Comparative Study on Extracting Alumina from Circulating Fluidized

Nov 19, 2013 - Fluidized-Bed and Pulverized-Coal Fly Ashes through Salt ... Up to 93.3% of the ..... This explains why up to 92.3% of the alumina coul...
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Comparative Study on Extracting Alumina from Circulating Fluidized-Bed and Pulverized-Coal Fly Ashes through Salt Activation Chunbin Guo, Jingjing Zou, Cundi Wei,* and Yinshan Jiang Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering, Jilin University, 5988 People’s Avenue, Changchun 130025, People’s Republic of China ABSTRACT: Alumina was extracted from mixtures of circulating fluidized-bed fly ash (CFBFA)/pulverized-coal fly ash (PCFA) and sodium pyrosulfate (Na2S2O7) through salt-roasting activation. The effects of the sodium pyrosulfate amount, calcining temperature, and calcining time on the efficiency of extracting alumina from CFBFA and PCFA were analyzed and compared. The results showed that adding Na2S2O7 to CFBFA and PCFA released alumina in the form of Na3Al(SO4)3. Up to 93.3% of the alumina was extracted from the PCFA calcined at 420 °C for 2 h when n(Al2O3)/n(Na2S2O7) = 1:3; however, 92.8% of the alumina was extracted from the CFBFA calcined under the same conditions when n(Al2O3)/n(Na2S2O7) = 1:5, meaning that the CFBFA consumed more sodium pyrosulfate than the PCFA because the Al−O−Si bonds in the CFBFA were more active than those in the PCFA. A new amorphous aluminosilicate phase formed while the CFBFA was calcining with sodium pyrosulfate. Meanwhile, Na2Ca(SO2)2 and CaAl2O4 formed at the beginning of the reaction, which prevented the alumina from dissolving and provided resistance against mass transfer.

1. INTRODUCTION Coal fly ash (FA) is a major solid waste product of coal-fired power plants;1−4 huge amounts of it may be produced during power generation. About 800 million tons of coal FA is annually generated worldwide.5−7 Coal-fired power plants generate a major portion of China’s electricity and annually discharge more than 4 × 108 tons of FA.8,9 FA-based materials can be beneficially used in construction, waste stabilization, and even agricultural soil amendment. Only 25% of the FA produced is used globally;10 most is released into the environment through various mechanisms, the most common of which is by dumping it directly onto land, which poses a serious threat to the environment.11 Therefore, developing an efficient and safe method of disposing of FA is of great interest. FA is a potential substitute for bauxite in alumina production17,18 because it contains ∼10−55 wt % Al2O3.12−16 Extracting Al2O3 from FA has attracted significant attention. The extraction methods mainly involve leaching the FA with acid19,20 or alkali21,22 solutions or a combination of both,23 and each shows unique advantages. Nevertheless, few researchers have investigated salt activation24−31 as a method of extracting alumina, especially from FA generated from various combustion systems. The composition and properties of FA are related to the quality of the coal burned and the combustion process used.32 The conventional pulverized-coal (PC) and circulating fluidized-bed (CFB) combustors are most common. Although PCfired boilers still dominate the electric power industry, which produces about 50% of the world’s electricity, CFB combustors are being used as a new combustion technology in increasingly more power plants. Fine coal particles are put into a combustor and are directly fired in air during PC combustion. Limestone is always added to the CFB during circulating combustion. The combustion temperature in CFB boilers is always lower than that in conventional PC boilers.33 The ashes generated by CFB and PC boilers show some differences because of the different © 2013 American Chemical Society

combustion temperatures and modes and the size of the feedcoal particles used for the boilers; all of these parameters directly affect alumina extraction. Sodium pyrosulfate (Na2S2O7) roasting is an effective new method of activating FA, where alumina is separated from silica by converting the alumina in various phases of FA into soluble Na3Al(SO4)3 when a mixture of sodium pyrosulfate and FA is calcined. Most of the impurity elements, such as Ca and Si, remain in the residue when the calcined mixture is filtered, and potassium permanganate can be used to remove the iron in the solution.34 High-purity alumina is subsequently produced by calcining Al2(SO4)3·nH2O, which can be obtained using evaporation crystallization,35 at 1000 °C for 2 h. This method of extracting Al2O3 from FA is simple, involves the use of simple equipment, and produces recyclable sulfates and small amounts of residue. Furthermore, the whole process can be easily industrialized.36 In this work, we investigated the relations between the activation parameters on the one hand and the phase composition and efficiencies of extracting alumina from circulating fluidized-bed fly ash (CFBFA) and pulverized-coal fly ash (PCFA) on the other hand. The calcined CFBFA and PCFA were characterized using X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectroscopy, and thermogravimetric analysis and differential scanning calorimetry (TGA−DSC). The mechanisms for extracting alumina from CFBFA and PCFA and the characteristics of the alumina extracted under different experimental conditions were also investigated. Received: August 19, 2013 Revised: November 18, 2013 Published: November 19, 2013 7868

