Synergistic Mechanisms of CaCl2 and CaO on the Vaporization of Cs

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Synergistic Mechanisms of CaCl2 and CaO on the Vaporization of Cs from Cs-Doped Ash during Thermal Treatment Facun Jiao,†,‡ Norikazu Kinoshita,§ Masato Kawaguchi,§ Motoyuki Asada,§ Maki Honda,∥ Keisuke Sueki,⊥ Yoshihiko Ninomiya,*,† Dmitry Sergeev,# Marc Blas̈ ing,# and Michael Müller# †

Department of Applied Chemistry, Chubu University, 1200 Matsumoto-Cho, Kasugai, Aichi 487-8501, Japan School of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, People’s Republic of China § Institute of Technology, Shimizu Corporation, 3-4-17 Etchujima Koto-ku, Tokyo 135-8530, Japan ∥ Graduate School of Pure and Applied Sciences and ⊥Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan # Institute for Energy Research (IEK-2), Leo-Brandt-Straße 1, 52425 Jülich, Germany ‡

ABSTRACT: This study aimed to clarify the roles of CaCl2, CaO, and their mixture in the vaporization of Cs from Cs-doped ash during thermal treatment. In particular, potential mechanisms of the synergistic effect of the addition of a mixture of CaCl2 and CaO on Cs vaporization were investigated. Vaporization experiments were carried out in a lab-scale horizontal furnace at 900, 1000, and 1100 °C. The results indicated that adding a mixture of CaCl2 and CaO produced a synergistic effect on Cs vaporization when the reaction temperature was above 1000 °C and the vaporization ratio was noticeably increased in comparison to that when adding CaCl2 or CaO alone. The formation of wadalite (Ca6Al5Si2O16Cl3) and/or igumnovite [Ca3Al2(SiO4)2Cl4], derived from chemical reactions among CaCl2, CaO, and aluminosilicates in the Cs-doped ash, delayed the release of Cl during thermal treatment, thus extending the contact time of Cs and gaseous Cl. Furthermore, CaO destabilized the aluminosilicate structure, resulting in a higher volatility and reactivity of Cs, and thus, a reaction readily occurred between activated Cs and gaseous Cl released from the decomposition of wadalite and/or igumnovite.

1. INTRODUCTION A large amount of radionuclides, particularly 137Cs and 134Cs, were released from the accident at the Fukushima Daiichi Nuclear Power Plant (PDNPP) on March 11, 2011, resulting in radioactive contamination of the northeastern area of Japan.1−3 As a result, the soil, crops, trees, structural timbers, and water were polluted with radioactive cesium through diffusion by dry and wet deposition. 4−10 As a result of the vigorous decontamination work in these areas by the Japanese government, massive generation of Cs-contaminated combustible solid wastes is collected and accumulated. To deal with such radioactive waste safely, an incineration treatment is considered to be a reasonable solution. However, the produced ashes from incineration contain high levels of radioactive cesium relative to raw wastes, thus posing a serious challenge to their safe disposal and management. Two major chemical species of Cs are produced in the incineration process: CsCl and Cs aluminosilicate.11,12 CsCl is the predominant Cs-bearing species in fly ash, formed through the vaporization−condensation process in the heat recovery chamber of an incinerator, while Cs aluminosilicate is generated via the gas−solid reaction between gaseous Cs and inorganic matter in the combustion chamber and discharged as bottom ash. Their partitioning is greatly dependent upon the Cl content in the raw wastes and the temperature during incineration.13 Water-soluble CsCl can be removed via water extraction. Removed Cs in solution must be further captured by solid absorbents and then stored in a small volume.14−16 However, water extraction is unsuitable for the removal of © XXXX American Chemical Society

water-insoluble Cs species, such as Cs aluminosilicate. Thermal treatment with the addition of Cl-containing additives has been widely employed to vaporize heavy metals in incineration ashes17−20 and is thus a promising method to remove Cs from incineration ashes.11,15,21 Our previous studies investigated the Cs vaporization behavior of a raw ash and powdered/granulated Cs-doped ash samples containing non-radioactive Cs during thermal treatment with the addition of CaCl2 or MgCl2.22−24 It has been found that CaCl2 was more successful at vaporizing Cs than MgCl2. The main route for Cs vaporization with added CaCl2 involves the formation of an intermediate compound, CsCaCl3, through a chemical reaction between CaCl2 and Cs in the ash, followed by the subsequent decomposition of this compound. Accordingly, thermal treatment with the addition of CaCl2 is a proposed method to evaporate water-insoluble Cs from Cs-contaminated ash. However, a high concentration of chlorine gas produced from the decomposition of CaCl2 potentially causes hot corrosion of treatment facilities and environmental pollution. Therefore, reduction of CaCl2 addition during thermal treatment for Cs vaporization is significant upon further application of this process. We recently reported that the addition of the CaCl2 and CaO mixture can promote the vaporization of Cs in a Cs-doped ash compared to the addition of CaCl2 alone. Cl release was delayed by added Received: February 21, 2018 Revised: March 25, 2018

