Encapsulated behavior and extraction ability of uranium in coal ash: A quantitative investigation with SiO2-Al2O3-Fe2O3-CaO system

Fuel 259 (2020) 116225

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Encapsulated behavior and extraction ability of uranium in coal ash: A quantitative investigation with SiO2-Al2O3-Fe2O3-CaO system

T

Zhe Yanga, Changxiang Wanga, Yumei Lia,b, Sen Yanga, Yangyang Zhanga, Ye Tanga, Wei Zhanga, ⁎ Danqing Liua,b, Yilian Lia, a b

School of Environmental Studies, China University of Geosciences, Wuhan 430074, China State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Uranium-rich coal Synthetic ash Modes of occurrence Combustion

Uranium-rich coal ash (CA) receives much attention as the supplementary nuclear resource also its potential environmental risk. In this study, experiments were conducted to investigate the influences of major oxides on the U mobility in synthetic ash (SA) via sequential extraction and SiO2-Al2O3-Fe2O3-CaO system. Overall, modes of occurrence of U were governed by the complicated interactions among U and major oxides. Molten CaO could remarkably activate the refractory minerals and resulted in a large proportion of active U, while it was opposite for Fe2O3 due to the formation of amorphous Fe-Si depletion with the high strength. However, the activated and the encapsulated behaviors of U were both weakened if provided the sufficient Al2O3, by considering a passive immobilization of U in Si-Al matrix. Based on aforementioned facts, uranium mobility was synergistically controlled by various influences especially upon a medium SiO2 content. Ultimately, an equation, α = MCa/ (MSi + MFe + MAl) + MAl/100, was established to describe the extracting ability of U from SA with a known composition. It is most efficient to extract U when α is 0.75 once the U-rich CA is employed to synthesize supplementary nuclear resources.

1. Introduction

consumption [1]. It is estimated that coal still plays a crucial role on China’s energy supplement in the near future, resulting in a large amount of coal ash (CA, coal combustion residue). Although CA can be reused to synthesize advanced microporous adsorbents and other

Statistically, there were 2.7 billion tons of coal consumed in China at the end of 2017 and constituted by 60.4% of total energy

⁎

Corresponding author. E-mail address: [email protected] (Y. Li).

https://doi.org/10.1016/j.fuel.2019.116225 Received 24 May 2019; Received in revised form 16 September 2019; Accepted 17 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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construction materials [2,3], it was still disposed in the long-term surface impoundments and landfills [4]. In recent years, uranium-rich CA has been receiving significant attention as a supplementary nuclear resource [5] also its potential environmental risk [6]. Uranium content in Chinese coal is 2.43 μg/g in average [7], whereas can be comparable to hundreds and thousands in certain coal deposits in China [8–11]. With the decreasing reserve of U in conventional deposits, uranium-rich coal is regarded as the promising nuclear source and the cut-off grade is suggested to be 1000 μg/g (ash basis) [12]. As not being a volatile element, uranium content in CA is at least 4 times higher than in coal [13,14], so that U-rich CA is considered as a potential nuclear resource with the technology improved [5,15]. It was reported that U recovery from coal-fired power plants had been applied in industry in China [16]. Likewise, CA, containing strategic metals, metalloids, and rare earth elements, may be a potential source in the future[12,17]and therefore, a large amount of researchers was devoted to recovery those valuable elements [18–21]. Uranium compositions mainly consist of U-oxide [22] and uranate [23] in CA, however, they tend to incorporate into amorphous Al-bearing phase [14], silicates [24], Si-Al glass [25], Fe-rich multi-element eutectic [26], and Fe-oxide [22], resulting in a low extraction efficiency of U especially when the CA is prepared at a high temperature [13,14]. Furthermore, the degree of encapsulated behavior of U, closely associating with the sintering characteristic of CA, varies widely depending on the ash composition and other factors including ash size, combustion temperature, atmosphere, and pressure [27–30]. Hereinto, combustion temperature plays a critical role on determinations of the sintering and the fusion behavior of CA and synthetic ash (SA) [31–33]. Alkali metals, alkaline earth metals, and iron oxides can integrate with aluminosilicate to form low melting-point eutectics and multi-element agglomerations during coal combustion [34–36]. Formations of Fe-integrated SiO2/Si-Al glass can strengthen the encapsulated behaviors of U and other trace elements [37–39] and thus, the mode of occurrence of them in CA is dominated by residue [14,25]. On the other side, with the developments of calcium looping CO2 capture [40] and biomass-coal power system [41], the change of fuel composition might influence the species and the distribution of trace elements in CA. Though the mobility and the leachability of U had been studied, few researches devoted to quantitatively investigate the potential leachability of U owing to the different varieties of fuel itself. On these bases, experiments were conducted to study modes of occurrence of U in CA/SA, which was prepared with SiO2-Al2O3-Fe2O3-CaO system and combusted at 1100 °C. Interaction mechanisms among U and major oxides were further described, coupled with X-ray diffraction (XRD) and field emission scanning electron microscopy-energy dispersive Xray spectroscopy (FESEM-EDS) analyses. Ultimately, a mathematics model was established to quantitatively evaluate the potential leachability of U from U-rich CA, by considering the different ash treatments.

