Energy & Fuels 2006, 20, 2037-2045
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Rheological Properties of Asphaltite-Water Slurries Cahit Hicyilmaz, Savas¸ O ¨ zu¨n, and N. Emre Altun* Middle East Technical UniVersity, Department of Mining Engineering, 06531 Ankara, Turkey ReceiVed February 14, 2006. ReVised Manuscript ReceiVed May 17, 2006
In this study, the rheological characteristics of asphaltite-water slurries (AWSs) were investigated with respect to some of the most important parameters in the preparation of slurries. The effects of pulp density, chemical addition, pulp pH, and particle size on the rheological behavior and viscosity of AWSs were studied. The role of demineralization of asphaltite was also investigated, and rheological properties of raw and relatively demineralized asphaltites were compared. Results showed that viscosity of the AWSs was negatively influenced by increases in the pulp density and as the mean particle size decreased from 104.81 to 15.39 µm. Increases in pH provided reduced viscosity values. The effects of dispersing and stabilizing agents were studied with a chemical mixture including 90% polystyrene sulfonate (PSS) as the dispersant and 10% Na-carboxylmethylcellulose (Na-CMC) as the stabilizer. The change in the viscosity was also investigated as a function of the dosage of the chemical mixture used. Minimum viscosity was achieved with a 1.1% chemical mixture addition, while excess dosages resulted in adverse effects and thickening of the slurry. Studies with raw and demineralized samples showed that mineral matter and hydrophobic aggregation of particles are critical factors, significantly affecting the rheological characteristics of AWSs.
1. Introduction The increasing demand toward petroleum and its increasing price has forced scientists to search for new sources of energy. Coal-water slurries (CWSs) were suggested as a substitute for oil, and many researches were conducted about the CWSs and their rheological properties. Using coal in the form of slurries also has some advantages for the evaluation and utilization of coal fines that could be stored without the risk of coal dust explosion and could be pumped through pipes and combusted like fuel oil.1-4 In addition, the use of CWS as a fuel in modified diesel engines was also investigated and found to be the most economic alternative.5-7 CWS preparation processes were developed and used commercially in large-scale plants in China, Russia, Japan, and Italy. Other countries such as Canada, France, U.K., and Germany also facilitated CWS technology.2 The studies about CWSs were focused on their rheological properties. Because CWSs have been tested successfully and operated in various boilers, it has been found that the stability of the CWSs varies from coal to coal and the stability is extremely low for some coals, limiting their utilization in industry. Ideally, CWSs should be relatively stable at static and dynamic conditions and exhibit quite good rheological behavior. Viscosity is one of the most important rheological properties, and it is required to be as low as possible with the highest allowable pulp density for pumping and atomization to the boiler * To whom correspondence should be addressed. Telephone: +90-312210-5811. Fax: +90-312-210-5822. E-mail:
[email protected]. (1) Aktas¸ , Z.; Woodburn, E. T. Fuel Process. Technol. 2000, 62, 1-15. (2) Li, Y.-X.; Li, B.-Q. Fuel 2000, 79, 235-241. (3) Ates¸ ok, G.; Boylu, F.; Sirkeci, A. A.; Dincer, H. Fuel 2002, 81, 1855-1858. (4) Turian, R. M.; Attal, J. F.; Sung, D.-J.; Wedgewood, L. E. Fuel 2002, 81, 2019-2033. (5) Hsu, B. D.; Leonard, G. L.; Johnson, R. N. Coal-Fuelled Diesel Engines; SME: New York, 1989; ICE-Vol. 7. (6) Caton, J. A.; Webb, H. A. Coal-Fuelled Diesel Engines; ASME: New York, 1993; ICE-Vol. 19. (7) Urban, C. M.; Mecredy, H. E.; Ryan, T. W.; Jett, B. T. ASME; New York, 1998; January, paper number 88-ICE-28.
