Novel two-step process for synthesising β-SiC whiskers from coal fly ash and water glass

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Novel two-step process for synthesising β-SiC whiskers from coal fly ash and water glass ⁎

Yang Luoa,b, Shili Zhenga, Shuhua Maa, , Chunli Liua,b, Jian Dinga, Xiaohui Wanga,

⁎

a

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100049, China

A R T I C LE I N FO

A B S T R A C T

Keywords: SiC whisker Coal fly ash Water glass Alkali-activation Carbothermic reduction

Highly crystalline, high-purity β-SiC whiskers were successfully synthesised from a mixed sol of coal fly ash (CFA) and water glass (a silicon resource), and activated carbon (a carbon source) in a carbothermic reduction. The efficiency of the process is over 70%. The alkali activation effects of the water glass on CFA were characterised by FTIR and MAS-NMR. In the activation reaction, silicate frameworks of water glass and CFA became connected to form a large network, producing a stable, uniform sol system. Experiments and thermodynamic analyses were used to study the effects of Na2O, Al2O3, and Fe2O3 on the synthesis of β-SiC whiskers, revealing that Na2O does not affect the synthesis process while Al2O3 and Fe2O3 promote the elongation of the SiC whiskers. The results of this study may not only help to reduce the production cost of SiC whiskers but also allow value to be extracted from waste.

1. Introduction Coal fly ash (CFA) is one of the residues generated by the combustion of coal [1]. In some countries where coal remains a major source of energy, the large amounts of CFA that are produced constitute a major environmental problem. For example, China's CFA production exceeded 580 million tons in 2015 [2]. Although as much of 45% of CFA is recycled [3], the disposal of the remaining CFA still causes major environmental damage, including pollution of the soil, ground water, and air [4,5]. In addition, the potentially toxic substances in CFA can leach out into the soil and groundwater in an acid rain environment, and subsequently accumulate in the food chain [1,6]. Over the past several decades, CFA has been successfully used in a wide range of applications, including building materials [7,8], mine backfill [9], soil improvement agents [10], and plastic/rubber fillers [11]. However, although many research efforts have set out to utilise CFA in conventional and low-end applications, there have been very few reports addressing higher-value applications of CFA. As CFA contains a significant amount of SiO2, it could be used as a valuable and rich alternative source of silicon for the fabrication of higher value-added products, such as silicon carbide (SiC). It is well known that SiC ceramics offer many excellent room-temperature mechanical properties, such as a high bending strength,

excellent oxidation resistance, superior corrosion resistance, good wear resistance, and a low a friction coefficient [12]. In addition, their hightemperature mechanical properties (mechanical strength, resistance to creep, etc.) are the best among currently known ceramic materials [13]. Among the various forms of SiC, fibrous SiC is of particular importance because it is widely used as the reinforcing phase in composite materials [14,15]. The SiC manufacturing process was initially developed by Acheson in 1892. In the Acheson process, polycrystalline SiC with an irregular shape is obtained by mixing silica sand with carbon coke, which are then sintered in an electric arc furnace at 2600 °C. Due to the limitations of the conventional Acheson process, however, a number of new methods have been developed to synthesise fibrous SiC, such as carbothermic reduction, chemical vapor deposition (CVD) [16], arc discharge [17], and the thermal decomposition of silane compounds [18]. Among these new methods, the carbothermic reduction technique is regarded as being the most economically viable method owing to its efficiency and simplicity [19]. In simple terms, the carbothermic reduction technique uses a carbon source to reduce silica at a high temperature. The feasibility of this technique has been proven by many researchers [20,21]. Recently, the use of waste materials, such as ewaste glass [22], macadamia shell waste [23], and rice husks [24] as sources of silicon has been investigated to further reduce production costs. Despite excellent progress in this area, both the sintering

Abbreviations: CFA, coal fly ash; V–S, vapor–solid; V–L–S, vapor–liquid–solid ⁎ Corresponding authors. E-mail addresses: [email protected] (S. Ma), [email protected] (X. Wang). https://doi.org/10.1016/j.ceramint.2018.03.082 Received 19 December 2017; Received in revised form 27 February 2018; Accepted 10 March 2018 0272-8842/ © 2018 Published by Elsevier Ltd.