2.2. Coal ash and synthetic ash SA, prepared by the various proportions (wt. %) of SiO2, Al2O3, Fe2O3, and CaO with the fixed Na2O (2%), was used to investigate the influence of major oxides on the U occurrence. Compositions of SA were supplied in the Supplementary materials (Table 1s, Table 2s, and Table 3s). Every 0.5 g of aforementioned SA was weighted and transferred into a 50 ml centrifuge tube. 2 ml of U standard solution (250 μg/ g) was added to the tube and thoroughly mixed in a gas bath shaker without lids at the condition of 200 rpm, 12 h, and 60 °C. The treated SA was dried to constant weight at 60 °C in a drying oven. Subsequently, the coal sample and the resulted SA were completely transferred in quartz crucibles, and combusted in a muffle furnace at regulated temperatures for 2 h. 0.5 g of obtained CA and SA were naturally cooled down to room temperature, and collected in the 50 ml centrifuge tubes for the following experiments and analyses. 2.3. Sequential extraction Modes of occurrence of U in samples are successively divided into 5 fractions (S1: ion exchangeable, S2: carbonates-bound, S3: Fe-Mn oxide-bound, S4: organic matter-bound, and S5: residual) based on Tessier sequential extraction method [43]. Extraction procedures to investigate the U occurrence only involved S1, S2, S3, and S5 because of no organic matter in SA. 0.5 g of sample was weighted and experimental procedures were conducted as requested, while the digestion of residue referred to the Chinese standard [44], which was fully described in the previous work [14]. Proportions of the U occurrences were projected into CaO-Al2O3-Fe2O3 ternary phase diagrams with the fixed SiO2 (30%, 50% and 70%) and Na2O (2%) contents, and their distributions were constructed by using Kriging interpolation in the software Surfer 12. 2.4. Quality assurance The number of parallel samples at least accounted for the 50% of total sample to ensure the accuracy of data. Quality assurances of sequential extraction analysis included blanks and regulated standards for every 20 samples. Simultaneously, the stability of ICP-MS was examined through the results of regulated and internal standards, which were guaranteed at an acceptable range (80–120%) during operation process. 2.5. Analytic measurement The studied sample was scanned at a 2θ interval of 5–70° with a step size of 0.01° to determine its mineralogy by X-ray diffraction (XRD, D8FOCUS, Bruker Company, Germany) upon Cu Kα radiation. Loss on ignition (LOI) of coal samples was calculated by combusting samples at 815 °C to constant weight. Major compositions of coal samples (815 °C, ash basis) were analyzed by X-ray fluorescence spectrometer (XRF, AXIOSmAX, PANalytical B.V. Company, Netherlands) with the detection limits of 0.03% for those major oxides, referring to the procedures outlined by Dai et al. [11,45]. Morphology and composition of samples were characterized by field emission scanning electron microscope (FESEM, SU8010, Hitachi, Japan) in conjunction with energy dispersive X-ray spectroscopy (EDS). Studied samples were firstly coated with Au, and images were captured via a backscattered electron detector at a 10 kV accelerating voltage. Aqueous samples were acidized with 2% HNO3, uranium content in which was determined by inductively coupled plasma-mass spectrometry (ICP-MS, ELAN DRC-Ⅱ, PerkinElmer Company, USA). The determination of U content in residue, following the digestion procedure based on a combined solution of HNO3-HFHClO4 (10:5:2, 17 ml) in a 50 ml PTEE beaker at the regulated temperature [14], referred to Chinese standard reference (DZ/T 0279.62016) [44]. The R2 of established standard curves should be guaranteed

