Study of extracting alumina from high-alumina PC fly ash by a hydro-chemical process

    Study of extracting alumina from high-alumina PC fly ash by a hydro-chemical process Jian Ding, Shuhua Ma, Shili Zheng, Yi Zhang, Zongli Xie, Shirley Shen, Zhongkai Liu PII: DOI: Reference:

S0304-386X(16)30025-1 doi: 10.1016/j.hydromet.2016.01.025 HYDROM 4278

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

29 July 2015 30 November 2015 17 January 2016

Please cite this article as: Ding, Jian, Ma, Shuhua, Zheng, Shili, Zhang, Yi, Xie, Zongli, Shen, Shirley, Liu, Zhongkai, Study of extracting alumina from highalumina PC fly ash by a hydro-chemical process, Hydrometallurgy (2016), doi: 10.1016/j.hydromet.2016.01.025

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ACCEPTED MANUSCRIPT Study of extracting alumina from high-alumina PC fly ash by a hydro-chemical process

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Jian Dinga,b , Shuhua Mab,*, Shili Zhengb, Yi Zhangb, Zongli Xied, Shirley Shend , Zhongkai Liub,c, a School of Materials and Metallurgy, Northeastern University, Shenyang 110819, People’s Republic of China b National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China c School of Chemical and Environmental Engineering, China University of Mining Technology(Beijing), Beijing 100083, People’s Republic of China d CSIRO Manufacturing, Private Bag 10, Clayton South, VIC 3169, Australia * Corresponding author: Shuhua Ma, E-mail: [email protected], Tel./Fax: 86-10-82544856

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Abstract Alumina extraction from high-alumina pulverized coal (PC) fly ash by a mild hydro-chemical process characterized by a high alkaline sodium aluminate solution was studied. A high alumina extraction efficiency of 96.03% has been achieved. The effects of temperature and reaction time on alumina extraction ratio, the phase transformation and the morphology change of the extraction residues have been investigated. The alumina extracting process involves two steps: transformation from mullite and corundum to an intermediate product aluminasilicate, Na8Al6Si6O24(OH)2(H2O)2, and then from the aluminasilicate to calcium sodium hydrate silicate (NaCaHSiO4). The reaction temperature has been found to be critical for the alumina extraction and especially for the decomposition of the intermediate product Na8Al6Si6O24(OH)2(H2O)2 at high temperatures (>250℃). With the aid of a specially designed rapid real-time sampling device, the kinetics of the decomposition of Na8Al6Si6O24(OH)2(H2O)2 at high temperatures has also been studied and fitted by the Avrami–Erofeev equation. Furthermore, some engineering suggestions about the reactors design and prevention of pipe scaling have been proposed based on the kinetic results. Keywords: fly ash; alumina extraction; kinetics; hydro-chemical process

1. Introduction Fly ash is an ultrafine solid residue generated during high temperature combustion of coal in coal-fired power plants (Shemi et al., 2012). It is the largest industrial solid waste in China (ISMSWU, 2011) and annual production of fly ash in 1

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China is currently estimated to be 500 Mt (Yao et al., 2014). Efficient disposal of coal fly ash has been a worldwide issue because of the massive amount of ash produced and its harmful effects on the environment (Cobo et al., 2009). Fly ash is generally stored up in landfills. This easily causes a huge negative environmental impact mainly including the pollution of soils and groundwater (Li et al., 2014). It was estimated that the storage of fly ash in China could be 2.5 billion tons by the end of 2014, a very huge volume. Therefore there is an urgent need to develop more and new methods to utilize fly ash more effectively to achieve significant economic and environmental benefits.

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Typical fly ash only contains 20-30% alumina (Al2O3). But in the Inner Mongolia and Northern Shanxi of China, a type of high-alumina fly ash (HAFA) in which the alumina content is generally about 30-50% has emerged with the construction of coal-fired power plants (Cao et al., 2008), which is equivalent to the Al2O3 content of middle and low-grade bauxite. Now, over 50 Mt high-alumina fly ash (HAFA) is piled up as a waste in Inner Mongolia alone every year (Qi et al., 2011), in which the alumina was about 25 Mt, equal to the half of the annual alumina production in China. On the other hand, the contradiction of supply shortage of bauxite and alumina demand in China has become increasingly prominent (Sun et al., 2013). At present, more than 50% of alumina production in China has to rely on import aluminum resources (Ministry of Land and Resources of the People's Republic of China, 2013). Just due to the high content of alumina in HAFA and shortage of bauxite deposits in China, extraction of alumina from HAFA has been the research focus in the last decade. A number of processes for extracting alumina from HAFA have been reported and can be broadly classified into acid and alkali processes (Tang et al., World of Coal Ash Conference (WOCA), 2013).