Please cite this article as: Luo, Y., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.03.082

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temperature and costs require further reduction, while from an industrial perspective, the product properties need to be improved. The progress of a carbothermic reduction reaction is greatly influenced by the crystallinity of the reactants and the distribution of the carbon within the silica. To promote the reactivity, several researchers have used a silica sol as a silicon resource [25,26] and amorphous carbon as a carbon source [27,28]. The use of a silica sol as a silicon resource has three main advantages. First, the silica sol is highly miscible with the carbon source. Second, the amorphous phase has a higher reaction activity, such that the reaction occurs as a relatively low temperature. Third, water-soluble additives can be evenly distributed throughout the raw material system if necessary. However, to the best of the authors’ knowledge, very few studies have attempted to use cheaper water glass in place of the silica gel. In the present study, a uniform and stable sol made of CFA and water glass, as well as activated carbon, were used as the silicon resource and carbon source, respectively, for the synthesis of β-SiC whiskers through a carbothermic reduction. The goal of the present study is to combine the environmental advantages of consuming hazardous waste with the economic advantages of using low-cost raw materials. We first investigated the alkali activation of CFA using water glass, and then addressed the nature of the fabricated SiC whiskers. Subsequently, the effects of Na2O, Al2O3, and Fe2O3 impurities on the carbothermic reduction reaction were discussed in detail. Finally, the growth mechanism of the SiC whiskers was interpreted. 2. Experimental Fig. 1. Fabrication of SiC whiskers from coal fly ash.

2.1. Materials 2.3. Characterisation techniques The CFA used in this experiment was sourced from the pulverised coal boiler of a thermoelectric power plant located in Inner Mongolia, China. Technical-grade water glass solution (Beijing Chemical Industry Group Co., Ltd., China) was used as the CFA alkali activator without further purification. The coal-derived activated carbon, hydrofluoric acid (40.0 wt%), and nitric acid (65.0–68.0 wt%) were all of reagent grade (Xilong Chemical Co., Ltd., China) and were also used as received without further purification.

The chemical compositions of the CFA, water glass solution, and SiC product were analysed by inductively coupled plasma-optical emission spectrometry (ICP-OES). Furthermore, in the pre-processing stage, autoclave mixed-acid leaching is required for the CFA and SiC samples in the chemical composition test. The C, H, N, S, and O contents of the activated carbon were determined using a CHNS-O analyser. The particle size distributions of the CFA and activated carbon were obtained by using a laser particle size analyser. The raw materials, intermediate products, and final products were analysed by X-ray diffraction (XRD) to identify the crystalline phases. The morphologies of these samples were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while the sample compositions at given points were analysed by energy-dispersive X-ray spectroscopy (EDS). The Fourier transform infrared spectra (FTIR), the solid-state 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and the X-ray photoelectron spectroscopy (XPS) were also adopted in this study. The distribution of different elements in the SiC products was analysed using an electron-probe micro-analyser (EPMA). The detailed parameters of the specific characterisation are presented in the Supplementary material.

2.2. Experimental procedures A schematic diagram of the fabrication of SiC whiskers is shown in Fig. 1. In general, step 1 involves the alkali activation of the CFA by the water glass, while step 2 is the carbothermal reduction process which produces the β-SiC whiskers. The technological parameters were optimised according to the results of the initial experiments and economic feasibilities. In step 1, the water glass solution and CFA were mixed at a mass ratio of 10:1 and stirred under the determined conditions (25 °C; 5 h; 400 rpm) to produce a stable sol. In step 2, the activated carbon was added to the resulting alkaline mixed sol to form a uniform mixed gel. The molar ratio of activated carbon to SiO2 in the mixed sol was 4:1. The obtained gel sample was dried in an oven at 105 °C for 12 h prior to sintering. The carbothermal reduction occurred during the sintering process which was performed in a high-temperature corundum tube furnace (Model GSL-1600X, Hefei Kejing Materials Technology Co., Ltd., China) heated at a rate of 10 °C/min up to 1500 °C. It was then held at this temperature for 4 h, and then cooled to room temperature at a rate of 5 °C/min. The entire heating-insulation-cooling process was performed under a steady flow of argon (1 L/min). Subsequently, the resulting powders were heated to 700 °C in air for 3 h to eliminate any excess activated carbon. Finally, β-SiC whiskers were obtained after leaching with 20.0 wt% hydrofluoric acid (25 °C; 2 h; 20 mL/g; 300 rpm), and then drying. The more detailed experimental procedures are presented in the Supplementary material.

