Magnetic mineralogy and grain-size dependence of hysteresis parameters of single spherules from...

Physics of the Earth and Planetary Interiors 154 (2006) 255–265

Magnetic mineralogy and grain-size dependence of hysteresisparameters of single spherules from industrial waste products

Diana Jordanovaa,∗, Neli Jordanovaa, Viktor Hoffmannb

a Geophysical Institute, BAS, Acad. G. Bonchev Str., Block 3, 1113 Sofia, Bulgariab Institute of Geosciences, University Tuebingen, Sigwartstrasse 10, 72076 Tuebingen, Germany

Received 2 December 2004; received in revised form 25 May 2005; accepted 7 November 2005

Abstract

The use of magnetic susceptibility as a proxy for the relative degree of industrial pollution of soils and sediments is only qualitativeand complications arise when unmixing the contributions from different sources. Detailed characterization of anthropogenic magneticphases is needed in order to discriminate the input from different sources. The present study investigates magnetic fractions fromfly ashes from different thermoelectric power plants and lagooned ash from metal works in Bulgaria, to further our knowledgeof potential sources. Thermomagnetic analysis of saturation magnetization (Ms) and magnetic susceptibility (K) revealed a low-substituted magnetite-like phase in power-plant fly ashes and magnetite/maghemite phases in lagooned ash. Magnetic hysteresismeasurements on single spherules from the wastes are combined with subsequent SEM/EDX examination of the same grains,allowing for the grain size dependence of the hysteresis parameters to be directly obtained for the studied anthropogenic phases. Theresults suggest that the structure-sensitive magnetic parameters for the lagooned ash show stronger size dependence compared to thep hropogenicp of differentc©

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ublished data for synthetic crushed magnetites, which is due to the complex internal structure and stress level in the anthases formed under fast cooling conditions. Moreover, the Day plot for the studied single grains shows the presenceorrelation lines for fly ashes and the lagooned ash, i.e. it discriminates between the sources.2006 Elsevier B.V. All rights reserved.

eywords: Fly ash; Lagooned ash; Single grains; Hysteresis parameters; Magnetic mineralogy

. Introduction

Coal-burning thermoelectric power plants constitutene of the major sources of atmospheric pollution with

oxic gases (SOx, NOx, CO2), as well as particulate pollu-ants. Most of the residual unburned material called “flysh” is captured by systems of mechanical and/or electri-al precipitators. This waste material is represented byottom-ash (BA) and slag from under the combustion

∗ Corresponding author. Fax: +359 2 971 3005.E-mail addresses: [email protected] (D. Jordanova),

[email protected] (V. Hoffmann).

chambers, fly ash (FA) from the hoppers of electrosand mechanical collectors and lagooned ash (LA) fdisposal sites, and usually contains significant amoof heavy metals. The metal processing industry, whialso connected with high-temperature burning procein the blast furnaces, is another major source of envmental pollution. During the high temperature processpherical particles are usually formed (McLennan et al2000; Sokol et al., 2002). The different morphology anchemical composition of the spherules (dense, vesicpleurospheres, cenospheres, etc.) depends on the plar conditions, e.g. the viscosity of the melt, gases ware formed during burning as a result of decomposof organic material, carbonates, sulfides, etc.

031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.pepi.2005.06.015

256 D. Jordanova et al. / Physics of the Earth and Planetary Interiors 154 (2006) 255–265

Detailed investigations on single grains from fly ashesare performed for the first time in order to better char-acterize their behavior either as a waste product or assecondary raw material for cement, construction, miningindustries, etc. The magnetic properties of bulk mate-rial although well documented, may provide misleadingmagnetic signatures as their signals are dominated bysmall particles at the expence of the larger spherules.There are few studies on the properties of single grainsfrom fly ashes from coal burning power plants (Hockand Lichtman, 1982; Giere et al., 2003; Veneva et al.,2004; Goddu et al., 2004), though a wealth of literatureis dedicated to characterization of bulk properties of thefly ashes (e.g.Norton et al., 1986; Gomes et al., 1999;Hower et al., 1999; Sokol et al., 2002; Kukier et al.,2003).

The aim of the present study is to compare themagnetic characteristics of single strongly magneticspherules from lagooned ash (LA) from blast furnacesof the biggest metal works complex in Bulgaria –Kremikovtzi, and from the FAs from three thermoelec-tric power plants in the towns of Russe and Devnja, andpower plant Maritza East 1 – 60 kmsoutheast from thetown of Stara Zagora.

2. Samples and methods

Detailed mineralogical, chemical and micro-structural investigations were carried out on the flyashes analyzed in our study (Vassilev, 1992; Vassilev

of

the fly ashes from the three power plants, burningconditions, coals used and sampling locations are sum-marized inTable 1, according toVassilev and Vassileva(1996, 1997). Considering their bulk composition, theFA samples can be classified as low-Ca class F flyashes. They were collected in 1984 from the hoppersof electrostatic filters of the electrical precipitators andkindly provided by Dr. St. Vassilev (Central Laboratoryof Mineralogy and Crystallography, BAS).