for a better combustion. It also affects the economics of the process because the lower the viscosity, the higher the amount of solid particles that can be loaded to the slurry.8-10 A typical CWS consists of 60-75% coal, 25-40% water, and 1% chemical additives. Different parameters, such as particle-size distribution and morphology, type and amount of chemical additives, ζ potential and pH, mineral matter in coal, and maturity of coal, have very important roles in the viscosity of the CWSs. Particle-size distribution is one of the most important parameters for CWSs. Ferrini et al.,11 Boylu et al.,12 and Ates¸ ok et al.3 showed that particle size was reversely proportional with viscosity; i.e., as the particle size decreases, viscosity increases. However, because of the sedimentation and limitations about the particle size during the atomization and combustion of CWSs, it was indicated that particles should be less than 74 µm (mean particle diameter is around 20-30 µm).13-16 The particle size range was also investigated by Keller (8) Laskowski, J. S. Coal Prep. 1999, 21, 105-123. (9) Laskowski, J. S. Rheological Measurements in Mineral Related Research; IX Balkan Mineral Processing Congress: I˙ stanbul, Turkey, Beril Ofset, 2001; pp 41-57. (10) Dinc¸ er, H.; Boylu, F.; Sirkeci, A. A.; Ates¸ ok, G. Int. J. Miner. Process. 2003, 70, 41-51. (11) Ferrini, F.; Battara, V.; Donati, E.; Piccinini, C. Optimization of Particle Grading for High Concentration Coal Slurry; Proceedings of the 9th International Conference on Hydraulic Transport of Solids in Pipes, Rome, Italy, 1984; paper B2. (12) Boylu, F.; Dincer, H.; Ates¸ ok, G. Fuel Process. Technol. 2003, 85, 241-250. (13) Allen, J. W. Burner for Coal/Water Slurry Firing Modern Power System; 1984; pp 43-45. (14) Allen, J. W.; Rennie, A. G.; Wellborne, M. C. Atomisation of Coal Water Mixtures; International Chemical Engineering Symposium Series 95; 1985; pp 101-113. (15) Dubin, G. W.; Knell, E. W.; Rakitsky, W. G. CWF Production: Storage/Transportation Consideration and Experience; International Workshop on Coal-Liquid Fuels Technology; Halifax, Canada, October 1418, 1985; pp 156-180. (16) Kefa, C.; Guoquang, H.; Mingjiang, N. Pipeline ConVeyance and Fluidized Bed Combution of Water Mixture with High Viscosity; II. European Conference on Coal Liquid Mixtures. International Chemical Engineering Symposium Series; 1985; Vol. 95, pp 87-99.
10.1021/ef060063o CCC: $33.50 © 2006 American Chemical Society Published on Web 07/12/2006
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Table 1. Proximate and Ultimate Analyses of Avgamasya Asphaltite proximate analyses
weight (%, air dried)
ultimate analyses
weight (%, air dried)
moisture ash volatile matter fixed carbon gross calorific value (kcal kg-1)
0.37 45.74 31.11 22.78 4638
C H N O S
46.5 3.33 0.85 1.38 5.75
and Keller.17 It was found that, when the size range increases, viscosity values drop at low shear rates. ζ potential and pH play important roles in the preparation of CWSs because high values of ζ potential lead to high dispersion of particles and therefore to low viscosities.18 The effect of mineral matter and cleaning of it on the preparation of CWSs was also investigated. The viscosities of runoff-mine (ROM) slurries were significantly higher compared to the CWSs prepared with cleaned (washed) coals.1 Boger et al.19 proposed that, as the coal rank increases, substantial increases in the apparent viscosity of the CWS were observed. Al Taweel and Fadaly20 stated that bituminous coals with lower oxygen/carbon (O/C) ratios cause better fluidity compared to younger coals with higher O/C ratios. Leong and Boger21 and Skolnik and Scheffee22 stated that, to prepare CWSs with high solid content using low rank coals, the coal should have low porosity and be highly hydrophobic. Although there are many studies about CWSs, no research has been conducted on asphaltite-water slurries (AWSs). Turkey has large sources of asphaltite of around 82 million tons.23 The reserves have been well-known and mined since 1974 for domestic indoor heating. Asphaltite is petroleumoriginated, a hard and a blackish material with a relatively high softening point of about 200-315 °C. Asphaltite can be solubilized in carbon disulfide. Low oxygen content of asphaltites is one of the most important pieces of evidence to distinguish its origin from coal. The object of this study was to investigate the possibility of preparing AWSs with high pulp density and low viscosity characteristics. In this respect, the effects of pulp density, chemical additive amounts, pH, particle-size distribution of the feed material, and mineral matter (or ash content) of asphaltite on the viscosity of AWSs were studied. 2. Experimental Section 2.1. Asphaltite Sample. Asphaltite from the Avgamasya vein of S¸ ırnak (vein 3-4) was used throughout the study. The sample was characterized by proximate, ultimate, mineralogical, and X-ray diffraction (XRD) analyses. The XRD spectrum was obtained in the 5-75° range, using a powdered asphaltite sample. The results of the proximate and ultimate analyses are seen in Table 1. The XRD spectrum of raw Avgamasya asphaltite is given in Figure 1. (17) Keller, D. S.; Keller, D. V., Jr. J. Rheol. 1991, 35, 1583-1607. (18) Siffert, B.; Hamleh, T. Colloids Surf. 1989, 35, 27-40. (19) Boger, D. V.; Leong, Y. K.; Mainwarin, D. E.; Christie, G. B. Victorian Brown Coal-Water Suspensions as Liquid Fuels; III. European Conference on Coal Liquid Mixtures, International Chemical Engineering Symposium Series, number 107; Malmo¨, Sweden, October 14-15, 1985; pp 1-12. (20) Al Taweel, A. M.; Fadaly, O. Technical Report no. 0.07 (TR-0.07); Technical University of Nova Scotia: Halifax, Nova Scotia, Canada, 1985; pp 1-62. (21) Leong, Y. K.; Boger, D. V.; Christie, G. B.; Mainwarin, D. E. Rheol. Acta 1993, 32, 277-285. (22) Skolnik, E. G.; Scheffee, R. S. Suitability Of Coals For CW; Fifth International Workshop on Coal-Liquid Fuels Technology, Halifax, Nova Scotia, Canada, October 14-18, 1985; pp 265-276. (23) Go¨nenc¸ , O. Asphaltites and Asphaltites ReserVes in Turkey; MTA Report, MTA: Ankara, Turkey, 1990.
