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Comprehensive Characterization of Pond Ash and PondAsh Slurries for Hydraulic Stowing in Underground CoalMinesDevi Prasad Mishra a & Samir Kumar Das ba Department of Mining Engineering , Indian School of Mines , Dhanbad , Jharkhand , Indiab Dr. B. R. Ambedkar National Institute of Technology , Jalandhar , Punjab , IndiaAccepted author version posted online: 03 Mar 2014.Published online: 10 Jul 2014.

To cite this article: Devi Prasad Mishra & Samir Kumar Das (2014) Comprehensive Characterization of Pond Ash and Pond AshSlurries for Hydraulic Stowing in Underground Coal Mines, Particulate Science and Technology: An International Journal, 32:5,456-465, DOI: 10.1080/02726351.2014.894162

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Comprehensive Characterization of Pond Ash and Pond AshSlurries for Hydraulic Stowing in Underground Coal Mines

DEVI PRASAD MISHRA1 and SAMIR KUMAR DAS2

1Department of Mining Engineering, Indian School of Mines, Dhanbad, Jharkhand, India2Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India

In this study, the physicochemical and self-heating characteristics of pond ashes from Talcher Thermal Power Station (TTPS),located in the Angul district of the Indian state of Odisha are studied. The study revealed that the TTPS pond ashes belong to ClassF fly ash category consisting mainly of SiO2, Al2O3, and Fe2O3, with a small amount of CaO. The presence of mineral phases,namely, quartz, mullite, magnetite, and hematite in the pond ash are confirmed by x-ray diffraction. Based on the pond ash proper-ties, various properties of the slurries of one representative pond ash, namely, density, volume concentration, and viscosity aredetermined by varying the ash concentration from 45% to 65% with an increment of 5%. As well, the critical deposition velocitiesof slurries as a function of slurry concentration and pipe diameter are determined. The results showed that the slurry viscosityincreases exponentially with increase in solid content and the critical velocity increases linearly with increase in slurry concentrationand pipe diameter. The relationships generated through regression analysis can be used to quickly predict the critical velocity ofsimilar types of ash slurries for any known slurry concentrations and pipe sizes.

Keywords: Critical velocity, fly ash, hydraulic stowing, physicochemical properties, pond ash, slurry viscosity

Introduction

Typically, two kinds of ashes, namely, fly ash and bottomash, are produced in the thermal power plants. They are thor-oughly mixed with water and transported through pipelinesto the onsite storage ponds, called ash ponds. The ash lockedin the ash ponds is known as pond ash. Although these coalashes possess different engineering properties, they are syno-nymously called ‘‘fly ash.’’ The large volume of unutilizedpond ash occupies vast stretches of precious land causingsignificant economic and environmental problems. Pondash is sufficiently available at a lower cost and utilized in bulkin several geotechnical applications such as the constructionof roads and highway embankments, in mine stowing andbackfilling, land development, ash dyke raising, filling lowlying areas for developing residential and industrial sites,etc. (Pandian 2004; Kim et al. 2005; Jakka et al. 2010). Theuse of pond ash as a stowing material in underground mineshas been illustrated throughout the literature (Kumar et al.2003; Rahman 2005; Ghosh et al. 2006; Mishra 2007; Mishraand Das 2010).

For effective stowing, it is desirable that the stowingmaterial should be chemically inert and devoid of carbon-aceous matter. Presence of organic carbonaceous matter