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In the acid process, the HAFA first reacts with either hydrochloric acid or sulfuric acid, to produce either aluminum chloride or aluminum sulfate (Yao et al., 2014; Wu et al., 2012; Guo et al., 2013; Bai et al., 2011). Then the aluminum-containing salts are separated from the acid medium and subsequently decomposed to produce alumina. An alumina extraction efficiency of ~80% has been reported (Wu et al., 2012) for the hydrochloric acid process. Meanwhile, a thermal activation method to extract alumina from HAFA using NaHSO4 was developed (Guo et al., 2014), and the efficiency of extracting alumina from HAFA could reach 90%. However, these acidic processes inevitably cause serious equipment corrosion and acid smog problems, and there are requirement for expensive acid-resistant and air-tight processing equipments. In addition, the impurities such as iron oxide and calcium oxide are difficult to be removed from the leaching solution. Besides, the utilization of the silicon slag containing Cl- or SO42- is difficult to treat. Therefore, the acid leaching process is not very practical to produce alumina at industrial scale. Alkali process mainly includes two methods: sintering and hydro-chemical process. The sintering process involves a high-temperature reaction of coal fly ash with powdered sintering agents such as lime and/or soda to form soluble alumina 2

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compounds. The sintered product is then leached with sodium hydroxide or sodium carbonate solution (Yao et al., 2014). It is characterized by high alumina recovery rate (~85%) and recyclable reaction medium, which also shows favorable value and potential prospect in industrial application. The disadvantages, however, are high reaction temperature (~1300 ℃ ) and consequently a high production cost. Additionally, the abundant slag, contains 2-4% Na2O, is very difficult to be reused (Tang et al., World of Coal Ash Conference (WOCA), 2013). The hydro-chemical process, first developed by scientists from the former Soviet Union, can separate alumina from silica in a whole wet process by generating NaCaHSiO4, an insoluble alumina-free compound (Bi, 2006). This alkaline process has been successfully implemented for treating low-grade bauxite and red mud (Zhong et al., 2009). Extraction of alumina from circulating fluidized bed (CFB) HAFA by a mild hydro-chemical process under various conditions has been reported by Yang and coworkers (Yang et al., 2014) recently. A favorable high Al2O3 extraction efficiency of more than 90% was achieved. However, the research on the high-valued utilization of fly ash is still in its early stage, and there are still many issues to be solved in both theory and practice. For example, in order to design the reactors on an industrial scale, it is necessary to carry out a detailed study on kinetics of the digestion process especially at high temperatures. In view of these aspects, this paper will investigate the effects of temperature and reaction time on alumina extraction ratio in the process of treating HAFA by the mild hydro-chemical method firstly. Then the phase transformation and the morphology change of the extraction residues will be explored systematically. At last, the reaction kinetics of this alkaline digestion process at the high temperature region will be calculated and fitted with theoretical equations.

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2. Experimental

2.1 Materials The HAFA used in this study was obtained from a pulverized coal boiler located in Inner Mongolia, China. Fly ash samples were collected using the cone and quartering method and dried in an oven at 105°C for 24 h prior to experiments and analyses. Chemical reagents, sodium hydroxide and calcium hydroxide of reagent grade were purchased from Xilong Chemical Co. Ltd. and used without further purification. 2.2 Experimental apparatus and sampling The alumina extraction experiments were carried out in a 5000 mL stainless steel autoclave with a specially designed real time sampling apparatus and pure nickel protective lining as illustrated in Fig. 1. The reactor was heated in an electro-thermal furnace, equipped with a mechanical agitator and a temperature controlling system to maintain a desired temperature with an error of ±0.2℃. A sampling pipe was inserted to the bottom of the reactor to enable real time sampling during the reaction. 3

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A reverse blow valve, which was connected with the air bottle, was set in the upper part of the sampling pipe to blow away the residue in sampling pipe after each sampling. During sampling, the valves in the sampling pipe were opened and the slurry was sucked from the bottom of the reactor by negative pressure through the sampling pipe. After taking a certain amount of sample from the reactor, the valves in the sampling pipe were closed and the sample was stored in buffer tank which was located at the top of the sampling pipe. The residue in sampling pipe was back-flushed into the reactor after each sampling through the reverse blow valve.

