Role of additives in improved thermal activation of coal fly ash for alumina extraction

Fuel Processing Technology 110 (2013) 114–121

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Role of additives in improved thermal activation of coal fly ash for alumina extraction Yanxia Guo a, Yaoyao Li a, Fangqin Cheng a,⁎, Miao Wang a, Xuming Wang b a

State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Institute of Resources and Environment Engineering, College of Environmental & Resource Sciences, Shanxi University, Taiyuan 030006, China b Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, Salt Lake City, UT, USA

a r t i c l e

i n f o

Article history: Received 16 April 2012 Received in revised form 14 November 2012 Accepted 2 December 2012 Available online 27 December 2012 Keywords: Coal fly ash Thermal activation Alumina extraction NaOH Na2CO3

a b s t r a c t The extraction of alumina from coal fly ash is a good direction for its value-added utilization. The presence of the inert matters with high degree of polymerization, such as mullite and other aluminosilicates, makes the reactivity of coal fly ash very poor. The activation of coal fly ash is necessary before its utilization. The thermal activation calcination with the addition of NaOH and Na2CO3 was carried out in this research. The results showed that the addition of NaOH and Na2CO3 improved the alumina extraction evidently. The maximum alumina extraction reached ~60% when calcination at 600–900 °C with the addition of NaOH and that could reach 82% at 900 °C with the addition of Na2CO3. Detailed analysis and characterization was carried out by using thermal gravimetric and differential scanning calorimetric analysis (TG-DSC) and X-ray diffraction (XRD). The results indicated that NaOH and Na2CO3 facilitated the decomposition of the polymeric phases. At lower temperatures (b 600 °C), NaOH played a main role while Na2CO3 did at higher temperatures (> 700 °C). As a result, the mixed additives containing NaOH and Na2CO3 made alumina extraction attain 95% at 700 °C. © 2012 Published by Elsevier B.V.

1. Introduction Coal fly ash (CFA) is generated during high temperature combustion of coal in coal-fired power plants, and also in the smelting, and chemical industries [1]. Currently, about 800 million tons of coal fly ash has been generated in the world [1–3]. In China, electricity generation is majorly from coal-fired power plants. The annual fly ash discharge is more than 4 × 10 8 tons in China [4,5]. The fly ash is the major coal solid waste, beside coal waste and coal slime. Due to the low utilization rate, coal fly ash is generally dumped in lands which pose a serious threat to the environment [6]. Therefore, efficient and safe disposal of CFA is of environmental concern in the whole world. CFA contains about 10–55 wt.% of Al2O3 [7–11] and is a potential substitute of bauxite for alumina production [12,13]. Extracting Al2O3 from CFA has attracted many researchers' attentions [14–18]. Since the Al/Si weight ratio in CFA is less than 1, acid leaching process other than bayer process or other alkali methods is generally preferred based on its lower cost and lower material consumption [19–21]. The main reactions are as follows (take HCl for instance): 6HCI þ Al2 O3 ¼ 2AlCl3 þ 3H2 O Nevertheless, CFA is formed at very high temperature and it is mainly made up of mullite, quartz and other amorphous phases. The mullite as a major source of aluminum in CFA is considered a non-reactive matter [2]. It is difficult to extract alumina using direct acid leaching method only ⁎ Corresponding author. Tel.: +86 351 7016893; fax: +86 351 7016893. E-mail address: [email protected] (F. Cheng). 0378-3820/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.fuproc.2012.12.003

after a vigorous treatment through some methods [22–24]. Thermal activation by calcination the samples under high temperature is a common method to motivate the reactivity of CFA to enhance alumina extraction [25]. However, the application of the thermal method is restricted because of the higher calcination temperature and lower activation efficiency. Literatures showed that the addition of certain additives in CFA during the thermal treatment could reduce the calcination temperature and improves alumina extraction [14,26]. Lime and calcium oxide were studied as the common additives. For example, the fly ash mixed with calcium oxide and fine coal, then calcined at 1000–1200 °C, an alumina extraction efficiency of 85% can be achieved using sulphuric acid leaching [14]. In Gabler's study [26] 90% alumina dissolution can be obtained using lime as the additive of low-calcium ashes, calcined at 1100 °C, then leached using sulphuric acid. Although the improved alumina extraction was obtained by using thermal activation, the calcination temperature is still high. In this regard, research on high effective active additive to reduce the calcination temperature is significant to improve Al2O3 extraction from the fly ash using thermal activation. Na-containing substances, such as NaOH, Na2CO3, NaCl, Na2B4O7⋅ 10H2O, have good chemical reactivity. They are often used as the fluxing agent for aluminosilicate minerals or ashes to form eutectoid [27–30] or as a solvent to extract minerals from slag [31–33]. The researches proved that Na-containing substances can motivate the chemical reactivity of the aluminosilicate or the slag. Literatures showed that the Na-containing substances such as NaOH and Na2CO3 are also often used to motivate the activity of the CFA [4,34–36]. It has been reported that NaOH or Na2CO3 as the sintering additives was used to improve the extraction of aluminum or silicon in the water or alkaline leaching processes [37–39]. These researches