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Table 1. Compositions of FA Samples (wt %) sample

SiO2

Al2O3

TiO2

CaO

MgO

Fe2O3

FeO

K2O

Na2O

LOS

PCFA CFBFA

36.30 44.74

51.70 45.94

1.75 0.72

2.68 1.97

0.24 0.06

1.28 0.67

0.30 0.55

0.34 0.25

0.09 0.18

2.21 3.46

2. EXPERIMENTAL SECTION

ω(Al 2O3) =

2.1. Materials and Reagents. The PCFA and CFBFA raw materials used in this study were obtained from the Jungar Power Plant in Inner Mongolia, China. The same coal is used for all of the boilers there. The samples were collected from the part of the boilers located after the baghouse filters of PC and CFB combustors. About 10 kg of material was collected at 2 h intervals over 3 days for each ash sample. The samples were immediately stored in polyvinylchloride (PVC) Ziplock bags to prevent the samples from becoming contaminated and from absorbing water from the air. Duplicate samples from each day were mixed to obtain a representative sample. The chemical components of the samples, listed in Table 1, were determined at The First Geological Survey, Changchun, Jilin, China. The accuracy of chemical compositions was evaluated using the Chinese standard GB/T14506-2010 (silicate rock). The analytical results were averaged over all 3 days to represent each FA. The uncertainty in the chemical composition of each analyzed component was estimated within ±3%. Table 1 lists alumina and silica, whose sum accounts for more than 85 wt % of the FA, as the main chemical components of the PCFA and CFBFA. The alumina content of the PCFA reaches 51.7 wt %, higher than that of the CFBFA. This unique characteristic makes PCFA a potential alternative to bauxite for alumina production. In addition, both samples contained trace amounts of Fe2O3, FeO, CaO, MgO, K2O, Na2O, TiO2, unburnt carbon, etc. Sodium pyrosulfate used in this study is an analytical-grade reagent purchased from the East China Reagent Factory, Tianjin, China. All other chemicals were commercially available analytical-grade reagents. 2.2. Experimental Procedure. Two samples of FA were ground and passed through a 200-mesh sieve. The milled FA samples were dried in an oven at 110 °C for 24 h and then well-mixed with a measured amount of sodium pyrosulfate to use thermal analysis to determine the thermal characteristics of the reaction mixtures. Sodium pyrosulfate was mixed with CFBFA and PCFA in various Al2O3/Na2S2O7. The mixtures were placed in covered alumina crucibles, then heated in air at various temperatures for various lengths of time, and subsequently cooled to room temperature. The CFBFA and PCFA were filtered and washed with distilled water, and the alumina concentrations in the filtrates were determined using chemical analysis. All of the experiments were replicated 10 times, and the mean data were used. The uncertainty in the data was estimated within ±2% for the analyzed alumina. 2.3. Characterizations. The crystalline phases in the FA and calcined samples were characterized using XRD (DX-2700, Dandong Fangyuan, Dandong, China) with Cu Kα radiation. The acceleration voltage was 35 kV, and the electrical current was 25 mA. The scans ranged from 10° to 60° at 3°/min. FTIR spectroscopy (Nexus-6700 spectrometer equipped with a FTIR-grade KBr beam splitter, Thermo Electron Corporation) was conducted on the samples at room temperature. The spectra were acquired in the range of 4000−400 cm−1 wavenumber with 2 cm−1 resolution. The background spectrum for KBr was also recorded under the same conditions for reference. The samples were analyzed using TGA−DSC under nitrogen in an integrated thermal analyzer (STA449C, NETZSCH, Germany). A 10 mg sample was loaded into the thermal analyzer and heated between 25 and 1000 °C at 5 °C/min. The concentrations of aluminum in the filtrates were determined using ethylenediaminetetraacetate formation constant (EDTA−KF) titrimetric analysis.37 Alumina was used as the standard material to determine the concentrations of aluminum. The alumina extraction [ω(Al2O3)] was calculated according to eq 1

mF(Al 2O3) mFA (Al 2O3)

(1)

,where mF(Al2O3) and mFA(Al2O3) denote the masses of Al2O3 in the filtrate and FA, respectively.