A

DOI: 10.1021/acs.energyfuels.8b00620 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels CaO according to the results of molecular beam mass spectrometry (MBMS) analysis, which probably affected reactions between Cs and Cl.25 However, the synergistic mechanisms of CaCl2 and CaO on the vaporization of Cs during thermal treatment have not been clarified from this measurement. In addition, the reason for the delay of Cl release caused by CaO during thermal treatment was not explained systematically. In the present work, to elucidate the role of CaO in Cs vaporization during thermal treatment with added CaCl2, thermal treatments of a Cs-doped ash, containing stable Cs (133Cs), with the addition of CaCl2, CaO, and their mixtures were examined in a lab-scale electrical-heating horizontal furnace. The resulting treated ash was subjected to X-ray diffraction (XRD) to determine the crystalline species and atomic absorption spectroscopy (AAS) to measure the Cs concentration. Knudsen effusion mass spectrometry (KEMS) was employed to examine the propensity of Cs to vaporize in Cs-doped ash as a function of the CaO content added during thermal treatment. The purpose of this study was to develop an applicable method to reduce the amount of CaCl2 required when thermal treatment was employed for Cs removal from Cs-contaminated incineration ashes.

Figure 1. XRD pattern of the Cs-doped ash.

2. EXPERIMENTAL SECTION 2.1. Cs-Doped Ash Preparation. The Cs-doped ash was prepared using a bottom ash (from coal combustion at 1400 °C) mixed with Cs2CO3 (chemical reagent, purity of >95%, Wako Pure Chemical Industries, Ltd., Osaka, Japan). The bottom ash was first fully mixed with Cs2CO3 and then heated at 900 °C in a muffle furnace for 2 h. This mixing and heating procedure was repeated twice. The distribution of Cs in the Cs-doped ash is given elsewhere.22 The elemental composition of the Cs-doped ash is shown in Table 1, and

Figure 2. Schematic representation of the horizontal furnace. with a length of 600 mm and an inner diameter of 28 mm. A thermocouple was installed in the middle of the furnace to continuously monitor the reaction temperature. Approximately 1.5 g of the Cs-doped ash was first mixed with different amounts of CaCl2· 2H2O (purity of >99%, Kanto Chemical Co., Inc., Kyoto, Japan), CaO (reagent grade, Sigma-Aldrich Co., Germany), or their mixture and then transferred to a combustion boat (99.6% Al2O3). When the reaction temperature reached the desired value, the ash-laden combustion boat was quickly moved to the middle of the reaction tube. Then, air was injected into the reaction tube at a rate of 400 mL/ min to remove the vaporized compounds. When a dwell time of 30 min was reached, the ash sample was removed from the reactor and allowed to cool naturally in an air atmosphere. The reaction temperatures were 900, 1000, and 1100 °C. In addition to a quartz filter placed near the outlet of the reactor to collect deposits for further morphological and elemental analyses by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM− EDX), an impinger containing 1 mol/L HNO3 was connected at the outlet of the reactor to capture the vaporized species in the flue gas before it was discharged into the ambient environment. The deposits condensed on the reactor, quartz filter, and gas line were carefully washed by 1 mol/L HNO3. Such washing solution was mixed with the solution in the impinger and then measured by AAS for Cs determination. The ash resulting after thermal treatment was successively subjected to pulverization, microwave-assisted acid digestion, and AAS analysis. The total recovery, defined as the percentage of the total amount of Cs in the treated ash, the solution in the impinger, and that deposited on the reaction tube, filter, and gas line to the initial amount in the ash before treatment, reached 90%. Calculations of the content of CaCl2 and CaO were carried out according to eqs 1 and 2, respectively. Here, MWCaO and MWCaCl2·2H2O denote the molecular weight of CaO and CaCl2·2H2O, respectively. In eq 1, to estimate the increase of the treated ash amount compared to that before thermal treatment, MWCaO/MWCaCl2·2H2O rather than MWCaCl2·2H2O was used in terms of a viewpoint that CaCl2 is decomposed to CaO remaining in the treated ash. The amount of added CaCl2 was 2, 5, and 10%, while the amount of CaO was 10, 20, and 30%.

Table 1. Composition of Cs-Doped Ash (wt %) species

composition

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 P2O5 TiO2 MnO2 Cl Cs (mg/kg)

47.82 36.07 5.6 2.47 0.86 1.13 0.53 0.63 0.76 3.12 0.028 0.044 4160

the XRD spectrum of the Cs-doped ash is demonstrated in Figure 1. The major crystalline compounds are mullite (Al2Si2O13) and quartz (SiO2), which is consistent with the ash composition analysis, in which the two elements Si and Al are predominant in the Cs-doped ash, as shown in Table 1. The Cs concentration in the Cs-doped ash was 4160 mg/kg, as determined by AAS. Approximately 0.3% of Cs was watersoluble, according to a leaching test [liquid/solid ratio of 50 (mL/g) and shaking time of 6 h at room temperature], indicating the combination of Cs with aluminosilicate in the prepared Cs-doped ash.22 2.2. Experimental Setup and Methods. Thermal treatment experiments were conducted in a lab-scale electrical-heating horizontal furnace, the schematic representation of which is given in Figure 2. The furnace consists of a heater and a reaction tube (99.6% Al2O3) B