Fig. 1 A schematical illustration of the experimental apparatus

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1- Sampling Device; 2- Reverse Blow Valve; 3-Reaction Vessel; 4- Temperature Controller

To eliminate the effects of external diffusion, effect of the stirring speed on the alumina extraction efficiency was investigated firstly. The result shows that there is no significant difference on the extraction rate under the stirring speed between 250 ~ 350 rpm and the stirring speed of 300 rpm was chosen for all the rest experiments. Each experiment was performed by mixing 210 g of HAFA, 115 g of calcium hydroxide with a predetermined volume of sodium aluminate solution with a NaOH concentration of 45% and a caustic ratio (molar ratio of Na2O to Al2O3) of 25. The sodium aluminate solution with certain composition was prepared by dissolving and commixing the pure substances of H2O, NaOH and Al(OH)3. Then the mixture was stirred at a constant speed of 300 rpm under the atmospheric pressure and samples were taken periodically by the procedure described above. The samples were filtered to obtain a liquid fraction and a solid residue for the subsequent analyses. The solid residue was washed thoroughly with deionized water at 80℃ and then 4

ACCEPTED MANUSCRIPT dried in an oven at 120℃ for 12 hours prior to the analyses. And the contents of the solid residue obtained at each point were all tested for twice, respectively.

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2.3 Analysis Methods The dried solid residue was characterized by X-ray diffraction (XRD, X’Pert Pro MPD, Panalytical ) at 40 kV, 30 mA, and 2θ of 5°–90º using Cu Kα as the X-ray source for identifying the crystalline phases. Inductively coupled plasma-optical emission spectrometry (ICP-OES, Optimal 5300DV, PerkinElmer) at 1300 W with a carrier gas flow of 0.08 L/min and peristaltic pump flow of 1.5 mL/min was carried out for analyzing the chemical composition of both liquid fraction and solid residue. A kind of alkaline fusion and acid leaching of the solid residue using ICP-OES to determine the contents of Fe2O3, SiO2, CaO, MgO, Al2O3, TiO2 was established (Todand et al., 1995). We used the mixture of Na2CO3 and Na2B4O7•10H2O as the molten solvent. A certain amount of the solid residue was mixed with the molten solvent and then the sample was calcined in muffle furnace at 950℃ for 15min. After sintering of high temperature, the major elements, such as Fe, Si, Ca, Mg, Al, Ti, were all converted into sodium salt. At last, the sintering products then react with diluted hydrochloric acid solution. After the acid leaching, the solution was diluted with water to 100mL.We can calculate the compositions of the solid residue by measuring the contents of the final solution using ICP-OES. The particle size of HAFA was analyzed by a Mastersizer 2000 laser particle size analyzer (Malvern Instruments). The morphological characteristics of residues were studied with a scanning electron microscopy (SEM) with EDS (Sirion 200, FEI). As the liquid volume was constantly changing during the continuous sampling process, it was difficult to calculate the yields of alumina extraction based on the contents of Al2O3 in alkali liquor. So TiO2 content in the residues was used as an internal standard which is not soluble in alkali liquor, and the contents of Al2O3 and TiO2 in the residues were used to calculate the extraction efficiency, which was obtained by the following formula:  C0 TiO  C Al O   2  2 3  % = 1100  C TiO   C0 Al O   2 2 3  

(1)

where η is the extraction efficiency of aluminum, C0 TiO  and C0 Al O  denotes 2 2 3 the content of TiO2 and Al2O3 before digestion, respectively, C TiO  and C Al O  2 2 3 denotes the content of TiO2 and Al2O3 in the residues obtained after digestion, respectively.

3. Results and discussion 3.1 Characterization of the raw materials The chemical composition of the fly ash is listed in Table 1. It mainly contains 5

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Al2O3, SiO2, Fe2O3, TiO2, and CaO. The contents of Al2O3 and SiO2, the two key components, were found to be 49.50 and 42.25 wt%, respectively, with a combined content of more than 90 wt%. Especially, the high content of alumina in the fly ash implies that it is a typical HAFA. The mass ratio of alumina (Al2O3) to silica (SiO2) (A/S) is 1.17, far lower than the theoretical value of 2.55 for mullite (3Al2O3·2SiO2) due to the presence of amorphous silica. Mineralogical analysis of the HAFA shown in Fig. 2 indicates that the sample mainly consists of two crystalline phases, mullite [3Al2O3·2SiO2] (reference code of 00-006-0258) and corundum [Al2O3] (reference code of 01-089-3072), but with low crystallinity. The broad diffraction peaks at ~22o in the XRD pattern indicates the presence of a considerable amorphous phase in the fly ash. No silica-bearing crystalline phase was observed in the sample, revealing that silica in the fly ash is dominant by an amorphous glass phase. The SEM images of the HAFA sample in Fig. 3 shows spherical particles with a smooth surface and variable particle sizes.