2. Materials and methods 2.1. Coal sample and reagents Uranium-rich coal samples were collected from four coal deposits, located in three villages (MW: Mengwang, DZ: Dazhai, and ZT: Zhangtuo) of Lincang, Yunnan, Southwestern China. Samples were firstly dried to constant weight at 105 °C, crushed by a planetary ball mill to pass through a 100-mesh sieve, and stored in a desiccator for future use. The contents of major oxides and U in studied coals were well documented at the previous study [42] and listed in Table 1. SiO2, Al2O3, Fe2O3, CaO, and other chemical agents were supplied by Sinopharm Chemical Reagent Co., Ltd. Deionized water was prepared with TKA GenPure system for the subsequent experiment and analysis. 2

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Table 1 Major composition of coal samples (wt.%, ash basis). Sample

MW1 MW2 DZ ZT

Element composition SiO2

Al2O3

Fe2O3

CaO

K2O

Al/Si*

Fe/Si

Ca/Si

U**

LOI

40.4 56.7 58.4 78.6

19.0 23.0 13.4 7.55

17.0 6.22 6.97 8.10

6.35 2.35 7.85 2.20

2.40 2.53 2.25 0.172

0.55 0.48 0.27 0.11

0.32 0.08 0.09 0.08

0.17 0.04 0.14 0.03

101.22 112.56 60.86 14.23

88.90 78.39 62.66 74.73

* The mole ratios of Al/Si, Fe/Si and Ca/Si. ** The uranium contents (μg/g) in four coals are 41.7, 46.3, 25.0 and 5.86 times higher than the average content in Chinese coal [7], respectively.

to > 0.999 with 5 points at least, and simultaneously, certified reference materials (GSS-4, GSS-7, and GSS-16) were used to calibrate the U content.

[31,46], it was difficult to crush the CAs prepared at 1000 °C and 1100 °C. Fig. 1 showed that exchangeable and carbonates-bound U transferred into Fe-Mn oxide-bound and residual U as the temperature raised, so that two sudden changes of U occurrences were observed especially at 800 °C and 1100 °C. Furthermore, modes of occurrence of U involved a three-stage transformation during coal combustion, including that 1) exchangeable and organics-bound U are oxidized to Uoxide that tends to interact with Ca2+ to form uranate (from S1 and S4 to S2); 2) uranate integrates with Fe-oxide and aluminosilicate owing to the decomposition of carbonate minerals once the temperature exceeds 700 °C (from S2 to S3 and S5) [46]; 3) the compact Si-Al matrix is generated at 1100 °C to encapsulate U-bearing fractions (from S2 and S3 to S5) [25]. Compared with other samples, DZ, characterized by a medium Al/Si, a low Fe/Si, and a high Ca/Si ratio, possesses a large

3. Results 3.1. Modes of occurrence of uranium in CA According to Table 1, coal samples are mainly composed of SiO2, Al2O3, Fe2O3, and CaO along with small amounts of K2O and trace elements. Hence, SA nearly represents a large proportion of natural ashes in this study. The decomposition of CaCO3 might promote the transfers of carbonates-bound U into other occurrences at 800 °C [46]. Due to the concurrent decreases in bulk density and porosity of CA [31]

Fig. 1. Modes of occurrence of uranium in the CAs (a: WM1, b: WM2, c: DZ and d: ZT) prepared at different temperatures [42]. 3

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proportion of active U even followed by the combustion at 1000 °C. Moreover, as the lowest ratios of Al/Si and Fe/Si in ZT, it was noticed a small amount of carbonates-bound U at 1100 °C without being totally encapsulated in Si-Al matrix and Si-Fe depletion. Hence, it is reasonable to conjecture that the ash components can influence modes of occurrence of U in CA.

