Research on Reaction between SiC and Fe2O3

Yong Hou, Guo-Hua Zhang* and Kuo-Chih Chou

The State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China

In the present study, the solid state reaction between silicon carbide (SiC) and ferric oxide (Fe2O3) under different molar ratios and dif-ferent 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°C) for 30 minutes. The results of this study may provide a basis for the better use of SiC containing wastes.　[doi:10.2320/matertrans.M2017159]

(Received May 22, 2017; Accepted October 5, 2017; Published December 25, 2017)

Keywords:　 silicon carbide, ferric oxide, reduction

1.　 Introduction

Silicon carbide (SiC) has been widely used in many �elds due to its stable chemical properties, high thermal conduc-tivity, low thermal expansion coef�cient and good wear re-sistance. 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-gasi�cation-generation process, so as to limit some corrosive or toxic particles emitted into the envi-ronment. The porous SiC based ceramic �lter 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 in�ltration using porous Si3N4 ceramic as preform at 2073 K (1800°C). 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 sili-con, silicon dioxide, silicon carbide and a small amount of iron. At present, the recycling of cutting waste is generally limited to cutting �uid 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 bene�ts, 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 pyrome-tallurigal 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 deoxi-dizers decreases by half. Besides, it is well known that struc-turally sound steel ingots are produced by the conventional electroslag process using prereduced iron ore pellets con-taining 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 �ux to prevent oxygen transfer from the �ux to the ingot thus preventing blowhole porosity caused by the oxygen and allowing production of a structur-ally sound ingot. This not only deoxidizes the melt, but also permits the production of speci�c alloy steel compositions6).

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

2.　 Materials and Experimental Procedures

2.1　 MaterialsUltra-�ne silicon carbide (SiC, 2 μm, Wt pct >99.8) and

ferric oxide (Fe2O3, 1 μm, Wt 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

* Corresponding author, E-mail: ghzhang_ustb@163.com Fig. 1　X-ray diffraction patterns of studied ultra-�ne SiC powders.

Materials Transactions, Vol. 59, No. 1 (2018) pp. 98 to 103 ©2017 The Japan Institute of Metals and Materials

Fig. 1, it can be seen that the used SiC belongs to α-SiC, which is a thermodynamically stable phase. In order to iden-tify 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-�ne SiC pow-ders and Fe2O3 powders with a molar ratio of 1:1 were �rst measured and shown in Fig. 2 from room temperature to 1473 K (1200°C) 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 �rst main peak indicates an initial weight loss from 1223 K (950°C) and the second one shows a further weight loss from 1423 K (1150°C) in the heating process. Besides, the �rst main peak reveals the reaction of SiC and Fe2O3 to generate Fe, SiO2 and CO. And the second main peak may reveal the genera-tion of liquid iron after the carburization process which de-creases the melting point of Fe.

2.2　 Experimental proceduresFigure 3 shows the schematic diagram of the isothermal

experimental apparatus. SiC and Fe2O3 were uniformly mixed in a blender to assure the composition homogeniza-tion. 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).

When the desired temperature was achieved, the cylindri-cal briquettes with an alumina crucible were put into the constant temperature zone of the furnace with the heating el-ements of SiC, and reacted for some time before being taken

out and cooled down to the room temperature under the pro-tection of argon gas. Throughout the run, a �owing Ar atmo-sphere 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°C) under the molar ratio of SiC and Fe2O3 φ =  1:1. The weight loss rate was calculated with the following equation

η =m0 − mt

m0× 100 pct (1)

where m0 and mt 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 reac-tion time was prolonged to 120 min, the weight loss rate in-creased by only 0.25 pct. Thereby, there was a little change of the weight loss rate after 30 min. Therefore, in the follow-ing experiments, the reaction time was set to 30 min. Then, the reaction mechanism between SiC and Fe2O3 under dif-ferent temperatures and different molar ratios were studied. Since the initial reaction temperature between SiC and Fe2O3 is about 1223 K (950°C), temperatures of 1273 K (1000°C), 1323 K (1050°C), 1373 K (1100°C), 1423 K (1150°C) and 1473 K (1200°C) were selected for isothermal experiments. The molar ratio of SiC and Fe2O3 at each tem-perature were 0.8:1, 1:1, and 1.2:1 respectively.