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Table 1 Chemical composition of the high-alumina fly ash Composition

Al2O3

SiO2

Fe2O3

TiO2

CaO

MgO

LOI

Content (wt%)

49.50

42.25

2.31

1.78

1.35

0.49

2.44

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Note: LOI is the loss on ignition

Fig. 2 XRD pattern of the high-alumina fly ash

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Fig. 3 A SEM micrograph of the high-alumina fly ash

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3.2 Effect of reaction temperatures Figure 4 shows the effect of reaction temperatures on the alumina extraction in a sodium aluminates solution via the hydro-chemical process. Experiments were performed at an initial caustic ratio of 25, a reaction temperature range from 180 280°C, V (liquid)-to-M (fly ash) ratio of 13:1, a NaOH concentration of 45%, a calcium hydroxide to silicate ratio of 1.05:1.0, and a residence time of 2h according to the literature (Yang et al., 2014). It is found that temperature has significant effect on the alumina extraction and there is a step change at 250℃. In the lower temperature region, alumina extraction efficiency was firstly increased from 23.32% to 33.56% when temperature was increased from 180 to 210℃ and then remained constant at ~33% when temperature was further increased to 250 ℃ . But in the higher temperature region, there was a step increase in alumina extraction efficiency from 33.56% to 96.03% when temperature was increased from 250 to 280℃. It indicates that 250℃ is a critical turning point during the hydro-chemical extraction process for this type of HAFA. So, it can be concluded that the high reaction temperature over 250℃ can enhance the digestion efficiency of alumina in the mild hydro-chemical process dramatically.

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Fig. 4 Effect of reaction temperatures on the alumina extraction in a sodium aluminates

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solution with a concentration NaOH 45% and an initial caustic ratio of 25

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To clarify the reaction mechanism further, phase transformation of leached residues at various reaction temperatures were analyzed by XRD, as shown in Fig. 5. When the reaction temperature reached 180℃, the phases of leached residue are transformed. The majority of main characteristic peaks exist in XRD patterns of fly ash disappear and characteristic peaks of Na8(Al6Si6O24)(OH)2(H2O)2 emerge. But several weak peaks at ~25.57o and ~43.35o still exist, which demonstrates that a small amount of corundum (reference code of 01-089-3072) in the fly ash did not dissolve in the alkaline solution. When the reaction temperature is 210℃, the peaks of Na8(Al6Si6O24)(OH)2(H2O)2 still exist, and their relative intensity increases. But the peaks attributed to corundum disappear, as causes a higher alumina extraction rate than that at 180℃. We had carried out experiments and found that very little of the corundum powder dissolved in a NaOH solution of 50.10% at 200℃ for 6h. But since the generation time of corundum in fly ash is very short causing imperfect crystallite shape of corundum and more solvent in alkali liquor. When the temperature is increased to 240℃, the change of diffraction apex of leached residue compared to that at 210℃ is little. However, when the reaction temperature reaches 260℃, the peak intensity of Na8(Al6Si6O24)(OH)2(H2O)2 had a reducing tendency. In the meantime, a new peak of NaCaHSiO4 characterized at 2θ=31.35° appears. As the reaction temperature is further increased to 280 ℃ , the peaks of Na8(Al6Si6O24)(OH)2(H2O)2 disappear and peaks of NaCaHSiO4 enhance significantly. It is worth mentioning that Yang et al. also reported presence of 8

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Na8(Al6Si6O24)(OH)2(H2O)2 as the aluminum-containing intermediate products during a similar evaluation process (Yang et al., 2014). However, in that study, the Ca3Al2(SiO4)3 is also reported as another intermediate product. This difference from this study could be attributed to different types of the HAFA used in the two studies. It was also thought that the Ca3Al2(SiO4)3, as a hydroxy-free minerals, should not exist in this hydro-chemical process at such a temperature. Although structure of Na8(Al6Si6O24)(OH)2(H2O)2 has already been studied extensively (Hassan et. al, 1982; Engelhardt et al., 1992), little has been done on the decomposition of the Na8(Al6Si6O24)(OH)2(H2O)2, especially under a high-alkali hydrothermal condition.

