-1) > σ 0(2-1)

Theoretical and practical study of a stability test: application to highly concentrated coal-water suspensions

A single test allowing the stability determination of highly concentrated suspensions was elaborated by applying the technique of penetrometry. It consists of dropping a rod with known dimensions into the suspension with no initial speed. A mathematical model was developed and its validity tested using 75% coal slurries in water. The penetration of the rod is described by the equation: m d 2 x d t 2 + C r s p 2 ( d x d t ) 2 + s p g x = m g Approximated solutions of the rod equation were found and precise solutions proposed in particular cases. This technique reveals not only the stability of the suspension but also the different features (homogeneity, viscosity, presence of a sediment, etc.) of the dispersions. A good agreement was observed between the viscosity obtained by our technique and the experimental viscosity measured by a viscosimeter (Rotovisco).

Mechanism of blockage during pumping of concentrated water-coal suspensions

Mechanism of blockage during pumping of concentrated water-coal suspensions

Concentrated coal—water suspensions containing non-ionic surfactants and polyelectrolytes 1. A preliminary study using rheology and adsorption isotherms

The influence of addition of dispersing agents on the properties of coal—water suspensions has been investigated using adsorption isotherms, and rheological (steady state and oscillatory techniques) measurements. Four different dispersants were investigated - two non-ionic ethoxylated surfactants and two anionic polyelectrolytes (lignosulphonate and naphthalene formaldehyde sulphonated condensates). The non-ionic dispersants showed an initial increase in the complex modulus and viscosity, with further decrease on ??? the dispersant concentration. The initial increase was attributed to adsorption of the non-ionic dispersant with the polyethylene oxide (PEO) chain partly directed towards the surface (probably by hydrogen bonding with polar groups such as CO and -COOH) leaving the hydrocarbon chain pointing towards the bulk solution. This makes the coal surface initially more hydrophobic resulting in its enhanced flocculation. On further addition of the non-ionic dispersant, adsorption occurs by hydrophobic bonding between the alkyl group and the hydrophobic portions of the coal surface. These result in a hydrophilic surface and the PEO chains provide steric stabilisation. With the anionic polyelectrolytes, adsorption occurs via the hydrophobic groups present on the chains leaving the charged chains dangling from the surface. This results in a combination of electrostatic and steric stabilisation. Adsorption isotherms showed good correlation with the rheological results, namely a rapid reduction in absolute shear modulus and viscosity at the concentration at which the steep rise in adsorption occurs. A comparison between the various dispersants was obtained from the complex modulus, | G*|, and viscosity, η pl volume fraction curves. The non-ionic dispersants showed a steep rise in | G*| and η pl at a critical volume fraction of coal that was lower than that obtained with the anionic polyelectrolyte. A more quantitative comparison was obtained from the maximum packing fraction φ p which could be calculated from the plot of | G*| −0.5 and η −0.5 pl vs φ c. The maximum packing fraction was higher with the anionic polyelectrolytes when compared with the non-ionic dispersants. However,

The effect of particle size distribution on the antithixotropic and shear thickening properties of coal–water dispersions

The shear thickening rheology of coal–water suspensions prepared from six different naturally generated particle size distributions with mass median particle diameter d50 from 0.8 to 28 μm were investigated. Refined coal samples were thoroughly deflocculated with an anionic dispersant in water at particle concentrations from 47.7 to 56.4 vol %. The variations in flow behavior of the dispersions were observed in Couette flow as a function of shear rate in ramp tests and of time as a constant rate of shear was applied. Ramp test results indicated a decrease in the severity of shear thickening with increasing particle size distribution. Discontinuous shear thickening that occurred with the smaller particle sized samples was found to be approximated by a single function of the particle concentration reduced by the maximum packing fraction. The experimental results from constant shear rate experiments were compared in terms of the rate of antithixotropy, the extent of stress increase, and the total energy dissipated prior to shear thickening, as determined from characteristic points in the stress‐time rheograms. Significant differences in the shape of the antithixotropic curves were observed for the different particle size distributions. The magnitude of stress increase observed during the antithixotropic event was found to diminish with increasing particle size distributions which limited the detection of the antithixotropic event at the larger sizes. The extent of stress increase for the smaller particle sized samples, i.e., d50 < 3 μm, was such that the antithixotropic shear thickening appeared to dominate the flow behavior. A bimodal mixture of particles at d50 = 0.77 and 7.9 μm was also examined. An 8% increase in volumetric particle concentration was observed when compared to a monomodal suspension with equivalent d50 and average viscosity. No antithixotropic tendencies were observed with the bimodal suspension.

Interations between particles in suspension : application to coal-water suspensions

We showed, using theoretical calculations that the interaction energy VT between coal particles to get stable coal-water suspensions requires an ionic strength less than 5. 10-3 M, a surface potential higher than 50 mV and a particles size around 1 µm. The experimentation confirms the theoretical previsions and also shows that the surface charge of a coal such as that of Freyming is negative if pH > 2,9. Moreover, the use of lignosulfonates coupled with sodium tripolyphosphate gives fluid and very stable coal-water slurries.

Préparation des mélanges charbon-eau: Effet de la nature du charbon et amélioration de la qualité des suspensions par action sur les propriétés des particules

Since 1983, CERCHAR has been studying coal-water mixtures (CWM) with the objective of valorizing some waste and fine products from washing plants. It appeared through this study that the nature of coal is a decisive parameter in the formulation and preparation of coal-water suspensions. Indeed, in identical operatory conditions, the high-rank coals (bituminous and anthracite) give more concentrated suspensions than those realized from low-rank coals (lignite and high volatile bituminous). This difference in behaviour can be explained by properties related to rank such as ratio of oxygen/carbon (O/C), capacity of water absorption, state of surface and shape of particles. Diluted milling was the preparation mode used (50 to 55 wt.% of solids), followed by filtration and mixing with additives. With this procedure, the desired rheological properties were not obtained when using low-rank coals. Another procedure was devised in which the coal is wet milled in the presence of additives and at high coal concentration (about 70 wt.% of solids). The interest of this procedure is to simplify drastically the process flow sheet, but the control of the wet milling at a high concentration is much more difficult. CERCHAR succeeded in developing a two-stage, high-slurry concentration, wet-milling process allowing the preparation of better quality, high-concentration suspensions than those obtained from the classical procedure. This innovative process was patented. The quality improvement can be explained by the effect of this procedure on particle size distribution, state of surface and shape of particles.

Rheology of brown coal-water suspensions

Techniques for measuring the fundamental flow properties of as-mined Victorian brown coal suspensions are developed. Flow properties are presented for Morwell, Yallourn and Loy Yang coals as a function of concentration for fixed particle size distribution. Even at the relatively low solids concentration of 20 to 30 percent by weight, the suspensions exhibit complex non-Newtonian characteristics. Generally at high concentration, the suspensions are thixotropic with a shear rate dependent viscosity and exhibit a yield stress. The inherent thixotropy of the coals can be exploited and it is technically but perhaps not economically feasible to transport the as-mined coals in a pipeline. The power requirement to pump the coals in a pipeline is found to be 10 to 30 times that required to pump black coal in the Black Mesa pipeline in the U.S.A. The differences in the rheological properties of the three coals is related to the surface properties of the coal particle — expressed in terms of carboxylate and inorganic cation content, and to differences in the pore volume of the coals. The flow characteristics of one coal can be converted to that of another simply by varying either the carboxylate or ionic content of the suspension.

Theoretical and practical study of a stability test: application to highly concentrated coal-water suspensions

Theoretical and practical study of a stability test: application to highly concentrated coal-water suspensions