proportions of U as SiO2 content raised were simultaneously noticed in exchangeable, carbonates-bound, and Fe-Mn oxide-bound U. Thereby, increasing SiO2 contributed to the transfer of active U into residual. 3.2.4. Residual uranium in SA In view of the common hosts of encapsulated U in silicate [24], Si-Al matrix [25], amorphous phase [14], and Fe-oxide [22] in CA, as shown in Fig. 1, nearly all of active U transferred into residual followed by the combustion at 1100 °C. Proportions of residual U in SiO2-Fe2O3-constituted regions were much higher than in SiO2-Al2O3-constituted, and significantly increased with Fe2O3 content raised (Fig. 5). Furthermore, increasing CaO content could improve the proportion of active U and therefore, residual U tended to accumulate at the certain regions, the SA in which was prepared with the limited CaO content. Although proportions of residual U obviously increased at the higher SiO2, there was a complicated distribution of residual U at 50% SiO2 by comparing the regular at 30% and 70% SiO2 contents. Hereinto, proportions of U at the left corner, the right corner, the top corner, and the central area were governed by Fe2O3, Al2O3, CaO, and their combinations (Fig. 5(b), auxiliary line), respectively. Based on the limited contents of CaO and Fe2O3, influences of Si-Al glass on U occurrence are amplified and should be emphasized due to its largest abundance in natural ashes.

3.2. Modes of occurrence of uranium in the SA prepared at 1100 °C Although some researchers devoted to improve the leachability of U from CA [14,24], influences of ash composition on the U leachability are seldom investigated. In addition, drawbacks such as, energy wastes and potential environmental risks, limit the application of extra-added additives to improve the leaching efficiency of U through the roasting process. Therefore, uranium occurrences in the SA prepared at 1100 °C were employed to quantitatively investigate the U leachability and the complicated interactions among U and major oxides. 3.2.1. Exchangeable uranium in SA Major components of each SA were listed in the Supplementary materials, and color-filled contours represented the different intervals of U proportion. Uranium tends to interact with alkali metals, alkaline earth metals, Fe-oxides, and is eventually encapsulated in bulk minerals with a stable chemical property. As shown in Fig. 2, a small proportion of exchangeable U was observed by comparing with other occurrences and tended to accumulate at certain regions, where the SA was characterized by the low CaO and Fe2O3 but the high Al2O3 contents. Furthermore, increasing SiO2 content also leaded to the decreasing proportion of exchangeable U.

3.3. Influence of major components on uranium occurrence in SA Proportions of the U occurrences were collected and projected into a coordinate system with the abscissas of Ca/Si, Al/Si and Fe/Si mole ratios to further illustrate the interaction mechanisms among U and major oxides.

3.2.2. Carbonates-bound uranium in SA Uranium can be extracted as the form of carbonates-bound U for the high acid solubilities of U-oxide and uranate. According to Fig. 3, proportions of carbonates-bound U involved a dynamic trend as the ash composition changed. Increasing CaO content can improve the U leachability in CA [24], whereas the excess stimulated the occurrence of Fe-Mn oxide-bound U instead of carbonates-bound especially in SAA ~ F. Carbonates-bound U tended to accumulate at certain regions, the SA in which was characterized by a high Al/Fe ratio upon a fixed CaO content (Fig. 3(a), auxiliary line). However, except for the SA prepared without Fe2O3, proportions of carbonates-bound U were significantly decreased to almost < 10% with raising SiO2 content to 70%.

3.3.1. Influence of CaO on uranium occurrence 1) Residual uranium Modes of occurrence of U are divided into two fractions, namely, active U (the sum of S1 to S3, in this study) and residual U (S5). Overall, it was effective to extract U when the Ca/Si ratio was ≥1.5 by considering a saturated proportion of residual U (Fig. 6(d)). Increasing Ca/ Si ratio promoted to decrease the proportion of residual U, whereas some differences, attributed to the more effective activation of Ca2+ on Si-Al matrix than on Fe-Si depletion, were also noticed upon a same Ca/ Si ratio. Given the aforementioned phenomenon, proportions of residual U were obviously increased with replacing Al2O3 by Fe2O3.