3. RESULTS AND DISCUSSION 3.1. PCFA and CFBFA Characterization. The crystallographic components of the PCFA and CFBFA samples were determined using XRD. Figure 1 shows that the major

Figure 1. XRD patterns for PCFA and CFBFA.

crystalline phases in the PCFA samples were mullite and corundum and that the samples showed relatively high crystallinity. The XRD pattern for the PCFA sample shows peaks at 2θ = 16.5°, 25.9°, 26.2°, 35.2°, and 40.8°, which can be indexed to mullite, and peaks at 2θ = 25.6°, 37.8°, 43.4°, and 64.5°, arising from corundum. An obvious embossment appears at 2θ ≈ 22°, similar to the peak associated with tridymite and cristobalite.38 The high percentage of the glass phase in the PCFA sample is due to the PCFA rapidly cooling from temperatures above 1200 °C, and the broad, weak embossment from 10 to 25 °C also indicates the siliceous glass phase in the samples. In contrast to the PCFA sample, the CFBFA sample contains a large proportion of an amorphous phase, which is strongly evidenced by several broad peaks in the XRD pattern, and shows weak indication of a crystalline mineral. Quartz may be the primary component in CFBFA because of its high melting point (1713 °C). The formation of calcium sulfate is attributed to limestone-assisted desulfurization during coal firing in the CFB boiler. The calcite in the CFBFA may be a secondary phase formed from CaO at low temperatures.39 FTIR spectra for the PCFA and CFBFA samples are presented in Figure 2. The spectra are similar, each showing three obvious absorption peaks at ∼1099, ∼562, and ∼459 cm−1. The strongest peak at 1099 cm−1 is assigned to Si−O−Si and Al−O−Si antisymmetric stretching vibrations. The bands 7869

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mass loss below 130 °C shows endothermic DSC peaks at 50 and 125 °C, indicating ∼3.0% mass loss, and another peak at 55 °C, indicating ∼3.2% mass loss, all of which are due to the physically adsorbed water evaporating from the sample. The second-stage mass loss between 130 and 245 °C shows endothermic DSC peaks at 192 and 197 °C, indicating mass losses of 2.4 and 2.1%, respectively, which can be assigned to water evaporating from the Na2S2O7 crystal. The third-stage mass loss in the range of 245−515 °C shows endothermic DSC peaks at 396 and 401 °C, indicating mass losses of ∼13.5 and ∼17.02%, respectively, suggesting that decomposing Na2S2O7 had actively reacted with coal FA. The endothermic DSC peaks at 637 and 635 °C are associated with the production of aluminum and sodium sulfates through the thermal decomposition of Na3Al(SO4)3. The DSC curves indicate that a significant thermal effect happens with an increasing temperature and that the peak temperature of PCFA is higher than that of CFBFA. Despite the difference, the PCFA and CFBFA showed similar reaction mechanisms when Na2S2O7 was added. 3.3. Effect of Calcination Activation on Alumina Extraction. 3.3.1. Effect of Calcination Additives on Alumina Extraction. Sodium pyrosulfate and dry FA were accurately weighed, so that the Na2S2O7/Al2O3 ratio of the mixtures was 1:1, 2:1, 3:1, 4:1, and 5:1, to investigate the effect of Na2S2O7/ Al2O3 on alumina extraction. PCFA (10 g) and CFBFA (10 g) were mixed with sodium pyrosulfate in separate ceramic mortars. The mixtures were transferred to separate crucibles and were heated at 300 °C/h to 420 °C, where they were maintained for 2 h and subsequently cooled. The products were then dispersed in a 1:3 solid/liquid [m (g)/V (mL)] ratio in water at 98 °C for 30 min and then filtered and washed with distilled water. The filter residues were dried under vacuum at 105 °C for 24 h. The alumina concentrations in the filtrates were analyzed, and the results are shown in Figure 4. The amount of alumina extracted from PCFA clearly increases with increasing Na2S2O7/Al2O3, reaching 92.3% when Na2S2O7/ Al2O3 was 3:1. The amount of alumina extracted from CFBFA was 78.8% at the same Na2S2O7/Al2O3. More than 90% of the alumina was extracted from the CFBFA when Na2S2O7/Al2O3 was increased to 5:1. This result is in contrast to the hypothesis