causes auto-oxidation and is responsible for spontaneousheating. Apart from this, particle shape, size, density, per-meability, and water-holding capacity of the material playan important role in hydraulic stowing. Therefore, character-ization of pond ash is essential to assess its suitability forstowing in underground mines. The characteristics of coalash are greatly influenced by the geological and geographicalcharacteristics of the coal deposit, combustion conditions,and removal efficiencies of the control devices (Gupta et al.1998; Liu et al. 2004; Sarkar et al. 2006). Fly ash is mainlycomposed of silt-sized particles, typically range from lessthan 1 mm to 150 mm in size (Metcalfe et al. 2006; Siddique2007). Generally, the specific gravity of coal ash lies around2.0 and for Indian fly ashes it varies widely from 1.46 to2.66 (Pandian et al. 1998; Pandian 2004). Fly ash primarilyconsists of SiO2, Al2O3, Fe2O3, and CaO with varyingamounts of unburned carbon as measured by the loss onignition (LOI) test (Pandian and Balasubramonian 2000;Liu et al. 2004; Metcalfe et al. 2006; Siddique 2007;Ahmaruzzaman 2010). Quartz (SiO2) and mullite (Al6Si2O13)are the major crystalline phases and iron oxides, that is,magnetite (Fe3O4) and hematite (Fe2O3) are the minor phasespresent in fly ash (Monte and Sabbioni 1984; White andCase 1990; Khanra et al. 1998; Lee et al. 1999; Morenoet al. 2005; Siddique 2007; Koukouzas et al. 2007).

Hydraulic stowing is considered as the most effectivemeans of backfilling underground mines, in which a largeamount of solids of varying size, shape, and density aretransported economically underground in the form of slurry

Address correspondence to: Devi Prasad Mishra, Departmentof Mining Engineering, Indian School of Mines, Dhanbad 826004, Jharkhand, India. E-mail: devi_agl@yahoo.comColor versions of one or more of the figures in the article can befound online at www.tandfonline.com/upst.

Particulate Science and Technology, 32: 456–465

Copyright # 2014 Taylor & Francis Group, LLC

ISSN: 0272-6351 print=1548-0046 online

DOI: 10.1080/02726351.2014.894162

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through pipelines. The variables, namely, flow velocity ofmixture, density of solid particle, characteristic particlediameter, density and dynamic viscosity of carrier fluidand mixture, volumetric concentration of solid particles inthe mixture, pipe diameter, shape of solid particle, and gravi-tational constant, play a major role in efficient transpor-tation of solid-liquid mixtures through pipeline (Kokpınarand Gogus 2001). Therefore, the knowledge of above systemoperating parameters is essential before transporting thepond ash–water slurry in pipelines. Generally, the coal flyash slurries exhibit non-Newtonian or shear thinning rheolo-gical behavior, that is, a decrease in viscosity with increasingshear rate (Mishra et al. 2005; Senapati et al. 2010). How-ever, Blissett and Rowson (2013) observed a complexnon-Newtonian behavior including shear thinning and shearthickening of the fly ash slurries. Viscosity, one of the mainslurry properties, determines whether the flow regime islaminar, transitional, or turbulent. Knowledge of viscosityis useful for accurate computation of pressure losses throughpipeline, selection of pump unit, and better flow rate control(Nagala and Boufaida 2004). During pipeline transportationof slurries, the flow energy must be sufficient to preventtransient formation of bed of settled solids, which causesflow and pressure fluctuations in pipeline resulting in anunstable flow condition. This can be prevented by providingsufficient flow energy or maintaining the slurry flow abovecritical velocity to retain the solid particles in suspension.Critical flow velocity of the slurry in pipeline is the minimumflow velocity corresponding to minimum head loss. Itdepends on the flow conditions and the characteristics ofslurry rheology, solids, and pipeline. The pressure drop isminimum at critical velocity resulting in most energyefficient pipeline transportation of the slurries (Wasp et al.1977). Furthermore, at critical velocity, the excessive pipeabrasion, head loss, and plunging are reduced (Kokpınarand Gogus 2001).

In this study, the physicochemical properties of pondashes are investigated. The physical properties of pond ashslurries are determined through experiments andmathematical computations. The objective of the study isto provide engineering data relative to the requirements forhydraulic transportation of pond ash for possible use inmine stowing.

