Research on Reaction between SiC and Fe₂O₃

Abstract

In the present study, the solid state reaction between silicon carbide (SiC) and ferric oxide (Fe₂O₃) under different molar ratios and different reaction temperatures have been investigated. It was found that Fe and SiO₂ were generated by the reaction between SiC and Fe₂O₃ with molar ratio of 1.2:1 at 1473 K (1200℃) for 30 minutes. The results of this study may provide a basis for the better use of SiC containing wastes.

1. Introduction

Silicon carbide (SiC) has been widely used in many fields due to its stable chemical properties, high thermal conductivity, low thermal expansion coefficient and good wear resistance. For example, silicon carbide is used as abrasive during wire cutting process of polysilicon to prepare solar grade polycrystalline silicon chip. In addition, people have tended to apply porous SiC-based ceramics to diesel exhaust systems or coal-gasification-generation process, so as to limit some corrosive or toxic particles emitted into the environment. The porous SiC based ceramic filter can not only withstand mechanical stress or thermal shock during pulse cleaning, but also resist the chemical attack of various gases at high temperature¹⁾. Besides, silicon carbide is also used for the preparation of composite materials. Jiang et al.²⁾ used self-propagating high-temperature combustion synthesis a-Si₃N₄ powder and appropriate amount of β-SiC powder by pressureless sintering to prepare SiC/Si₃N₄ composite. Zheng et al.³⁾ fabricated Si₃N₄-SiC composite ceramics by chemical vapor infiltration using porous Si₃N₄ ceramic as preform at 2073 K (1800℃). Fe/SiC bulk nanocomposites were fabricated by selective laser melting directly from a simple mixture of microsized Fe and SiC, which exhibited much higher strength than pure Fe⁴⁾.

However, polysilicon cutting process using silicon carbide as cutting abrasive will produce a large number of cutting waste. The main components of these cutting wastes are silicon, silicon dioxide, silicon carbide and a small amount of iron. At present, the recycling of cutting waste is generally limited to cutting fluid and a small amount of silicon carbide abrasive, but the recovery method of residual silicon carbide in slurry is not yet mature. The residue after recovering is often discarded as industrial waste. Besides cutting wastes, silicon carbide based refractories after being discarded will also produce a large number of silicon carbide. Recycling silicon carbide, not only can bring some economic benefits, but also decrease environmental loading. In order to make full use of these silicon carbide containing wastes, many companies began to use them as a deoxidizer in the pyrometallurigal processes. Lazutkina et al.⁵⁾ has investigated the deoxidation rate of liquid steel by silicon carbide, and found that the quantity of oxidized metal after introducing deoxidizers decreases by half. Besides, it is well known that structurally sound steel ingots are produced by the conventional electroslag process using prereduced iron ore pellets containing as much as 2.8 pct oxygen pressed into a bar shape as a consumable electrode. A carbon source, such as silicon carbide, is dispersed in the flux to prevent oxygen transfer from the flux to the ingot thus preventing blowhole porosity caused by the oxygen and allowing production of a structurally sound ingot. This not only deoxidizes the melt, but also permits the production of specific alloy steel compositions⁶⁾.

Although silicon carbide has been widely used as a reducing agent, there is little investigation about the related reaction process. In this study, reaction between ferric oxide and silicon carbide was stuied in the temperature range of 1223 K (950℃) to 1473 K (1200℃) in details, results of which may provide a basis for the better use of SiC containing wastes.

2. Materials and Experimental Procedures

2.1 Materials

Ultra-fine silicon carbide (SiC, 2 μm, W_t pct >99.8) and ferric oxide (Fe₂O₃, 1 μm, W_t pct>99.6) were supplied by Sinopharm Chemical Reagent BeijingCo., Ltd. The X-ray diffraction (XRD) patterns of SiC are shown in Fig. 1. From Fig. 1, it can be seen that the used SiC belongs to α-SiC, which is a thermodynamically stable phase. In order to identify the isothermal reaction temperature between α-SiC and Fe₂O₃, the non-isothermal Thermogravimetric Analysis (TG) curve and non-isothermal Differential Thermal Analysis (DTA) curve of the mixture of ultra-fine SiC powders and Fe₂O₃ powders with a molar ratio of 1:1 were first measured and shown in Fig. 2 from room temperature to 1473 K (1200℃) with the heating rate of 10 K/min under high purity Ar atmosphere. From Fig. 2, it can be seen that there are two main peaks in the DTA curve. The first main peak indicates an initial weight loss from 1223 K (950℃) and the second one shows a further weight loss from 1423 K (1150℃) in the heating process. Besides, the first main peak reveals the reaction of SiC and Fe₂O₃ to generate Fe, SiO₂ and CO. And the second main peak may reveal the generation of liquid iron after the carburization process which decreases the melting point of Fe.

