Stabilization and Rheological Behavior of Fly Ash-Water Slurry Using a Natural Dispersant in Pipeline Transportation.

Effective transportation of fly ash-water slurry through a pipeline from its generation site, a power plant, to a storage site by replacing commercial surfactants such as cetyl trimethyl ammonium bromide and sodium dodecyl sulfate by a natural dispersant extracted from Sapindus laurifolia was studied. The stability of fly ash slurry was determined from its rheological parameters, dispersant concentration, and stabilization mechanism. From surface tensiometric data, the critical micelle concentration of the dispersant was obtained to be 0.017 g/cc. The stabilization of high-concentration fly ash slurry has been studied through its rheological behavior by variation of temperature and dispersant and ash concentration. The rheological result obtained for fly ash concentrations in the range of 50-65% slurry was best justified by the Bingham plastic model. The wettability of fly ash particles is increased in the presence of dispersants, which is inferred from reduction of the surface tension value. The stabilization mechanism of the slurry is explained by a steric factor as indicated by the decrease in the zeta potential value. Air pollution is minimized at its destination site due to agglomeration of fly ash particles, which is confirmed from the SEM microphotograph.

Introduction

Most of the energy requirement of India comes from burning fossil fuel. In more than 60% of the thermal power plants in India, a fine-grained particulate material known as fly ash is produced from burning of coal in a coal-fired boiler in the power plant and gets carried off in the flue gas. The fly ash is collected from the flue gas before it is released into the atmosphere, causing air pollution by means of particulate air pollution control equipment. Even though the massive production rate of fly ash in India attained only a utilization rate of around 45% in the cement industry, those from the production of building materials and others are disposed of onto land through landfills or ash ponds. It has been reported by some researchers that fly ash-derived particles may be used in potential drug delivery systems,[1,2] anti-biofouling systems,[3] and low-cost adsorbents for removal of organic dyes and geolite synthesis.[4] Many environmental problems related to soil, air, and water pollution escalate due to fly ash. Fly ashes are usually disposed of onto ash ponds in a slurry form through pipelines from thermal power plants. Such a type of disposal system requires a large amount of water and energy, which makes the process uneconomical as fly ashes with concentrations varying from 10 to 15% are transported. The economical transportation of solid concentration slurry should be high, which in turn requires less amount of water, a low energy requirement for pumping, and low-energy wastage, which is an environmentally friendly and emerging trend.[5−7] This causes an increase in slurry viscosity and yield stress; therefore, more pumping power is required for transportation and it is very much essential to know the rheological behavior of the slurry at various ash concentrations. Furthermore, it is also necessary to study the surface effect of the surfactant and other microscopic particles.[8] The transportation of high-volume fly ash slurry through long distances in pipelines is always challenged by friction loss, elevated energy consumption, and high settlement rate in the pipelines. There have been several attempts to transport high-concentration fly ash slurry in pipelines all over the world. In the past decade, a number of techniques have been investigated, which alter the flow behavior of slurry either by reducing the pressure drop or reducing adversely the impact of rheological properties. The flow behavior of fly ash slurry depends on different factors, for instance, shape of particles, particle size distribution, solid concentration in slurry, and slurry viscosity.[9] Viscosity and shear stress of slurry could be decreased by adding different additives, thereby enhancing the possibilities of a high percentage of solids in the slurry transportation.[10] Many investigators have examined the effects of various additives on the rheological behavior of fly ash slurry at high concentrations during pipeline transportation. A substantial reduction in the viscosity of the slurry was observed by using 0.1% sodium hexametaphosphate by mass.[6] The fly ash slurry stabilization by considering the combined effect of 0.1% sodium hexametaphosphate and particle size distribution at different ash concentrations were reported, which ensue non-Newtonian rheology of the slurry.[11] The reduction of Bingham plastic viscosity, yield stress by using sodium carbonate (0.2% by mass) and Henko detergent (5:1) as an additive in fly ash concentration from 50 to 70% by mass.[12] The outcomes of dispersants such as cetyl trimethyl ammonium bromide (CTAB) and sodium salicylate on stabilization of fly ash slurry in the solid concentration range from 20 to 40% at varying temperatures from 20 to 40 °C reported that the apparent viscosity and shear stress decrease with the increase in temperature even without an additive and the optimized additive concentration in the range of 0.2 to 0.3% by mass.[13−16]