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CaCl 2 (%) =

amount of ash × 100

CaO (%) =

amount of CaO × 100 amount of ash

(1) (2)

The removal efficiency of Cs from Cs-doped ash with additives during thermal treatment was evaluated by the vaporization ratio, which is defined according to eq 3 m − m2 vaporization ratio (%) = 1 × 100 m1 (3) where m1 and m2 denote the Cs amount in the Cs-doped ash before and after treatment, respectively. 2.3. Characterization of Samples. The chemical composition of the Cs-doped ash and treated ash was determined by XRF (Rigaku RINT). The crystalline compounds in the Cs-doped ash and the treated ashes were analyzed by XRD using a Cu target (40 kV and 40 mA). The Cs concentration in the ash samples was measured by AAS (Shimadzu AA6200) after acid digestion using a microwave digestion procedure. The detailed microwave digestion procedure is described in our previous study.22 The vapor pressure of Cs was determined by a modified CH5 magnetic Knudsen effusion mass spectrometer (Finnigan MAT). The details of the KEMS instrument are given elsewhere.26 In this study, approximately 70 mg of the Cs-doped ash sample was used in each experiment. Before KEMS measurement, the samples were heated to 1200 °C in a differential thermal analyzer to evaporate CO2 and SO2 impurities. Vaporization of the samples from the Knudsen cell (made of iridium) was carried out under high-vacuum conditions (10−11 atm) in two temperature modes: (1) polythermal, i.e., from 1060 to 1200 °C with 20 °C steps, a dwell time of 25 min, and 2 repeated heating− cooling cycles, and (2) isothermal, i.e., holding at 1230 °C for 66 h with 1 h steps. The vapor pressure (P, Pa) of Cs was calculated from the Cs+ ion currents according to the following equation: PCs =

kNiICs+T σCs

Figure 3. Cs vaporization ratio of the Cs-doped ash with added (a) CaCl2 and (b) CaO as well as their dependence upon the reaction temperature. The reaction time was 30 min.

electron microscopy (SEM, JEOL JSM6500) coupled with energy-dispersive X-ray spectroscopy (EDX). Figure 4 shows the morphology of the deposit and its chemical composition. The major elements include Na, K, Cs, and Cl, according to EDX analysis. The molar ratio of (Na + K + Cs) to Cl was 0.97, indicating that chlorides were the predominate species of these alkali elements. With regard to CaO addition, an increase in the CaO content and/or the reaction temperature slightly improved the Cs vaporization. However, the Cs vaporization ratio of the ash with added CaO was much lower than that with added CaCl2, and 12% Cs was vaporized from the Cs-doped ash with 30% CaO addition at 1100 °C, as shown in Figure 3b. The slag basicity (CaO/SiO2) has been found to significantly affect the Cs vaporization behavior. An increase in the content of CaO alters the slag basicity and promotes the vaporization of Cs.28 To further confirm the propensity for Cs vaporization as a function of CaO addition, KEMS experiments were performed on the Cs-doped ash mixed with different contents of CaO (0, 10, and 20%). Figure 5 presents the results of the polythermal measurements of the vapor pressure of Cs. The Cs pressure in the initial stage is higher for samples with additional CaO, which indicates that the Cs vaporization propensity is promoted with an increase in the amount of added CaO and the reaction temperature. This observation was in line with that in Figure 3b. In addition, the experiment showed that the vapor pressure was not stable and decreased during the cycles. Therefore, isothermal measurements of the time dependence of the vapor pressure of Cs at 1230 °C were carried out on the raw Cs-doped ash and the ash with 20% CaO addition (Figure 6). The results are in good agreement with the polythermal experiment. At the beginning, the vapor pressure of Cs in the 20% CaO sample was higher, and this sample had a higher

(4)

where kNi represents the sensitivity constant of the instrument measured with pure Ni, ICs+ denotes the measured ion current of Cs+, T is the temperature, and σCs is the ionization cross section of Cs taken from ref 27 at an electron ionization energy of 50 eV.