3. Results and discussion 3.1. Characterisation of raw materials To develop a comprehensive understanding of the raw materials, their chemical compositions, mineral phases, particle sizes, and micro morphologies were studied using the corresponding characterisation methods. CFA (Table 1) contains various chemical compounds, but consists mainly of 21.47 wt% Al2O3, 55.57 wt% SiO2, and 6.80 wt% Fe2O3. Therefore, CFA can be treated as a SiO2-rich secondary resource. The loss on ignition (LOI) value of 2.83 wt% represents the carbon residue. The results of an XRD mineralogical analysis of CFA, shown in Fig. 2(a), 2

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Table 1 Chemical composition of CFA (wt%).

Table 2 Chemical composition of water glass (wt%).

Constituent

Al2O3

SiO2

Fe2O3

TiO2

CaO

MgO

Na2O

K2 O

LOI

Constituent

SiO2

Na2O

Al2O3

Fe2O3

Content

21.47

55.57

6.80

0.01

5.12

2.97

3.42

1.22

2.83

Content

30.24

9.76

1.9

0.03

LOI: loss on ignition. Table 3 Elemental composition of activated carbon (wt%).

indicates that there are two major crystalline phases: quartz (SiO2, JCPDS card no. 01-083-0539) and hematite (Fe2O3, JCPDS card no. 01085-0599), plus one considerable amorphous phase (the broad diffraction peak at 2θ = 18–34°). The centre position of the broad diffraction peak overlaps that of the diffraction peak of quartz, which means that the amorphous phase is dominated by the amorphous SiO2. The laser particle size analysis result in Fig. 2(b) shows that the particle distribution of the CFA exhibits an approximately normal distribution with a d(0.5) of 10.695 µm, indicating that the granularity of the CFA is very fine. The SEM image in Fig. 2(c) reveals that CFA particles have globular shapes with smooth surfaces, which is attributed to the abrupt temperature drop during their high-temperature forming process [29]. The chemical compositions of water glass are listed in Table 2. The largest single chemical component of water glass is sodium silicate. The modulus indicates the molar ratio of SiO2 and Na2O, with calculations finding this to be 3.20. Moreover, there is a small amount of Al2O3 and Fe2O3 in the water glass solution. The water glass solution is alkaline, such that it can react with the silicate structure of the CFA. Table 3 indicates that C is the major constituent of the activated carbon, with H, N, S, and O being minor components. When the XRD pattern shown in Fig. 2(a) is also considered, the activated carbon can be seen to be an amorphous carbon source with high reactivity. The activated carbon particles have a d(0.5) of 23.758 µm and an irregular appearance, as shown in Fig. 2(b) and (d), respectively.

Constituent

C

H

N

S

O

OI

Content

93.18

0.42

0.52

0.15

2.53

3.20

OI: other impurities.

Fig. 3. FTIR spectra of CFA, water glass, and mixed sample.

Fig. 2. CFA and activated carbon characterisation results: (a) XRD patterns, (b) particle size distribution, (c) SEM image of CFA, (d) SEM image of activated carbon.

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Fig. 4. MAS-NMR spectra of CFA, water glass, and mixed sample.