With respect to the charge on the surface, three types of dispersants (cationic, anionic, and nonionic polymers such as cetylpyridinium chloride, sodium dodecyl sulfate (SDS), and Triton X-100, respectively) are established to reduce viscosity, and a shear stress of 40% mass concentration of fly ash slurry was reported.[17] The flow behaviors of high-concentration fly ash slurries with a solid concentration of 32 to 49% can be illustrated by the non-Newtonian power law model as they are greatly pseudo-plastic in nature and the relative viscosity of slurry depends on the concentration of the solid volume fraction, particle size, and their distribution as examined by Senapati et al.[18] They have developed a model for power plant ash slurry, integrating the maximum solid fraction, median particle size, coefficient of uniformity, and shear rate power law index to predict the apparent viscosity. Particle gradation has a remarkable effect on the ash concentration and pressure drop observed by Kumar et al.[19] that comparatively less energy required for transportation and even less energy than fine ash slurry for optimized particle size distribution that is the ratio of fly ash and bottom ash in the range 4:1 to 3:2. The mixture of surfactants tris(2-hydroxy-ethyl) tallow alkyl ammonium acetate [tallow alkyl N-(C2H4)(OH)3], sodium salicylate used as a counterion, and copper hydroxide [Cu(OH)2] in a certain ratio can stabilize the fly ash slurry to a considerable extent.[20] Elizabet et al.[21] have investigated the effect of an anionic surfactant, sodium lauryl sulfate, on the fly ash surface at different concentrations and a temperature range from 50 to 80 °C. They reported that fly ash treated with a surfactant was more hydrophobic than untreated fly ash due to the remarkable reduction of the degree of agglomeration.

A high concentration of ∼300 g/L fly ash slurry can competently be stabilized by an equimolar mixture of biopolymers hydroxypropyl guar gum and xanthan gum. The stability of fly ash–water slurry can be increased by increasing the biopolymer concentration in a time period of ∼72 h, which is confirmed by the elastic modulus value.[22] In addition to the cationic surfactant and biopolymer, anionic surfactant SDS also exhibits an excellent stabilizing effect on microfly ash slurry, which inhibited spontaneous coal combustion.[13,23]

From literature reviews, a conclusion that most of the additives or dispersants used in fly ash slurry stabilization are synthetic in nature and not eco-friendly can be drawn. Therefore, in the present investigation, an attempt has been made to utilize a natural dispersant from the plant, Sapindus laurifolia (S. laurifolia). It is saponin rich and typically in profusion in India. Saponin is a naturally occurring nonionic surfactant, which contains a hydrophilic component described as glyconic consisting of polysaccharides, such as rhamnose, pentose, galactose, and glucose, and a hydrophobic component described as aglyconic. An aglycon is made up of steroids and triterpenes, which are linked with a polysaccharide unit through oxygen and so on to form macromolecules.[24] In this paper, we have reported the rheological behavior of fly ash slurry using the aqueous extract of S. laurifolia as a dispersant. Also, the agglomeration of fly ash particles after their transportation to the destination site is observed, which leads to minimization of air pollution.

Results and Discussion

Optimization of S. laurifolia Concentration

The dispersants are the molecules that experience steric hindrance and/or electrostatic repulsion while adsorbing on the fly ash surface.[13,25] The mechanisms of stabilization of the slurry rely on the susceptibility of the dispersant to be attached on the fly ash surface. The presence of surfactants in solution reduces the surface tension or interfacial tension of the solution. Thus, surface tension of the solvent consistently decreases with a gradual increase in the dispersant concentration. From Figure , it is observed that the minimum surface tension is achieved at 0.017 g/cc, which is the critical micelle concentration (CMC) of the dispersant. A correlation can be established between van der Waals forces of attraction among the fly ash particle and the amount of dispersant adsorbed. It has been found that by adding more of the dispersant to the slurry, the magnitude of van der Waals forces of attraction among the fly ash particles can be minimized. From Figure , it is observed that by increasing the concentration of dispersants from 0.010 to 0.017 g/cc containing a 60% weight fraction of fly ash, the apparent viscosity of the fly ash slurry decreases from 1680 to 503 mPa. Upon further increasing the S. laurifolia concentration, there is no appreciable reduction of the apparent viscosity and plateau values are obtained, which may be due to the formation of micelles and no further adsorption of dispersants on the fly ash surface.[26]

Figure 1

Plot of dispersant concentration versus surface tension.

Figure 2

Plot of dispersant concentration versus apparent viscosity.