3.2.3. Fe-Mn oxide-bound uranium in SA Unexpectedly, proportions of Fe-Mn oxide-bound U were mainly governed by CaO content in SA, and continuously increased with raising CaO content (Fig. 4). It well suggests that the determination of element occurrence in CA depends not only on the binding priority of element to minerals but also on the solubility of element-bearing fractions. On the other hand, the similar regularity was observed at different SiO2 contents, while the phenomena about the decreasing

2) Active uranium On the other side, proportions of exchangeable and Fe-Mn oxidebound U both showed a regular trend as Ca/Si ratio increased. Due to the strong reaction between U and CaO during coal combustion [23], proportions of exchangeable U significantly dropped down and

Fig. 2. Distribution of ion exchangeable uranium in the SA (a: 30% SiO2, b: 50% SiO2 and c: 70% SiO2 content) prepared at SiO2-Al2O3-Fe2O3-CaO system. 4

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Fig. 3. Distribution of carbonates-bound uranium in the SA (a: 30% SiO2, b: 50% SiO2 and c: 70% SiO2 content) prepared at SiO2-Al2O3-Fe2O3-CaO system.

transferred to other active occurrences if provided a small amount of CaO (Fig. 6). U-oxide and uranate can be theoretically extracted as the form of carbonates-bound U for their acid solubilities, however, the relatively lower proportions of carbonates-bound U were found in the SAs prepared with 50% and 70% SiO2. Interestingly, the proportion trend of carbonates-bound U was firstly increased, peaked at the Ca/Si ratio = 1.0, and decreased with the ratio further raised. Fixed Ca/Si ratio, substituting Fe2O3 for Al2O3 was responsible for the proportion differences owing to the higher acid solubility of CaO-SiO2-Al2O3-based mineral than CaO-SiO2-Fe2O3-based. Proportions of Fe-Mn oxide-bound U were continuously increased as Ca/Si ratio raised, and the similar phenomenon of heavy metals was also found in the co-combustion process [47]. Although Fe-Mn oxidebound U presented the same trends upon 30%, 50% and 70% SiO2, the reason for some observable differences especially when the Ca/Si ratio was 1.0 (Fig. 6(c)) needed to be discussed by combining other characteristic methods.

decrease in the SA prepared without CaO, and 2) the sudden decrease in the SA prepared with CaO. Since the formation of Fe-Si depletion promoted to decrease the U mobility, proportion differences of exchangeable U gradually narrowed with Fe/Si ratio raised. Although U could both react with Fe2O3 and CaO within the combustion process, the integrating priority of U with Fe2O3 was much lower than with CaO according to little exchangeable U if provided a small amount of CaO. On the other side, carbonates-bound and Fe-Mn oxide-bound U both showed decreasing trends as Fe/Si ratio raised, but their proportions were also governed by SiO2 content (Fig. 7(b, c)). Complicated interactions among Fe2O3 and other oxides at 1100 °C synergistically influenced the ash property and the U occurrence. For instance, the extraadded CaO and Al2O3 could improve U mobility in varying degree especially upon 30% SiO2, leading to the proportion differences in all modes of occurrence of U at the same Fe/Si ratio.

3.3.2. Influence of Fe2O3 on uranium occurrence

1) Residual uranium

1) Residual uranium

The host of U in Si-Al glass could not be ignored, by considering its largest abundance in CA. Residual U presented a stabilized trend with raising Al/Si ratio but its proportion was distinguished upon different SiO2 contents (Fig. 8(d), auxiliary line). Moreover, fixed the Al/Si ratio around 1.0, proportions of residual U, lying between those in the SAs constituted by SiO2-Al2O3-Fe2O3-based and SiO2-Al2O3-CaO-based mineral, were remarkably increased with replacing CaO by Fe2O3. It is thus reasonable to conjecture that increasing Al/Si ratio competed the host of encapsulated U in Si-Al matrix with a neutralized mobility.

3.3.3. Influence of Al2O3 on uranium occurrence

Proportions of residual U presented a similar trend as Fe/Si ratio raised, whereas they were quite different with a fixed Fe/Si ratio but various SiO2 contents (Fig. 7(d)). Though SiO2 seldom deform at 1100 °C for a high-melting point, raising SiO2 content contributed to the increasing proportion of residual U, which indicated that the formation of Si-Fe depletion, rather than Fe2O3 and SiO2, significantly stimulated the encapsulated behavior of U. On the other side, proportions of residual U increased with replacing CaO by Al2O3 (auxiliary line in Fig. 7(d)), resulting from the immobilization of U in Si-Al matrix.

