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Distribution and mode of occurrence of uranium in bottom ashderived from high-germanium coals

Yinglong Sun1, Guangxia Qi1,2, Xuefei Lei1, Hui Xu1, Lei Li1, Chao Yuan1, Yi Wang1,⁎

1. Department of Environmental Engineering, School of Environment, Tsinghua University, Beijing 100084, China.E–mail: [email protected]. Department of Environmental Science and Engineering, School of Food and Chemical Engineering,Beijing Technology and Business University, Beijing 100048, China

A R T I C L E I N F O

⁎ Corresponding author. E-mail: yi_wang@tsi

http://dx.doi.org/10.1016/j.jes.2015.07.0091001-0742/© 2015 The Research Center for Ec

A B S T R A C T

Article history:Received 25 May 2015Revised 16 July 2015Accepted 17 July 2015Available online 26 September 2015

The radioactivity of uranium in radioactive coal bottom ash (CBA) may be a potential dangerto the ambient environment and human health. Concerning the limited research on thedistribution andmode of occurrence of uranium in CBA, we herein report our investigationsinto this topic using a number of techniques including a five-step Tessier sequentialextraction, hydrogen fluoride (HF) leaching, Siroquant (Rietveld) quantification, magneticseparation, and electron probe microanalysis (EPMA). The Tessier sequential extractionshowed that the uranium in the residual and Fe–Mn oxide fractions was dominant (59.1%and 34.9%, respectively). The former was mainly incorporated into aluminosilicates,retained with glass and cristobalite, whereas the latter was especially enriched in themagnetic fraction, of which about 50% was present with magnetite (Fe3O4) and the rest inother iron oxides. In addition, the uranium in the magnetic fraction was 2.6 times that inthe non-magnetic fraction. The experimental findings in this work may be important forestablishing an effective strategy to reduce radioactivity from CBA for the protection of ourlocal environment.© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.

Published by Elsevier B.V.

Keywords:Coal bottom ashUraniumRadioactivityTessier sequential extractionMagnetic separation

Introduction

Lignite is often used as fuel for many small to middle scalepitheadpower plants, and in some cases, as a raw resource for Gesmelters (Papastefanou, 2010; Dai et al., 2014c). However, inLincang, Yunnan province, China, the average radioactivity ofuranium (U) in lignite can reach 87.1 Bq/kg, much higher thanthat of the other types of coal (such as low-rank coals,middle-rank coals and high-rank coals) (Xiong et al., 2007; Yu,2007). After burning, the natural radioactivity level of coalcombustion ash is 4–10 times higher than that of the feed coals(Bhangare et al., 2014; Tripathi et al., 2013), which may beextremely dangerous for the surrounding environment and

nghua.edu.cn (Yi Wang).

o-Environmental Science

human health. For example, the enrichment and transformationof radionuclides in coal fly ash and bottom ash has alreadycaused secondary pollution, and has negatively impacted thelocal environment and humanhealth in Yunnan province, China(Yu, 2007).

Some late Permian coals are highly enriched in uranium (Daiet al., 2008, 2013a, 2013b, 2015a). Unfortunately, the radioactivityof their combustion residue (e.g., bottom ash) has not yet beenstudied in a great detail. In comparison with the abundant use offly ash in construction materials (Dai et al., 2012; Camilleri et al.,2006; Eze et al., 2013; Lima et al., 2012), the coal bottom ash (CBA)is still stocked in piles close to coal fields, and could generatenegative impacts on the surrounding environment including air,

s, Chinese Academy of Sciences. Published by Elsevier B.V.

92 J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S 4 3 ( 2 0 1 6 ) 9 1 – 9 8

soil, ground water and human health (Bartoňová and Klika, 2014;Lanzerstorfer, 2015; Liu et al., 2011). Therefore, it is essential toprocess the relatively highly radioactive CBA prior to itsreutilization and to minimize its negative influence on theambient environment.