Figure 2. FTIR spectra for PCFA and CFBFA samples.

centered at 562 and ∼459 cm−1 represent Al(VI)−O−Si stretching and Si−O bending vibrations,40 respectively. Several weak adsorption bands at ∼739, ∼874, and ∼1143 cm−1 are assigned to the stretching vibration of Al(IV)−O−Si and vibration stretch of Si−O. The vibration at 814 cm−1 is assigned to the Al−O stretch and the stretching modes involving mainly tetrahedral atoms.41 This result indicates that the PCFA sample contains a higher percentage of crystalline phases than the CFBFA sample and that the major component of the crystalline phase was mullite, which is consistent with the XRD patterns for the FA samples, as shown in Figure 1. Although the XRD/FTIR spectra suggest that the chemical compositions of the PCFA and CFBFA samples were similar, the phase compositions of the PCFA and CFBFA samples were obviously different. We therefore assume that CFBFA is more active than PCFA. 3.2. TGA−DSC Characterization. Figure 3 shows the weight-loss and heat-flow data obtained from the TGA−DSC analysis of the PCFA and CFBFA samples to which Na2S2O7 was added. The DSC curves show a series of exothermic and endothermic peaks in the range of 50−650 °C. The first-stage

Figure 3. TGA−DSC curves for pure FA samples and those mixed with sodium pyrosulfate.

Figure 4. Effect of the Na2S2O7/Al2O3 ratio on aluminum extraction. 7870

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This phenomenon indicates that the Al−O−Si bond, which is highly active in CFBFA, broke and then recombined to form a new aluminosilicate, which required more sodium pyrosulfate to activate the bond. This explains why up to 92.3% of the alumina could be extracted from PCFA, while only up to 78.8% could be extracted from CFBFA when Na2S2O7/Al2O3 was 3:1. Almost all of the Al−O−Si bonds had broken, transforming the alumina into Na3Al(SO4)3, when Na2S2O7/Al2O3 was further increased to 5:1. 3.3.2. Effect of the Calcination Temperature on Alumina Extraction. Sodium pyrosulfate was mixed with separate 10 g samples of CFBFA and PCFA, so that Na2S2O7/Al2O3 = 3:1, to determine how the calcination temperature affected alumina extraction. The mixtures were heated at 320, 380, 420, and 460 °C for 2 h and subsequently cooled. The products were then dissolved in water, filtered, and washed with distilled water. The alumina extraction rates are shown in Figure 6. The amounts of

that aluminum was more active in CFBFA than in PCFA, which could react with sodium pyrosulfate when Na2S2O7/Al2O = 3:1 to form Na3Al(SO4)3. Figure 5 displays FTIR spectra in the region of 4000−400 cm−1 for the filter residues prepared using various Na2S2O7/

Figure 6. Effect of the calcination temperature on alumina extraction.

alumina extracted from the CFBFA and PCFA samples calcined at various temperatures were almost identical. Slightly more alumina was extracted from CFBFA than from PCFA for the samples calcined at 320 °C, but the opposite results were obtained for the samples calcined from 380 to 460 °C. The optimal calcination temperature for CFBFA and PCFA was 420 °C. XRD was used to analyze the products calcined at various temperatures, to compare the characteristics of the CFBFA and PCFA calcinations. The XRD patterns obtained for the calcined PCFA and CFBFA are shown in Figures 7 and 8, respectively. The calcined PCFA was mainly composed of sodium pyrosulfate and mullite, and only 3.9% of the alumina was extracted from the sample calcined at 320 °C. However, peaks characteristic of Na3Al(SO4)3 began appearing in the XRD pattern for the CFBFA calcined at 320 °C, and 10.2% of the alumina was extracted from this sample, indicating that, although most of the alumina in the calcined PCFA sample had not reacted with sodium pyrosulfate, part of the highly active alumina in the calcined CFBFA sample had begun to react. The intensity of the peaks attributed to sodium pyrosulfate and mullite decreased when the calcination temperature was increased to 380 °C. Meanwhile, Na3Al(SO4)3

Figure 5. FTIR spectra for PCFA, CFBFA, and corresponding sodium pyrosulfate mixtures.