Fig. 5 XRD patterns of fly ash residue obtained in the course of increasing temperature process a, 180℃; b, 210℃; c, 250℃; d, 260℃; e, 280℃.

3.3 Analysis of digestion mechanism According to the phase transformation of the slags，the reaction mechanism for extracting alumina from high alumina PC fly ash by the mild hydro-chemical process can be explained as the following two-stage process: The first stage: dissolution of active minerals and formation of Na8Al6Si6O24(OH)2(H2O)2 at temperatures below 250℃ SiO2+2NaOH→Na2SiO3+H2O (2) 3Al2O3·2SiO2 +10NaOH→6NaAlO2 + 2Na2SiO3+5H2O (3) 6NaAlO2+6Na2SiO3+8H2O→Na8Al6Si6O24(OH)2(H2O)2+10NaOH (4) Al2O3 (corundum)+2NaOH→2NaAlO2+H2O (5) The second stage: Na8Al6Si6O24(OH)2(H2O)2 decomposition at temperatures higher than 250℃ 9

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Na8Al6Si6O24(OH)2(H2O)2+6Ca(OH)2+4NaOH→6NaCaHSiO4+6NaAlO2+8H2O (6) After this two-stage hydro-chemical process, Al2O3 in HAFA will be leached into the solution in the form of NaAlO2 while SiO2 in HAFA is incorporated into the precipitate in the form of NaCaHSiO4. At lower temperatures (<250℃), NaAlO2 is mainly formed from the direct reaction of Al2O3 with NaOH. NaAlO2 formed is unstable and some will react further to form insoluble Na8Al6Si6O24(OH)2(H2O)2 at the temperatures lower than 250 ℃ . At the temperatures higher than 250 ℃ , Na8Al6Si6O24(OH)2(H2O)2 becomes unstable and will decompose to form soluble NaAlO2 again. It is also confirmed that the decomposition reaction of Na8Al6Si6O24(OH)2(H2O)2 is very sensitive to the temperature change. A further study on the decomposition reaction of Na8Al6Si6O24(OH)2(H2O)2 at a high temperature region (>250°C) will have important guideline significance in parameter adjustment and scale up production.

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3.4 Kinetics of decomposition of Na8Al6Si6O24(OH)2(H2O)2 at high temperature region 3.4.1 Confirmation of steady state at 250℃

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In order to verify whether the reaction system at 250℃ has reached equilibrium which is the basis for the following kinetics research at high temperatures，samples were taken at various holding times (0, 20 and 50 min) when the temperature reached 250 ℃ under the same conditions as described above. The phase chemical compositions of the fly ash residue obtained at various holding time were analyzed and the results are shown in Fig. 6 and Table 2.

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ACCEPTED MANUSCRIPT Fig. 6 XRD patterns of the fly ash residues obtained during the course of holding at 250℃ for a，0min；b，20min；and c，50min

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As shown in Fig. 6, there is no significant change in the X-ray diffraction patterns of the fly ash residue held at 250 ℃ for various times (0 - 50 min). The position and relative intensity of the diffraction peaks remained unchanged regardless of the holding time. In addition, chemical compositions of the fly ash residues obtained at 250℃ for 0 min and 50 min are also the same (shown in Table 2). Combining these together, it can be concluded that the reaction system has reached a steady state at 250℃ with Na8Al6Si6O24(OH)2(H2O)2 and Ca(OH)2 being the main chemical compounds.

Al2O3

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Table 2 The comparison of chemical compositions of the fly ash residues obtained during the

SiO2

Fe2O3

TiO2

CaO

Na2O

A/S

250℃-0min

19.82

21.71

0.46

1

26.15

13.67

0.91

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19.57

21.53

0.45

1.01

26.50

14.02

0.91

Composition

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course of holding at 250℃ for either 0 min or 50 min.

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Fig. 7 shows the XRD patterns of fly ash residues obtained at 250℃ and 255℃ for 50 min. The two XRD patterns are different: the first one, at 250℃, consists of only Na8Al6Si6O24(OH)2(H2O)2 and Ca(OH)2 while the second one, at 255℃, consists of an additional NaCaHSiO4 phase apart from those two phases. It suggests that Na8Al6Si6O24(OH)2(H2O)2 began to react with Ca(OH)2 and decompose at a temperature higher than 250℃, for example at 255℃.