3.2. Alkali activation of coal fly ash To fully understand the alkali activation effect, CFA, water glass, and the water glass/CFA sample, mixed at a mass ratio of 10:1, were examined using FTIR and NMR. Fig. 3 shows the corresponding FTIR spectra. A broad absorption peak at 3420 cm−1 and the small absorption peak at 1660 cm−1 for all the samples can be attributed to the stretching vibration and bending vibration of the -OH groups [30]. The significant absorption peaks at 1084 cm−1 and 781 cm−1, corresponding to the asymmetrical stretching vibration and symmetrical stretching vibration of the Si–O bonds, respectively [31], differ significantly depending on the sample. For the CFA, the stronger absorption peaks are attributed to the silicate structure with a higher degree of polymerisation. When the CFA is added to the water glass, there is a peak range expand centred at 1084 cm−1 and an increase in the peak intensity at 781 cm−1 for the mixed sample, indicating an increase in the structure of the Si-O-Si [32]. Moreover, Al2O3 is a very common impurity in a silicate, and some subtle changes related to the Al–O bonds can also be identified. The stretching vibration peak of the Al–O bonds in [AlO6] (aluminium-oxygen octahedron) appears at 618 cm−1 [33], with the intensity of the CFA alone and water glass alone evidently higher than the corresponding intensity of the mixed sample. This phenomenon is caused by the [AlO6] being transformed into [AlO4] (aluminium-oxygen tetrahedron) and incorporated into the [SiO4] (silicon-oxygen octahedron) framework in the alkali activation reaction. Nevertheless, there is no independent absorption peak of the Al–O bonds in [AlO4] because [AlO4] has a very similar structure to [SiO4] [34]. Interestingly, the bond length of the Al–O bonds in [AlO4] (1.75 Å) is different from that of the Si–O bonds in [SiO4] (1.61 Å), causing the peaks of the Si–O bonds to shift towards lower wavenumber [34,35]. This is consistent with the spectrum characteristics of the mixed sample. The [AlO6] exists in the gaps in the silicate framework while the [AlO4] connects with the [SiO4] to increase the degree of polymerisation [36]. The Al2O3 impurity exists in the water glass in the form of [AlO6], such that self-crosslinking does not occur without the addition of CFA. During the alkali activation, the cross-linking between the silicate structures of the CFA and water glass, as well as the internal linking in the silicate structure of the water glass, occurred with the assistance of the interconnect function of the [AlO4]. In addition, there are obvious peak intensities at 1400 cm−1 for CFA and water glass, which can be attributed to the C-H bending vibration modes of some volatile organic impurities [30]. This organic matter is believed to be introduced in water glass production and have volatilised in the mixing process. The absorption peak at 1460 cm−1 for the mixed sample corresponds to the asymmetrical stretching vibration in the [CO32-] and results from the reaction between the CO2 in the air and the sodium silicate, as a result of the stirring [31].

Fig. 5. XPS spectrum of synthesised SiC products: (a) general spectrum, (b) high-resolution spectra of silicon region, (c) high-resolution spectra of carbon region.

By deconvolving the 29Si-NMR spectrum of these three samples (Fig. 4), the different framework silicates can be distinguished according to their characteristic chemical shifts. For the water glass and 4

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Fig. 6. (a) XRD pattern and (b) FTIR spectrum of synthesised SiC products.

mixed samples, the peaks appear mainly in the area from −70 to −100 ppm, indicating that the one-dimensional chain structures of SiO-Si (Q1 and Q2) and the two-dimensional layer structures of Si-O-Si (Q3) dominate in these two samples. The CFA peak appears mainly in the region from −100 to −120 ppm, which is caused by the high polymerised silicate framework (Q4) [32]. The smaller peak area of the CFA is due to there being less silicon in the CFA. More importantly, after the alkali activation reaction between the water glass and the CFA, the peak shifts to the left and the intensity for a range of −100 to −120 ppm clearly improves for the mixed sample. This can be ascribed to the appearance of more three-dimensional network structures (Q4) [37]. In general, this can be interpreted as the silicate polymerisation in the sol being partially triggered as the silicate framework of the CFA cross-linking with the silicate structure of water glass, which is in accordance with the results of the above FTIR analyses. The CFA and water glass form a stable sol system due to the intercomponent chemical binding effect. In the alkali activation reaction, on the one hand, [AlO6] in CFA and water glass drift away from the silicate frameworks and transform into [AlO4]; on the other hand, the threedimensional network structure of the CFA was partially destroyed, while the reaction activity of the end groups was improved. With the help of the [AlO4], the silicate frameworks of the water glass and CFA were connected to form a larger network with many local defects. This huge silicate network not only has a high silicon content, but can also wrap around the fine activated carbon particles in the following step to form a silicon–carbon evenly distributed gel system, which is beneficial to the carbon reduction reaction.