Experimental

Materials

Three pond ash samples (designated as P1, P2, and P3) werecollected in airtight plastic bags from different ash ponds ofTalcher Thermal Power Station (TTPS) for experimen-tation. TTPS, a subsidiary of National Thermal PowerCorporation (NTPC), Ltd., is located in the state of Odisha,India, with the installed capacity of 460MW. It receivesbituminous coal from Jagannath Opencast Mines ofMahanadi Coalfields, Ltd. (MCL), Talcher for power gener-ation and generates about 9.0Mt of coal ash annually. Theunutilized fly ash of TTPS is mainly disposed in ash pondsand landfills.

Characterization of the Pond Ash

Study of Physicochemical Properties

The bulk density (q) of the pond ashes is determined andparticle density (qs) is determined by water pycnometer asper IS: 2386 (Part III). The porosity (U) is calculated byusing the following relationship:

U ¼ ð1� qqsÞ � 100%: ð1Þ

The particle morphology of the pond ashes is analyzedfrom the micrographs obtained with the help of a scanningelectron microscope (SEM; JEOL JSM-5800). The particlesize analysis is done using a Malvern 3601 particle sizeanalyzer. The coefficient of permeability is determined by aconstant-head permeameter as per ASTM D-2434 and thewater-holding capacity is determined by Keen Box Method.

The chemical composition of TTPS pond ashes is deter-mined using SEM with energy dispersive x-ray microanalyzer(Oxford ISIS-300) and auto-sputter coater. The unburnedcarbon content of the ashes is determined by LOI test asper ASTM C-311 subjecting the oven-dried samples to 750�50�C in a muffle furnace. The LOI is considered as a standardindex for carbon content of fly ash and natural pozzolans. Themineral phases present in the ash samples are identified by thex-ray diffraction technique using Philips X’Pert diffract-ometer (Model: PW 1710), operated at 40 kV and 30mA uti-lizing Cu Ka radiation (k¼ 1.542 A). The detector is scannedover a scattering angle (2h) range from 10� to 70�, with a 0.05�

step size and a dwell time of 2 sec per step. From the powderdiffraction patterns, the crystalline phases and minerals asso-ciated with the pond ashes are identified by comparing thepeak positions and intensities with those in the Joint Commit-tee on Powder Diffraction Standards data files.

Study of Self-Heating Characteristic of the Pond Ash

It is desirable that the stowing materials should not possessself-heating or spontaneous heating tendency. The self-heating characteristic of the pond ashes is assessed usingcrossing point temperature (CPT) apparatus commonly usedfor measuring the susceptibility of coal to spontaneous com-bustion (Ramlu 1997). The temperature at which the sampleand furnace coincide is known as the crossing point tem-perature. It signifies the relative tendency of the materialtoward spontaneous combustion. In this test, 4 g ash sample(�100þ 200 mesh size) kept in a reaction tube between twolayers of glass wool was heated in a furnace at a constant rateof 1�C=min while circulating air into the reaction tubethrough a pipe at a flow rate of 80ml=min using an air motor.The furnace and ash sample temperatures are noted at 5-minintervals. The CPT curves (Figure 4) are drawn by plottingthe furnace and sample temperatures against time.

Characterization of the Pond Ash Slurries

Study of Physical Properties

The pond ash P2 is used for making pond ash–water slurriesin five different weight concentrations (Cw), varying the ashconcentration from 45% to 65% in increments of 5%. Thephysical properties of pond ash slurries, that is, volumetric

Comprehensive Characterization of Pond Ash and Pond Ash Slurries 457

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concentration of solid, density, and viscosity, which aremainly used in slurry flow calculations and design of pipingsystem, are calculated. The density of pond ash slurries (qm)is determined from experiment using density meter andfrom the most commonly used relation (Wasp et al. 1977;Abulnaga 2002)

qm ¼ 100Cw

qsþ 100�Cw

ql

; ð2Þ

where Cw is the weight concentration of solid (%), qm is thedensity of slurry (kg=m3), ql is the density of liquid (taken as1000 kg=m3 for water), and qs is the particle density of solid(for the pond ash P2 used, it is 2000 kg=m