Al2O3. The characteristic peaks of mullite at 874.82, 739.17, and 562.21 cm−1 in the spectrum for PCFA and those of partial kaolinite at 814.24 and 563.50 cm−1 in that for CFBFA gradually weaken with increasing sodium pyrosulfate content and merge into new vibration peaks at 808.30 and 801.05 cm−1, respectively, which are attributed to the vibration stretch of Si− O. The characteristic peaks of mullite disappeared in the spectrum for PCFA when Na2S2O7/Al2O3 was increased to 3:1. However, the peak at 563.50 cm−1 associated with the Al(VI)− O−Si vibration shifted toward lower frequency at ca. 602.31 cm−1, which is assigned to the stretching vibration of Al−O−Si. The peak at 563.50 cm−1 continued shifting toward lower frequency with an increasing sodium pyrosulfate content. The peaks at 563.50 and 814 cm−1 merged into a peak at 801 cm−1, attributed to Si−O vibration, when Na2S2O7/ Al2O3 was 5:1. 7871

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intensity of the peaks attributed to Na3Al(SO4)3 reached a maximum in the patterns for both ashes, and 93.3 and 78.8% of the alumina were extracted from the calcined PCFA and CFBFA samples, respectively. These results show that the PCFA and CFBFA had almost completely reacted with sodium pyrosulfate. The XRD patterns for the PCFA and CFBFA calcined at 460 °C still showed peaks corresponding to Na3Al(SO4)3, and the peaks did not show any obvious change in intensity; 94.5 and 80.1% of the alumina were extracted from the PCFA and CFBFA, respectively. 3.3.3. Effect of the Calcination Time on Alumina Extraction. Sodium pyrosulfate was mixed with separate 10 g samples of CFBFA and PCFA, so that Na2S2O7/Al2O3 was 3:1. The mixtures were heated at 420 °C for 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 h and were subsequently cooled. The calcined CFBFA and PCFA were then washed with distilled water and filtered. The efficiencies for extracting alumina from the CFBFA and PCFA are shown in Figure 9. The calcination time similarly affected the amounts of alumina extracted from CFBFA and PCFA.

Figure 7. XRD patterns for PCFA calcined with sodium pyrosulfate at various temperatures.

Figure 9. Effect of the calcination time on aluminum extraction. Figure 8. XRD patterns for CFBFA calcined with sodium pyrosulfate at various temperatures.

Figures 10 and 11 show XRD patterns for the PCFA and CFBFA calcined for various lengths of time, respectively. Sodium pyrosulfate and sodium hydrogen sulfate were the main phases in the PCFA and CFBFA calcined for 0.5 h, indicating that the reactions between the PCFA/CFBFA and sodium pyrosulfate were incomplete after 0.5 h. Peaks characteristic of sodium pyrosulfate and sodium hydrogen sulfate disappeared from the XRD patterns, and the intensities of the peaks attributed to Na3Al(SO4)3 did not significantly change when the calcination times were prolonged to 1.0, 1.5, 2.0, 2.5, and 3.0 h, indicating that a new phase had not formed. These results suggest that the reaction between the PCFA/CFBFA and sodium pyrosulfate was incomplete until the calcination time was longer than 1 h. The optimal calcination time should be 2 h in practice to ensure that the reaction is more complete. 3.4. Mechanism for Salt Activation of FA Samples. Two previous reports43,44 indicate that adding pyrosulfates, such as sodium pyrosulfate (Na2S 2O7 ) and potassium pyrosulfate (K2S2O7), to aluminosilicate minerals or coal FAs could form the sulfate phase when the mixture is melted. Pyrosulfates are ionic compounds whose sulfur anions are oxidized. The pyrosulfate anion, S2O72−, is the dimer of the sulfate anion, SO42−, which has one bridging oxygen atom. The

had formed as a product of the calcined PCFA, and 57.6% of the alumina was extracted from the PCFA calcined at 380 °C. The peak attributed to Na3Al(SO4)3 was more intense in the XRD pattern for the calcined CFBFA than in the peak for the calcined PCA, and peaks associated with new phases, such as Na2Ca(SO4)2 and CaAl2O4, appeared in the pattern for the former. Further, 43.3% of the alumina was extracted from the CFBFA calcined at 380 °C. These results indicate that the PCFA and CFBFA calcined at 380 °C had not completely reacted with sodium pyrosulfate. Na2Ca(SO4)2 probably formed because Ca(SO4)2 in the calcined CFBFA reacted with Na2SO4, which had decomposed from sodium pyrosulfate. CaAl2O4 formed through the reaction between the activated alumina and calcium oxide. Both new compounds that formed during calcination formed barriers, preventing the alumina from dissolving, and provided resistance against mass transfer.42 This is one of the reasons less alumina was extracted from CFBFA than from PCFA. All of the peaks attributed to sodium pyrosulfate and mullite completely disappeared from the XRD pattern for the PCFA calcined at 420 °C. Meanwhile, the 7872