2) Active uranium As shown in Fig. 8(a), a large amount of exchangeable U was observed in the SA only prepared with Al2O3-SiO2-based mineral, different from the little with the existences of CaO and Fe2O3. Carbonates-bound U converged at the medium proportion as Al/Si ratio raised (Fig. 8(b)),

2) Active uranium Fig. 7(a) showed that proportions of exchangeable U presented two distinguished trends as Fe/Si ratio raised, namely, 1) the gradual

Fig. 4. Distribution of Fe-Mn oxide-bound uranium in the SA (a: 30% SiO2, b: 50% SiO2 and c: 70% SiO2 content) prepared at SiO2-Al2O3-Fe2O3-CaO system. 5

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Fig. 5. Distribution of residual uranium in the SA (a: 30% SiO2, b: 50% SiO2 and c: 70% SiO2 content) prepared at SiO2-Al2O3-Fe2O3-CaO system.

but an elevated proportion of carbonates-bound U was observed in the SA prepared at the higher SiO2 content. Moreover, the similar trends of Fe-Mn oxide-bound U were noticed in Fig. 7(c) and Fig. 8(c), which indicated the integrations of Al2O3 and Fe2O3 with SiO2 both played critical roles on the transfers of Fe-Mn oxide-bound U into residual. However, there were proportion differences of Fe-Mn oxide-bound U at the same Al/Si ratio ascribed to the opposite influences by Fe2O3 and CaO.

proportion of active/residual U was found with raising SiO2 content. It well suggested that increasing SiO2 content promoted the hosts of U in Si-Al matrix and Fe-Si depletion, but instead limited the improvement efficiency of CaO on U mobility. Furthermore, sequences of U to integrate with studied oxides were described as follows: CaO > Fe2O3 > Al2O3 > SiO2, by summarizing the aforementioned results.

3.3.4. Influence of SiO2 on uranium occurrence As shown in Fig. 6, Fig. 7, and Fig. 8, although the proportion trends of U were similar upon different SiO2 contents, a lower/higher

4.1. Interaction mechanisms between uranium and ash components

4. Discussion

To study the interaction mechanisms between U and ash

Fig 6. Changing characteristic of uranium occurrence (a: exchangeable uranium, b: carbonates-bound uranium, c: Fe-Mn oxide-bound uranium, d: residual uranium) in the SA at different Ca/Si mole ratios. 6

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Fig. 7. Changing characteristic of uranium occurrence (a: exchangeable uranium, b: carbonates-bound uranium, c: Fe-Mn oxide-bound uranium, d: residual uranium) in the SA at different Fe/Si mole ratios.

components, mineralogy, morphology, and elemental composition of SA were also investigated by XRD and FESEM-EDS analyses, and the results were attached in the Supplementary materials. 4.1.1. Interaction mechanisms between uranium and CaO Uranium firstly interacts with CaO to from uranate [23], and subsequently integrates with silicate [24], Si-Al matrix [25], Fe-oxide [22], and amorphous phase [14] with stable properties, followed by the coal combustion. According to XRD patterns in Fig. 1s–3s, Ca2+ would combine with other oxides to form calcium silicate (larnite/wollastonite, Eq. (1) ~ (2)), CaO-Al2O3-based mineral (grossite, Eq. (3)), CaOFe2O3-based mineral (srebrodolskite, Eq. (4)), and also multi-element eutectic (Fig. 6s, Eq. (5)). These aforementioned minerals were all characterized by a higher acid solubility, compared with silicate, Si-Al glass, and Fe-oxide. On the other hand, increasing CaO content raises the ash fusion temperature, decreases the ash strength, and weakens the slagging behavior of SA owing to the formation of high-melting silicates [48]. On these bases, the improved acid solubility and the decreased ash strength might simultaneously accelerated the transfers of residual U into active. It needed to be emphasized that the activation efficiency of encapsulated U in Si-Al matrix via CaO was much stronger than in Fe-Si depletion. However, the excess CaO increased the acid assistance of SA, so that U tended to be extracted as the form of Fe-Mn oxide-bound U rather than carbonates-bound for a higher acidity of extracting solution. 2 CaO + SiO2 = Ca2SiO4