As there are many uranium-rich coals in China and othercountries, reducing radioactivity is becoming important. Themain method to extract uranium from coal bottom ash is acidleaching. The experimental results of Paul and Seferinoğluindicated that nearly 80% of uranium in coal ashes was leachedwith sulfuric acid after 14 days (Seferinoğlu et al., 2003; Paul et al.,2006), due to its predominant occurrence in uranium-organiccompounds in the original coal. El-Hamid et al. (2014) reportedthat more than 97.1% leaching of the uranium in petroleum ashcould be achieved, using a high sulfuric acid concentration(200 g/L) with 6% vol.% MnO2 oxidant and 6 hr of agitation.However, direct acid leaching of uranium from many other coalbottom ashes is difficult. Lei et al. (2014) were only able to leachless than 20% of the uranium from their samples. Zielinski et al.(2007) compared the leaching conditions of uranium and arsenicin coal ash and found that leaching of arsenic with a carbonatebuffer solution was rapid and efficient (the leaching rate was49%). In contrast, U barely leached (7%) in 2 weeks. Mostexplanations for the low leaching efficiency of uranium in CBAinvolve the relative insolubility of uranium residing in particleswithina glassymatrix (Zielinski et al., 2007; Zielinski andBudahn,1998).

Thus, extraction of uranium from coal ashes greatly differswith coals and regions, but a uniform standard extractionmethod has not been developed for uranium-rich bottom ash(Zhang et al., 2008). Different uranium extraction methods,which depend on the combustion conditions (e.g., combustiontemperatures, categories of raw coal, furnace types) andmodesof occurrence of uranium in raw coals, would lead to differentleaching efficiencies. Therefore, to effectively extract uraniumand reduce radioactivity fromCBA, the distribution andmodeofoccurrence of uranium in bottom ash must be known.

The purpose of this work is to investigate the distributionand mode of occurrence of high uranium bottom ash. Thesamples were supplied from Lincang, southwestern China, anda number of extraction and analytical techniques were utilizedincluding a five-step Tessier sequential extraction, hydrogenfluoride (HF) leaching, Siroquant (Rietveld) quantification,magnetic separation, and electron probe microanalysis(EPMA). The experimental findings in this work are not onlyimportant for understanding the distribution of uranium inbottom ash, but also for establishing an effective strategy toreduce radioactivity in CBA and protect the local environment.

1. Materials and methods

1.1. Samples and reagents

The CBA samples were obtained from two different germani-um (Ge) smelters in Lincang, Yunnan Province, China (sam-ples no. 1 and no. 2). The samples were crushed with a ballmill, and then passed through a 500-mesh standard sieve(<25 μm in diameter). The fine powder samples were dried at105°C in a forced air oven to constant weight and stored in a

desiccator until further use. The CBA was characterized as auranium-rich (374 ppm) material with low-level radioactivity(gross alpha decay (α) of 3.08 Bq/g, and gross beta decay (β) of11.83 Bq/g). The two samples had no significant differences interms of components and characteristics, therefore, they werecombined together for further investigation.

1.2. Tessier sequential extraction of uranium in the coalbottom ash

The combined samples were analyzed below. Tessier sequentialextraction procedures were used to fractionate the uranium inthe CBA into five components: exchangeable, bound tocarbonates, bound to iron and manganese oxides, bound toorganicmatter, and remaining in residue (Tessier et al., 1979).The experimental procedures were analogous to thosedescribed in the literature (Wan et al., 2006; Smeda andZyrnicki, 2002; Landsberger et al., 1995; Bódog et al., 1996).Briefly, (1) the CBA was extracted at room temperature for3 hr with a sodium acetate solution (1 mol/L CH3COONa,pH 8.2) under continuous agitation; (2) the residue from (1)was leached at 50°C with a 1 mol/L sodium acetate solutionadjusted to pH 5.0 with acetic acid (CH3COOH). Continuousagitation wasmaintained for 5 hr; (3) the residue from (2) wasextracted with a 0.04 mol/L NH2OH–HCl solution in 25% (V/V)acetic acid. The extraction occurred at 60°C under continuousagitation for 8 hr; (4) a solution of 0.02 mol/L HNO3 and 30%H2O2 adjusted to pH 2 with HNO3 was added to the residuefrom step (3), and the mixture was heated at 85°C for 2 hrunder continuous agitation. NH4Ac was then added and thesample was heated again to 65°C for 6 hr under continuousagitation; and (5) the residue from (4) was digested with amixture of HF, HNO3 and HClO4 for total metal analysis. Eachstep was repeated four times, and the leachate was collectedseparately to measure the concentration of uranium andother major metals.