Effect of Fly Ash Loading on Apparent Viscosity of Slurry

The apparent viscosity of the slurry can be altered by varying the concentration of fly ash in the slurry, which is very important in pipeline transportation. The viscosity of slurry is the resultant effects of van der Waals forces among fly ash particles. Thus, the viscosity increases with the rise in the amount of fly ash loading as they get agglomerated at higher concentration. The indispensable condition for better slurry transportation is during the transportation viscosity, which should be low and high at the destination end for better sedimentation.[7,26]Figure describes the deviation of the apparent viscosity of the slurry by adding different amounts of fly ash in the range of 50–65% in the presence of an optimized dispersant concentration (CMC) of 0.017 g/cc at a shear rate of 80 s–1. With the increase in fly ash concentration, the apparent viscosity gradually increases up to 64% concentration of fly ash in the slurry. Beyond the studied range of concentrations, the apparent viscosity is not suitable for pipeline transportation due to agglomeration of ash particles.

Figure 3

Plot of fly ash concentration versus apparent viscosity.

Rheological Behavior of the Fly Ash Slurry

For very low-concentration slurry, a straight line passing through the origin is obtained from the plot between shear stress and shear rate. The slope of the line corresponds to the viscosity of the slurry, confirming the slurry as a Newtonian fluid. On the measurement of shear stress, varying the shear rate of the fly ash–water slurry containing different solid weight ratios of fly ash in the presence of 0.017 g/ cc saponin, we have compared the flow characteristic of slurry with CTAB, a commercial dispersant.[14] In the presence of both surfactants, the fly ash–water slurry shows a linear relation between shear stress and shear rate with an early shear yield stress value, indicating a non-Newtonian Bingham plastic fluid in a solid weight fraction range of 50–64%.[27,28] The slurry follows the equation of the Bingham plastic modelwhere γ and τ indicate the applied shear rate and shear stress, respectively. τ0 is the yield stress, and η is defined as the Bingham viscosity. The initial threshold of shear stress and yield stress of slurry increases with the increase in the solid weight fraction of fly ash in the fly ash–water slurry, which may be attributed to the increase in particle–particle interactions due to greater packing of fly ash. It is confirmed from Figure a,b that the natural dispersant, saponin from S. laurifolia, exhibits a similar type of flow characteristic to CTAB.

Figure 4

(a) Plot of shear stress versus shear rate of fly ash slurry in the presence of a natural dispersant. (b) Plot of shear stress versus shear rate of fly ash slurry in the presence of commercial surfactant CTAB.

Effect of Temperature on Apparent Viscosity

There is a correlation between the viscosity of the fluid and ease of movement of molecules with respect to one another. With the increase in temperature, the apparent viscosity of the slurry decreases due to the decrease in cohesive force between the particles and the increase in dissolving capacity of the dispersant.[14,29] Therefore, the yield stress decreases with the rise in the temperature. In our present investigation, Figure describes an exponential decrease in viscosity with the increase in temperature. This may be due to the decrease in interparticle attraction as the kinetic energy of fly ash particle increases. Also, first, moving the suspended sugar chain of saponin at the fly ash–water interface increases the mobility of the fly ash particle.[29] The Arrhenius expression (eq ) describes the relation between viscosity and temperature that may be represented by a simple relation[30] as

Figure 5

Plot of apparent viscosity versus temperature.

On rearrangement, eq yields eq

The above equation represents the apparent viscosity at a particular shear rate where T is the temperature in kelvin of the slurry, E is the activation energy for the fluid flow, R is the universal gas constant, and A is the fitting parameter.

Surface Activity of the Fly Ash Slurry

Dispersants are surface active agents that reduce the interfacial tension between solid particles and the liquid in the slurry system by lowering the surface tension of a liquid, which improves the wettability of fly ash particles. The stabilization of the fly ash slurry primarily depends upon the binding tendency of the dispersant to the fly ash surface. The more the dispersant is at ease in the solution, the less is its tendency to be adsorbed onto the fly ash surface. Therefore, the study of solution behavior of the dispersant in the mixture is essential. The effect of surface tension by the dispersant depends on the replacement of solvent molecules at the surface of solution, that is, the air–water interface. With the increase in the exchange of solvent molecules by the surfactant at the interface, the surface tension decreases.[29] The surface tension decreases with the increase in the dispersant molecules in the mixture and hence in the dispersant concentration in the mixture, which indicate the segregation of the interface that was inhibited beyond CMC. The gradual decrease in the surface tension with the increase in dispersant concentration in the slurry has been observed with 20% fly ash concentration, and the minimum surface tension value of 40 mN/m at a CMC of dispersant at 0.017 g/cc was attained and was immutable thereafter, as observed from Figure .[31,32]

Figure 6

Plot of surface tension versus dispersant concentration.