Table 1 shows the proximate and ultimate analyses results, indicating that the Avgamasya asphaltite sample contains a high amount of ash and sulfur and a low amount of moisture. The calorific value of Avgamasya asphaltite (Table 1) was retained using the oxygen bomb method with a Parr oxygen bomb calorimeter. Mineralogical analysis, carried out with a thin-section microscopic investigation, indicated that carbonate minerals such as calcite, dolomite, ankerite, and siderite within 5-10 µm to 1-2 mm were presented with trace amounts of mica, quartz, and clay minerals. Pyrite was observed in around 20-25 µm in agglomerated and dispersed form within the sample. Sphalerite and titanium minerals such as rutile and anatese were other constituents of the mineral matter in Avgamasya asphaltite. In the XRD spectrum of raw asphaltite, the peaks at 29.4°, 39.4°, and 43.2° corresponded to calcite and the peaks at 30.9°, 41.1°, and 44.9° showed dolomite. The peaks at 27.5°, 36° and 37°, 48.5° were due to titanium minerals, rutile and anatese, respectively. The peaks at 33° and 44.9° showed marcasite content, which is a sulfide mineral. Quartz was recognized with a low intensity peak at 26.5°. Also, the low intensity, successive peaks at 50.5° and 51.1° pointed feldspar content in asphaltite. The peaks at 56.2° and 61.6° showed pyrite. In view of the XRD spectrum, carbonate minerals, calcite and dolomite, were the main inorganic constituents in Avgamasya asphaltite. Sulfide minerals, pyrite and marcasite, also existed in significant amounts. The presence of titanium minerals, rutile and anatase, was one of the unique characteristics of Avgamasya asphaltite. The peak intensities of rutile and anatese revealed that these minerals occurred in distinguished amounts compared to lignites and most coals. Quartz and feldspar were of minor importance compared to other inorganic minerals. 2.2. Chemical Additives. Two chemicals, namely, Na-carboxylmethylcellulose (Na-CMC) and polystyrene sulfonate (PSS) were used in the study. PSS was used as the dispersing agent, while Na-CMC was used as a stabilizer during the preparation of AWSs. On the basis of the previous investigations, chemical agents were used with a proportion of 1 wt % of solid at the beginning of the study, consisting of 90% PSS and 10% Na-CMC.3 The chemical structures of PSS and Na-CMC are given in Figure 2. 2.3. General Methods. The viscosity measurements were performed using a RVDV-II +Pro Model Brookfield rotating viscometer with a SC4-27-type spindle. It should be noted that the viscosity of the AWS is subject to variation at different shear rates and with different spindle types. Thus, the measured viscosity values are “apparent viscosities”. The apparent viscosity of the slurries was measured through 960 s. The effect of the shear rate between 17 and 68 s-1 to viscosity was also investigated. The relationship between the viscosity, shear rate, and shear stress is as follows: γ ) SRC × n
(1)
τ ) VTC × SMC × SRC × T
(2)
η ) (100/n) × VTC × SRC × T
(3)
where SRC is the shear rate constant (0.34 for the used instrument), n is the rotational speed (in rpm), VTC is the viscosity torque constant (1.0 for the used instrument), SMC is the spindle multiplier constant (25 for the used instrument), T is the torque (in dyne cm-2), γ is the shear rate (in s-1), τ is the shear stress (in dyne cm-2 or centipoise s-1), and η is the viscosity (in centipoise or centidyne s cm-2). For the viscosity measurements, the asphaltite sample was ground down to 17.85 µm average particle size with a 19.3 Ø × 30.3 cm laboratory rod mill. The sample was also ground for different periods of 3, 5, 10, and 15 min to obtain asphaltite fines with different mean particle sizes to investigate the effect of the particle size. The particle-size distributions were determined with a Symtatec Laser Sizer (Helos H-1305). Junke and Kunkel RW 20-type mechanical mixer, having a 6.48 cm impeller, was used for the preparation of AWSs. The ground sample was mixed with distilled water and chemical additives (PSS plus Na-CMC) at a 800 rpm rotational speed for 15 min. At the end of mixing, the homogenized
Rheological Properties of AWS
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Figure 1. XRD spectrum of raw Avgamasya asphaltite (Q, quartz; R, rutile; C, calcite; D, dolomite; M, marcasite; A, anatase; F, feldspar; P, pyrite).