The appearance of a weak peak corresponding to Na 1s could be due to there being a small amount of Na residue on the surface of the sample. As shown in Fig. 5(b), the high-resolution Si 2p peak can be deconvoluted into two subpeaks at 100.9 and 103.0 eV, which are associated with the SiC and SiO2, respectively [39]. The confirmed SiO2 impurity arises from the surface oxidation layer of the SiC or the trace SiO2 residue. The high-resolution C 1s peak can be deconvoluted into two peaks at 283.0 and 284.6 eV, corresponding to the SiC and C, respectively [39]. The C peak may be derived from a combination of air pollutants and a little residual carbon. The XRD pattern for the obtained products, shown in Fig. 6(a), exhibits several sharp diffraction peaks assigned to the (111), (220), (311), (200), and (222) planes of the β-SiC (JCPDS card no. 01-0731708). According to computer simulation results obtained by Pujar and Cawley [40], the small diffraction peak near 2θ = 33.84°, marked SF, results from the stacking faults in the whisker structure. The stacking faults are a type of planar defect which characterises the disordering of the crystallographic planes (Si and C atomic planes in the present study). The alternative view is that this additional peak can be ascribed to the existence of the 2H polytype [22,41]. However, the presence of the 2H polytype should be accompanied by another stronger diffraction peak near 2θ = 38.38° which is not observed in this XRD pattern. Therefore, we can say that the peak near 2θ = 33.84° is caused by the stacking faults. The FTIR spectrum of the resulting products was used to further confirm their compositions. Fig. 6(b) shows only one intensive absorption valley at 819–910 cm−1 which is attributed to the stretching vibration of the Si–C bonds [19,22]. Note that no trace of SiO2 is observed in either the XRD pattern or the FTIR spectrum, which means that the amount of SiO2 impurities in the SiC products is very low. The surface morphology of the products was characterised by SEM, as shown in Fig. 7. The SiC grains are all fibrous, and have both smooth and hammered surface morphologies, with diameters of several hundreds of nanometres and lengths of up to several tens of micrometres, as shown in Fig. 7(a). In terms of their morphology, SiC grains can be divided into three types: (1) straight fibres with a columnar structure, as shown in Fig. 7(b) and (c); (2) straight fibres with a bamboo-like structure, as shown in Fig. 7(d) and (e); (3) bent fibres with an irregular structure, as shown in Fig. 7(f) and (g). The different micro morphologies of the SiC fibres are attributed to their different local growth conditions, discussed in the Supplementary material. To further understand the crystal structural characteristics of the fibres, TEM with selected area electron diffraction (SAED) was carried out to characterise these fibres (Fig. 8). For all three types of fibre, the well-defined SAED

3.3. Characterisation of synthesised SiC products According to the conversion rate of SiO2, the efficiency of the process is over 70%. To develop a comprehensive understanding of the nature of the final products synthesised by a carbothermic reduction, XPS, XRD, FTIR, SEM, and TEM were applied to characterise them. The elemental compositions of the obtained sample and the chemical bonding of the Si and C elements were identified using XPS analysis (Fig. 5). The peaks corresponding to the Si 2p, Si 2s, C 1s, N 1s, O 1s, and Na 1s can be observed in the general spectrum, shown in Fig. 5(a). The peak corresponding to O 1s might be related to a small amount of SiO2 impurity. Furthermore, the presence of the O 1s and N 1s may be derived from the airborne pollutants. Because XPS is highly sensitive to the sample surface, the impure peaks of the O 1s, N 1s, and C 1s will appear in almost any sample that has ever been exposed to the air [38]. 5

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Fig. 7. SEM images of synthesised SiC products.

images of these two types of SiC whiskers, shown in Fig. 8(c) and (e) show that they have a good crystallinity with distinct 0.25-mm lattice fringes, corresponding to the (111) planes of the β-SiC phase, which determines the growth direction to be [111] from another perspective.

patterns indicate typical single-crystal structures, implying that each fibre is a whisker of SiC. The growth directions of both the Type 1 and Type 2 whiskers can be easily confirmed as being [111] by indexing the lattice planes in the SAED patterns. The lattice-resolved HR-TEM

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Fig. 8. TEM images of synthesised SiC products.

always [111] with lattice fringes of 0.25 nm. The results obtained regarding the growth direction in the present study are consistent with those in the literature [19,22,42,43]. Moreover, the presence of a high concentration of stacking faults perpendicular to the SiC whisker axis is confirmed by the TEM images, which is in keeping with the above XRD

However, because the Type 3 whisker bends too many times, it is difficult to index its superposed SAED pattern. The lattice fringe measuring method was adopted to determine the growth direction of the Type 3 whisker, as shown in Fig. 8(g). As a result, it can be concluded that, even though the Type 3 whisker bends, its growth direction is 7

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therefore, the synthesised SiC products are whiskers with a good crystalline form and a high degree of purity.