3. RESULTS AND DISCUSSION 3.1. Effects of CaCl2 and CaO Addition on Cs Vaporization. Figure 3 shows the Cs vaporization ratio as a function of the amount of CaCl2 and CaO addition and the dependence upon the reaction temperature. As shown in Figure 3a, regardless of the reaction temperature, an increase in the CaCl2 content enhanced the Cs vaporization. The Cs vaporization ratio increased from 18% with 2% CaCl2 addition to 70% with 10% CaCl2 addition at 900 °C. On the other hand, an increase in the reaction temperature from 900 to 1100 °C slightly enhanced the Cs vaporization. For instance, the Cs vaporization ratio increased from approximately 40% at 900 °C to 50% at 1100 °C with 5% CaCl2 addition, as demonstrated in Figure 3a. According to our previous study, Cs vaporization from ash with the addition of CaCl2 is mainly attributed to the formation of an intermediate compound, CsCaCl3, through the chemical reaction of Cs in the ash and CaCl2. The formed intermediate subsequently undergoes decomposition and releases gaseous CsCl.22,24 A quartz filter was placed downstream of the reactor to collect solid particulates formed from the condensation of the vaporized compounds upon flue gas cooling. The collected deposit was analyzed by scanning C


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Figure 6. Isothermal time dependence of the Cs vapor pressure from the raw Cs-doped ash and with 20% CaO addition.

of the competitive reactions of Ca and Cs with the aluminosilicates. Therefore, Cs in the ash with CaO addition possessed a relatively higher activity and, thus, preferentially vaporized. 3.2. Effect of the Mixture of CaCl2 and CaO Addition on Cs Vaporization. Figure 7 demonstrates the Cs vaporization ratio in the Cs-doped ash treated with a mixture of CaCl2 and CaO at different temperatures. The calculated results obtained on the basis of the assumption that there were no extra interactions between CaCl2 and CaO during thermal treatment are also shown for comparison. In the case of 2% CaCl2 mixed with different contents of CaO (10−30%), as shown in Figure 7a, the Cs vaporization ratios with 10, 20, and 30% CaO addition were comparable at 900 °C. An increase in the reaction temperature to 1000 and 1100 °C resulted in an increase in Cs vaporization, relative to that at 900 °C. Additionally, an increase in the CaO content from 10 to 30% at 1000 or 1100 °C promoted the vaporization of Cs. Comparing the Cs vaporization ratios of the experimental results and the calculation results shows that the experimental ratios are higher than the calculated ratios, and the difference between the ratios increased with an increase in the reaction temperature. This result clearly implies that adding a mixture of CaCl2 and CaO into the Cs-doped ash produced a synergistic effect on Cs vaporization during thermal treatment; this synergistic effect was also highly dependent upon the reaction temperature. Similar results can be observed in the case of 5 and 10% CaCl2 mixed with different CaO contents (panels b and c of Figure 7). With regard to the effect of the CaO content in the mixture of CaO and 5 or 10% CaCl2 on Cs vaporization at 900 °C, Cs vaporization was inhibited when the CaO content was increased from 10 to 30% (panels b and c of Figure 7) compared to the trivial effect when 2% CaCl2 was added (Figure 7a). For instance, the Cs vaporization ratio decreased from 61% in the case of 10% CaO to 48% in the case of 30% CaO addition for the sample with 5% CaCl2 addition at 900 °C (Figure 7b), while the ratio was reduced from 68 to 55% for the sample with 10% CaCl2 addition (Figure 7c). This phenomenon will be discussed in section 3.3. 3.3. Mechanisms of Cs Vaporization with the Addition of a Mixture of CaCl2 and CaO. 3.3.1. Cl Release Behavior during Thermal Treatment. As mentioned in the above discussion, Cs was vaporized as gaseous CsCl during thermal treatment with the addition of CaCl2. Therefore, the Cl content

Figure 4. SEM−EDX analysis of the deposit collected by the quartz filter. Experimental conditions: Cs-doped ash was mixed with 10% CaCl2 and then treated at 1100 °C for 30 min.

Figure 5. Temperature dependence of the vapor pressure of Cs from Cs-doped ashes with different CaO concentrations.

vaporization rate. After 27 h, the pressure of this sample decreased to 1.7 × 10−5 Pa, which is 2 orders of magnitude lower than the pressure of the raw ash, and the ratio did not undergo anymore changes. Thus, after 27 h, most Cs was evaporated from the sample with 20% CaO. This result was confirmed by inductively coupled plasma mass spectrometry (ICP−MS) analysis of the samples before and after the KEMS experiments, which showed that the Cs concentration in the untreated sample decreased by only 41%, whereas the Cs concentration in the sample with 20% CaO decreased by 98%. The KEMS results potentially indicate that Cs in the Cs-doped ash with added CaO became unstable during heating as a result D


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Figure 7. Effect of the addition of the CaCl2 and CaO mixture on Cs vaporization at different reaction temperatures. The calculated results were obtained on the basis of the assumption that there were no extra interactions between CaCl2 and CaO during thermal treatment. The reaction time was 30 min.

in the treated ash is significant for Cs vaporization. Cl in the treated ash was measured by X-ray fluorescence (XRF), and the vaporization ratio of Cl was calculated on the basis of eq 3, where m1 and m2 denote the Cl amount in the ash before and after treatment, respectively. Figure 8 shows the Cl vaporization

Figure 8. Effect of the CaO addition on Cl release from the Cs-doped ash during thermal treatment with the addition of CaCl2 at different reaction temperatures.