Figure 3. Change of shear stress versus shear rate as a function of the AWS pulp density.
Figure 2. Chemical structures of (a) Na-CMC and (b) PSS. Table 2. Average Particle Size and Specific Surface Area of Avgamasya Asphaltite average particle size (µm)
specific surface area (m2 g-1)
104.81 40.41 17.85 15.39
1.60 2.02 2.48 2.86
slurry was immediately placed into the viscometer cup and its viscosity was measured. The natural pH value of the slurry was around 6.5, and it was adjusted to the desired value either by HCl or NaOH addition to the pulp. The measurements were done at ambient temperature, which was around 22 ( 2 °C. The specific surface area of the sample was determined by the BET nitrogen adsorption method with a Micromeritics Flowsorb-2300 model instrument. Table 2 shows the specific surface area of the asphaltite sample ground to different sizes. ζ-potential measurements were carried out with a Rank Brothers microelectrophoresis instrument.
3. Results and Discussion The rheological behavior as a function of the concerned parameters was characterized from three viewpoints. First, the shear stress versus shear rate profiles were drawn to find out the Newtonian or non-Newtonian flow characteristics of AWS.
Figure 4. Change of viscosity with time as a function of the AWS pulp density.
Then, the rheological behavior was evaluated as a function of time. The viscosity profiles were drawn over a certain time period, and the rheological characteristics were determined from the view of thioxotropic or rheopectic behavior. Last, the viscosity value, achieved at the 480th second of measurement, was compared for different conditions. 3.1. Effect of Pulp Density. The effect of the pulp density was studied with 55, 60, 65, and 70% asphaltite by weight. The shear stress versus shear rate profiles of AWSs with different pulp densities are seen in Figure 3. Figures 4 and 5 show the change of the viscosity with time for AWSs of varying pulp
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Figure 5. Change of viscosity as a function of the AWS pulp density (at the 480th second).
densities and the change of viscosity as a function of the pulp density at the 480th second of measurement, respectively. The proportion of the change of shear stress with the variation of the shear rate is not constant at all pulp densities (Figure 3). Such a behavior indicates that AWS has non-Newtonian flow characteristics irrespective of the pulp density; i.e., the viscosity of AWS changes as a function of the shear rate. This behavior is attributed to the fact that AWS is a nonuniform slurry, constituted of solid particles with varying particle sizes, shapes, and morphologies. Thus, the alignment of the particles during the flow of AWS may vary as the particles pass by each other and require forces with different magnitudes to maintain, at least, a continuous flow. The alignment is determined by the shear rate applied, because the force required for moving the particles varies instantaneously with the surface, shape, and size characteristics of the particles. AWS displayed characteristic rheological behavior with time as the pulp density was varied. It was seen that the viscosity of AWS increased with time at 55% pulp density, showing ‘‘rheopectic” behavior. When the pulp density was increased from 55 to 60%, the viscosity of AWS decreased until the 630th second of measurement and showed a slight increase after this time. Thus, 60% pulp density would be treated as a turning point from ‘‘rheopexy” to ‘‘thixotropy”. When the pulp density was further increased to 65%, viscosity tended to decrease with time and AWS displayed ‘‘thixotropic” behavior. At 70% pulp density, ‘‘thixotropic” behavior became apparent (Figure 4). When the viscosity values at the 480th second of measurement were evaluated as a function of the pulp density, it was seen that the flow characteristics of AWS were negatively influenced by the increases in the pulp density and the viscosity of AWS significantly increased as the pulp density was raised from 55 to 70% (Figure 5). The increased resistance against flow was due to the increased contact and friction between particles at higher solid content. The relative decrease of the amount of water between the particles with an increase in the pulp density is another factor that increases the viscosity. It should also be noted that the lubricating effect of water flow becomes limited because the extent of the free space between the particles is comparatively lower at high solid content. In addition, high pulp density (70% in the current case) caused an insufficient mixing of the pulp, and an irregular, fluctuating viscosity profile with time was achieved because of this. It is reported that, for a favorable flow regime with solid fuel-water slurries, the viscosity value should be lower than 1000 centipoise. As seen in Figures 4 and 5, this limit is exceeded only with 70% pulp density. In this respect, it can be claimed that
Hicyilmaz et al.
Figure 6. Change of shear stress versus shear rate as a function of the AWS chemical content.