Y. Guo et al. / Fuel Processing Technology 110 (2013) 114–121

proved that NaOH and Na2CO3 can be used as a chemical activator for the CFA. However, few researchers focus the attentions on Na-containing substances as the calcination additive in acid leaching processes for alumina extraction. Our previous study showed that NaOH as the calcination additive of coal fly ash improved the alumina extraction and decreased the thermal activation temperature greatly when using hydrochloric acid as the leachant [40]. Our recent work showed that an improved Al2O3 extraction from the fly ash was also achieved using Na2CO3 as the calcination additive in the same acid leaching process. But the mechanisms of the effect of NaOH and Na2CO3 on the transformation of the phases and the influence on the Al2O3 extraction are still unclear. Information in this regard is of importance to develop the alumina extraction technique from coal fly ash economically and effectively. In this paper, alumina extraction from coal fly ash using hydrochloric acid as the leachant was performed. The research focused on the effect of Na2CO3 and NaOH additives on the thermal activation efficiency. Based on thermal gravimetric and differential scanning calorimetric analysis (TG-DSC), X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis techniques, the mechanisms on coal fly ash activation, aluminum extraction and phase transformation during activation were discussed in detail.

2. Experimental 2.1. Materials

2.2.2. Alumina extraction from coal fly ash The as-received and activated coal fly ash samples as shown in Section 1.2.1 were used to extract alumina. The leaching processes as follows: 20 wt.% hydrochloric acid solutions were added in a 4-mouth flash with coal fly ash at a solid–liquid weight ratio of 1:3. The flask was heated to 100 °C in an electric jacket and stirred for 2 h. After stood for 2 h, the flask was cooled to room temperature. Then the mixture was filtered and washed with deionized water for many times until Cl− could not be detected from the filtrate using AgNO3. After that, the filtrate and insoluble residue were collected. The content of Al3+ was determined using copper salts back-titration [41]. The dissolution of Al3+ in hydrochloric acid solution was the indicator to evaluate the activation efficiency of coal fly ash. The method of copper salts back-titration is described as follows: an excessive amount of standard Ethylene diamine tetra acetic acid (EDTA) solution was added into the filtrate. Pyridine-azo naphthol (PAN) was added as an indicator. CuSO4 solution was added until the color of the filtrate changed from green to purple. It is needed to note that the filtrate contains a large amount of Fe3+ in addition to Al3+. The presence of Fe3+ would disturb the determination of Al3+ by coordinating with EDTA. To exclude the interference of Fe3+, Fe3+ need to be coordinated with EDTA firstly with sulfosalicylic acid as an indicator before the measurement of Al3+. The alumina extraction (ω(Al2O3)) was calculated according to Eq. (1). ωðAl2 O3 Þ ¼

Coal fly ash (CFA) samples are collected from the electrostatic precipitator of the First Coal-fired Power Plant in Taiyuan, Shanxi. The chemical compositions are shown in Table 1. Analytical grade reagents: Ca(OH)2 (Xudong Chemical Factory in Chaoyang District, Beijing), NaOH (Hengxing Chemical Reagent Co., Ltd. Tianjin), aqueous ammonia (Chemical Fertilizer Plant Reagent Factory in Taiyuan), Hydrochloric acid (Chemical Plant in Beijing), ethylene diamine tetraacetic acid (EDTA, Taixing reagent Factory in Tianjin), CuSO4 (Chemical Plant, Jiaozuo City in He'nan), oxalic acid (Tianda Chemical Reagent in Dongli District, Tianjin), NaAc (Tianjin chemical Regent Co., Ltd.), Pyridine-azo naphthol (PAN, Shanghai Reagent plant), sulfosalicylic acid (Beijing chemical Plant). Na2CO3 is industrial soda (Qinghai alkali Industry).

115

¼

mF ðAl2 O3 Þ mCFA ðAl2 O3 Þ ½cðEDTAÞ•VðEDTAÞ−cðCuSO4 Þ•V ðCuSO4 Þ  27  M ðAl2 O3 Þ

102 54

ð1Þ

Where, mF(Al2O3) and mCFA(Al2O3) denoted the mass of Al2O3 in the filtrate and in coal fly ash respectively, c and V denoted the concentration and volume of the reagent. M(Al2O3) denoted the molecular weight of Al2O3. The insoluble residue needs to be dried and collected. Corresponding to the marks of the coal fly ash samples, the residues were denoted as R-FA, R-FA-Cal(-T), R-FA-Na2CO3(-T), R-FA-NaOH(-T) and R-FA-Na2CO3 + NaOH(-T). 2.3. Sample characterization

2.2. Methods and processes 2.2.1. The activation of coal fly ash Coal fly ash was activated by thermal activation and thermal treatment with the addition of additives before leached using hydrochloric acid. The samples were made and denoted respectively as FA (the as-received coal fly ash without any treatment before acid leached), FA-Cal(-T) (coal fly ash was activated thermally at a desired temperature (T denote the calcination temperature, below is the same)), FA-Na2CO3(-T) (the sample was activated thermally with the addition of Na2CO3), FA-NaOH(-T) (the sample was activated thermally with the addition of NaOH) and FA-Na2CO3 + NaOH(-T) (the sample was activated thermally with the addition of NaOH and Na2CO3 at a weight ratio of 1:1). Among them, the proportion of coal fly ash and additives was 1:1 (weight ratio). The coal fly ash samples were calcined in a muffle furnace (SX2-12-10) at the desired temperature for 2 h.