Zeta Potential Study for Measurement of Surface Charge

Yield stress, apparent viscosity, and stability of fly ash slurry depend upon the type of functional group and magnitude of surface charge on the fly ash particle. Thus, the flow behavior of the slurry depends upon the surface characteristics of the fly ash particle, which can be studied by measuring the surface charge.[30,32] The surface charge is determined on the basis of the magnitude of electrostatic repulsion and attraction between the fly ash particles when they are kept in an electrolytic cell. Figure represents the result of zeta potential of the fly ash sample as a function of adsorbed dispersant. In the absence of dispersants, the surface charge of the fly ash particle was found to be −29 mV, which confirmed the negatively charged fly ash surface. With the increase in concentration of the dispersant, the zeta potential of fly ash slurry decreases, as observed from Figure . When the nonpolar moiety of saponin adsorbs on the fly ash surface, reduction of exposed surface charge on the fly ash particle occurs, which may be the reason for the decreasing trend of zeta potential. Also, the projected sugar chain may displace the original shear plane mechanically, which causes the decrease in the zeta potential value. Several researchers have observed this type of decreasing trend in the zeta potential value due to the adsorption of the dispersant on solid surfaces.[32−35]

Figure 7

Plot of zeta potential versus of dispersant concentration.

Stabilization Mechanism of Fly Ash Slurry

In the preparation of stable high-concentration fly ash slurry, particles should not be agglomerated to each other, which can be achieved by creating a repulsive barrier between the fly ash particles. Stabilization of fly ash slurry depends upon the extent of stabilization of silica (SiO2) and alumina (Al2O3) particles because of the major contribution up to 54.6 and 32.8%, respectively. The mechanism of stabilization can be explained either by a steric or electrostatic factor.[24] In our present investigation, the zeta potential of slurry decreases from −29 to −13 mV at CMC of the dispersant, and the mechanism of stabilization may be due to steric repulsion instead of electrostatic repulsion. Due to a certain kind of specific interaction with silanol hydrogen, the hydrophobic part of the dispersant is attached to the fly ash surface and the hydrophilic sugar chain is hydrated, as shown in Figure a,b, respectively. This type of interaction creates a steric barrier around each particle and inhibits particle–particle association.[24,29,36−38]

Figure 8

(a) Mechanism of interaction between fly ash particles with dispersants. (b) Schematic diagram of interaction between fly ash particles with dispersants during transportation.

Comparative Cost Analysis with Commercial Dispersant

The cost of additive S. laurifolia fruits (Tables and 2) used in the viscosity reduction of fly ash–water slurry has been estimated by comparing its cost with well-known commercial dispersants CTAB and SDS.[13] From Figure a, the lowest viscosity is observed at 0.325 × 10–3 g/cc CTAB concentration, and no significant change in viscosity is observed with a further increase in CTAB concentration. Similarly, from Figure b, the lowest viscosity is observed at 2.34 × 10–3 g/cc SDS concentration. Since the CMCs of CTAB and SDS concentrations are 0.9 mM (0.328 × 10 –3 g/cc) and 8.2 mM (2.34 × 10–3 g/cc), respectively,[39] the optimized concentration for pipeline transportation may be their CMC. From viscosity measurement and CMC results, the stabilizing effect of 1 kg of CTAB is equivalent to 51.82 kg of S. laurifolia fruits. Thus, the estimated overall cost of the dispersant CTAB is determined to be ∼3.6-fold in comparison to that of a S. laurifolia fruit. The cost of a S. laurifolia fruit is ∼0.72 U.S. dollars per kilogram in India, whereas the CTAB (Merck, India) costs ∼186 U.S. dollars (per kg). Similarly, by comparing with SDS, 7.26 kg of S. laurifolia fruits has an equal stabilizing effect on fly ash slurry with 1 kg of SDS.

Table 1

Comparative Cost Analysis of S. laurifolia with CTAB

Sl. no	surfactant	amount of additive (kg)	CMC (g/cc)	price, (USD)
1	CTAB	1.0	0.328 × 10–3	186
2	S. laurifolia fruit	51.829	0.017	50.65

Table 2

Comparative Cost Analysis of S. laurifolia with SDS

Sl. no	surfactant	amount of Additive (kg)	CMC (g/cc)	price (USD)
1	SDS	1.0	2.34 × 10–3	355.27
2	Sapindous laurifolia fruit	7.264	0.017	7.09

Figure 9

(a) Plot of apparent viscosity versus concentration of commercial surfactant CTAB at 60% fly ash loading in the slurry. (b) Plot of apparent viscosity versus concentration of commercial surfactant SDS at 60% fly ash loading in the slurry.