Figure 7. Change of viscosity with time as a function of the AWS chemical content.
favorable flow characteristics could be achieved up to 65% pulp density with AWS, and the succeeding parts of the study were carried out at this solids ratio. 3.2. Effect of Chemical Content. The chemicals used for enhancing the dispersion between the asphaltite particles and providing a better means of slurry stability were added with varying proportions from 0 to 1.5 wt %. Although the chemical addition amount was varied, the relative proportions of PSS and Na-CMC in different dosages were kept constant (0.9% PSS and 0.1% Na-CMC). The shear stress versus shear rate profiles for different chemical addition amounts are given in Figure 6. At all dosages, AWS had a non-Newtonian flow character. This behavior arose because of the nonuniform physical characteristics of the AWSs, as stated in details before. Also, at all dosages, AWS preserved its ‘‘thioxotropic” behavior; i.e., the viscosity of AWS tended to decrease with time (Figure 7). A fluctuating viscosity profile was obtained when chemicals were not used (0% chemical). Such behavior was also observed at a 0.5% chemical addition but to a lower extent. The fluctuating behavior points toward the lacking of stability in the absence of Na-CMC or insufficient Na-CMC concentration and indicates the importance of a chemical additive in maintaining the stability of AWS. The viscosity of AWS decreased with time as the chemical addition was increased from 0 to 1.1%, and the lowest viscosity values were obtained at a 1.1% chemical addition. When the chemical addition amount was further increased to 1.3 and 1.5%,
Rheological Properties of AWS
Figure 8. Change of viscosity as a function of the AWS chemical content (at the 480th second).
viscosities increased (Figures 7 and 8). Achieving a fossil-fuel slurry with optimum viscosity and stability is a delicate balance between dispersion and aggregation. In other words, a welldispersed system without stability exhibits irregular viscosity characteristics. An extensively aggregated system, on the other hand, would preserve a suspension form for prolonged periods; however, obtaining favorable flow behavior is an issue. In this respect, regulating dispersant and particularly stabilizer concentrations during the preparation of such slurries is probably one of the most critical aspects.9 The stability of the slurry is achieved by a limited extent of aggregation between particles, mostly under the bridging effect of the polymer. The increase in the viscosity at higher chemical concentrations than 1.1% is attributed to a higher stabilizer (Na-CMC) concentration. NaCMC is a strong polymer. The presence of an excess Na-CMC concentration would cause a bridging mechanism between asphaltite particles, leading to the formation of a network of aggregates or flocculates. This would have resulted in an increase in the viscosity of AWS. It was also reported in previous studies that the use of an optimum dosage of Na-CMC is one of the critical issues during the preparation of CWSs, and an excess dosage would easily result in thickening of the slurry (increase in viscosity) and/or rapid settlement of solid particles.10,24 Figure 8 also shows the effect of the chemical addition amount on the viscosity of AWS. The decrease in the viscosity up to a 1.1% chemical dosage is clearly seen. The reduction in the viscosity of AWS is due to a better means of dispersion between the asphaltite particles under the effect of PSS. Without the aid of a dispersing agent, the liability of the asphaltite particles toward coagulation is very high on account of their natural hydrophobic character. The measurement of the ζ potential reveals the change of the surface potential of asphaltite particles at different PSS dosages (Figure 9). Here, it should be noted that, although ζ potential measurements were started at neutral pH of the slurry, PSS addition sharply decreased the pulp pH to around 3.80 and this value was approximately preserved at all PSS dosages. Asphaltite particles had a negative surface potential at all PSS dosages and changed from -31.92 to -60.23 mV as the PSS dosage was increased from 0.40 to 2.00%. This showed that at higher PSS dosages the absolute magnitude of the surface potential increased. It is well-known that the greater the negative surface potential of coal particles, the higher the extent of their dispersion in the slurry.3,25 This condition also applies for AWS, because asphaltite particles are naturally hydrophobic and negatively charged in the pulp. The
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Figure 9. ζ potential of asphaltite particles as a function of the PSS dosage.
Figure 10. Change of shear stress versus shear rate as a function of AWS pH (with a 1.1% chemical mixture of 90% PSS plus 10% NaCMC).