The phase compositions of the samples were analyzed us-ing 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 scan-ning 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 pol-ished to remove the organic substance covered on the sur-face of samples. Finally, the samples were treated by ultra-sonic cleaning.

Fig. 2　TG and DTA curves of the mixture of ultra-�ne SiC powders and Fe2O3 powders under high purity Ar atmosphere (heating rate: 10 K/min).

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

Fig. 4　The experimental curve of weight loss rate versus reaction time (φ =  1:1; 1223 K (950°C)).

99Research on Reaction between SiC and Fe2O3

3.　 Results and Discussion

3.1　 Weight loss rateIf assuming that SiC and Fe2O3 with a molar ratio of 1:1

are all reacted to generate Fe and SiO2, the maximum theo-retical 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 rea-sons may be that the reaction rate is higher at higher tem-peratures. 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 lit-tle smaller than that of φ  =  1:1.The difference is resulted from the constant value of m0 but the decrease of m0-mt. The plateau in Fig. 5 should be due to the experimental error.

3.2　 X-ray diffraction analysesFigures 6, 7 and 8 show the XRD patterns of products ob-

tained 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 in-crease with the rise of reaction temperature. When the reac-tion temperature is 1223 K (950°C), it can be known from Fig. 6 that Fe3Si and Fe2SiO4 as two intermediate products are formed during the reduction process. However, the con-tents of Fe3Si and Fe2SiO4 decrease gradually with the in-crease of temperature. When the reaction temperature is 1423 K (1150°C), no Fe3Si phase exists, but there is still a small amount of Fe2SiO4 at 1473 K (1200°C). 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°C) 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 �nal products are Fe and SiO2. It should be pointed out that some diffrac-tion peaks of Fe2SiO4 are slightly overlapped with these of SiO2. For the sample held for 60 min, those peaks belong to

Fig. 5　The experimental curves of weight loss rate versus reaction tem-perature under the molar ratio of 0.8:1, 1:1, 1.2:1.

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°C) for different time, φ =  1:1.

Fig. 8　XRD patterns of samples with different molar ratios reacted at 1473 K (1200°C) for 30 min.

100 Y. Hou, G.-H. Zhang and K.-C. Chou

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°C) 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°C) 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, com-bining 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.

3.3　 The microstructure of the reduction productFigure 9 shows the SEM photos of samples with the mo-

lar 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 con-sists of approximately 92.4 pct Fe and 7.6 pct C, which indi-cates that the bright phase is composed of Fe phase. The re-gion 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°C), the inter-mediate 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°C) for 10 min and 60 min, respectively. Compared with Fig. 9 (c), contents of Fe and SiO2 increase gradually with the exten-sion of reaction time. When the reaction time is 60 min, Fe2SiO4 disappears, as shown in Fig. 10 (b). The SEM im-ages of samples with different molar ratios reacted at 1473 K (1200°C) for 30 min are shown in Fig. 11. Combining Fig. 11 with Fig. 9 (c), as the molar ratio in-creased from 0.8:1 to 1.2:1, the areas of Fe and SiO2 in-crease, while that of Fe2SiO4 decreases. Therefore, by en-hancing the reaction temperature and the reaction time, or increasing the molar ratio of SiC to Fe2O3, the �nal products could be gradually transformed to Fe phase and SiO2 phase. The results of SEM and EDS analyses are in good agree-ment with those of the XRD curves shown in Fig. 6 to Fig. 8.