Table 1 Chemical composition of local fly ash. Component SiO2 Contents (wt.%)

Al2O3 Fe2O3 CaO MgO TiO2 K2O

48.52 33.59 5.66

2.12 0.39

P2O5 Na2O Loss on ignition

1.57 1.48 0.18

0.31

5.22

The phase analysis of the coal fly ash was performed by using an X-ray diffraction analyzer (D/MAX2500PC XRD analyzer of Tokyo Rigaku Co.). The accelerate voltage was 40 kV, the electrical current was 100 mA and scan ranges from 10° to 80° at the speed of 4°/min. Thermal gravimetric and differential scanning calorimetric analysis (TG-DSC) analysis was carried out in a thermo gravimetric analyzer (TGA92, SETARAM, France) in an argon atmosphere. The sample loading was 10 mg and the heating rate was 3 °C/min. The weight changing with time was recorded on a personal computer. Chemical compositions of CFA and its acid-leached residues were analyzed by X-ray fluorescence analyzer spectrometer (Simultix 12 XRF spectrometer of Tokyo Rigaku Co.). The voltage was 50 kV, the electrical current was 40 mA and the integral time was 40 s. 3. Results and discussion 3.1. Effect of activation on the alumina extraction 3.1.1. Alumina extraction from as-received and calcined coal fly ash Fig. 1 shows the alumina extraction from the as-received and the calcined coal fly ash (CFA) samples using hydrochloric acid (HCl). The calcined CFA sample was obtained by calcination the as-received CFA at 900 °C for 2 h. The alumina extraction from the as-received CFA was very low and the value was less than 5% even though the concentrated hydrochloric acid was used. For the calcined CFA, the

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extraction of Al2O3 was slightly improved but the extraction is no more than 6%. Our previous work also showed that thermal activation by calcinations at the temperature of 150–900 °C contributed little to the alumina extraction and the maximum alumina extraction was only about 5% [40]. The results indicated that alumina extraction from CFA and the calcined CFA is not suitable for high recovery. It is approved that the initial extraction of alumina is from the exterior amorphous aluminum-containing phases and more amount of alumina in crystalline mullite phase is of insolubility in mineralized acids such as hydrochloric acid and sulphuric acid [14,21]. These results suggest that the amount of amorphous aluminum-containing phases in CFA of this work is little and the majority aluminum is existed in crystalline mullite phases. The thermal treatment only has little effect on the decomposition or disaggregation of mullite. 3.1.2. Effect of calcination additives on alumina extraction The previous work showed that NaOH as the additive improved the alumina extraction and decreased the thermal activation temperature significantly [40]. In order to contrast the improvement effect from various additives more clearly, the data about NaOH as the additive from the literature [40] was cited in Fig. 2. It shows that the improvement of NaOH (FA-NaOH) was more evident at lower temperatures by the evidence that the alumina extraction reached 53% at 400 °C and 60% at 600 °C. But the alumina extraction changed little with the temperature increase from 600 °C to 900 °C. Fig. 2 also shows that the alumina extraction rate of coal fly ash with the addition of Na2CO3 (FA-Na2CO3). With Na2CO3, the alumina extraction increased little at a lower calcination temperature (lower than 400 °C). On the other hand, at a higher calcination temperature (>500 °C), the alumina extraction increases rapidly. The alumina extraction increases from 18% at 500 °C to 73% at 700 °C, and 82% at 900 °C. The results show that both Na2CO3 and NaOH can significantly improve the alumina extraction. When NaOH was used as additive, a better improvement was observed at lower calcination temperatures (~400 °C). On the contrary, the action of Na2CO3 was more obvious at higher calcination temperatures (>700 °C). 3.1.3. Composition analysis of residues Table 2 shows the compositions of the leached residues analyzed by XRF. It can be seen that the contents of SiO2, Al2O3 and Fe2O3, etc. in leached residues are different for the samples with different additive and calcination temperature. The results clearly show that SiO2 is concentrated in the residues due to metals (such as Al and Fe) dissolution during the leaching processing. In order to clarify