The total amount of particulate matter emitted inthe atmosphere from the three blast furnaces in Bul-garia at Kremikovtzi industrial complex near Sofia is4.65 t/24 h (Douchanov, 2002). The slag is disposed inan active dumping site. The resulting lagooned ash wassampled in 2002 at one location. Only one locationwas sampled due to accessibility problems. Its grainsize distribution is probably affected by local sedimenta-tion.

Magnetic extracts were made from the FA samplesusing a hand magnet. The LA was not extracted becauseall material was attracted by the magnet. Initial exami-nation of the extracts and LA sample by optical micro-scope revealed that around 80% of all the grains werespherical or semi-spherical particles of various sizes.Using thin wooden sticks, single grains were picked outunder the microscope. The particles were selected bytheir reflectance (grayish or bright). A set of 60 sin-gle grains was obtained in this way. The single grainswhich we have studied fall in the coarse grain size range(50–600�m). According to another study (Matsunage et

m-

dent s era-agnet

onTp

el D

ousals

S

ousals

S

aterial

and Vassileva, 1996, 1997). The basic chemistry

Table 1Mass-specific magnetic susceptibility (χ), percent frequency depenture/transformation temperature/structural effects) from thermomfly ashes and lagooned ash

Source Type of wastestudied

Sourcematerial/producti

Kremikovtzi blastfurnaces

Lagooned ash fromdepository

Production of steprofiles

Maritza East 1 powerplant (sample 50)

Fly ashes fromelectrostaticprecipitators(electrofilters 1–7)

Lignite coals

Russe power plant(sample 651)

Fly ashes fromelectrostaticprecipitators(electrofilter 8)

Ukrainian bituminand anthracitic co

Devnja power plant(sample 950)

Fly ashes fromelectrostaticprecipitators(electrofilter 6)

Ukrainian bituminand anthracitic co

The type of coals used, type of fuel processing and the waste m

al., 2002), higher crystallinity spheres with more co

usceptibility (χFD, %) and the obtained inflection points (Curie tempic analyses of saturation magnetizationMs(T) and susceptibilityK(T) for the

ype of fuelrocessing

χ (× 10−8

m3/kg)χFD

(%)Inflection points (◦C)on the heating curves

ry discharge 15296.5 0.0 Ms(T) = 361, 520;K(T) = 382, 586

Dry discharge boilers 1238 1.3 Ms(T) = 152, 540,670;K(T) = 506, 580

lag discharge boilers 2072 1.9 Ms(T) = 556;K(T) = 512, 583

lag discharge boilers 2301 1.8 K(T) = 500, 571

studied are indicated in the columns 1–4.

D. Jordanova et al. / Physics of the Earth and Planetary Interiors 154 (2006) 255–265 257

plicated morphology and composition are found in thecoarse grained fraction of the fly ashes. At the same time,the relative proportion of the magnetic fraction withinthe grains increases with grain size (Tipse et al., 2002),giving a higher contribution to the total magnetic suscep-tibility. Consequently, the lower limit of the grain sizes

which we were able to extract and measure meets rea-sonably the bulk magnetic response of FAs. Additionally,measurements of the frequency dependent magnetic sus-ceptibility of the bulk FAs and LA were performed forevaluation of the relative importance of SP fraction tothe total signal.

FK

ig. 1. High temperature thermomagnetic analyses of saturation magn(T) (heating in air). Heating curve is indicated by bold line, cooling—b

etizationMs(T) (saturation field of 1 T, heating in vacuum) and susceptibilityy thin line.


Magnetic hysteresis measurements, isothermal rema-nent magnetization (IRM) acquisition starting from aninitially alternating-field (AF) demagnetized state andsubsequent direct current (dc) demagnetization were car-ried out for the single grains. A high sensitivity alter-nating gradient magnetometer (AGM) with a maximumfield of 1 T was used, allowing for the measurementof weak signals (sensitivity—1× 10−11 A m2). Typicalvalues for magnetic moment of the spherules were inthe range 2× 10−9 to 20× 10−6 A m2. The same grainsfrom the LA sample (34 grains) and a sub-set from theFA samples (15 grains) were observed using a scanningelectron microscopy (SEM) fitted with EDX facilities(LEO 1450VP with INCA system, Oxford Instruments).EDX analysis data are normalized to 100% assuminganhydrous material. Thermomagnetic analysis of mag-netic susceptibilityK(T) for bulk ash material (heatingin air; heating rate of 11◦C/min) and temperature depen-dence of saturation magnetizationMs(T) (vibrating sam-ple magnetometer (VSM) with saturation field of 1 T,heating rate 6◦C/min, in vacuum) were used for mag-netic phase identification. Determination of the Curietemperatures fromK(T) curves was done using the inflec-tion point on the first derivative of susceptibility withtemperature. Curie temperatures from theMs(T) curveswere determined using the intersection point of the linearsegments interpolating the thermomagnetic curve closeto the ferro-paramagnetic transition.