ratio as a function of the content of CaO and CaCl2 added in the ash. Regardless of the CaCl2 and CaO contents, an increase in the reaction temperature enhanced the Cl vaporization, as expected. At a certain CaCl2 content and reaction temperature, the Cl vaporization ratio decreased with an increase in the CaO content. Taking the case of 2% CaCl2 at 900 °C as an example (Figure 8a), the Cl vaporization ratio decreased from 90% without CaO addition to 10% with 30% CaO addition, suggesting that the Cl release was inhibited by CaO addition. 3.3.2. Cl Species in the Treated Ash. XRD analysis was conducted to specify the crystalline species in the treated ash. Figure 9 shows the XRD spectra of the treated ash with the addition of CaCl2 and CaO separately. At 900 °C (Figure 9a), 10% CaO addition caused a minor amount of gehlenite (Ca2Al2SiO7) formation via the chemical reaction of CaO with

Figure 9. XRD patterns of the Cs-doped ash mixed with (a) CaO and (b) CaCl2. The reaction temperatures were 900, 1000, and 1100 °C. The reaction time was 30 min.

the aluminosilicates [e.g., mullite (Al2Si2O13)] in the Cs-doped ash. When the CaO content rose to 20%, portlandite [Ca(OH)2] was detected, which was formed from the hydration of unreacted CaO in the treated ash after cooling. The dominant species in the treated ash with the addition of 30% CaO was lime (CaO), reflecting a drastic excess of CaO under this condition. An increase in the reaction temperature to 1000 and 1100 °C caused the formation of two major Ca E


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(Ca6Al5Si2O16Cl3) was produced in this process through a chemical reaction between hydrogrossular and HCl gas.

aluminosilicates, gehlenite (Ca 2 Al 2 SiO 7 ) and anorthite (CaAl2Si2O8), regardless of the added CaO content, whereas the CaO content in the treated ash was very minor, which was supported by the fact that the intensity of the CaO peaks were very weak relative to that of the Ca aluminosilicate peaks. In the case of CaCl2 addition (Figure 9b), apart from the mullite and quartz phases from the original Cs-doped ash, the major species in the treated ash was anorthite, the peaks of which were intensified upon an increase in the reaction temperature and added CaCl2 content. Figure 10 demonstrates the XRD patterns of the Cs-doped ash mixed with different CaCl2 and CaO contents. In addition

9CaO + 3CaCl 2 + 4SiO2 + 5Al 2O3 → 2Ca6Al5Si 2O16 Cl3 (5)

CaO + 2CaCl 2 + 2SiO2 + Al 2O3 → Ca3Al 2(SiO4 )2 Cl4 (6)

The formation of Cl−Ca−Si−Al−O greatly affected Cl release during thermal treatment. Understanding the relative content of Cl−Ca−Si−Al−O in the treated ash depending upon the experimental conditions, including reaction temperature and added CaCl2 or CaO content, can assist in clarifying the Cl release behavior. Because the major species in the treated ash included mullite, quartz, anorthite, gehlenite, and Cl−Ca−Si− Al−O, according to the XRD analysis, the relative content of Cl−Ca−Si−Al−O was evaluated using eq 7 A relative content of Cl−Ca−Al−Si−O (%) = 1 × 100 ∑A (7)

where A1 refers to the peak area of Cl−Ca−Si−Al−O in the XRD spectrum of each treated ash sample and ∑A refers to the total peak area of mullite, quartz, anorthite, gehlenite, and Cl− Ca−Si−Al−O for each treated ash sample. The calculation results are shown in Figure 11. The relative content of Cl−Ca−

Figure 11. Relative content of Cl−Ca−Si−Al−O in the treated ash derived from the Cs-doped ash with the addition of the CaO and CaCl2 mixture.

Si−Al−O increased with increased CaO addition, regardless of the CaCl2 content or reaction temperature. An increase in the CaCl2 content facilitated the formation of Cl−Ca−Si−Al−O at a certain CaO content, particularly at 900 and 1000 °C. Both of these results and those of Cl release in Figure 8 show that Cl vaporization was affected by the formation of Cl−Ca−Si−Al− O. Furthermore, the reaction temperature exhibited a negative effect on the formation of Cl−Ca−Si−Al−O under an identical content of CaO and CaCl2. For instance, when the contents of CaO and CaCl2 were 20 and 5%, respectively, the relative content of Cl−Ca−Si−Al−O decreased from 45% at 900 °C to 10% at 1100 °C, as shown in Figure 11b. This result can be attributed to two possible reasons. One is that the formation of Cl−Ca−Si−Al−O, produced through reactions 5 and 6, is favorable at a relatively low temperature, such as 900 °C, under the conditions in this study. The other reason is that the formation of Cl−Ca−Si−Al−O can occur at high temperatures, such as 1100 °C. However, the subsequent decomposition of this compound resulted in a decrease in its relative content.