the changes of these components more clearly, the SiO2, Al2O3, and Fe2O3 distribution in the residue, which is the weight ratio of the matter in residue to that in untreated samples, are shown in Table 3. It can be seen that more than 92% SiO2 still distributes in the residue. This implies that after thermal treatment the SiO2 dissolution does not change much for all samples, which is consistent with the result in literature which Si could not be leached by acid from coal or coal-ashes [42]. For the Al2O3 distribution in residue, it was found that there were remarkable changes among the samples. Theoretically, the value of Al2O3 distribution in residue and in solution should be equivalent to 100%. Comparing the Al2O3 distribution in the solution equaling to 1 subtract Al2O3 distribution in residue and the extraction data in Fig 2, there are some differences between the two data sites. One of the reasons might be the differences existed in two types of analytical methods. The Al2O3 distribution was based on XRF analytical results and alumina extraction was calculated using chemical analysis data by copper salts back-titration method as described in Section 1.2.2. The other reason might be due to experimental error. Although there is small difference between two methods all the data show the same trends. The results indicate that the addition of Na2CO3 and NaOH improved the Al2O3 dissolution in HCl. It is interesting notice that from Fe2O3 distribution in Table 3 after activated by calcination with the addition of additive (Na2CO3 or NaOH), Fe2O3 distribution in the residues decreased significantly with the increasing of calcinations temperature, which indicated that the reactivity of Fe2O3 was improved with the activation of CFA. Apparently, Fe2O3 has a higher solubility than Al2O3.

3.2. Gravimetric and calorimetric changes during thermal treatment (TG-DSC characterization) To further understand the activation effect of the additives on coal fly ash, analysis using thermogravimetric and differential scanning calorimetric analysis (TG-DSC) during the calcination treatments were performed. Figs. 3–5 show the weight loss and heat flow data obtained from TG-DSC analysis for the as-received CFA, the ash with the addition of Na2CO3 and NaOH respectively. As shown in Fig. 3, for the as-received CFA, there was only slight weight loss (about 5%) at 500–800 °C and an exothermic peak appeared at the same temperature ranges, which should be ascribed to the combustion of unburnt carbon in coal fly ash [43,44]. With the addition of Na2CO3, the weight loss occurred at 100–200 °C and 400–950 °C and the total amount of weight loss reached about 25%

30 FA FA-Cal-900

100 90

Extraction of Al2O3 (%)

Extraction of Al2O3 (%)

25 20 15 10 5 0

FA-NaOH* FA-Na2CO3

80 70 60 50 40 30 20 10 0

0

5

10

15

20

25

30

35

40

Concentration of HCl (%) Fig. 1. Extraction of Al2O3 for the as-received CFA (FA) and calcined CFA at 900 °C (FA-Cal-900) with various concentration of HCl leaching.

0

100 200 300 400 500 600 700 800 900 1000

Temperature (oC)) Fig. 2. Extraction of Al2O3 from coal fly ash treated by calcinations with the addition of additive. (*Data were selected from reference [40]).

Y. Guo et al. / Fuel Processing Technology 110 (2013) 114–121

117

Table 2 Composition analysis of the leaching residues of coal fly ash with hydrochloric acid. Samples

SiO2/wt.%

Al2O3/wt.%

Fe2O3/wt.%

Na2O/wt.%

CaO/wt.%

K2O/wt.%

MgO/wt.%

P2O5/wt.%

TiO2/wt.%

R-FA R-FAR-FAR-FAR-FAR-FA-

50.42 55.15 68.55 69.33 66.68 70.26

32.82 32.74 23.6 5.6 15.21 11.76

5.1 3.2 1.15 0.69 1.77 1.1

0.303 0.139 0.825 8.53 0.277 0.97

2.58 0.894 0.396 0.46 0.651 0.52

1.88 1.73 0.473 0.333 0.931 0.702

0.388 0.151 0.033 0.035 0.056 0.033

0.312 / 0.107 0.058 0.112 0.101

1.64 1.52 0.781 0.737 0.88 1.11

(Fig. 4). By comparing with Fig. 3, the weight loss (~4%) at 100–200 °C accompanied with an endothermic peak should be the evaporation of water adsorbed in Na2CO3 [45]. The weight loss (~21%) at 400–950 °C was much higher than the loss from the combustion of unburnt carbon (~5%) as shown in Fig. 3. Accordingly, a series of exothermic and endothermic peaks appeared at 400–950 °C in the DSC curves. As shown in Fig. 2 there is a significant increase in Al2O3 extraction with the addition of Na2CO3 especially at >400 °C. The results suggested that very complicated reactions between coal fly ash and Na2CO3 might occur, which facilitated the decomposition of mullite in coal fly ash and improved the extraction of Al2O3 in the acid solution. With the addition of NaOH (Fig. 5), coal fly ash lost weight in the temperature intervals of 80–150 °C, 270–410 °C and 750–950 °C. The total amount of the weight loss was about 15%. Comparing Fig. 3 with Fig. 5, it was observed that more complicated exothermic and endothermic reactions occurred at 270–410 °C. Considering the melting point of NaOH (318.4 °C), these reactions occurred at the melting temperature ranges of NaOH. Combined with the results shown in Fig. 2, the addition of NaOH improved the alumina extraction at ~ 400 °C significantly, it suggested that NaOH in melting state would react with coal fly ash more actively, which facilitated the decomposition of mullite and the extraction of Al2O3. In the melting state, NaOH causing the decomposition of mullite in coal fly ash was reported in literature [37]. 3.3. Phase transformation characterization by XRD In order to clarify the activation effect of Na2CO3 and NaOH on coal fly ash and further clarify the activation mechanisms, X-ray diffraction (XRD) technique was used to analyze the crystallographic texture of coal fly ash. Fig. 6 shows that the XRD spectra for coal fly ash before and after calcination are under different temperatures. It can be seen that the as-received coal fly ash (FA) is mainly made up of mullite (marked as 1) and quartz (marked as 2). After calcination even at 900 °C, the main phases of coal fly ash still are mullite and quartz. It is indicated that the calcination cannot decompose the polymeric phases with high stability in coal fly ash. This is the reason why Al2O3 is difficult to be extracted from the calcined CFA as shown in Fig. 1. The XRD spectra of coal fly ash with the addition of Na2CO3 are also shown in Fig. 6. It can be seen that Na2CO3 improved the transformation of the polymeric phases under the thermal treatment