Conclusions

The pipeline transportation of fly ash from the thermal power plant to its destination site such as an ash pond is an economical method with a minimum negative impact on the environment by utilizing an aqueous extract of S. laurifolia, wisely replacing well-known commercial dispersants CTAB and SDS. This natural dispersant stabilizes the fly ash slurry to the maximum extent at 0.017 g/cc, which is the CMC. Wettability of fly ash slurry is increased, which is confirmed from the reduction of the surface tension value from 70.6 to 40 mN/m. The decreased value of the zeta potential from −29 to −13 mV confirmed that the stabilization mechanism is of a steric type instead of electrostatic repulsion. The SEM image (Figure ) of the air-dried slurry indicates that there may be agglomeration of fly ash particles in the presence of the dispersant S. laurifolia.

Figure 10

SEM microphotograph of agglomeration of fly ash in the presence of dispersants.

Materials and Methods

Procurement of Fly Ash Sample

The fly ash sample was procured from Jindal Steel, Chhattisgarh. The fly ash physical characteristic properties are given in Table , and the chemical assay is summarized in Table . The particle size distribution of the fly ash sample was measured by a particle size analyzer (Malvern, Pvt. Ltd.), and its peak pattern is presented in Figure . The surface topography of the fly ash sample was examined based on the SEM (JEOL, JSM-7100F) microphotograph and is shown in Figure .

Table 3

Physical Analysis Result of Fly Ash

parameters	amount
specific gravity	2.35
specific surface area	1.30 m2/gm
moisture content	0.22
wet density	1.90 g/cm3
turbidity	430 NTU

Table 4

Chemical Analysis Result of Fly Ash

compound	concentration (%)
SiO2	54.6
Al2O3	32.8
Fe2O3	1.99
CaO	0.81
K2O	1.52
TiO2	2.03
P2O5	0.006
Na2O	0.68
MgO	2.48

Figure 11

Particle size distribution of the fly ash sample (ref (36)).

Figure 12

SEM photomicrograph of the fly ash sample.

Preparation of Aqueous Extract of S. laurifolia

The additive S. laurifolia was collected from the Eastern sector forest of Odisha, India. Dry fruits of S. laurifolia (10 g) were taken, and the pericarp of the fruits was removed, dried, and powdered. The pericarp of S. laurifolia was changed to a powder form and then dissolved in a desired volume (100 mL) of water. The aliquot was subjected to agitation for a duration of 3 h using a magnetic stirrer. Then, the supernatant solution obtained was centrifuged (centrifuge, Eppendorf, Pvt. Ltd.) and filtered to extract the active component as saponin into the corresponding aqueous medium. This extract was utilized as the dispersant in the preparation of fly ash–water slurry.

Surface Activity of the Aqueous Extract of S. laurifolia

The surface tension of the aqueous extract was measured using a surface tensiometer (Kyowa-350, Japan). The varying trend of the surface tension of the aqueous phase is plotted with depressant concentration, and results are shown in Figure . The trend shows a quick drop of surface tension while increasing the depressant dose. The pure water surface tension is 72 mN/m and gets saturated until a minimum value of 40 mN/m, where the concentration of depressant reaches 0.017 g/cc (1.7 wt %).

Measurement of Surface Charge on Fly Ash Particles

The probe 24V (52–60 Hz) T3A attached to a microprocessor was used in measuring the zeta potential for the fly ash slurry sample. The condition of 10% S/L of fly ash with deionized water was maintained and then subjected to stirring at 400 rpm for 30 min at a temperature of 25 °C. Subsequently, 1 mL of the resulting sample was taken for zeta potential analysis. Similarly, the zeta potential of fly ash along with a dispersant sample was determined at varying doses at the same conditions. Most of the experiments were investigated in triplicate; furthermore, the mean values were considered and reported.

Measurement of Rheological Behavior

The rheology experimental study for fly ash slurry was examined using a HAAKE RheoStress 1(Thermo Scientific rheometer). For the test, the slurry was prepared by slowly adding fly ash ranging from 50 to 65% by weight in distilled water with continuous stirring of the mixture for 5 to 10 min. In this study, ∼30 mL of the slurry sample was introduced into a cleaned rheology cup in which the temperature was maintained up to 30 °C. The experiment was tested twice, and the average output data was further analyzed. The relationship of shear stress and shear rate showed the best fit to the Bingham plastic model.