observation of higher values of surface potential with PSS addition implies that the dispersing effect by PSS relied mostly on an electrostatic mechanism that makes asphaltite particles gain a highly negative charge. Thus, an adequate dispersion phase is achieved as the negative charge of asphaltite particles becomes higher and the particles repel each other. 3.3. Effect of Pulp pH. The effect of pH on the rheological properties of AWS was studied with a 1.1% chemical addition, because the lowest viscosity value was obtained at this dosage. The shear stress versus shear rate profiles for AWSs prepared at different pH conditions are seen in Figure 10. At all pH values, AWS had non-Newtonian flow behavior. When the viscosity versus time profiles of AWS were evaluated as a function of pH, it was seen that the ‘‘thioxotropic” character was preserved at acidic, neutral, and alkaline conditions (Figure 11). Lower viscosity values were attained throughout the entire period as the pH was increased. Lowest viscosity values were obtained as the slurry became alkaline and particularly at pH 11.65 (Figure 11). It is also seen in Figure 12 that alkaline pH has an apparently favoring effect on lowering the viscosity. The viscosity of AWS was 737.5 centipoise at pH 5.73, while it decreased to 527.5 centipoise when the pH (24) Usui, H.; Saeki, T.; Hayashi, K.; Tamura, T. Coal Prep. 1991, 18, 201. (25) New Energy and Industrial Technology Development Organisation (NEDO). CWM in Japan, 1997.
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Figure 11. Change of viscosity with time as a function of AWS pH (with a 1.1% chemical mixture of 90% PSS plus 10% Na-CMC).
Figure 14. Particle-size distribution and average particle sizes of asphaltite fines ground for different periods.
Figure 12. Change of viscosity as a function of AWS pH (at the 480th second).
Figure 15. Change of shear stress versus shear rate as a function of the AWS particle-size distribution.
Figure 13. ζ potential of asphaltite particles as a function of slurry pH (with a 1.1% chemical mixture of 90% PSS plus 10% Na-CMC).
was 11.65 at the 480th second of measurement. Lower viscosity values at alkaline pH show that the repelling forces between the particles become greater at alkaline conditions; i.e., a higher extent of dispersion is achieved because of the increased surface potential of asphaltite particles. The ζ potential measurements performed at different pH conditions also confirm this claim (Figure 13). The ζ potential was -18.45 mV at highly acidic pH (3.79), while it was recorded as -56.01 mV at highly alkaline pH (10.34), corresponding to an absolute increase in
the surface potential of asphaltite particles (Figure 13). The increase in the negative surface potential is a result of the increase in the amount of OH- ions in the pulp as the pH becomes alkaline. 3.4. Effect of Particle Size. The effect of the particle size on the rheological properties of AWS were investigated at 65% pulp density with a 1.1% chemical addition and at highly alkaline pH conditions (pH 11.65) on account of the lowest viscosity values obtained. For the investigation of the particle size effect, four different slurries were prepared using asphaltite fines ground for 3, 5, 10, and 15 min. The average particle size of asphaltite after 3, 5, 10, and 15 min of grinding were 104.81, 40.41, 17.85, and 15.39 µm, respectively. The log-log particlesize distributions for 3, 5, 10, and 15 min of grinding are given in Figure 14. Figure 15 shows the shear stress versus shear rate profiles for AWSs having different particle-size distributions with varying mean sizes. All AWSs exhibited non-Newtonian behavior with different extents. The coarsest particle-size distribution, obtained after 3 min of grinding (mean size ) 104.81 µm), showed almost no change in its viscosity value as the shear rate was varied and characterized with its strong nonNewtonian character. As the particle-size distribution and the mean particle sizes became lower, the extent of non-Newtonian flow character increased (Figure 15). The viscosity of AWS with the coarsest particle-size distribution remained almost the same until 500th second of measurement and exhibited ‘‘rheopectic” behavior after 500 s (Figure 16). The viscosity versus time
Rheological Properties of AWS
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Figure 16. Change of viscosity with time as a function of the AWS particle-size distribution.
Figure 17. Change of viscosity as a function of the AWS particlesize distribution (at the 480th second).
profiles tended to shift into a ‘‘thixotropic” character as finer particle-size distributions and lower mean sizes were obtained after prolonged grinding periods. It should also be noted that higher viscosity values over the entire range of measurement were recorded as the particle-size distribution became finer (Figure 16). The highest viscosity values were attained with AWS prepared with asphaltite fines obtained after 15 min of grinding. Figure 17 shows the negative effect of particle-size reduction on the viscosity of AWS. It was observed that the viscosity of AWS apparently increased from 270 to 825 centipoise as the particle-size distribution became finer and the mean particle size decreased from 104.81 to 15.39 µm. The increase in the viscosity of AWS with a decreasing particle size is in conformity with previous results obtained by Ates¸ ok et al.3, Ferini et al.,11 and Boylu et al.12 for CWSs. The increase in the viscosity with a decrease in the particle size is related to the volume fraction aspect. The volume fraction is defined as the ratio of the volume of a known amount of particles after vibrating in a graduated cylinder for a certain period of time to the initial bulk volume. The volume fraction is simply found according to the following equation:
VR ) VF/VI
(4)
where VR is the volumetric ratio, VF is the volume after vibrating in the graduated cylinder (in cm3), and VI is the bulk volume (in cm3).