3).The slurry viscosity (l) is a complex function of liquid

viscosity, solid volume fraction (/), strain rate (in caseof non-Newtonian slurry), temperature, slurry chemistry,etc. (Ebadian et al. 2001). The following l–/ relationshipfirst proposed by Einstein (1906) is used for estimating theviscosity of laminar slurry

lr ¼ 1þ 2:5/; ð3Þ

where lr is the relative viscosity of the slurry, which is theratio of the slurry viscosity (l) to the viscosity of puresuspending fluid (lf). For water, the value of lf is taken as0.001 Pa � s. The main drawback of Einstein’s equation isthat it does not take into consideration the particle sizeand interaction between the particles and, hence, is validonly for dilute suspensions.

Several empirical models commonly used for calculationof viscosity of concentrated slurries including that of flyash have been used in this study. The following polynomialexpression, a modification of the above Einstein’s formula,proposed by Thomas (1965) is more commonly used forestimation of viscosity of concentrated suspensions:

lr ¼ f1þ 2:5/þ 10:05/2 þ 0:00273expð16:6/Þg: ð4ÞFor more concentrated slurries, the following model pro-

posed by Krieger and Dougherty (1959) is extensively used

lr ¼ 1� //m

� ��½l�/m

; ð5Þ

where /m is the maximum volume fraction of solids in theslurry and [l] is the intrinsic viscosity of the slurry.

Chong et al. (1971) developed the following modelfor concentrated slurry that is equally applicable for poly-dispersed suspension:

lr ¼ 1þ 0:75

//m

1� //m

� �24

352

: ð6Þ

Dabak and Yucel (1986) developed the following modelinvolving the adjustable parameters [l] and N:

lr ¼ 1þ l½ �//m

N /m � /ð Þ

� �N; ð7Þ

where N is a suspension dependent parameter. Under highshear rate condition N is taken as 2 and has been used inthe above equation.

Mori and Ototake (1956) proposed an equation forpredicting the viscosity of slurries with wide range ofconcentrations as

lr ¼ 1þ 3/

1� /0:52

: ð8Þ

This relationship was derived based on the assumptionthat the monosized spherical particles in the slurry areoriented in a regular arrangement. Considering randomarrangement of irregularly dispersed particles, Yoshida et al.(2013) slightly modified the above equation by replacing 0.52with 0.57 and obtained satisfactory results.

Senapati et al. (2010) proposed a model incorporatingmaximum solids fraction (/m), power-law index (n), medianparticle size (D50), coefficient of uniformity (CU), shear rate (c)to predict viscosity of fly ash slurries at higher concentration as:

lr ¼10CU

D501þ l½ �

c0:4/

/m � /

� �� �3:5: ð9Þ

In the aforementioned equations, the volume fraction ofsolid in slurries (/), which is the ratio of the solid volumeto the suspension volume, is estimated using the relationship(Wasp et al. 1977)

/ ¼ Cw:qm100qS

; ð10Þ

where Cw is the weight concentration of solid (%), qm is thedensity of slurry (kg=m3), and qs is the particle density ofsolid (kg=m3). The value of [l] is 2.5 for spherical rigid par-ticles (Senapati et al. 2010; Blissett and Rowson 2013) andhas been used in this study. The maximum solids volumefraction (/m) indicates the threshold flowability of the slurryas lr approaches infinity. Several researchers have deter-mined the value of /m as 0.62 (Barnes at al. 1989; Senapatiet al. 2010) and it has been used in this study for calculatingthe pond ash slurry viscosity. In Equation (9), the value ofD50 and CU determined for the pond ash (P2) as 28.1 mmand 3.89, respectively, have been used and the shear rate(c) value has been considered as 200 s�1.