1.3. HF leaching of the coal bottom ash residue

A total of 5 g of the residual Tessier fraction was placed into a150 mLTeflon beaker, and 80 mLof anHF solution (10%, 8%, 6%,4%, or 2%, V/V) was added. The resulting suspension wasmagnetically stirred at 500 r/min for 20 min at room tempera-ture, and then the slurry was centrifuged. The supernatant wascollected and the residue washed with 80 mL of distilled water.The supernatant and washing solution weremixed and dilutedup to 500 mL, and then a 10 mL-aliquot was withdrawn for auranium content analysis. The solid residue was furtherwashed twice, dried, and the weight of the residue wasrecorded. The leaching experiment was performed in duplicate.

1.4. Magnetic separation of the coal bottom ash

A total of 20 g of CBA was added to 2.0 L of distilled water, andthe slurry was stirred vigorously with a magnetic rod (3000 G).This procedure was repeated until no more magnetic fractionadhered to the magnet. Then the residual parts were furthersubjected to a wet-type high intensity magnetic separator witha magnetic field intensity of 15,000 to 20,000 G to collect theweakly magnetic fractions. The weakly magnetic, strongly

Table 1 – Elemental abundances (oxides of major elements)in the coal bottomash (CBA) fromLincang, Yunnan Province,China.

Elements Sample no. 1 (wt.%) Sample no. 2 (wt.%)

SiO2 65.7 64.1Al2O3 18.6 19.2CaO 6.92 6.83Loss on ignition 4.11 5.24Fe2O3 4.35 5.33K2O 2.38 2.54MgO 0.88 0.99TiO2 0.34 0.54Na2O 0.17 0.21MnO 0.10 0.20WO3 0.065 0.074BaO 0.09 0.08P2O5 0.096 0.055U3O8 0.04 0.04SrO 0.04 0.08Rb2O 0.031 0.054Cr2O3 0.034 0.076ZrO2 0.026 0.063GeO2 0.027 0.042ZnO 0.021 0.030NbO 0.017 0.013NiO 0.016 0.017

93J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S 4 3 ( 2 0 1 6 ) 9 1 – 9 8

magnetic, and non-magnetic fractions were oven-dried toconstant weight, and their mass was recorded. In addition, thetotal iron (TFeO, expressed as an iron oxide) and uraniumcontents in these three fractions were measured.

1.5. Analytical methods and characterization of the coalbottom ash

Chemical compositions of the coal bottom ash were deter-mined by an X-ray fluorescence spectrometer (XRF) (XRF-1800,Shimadzu Company, Japan). All of themajor elemental resultsin the ash were listed as oxides.

The CBA samples were crushed and ground with a pestleandmortar, and each powdered sample was subjected to X-raydiffraction (XRD) analysis (D8 Advance X-ray Diffractometersystem, Bruker Company, Germany) with Cu Kα radiation.

The uranium content in the CBA was determined by aninductively coupled plasma mass spectrometer (ICP-MS)(Xseries2, Thermo Scientific Company, USA). Samples foranalysis were subjected to microwave digestion with a mixtureof HNO3, HF, and HCl at a volume ratio of 3/1/1.

The quantitative composition ofmineralogical phases in thesamples was obtained with Siroquant™ software (Common-wealth Scientific and Industrial Research Organisation, Sydney,Australia), which was developed by Taylor (1991) based on theprinciples for powder X-ray diffractogram profiling establishedby Rietveld (1969). Further details related to the use of thistechnique for coal combustion products were provided byWardet al. (1999, 2001) and Dai et al. (2014b, 2015b). Metakaolin andtridymite were consistent in representing the amorphous orglassy phase in the fly ash in the Siroquant quantitativeanalysis (Ward and French, 2006). In this work, tridymite waschosen for glass interpretation.