3. Experimental results

lesment

ge oashrder

ndavior

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heatingK(T) run for Kremikovtzi LA sample (Fig. 1a)shows a kink at 380◦C together with a “magnetite” Curietemperature (Tc) of 586◦C. High-temperature depen-dence of saturation magnetizationMs(T) for the FAsfrom Russe and Maritza East 1 power plants (Fig. 1b andc) reveal one majorTc of around 540–556◦C (Table 1).Bulk FA from Maritza East 1 power plant shows a smallcontribution from a phase withTc of 670◦C (see theinset with an enlarged high-T part of the heating run) anda kink at 150◦C. Both samples are characterized by anincrease ofMs at room temperature after the heating run,while the major “magnetite”Tc is preserved. Lagoonedash from Kremikovtzi (Fig. 1a) is characterized byinflection points at 360 and 520◦C on theMs(T) curves.A general observation from all thermomagnetic runs(Fig. 1) is thatMs(T) gives lower estimations (of about20◦C) of the inflection points (Curie temperaturesor structural effects), in comparison with the onesobtained from the minimum on the first derivative oftheK(T) curves. In addition, low-temperatureK(T) runson the FA samples (Fig. 2) did not show a Verweytransition.

The Day diagram (Day et al., 1977) is used to depictthe data from the hysteresis measurements.Fig. 3a showsthe distribution of the data points for the single par-ticles from different waste materials. Experimentallyobserved systematic log–linear distribution of the datafor LA from Kremikovtzi, spanning MD-PSD regions(Fig. 3a) is well separated from the FA data points

s

Magnetic susceptibility of the FA and LA sampwas measured in order to estimate the relative enrichof the material with strongly magnetic minerals.Table 1demonstrates that the fly ashes possess similar ranmass specific susceptibilities, while the lagoonedfrom the metal works is characterized by almost an oof magnitude higher signal.

Magnetic mineralogy of the bulk fly ashes alagooned ash is deduced from the thermal behand Curie temperatures identified throughK(T) andMs(T) analyses, shown inFig. 1 and summarizedthe last column ofTable 1. The presence of reversibheating–cooling cycle on the susceptibility curvespreservation of the same features on both curvesgests no chemical alteration.

The major phase in the fly ashes from the power pis characterized by Curie temperatures in the inte570–586◦C, as deduced from the temperature dedence of susceptibility (Fig. 1, right column). At thesame timeK(T) analysis reveals a second feature at lotemperatures of about 500–512◦C. Thermomagnet

f

Fig. 2. Low temperature thermomagneticK(T) analysis for FA samplefrom Devnja (a) and Maritza East 1 (b) power plants.


Fig. 3. (a) Day diagram for the LA from Kremikovtzi and theFAs from Russe, Devnja and Maritza East 1 power plants; (b)“squareness–coercivity plot” ofTauxe et al. (2002). Stars representthe corresponding data points for bulk magnetic extracts of the sameashes.

(Russe, Devnja and Maritza East 1). The FA samplesdisplay “harder” magnetic behavior.Fig. 3b presentingthe so-called “squareness–coercivity plot” ofTauxe etal. (2002), reveals strong discrimination according to thetype of waste (fly ashes from power plants and from blastfurnaces in metal works) based on coercive forceBc val-ues. Fly ashes are characterized by coercivities generallyhigher than 10 mT, although the physical grain sizes ofthe spherules are in the same range both for measuredFA and LA.

Calculation of the magnetization values per unit vol-ume (Ms, saturation remanenceMrs) was estimated usingthe particles’ diameters obtained from the SEM obser-vations. It was not possible to determine mass of an indi-vidual spherule with a mean diameter of about hundredmicrons using the balance available in the laboratory.Thus, we made a rough supposition that the physical vol-

Fig. 4. (a) Grain size dependence of saturation magnetizationMs; (b)Mrs vs.Ms plot; (c) Bc vs.Mrs plot.

ume of the particle is equal to the volume of the magneticphase. The results presented inFig. 4a for the collectionof single grains show the obtainedMs values as a func-tion of grains’ diameter. Comparing the calculated valueswith that of magnetite (480 kA/m), quite low saturationmagnetization values are obtained for part of the grains.TheMs values for single spherules from the FA samplesdisplay a much higher degree of scatter as a function of


Fig. 5. Grain size dependence of coercivitiesBc andBcr (a); saturation remanenceMrs (b); the ratiosBcr/Bc andMrs/Ms (c). Regression lines arecalculated for LA points and only statistically significant correlations are shown.

grain diameter. No systematic relation betweenMs andMrs is revealed (Fig. 4b). On the other hand, quite goodlinear dependence is observed between structure sensi-tive parametersMrs andBc for the LA grains, but not forthe FA grains (Fig. 4c). Looking for further relationshipbetween physical size of the spherules and their mag-netic characteristics, we chose to discard the particlesfor which we obtained anomalously lowMs estimation(seeFig. 4a). Thus, the corresponding plots of magnetichysteresis parameters:Bc, coercivity of remanenceBcr,Mrs and the ratiosMrs/Ms and Bcr/Bc as a function ofgrain diameter for 32 single grains from LA are pre-sented inFig. 5. The strongest grain size dependence isseen for theBc andMrs parameters, while the others areless size-dependent.