Study of Critical Deposition Velocity

The following Durand’s (1952) correlation is mostly used fordetermination of critical velocity (Vc) of the slurries:

Vc ¼ FL½2gDðqs � qlql

Þ�0:5; ð11Þ

where FL is a dimensionless factor that depends on particlesize and concentration, D is pipe diameter, qs is the densityof transported solid particles, and ql is the density of carrierfluid.

Since Equation (11) does not take into account the solidconcentration and particle diameter, following Wasp’s(1977) formula, a modification of Durand’s (1952) formula,which includes the effect of particle concentration and meanparticle diameter on critical flow velocity, has been used in

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this study for estimation of Vc:

Vc ¼ 3:116/0:186½2gDðqs � qlql

Þ�12ðdDÞ16; ð12Þ

where Vc is the critical velocity (m=s), / is the solid volumefraction, D is the pipe diameter (m), d is the average particlediameter (m), qs is the density of transported solid particles(kg=m3) and ql is the density of carrier fluid (taken as1000 kg=m3 for water). Herein, the average particle diameter(d) of the pond ash (P2) used is taken as 0.028mm(D50¼ 28.1 mm, as determined from particle size analysis).The critical velocity of pond ash slurries are calculated forfive different test pipe diameters, that is, 80, 100, 125, 150,and 200mm.

Results and Discussion

Physical Properties of the Pond Ash

The scanning electron micrographs at 1000 magnificationshown in Figure 1 revealed that the pond ash samples mostlyconsist of spherical particles. Some of the ash particles areirregular in shape and porous in nature. The irregularlyshaped particles are primarily of crushed bottom ash.Agglomeration of fine particles and sticking of finer particlesto the coarser particles are also noticeable in the micro-graphs. The less abrasive spherical particle morphology helpsthe ash to flow and blend freely in ash–water slurries.Therefore, it can significantly improve rheology of the slurryresulting in frictionless flow in the stowing range and reduce

wear and tear of the pipeline. The physical properties of ashsamples are given in Table 1. The particle density (qs) of thepond ashes varies from 2000 to 2150 kg=m3. This suggeststhat pond ash is lighter than river sand and this lightnesscharacteristic of pond ash will be favorable for hydraulictransportation through pipes due to less head loss and lessenergy and transportation costs (Ghosh et al. 2006). In otherwords, energy consumption associated with its transportationduring stowing will be less as compared to the transportationof same volume of sand. The bulk density of pond ashesranges between 0.95 and 1.05 gm=cm3 and the porosity ofbulk pond ash varies in the range of 51.16% to 54.21%.

The particle size distribution curves of the pond ashes areshown in Figure 2. The particle size analysis results (Table 1)indicate that the specific surface area of samples varies in therange of 0.2435 to 0.3732m2=cm3. Pond ash P3 is the coarsestand pond ash P1 is the finest amongst all the samples, withmean particle diameters (D50) of 33.44 mm and 23.44 mm,respectively. TheD30 andD60 values of the samples are deter-mined from the particle size distribution curves and the coef-ficient of uniformity (CU) and coefficient of curvature (CC)are calculated using the following formulae:

CU ¼ D60=D10; ð13Þand

CC ¼ D30 �D30ð Þ= D60 �D10ð Þ: ð14Þ

Since, CU< 6 and 1<CC< 3, the pond ashes are poorlygraded as per the classification and gradation of soils by

Fig. 1. Scanning electron micrographs of the pond ash samples: a) P1, b) P2, and c) P3 at 1000 magnification..

Comprehensive Characterization of Pond Ash and Pond Ash Slurries 459

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ASTMD-2487. TTPS pond ashes possess high water-holdingcapacity varying in the range of 62.77% to 66.79%. Theircoefficient of permeability varies in the range 3.3507 to7.2272� 10�4 cm=s, which is very low and equivalent to thepermeability of silts. Since pond ash P1 is the finest of all,it possesses least coefficient of permeability of 3.3507� 10�4

cm=s. It is also noticeable that the pond ash of comparativelycoarser particle size possesses higher WHC. This clarifies thatwater absorption is a function of particle size and the ashcontaining larger amounts of fine particles possesses lowerwater absorption capacity (Iyer and Stanmore 1999).