3.4. Effects of Na2O, Al2O3 and Fe2O3 The Na2O component of the water glass and the two main impurity components, Al2O3 and Fe2O3, in the CFA may affect the synthesis of the SiC whiskers. Therefore, the effects of Na2O, Al2O3, and Fe2O3 on the carbothermic reduction process were studied. Given that Na2O is a highly volatile component in the carbothermic reduction process, the XRD patterns of the samples synthesised at various temperatures were used to analyse the effects and the paths of the Na2O, as shown in Fig. 9. There are no peaks corresponding to the Nacontaining phase in the XRD patterns, except for the broad diffraction peaks related to the sodium silicate gel at 1200 and 1300 °C, respectively. When the temperature exceeds 1350 °C, the amorphous sodium silicate phase is completely removed. This phenomenon is due to the volatilisation of the Na2O (starting at approximately 1132 °C) and the evaporation of the Na formed by the carbothermic reduction of the Na2O (starting at approximately 882 °C). The presence of sodium carbonate deposits on the inner wall of the furnace tube, identified by XRD analysis, confirms the escape of Na2O and Na. The analysis results show that the Na2O component of the water glass does not affect the synthesis of the SiC whiskers because it has been removed before the temperature rises to the carbothermic reduction temperature of SiO2. The effects of Al2O3 and Fe2O3 on the carbothermic reduction process were studied through examination of the XRD patterns obtained at different temperatures, as shown in Fig. 9, combined with the corresponding thermodynamic analyses. When the sintering temperatures are 1200 and 1300 °C, some Fe2O3 exists in the form of ferrosilicon (Fe2Si, JCPDS card no. 00-026-1141), while the other part solubilises in the amorphous phase with Al2O3. At a temperature of 1350 °C, the mullite phase (3Al2O3·2SiO2, JCPDS card no. 01-079-1454) begins to appear, accompanied by an increase in the amount of quartz that is

Fig. 9. XRD patterns of products (without HF leaching) synthesised at different temperatures for 4 h.

analysis results. The interface energy of the (111) planes is much lower than that of the other lattice planes in the β-SiC phase [44]. Hence, the β-SiC whiskers can spontaneously grow along the [111] direction, while the stacking faults can be easily inserted into the (111) planes. Overall,

Fig. 10. SEM images and EDS analysis results of rounded Fe-Si tips of β-SiC whiskers.

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Fig. 11. EPMA surface scanning images of SiC products after hydrofluoric acid leaching.

reaction temperature and prolongs the reaction time, the delayed carbothermic reduction reaction can avoid the overgeneration of SiC nuclei, which is beneficial to the subsequent whisker growth. To better understand how the reaction process varies with the temperature, relevant thermodynamic analyses of the simple carbothermal reduction of SiO2 are carried out in the Supplementary material. If the effects of Al2O3 and Fe2O3 are considered, however, the actual carbothermal reduction reaction with CFA becomes more complex. Thermodynamically the Al2O3 reacts with the SiO2 to produce mullite at 303 °C, seeing Reaction 8 in the Supplementary material. However, it usually forms above 1400 °C because of the dynamic obstacles between the Al2O3 and SiO2 [45]. The Fe2O3 facilitates the formation of mullite in two ways: the fluxing action and lattice activation, promoting the mass transfer and reduction of the formation activation energy, respectively [46]. Because of the similar ionic radii and electronegativities of Al3+ (having a six-coordinate ionic radius of 0.0535 nm, a four-coordinate ionic radius of 0.039 nm, and an electronegativity of 1.61) and Fe3+ (six-coordinate ionic radius of 0.0645 nm, four-coordinate ionic radius of 0.049 nm, and an electronegativity of 1.83), the substitution of Al3+ with Fe3+ occurs, and this results in the lattice activation [47]. The thermodynamic data indicate that the carbothermal reduction of mullite requires a higher temperature (1698 °C, Reaction 7 in the Supplementary material), which is in