The grains’ morphology and quantitative chemicalcomposition deduced from the SEM/EDX observationshelped to reveal the role of several different factorswhich influence the magnetic behavior of anthropogenic

spherules.Fig. 6 shows typical examples of the vari-eties of grains’ morphology found for the particles fromRusse power plant and Kremikovtzi industrial complex.SEM microphotographs (back scattered electron mode)reveal particular surface morphology of the spherules(smaller grains attached to the surface, porous structures,holes) which is due to the high-temperature processes inwhich the ashes are formed. EDX spectra show that bothpure Fe-containing particles and complex mixture ofalumino-silicates and Fe-bearing compounds are present(Fig. 6).

4. Discussion

4.1. Bulk low-field magnetic susceptibility and itsfrequency dependence

The obtained values of the mass-specific low-fieldmagnetic susceptibility (χ) of the ashes FA and LA, fall


Fig. 6. Microphotographs and EDX spectra of single spherules from: (a) Russe power plant—weakly magnetic porous Si–Fe–C particle,d = 227�m;(b) Russe power plant—strongly magnetic Fe spherule with adhered smaller particles on the surface,d = 187�m; (c) Kremikovtzi LA—the biggestspherule studied (diameter of 567�m). White dots indicate the points of EDX analysis.

in the range of values reported in other studies of fly ashesfrom coal-burning power plants (Dekkers and Pietersen,1992; Kapicka et al., 2000; Veneva et al., 2004). An esti-mation of the proportion of magnetite-like phase basedon the measured mass-specific magnetic susceptibilityof the bulk ashes and reported typical values for mag-netite (Hunt et al., 1995), shows some 2.2–4.0% Fe3O4,while the corresponding estimation for the LA gives42.5% Fe3O4. Using measuredMs at room tempera-ture from theMs(T) analysis (Fig. 1), the estimation ofthe proportion of magnetite is lower – 1.6–2.7% for FA,and 26% for LA. This difference may be due to sev-eral reasons. One major uncertainty is the susceptibilityvalue of magnetite. The range of reported values (Hunt etal., 1995) is very broad (20–110× 10−5 m3/kg), so theaverage value taken may not be the best. On the otherhand, the reported values for magnetite percentage inDekkers and Pietersen (1992)for FAs with similar bulksusceptibilities are systematically higher. The latter canbe explained by the fact that in our case we have anotherphase with substitutions which lowersMs. Another pos-

sibility is that the discrepancy is due to the presence ofultra fine grained SP particles in our samples, increasingthe measured susceptibility. Frequency dependent sus-ceptibilities of 1.3–1.9% (Table 1) suggest about 10%SP fraction according to the semi-quantitative modelof Dearing et al. (1997). The information presented inTable 1suggests that the major factor which plays role forthe magnetic enhancement of fly ashes is the type of coalsused. Fly ashes from the Russe and Devnja power plantswhich both utilize Ukrainian bituminous and anthraciticcoals, show relatively similarχ-values, while FA sam-ples from Maritza East 1, using lignite coals exhibitsignificantly lower susceptibility. On the other hand, LAfrom Kremikovtzi is characterized by an order of mag-nitude higherχ-value, which is to be expected for wastefrom high-temperature blast furnaces. Here we obtaineda better correlation between the estimated percentageof Fe3O4 from Ms(T) and susceptibility measurements;26.0 and 26.8%, respectively. The lagooned ash did notcontain SP fraction, as the frequency dependent suscep-tibility was zero (Table 1).


4.2. Discrepancies between the Curie temperaturesobtained through temperature dependence oflow-field magnetic susceptibility and saturationmagnetization

As pointed out in previous studies (Ryall and Hall,1980; Moskowitz, 1981, 1987; Petrovsky, 2003), use ofdifferent methods for estimation of Curie temperaturesmay lead to significant discrepancies in the obtainedvalues and difficulties for inter-laboratory comparisons.Moreover, identification of Curie temperatures by mea-suring different magnetic characteristics (e.g. magneticsusceptibility and saturation magnetization) may intro-duce additional complications because of the differentphysical processes (Belov, 1959). The definition of theCurie point (Tc) from molecular field theory is the tem-perature at which long-range alignment of magneticmoments disappears, e.g.Ms is approaching zero. Thus,Tc in case ofMs(T) analysis should be determined asthe point of intersection of the tangent toM in thevicinity of Tc and temperature axis (O’Reilly, 1984;Dunlop andOzdemir, 1997). In case of using susceptibil-ity temperature dependence forTc estimation, accordingto the Curie–Weiss law [χp = C/(T − Tc)], aboveTc weshould have only paramagneticM–H linear dependence(O’Reilly, 1984). In practice, however, looking for theonset to a linearM–H dependence of 1/χ = f(T) is com-plicated because of the high level of noise aboveTc,which makes the interpolation difficult. Another prob-lem arises when the material under investigation contains