(1)

CaO + SiO2 = CaSiO3

(2)

CaO + x Al2O3 = CaO·xAl2O3

(3)

2 CaO + Fe2O3 = Fe2Ca2O5

(4)

a CaO + b Fe2O3 + c Al2O3 + d SiO2 = aCaO·bFe2O3·cAl2O3·dSiO2

(5)

4.1.2. Interaction mechanisms between uranium and Fe2O3 U-oxide and uranate tend to integrate with Fe-oxides during coal combustion [22], while their integrations are depleted in SiO2 particles, leading to a low mobility of U [25]. The phenomenon about the host of U in Fe-Si compounds was also identified in sediments [49]. Since Fe2O3 particle can interact with ash to form Fe-bearing recrystallization phase with a high strength [50], it is difficult to crush the SA prepared by large amounts of Fe2O3. According to XRD analyses in Fig. 1s ~ 3s, mineral compositions of SA-M/Q are mainly constituted by quartz, cristobalite, and hematite. Moreover, Fe-oxide bridges intact minerals, such as lime, quartz, and aluminosilicate, to form amorphous largesized eutectics with dense structures (Fig. 6s and Fig. 7s). The host of U in Fe-integrated Si-Al glass is responsible for the difficulty to leach U from the CA prepared at 1100 °C [22,25]. Especially at SA-Q, glassy eutectics simultaneously encapsulated some fine particles and obstructed the dissolution of U-bearing components (Fig. 8s), the result of which was consistent with the accumulate host of REEs in Fe-Si depletion [51]. In addition, Fe2O3 was partially gasified and condensed within combustion process [52], leading to the formation of spherical particle with an iron-rich core (Fig. 8s). There was a biased interaction between ash particle and extra-added MgO to replace the supposed one 7

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Fig. 8. Changing characteristic of uranium occurrence (a: exchangeable uranium, b: carbonates-bound uranium, c: Fe-Mn oxide-bound uranium, d: residual uranium) in the SA at different Al/Si mole ratios.

for Fe-oxide [53]. Likewise, interactions of CaO and Al2O3 with SiO2, preventing the formation of Fe-Si depletion, were beneficial to improve the U leachability.

interactions among SiO2 and other oxides, contributed to the different modes of occurrence of U in SA. Decreasing SiO2 content might stimulate its reactions with alkaline fractions to form multi-element eutectics, which leaded to an improved solubility of mineral and an increased mobility of U from SA. On the other side, the excess SiO2 obstructed the activations of Ca2+ on Si-Al glass/Fe-oxide and accelerated the formation of Fe-Si depletion, so that proportions of residual U were much higher at 70% SiO2 content (Fig. 5(c)).

4.1.3. Interaction mechanisms between uranium and Al2O3 According to XRD and FESEM-EDS analyses, mineral compositions of SA-V consisted of quartz, corundum, and aluminosilicate, however, Al2O3 could interact with CaO, SiO2, Fe2O3, and their combinations to form grossite, amorphous Si-Al glass, and multi-element eutectic (Fig. 1s, 2s, 3s, 7s, 9s). Hereinto, mineral compositions of SA are mainly constituted by independent SiO2, lowing melting eutectic, and largesized agglomeration when the Al/Si ratio is < 0.22, 0.29–0.67 and > 2.0, respectively [54]. In view of that U hardly react with Al2O3 and SiO2 during combustion [55], a large amount of active U was observed in SA-V, which differed from the little in SA-A and SA-Q. However, the passive immobilization of U in Si-Al matrix was responsible for the increasing proportion of residual U as Al/Si ratio raised [14,25]. Because ash sintering was prevented if provided sufficient amounts of Al2O3 for the formation of refractory corundum, it was less efficient for U to host in Si-Al matrix than in Fe-Si depletion. Moreover, there was an obvious activation of Ca2+ on Si-Al matrix contributing to the release of encapsulated U.