A representative coal combustion bottom ash sample wassubjected to an electron probe micro-analyzer (EPMA) todetermine its distribution of elements (U, Al, Si, Fe, and O) inCBA. Energy dispersive spectrometer (EDS) and back scatteredelectron (BSE) analysis results were both obtained using thesame instrument as EPMA. Samples were analyzed on anEPMA analyzer (JXA-8230, JEOL Company, Japan). The accel-erating voltage was 20 kV with beam current of 10−7 A.

2. Results and discussion

2.1. Characteristics of the coal bottom ash

Analogous to most coal combustion ashes, the major compo-nents of theCBA sampleswere SiO2 andAl2O3,which accountedfor 83–84 weight percent (wt.%) of the total mass (Table 1).However, the concentration of uranium (374 mg/kg) wascomparatively high and close to that of some low-gradeuranium ores. From the XRD pattern of the CBA provided inFig. S1 of the Supplementary data, it can be seen that the solidparticles were composed mainly of amorphous aluminosilicateglass (56.9 and 56.7 wt.%) and crystalline phases includingquartz (18.7 and 19.7 wt.%), mullite (12.9 and 13.7 wt.%),K-feldspar (8.1 and 5.0 wt.%), and a trace amount of cristobalite(3.5 and 4.9 wt.%) (Table 2).

2.2. Distribution of uranium in the coal bottom ash

According to the Tessier sequential procedures, the uranium inthe CBA samples was mainly present in the Fe–Mn oxidefraction and residual fraction (94%, Table 3). The exchangeableuranium was weakly adsorbed on the surface of the CBAparticles, and the uranium bound to carbonates and organicmatter was negligible. These results are consistent with theliterature, in that uranium can be concentrated in the undis-solved aluminosilicate matrix (Smeda and Zyrnicki, 2002). Asreported by Duff et al. (2002), uranium usually remains in theresidual fraction, since it tends to occur in the primary andsecondary silicates and other stableminerals. Since the contentof Mn in CBA was less than 0.1–0.2 wt.% (see XRF data inTable 1), the uranium should be mainly bound to Fe oxides dueto its high affinity for Fe-oxide minerals (Duff et al., 2002).

Most of the uranium (59.1% ± 2.4%) in CBA remained in theresidual phase (Table 3). This fraction of uranium is consid-ered to be unavailable for acid leaching because it is mostlyentrapped in aluminosilicates in CBA and cannot be easilyleached by most chemical reagents under general conditions.Therefore, it is not substantially extracted except by dissolv-ing the aluminosilicates with HF. Hence, it is commonlyconcluded that this fraction of uranium will not chemicallyimpact the ambient environment. However, this also leads todifficulty in extracting and possibly utilizing the vast amountof uranium in the residual fraction.

The enrichment of uranium in Fe oxides has been confirmedby previous studies. Gieré et al. (2003) has demonstrated that incoal ash, the Fe-rich particles were considerably enriched inuranium, and the concentration was usually 2–3 times higherthan that of Fe-poor particles. Zielinski and Budahn (1998) alsofound that the Fe-rich particles in coal ash had generally higherconcentration of uranium than that of other particles.

Table 3 – Percentages of various forms of uranium in CBA byTessier sequential procedure in the combined sample.

Fractions Percentage (%)

Exchangeable 2.7 ± 0.1Bound to carbonates 0.5 ± 0.2Bound to Fe oxides 34.9 ± 1.4Bound to organic matter 2.6 ± 0.7Residual 59.1 ± 2.4Total 99.8 ± 3.2

CBA: coal bottom ash.

Table 2 – Crystalline phases in CBA.

Crystalline Quartz (wt.%) Mullite (wt.%) K-feldspar (wt.%) Cristobalite (wt.%) Glass (wt.%)

Sample no. 1 18.7 12.9 8.1 3.5 56.9Sample no. 2 19.7 13.7 5.0 4.9 56.7



The uranium bound to carbonates most likely exists in theform of precipitates or co-precipitates with carbonates in CBA,and is a loosely bound phase and liable to change withenvironmental conditions (Filgueiras et al., 2002).