tak-

heticylyeasetive

ints

aon-

y, a

dropeak,scep-tureag-

FAs from the power plants show two inflection pointson the heatingK(T) curves—(500–512◦C) and the finalloss at (570–586◦C). As far as the peculiarity at about510◦C is not identified onMs(T) curve (Fig. 1b–d),it represents structural and/or grain size effect, whichis detected through temperature dependence of sus-ceptibility, or it is a real phase of thermally unstablesubstituted spinels, which exsolve during heatingexperiment (Dekkers and Pietersen, 1992; Vassilev,1992). Exploring these two possibilities, one should alsotake into account the sensitivity ofK(T) dependence tocation ordering of the spinels (O’Reilly, 1984; Harisonand Putnis, 1999b). Especially in quenched materials,such as the FA samples, where a highly disordered stateis preserved after quenching, and the exact degree oforder/disorder depends very much on the quench rate(Harison and Putnis, 1999b). During laboratory heatingin the Kappabridge, ordering process may occur, whichresults in the observed peculiarity at about 510◦C,disappearing on cooling (Fig. 1). The finalTc indicatesmagnetite presence. The absence of Verwey transition(Fig. 2) on low-temperatureK(T) curves suggests highdegree of non-stoichiometry (Aragon et al., 1985;Ozdemir et al., 1993; Muxworthy and McClelland,2000).

Variations of saturation magnetizationMs with tem-perature for FA and LA samples (Fig. 1) exhibit asimpler temperature dependence. The observed system-atically lower final Tc’s in comparison with the onesfrom K(T) experiments (Table 1) suggests a common

thatt in

rtyer-ring

unsausetheac--ors

gherse

n-

we

ng)xso-

two or more ferromagnetic phases with differentTc’s.Thus, during heating above the phase with the lowesTc,still other phases contribute to the total signal and ming it difficult to identify the firstTc. Another approacis commonly used; according to theory ferromagnsusceptibilityχ ∼ Ms(T)/k(T) (k, magnetic anisotropconstant), and aboveTc the susceptibility drops rapidaccording to the Curie–Weiss law. This sharp decris best expressed on the curve of the first derivadχ/dT. Another graphical determination ofTc is doneby extrapolating (using a straight line to the data poimmediately surrounding the point of inflection) theχ–Tcurve to theχ axis (Harison and Putnis, 1996, 1999).From the point of view of physical processes respsible for the temperature variations of susceptibilitmore correct way to determineTc is the “first derivativeapproach”. However, in cases with a very sharpof susceptibility after a well-expressed Hopkinson pboth methods give the same estimate. In addition sutibility may also underestimate the Curie temperadue to low unblocking temperatures of superparamnetic/SD grains.

underlying reason for such behavior. Consideringthe calibration of the temperature scale is correcboth instruments, then the obtained discrepancy inTc’sof about 40◦C should be due to materials’ propeor different experimental conditions. The main diffence in the two experimental setups is the measuatmosphere. Heating in air during susceptibility rresulted in some high-temperature oxidation, becof the changed color from grey-black to reddish atend of the cycle. On the other hand, heating in vuum (8× 10−2 Pa) duringMs(T) runs prevents oxidation, but the almost twice as slow heating rate favgreater degree of cation order. Consequently, hiTc’s in comparison withK(T) experiments and increain Ms after cooling should be observed (Harison andPutnis, 1999b). This supposition is correct if we cosider theTc’s of 520–556◦C identified onMs(T) curvesto correspond to the inflection points at 500–512◦Con K(T) runs. From this point of argumentation,could suppose that the magnetiteTc’s on K(T) curvesis due to a newly formed (during laboratory heatimore Fe-rich phase as a result of high-temperature e


lution during heating of the original substituted Fe-oxides.

Magnetic mineralogy of the lagooned ash fromKremikovtzi reflects the influence of the secondary alter-ations of the material as a result of its transport, sep-aration during discharge, storage and weathering. Theidentified inflection point at 360◦C onMs(T) and 380◦ConK(T) heating curves, which disappears on the coolingsuggests that it represents maghemite inversion temper-ature. Maghemite is a common product of low temper-ature oxidation (O’Reilly, 1984; Dunlop andOzdemir,1997), which probably took place during the LA samplesstorage in the depository.

Hematite is only identified for the fly ash from Mar-itza East 1 power plant (see the enlarged scale in the insetgraph inFig. 1b). This sample differs from the other twoFA samples in that it is a composite from all electrostaticfilters from a dry discharge boiler. Thus, one could sup-pose that coarser grains which are more abundant containalso more hematite particles, or the technological processof dry discharge of the fuel favors increased oxidationduring burning. The low-temperature inflection point at152◦C may be either an effect of dehydration, or sec-ondary (formed after the emission of the ash from thechimney) Fe-oxyhydroxide.