Fig. 1

X-ray diffraction patterns of studied ultra-fine SiC powders.

Fig. 2

TG and DTA curves of the mixture of ultra-fine SiC powders and Fe₂O₃ powders under high purity Ar atmosphere (heating rate: 10 K/min).

2.2 Experimental procedures

Figure 3 shows the schematic diagram of the isothermal experimental apparatus. SiC and Fe₂O₃ were uniformly mixed in a blender to assure the composition homogenization. Then, the mixtures were put into cylindrical briquettes (diameter 18 mm, thickness 5 mm) by utilizing a stainless steel mold under a pressure of 250 MPa. The total mass of cylindrical briquettes was around 10 g (±0.5 g).

Fig. 3

Schematic diagram of the isothermal experimental apparatus: 1. quartz tube; 2. electric resistance furnace; 3. alumina crucible; 4. Samples; 5. Thermocouple;

When the desired temperature was achieved, the cylindrical briquettes with an alumina crucible were put into the constant temperature zone of the furnace with the heating elements of SiC, and reacted for some time before being taken out and cooled down to the room temperature under the protection of argon gas. Throughout the run, a flowing Ar atmosphere was introduced into the furnace at 400 mL/min. In order to determine the optimum reaction time, experiments with different reaction time (10 min, 20 min, 30 min, 60 min, 120 min) were conducted at 1223 K (950℃) under the molar ratio of SiC and Fe₂O₃ φ = 1:1. The weight loss rate was calculated with the following equation   

\[\eta = \frac{m_0 - m_t}{m_0} \times 100 \ pct\](1)

where m₀ and m_t are the masses of the initial sample and that reacted for a period of time of t, respectively. The variation of weight loss rate against time is shown in Fig. 4. As could be seen from Fig. 4, when the reaction time was 30 min, the weight loss rate had reached 10.73 pct; while when the reaction time was prolonged to 120 min, the weight loss rate increased by only 0.25 pct. Thereby, there was a little change of the weight loss rate after 30 min. Therefore, in the following experiments, the reaction time was set to 30 min. Then, the reaction mechanism between SiC and Fe₂O₃ under different temperatures and different molar ratios were studied. Since the initial reaction temperature between SiC and Fe₂O₃ is about 1223 K (950℃), temperatures of 1273 K (1000℃), 1323 K (1050℃), 1373 K (1100℃), 1423 K (1150℃) and 1473 K (1200℃) were selected for isothermal experiments. The molar ratio of SiC and Fe₂O₃ at each temperature were 0.8:1, 1:1, and 1.2:1 respectively.

Fig. 4

The experimental curve of weight loss rate versus reaction time (φ = 1:1; 1223 K (950℃)).

The phase compositions of the samples were analyzed using X-ray diffraction (XRD, Rigaku Ultima IV) with Cu Ka radiation (k = 1.5406 A°). The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The morphologies of the samples were examined using a scanning electron microscope (SEM, Mineral Liberation Analyzer 250, voltage 200 V to 30 kV). Before the analysis, the samples were put into a PVA tube, and then the mixed liquid of epoxy resin and triethanolamine was poured into the tube until it froze. Then, the obtained samples were polished to remove the organic substance covered on the surface of samples. Finally, the samples were treated by ultrasonic cleaning.

3. Results and Discussion

3.1 Weight loss rate

If assuming that SiC and Fe₂O₃ with a molar ratio of 1:1 are all reacted to generate Fe and SiO₂, the maximum theoretical weight loss rate of the reaction is 14 pct. In this work, the experimental curves of weight loss rate versus reaction temperature under the molar ratio of 0.8: 1, 1:1, 1.2:1 are shown in Fig. 5. When the mixing molar ratio φ = 1:1, as shown in Fig. 5, the weight loss rate after reacting for 30 min increases with the increase of temperature. The reasons may be that the reaction rate is higher at higher temperatures. When the samples with a molar ratio 1:1 react at 1473 K for 30 min, the weight loss rate of the reaction is 13.66 pct, which is close to the theoretical weight loss rate. Furthermore, the results for the cases with the mixing molar ratio φ = 0.8:1 and 1.2:1, as shown in Fig. 5, are similar to those for the case of φ = 1:1. The only difference is that the weight loss rates in the cases of φ = 0.8:1 and 1.2:1 are a little smaller than that of φ = 1:1.The difference is resulted from the constant value of m₀ but the decrease of m₀-m_t. The plateau in Fig. 5 should be due to the experimental error.