Figure 18. Change of shear stress versus shear rate as a function of the chemical content of AWS obtained with demineralized asphaltite.
The finer the particle-size distribution, the lower the volumetric ratio becomes. This is because a more compact network is achieved as the ultrafine and fine particles are easily located within the voids between relatively coarse particles. The liability of the individual particles to agglomerate increases as a result of this compact structure and reduced distance between the particles, i.e., increased extent of interparticular attraction. Also, as the fine particles fill the voids, the amount of free space available for the penetration and free flow of water abruptly decreases. This reduces the lubricating effect of water during the bypass of particles along with each other and increases the interparticular friction, and thus, the viscosity of AWS. However, it is a necessity to grind asphaltite or coal to very fine sizes for the preparation of fuel-water slurries to prevent possible sedimentation during pumping and storage and to provide appropriate atomization and combustion of fuel-water slurries in the boilers. 3.5. Effect of Inorganic Matter Content. As revealed by the XRD analysis, asphaltite involves a variety of inorganic matter, such as carbonate, sulfide, silicate, and titanium minerals. These minerals constitute its ash content. To investigate whether the inorganic matter imposes effects on the rheological behavior of AWS, it was attempted to separate these ash-making constituents to the greatest possible extent and rheological measurements were performed with relatively clean samples of higher organic content. Using flotation, the ash content of asphaltite was decreased from around 45 to 35.40%. No reagents were used during flotation to not affect the original particle surface characteristics. The rheological behavior of demineralized samples was investigated as a function of the chemical (PSS plus Na-CMC) addition amount and pulp pH. The results with respect to the chemical addition amount are seen in Figures 18-20. As seen in Figure 18, the non-Newtonian character of the AWS was preserved after the removal of mineral matter. Viscosity versus time profiles in Figure 19 showed that viscosity gradually decreased when the chemical addition amount was increased up to 1.3%. After the lowest viscosity values were obtained at a 1.3% chemical addition, the viscosity of AWS increased when the dosage was 1.5%. The dosage of 1.3%, which provided the lowest viscosity values with demineralized asphaltite, was higher than the dosage (1.1%) that provided the lowest viscosity values with the raw asphaltite (Figure 7). This showed that the amount of chemicals required for a welldispersed and stable slurry phase increased after the removal of the inorganic matter. The favorable effect of a 1.3% chemical
2044 Energy & Fuels, Vol. 20, No. 5, 2006
Figure 19. Change of viscosity with time as a function of the chemical content of AWS obtained with demineralized asphaltite.
Figure 20. Change of viscosity as a function of the chemical content of AWS obtained with raw and demineralized asphaltite (at the 480th second).
addition amount is also seen in Figure 20, which shows the change of viscosity of AWSs with different chemical contents at the 480th second of measurement. Figure 19 shows that, regardless of the chemical addition amount, AWSs remain having a ‘‘thixotropic” behavior after demineralization; i.e., the viscosity of the slurry decreases with time. However, it is crucial to note that the viscosity of AWS prepared with demineralized samples is higher than the viscosity of AWS obtained with raw asphaltite at all chemical addition amounts (Figure 20). This observation claims that the viscosity of AWS apparently increases after the separation of the inorganic matter. The effects of pulp pH on the rheological characteristics of demineralized asphaltite are seen in Figures 21-23. As with raw asphaltite, the non-Newtonian behavior of AWS was preserved with demineralized asphaltite at all pH values (Figure 21). Also, the AWSs maintained their ‘‘thioxotropic” character with demineralized asphaltite regardless of the pulp pH (Figure 22). As the pulp pH increased from 6.42 to 10.14, the viscosity of AWSs prepared with demineralized asphaltite decreased (Figures 22 and 23). When the pulp was made more alkaline with a further increase of pH to 11.59, AWS had a higher viscosity and a fluctuating viscosity versus time profile because of gas evolution at high alkalinity. Figures 22 and 23 show that the viscosities of demineralized asphaltite are higher at all pH values compared with raw asphaltite (Figures 11 and 12). This observation confirms the previous findings in the study that demineralization increases the resistance of AWS against
Hicyilmaz et al.
Figure 21. Change of shear stress versus shear rate as a function of the pH of AWS obtained with demineralized asphaltite.
Figure 22. Change of viscosity with time as a function of the pH of AWS obtained with demineralized asphaltite.
Figure 23. Change of viscosity as a function of the pH of AWS obtained with raw and demineralized asphaltite (at the 480th second).
flowing. It should also be noted that the increase in the viscosity after demineralization is a distinct characteristic for asphaltite and does not conform with the observations reported by Aktas¸ and Woodburn1 for demineralized coal samples. Deminerilization through flotation causes two major changes in the characteristics of asphaltite: (1) the organic content relatively increases because of the decrease in the inorganic matter amount, and (2) the surfaces of asphaltite particles would undergo oxidation because of the wet separation (flotation) and the following
Rheological Properties of AWS
Energy & Fuels, Vol. 20, No. 5, 2006 2045
Figure 24. Change of viscosity with time as a function of the extent of oxidation of asphaltite.