4.3. Grain-size dependence of hysteresisparameters and ratios

The estimation ofMs using volume estimation deter-m wasu ag-n s inM etite( irstlyi s areh eticp nch-i e ofs lidsv uilib-rT na

s es;o er,f lin-e ev-ea e

so-called “squareness–coercivity plot” ofTauxe et al.(2002) (Fig. 3b) supports the above supposition aboutthe role of stress according to their modeling experi-ments. The data for the single spherules from the powerplants plot in the area predicted for assemblages of uni-axial SD grains with increasing length/width ratio and aSP component (Tauxe et al., 2002). On the other hand,the same behavior is observed for exsolved structureswithin the glass matrix, which is the common way ofoccurrence of the magnetic phase in fly ashes (Sokolet al., 2002). Such magnetic “superstates”, which actcollectively like much larger magnetized regions due tointeractions, are observed experimentally in magnetite-ulvospinel exsolutions (Harison et al., 2002). The highervalues of the ratioMrs/Ms andBc obtained for FA par-ticles (Fig. 3b) are probably due to the fact that the FAparticles experienced faster cooling, and had a relativelyhigher concentration of Al–Si within them giving rise toa fine crystallized magnetic phase (Norton et al., 1986;Vassilev, 1992). The elemental composition data for thespherules and the presence of inhomogeneous structuresas holes, smaller grains, etc. (seeFig. 6), suggest theimportance of microstructures. Very strongly magneticFe-rich particles and grains with significant Ca, Mg andMn content are identified. Similar composition and mor-phology of the fly ash particles are reported in a numberof studies (Gomes et al., 1999; Sokol et al., 2002; Kukieret al., 2003). The exact elemental composition obviouslydepends on various factors like the initial coal’s compo-sition, burning conditions, etc. In our particular case,

ba-froms of

llyomenceemd bypari-and

,the

coer-ncedata,-the

,ear.89

ined from SEM images, although controversial,sed as the only way to obtain magnetization from metic moments measured. The significant variations for some spherules compared to that of magn

Fig. 4a), may be due to several possible reasons. Ft may be an incorrect supposition that the spheruleomogeneous (they may be hollow), or the magnhase present is not stoichiometric magnetite. Que

ng from high temperature may cause the presencignificant disorder of the cation distribution in soolutions, which causes also systematically lowerMsalues as compared to the calculated ones for eqium cation distribution (Harison and Putnis, 1999b).he absence of a single linear relationship betweeMsndMrs (Fig. 4b) favors such explanation as well.

The correlation plot ofMrs/Ms versusBcr/Bc (Fig. 3a)hows the presence of two different correlation linne very well defined for LA grains and the oth

or the FAs from the power plants. This particularar distribution reveals the role of the different lls of internal stress within the spherules (Dunlopnd Ozdemir, 1997). The discriminative power of th

the strong discrimination between LA and FA is probly due to the fact that the FA samples are wastescoal burning, while LA comes from the blast furnacemetal works.

Taking into account the origin and the typicavery fast cooling (quenching) of the spherules frhigh temperatures, it is logical to suppose the presof significant stress within the grains. This put thcloser to the magnetic state of sized grains preparecrushing and glass–ceramic method. Thus, a comson with established size relationships for crushedglass–ceramic magnetite grains (Dunlop andOzdemir1997) is considered. Grain size dependences ofmost structure-sensitive hysteresis parameters –cive forceBc and the normalized saturation remaneMrs/Ms – are stronger, as compared to the publishedvarying asd−0.78 andd−0.74 (Fig. 5a and c). This similarity in the two power laws is in accordance withtheoretical relation betweenMrs andBc for MD grains,e.g.Mrs∼ Bc/N (Chikazumi, 1983; Dunlop andOzdemir1997). It is further proved by the obtained good linMrs–Bc correlation with regression coefficient of 0


(Fig. 4c). The estimatedN value from the above rela-tionship is 0.37, which is quite close to the theoreticaldemagnetizing factor (N = 1/3) for uniformly magnetizedsphere (Dunlop andOzdemir, 1997). If we consider thatstronger dependence ofd−0.9 is observed for unnormal-izedMrs values (Fig. 5b) and there is no uniqueMs–Mrscorrelation line (Fig. 3b), it follows that the main control-ling factor for the observed “harder” magnetic propertiesof the spherules compared to crushed magnetites is theoccurrence of the magnetic phase within the spherules,namely dendritic and skeletal structures within the alu-minosilicate matrix. These three-dimensional structures,give rise to complicated magnetic signatures depen-dent on size, internal stress, shape and composition.Similar conclusion has been drawn from detailed mag-netic, microchemical and microscopic investigation onanthropogenic particles from polluted sediments fromthe Danube river (Jordanova et al., 2004). The obtainedstronger size dependence of the hysteresis parametersin comparison with magnetite data (Day et al., 1977;Worm and Markert, 1987) are in better agreement withthe Bc andMrs size dependence (d−1) of crushed syn-thetic titanomagnetites withx = 0.2, 0.4 and 0.6 byDay etal. (1977). This supports the hypothesis that the magneticphases in the studied FA samples and LA are substituted.