Uranium bound to organic matter in the Tessier procedureaccounted for 2.6% of the total uranium (Table 3). Organiccomponents in CBA are typically represented by slightlychanged and coked coal particles (Vassilev and Vassileva,1996). Given that the organic matter in CBA is predominantlyunburned carbon, the uranium is mainly sorbed by electrondonating acceptor complexation reactions at the edge sites(Yakout et al., 2013).

2.3. Distribution and mode of occurrence of uranium inFe oxides

2.3.1. Distribution of uranium in different magnetic fractionsTo further investigate the distribution of uranium bound to Feoxides, the samples were weighed and the uranium contentswere determined in various magnetic fractions (Fig. 1). Theuranium content in the fraction increased from 263 to1057 mg/kg with an increase in magnetism. However, 55.1%

Strongly magnetic Weakly magnetic 1 Weakly magnetic 2 Non-magnetic

200

400

600

800

1000

1200Uranium contentDistribution of uranium

Category of fraction

Ura

nium

con

tent

(m

g/kg

)

0

10

20

30

40

50

60

Dis

trib

utio

n of

ura

nium

(%

)

Fig. 1 – Distribution of uranium in different magnetic fractionsin the combined sample.

of uranium still remained in the non-magnetic fraction due toits large mass proportion (79.3%) (Table 4). Although the massratio of the strongly magnetic fraction was only 8.2%, itcontained 23.1% of the total uranium (Table 4). On the basis ofa correlation analysis of uranium and the major chemicalcomponents in the fraction (Table S1 and Fig. S2), the uraniumcontent was found to be significantly correlated to the Fecontent (0.993 Pearson coefficient). Therefore, the uraniumwas especially enriched in Fe oxides in the CBA.

In the Tessier procedures, the mass ratio of uranium boundto Fe oxides was 34.9%, lower than that in the magneticfractions (45%), which are rich in Fe (TFeO 6%–11.8%). Thisindicates that theuraniumwouldnot be simply bound to free Feoxides, but partiallywith Fe oxides thatmight be surrounded byvitreous glass.

2.3.2. Mode of occurrence of uranium bound to Fe oxidesThe uranium fraction in the Tessier procedure is the reduciblefraction (U6+) that coexists with Fe and Mn oxides. Heavy metalions can be scavenged by Fe and Mn oxides through one or acombination of the following mechanisms: co-precipitation,adsorption, surface complex formation, ion exchange, andpenetration of the lattice (Duff et al., 2002; Ma et al., 2012). Inthe present research, the mechanism for the coexistence ofuranium with Fe–Mn oxides probably involved co-precipitationand adsorption, since high valence uranium (U6+) compoundshave a high affinity for Fe-oxide minerals, and these speciesbecome less stable under reducing conditions. Therefore, byreactingwithNH2OH–HCl, both high-valence uranium (U6+) andiron (Fe3+) were reduced to U4+ and Fe2+, and the extraction ofFe-oxide-bounduraniumbecamemore effective (Ma et al., 2012;Duff et al., 2002). However, in this study, the extractionefficiency of uranium bound to Fe–Mn oxide was 34.9%,indicating that only one third of the uranium was leachable bythese chemical procedures. The remaining iron oxides mayhave been entrapped in silicate glass and crystalline phases intheCBAand couldnot be leached by these chemical procedures.

The quantitative XRD analyses of the magnetic fractionsare shown in Table 5, which indicate that in the magneticfractions of CBA, crystalline phases (such as quartz, mullite,cristobalite and glass) still occur. However, a new mineral

Table 4 – Mass ratio and uranium distribution in variousmagnetic fractions of CBA in the combined sample.

Magneticfraction

Mass ratio(%)

Distribution ofuranium (%)

Strongly magnetic 8.2 23.1Weakly magnetic 1 7.6 15.3Weakly magnetic 2 4.9 6.5Non-magnetic 79.3 55.1


Table 5 – Composition of mineral phases by Rietveld Siroquant in various magnetic fractions of CBA in the combined sample.