3.4　 Thermodynamic calculationAccording to the thermodynamic calculation by FactPS

and FToxid database from FactSage 7.0, the �nal 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 dif-ferent temperatures are given by Table 2. As can be seen from Table 2, the �nal products are different at different temperatures. When the temperature is ranged from 1223 K (950°C) to 1423 K (1150°C), the �nal product is Fe, SiO2,

Fig. 9　SEM of samples with the molar ratio of 1:1 reacted at different temperatures for 30 min: (a) 1223 K (950°C), (b) 1423 K (1150°C), and (c) 1473 K (1200°C).

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°C) for different time: (a) 10 min and (b) 60 min.

Fig. 11　SEM of samples with different molar ratio reacted at 1473 K (1200°C) for 30 min: (a) φ =  0.8:1 and (b) φ =  1.2:1.

101Research on Reaction between SiC and Fe2O3

Fe2SiO4, CO and CO2. However, with the increase of tem-perature to 1473 K (1200°C), the �nal 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°C) and 1423 K (1150°C) for 30 min-utes, respectively, and the results were shown in Fig. 12. When the reaction temperature is 1223 K (950°C), it can be seen from Fig. 12 that there is still a small amount of Fe2O3 residue. With increasing the reaction temperature, Fe2O3 dis-appears and the �nal products are Fe, SiO2 and Fe2SiO4. Combined with the XRD pattern of Fig. 12, it can be con-cluded that the calculated results are consistent with the ex-perimental results in the temperature range of 1223 K (950°C) to 1423 K (1150°C). But there is a difference at 1473 K (1200°C). During the experiment, a small amount of liquid slag was found in the �nal product. When the molar ratio of SiC to Fe2O3 increases from 0.8:1 to 1:1, the ther-modynamic calculation results of FactSage 7.0 are also given by Table 2. From Table 2, it can be seen that the �nal products are always Fe, SiO2, CO at lower temperatures. But when the temperature is 1373 K (1100°C) 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 re-duction process and the unreacted SiC could react to gener-ate Fe3Si and C7–9). 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 �nal products are different at different tem-peratures. When the temperature range is 1223 K (950°C) to 1323 K (1050°C), the �nal product is Fe, SiO2, Fe3C and CO. With the increase of temperature to 1373 K (1100°C), a small amount of liquid Fe phase occurs. The liquid Fe phase will be further increased when the temperature is 1423 K (1150°C) 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°C) and rapidly decompose at temperature over 900 K (627°C)10)

3Fe(s) + SiC(s) = Fe3Si(s) + C(s) (2)

3Fe3Si(s) + 2Fe2O3(s) = 13Fe(s) + 3SiO2(s) (3)

3.5　 Process discussionAs 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 signi�cant role, in which process CO2 acts as a porter. However, in the reduc-tion 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 reac-tion, as shown by eqs. (4) and (5). And in the two-step pro-cess, there are some side reactions, such as eqs. (2), (3).

3Fe2O3 + 2SiC = 4Fe + SiO2 + Fe2SiO4 + CO + CO2 (4)

3Fe2SiO4 + 2SiC = 6Fe + 5SiO2 + 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°C) to 1473 K (1200°C) for different reaction time was investigated. The following conclusions can be drawn.

(1) When the temperature is 1473 K, 30 min is suf�cient

Table 2　The �nal products and their corresponding stoichiometric coef�cients calculated by FactSage 7.0.

Temperature, T/KThe �nal 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 Fe3C1273 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 K0.60 CO, 1.20 SiO2, 0.18 C,

2.42 liquid Fe (0.83 Fe, 0.17 C)1473 K0.70 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.

102 Y. Hou, G.-H. Zhang and K.-C. Chou

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 �nal solid products after 30 min in the temperature range of 1223 K (950°C) to 1473 K (1200°C) 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 �nal phases are Fe and SiO2 at different temperatures.And when the molar ratio further increases to 1.2:1, the �nal solid products are Fe, SiO2 at 1473 K (1200°C).

(3) The reduction process of Fe2O3 may obey a two-step reaction. In the �rst 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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103Research on Reaction between SiC and Fe2O3