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for 50 min

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Fig. 7 XRD patterns of fly ash residue obtained in the course of holding at (a) 250℃ and (b) 255℃

3.4.2 Effect of reaction times on the decomposition of Na8Al6Si6O24(OH)2(H2O)2

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Based on the steady state principle of kinetics and experimental results, the digestion kinetics of HAFA at high temperatures (>250℃), which is actually the decomposition kinetics of Na8Al6Si6O24(OH)2(H2O)2, has also been investigated. Fig. 8 shows the effect of reaction times on the Na8Al6Si6O24(OH)2(H2O)2 decomposition under the conditions of an initial caustic ratio of 25, V (liquid)-to-M (fly ash) ratio of 13:1, an initial NaOH concentration of 45%, and a calcium hydroxide to silicate ratio of 1.05:1 and agitation speed of 300rpm.

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Fig. 8 Effect of the reaction time on the Na8Al6Si6O24(OH)2(H2O)2 decomposition in 45% NaOH

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solution at different temperatures. (Note: Decomposition rate is calculated based on the composition of fly ash residue at 250℃)

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The Na8Al6Si6O24(OH)2(H2O)2 decomposition reaction was carried out as formula (6). And as expected, the reaction rate of Na8Al6Si6O24(OH)2(H2O)2 decomposition increased with the increasing temperatures. The decomposition rate goes up to 72% when the sample was held at 260℃ for 100 min, while the decomposition rate rises to 81% after held at 270℃ in only 30 min, and lastly, it goes up to 92% after held at 280℃ in just 10 min. As in industrial application, the whole production process is continuous. It takes a certain residence time for the reaction slurry to complete the decomposition in the reactor. So the flow rate of the slurry and residence time will determine the size parameters of the reactors for industrial scale-up. A suitable residence time can achieve high alumina extraction efficiency, as well as high production efficiency. In addition, based on digestion mechanism analysis above, the Na8Al6Si6O24(OH)2(H2O)2 decomposition process is also the process of NaCaHSiO4 crystal growth. The Avrami–Erofeev equation, which was put forward by Avrami on the conception of “nucleation and growth of nuclei” (Avrami, 1939, 1940, 1941), has been used successfully to explain the leaching kinetic data of various solid–liquid reactions in many cases (Kahruman et al., 2006; Demirk et al., 2007; Rodriguez, 2007; Li et al., 2011; Zhang et al., 2011). In this paper, we also attempted to analyze the kinetics using the Avrami–Erofeev equation shown below:

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 -k 

t t0 

n

(7)

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Where α is the reaction extent, k is the rate constant for the reaction, t is the time, t0 is the induction time, and n is Avrami exponent depending on the reaction mechanism. The equation (7) can be converted to a linear expression as the following equation (8), and the kinetic information can be extracted from the slope and axis intercept.

ln  ln 1     n ln  t  t0   n ln k

(8)

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The data obtained at 255℃, 260℃ and 270℃ were used to analyze the kinetics and the results are reported in Fig. 9. Note that due to the limitation of the instrument， the incubation time (t0) was too short to be detected, so it was assumed that t0 = 0 in this work. The values of ln[−ln(1−α)] are plotted versus ln(t). The Avrami exponent (n) is equal to the slope of the straight lines and the rate constant (k) can be calculated from the intercepts as shown in Fig. 9 and Table 3.

Fig. 9 Avrami plots for isothermal reaction of Na8Al6Si6O24(OH)2(H2O)2 decomposition at 255, 260 and 270°C Table 3 Calculated data of the kinetics of Na8Al6Si6O24(OH)2(H2O)2 decomposition at 255, 260 and 270 °C

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Order, n

R2, linear regression fit

Rate constant, k

1.38204

0.011427113

0.99472

260

1.3714

0.015666667

0.99831

270

1.51708

0.054525406

0.99842

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The apparent activation energy is calculated by the Arrhenius equation according to the data listed in Table 3. As seen from the values of R2, the proposed Avrami–Erofeev equation reasonably fits to the experimental data. The kinetic data are plotted in Fig. 10, with natural logarithm of reaction rate versus 1/T*103. The apparent activation energy, equal to the slope of the straight line, is 255.55 kJ/mol with R2 of 0.96. The high apparent activation energy indicates that Na8Al6Si6O24(OH)2(H2O)2 decomposition is very sensitive to the reaction temperatures.