Figure 10. XRD patterns of the Cs-doped ash with the addition of the CaCl2 and CaO mixture. The reaction temperatures were 900, 1000, and 1100 °C. The reaction time was 30 min.

to the species specified in the treated ash when CaCl2 or CaO were added individually, the new species wadalite (Ca6Al5Si2O16Cl3) and/or igumnovite [Ca3Al2(SiO4)2Cl4], denoted Cl−Ca−Si−Al−O hereafter, were detected, and their formation likely occurred through chemical reactions among CaO, CaCl2, and the aluminosilicates in the Cs-doped ash, according to reactions 5 and 6. The formation of Cl−Ca−Si− Al−O under these conditions was not surprising, as Fujita et al.29,30 showed the same result in studies upon capturing HCl from flue gas using hydrogrossular [Ca3Al2(SiO4)0.8(OH)8.8] in the temperature range of 400−950 °C. Wadalite F


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Figure 12. Vaporization of (a and b) Cs and (c and d) Cl as a function of the reaction time during thermal treatment of Cs-doped ash with the addition of 5% CaCl2 and the 5% CaCl2 and 20% CaO mixture separately.

an initial increase after 5 min at 1000 °C and 2.5 min at 1100 °C. These observations imply that Cl−Ca−Si−Al−O was formed and subsequently decomposed/reacted at 1000 and 1100 °C. However, the decomposition/reaction rate of Cl− Ca−Si−Al−O at 1100 °C was higher than that at 1000 °C, as evidenced by the fact that the relative content of Cl−Ca−Si− Al−O was approximately 45% at 1000 °C after 10 min, whereas its content decreased to 12% at 1100 °C. 3.3.4. Effect of Cl−Ca−Si−Al−O Formation on the Vaporization of Cs. In terms of the above discussion, the formation of Cl−Ca−Si−Al−O and its decomposition/reaction during thermal treatment likely play a significant role in Cs vaporization when CaO and CaCl2 were both added to the ash. To further explore the effect of Cl−Ca−Si−Al−O on Cs vaporization, synthetic ash was prepared by mixing SiO2, Al2O3, CaO, and CaCl2·2H2O in a molar ratio of 1:1:1:2 and then treating this mixture at 900 °C for 10 h in a muffle furnace. The resulting product was subsequently washed with water (to remove unreacted CaCl2), air-dried at 110 °C in an oven, and subjected to XRF/XRD analysis. The XRF results indicated that the synthetic ash contained 8.89% Cl, which was waterinsoluble because the synthetic ash had been washed with water and was probably combined with aluminosilicates, as evidenced by XRD determination. The XRD pattern of the synthetic ash after washing with water is shown in Figure 15. Wadalite (Ca6Al5Si2O16Cl3) and/or igumnovite [Ca3Al2(SiO4)2Cl4] were the major Cl-bearing compounds in the synthetic ash. Therefore, this ash can be employed to examine the role of Cl−Ca−Si−Al−O in Cs vaporization. Vaporization experiments were conducted using the Cs-doped ash blended with the synthetic ash (after washing with water). The reaction time was 30 min. The blending ratios (mass ratio) of the synthetic ash to the Cs-doped ash were 30:70 and 60:40. The Cs vaporization results are demonstrated in Figure 16. The

3.3.3. Vaporization of Cs and Cl as a Function of the Reaction Time. To confirm the mechanism governing the Cl release behavior, vaporization experiments were carried out for different reaction times. The reaction temperatures were 900, 1000, and 1100 °C. Ash samples were prepared by mixing the Cs-doped ash with 5% CaCl2 or both 5% CaCl2 and 20% CaO. The Cs and Cl vaporization as a function of the reaction time at different temperatures is demonstrated in Figure 12. Apparently, the vaporization rate of Cs in the Cs-doped ash with the addition of a mixture of CaCl2 and CaO was much higher than that in the Cs-doped ash with only CaCl2 addition, even though the added CaCl2 content was the same in both cases. At a reaction time of 10 min at 1100 °C, the Cs vaporization ratio reached nearly 90% when the mixture of CaCl 2 and CaO was added, whereas the ratio was approximately 45% when the Cs-doped ash was mixed with only CaCl2 (panels a and b of Figure 12). Conversely, the Cl release from ash treated with only CaCl2 was higher than that from ash treated by a mixture of CaCl2 and CaO (panels c and d of Figure 12), indicating that CaO addition promoted Cs vaporization by controlling the Cl release. The ash samples thermally treated for different reaction times were also subjected to XRD analysis. The XRD patterns of these samples are shown in Figure 13. The compound Cl−Ca−Si−Al−O was clearly formed, and the peak intensities were remarkably dependent upon the reaction time. The relative content of Cl− Ca−Si−Al−O as a function of the reaction time and temperature was also evaluated, and the results are summarized in Figure 14. The relative content of Cl−Ca−Si−Al−O in the ash sample treated at 900 °C increased after 10 min and then remained constant over the reaction time until 90 min, suggesting that Cl−Ca−Si−Al−O was relatively stable or unreactive at this temperature. In contrast, its content in the ash sample treated at either 1000 or 1100 °C decreased following G


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Figure 15. XRD pattern of the synthetic ash (after washing with water) resulting from reactions among SiO2, Al2O3, CaO, and CaCl2.