Mineral phase Percentage (%)

Strongly magnetic Weakly magnetic 1 Weakly magnetic 2 Non-magnetic

Glass 53.8 73.8 66.6 55Quartz 21.5 5.3 13.8 31.1Mullite 9.9 17.4 13.6 10.1Cristobalite 3.1 1.6 2.3 3.7Others Magnetite (11.7) Albite (1.9) Albite (3.3)

Calcite (0.4)–



phase, magnetite, was detected, and its percentage was ashigh as 11.7%. This result is consistent with the mass fractionof Fe-oxide (11.8%) found by chemical analysis, meaning thatin the strongly magnetic fraction, almost all Fe-oxide exists inthe form of magnetite (the main component is Fe3O4) and thisfraction contains 23.1% of the total uranium. In the raw CBA,the magnetite could not be detected by powder XRD. The datademonstrated that after magnetic separation, the magnetitewas concentrated in the strongly magnetic fraction. Consid-ering the high concentration andmass ratio of uranium in thestrongly magnetic fraction, it is very clear that uraniumcoexists with magnetite (23.1%) and other Fe-oxides (21.9%).

To further verify the co-existence of uraniumwith Fe-oxides,a magnetic fraction sample was subjected to EPMA–EDSanalysis, and as shown in Fig. 2, the simultaneous presence ofFe and uranium was clearly identifiable.

a

b Spectrum 1

0 1 2 3 4 5 6 7 8

60 µm

Inte

nsity

(a.

u.)

Energy (keV)

Fig. 2 – (a) EPMA (electron probe micro-analyzer) image and(b) EDS (energy dispersive X-ray spectrometer) spectrum oftypical iron-rich fractions in the combined sample.

2.4. Mode of occurrence of uranium in residual fraction

Previous studies have confirmed that the HF solutions candissolve vitreous phases from coal fly ash, but not the crystallinephases, e.g., mullite, quartz, and hematite (Fernández-Jimenez etal., 2006). Palomo et al. (2004) has also demonstrated thedetermination of the vitreous content in the coal fly ash usingHF. Dai et al. (2010) and Hulett et al. (1980) used 4% and 1% HF,respectively, to dissolve amorphous glasses in coal fly ash. UsingdilutedHF to dissolve amorphous silicate is based on the fact thatthe glass phase can dissolve rapidly in diluted HF, whereas thedissolution of crystalline mullite and quartz is much slower.

In this study, HF leaching of the residual phase was also usedto identify whether uranium is entrapped in the glass phase inthe residue fraction. Each HF-leached residue was weighed, andthe composition of each mineral phase was quantified bySiroquant, then the dissolution ratio of each mineral phase wascalculated. As indicated by the data in Table 6, the glass in theresidual fraction was efficiently dissolved (dissolution ratio of80.1%–93.2%) via 2%–10% of HF leaching in comparison with thecrystalline phase, e.g., quartz (dissolution ratio of 35.1%–55.3%),and this is consistent with research reported earlier(Fernández-Jimenez et al., 2006) (Table 7). The leaching rate ofuranium significantly correlated with that of glass andcristobalite (Pearson coefficients 0.989 and 0.973, respectively)(Table S2 and Fig. S3). Therefore, uranium in the residual fractionis mainly retained with the glass and cristobalite phases, mostprobably borne on the surface of these two phases (Glagolev,1962). The leaching of uranium was also significantly correlatedwith the leaching of silicon, aluminum and iron, with Pearsoncoefficients of 0.995, 0.984 and 0.998, respectively (Table S3 andFig. S4). In this case, uranium is expected to exist in the Fe/Al/Si/O-rich phases in the residual fraction (Fig. 3).

Although there are very few studies on the distribution andmodes of occurrence of uranium in bottom ash, a number of

Table 6 – Composition of mineral phases in the residueafter leaching with HF at various concentrations in thecombined sample (units: wt.%).

Mineralphase

Originalresidue

2% HFresidue

4% HFresidue

8% HFresidue

10% HFresidue

Quartz 21.7 40.5 41.2 45.1 54.8Cristobalite 4.9 6.5 6.6 8.6 8.7Mullite 16.7 20.5 21.9 28.5 17Glass 56.7 32.5 30.3 17.8 19.5

HF: hydrogen fluoride.