Fig. 5

The experimental curves of weight loss rate versus reaction temperature under the molar ratio of 0.8:1, 1:1, 1.2:1.

3.2 X-ray diffraction analyses

Figures 6, 7 and 8 show the XRD patterns of products obtained under different conditions. In the case of samples with a molar ratio 1:1 reacted at different temperatures for 30 min, as shown in Fig. 6, the peaks for Fe and SiO₂ increase with the rise of reaction temperature. When the reaction temperature is 1223 K (950℃), it can be known from Fig. 6 that Fe₃Si and Fe₂SiO₄ as two intermediate products are formed during the reduction process. However, the contents of Fe₃Si and Fe₂SiO₄ decrease gradually with the increase of temperature. When the reaction temperature is 1423 K (1150℃), no Fe₃Si phase exists, but there is still a small amount of Fe₂SiO₄ at 1473 K (1200℃). In order to study the transformation of the intermediate phase Fe₂SiO₄, the mixture samples of SiC and Fe₂O₃ with a molar ratio of 1:1 are sintered at 1473 K (1200℃) for 10 min and 60 min, respectively, and the results are shown in Fig. 7. As can be seen from Fig. 7, when the reaction time is 10 minutes, there is a still small amount of Fe₂O₃ residue, since the reation time is not enough for Fe₂O₃ to fully react with SiC. When the reaction time is extended to 60 minutes, Fe₂SiO₄ phase disappear, but Fe₂O₃ phase still exist. And the final products are Fe and SiO₂. It should be pointed out that some diffraction peaks of Fe₂SiO₄ are slightly overlapped with these of SiO₂. For the sample held for 60 min, those peaks belong to SiO₂. Besides, the existence of Fe₂O₃ phase may be due to partial oxidation of the generated iron. With increasing the molar ratio of SiC and Fe₂O₃ to 1.2:1, Fe₂SiO₄ disappear when the sample is reacted at 1473 K (1200℃) for 30 min, as shown in Fig. 8. In addition, the X-ray diffraction patterns for sample with a molar ratio of 0.8:1 reacted at 1473 K (1200℃) for 30 min are somewhat similar to the case of 1:1, as shown in Fig. 8. It is still clearly seen that the products are Fe, SiO₂ and Fe₂SiO₄ in the case of the molar ratio 0.8:1, which are the same as these in the case of 1:1. In short, combining with Figs. 6, 7 and 8, it can be found that under the conditions of small addition of silicon carbide, low reaction temperature and short reaction time, the intermediate phase Fe₂SiO₄ will be present; meanwhile, when the molar ratio of SiC and Fe₂O₃ is 1:1, Fe₃Si phase is also generated at lower temperature.

Fig. 6

XRD patterns of samples reacted at different temperatures for 30 min, φ = 1:1.

Fig. 7

XRD patterns of samples reacted at 1473 K (1200℃) for different time, φ = 1:1.

Fig. 8

XRD patterns of samples with different molar ratios reacted at 1473 K (1200℃) for 30 min.

3.3 The microstructure of the reduction product

Figure 9 shows the SEM photos of samples with the molar ratio of 1:1 after reacting at different temperatures for 30 min. From Fig. 9, the SEM image reveals three distinct regions which appear as bright, light gray, and dark gray. In order to identify the phases, EDS analyses were performed at different regions as shown in Table 1. The region 1 consists of approximately 92.4 pct Fe and 7.6 pct C, which indicates that the bright phase is composed of Fe phase. The region 2 is made up of 48.6 pct O, 39.2 pct Si, and 12.2 pct Fe, which indicates that the dark gray phase is mainly composed of SiO₂ phase. The region 3 is made up of 22.0 pct O, 16.4 pct Si and 61.6 pct Fe, which indicates that the light gray phase is mainly composed of Fe₂SiO₄ phase. However, as can be seen from the X-ray diffraction pattern in Fig. 6, when the reaction temperature is 1223 K (950℃), the intermediate phase Fe₃Si is produced, but it can not be found in Fig. 9 (a) because of its low content. Figure 9 shows that with the increase of the reaction temperature, the areas of Fe phase and SiO₂ phase increase, while the area of Fe₂SiO₄ decreases gradually. Figure 10 (a) and (b) show that samples with the molar ratio of 1:1 reacted at 1473 K (1200℃) for 10 min and 60 min, respectively. Compared with Fig. 9 (c), contents of Fe and SiO₂ increase gradually with the extension of reaction time. When the reaction time is 60 min, Fe₂SiO₄ disappears, as shown in Fig. 10 (b). The SEM images of samples with different molar ratios reacted at 1473 K (1200℃) for 30 min are shown in Fig. 11. Combining Fig. 11 with Fig. 9 (c), as the molar ratio increased from 0.8:1 to 1.2:1, the areas of Fe and SiO₂ increase, while that of Fe₂SiO₄ decreases. Therefore, by enhancing the reaction temperature and the reaction time, or increasing the molar ratio of SiC to Fe₂O₃, the final products could be gradually transformed to Fe phase and SiO₂ phase. The results of SEM and EDS analyses are in good agreement with those of the XRD curves shown in Fig. 6 to Fig. 8.