4.4. Implications for use of single grain magneticmeasurements for source discrimination

In any experimental study which deals with a limitedsiblethe

y, weresisruleseentsby

itialeticore

f thehro-eraleticos-bulk

mentionsti-ls. Inom

a metal works is characterized by high magnetizationwith very low coercivities.

Acknowledgements

This research has been supported by a MarieCurie Fellowship of the European Community pro-gram “Improving Human Potential” under contract no.HPMF-CT-2000-01084 and a Marie Curie Reintegra-tion grant “MAPHOPE” (contract no. MERG-CT-2004-508786). The authors thank Dr. M. Funaki from NIPR(Tokyo) for using the laboratory facilities and his kindcooperation. The very helpful and constructive com-ments by M. Dekkers and English improvement andsuggestions by A. Muxworthy are highly appreciated.

References

Aragon, R., Buttrey, D., Shepherd, J., Honig, J., 1985. Influence of non-stoichiometry on the Verwey transition. Phys. Rev. B 31, 430–436.

Belov, K., 1959. Magnetic Transitions. Publ. House of Physics andMathematics, Moskva, p. 259 (in Russian).

Chikazumi, S., 1983. Physics of Ferromagnetism, vol. 1: MagneticProperties of the Matter. Mir, Moskva, p. 302 (in Russian).

Dearing, J., Bird, P., Dann, R., Benjamin, S., 1997. Secondary ferri-magnetic minerals in Welsh soils: a comparison of mineral mag-netic detection methods and implications for mineral formation.Geophys. J. Int. 130, 727–736.

Day, R., Fuller, M.D., Shmidt, V.A., 1977. Hysteresis properties oftitanomagnetites: grain size and composition dependence. Phys.

a flytially47.tion

and

sier,lico-pti-. 29,

em-88,

, F.,namys.

etite-lities,

m of

hem.

number of single objects considered to be responfor the bulk/macroscopic behavior, the question ofdegree of representativeness arises. In this studexamine this issue by comparing magnetic hysteparameters from bulk samples and individual sphefrom these same samples (stars inFig. 3a and b). Thdata points which correspond to the bulk measuremfall approximately in the middle of the area occupiedthe single grain data. This agrees well with the inhypothesis that the main contribution to the magnsignal comes from the bigger spherules, in which mcrystalline magnetic phases occur. On the basis odata from our single grains, the possible origin of antpogenic pollution in an area under influence of sevdifferent local sources can be identified; the magnfraction of fly ashes from coal-burning power plants psess high magnetic stability and thus, the measuredparameters along contaminated soil profile or sedicore will show “harder” magnetic behavior. In additlowerTc’s will also be found, corresponding to a subtuted magnetic phase in the uppermost polluted levecontrast, the bulk magnetic properties of pollution fr

Earth Planet. Interiors 13, 260–267.Dekkers, M., Pietersen, H.S., 1992. Magnetic properties of low-C

ash: a rapid tool for Fe-assessment and a survey for potenhazardous elements. Mater. Res. Soc. Symp. Proc. 245, 37–

Douchanov, D., 2002. Environmental problems and control of polluin iron industry. J. Environ. Protect. Ecol. 3 (1), 92–100.

Dunlop, D.,Ozdemir,O., 1997. Rock Magnetism. FundamentalsFrontiers. Cambridge University Press, p. 573.

Gomes, S., Francois, M., Abdelmoula, M., Refait, Ph., PellisC., Evrard, O., 1999. Characterization of magnetite in sialuminous fly ash by SEM, TEM, XRD, magnetic suscebility and Mossbauer spectroscopy. Cement Concrete Res1705–1711.

Giere, R., Carleton, L., Lumpkin, G., 2003. Micro- and nanochistry of fly ash from a coal-fired power plant. Am. Mineral.1853–1865.

Goddu, Srinivasa Rao, Appel, E., Jordanova, D., Wehland2004. Magnetic properties of road dust from Visakhapat(India)—relationship to industrial pollution and road traffic. PhChem. Earth 29, 985–995.

Harison, R., Putnis, A., 1996. Magnetic properties of the magnspinel solid solution: Curie temperatures, magnetic susceptibiand cation ordering. Am. Mineral. 81, 375–384.

Harison, R., Putnis, A., 1999a. Determination of the mechaniscation ordering in magnesioferrite (MgFe2O4) from the time- andtemperature-dependence of magnetic susceptibility. Phys. CMiner. 26, 322–332.


Harison, R., Putnis, A., 1999b. The magnetic properties and crys-tal chemistry of oxide spinel solid solutions. Surv. Geophys. 19,461–520.

Harison, R., Dunin-Borkowski, R., Putnis, A., 2002. Direct imagingof nanoscale magnetic interactions in minerals. PNAS 99 (26),16556–16561, doi:10.1073/pnas.262514499.