conditions. The mullite (1) and quartz (2) gradually transformed into other phases with the temperature change. After calcination at 400 °C, coal fly ash still contain mullite (1), which indicated that Na2CO3 contribute little to the decomposition of mullite at b400 °C. The results consist with those shown in Fig. 4, which shows that coal fly ash has no caloric changes other than the heat lose caused by water evaporation. As a result, the Al2O3 extraction is not changed much at 400 °C (Fig. 2). With the temperature increase, the peak intensity of mullite and quartz decreased or disappeared at 600 °C– 700 °C, meanwhile NaAlSiO4 (sodium aluminum silicate, 4) and Na2SiO3 (sodium silicate, 5) began to appear. It indicated that the polymeric phases in coal fly ash decomposed with the action of Na2CO3 under these calcination conditions. Correspondingly, the extraction of Al2O3 increased to about 40% at 600 °C and 73% at 700 °C respectively (Fig. 2). The results indicated that the presence of the stable polymeric phases such as the mullite and the quartz is the main reason of the poor reactivity of coal fly ash [21,46]. When the calcination temperature increased to 800 °C, Na2CO3 (3) and NaAlSiO4 (sodium aluminum silicate, 4) disappeared and the new phases such as NaAlSiO4 (nepheline, 6) and Al2O3 (nordstrandite, 7) appeared. The results indicate that NaAlSiO4 (sodium aluminum silicate, 4) transformed into NaAlSiO4 (nepheline, 6) and Al2O3 (nordstrandite, 7) under the action of Na2CO3. With the temperature increasing to 900 °C, NaAlSiO4 (nepheline, 6) became the main Al-containing phase and the extraction of Al2O3 increased to 82% (Fig. 2). The results are in good agreement with the conclusion obtained by Kosminski and Ross et al. who found that NaAlSiO4 (nepheline, 6) is soluble in acid [47]. Wang [48] and Li [49] both detected nepheline phase when sintering the mixture of coal fly ash and Na2CO3 at 800–900 °C. For Si-containing phases such as Na2SiO3 and NaAlSiO4 (nepheline) formed during the activation, although these matters are soluble in HCl, H2SiO3 produced is insoluble. So Si is mainly distributed in the residues (>90%) as shown in Section 2.1.3. In summary, the action of Na2CO3 on the decomposition of polymeric phases in CFA was weak at b600 °C by the evidence that the

TG (wt%)

Na2CO3-400 Na2CO3- 600 Na2CO3-900 NaOH-400 NaOH-600

Samples

R-FA R-FAR-FAR-FAR-FAR-FA-

Na2CO3-400 Na2CO3-600 Na2CO3-900 NaOH-400 NaOH-600

Total residue weight (g) 9.93 9.19 6.67 6.79 7.03 6.54

SiO2

Al2O3

Fe2O3

Wt.⁎ (g)

Distr.⁎ (g)

Wt. (g)

Distr. (%)

Wt. (g)

Distr. (%)

5.00 5.07 4.57 4.70 4.69 4.60

103.1 104.5 94.2 96.9 96.7 94.8

3.17 3.01 1.57 0.38 1.07 0.77

94.3 89.6 46.7 11.3 31.8 22.9

0.51 0.29 0.08 0.05 0.12 0.07

89.5 50.9 14.0 8.8 21.1 12.3

Distr.: the distribution of the matter in the residue, %. ⁎ Wt: denote the weight in residues, g;

DSC (mW/mg)

Table 3 SiO2, Al2O3 and Fe2O3 distribution in the residue.

100 95 90 85 80 75 70 0 -1

exo

-2

700oC

-3 0

200

400

600

800

Temperature (oC)) Fig. 3. TG–DSC curves of as-received coal fly ash (FA).

1000

Y. Guo et al. / Fuel Processing Technology 110 (2013) 114–121

100 95 90 85 80 75 70 136oC

DSC

0

835oC

-1

Intensity (a.u.)