Table 7 – Dissolution ratio of mineral phases and leaching of uranium after leachingwith HF at various concentrations in thecombined sample.

HFconcentration

Leaching ofU (wt. %)

Dissolution ratio ofglass (wt.%)

Dissolution ratio ofquartz (wt.%)

Dissolution ratio ofmullite (wt.%)

Dissolution ratio ofcristobalite (wt.%)

10% 95.0 93.2 55.3 79.9 64.98% 93.0 92.3 50.1 63.3 61.84% 85.6 83.7 42.1 60.0 58.92% 81.2 80.1 35.1 57.3 53.8

HF: hydrogen fluoride.


research groups have reported that in coal, uranium is mostlyassociated with aluminosilicate and organic matter (Yang, 2009).For example, using a six-step sequential chemical extractionprocedure, Dai et al. (2004) studied uranium in Late Paleozoic coalin the Ordos Basin of China. Their results indicate that mosturanium (67%) occurred in association with aluminosilicate andorganics. In addition, in coal fly ash, uranium acts as a lithophile,meaning that uranium is mainly associated with silicates andother oxysalts (Dai et al., 2010). Zielinski et al. (2007) compared theexistence modes of uranium and arsenic, and found that theuranium mainly resided within the relatively insoluble glassymatrix of fly ash particles. In Zielinski and Finkelman (1997), theglassy components in fly ash were identified as themain host ofuranium, and the distribution of uranium in fly ash particles wasfairly uniform throughout the glassy matrix.

2.5. Strategy for uranium removal from the coal bottom ash

Undoubtedly, the recovery of uranium and the reduction ofthe radioactivity of CBA are essential before the ashes can be

Si

Fe U

Fig. 3 – BSE (back scattered electron) (the gray scale image) and EPrelated elements in the combined sample.

further utilized (e.g., as construction materials). The first stepis to dissolve the vitreous phases and expose the Fe-oxides tochemical reagents. Then, on leaching with chemical reagents,uranium would dissolve. Based on these findings, wediscussed a method in one of our previous studies for theremoval of uranium from CBA by calcination with CaCl2 andleaching with HNO3 (Lei et al., 2014).

Considering the continuous accumulation of CBA in theworld(particularly in China), the total content of uraniumwill be huge.As a typical example, in Xiaolongtan power station, approxi-mately 900,000 tons of fly and bottom ash is produced annually,and more than 5 million tons of recoverable ash has beenstockpiled. The average content of uranium is 200 ppm, andhence this station could generate 180 tons of uraniumevery year.The recovery of uranium from CBA is becoming more and morepractical to meet fuel demands for nuclear power. In addition,CBA is often enriched in Ge, W, U, Sr, Rb, Nb and some traceplatinummetals (Dai et al., 2014a, 2014c; Seredin andDai, 2014). Itis therefore important to recover these elements, especiallyuranium, for sustainable development.

O

Al

MA images of the residual fraction showing the distribution of


3. Conclusions

The distribution and mode of occurrence of uranium in a coalcombustion bottom ash were studied in this research. In thestudied CBA, the uranium coexists with Fe-oxides and theglass phase. Most of the uranium-containing Fe-oxides wereentrapped in vitreous aluminosilicate, and only one thirdexisted on the surface of the CBA particles. Uranium was alsoenriched in the magnetic fraction (45%), in which 23.1% oftotal uranium coexisted with magnetite, and the remainder(21.9%) co-existed with other Fe oxides or was surrounded bythe vitreous glass phase. In the residual fraction, uraniumwasexpected to be retained in the Fe/Al/Si/O-rich phases.

To recover uranium from CBA, the first and most importantstep is to destroy the vitreous materials surrounding theuranium-Fe-oxide so that these grains are exposed to chemicalreagents. By thoroughly understanding the distribution ofuranium in CBA, it will be possible to establish effectivestrategies to reduce radioactivity in CBA for environmentalsafety, and to recover the uranium from CBA for sustainabledevelopment.

Acknowledgments

The authors are sincerely grateful for the financial supportfrom the Talent Support Fund of Tsinghua University (No.413405001).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttp://dx.doi.org/10.1016/j.jes.2015.07.009.

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