Chemical Properties of the Pond Ash

The chemical compositions of TTPS pond ash samplespresented in Table 2 revealed that they are enriched

predominantly in SiO2 and Al2O3 with small amounts ofFe2O3, TiO2, K2O, CaO, and MgO. The SiO2, Al2O3, andFe2O3 content in the samples vary in the ranges of 59.89–61.85%, 30.48–31.69%, and 3.01–3.54%, respectively. Thesum of SiO2, Al2O3, and Fe2O3 accounts for more than90% of the total composition. The high SiO2 and Al2O3 con-tent impart good potential binding ability to the pond ash(Jakka et al. 2010) and may enhance the capacity of stowedpond ash mass in taking load of the overlying strata. Dueto negligible free lime (CaO) content (<1%), the ash samplespossess insignificant pozzolanic or cementing property. Theyalso contain very little amounts of unburned carbon (LOI)varying from 0.61% to 1.47% and hence there is no riskof spontaneous heating if utilized as a stowing materialin underground mines. Since the sum total of SiO2, Al2O3,and Fe2O3 is more than 70% and LOI is less than 6% inall the samples, they confirm to Class F fly ash categoryaccording to ASTM C-618 specifications.

The diffractograms of the pond ashes shown in Figure 3indicate the peaks characteristic of quartz (SiO2)-Q, mullite(Al6Si2O13)-M and iron oxides such as magnetite (Fe3O4)-Maand hematite (Fe2O3)-H, which occur in crystalline form(Shao et al. 2004). However, the most common phases andminerals found in the ashes include quartz and mullite. Themost intense peak near 2h¼ 26.66� is identified as the mainpeak due to quartz (101), the primary mineral present inpond ash. The peaks near 2h¼ 26.40� are identified as mulliteor aluminosilicate mineral, whereas, peaks occurring near2h¼ 16.4� are identified as refractory mullite (Sarkar et al.2006). Along with aluminosilicate mineral, the concurrenceof strong peaks close to 2h¼ 26.50� indicate quartz. Thepresence of heavy minerals like magnetite and hematite areindicated by their respective peaks near 2h of 35.4� and 33.2�.

Self-Heating Characteristic of the Pond Ash

The CPT curves shown in Figure 4 indicate that no samplecould attain the furnace temperature till heating up to

Fig. 2. Particle size distribution curves of the pond ash samples.

Table 1. Physical properties of pond ash samples

Parameters P1 P2 P3

Particle density (qs), kg=m3 2150 2000 2140

Bulk density (q), g=cm3 1.05 0.95 0.98Porosity (U), % 51.16 52.50 54.21Coefficient of permeability,k� 10�4 (cm=sec)

3.3507 5.5472 7.2272

Water-holding capacity(WHC), %

62.77 63.56 66.79

Particle size analysisSpecific surface area, m2=cc 0.3732 0.2930 0.2435D(4, 3), mm 28.18 35.17 41.83D(3,2), mm 16.08 20.48 24.64D90, mm 64.06 74.26 83.94D50, mm 23.44 28.10 33.44D10, mm 6.93 8.74 10.28D30, mm 14.59 19.61 22.75D60, mm 26.35 34.04 45.17Coefficient of uniformity, CU 3.80 3.89 4.39Coefficient of curvature, CC 1.16 1.29 1.11

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a temperature of 200�C. A difference of 1–4�C is noticeablebetween the furnace and ash temperatures during the test.Also the curves illustrate that both the furnace and ashtemperature lines move parallel and nowhere cross eachother within the maximum testing temperature of 200�C.Moreover, since the ashes contain very negligible amountsof carbon (0.61 to 1.47% obtained from the LOI test), theyare not susceptible to spontaneous heating and can be safelyutilized as a stowing material in the underground coalmines.