Table 4 Element compositions of SiC products obtained using autoclave mixed-acid leaching method (wt%). Constituent

Si

C

O

Na

Al

Fe

Content

69.83

27.43

2.57

0.13

0.02

0.02

evolved from the amorphous water glass. The doping of the Fe2O3 can accelerate the formation of mullite, resulting in the forming temperature being much lower than normal. The generation of SiC starts at around 1400 °C from the carbothermic reduction reaction of quartz. When the quartz is almost exhausted, SiC begins to be generated by the carbothermic reduction of mullite at 1450 °C, accompanied by the production of corundum (Al2O3, JCPDS card no. 00-046-1212). Meanwhile, the small amount of residual quartz transforms into cristobalite once the temperature exceeds 1400 °C. The crystal phases are dominated by β-SiC with a trace amount of cristobalite and corundum when the temperature reaches 1500 °C. A further increase in the temperature causes the β-SiC to better crystallise, as evidenced by its sharper diffraction peaks, without any phase transformation. Overall, therefore, the Al2O3 and Fe2O3 cause the formation of mullite at a lower temperature. Although the formation of mullite results in an increase in the

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good agreement with the results of XRD analyses. The appropriate delay in the carbothermal reduction reaction is conducive to increasing the length extension of the SiC whiskers, rather than increasing their number. The above analyses indicate that the Al2O3 and Fe2O3 in CFA should have a positive influence on the morphology of the SiC whiskers, but further research on this effect is required. Fig. 7(a) in Section 3.3 shows that most of the SiC whiskers have no alloy droplets on their ends. However, SEM images and corresponding EDS analyses reveal ferro-silicon alloy droplets on the ends of a small number of SiC whiskers, as shown in Fig. 10. Therefore, it is believed that the SiC whiskers formed in the present study are derived from the combined effect of the V–S and V–L–S growth mechanisms [48]. To demonstrate the efficacy of the hydrofluoric acid at removing impurities, the SiC products after hydrofluoric acid leaching were tested by EPMA and ICP-OES (after autoclave mixed-acid leaching pre-processing). The results are shown in Fig. 11 and Table 4, respectively. The results show that the amount of impurities is negligible while there is also a small amount of O present, which means that the residual Na2O, Al2O3, and Fe2O3 in the SiC whiskers can be almost completely removed by hydrofluoric acid leaching. The stubborn O may be present in the crystal structure of the SiC whiskers and be introduced as part of the carbothermal reduction [49].

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4. Conclusion In the present study, low-cost β-SiC whiskers were successfully synthesised through the carbothermal reduction of a mixture of CFA and water glass. The efficiency of the process is over 70%. The alkali activation effect of the water glass solution on the CFA was revealed at the micro level. In the activation reaction, the CFA and water glass can form a uniform and stable sol system given that the silicate frameworks of the water glass and CFA were connected to form a larger network with the help of [AlO4]. The highly reactive silicate network in which activated carbon particles were evenly distributed were found to be beneficial to the subsequent carbothermic reduction. The synthesised β-SiC products are whiskers with a good crystalline form and a high degree of purity. In addition, the effects of the Na2O in the water glass, as well as the Al2O3 and Fe2O3 in the CFA, were studied. Na2O does not affect the synthesis process because of its volatility; Al2O3 and Fe2O3 can induce the formation of a mullite mesophase and thereby promote the elongation of the SiC whiskers. The proposed method for fabricating SiC whiskers lays the basis for large-scale industrial applications with lower production costs, while transforming waste into a value-added product. Acknowledgements This work was supported by the Major State Basic Research Development Program of China [2013CB632601], and with the cooperation of the Xilingol Vocational College and Defar (plain) Technology Co., Ltd. In addition, we would like to thank Editage [www. editage.cn] for English language editing. Declarations of interest None. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint.2018.03.082. References [1] M. Ahmaruzzaman, A review on the utilization of fly ash, Prog. Energy Combust. 36 (2010) 327–363.

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