4.2. Mathematics model of uranium encapsulation in the SA prepared at 1100 °C According to the previous results and discussions, influences of ash components on the U mobility were systematically investigated. Integrations of CaO with SiO2, Al2O3, Fe2O3, and their agglomerations contributed to the improvements of mineral solubility as well as U mobility. On the contrary, Fe2O3 particle strongly associated with U at the initial combustion, and then incorporated into SiO2 to form amorphous Fe-Si depletion with a low chemical activity. Although U hardly react with SiO2 and Al2O3 during combustion, due to a passive immobilization of U in Si-Al matrix, the activated and encapsulated behaviors of U via the extra-added CaO and Fe2O3 were both weakened if provided the sufficient amount of Al2O3. Based on the aforementioned facts, an equation (Eq. (6)) was established to evaluate the extraction ability of U from SA, considering a relatively high determination coefficient (R2) between simulation results and experimental data (Table 2). The R2, fitted by polynomial and linear equations, were 0.91,

4.1.4. Interaction mechanisms between uranium and SiO2 Despite of little U in quartz followed by the combusted at 1100 °C [55], the change of mineral composition, resulting from the various 8

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5. Conclusion

Table 2 The mathematical expression of the derived fitting curves in Fig. 9. Fitting Equation

a

y= y= y=

13.13 X3 + 84.07 X2 82.70 X + 94.54 107.08 X + 100.18

y=

17.056 + 117.284 e

b c d

In this study, modes of occurrence of U in CA/SA were systematically investigated upon SiO2-Al2O3-Fe2O3-CaO system, and a reasonable mathematics model was established to evaluate its extraction ability from SA. The results leaded to conclusions as follow:

R2

Sample

169.32

X + 108.56

(X 0.035)/0.774

0.91 0.80 0.88 0.88

(1) Transfers of carbonates-bound U into Fe-Mn oxide-bound and residual were ascribed to the decomposition of carbonate and the formation of Si-Al glass with increasing the coal combustion temperature from 800 °C to 1100 °C. (2) Integrations of CaO with SiO2, Al2O3, Fe2O3, and their agglomerations contributed to the release of encapsulated U as the forms of active U, but the excess CaO could improve the acid resistance of minerals and resulted in the decrease of U mobility. (3) Uranium was strongly incorporated into amorphous Fe-Si depletion as Fe2O3 content raised, leading to a large proportion of residual U. (4) Extra-added Al2O3 simultaneously weakened the activated and the encapsulated behavior of U, while the changes of composition, microstructure, and morphology of SA leaded to the different proportions of U hosted in Si-Al glass. (5) A wide range of U mobility especially at the lower SiO2 content resulted from the complicated interactions among ash components. On the contrary, the limited activations of Ca2+ on Si-Al matrix and Fe-Si depletion might lower the proportion of active U with raising SiO2 content. (6) An equation, α = MCa/(MSi + MFe + MAl) + MAl/100, was established to evaluate the extraction ability of U from ash. It is most

0.80, and 0.88 at 30%, 50%, and 70% SiO2, respectively. It is efficient to enhance the value of α by increasing CaO content. Fig. 9(d) showed that the slope of fitting curve suddenly decreased and gradually stabilized with a saturated proportion of residual U. An intersection point of two analogous asymptotes was observed when α was nearly 0.75 and thus, it is beneficial to extract U at that value once the U-rich CA is considered as the supplementary nuclear resource. With the developments of calcium looping CO2 capture [40] and biomass-coal power system [41], appropriate combinations of calcium and biomass with fuel could simultaneously contribute to the efficient utilization of valuable elements in the combustion residues.

=

MCa M + Al MSi + MFe + MAl 100

(6)

where α represents the leachability of U, and MCa, MSi, MFe, and MAl are the mass fractions of CaO, SiO2, Fe2O3, and Al2O3 in SA (wt.%), respectively.

Fig. 9. Fitting results of uranium encapsulation in the SA prepared at 1100 °C (a: 30% SiO2, b: 50% SiO2 c: 70% SiO2 and d: collection). 9

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Z. Yang, et al.

efficient to extract U when α is 0.75 once the U-rich CA is considered as the supplementary nuclear resource.

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