Physical Properties of the Pond Ash Slurries

Table 3 summarizes the physical properties of pond ashslurries. The results indicate that the estimated slurry densi-ties closely match with the measured values, and thereforeused in the calculation of volume concentration of solid inthe slurries. It is estimated that the solid volume fractions(/) in the slurries varies from 0.29 in the case of slurry with45% concentration to 0.48 for the slurry with 65% concen-tration. The analysis of viscosity data revealed that differentmodels predicted the pond ash slurry viscosity differently andthe estimated viscosity ranges for each slurry concentration ispresented in Table 3. It is also noticeable that the range ofvariation of the estimated viscosity values is less at lower

slurry concentrations (/¼ 0.29 and 0.33) and more at higherconcentrations. The viscosity values of slurries predictedusing Thomas (1965), Chong et al. (1971), and Dabak andYucel (1986) models closely match. While Mori and Ototake(1956) and Senapati et al. (2010) models overestimated theviscosity, Krieger and Dougherty (1959) and modified Moriand Ototake (2013) models underestimated the viscosity ofthe slurries. However, the viscosity values estimated by Krie-ger and Dougherty and modified Mori and Ototake modelsvery closely matched. It is also observed that the Thomas’sequation, which was developed for systems of monodispersespheres, predicts well the viscosity of concentrated pond ashslurries as the slurry viscosities predicted using it fallwithin the values predicted by rest of the models.

The variation of pond ash slurry viscosity with slurryconcentration (/¼ 0.29–0.48) predicted using several empiri-cal models is shown in Figure 5. Figure 5 indicates that theviscosity increases more or less exponentially with solid con-tent and it is estimated in the ranges of 2.66–3.46� 10�3 Pa � sand 10.04–19.72� 10�3 Pa.s for the slurries with concen-trations of 45% and 65%, respectively. Beyond 50% ashconcentration (/¼ 0.33), a steep increase in slurry viscosityis noticeable. Rokita and Tomaszewski (1988) reported thatat /> 0.35, there is a rapid increase in the dynamic viscosityof ash–water slurries and initial tangential stress causing anincrease in the flow resistance and energy consumption. Overthis concentration, flow characteristic of the slurry is laminar.The slurry with higher viscosity would result in higher pressuredrop while moving through pipelines.

Critical Velocity of the Pond Ash Slurries

The effects of slurry concentration and pipe diameter oncritical velocity are evaluated. The critical velocity in the caseof slurries with 45% ash concentration varied in the range of0.823 to 1.117m=s, whereas, at 65% concentration, it variedin the range of 0.904 to 1.227m=s for the selected pipe sizes.The variation of critical velocity with slurry concentrationsand test pipes of five different nominal internal diameters,that is, 80, 100, 125, 150, and 200mm are shown inFigures 6 and 7, respectively. In both the cases, critical

Fig. 3. X-ray diffraction patterns of the pond ash samples: a) P1, b) P2, and c) P3.

Table 2. Chemical compositions of pond ash samples

Oxides

% (by mass)

P1 P2 P3

SiO2 59.94 61.85 59.89Al2O3 31.69 30.48 31.30Fe2O3 3.01 3.23 3.54TiO2 2.66 2.19 2.57CaO 0.91 0.72 0.85MgO 0.35 — —K2O 0.84 0.90 0.76P2O5 0.61 0.64 0.56MnO — — 0.53LOI 0.61 1.27 1.47