Fig. 10 Arrhenius plot of data for Na8Al6Si6O24(OH)2(H2O)2 decomposition during 255 °C to 270 °C

3.5 Effect of reaction times on morphology transformation Fig.11 illustrates leaching residues obtained at various holding times at 270℃. At the early stage of the reaction, the leaching residues were in a uniform size distribution and well-developed polyhedral crystals (Fig.11-a). The similar morphology was also observed on the synthesized basic hydrosodalite 15

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Na8[SiAlO4]6(OH)2(H2O)2 previously (Engelhardt et al., 1992). With the increasing reaction times, the amount of spherical particles start to decrease and that of inhomogeneous sticks gradually emerges and grows, as can been seen in the Fig. 11(b-f). At a reaction time >50 min, the leaching residues mainly contained NaCaHSiO4 as shown in the Fig. 11 (e-f). The morphology of NaCaHSiO4 is in good agreement with the previous research by Kenyon (Kenyon et al., 1992). He also reported NaCaHSiO4 was in the shape of square rods with uneven porous surfaces. On one hand, as shown in Fig. 11, it is found that the crystal of NaCaHSiO4 grows fairly fast. The shapes of leaching residues change from polyhedral to thin stick in less than 10 min. Industrial experience indicates that the faster the residues crystal growth velocity is, the more easily the scars formed on the pipe wall. Therefore, the pipe will be easily blocked with the quick crystal growth. So we can conjecture that the fast crystal growth velocity of NaCaHSiO4 would cause scars, especially on the flow pipes which are used to join the two different reactors together. If this type of blockage occurs, it will force aluminium smelters to shut down and cause a serious industrial accident. Therefore, the obtained kinetics results in this study is critically needed for the design of the reactor to make sure that the decomposition reaction of Na8Al6Si6O24(OH)2(H2O)2 is completed in a integrated reactor before entering the pipes. On the other hand, it is also found that the growth of NaCaHSiO4 crystal makes the filtration speed up and the rod-like structure of NaCaHSiO4 is quite favorite to the solid-liquid separation. It will be beneficial to the industrial production.

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Fig. 11 SEM micrographs of leaching residues obtained during the course of holding at 270℃ for a, 0min; b, 10min; c, 20min; d, 30min; e, 50min; and f, 60min. 17

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4. Conclusions

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The effect of reaction temperatures on the alumina extraction and phase transformation during temperature-rise period of high alumina PC fly ash has been investigated using a specially designed real-time sampling reactor. The Na8(Al6Si6O24)(OH)2(H2O)2 phase has been confirmed to be the important transition phase in this mild hydro-chemical process. The two stage mechanism of the mild hydro-chemical process for extracting alumina from high alumina PC fly ash has been proposed in the following order: the first stage of dissolution of active minerals and formation of Na8Al6Si6O24(OH)2(H2O)2 and the second stage of Na8Al6Si6O24(OH)2(H2O)2 decomposition. It is found that 250℃ is a critical turning-point for the decomposition of Na8Al6Si6O24(OH)2(H2O)2. The kinetics of Na8Al6Si6O24(OH)2(H2O)2 decomposition has also been studied at the second stage of 255, 260 and 270℃and it is regressed by the Avrami–Erofeev equation. The apparent activation energy for the decomposition of Na8Al6Si6O24(OH)2(H2O)2 at the studied conditions is 255.55 kJ/ mol. The decomposition process is found to be a very temperature sensitive process. The kinetics results will have important guideline significance in parameter adjustment and scale up production.

Acknowledgements

References

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The authors gratefully acknowledge the National Science and Technology Support Program of China under Grant No. 2012BAF03B01. [1] Shemi, A., et al., 2012. Alternative techniques for extracting alumina from coal fly ash.

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Minerals Engineering 34, 30-37. [2] National Development and Reform Commission of China, Implementing Scheme of Mainly Solid Waste Utilization (ISMSWU), Beijing, (2011) (in Chinese). [3] Yao, Z.T., et al., 2014. A review of the alumina recovery from coal fly ash, with a focus in China. Fuel 120, 74-85. [4] Cobo, M., et al., 2009. Characterization of fly ash from a hazardous waste incinerator in Medellin, Colombia. Journal of Hazardous Materials 168 (2), 1223-1232. [5] Li, J., et al., 2014. Synthesis of merlinoite from Chinese coal fly ashes and its potential utilization as slow release K-fertilizer. Journal of hazardous materials 265, 242-252. [6] Cao, D.Z., et al., 2008. Utilization of fly ash from coal-fired power plants in China. Journal of Zhejiang University Science A. 9 (5), 681-687. [7] Qi, L., Yuan, Y., 2011. Characteristics and the behavior in electrostatic precipitators of high-alumina coal fly ash from the Jungar power plant, Inner Mongolia, China. Journal of hazardous materials 192 (1), 222-225. [8] Sun, J.M., Chen, P., 2013. Resourcing utilization of high alumina fly ash. Advanced Materials Research 652, 2570-2575. 18

ACCEPTED MANUSCRIPT [9] Ministry of Land and Resources of the People's Republic of China, China mineral resources. Beijing, China: Geological Publishing House, 2013. [10] Tang, Z.H., et al., 2013. Current Status and Prospect of Fly Ash Utilization in China. World of Coal Ash Conference (WOCA), 1-7.