TG (wt%)

118

975oC

exo

-2 550

-3 0

200

400

oC

600

800

1000

1200

Temperature (oC))

1200 800 400 1200 800 400 1200 800 400 1200 800 400 1200 800 400 1200 800 400 1200 800 400 0

1

1 2

2

1

2

1

11

1

3 3 3

12 3

1

6 57

33

1

3 35 4 3 3

3 3

FA-Na2CO3-600

43 3

FA-Na2CO3-800

6

30

40

(1) Decomposition of mullite (b700 °C): 3Na2 CO3 þ 3Al2 O3 ·2SiO2 ðmulliteÞ þ 4SiO2 ðquartzÞ→NaAlSiO4 ðsodium aluminum silicateÞ þ CO2 ð2Þ

Formation of NaAlSiO4 (nepheline): (700 °C–900 °C): Na2 CO3 þ 3NaAlSiO4 ðsodium aluminum silicateÞ →NaAlSiO4 ðnephelineÞ þ 2Na2 SiO3 þ Al2 O3 þ CO2

100 95 90 85 80 75 70 117oC

DSC

987oC

294oC 400oC

-1 exo

315

-2

60

70

80

Fig. 6. XRD spectra of coal fly ash calcined under different conditions. 1-Al6Si2O13 (mullite); 2- SiO2(quartz); 3- Na2CO3(sodium carbonate); 4-NaAlSiO4(sodium aluminum silicate); 5-Na2SiO3(sodium silicate); 6-NaAlSiO4(nepheline); 7-Al2O3( nordstrandite).

(nepheline, 6) are found to be present in both cases, it can be noticed that the diffraction patterns of the samples differ in the intensity, number of peak characteristics and the reaction temperatures, which indicates variation in the reaction processes. The addition of NaOH made the polymeric phases in coal fly ash transformed significantly even at lower temperatures. The peak characteristics of quartz and mullite were found to disappear or fade once the temperature reached 400 °C. Na2CO3 (3), NaAlSiO4 (sodium aluminum silicate, 4), NaAlSi2O6 (jadeite, 8) and other phases were found to appear. Na2CO3 should be originated from the reaction between NaOH and carbonate salts in coal fly ash or CO2 produced from calcination process. The results were in agreement with those of DSC shown in Fig. 5 which indicates that the complicated reactions occurred at 270–400 °C. The obtained XRD diffraction patterns show that NaOH could advance the decomposition of mullite phase at lower temperatures. As a result, 53% of alumina extraction was obtained even at 400 °C (Fig. 2). NaAlSi2O6 (8) and NaAlSiO4 (4) disappeared or decreased at 600 °C–700 °C, which indicated that Al-Si bonds were destroyed further. With the temperature increasing to 800 °C, Na2CO3 disappeared and NaAlSiO4 (nepheline, 6) appeared, which might suggest that Na2CO3 formed played the key role on the formation of NaAlSiO4 (nepheline, 6). However, in XRD spectra, NaAlSiO4 (nepheline, 6) formed in this case were weaker than those formed with the addition of Na2CO3 (Fig. 6). It

Intensity (a.u.)

TG (wt%)

The obtained diffraction patterns of coal fly ash calcined with the addition of NaOH are cited from our previous work [40] and shown in Fig. 7. Compared with the case of adding Na2CO3, even though NaAlSiO4 (sodium aluminum silicate, 4), Na2SiO3 (sodium silicate, 5) and NaAlSiO4

65oC

50

2 Theta

mullite phase is still present after calcinations at 600 °C. With the calcination temperature increase, the decomposition effect from Na2CO3 increased gradually. The mullite phase disappeared completely and the NaAlSiO4 (sodium aluminum silicate) formed at 700 °C. The NaAlSiO4 (sodium aluminum silicate) decomposed further and transformed into NaAlSiO4 (nepheline) at >800 °C. The following reactions are proposed according to above discussion:

0

FA-Na2CO3-900 6

6

Fig. 4. TG–DSC curves of coal fly ash mixed with Na2CO3 (FA–Na2CO3).

Na2 CO3 þ SiO2 →Na2 SiO3 þ CO2

FA-Na2CO3-700

5 6

6

20

FA-Na2CO3-400

3

6

6

FA-Cal-900

1

3

4

1 FA

33

1

4

1

10

1 1

836oC

oC

1200 800 400 1200 800 400 1200 800 400 1200 800 400 1200 800 400 0

1

1

4

1

57

5

5

7

6 4

20

1

1

3 8

53

5 7 4 5 53

10 -3

FA

1 2

2

3

43

1

FA-NaOH-400

3

FA-NaOH-600 3

5 5 FA-NaOH-700

43 3

5 656 6

30

1

5

5

5

5

FA-NaOH-800

40

50

60

70

80

2 Theta 0

200

400

600

800

1000

Temperature (oC)) Fig. 5. TG–DSC curves of coal fly ash mixed with NaOH (FA–NaOH).

1200

Fig. 7. XRD spectra of coal fly ash calcined with the addition of NaOH. 1-Al6Si2O13 (mullite); 2- SiO2(quartz); 3- Na2CO3(sodium carbonate); 4-NaAlSiO4(sodium aluminum silicate); 5-Na2SiO3(sodium silicate); 6-NaAlSiO4(nepheline); 7-Al2O3( nordstrandite); 8-NaAlSi2O6(jadeite). (Data were selected from reference [40]).