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The main chemical reactions for roast activating a mixture of PCFA and sodium pyrosulfate are described as follows: Na 2S2 O7 → Na 2SO4 + SO3

(2)

Al 2O3 (corundum) + 3Na 2S2 O7 → 2Na3Al(SO4 )3

(3)

Al 6Si 2O13 (mullite) + 9Na 2S2O7 → 6Na3Al(SO4 )3 + 2SiO2

(4)

Al 2O3 (glass‐phase alumina) + 3Na 2S2 O7 → 2Na3Al(SO4 )3

(5)

The main chemical reactions for roast activating a mixture of CFBFA and sodium pyrosulfate are described as follows: Al4Si4O14 (metakaolinite) + Na 2S2 O7 → 4Na3Al(SO4 )3 + 4SiO2 Figure 10. XRD patterns for PCFA calcined with sodium pyrosulfate for various lengths of time.

(6)

CaSO4 + Na 2SO4 → Na 2Ca(SO4 )2

(7)

CaO + Al 2O3 → CaAl 2O4

(8)

The results of this work demonstrate that whether the aluminosilicate decomposes or is destroyed varies for FA species. The Al−O−Si bonds are amorphous in CFBFA, which are more active than the crystalline bonds in PCFA. The different combustion conditions that produce CFBFA and PCFA may lead to the different amounts of alumina extracted from them through salt-roasting activation.

4. CONCLUSION (1) The amounts of alumina extracted from CFBFA and PCFA through salt-roasting activation were analyzed and contrasted. The CFBFA/PCFA and sodium pyrosulfate mixtures can react to form water-soluble Na3Al(SO4)3 during calcination. This method can be used to extract alumina from FA. (2) The Na2S2O7/Al2O3, calcining temperature, and calcining time significantly affected the rate of alumina extraction. The efficiency of extracting alumina from PCFA can reach 93.3% when n(Na2S2O7)/n(Al2O3) = 3:1, the calcination temperature is 420 °C, and the calcination time is 2 h. The efficiency of extracting alumina from CFBFA is 78.8% for the same Na2S2O7/Al2O3. The efficiency of extracting alumina can reach 92.8% when Na2S2O7/Al2O3 is 5:1 for the same calcination temperature and time. (3) The FTIR spectra and XRD patterns indicated that the Al−O−Si bonds in CFBFA showed better activity than those in PCFA, which formed new amorphous aluminosilicate during calcination with sodium pyrosulfate. Thus, more sodium pyrosulfate was required to activate and decompose the newly formed aluminosilicate into Na3Al(SO4)3. Meanwhile, the formation of Na2Ca(SO4)2 and CaAl2O4 formed a barrier preventing alumina from dissolving and provided resistance against mass transfer at the beginning of the reaction. These two findings explain why less alumina was extracted from CFBFA than from PCFA.

Figure 11. XRD patterns for CFBFA calcined with sodium pyrosulfate for various lengths of time.

sulfate groups connect with adjacent Al3+ ions, forming complex three-dimensional networks, in which all of the oxygen atoms in the nearly perfectly tetrahedral SO42− groups are bonded with the aluminum ions, such that the oxygen atoms are not terminally bonded with aluminum, and all of the oxygen atoms bonded with aluminum ions are further connected to sulfur atoms (Al−O−S bridging). The sulfategroup tetrahedra have terminal S−O bonds significantly shorter than the Al-bridging S−O bonds. The Na+ ions are located between the AlO6 octahedra and the SO4 tetrahedra, completing the three-dimensional network of the Na3Al(SO4)3 structures.45 Levin et al.46 analyzed the Na2SO4−Al2(SO4)3 phase diagram and calculated the thermodynamics of the system to predict the mechanism for the salt activation of this system. The Na+ ions seem to act as electron acceptors that interact with oxygen atoms in aluminosilicate, and Al3+ is expected to form anionic−sulfate complexes through destroying the original lattice structure.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: 86-0431-85094856. E-mail: weicundi_jlu@ 163.com. Notes

The authors declare no competing financial interest. 7873

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant 41072025).

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