Hock, J., Lichtman, D., 1982. Studies of surface layers on single par-ticles of in-stack coal fly ash. Environ. Sci. Technol. 16, 423–427.

Hower, J.C., Rathbone, R.F., David Robertson, J., Peterson, G., Trim-ble, A.S., 1999. Petrology, mineralogy, and chemistry of magneti-cally separated sized fly ash. Fuel 78, 197–203.

Hunt, P., Moskowitz, B., Banerjee, S., 1995. Magnetic properties ofrocks and minerals. Rock Physics and Phase Relations. A Hand-book of Physical Constants, AGU Reference Shelf 3, pp. 189–204.

Jordanova, D., Hoffmann, V., Fehr, Th., 2004. Mineral magneticcharacterization of anthropogenic magnetic phases in the Danuberiver sediments (Bulgarian part). Earth Planet. Sci. Lett. 221,71–89.

Kapicka, A., Jordanova, N., Petrovsky, E., Ustjak, S., 2000. Magneticstability of power plant fly ash in different soil solutions. Phys.Chem. Earth A 25 (5), 431–436.

Kukier, U., Ishak, Ch., Sumner, M., Miller, W., 2003. Composition andelement solubility of magnetic and non-magnetic fly ash fractions.Environ. Pollut. 123, 255–266.

Matsunage, T., Kim, J.K., Hardcastle, S., Rohatgi, P.K., 2002. Crys-tallinity and selected properties of fly ash particles. Mater. Sci. Eng.A 325, 333–343.

McLennan, A.R., Bryant, G.W., Stanmore, B.R., Wall, T.F., 2000. Ashformation mechanism during pf combustion in reducing condi-tions. Energy Fuels 14, 150–159.

Moskowitz, B., 1981. Methods for estimating Curie temperatures oftitanomaghemites from experimentalJs–T data. Earth Planet. Sci.Lett. 53, 84–88.

Moskowitz, B., 1987. Towards resolving the inconsistencies in char-acteristic physical properties of synthetic titanomaghemites. Phys.

M w-netic

Norton, G., Markuszewski, R., Shanks, H., 1986. Morphological andchemical characterization of iron-rich fly ash fractions. Environ.Sci. Technol. 20, 409–413.

O’Reilly, W., 1984. Rock and Mineral Magnetism. Blakie. Chapmanand Hall, Glasgow, p. 219.

Ozdemir, O., Dunlop, D.J., Moskowitz, B.M., 1993. The effect of oxi-dation on the Verwey transition in magnetite. Geophys. Res. Lett.20, 1671–1674.

Petrovsky, E., 2003. Temperature dependence of magnetic susceptibil-ity and determination of Curie (Neel) temperature: are we correct ?IUGG 2003. Abstract book, Abstract GAI.03/03P/A14-001, Sap-poro, Japan.

Ryall, P., Hall, M., 1980. Iron loss in titanomagnetite during low tem-perature oxidation. J. Geomagn. Geoelectr. 32, 661–669.

Sokol, E.V., Kalugin, V., Nigmatulina, E., Volkova, N., Frenkel, A.,Maksimova, N., 2002. Ferrospheres from fly ashes of Chelyabinskcoals: chemical composition, morphology and formation condi-tions. Fuel 81, 867–876.

Tauxe, L., Bertram, H., Seberino, Ch., 2002. Physical interpretationof hysteresis loops: micromagnetic modeling of fine particle mag-netite. Geochem. Geophys. Geosyst. 3 (10), 1055.

Tipse, S.S., Schoenitz, M., Dreizin, E.L., 2002. Morphology and com-position of the fly ash particles produced in incineration of munic-ipal solid waste. Fuel Proc. Technol. 75, 173–184.

Vassilev, St., 1992. Phase mineralogy studies of solid waste productsfrom coal burning at some Bulgarian thermoelectric power plants.Fuel 71, 625–633.

Vassilev, St., Vassileva, Ch., 1996. Mineralogy of combustion wastesfrom coal-fired power stations. Fuel Proc. Technol. 47, 261–280.

Vassilev, St., Vassileva, Ch., 1997. Geochemistry of coals, coal ashesand combustion wastes from coal-fired power stations. Fuel Proc.Technol. 51, 19–45.

Veneva, L., Hoffmann, V., Jordanova, D., Jordanova, N., Fehr, Th.,2004. Rock magnetic, mineralogical and microstructural charac-terization of fly ashes from Bulgarian power plants and the nearby

s ofhys.

Earth Planet. Interiors 46, 173–183.uxworthy, A.R., McClelland, E., 2000. Review of the lo

temperature magnetic properties of magnetite from a rock magperspective. Geophys. J. Int. 140, 101–114.

anthropogenic soils. Phys. Chem. Earth 29, 1011–1023.Worm, H.-U., Markert, H., 1987. Magnetic hysteresis propertie

fine particle titanomagnetites precipitated in a silicate matrix. PEarth Planet. Interiors 46, 84–92.

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