Figure 16. Effect of blending the synthetic ash and the Cs-doped ash on the Cs vaporization ratio. The reaction time was 30 min.

reaction temperature was increased to 1100 °C. Cs vaporization in the blends of Cs-doped ash and synthetic ash most likely resulted from a chemical reaction of Cs in the Cs-doped ash with the chlorine gas generated from the decomposition of Cl− Ca−Si−Al−O in the synthetic ash. A chemical reaction between Cs in the Cs-doped ash and chlorine gas is possible. According to the research results of Nagano et al., chlorine gas was found to be effective for the vaporization of Cs from pollucite in the temperature range of 900−1200 °C.31 Pollucite is a main Cs-bearing species in Cs-doped ash22 and is also predominate in incineration bottom ash.11,15,21 As a result of the consumption of released chlorine by Cs, Cl−Ca−Si−Al−O continues to decompose and produce chlorine, despite its relatively high thermal stability. In the samples with added CaCl2, the reaction between CaCl2 and Cs in the Cs-doped ash to form CsCaCl3 was the major cause of Cs vaporization.22,24 When CaO and CaCl2 coexisted in the ash sample, the vaporization of Cs was attributed to two routes. One is the formation of CsCaCl3, which also occurred when only CaCl2 was added, while the other is the reaction between Cs in the Cs-doped ash and chlorine gas released from the decomposition of Cl−Ca−Si−Al−O. The formation of Cl−Ca−Si−Al−O delayed the release of Cl and, in turn, provided a relatively long contact time with Cs in the Cs-doped ash, thus promoting the vaporization of Cs. Additionally, CaO added in the Cs-doped ash during thermal treatment likely destroyed the local structure of Cs aluminosilicates, as mentioned in section 3.1, and thus, unstable Cs was prone to associate with Cl to form gaseous CsCl. As a result, Cs vaporization readily occurred when a mixture of CaO and CaCl2 was added to the ash during thermal treatment. The

Figure 13. XRD patterns of the Cs-doped ash with the addition of the 5% CaCl2 and 20% CaO mixture treated for different times and at different temperatures.

Figure 14. Relative content of Cl−Ca−Si−Al−O in treated ash samples as a function of the reaction time and temperature. The added CaO and CaCl2 contents were 20 and 5%, respectively.

blending ratio of the synthetic ash and the reaction temperature were found to influence Cs vaporization. An increase in the mass ratio of the synthetic ash in the blended ash led to a higher Cs vaporization ratio, regardless of the reaction temperature. Less than 15% of the Cs was vaporized at 900 °C, even if the ratio of the synthetic ash was 60% in the blended ash, whereas the ratio increased to approximately 70% when the H


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(CaAl2Si2O8) and/or gehlenite (Ca2Al2SiO7), according to the XRD analysis in Figure 9a. As per the discussion in section 3.3.2, Cs vaporization was affected by the formation of Cl−Ca− Si−Al−O via a chemical reaction among the ash, CaCl2, and CaO. However, Cl−Ca−Si−Al−O formation was probably not favorable in the reaction between CaCl2 and Ca aluminosilicates; thus, the synergistic effect on Cs vaporization was marginal. This result is of great importance to treat the ashes. The CaO content rather than the total Ca content in the ash is crucial when deciding the amount of CaCl2 to add to the ash during thermal treatment. The results in this study were based on the experiments on Cs-doped ash, which contain a higher concentration of Cs relative to that in raw ash. However, our previous studies have proven that the Cs vaporization behaviors of Cs-doped ash and raw ash are very similar.22 Therefore, the results observed in Cs-doped ash should also be observed in raw ash. Accordingly, two raw bottom ashes were employed for thermal treatment to verify the synergistic effect of the addition of the CaCl2 and CaO mixture. The CaCl2 and CaO contents were 5 and 20%, respectively. The elemental composition of the two bottom ashes is shown in Table 2. The Cs concentration was less than 5

formation of Cl−Ca−Si−Al−O consumed part of CaCl2 and thus, to some extent, prevented the formation of CsCaCl3, which is a possible reason for the decrease in the Cs vaporization ratio upon an increase in the CaO content at 900 °C at a reaction time of 30 min when the CaCl2 content was 5 and 10%, respectively (panels b and c of Figure 7). 3.4. Implications. The effect of individual CaCl2 and CaO addition and the addition of their mixture on Cs vaporization from Cs-doped ash was investigated. The addition of a mixture of CaCl2 and CaO was shown to produce a synergistic effect on Cs vaporization compared to the addition of the individual species when the reaction temperature was above 1000 °C. During the incineration processes, Ca compounds, such as CaCO3, are usually injected into the furnace to capture acid gases, such as SO2 and HCl,32,33 resulting in a large amount of Ca in the ashes. Ca in the incineration bottom ashes is normally combined with aluminosilicates. Therefore, it is unclear whether Ca aluminosilicates can also produce synergistic effects on Cs vaporization via a reaction with added CaCl2 during thermal treatment. Two experiments were designed to clarify the role of Ca (CaO or Ca aluminosilicates) in the ash on Cs vaporization when CaCl2 was used as a chlorinating reagent. In the first experiment (denoted the one-step experiment), the Csdoped ash was mixed with 2% CaCl2 and 20% CaO and then treated at 1100 °C for 30 min. In the second experiment (denoted the two-step experiment), the Cs-doped ash was initially mixed with 20% CaO and treated at 1100 °C for 30 min; the resulting treated ash was subsequently mixed with 2% CaCl2 and then subjected to a second thermal treatment at 1100 °C for 30 min. The Cs vaporization ratios in these two conditions are shown in Figure 17. Here, the calculated results