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[11] Wu, C.Y., et al., 2012. Extraction of aluminum by pressure acid leaching method from coal

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fly ash. Transactions of Nonferrous Metals Society of China 22 (9), 2282-2288. [12] Guo Y., et al., 2013. Role of additives in improved thermal activation of coal fly ash for

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alumina extraction. Fuel Processing Technology 110, 114-121.

[13] Bai, G.H., et al., 2011. Thermal decomposition of coal fly ash by concentrated sulfuric acid and alumina extraction process based on it. Fuel Processing Technology 92 (6), 1213-1219. [14] Guo, C., et al., 2014. Thermal activation of coal fly ash by sodium hydrogen sulfate for alumina extraction. Journal of Materials Science 49 (12), 4315-4322.

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[15] Bi, S.W., 2006. Alumina production process, Chemical Industry Press, Beijing. [16] Zhong, L., et al., 2009. Extraction of alumina and sodium oxide from red mud by a mild

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hydro-chemical process. Journal of hazardous materials 172 (2), 1629-1634. [17] Yang, Q.C., et al., 2014. Recovery of alumina from circulating fluidized bed combustion Al-rich fly ash using mild hydro-chemical process. Transactions of Nonferrous Metals Society of China 24 (4), 1187-1195.

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[18] Todand, M.M., et al., 1995. Microwave digestion and alkali fusion procedures for the

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determination of the platinum-group elements and gold in geological materials by ICP-MS. Chemical Geology 124(1), 21-36.

[19] Hassan, I., Grundy, H.D., 1983. Structure of basic sodalite, Na8Al6Si6O24(OH)2·2H2O. Acta

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Crystallographica Section C: Crystal Structure Communications 39 (1), 3-5. [20] Engelhardt, G., et al., 1992. The hydrosodalite system Na6+x[SiAlO4]6(OH)x·nH2O: formation, phase composition, and de-and rehydration studied by 1H,

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Na, and

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Si MAS-NMR

spectroscopy in tandem with thermal analysis, x-ray diffraction, and IR spectroscopy. Journal

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of the American Chemical Society 114 (4), 1173-1182. [21] Avrami, M., 1939. Kinetics of phase change. I General theory. The Journal of Chemical Physics 7(12), 1103-1112. [22] Avrami, M., 1940. Kinetics of phase change. II transformation-time relations for random distribution of nuclei. The Journal of Chemical Physics 8 (2), 212-224. [23] Avrami, M., 1941. Granulation, phase change, and microstructure kinetics of phase change. III. The Journal of chemical physics 9 (2), 177-184. [24] Kahruman C, Yusufoglu I., 2006. Leaching kinetics of synthetic CaWO4 in HCl solutions containing H3PO4 as chelating agent. Hydrometallurgy 81 (3), 182-189. [25] Demirkıran N., Künkül A., 2007. Dissolution kinetics of ulexite in perchloric acid solutions. International Journal of Mineral Processing 83 (1), 76-80. [26] Rodriguez M., et al., 2007. Kinetic study of ferrocolumbite dissolution in hydrofluoric acid medium. Hydrometallurgy 85 (2), 87-94. [27] Li, G., et al., 2011. Leaching of limonitic laterite ore by acidic thiosulfate solution. Minerals Engineering 24 (8), 859-863. [28] Zhang, R., et al. 2011. Research on NaCaHSiO4 decomposition in sodium hydroxide solution. 19

ACCEPTED MANUSCRIPT Hydrometallurgy 108(3), 205-213. [29] Kenyon, N. J., Weller, M. T., 2003. The effect of calcium on phase formation in the sodium aluminium silicate carbonate system and the structure of NaCaSiO3OH. Microporous and

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mesoporous materials 59 (2), 185-194

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

 A high alumina extraction efficiency of 96.03% has

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been achieved.

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 Effects of temperature and reaction time on alumina extraction has been studied.

 A two-stage theory was proposed as the reaction

kinetics

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decomposition

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 The

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mechanism for this process.

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Na8Al6Si6O24(OH)2(H2O)2 has been studied.

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 Engineering suggestions on reactors design and

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prevention of scaling have been offered.

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