Y. Guo et al. / Fuel Processing Technology 110 (2013) 114–121

(1) Decomposition of mullite and the formation of Na2CO3 (~400 °C):

100 FA-Na2CO3-NaOH

Extraction of Al2O3 (%)

indicated that the decomposition effect on mullite phase from NaOH was limited. Therefore the extraction of Al2O3 increased little with the temperature increasing as shown in Fig. 2. In summary, for the activation by the calcinations with the addition of NaOH, the decomposition of the polymeric phases in CFA could occur even at very low temperatures. With the action of NaOH, the polymeric matters such as NaAlSiO4 (4) and NaAlSi2O6 (8) further decomposed at 600 °C–800 °C. Na2CO3 formed during the calcination facilitated the formation of NaAlSiO4 (nepheline, 6). The following reactions might be proposed:

6NaOH þ 3Al2 O3 ·2SiO2 ðmulliteÞ þ 10SiO2 →6NaAlSi2 O6 ðjadeiteÞ þ 3H2 O

Na2 CO3 þ 3NaAlSiO4 ðsodium aluminum silicateÞ→NaAlSiO4 ðnephelineÞ þ2Na2 SiO3 þ Al2 O3 þ CO2

Actually, many references proved that the addition of sodiumcontaining matters such as NaCl [50], Na2SO4 [50], Na2B4O7⋅ 10H2O [51] and Na2CO3 [27,48,49] in aluminosilicate minerals or coal ashes could form NaAlSiO4 (nepheline) phase in the melting state. Li [51] analyzed Na2O–SiO2–Al2O3 phase diagram and predicted the reaction processes using quantum chemistry calculation. The view supported that Na + as an electron acceptor would interact with oxygen atom in mullite and form nepheline phase by destroying the original lattice [51]. As demonstrated by the results of this work, the destruction or decomposition action on aluminosilicate matters from Na + was accomplished by several steps and the processes varied with the change of Na-containing additive species. The decomposition of mullite in coal fly ash occurs at about 600 °C with the addition of Na2CO3 while that occurs at ~ 400 °C with NaOH, which results in a higher alumina extraction of FA-NaOH than that of FA-Na2CO3 at b 600 °C. With the calcination temperature increase to >700 °C, the decomposition action from Na2CO3 was stronger than that from NaOH. As a result, FA-Na2CO3 system yields a higher alumina extraction than FA– NaOH system at >700 °C. The difference might be originated from the diversity in the melting points of the additives. The melting point of NaOH and Na2CO3 are 318.4 °C and 851 °C respectively. When the calcination temperature is lower than the melting point of the additive, the solid-solid reactions between the fly ash and the additive would occur. Nevertheless, when reaching the melting point, the additive in molten state would flow and increase the contact interfaces with the ashes. This viewpoint was supported by Wang [52] who presented that the matters with low melting point make the reaction system containing more liquids and therefore improving the contact extent among the reaction matters. The melting point of NaOH is much lower than that of Na2CO3. So NaOH could activate coal fly ash at lower temperatures while Na2CO3 could only work at higher temperatures. As a result, the compound additives containing NaOH and Na2CO3 would have stronger activation effect since the decomposition effect of the polymeric phases might occur at both lower and higher temperatures.

70 60 50

300

400

500

600

700

800

900

1000

Temperature (oC)) Fig. 8. Extraction of Al2O3 with the addition of NaOH and Na2CO3 during calcinations.

3.4. Effect of the mixed additives on the extraction of Al2O3 and phases transformation Fig. 8 presents alumina extraction of coal fly ash mixed with the mixture additives containing NaOH and Na2CO3 with the weight ratio of NaOH, Na2CO3 and coal fly ash being 1:1:2. The extraction of Al2O3 was about 52% at 400 °C which was close to that of FA-NaOH (53%) at the same temperature. It suggests that NaOH play the role at the lower temperatures. When the temperatures increased to 600 °C and 700 °C, the alumina extraction reached about 80% and 95% respectively, which were higher than those of FA–NaOH and FA–Na2CO3 at the same temperature as shown in Fig. 2. It might indicate that the decomposition action on the polymeric phases from NaOH at low temperatures facilitated the further decomposition from Na2CO3 at high temperatures. To proof this hypothesis, XRD analysis was conducted. The obtained diffraction patterns of FA–NaOH–Na2CO3 were shown in Fig. 9. Although the representative characteristic peaks with low degree of polymerization were not found other than weak diffraction peaks of Al2SiO5 (sillimanite, 9) after calcination at 400 °C, the diffraction peaks of mullite became weak evidently. The presence of Na2CO3 might disturb the determination of the phases. When the calcination temperature increased to 600 °C, the phases such as NaAlSiO4 (sodium aluminum silicate, 4) and Na2SiO3 (sodium silicate, 5) appeared, which suggested that the decomposition of mullite occurred. With the temperature increase, NaAlSiO4 (nepheline, 6) with many diffraction patterns were

Intensity (a.u.)