Figure 25. Change of viscosity with time as a function of the inorganic matter content of asphaltite.
drying processes. Because the organic particles in asphaltite have a highly hydrophobic nature, an increase in the relative organic content would enhance hydrophobic aggregation and increase the viscosity. It was well-reported that hydrophobicity enhances the tendency of particles toward coagulation under the effects of strong hydrophobic attraction forces.26 Also, the oxidation of the surfaces of asphaltite particles would affect the viscosity. To test these possibilities, rheological experiments were carried out first with asphaltite samples oxidized for different periods. Next, the effect of the inorganic matter content was investigated with asphaltite samples of different ash contents. To test the effect of oxidation, ground asphaltite samples were wetted and left to oxidation in a laboratory furnace for 12 and 21 days. The viscosities of oxidized asphaltites were then compared with the viscosity of the fresh ground asphaltite. The viscosity versus time profiles for the fresh ground and oxidized samples are seen in Figure 24. It was seen that the viscosity decreased as the extent of oxidation increased and the lowest viscosity values were obtained with 21 days of oxidized asphaltite. It is known that oxidation of organic particles results in a loss of hydrophobicity. This is due to an increase in the amount of polar oxygen-rich functionals or elemental oxygen with a hydrophilic nature on the surfaces of particles. With an increase in the hydrophilic nature of asphaltite after oxidation, the hydrophobic attraction forces between organic particles decrease. This condition improves the dispersion of particles and lowers the viscosity. The effect of inorganic matter on the viscosity was tested with AWSs prepared with the raw asphaltite (43.96%) and reject and concentrate products of flotation. The reject product of flotation is the inorganic-matter-rich part, separated during demineralization, and involves 52.11% ash. The concentrate of flotation or demineralized product has the lowest ash of 35.40%. The viscosity versus time profiles with AWSs of different inorganic matter contents are given in Figure 25. It is seen that the higher the inorganic content, the lower the viscosity of the AWS. Inorganic particles display a hydrophilic surface behavior in a pulp. The increase in the hydrophilicity with higher inorganic matter decreases the effect of hydrophobic attractive forces between particles and the extent of hydrophobic aggregation. On the other hand, as the hydrophobic organic matter content relatively increases after demineralization, higher viscosity values were observed on account of increased hydrophobic aggregation. In view of these observations, it is clear
that inorganic matter content has a favorable effect on lowering the viscosity of AWS and that demineralization increases the resistance to flow.
(26) Xu, Z.; Yoon, R. H. J. Colloid Interface Sci. 1989, 132, 532-541.
4. Conclusion The results showed that pulp density, chemical addition amount, pulp pH, and particle size were important in obtaining a low viscosity AWS. Also, the viscosity of AWS is directly related to the inorganic content of asphaltite and the extent of hydrophobic aggregation. An increase in the pulp density had negative effects on viscosity because of the increased interparticular friction and resistance to flow. The use of dispersing (PSS) and stabilizing (Na-CMC) agents was inevitable in obtaining and maintaining a well-dispersed phase and in lowering the original viscosity values. However, the regulation of the stabilizer (Na-CMC) dosage was critical, and excess use resulted in an increase in viscosity. The function of the dispersing agent in lowering the viscosity relied on its contribution to the negative surface potential and, hence, increasing the interparticular repelling forces. A similar condition was observed as the pH increased. The viscosity decreased at alkaline conditions because of the higher negative surface potential of asphaltite particles under the effect of an increased OH- ion concentration in the slurry. The viscosity of AWS was directly related to the particle-size distribution, and an increase in fineness also increased the viscosity. This was related to the achievement of a more compact structure and the reduced lubricating effect of water with a decrease in the extent of voids and free spaces among asphalite particles. Viscosity is also a function of the relative hydrophobicity-hydrophilicity of particles. Any condition that enhances the hydrophilic nature of particles lowers the viscosity and would enable a higher solids loading in the preparation of AWS. The oxidation of particles lowers the viscosity, while demineralization and a relative increase of the organic content negatively affects viscosity. In other words, the aggregation, either because of chemical addition or surface properties of the sample, had negative effects on the viscosity of the slurry. In view of these findings, it was understood that evaluation of asphaltite fines in the form of AWSs would be an alternative utilization, as has been proven for lignites and/or coals. However, optimization of the related parameters remains to be the key issue to achieve an AWS with favorable flow, transportation, and combustion characteristics. EF060063O