Comprehensive Characterization of Pond Ash and Pond Ash Slurries 461

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velocity followed an increasing trend with increasing slurryconcentration and pipe diameter. A very good correlationbetween the critical velocity and the slurry concentrationand pipe diameter is obtained with correlation coefficientsof 0.999 and 0.996, respectively. In other words, the higherthe slurry concentration and larger the pipe diameter, thegreater the critical velocity requirement needed to keep pondash particles in suspension during pipeline transportationand vice versa. At critical flow velocity, since all the solidparticles in the slurry are in suspension and pressure dropin pipe flow is minimum, the flow of the pond ash slurries

should, therefore, be maintained above the critical velocitiesestimated for the test pipes to avoid potential pipe pluggingdue to the settling of solids. The relationships generatedthrough regression analysis shown in Figures 6 and 7 canbe used to predict the critical velocity of similar types ofash slurries for any other concentrations and pipe sizes. Last,an attempt should be made to experimentally validate theequation used in this study for prediction of critical velocity

Fig. 4. Crossing point temperature (CPT) curves of the pond ash samples: a) P1, b) P2, and c) P3.

Table 3. Physical properties of pond ash slurries

Slurry concentrationDensity of slurries

(qm), kg=m3 Estimated

viscosityrange (l)�10�3, Pa � s

Conc. byweight (Cw), %

Conc. byvolume (/) Measured Estimated

45 0.29 1265.8 1290.3 2.66–3.4650 0.33 1333.3 1333.3 3.25–4.4855 0.38 1369.9 1379.3 4.35–6.5460 0.43 1408.5 1428.6 6.25–10.4465 0.48 1470.6 1481.5 10.04–19.72

Fig. 5. Variation of viscosity of the pond ash slurries with solidconcentration.

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for the slurries using pond ash and see how well this relation-ship predicts experiment.

Summary and Conclusions

We investigated the characteristics of pond ashes and pondash slurries, which are of prime importance for the designof new pipelines or modification of the existing systemsfor efficient pipeline transportation of pond ash duringhydraulic stowing. The study revealed that the ash samplespossess spherical particle morphology, which will createa lubricating action due to ‘‘ball-bearing’’ effect and resultin a frictionless flow in the stowing range causing less wearand tear of the pipelines during hydraulic transportation.Also, due to light particle density and improved rheology,the energy cost in pipeline transportation of the pond ash

will be reduced. High water-holding capacity will facilitatemore water absorption and allow less water to drain outand thereby assists in reducing the pumping cost. Pond ashesare mainly composed of SiO2, Al2O3 with low amounts ofFe2O3 and little amounts of CaO, MgO, MnO, P2O5, K2O,and TiO2. The presence of SiO2 in abundance impartsstrength and CaO gives rise cementing properties to thestowed pond ash mass. Due to very negligible amount ofunburned carbon content, there is no risk of spontaneousheating if pond ash is utilized as a stowing material in under-ground coal mines. This has also been confirmed from theCPT test. Overall, the results indicate that the TTPS pondashes possess properties favorable for stowing in under-ground coal mines.

Characterization of pond ash slurries revealed that the esti-mated and measured pond ash slurry densities varied in theranges of 1290.3–1481.5 kg=m3 and 1265.8–1470.6 kg=m3,

Fig. 6. Variation of critical velocity of the pond ash slurries with solid concentration for different pipe diameters.

Fig. 7. Variation of critical velocity of the pond ash slurries with pipe diameter for different slurry concentrations.

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respectively. The viscosity of the slurries increases more orless exponentially with the solid content. The critical velocityincreased linearly with increasing slurry concentration andpipe diameter showing a very good correlation with respectivecorrelation coefficients of 0.999 and 0.996. It varied in therange of 0.823 to 1.117m=s in the case of slurries with 45%ash concentration and 0.904 to 1.227m=s at 65% concen-tration for the selected pipe sizes. The relationships generatedthrough regression analysis can be used to quickly predict thecritical velocity of similar types of ash slurries of any otherconcentrations and pipe sizes.

Funding

The authors are thankful to the Technology Information,Forecasting and Assessment Council (TIFAC), New Delhi,an autonomous organization under the Department ofScience and Technology (DST), India, for providing finan-cial support for carrying out this study.

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