Fig. 9

SEM of samples with the molar ratio of 1:1 reacted at different temperatures for 30 min: (a) 1223 K (950℃), (b) 1423 K (1150℃), and (c) 1473 K (1200℃).

Table 1 EDS results of different phases in Fig. 9.

regions	phase	element mass fraction (%)
1	Fe	92.4 Fe, 7.6 C
2	SiO2	48.6 O, 39.2 Si, 12.2 Fe
3	Fe2SiO4	22.0 O, 16.4 Si, 61.6 Fe

Fig. 10

SEM of samples with the molar ratio of 1:1 reacted at 1473 K (1200℃) for different time: (a) 10 min and (b) 60 min.

Fig. 11

SEM of samples with different molar ratio reacted at 1473 K (1200℃) for 30 min: (a) φ = 0.8:1 and (b) φ = 1.2:1.

3.4 Thermodynamic calculation

According to the thermodynamic calculation by FactPS and FToxid database from FactSage 7.0, the final products and their number of moles for sample with the molar ratio of 0.8:1 (taking 1 mol Fe₂O₃ for example) after reacting at different temperatures are given by Table 2. As can be seen from Table 2, the final products are different at different temperatures. When the temperature is ranged from 1223 K (950℃) to 1423 K (1150℃), the final product is Fe, SiO₂, Fe₂SiO₄, CO and CO₂. However, with the increase of temperature to 1473 K (1200℃), the final product changed to Fe, SiO₂, CO and CO₂, accompanied by the emergence of liquid slag phase. In order to check the consistency between the calculated results and the experimental results, the mixed samples of Fe₂O₃ and SiC with a molar ratio of 0.8:1 were sintered at 1223 K (950℃) and 1423 K (1150℃) for 30 minutes, respectively, and the results were shown in Fig. 12. When the reaction temperature is 1223 K (950℃), it can be seen from Fig. 12 that there is still a small amount of Fe₂O₃ residue. With increasing the reaction temperature, Fe₂O₃ disappears and the final products are Fe, SiO₂ and Fe₂SiO₄. Combined with the XRD pattern of Fig. 12, it can be concluded that the calculated results are consistent with the experimental results in the temperature range of 1223 K (950℃) to 1423 K (1150℃). But there is a difference at 1473 K (1200℃). During the experiment, a small amount of liquid slag was found in the final product. When the molar ratio of SiC to Fe₂O₃ increases from 0.8:1 to 1:1, the thermodynamic calculation results of FactSage 7.0 are also given by Table 2. From Table 2, it can be seen that the final products are always Fe, SiO₂, CO at lower temperatures. But when the temperature is 1373 K (1100℃) or higher, there is a liquid Fe phase. However, according to the results of XRD analyses in Fig. 6 and Fig. 7, it is found that there are two kinds of intermediate phases of Fe₃Si and Fe₂SiO₄ present in the reaction process. The Fe₃Si metastable phase may be generated according to eq. (2). Fe generated during the reduction process and the unreacted SiC could react to generate Fe₃Si and C^7–9). Then Fe₃Si and Fe₂O₃ react to generate Fe and SiO₂ according to eq. (3). When the molar ratio of SiC to Fe₂O₃ increases to 1.2:1 further, it can be seen from Table 2 that the final products are different at different temperatures. When the temperature range is 1223 K (950℃) to 1323 K (1050℃), the final product is Fe, SiO₂, Fe₃C and CO. With the increase of temperature to 1373 K (1100℃), a small amount of liquid Fe phase occurs. The liquid Fe phase will be further increased when the temperature is 1423 K (1150℃) or higher. However, according to the results of XRD analyses in Fig. 8, there is no Fe₃C in the products. The reason is that metastable Fe₃C phase will decompose into Fe and C under Ar atmosphere at 800 K (527℃) and rapidly decompose at temperature over 900 K (627℃)¹⁰⁾   

\[{\rm 3Fe(s)} + {\rm SiC(s)} = {\rm Fe_3 Si(s)} + {\rm C(s)}\](2)