(3) Formation of NaAlSiO4 (nepheline): (700 °C–800 °C):

80

30 200

2−

8NaOH þ Na2 CO3 þ 4NaAlSi2 O6 ðjadeiteÞ→2NaAlSiO4 ðsodium aluminum silicateÞ þ Al2 O3 ðnordstranditeÞ þ 6Na2 SiO3 þ 4H2 O þ CO2

90

40

2NaOH þ CO3 →Na2 CO3 þ H2 O (2) Further decomposition of NaAlSi2O6 (400 °C–600 °C):

119

1200 800 400 1200 800 400 1200 800 400 1200 800 400 0

FA

12 1

19

1

4

5

57

10

1 1

2

4 6

20

19

6

3

3 3 33

43

46

1

44

1

FA-Na2CO3-NaOH-400 3

FA-Na2CO3-NaOH-600 3 34 3

3

6 66 4 33 6 6

3

30

9

40

9

FA-Na2CO3-NaOH-700 50

6

60

70

80

2 Theta Fig. 9. XRD of FA–Na2CO 3 + NaOH calcined at different temperatures. 1-Al 6Si2O 13 (mullite); 2-SiO2(quartz); 3- Na2CO3(sodium carbonate); 4-NaAlSiO4(sodium aluminum silicate); 5-Na2SiO3(sodium silicate); 6-NaAlSiO4(nepheline); 7- Al2O3( nordstrandite); 8-NaAlSi2O6(jadeite); 9-Al2SiO5(sillimanite).

120

Y. Guo et al. / Fuel Processing Technology 110 (2013) 114–121

found at 700 °C. The results indicated that the compound additives containing NaOH and Na2CO3 could improve the decomposition of the polymeric phases in coal fly ash both at lower and higher temperatures. So FA–NaOH–Na2CO3 systems obtained a higher alumina extraction than FA–Na2CO3 and FA–NaOH did. 4. Conclusions (1) It is difficult to extract Al2O3 from coal fly ash since it mainly contains mullite and quartz with the steady and/or high degree of polymerization structure. Calcination alone contribute little to alumina extraction since the method cannot decompose the polymeric phases at b900 °C. The XRD analysis indicated that the mullite and quartz are very stable and the decomposition of mullite and quartz do not occurred even after high temperature calcination. (2) When Na2CO3 and NaOH were added as additives during calcination, the extraction of Al2O3 were improved greatly. The addition of Na2CO3 increased the extraction of Al2O3 rapidly from 5% to 73% at 700 °C and 82% at 900 °C. The improvement from NaOH was more evident at lower temperatures and the extraction of Al2O3 could reach 53% at a calcination temperature of 400 °C. Further increase calcination temperature, the increase in extraction of Al2O3 is not significant the maximum extraction was about ~ 60%. (3) The mechanism of improved Al2O3 extraction in the acid solution was studied. With the addition of Na2CO3, the mullite began to decompose at 600 °C and formed the NaAlSiO4 (nepheline) at >800 °C. In the case of NaOH addition, the decomposition of the mullite started at 400 °C. As a result, the extraction of Al2O3 with NaOH addition was much higher than that with Na2CO3 addition at 400 °C. Nevertheless, the decomposition effect from NaOH increased little at higher temperatures (>600 °C) on the fact that the low polymeric phases such as NaAlSiO4 (sodium aluminum silicate) and NaAlSiO4 (nepheline) formed were weaker than those with the addition of Na2CO3. (4) The mixed additives of Na2CO3 and NaOH improved alumina extraction more evidently than those with the addition of only Na2CO3 or NaOH. The extraction of Al2O3 reached 52% at 400 °C and 95% at 700 °C. The XRD pattern indicated that the decomposition of the mullite started at 400 °C and formed NaAlSiO4 (nepheline, 6) phase at 700 °C with the addition of the mixture. The temperatures are much lower than that required (800 °C) for the addition of only NaOH or Na2CO3. The action of the mixture additives combining mullite decomposition by NaOH at lower temperature and mullite decomposition by Na2CO3 at higher temperature could increase the efficiency of activation. A 95% of alumina extraction was obtained for the samples with the mixed additives containing Na2CO3 and NaOH at 700 °C. Acknowledgements This work was financially supported by the Shanxi Young Scientific and Technological Research Foundation (no. 2011021008-2), International Scientific and Technological Cooperation of China (2011DFA90830), National Hi-Tech Research and Development Program of China (863 Program, 2011AA06A103). References [1] M. Ahmaruzzaman, A review on the utilization of fly ash, Progress in Energy and Combustion Science 36 (2010) 327–363. [2] M. Izquierdo, X. Querol, Leaching behaviour of elements from coal combustion fly ash: an overview, International Journal of Coal Geology 94 (2012) 54–66. [3] M. Erol, S. Küçükbayrak, A. Ersoy-Meriçboyu, Comparison of the properties of glass, glass–ceramic and ceramic materials produced from coal fly ash, Journal of Hazardous Materials 153 (2008) 418–425.

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