Table 2. Composition of Two Bottom Ashes (wt %) composition species

bottom ash 1

bottom ash 2

SiO2 Al2O3 Fe2O3 K2O Na2O CaO MgO TiO2 SO3 P2O5 MnO Cs (mg/kg)

51.1 27.8 8.62 1.14 0.563 6.26 1.24 1.28 0.19 0.997 0.141 4.89

60.9 23.4 5.77 1.19 0.568 4.61 1.08 1.13 0.113 0.509 0.11 4.40

mg/kg, according to ICP−MS analysis (Agilent 8800). The experiment was conducted at 1100 °C for 30 min. The Cs vaporization ratio in the two raw ashes is shown in Figure 18. The Cs vaporization ratio in the two ashes containing the mixture of CaCl2 and CaO was much higher than that with CaCl2 or CaO alone, which was similar to the results observed in the Cs-doped ash experiments. According to the findings of

Figure 17. Effect of adding CaO on Cs vaporization. The calculated results were obtained on the basis of the assumption that there were no extra interactions between CaCl2 and CaO during thermal treatment. The one-step experiment denotes that the Cs-doped ash was mixed with both 2% CaCl2 and 20% CaO and then treated at 1100 °C for 30 min. The two-step experiment denotes that the Cs-doped ash was initially mixed with 20% CaO and then treated at 1100 °C for 30 min; the resulting treated ash was subsequently mixed with 2% CaCl2 and then subjected to a second thermal treatment at 1100 °C for 30 min.

were obtained on the basis of the assumption that there were no extra interactions between CaCl2 and CaO. The Cs vaporization ratio in the one-step experiment was much higher than the calculated ratio. However, the Cs vaporization ratio in the two-step experiment was very close to the calculated ratio, indicating the insignificance of the synergistic effect on Cs vaporization. In the first step of the two-step experiment, CaO was combined with aluminosilicate in the ash to form anorthite

Figure 18. Effect of the addition of 5% CaCl2, 20% CaO, and 5% CaCl2/20% CaO on the vaporization of Cs in two bottom ashes at 1100 °C for a reaction time of 30 min. I


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this study, adding a mixture of CaCl2 and CaO can greatly decrease the amount of CaCl2 added, which resulted in at least two advantages. One is that the cost could be reduced if thermal treatment with CaCl2 addition was adopted to treat the Cs-contaminated incineration ashes, and the other is that further treatment of the acid flue gas and corrosion of the treatment facilities during thermal treatment were highly mitigated.

4. CONCLUSION The synergistic effect of the addition of a mixture of CaCl2 and CaO on the vaporization of Cs from incineration ash during thermal treatment was systematically examined. The main conclusions are as follows: (1) The addition of mixtures of CaCl2 and CaO into the Cs-doped ash greatly promoted Cs vaporization compared to the individual addition of CaCl2 and CaO when the reaction temperature was over 1000 °C. An increase in the CaO content enhanced the synergistic effect of the addition of the CaCl2 and CaO mixture on Cs vaporization. However, only CaO addition resulted in the slight vaporization of Cs as a result of changes in the local structure of the Cs aluminosilicates. (2) Cl−Ca−Si−Al−O was formed through a chemical reaction among CaCl2, CaO, and the aluminosilicates when the mixture of CaCl2 and CaO was added to the Csdoped ash. The release of Cl was delayed as a result of the formation of thermally stable Cl−Ca−Si−Al−O, and thus, the contact time of Cs with gaseous Cl was extended. The presence of CaO in the ash destabilized Cs in the Cs aluminosilicates, and unstable Cs was readily extracted by gaseous Cl released from the decomposition of Cl−Ca−Si−Al−O.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-568-51-9178. Fax: +81-568-51-1499. E-mail: [email protected]. ORCID

Yoshihiko Ninomiya: 0000-0002-3523-9666 Marc Bläsing: 0000-0002-6116-1604 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from Grant A of Chubu University, Japan, and Japan Society for the Promotion of Science (JSPS) KAKENHI Grants JP15F15051 and JP16H02983.

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REFERENCES

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Synergistic Mechanisms of CaCl2 and CaO on the Vaporization of Cs

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