  

\[{\rm 3Fe_3 Si(s)} + {\rm 2Fe_2 O_3 (s)} = {\rm 13Fe(s)} + {\rm 3SiO_2 (s)}\](3)

Table 2 The final products and their corresponding stoichiometric coefficients calculated by FactSage 7.0.

Temperature, T/K	The final products
	0.8:1	1:1	1.2:1
1223 K	0.70 CO, 0.10 CO2, 1.50 Fe, 0.55 SiO2, 0.25 Fe2SiO4	1.00 CO, 2.00 Fe, 1.00 SiO2	0.60 CO, 0.20 Fe, 1.20 SiO2, 0.60 Fe3C
1273 K
1323 K
1373 K		1.00 CO, 1.98 Fe, 1.00 SiO2, 0.02 liquid Fe (0.86 Fe, 0.14C)	0.60 CO, 1.20 SiO2, 0.52 Fe3C, 0.51 liquid Fe (0.84 Fe, 0.16 C)
1423 K			0.60 CO, 1.20 SiO2, 0.18 C, 2.42 liquid Fe (0.83 Fe, 0.17 C)
1473 K	0.70 CO, 0.10 CO2, 1.50 Fe, 0.45 SiO2, 0.84 liquid slag (0.42 SiO2, 0.58 FeO)

Fig. 12

XRD patterns of samples reacted at different temperatures for 30 min, φ = 0.8:1.

3.5 Process discussion

As is known to all, during the reduction reaction of Fe₂O₃ by carbon, the generated CO₂ reacts with the remaining C to generate CO, which is used as the reducing agent actually. Therefore, the gas-solid reaction plays significant role, in which process CO₂ acts as a porter. However, in the reduction process of Fe₂O₃ by SiC, it is hard for CO₂ to react with SiC, since there is a SiO₂ layer on the surface of unreacted SiC. Therefore, the reaction between SiC and Fe₂O₃ should be a solid-solid reaction process. Besides, it is assumed that the reduction process of Fe₂O₃ may obey a two-step reaction, as shown by eqs. (4) and (5). And in the two-step process, there are some side reactions, such as eqs. (2), (3).   

\[{\rm 3Fe_2 O_3} + {\rm 2SiC} = {\rm 4Fe + SiO_2} + {\rm Fe_2 SiO_4} + {\rm CO} + {\rm CO_2}\](4)

  

\[{\rm 3Fe_2 SiO_4} + {\rm 2SiC = 6Fe} + {\rm 5SiO_2} + {\rm 2CO}\](5)

4. Conclusions

In this study, the reaction between SiC and Fe₂O₃ with different molar ratios in the temperature ranges of 1223 K (950℃) to 1473 K (1200℃) for different reaction time was investigated. The following conclusions can be drawn.

(1) When the temperature is 1473 K, 30 min is sufficient for this reaction to achieve the theoretical weight loss rate.

(2) During the reaction process, when the molar ratio of SiC to Fe₂O₃ is 0.8:1, the final solid products after 30 min in the temperature range of 1223 K (950℃) to 1473 K (1200℃) are always Fe, SiO₂, and Fe₂SiO₄.Whereas, when the molar ratio increases from 0.8:1 to 1:1, Fe₂SiO₄ and Fe₃Si metastable phases are formed. But the final phases are Fe and SiO₂ at different temperatures.And when the molar ratio further increases to 1.2:1, the final solid products are Fe, SiO₂ at 1473 K (1200℃).

(3) The reduction process of Fe₂O₃ may obey a two-step reaction. In the first step, the intermediate phase Fe₂SiO₄ is generated; Then, Fe₂SiO₄ is further reduced to Fe and SiO₂ by SiC. Besides, in the two-step process, there are some side reactions, such as the generation of Fe₃Si.

Acknowledgment

This work was supported by the Natural Science Foundation of China (No. U1460201 and 51474020).

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