Analysis on microcrystalline graphite and properties of MgO–C refractories with microcrystalline graphite

Abstract

The composition and microstructure of microcrystalline graphite were studied by X-ray diffraction, differential thermal analysis–thermogravimetry, SEM and energy dispersive analysis in this paper. The chemical composition of ash in microcrystalline graphite was also analysed in the study. The microcrystalline graphite was introduced in MgO–C refractories fabrication to investigate the influence of microcrystalline graphite on the main properties of MgO–C refractories. The oxidation resistance, thermal shock resistance, hot bending strength and expansion rate of MgO–C samples with microcrystalline graphite and flake graphite were investigated in this study. It is indicated that the proper addition amount of microcrystalline graphite in MgO–C refractories should be no more than 4 wt-%.

Introduction

Carbon containing refractories have been widely used in iron and steelmaking industry due to their outstanding performance, and flake graphite was used as carbon source in these refractories.^1–4 China not only has rich flake graphite deposits but also has rich microcrystalline graphite deposits. Microcrystalline graphite is now mainly used as raw materials for battery, casting and other industries,^5–7 which is rarely applied in metallurgical industry. On the other hand, microcrystalline graphite is directly used as fuel for cooking and heating in some areas of China, resulting in problem of resource wasting.

With decrement of flake graphite resources and cost increases in flake graphite containing refractories, ⁸ microcrystalline graphite has been paid more and more attention due to abundant resources and low price; however, the characteristics of its poor crystallinity ⁹ and high impurity content ¹⁰ inhibit its application and development in the high temperature industry. Nowadays, the researchers use alkali roasting ¹¹ or microwave assisted acid leaching method ¹² for the purification of microcrystalline graphite, which can make carbon content reached more than 99·5 wt-%. At the same time, researchers have performed lots of work on the development and application of microcrystalline graphite containing product.^13–15 Such development of new products expands the application of microcrystalline graphite, but it is still not enough for the usage amount increase in microcrystalline graphite.

MgO–C refractories have been extensively used in the metallurgical industry because of their excellent properties.^16–19 As the demand of clean steel increasing, the carbon content in carbon containing refractories should be reduced, especially MgO–C refractories.^20–22 With the decrease in superior graphite resources, it is worth studying on the application of microcrystalline graphite in low carbon containing MgO–C refractories.^23,24 The purpose of this research is to analyse the composition and microstructure of microcrystalline graphite and also investigate the influence of different carbon sources (microcrystalline graphite, flake graphite and mixture of both) on the properties of MgO–C refractories. The properties of MgO–C samples with different carbon sources were compared, and the proper content of microcrystalline graphite, which can be used in MgO–C refractories, was proposed also. Considering the cost effectiveness of MgO–C refractories, the microcrystalline graphite used in this study is 87 grades, which possesses large proportion of refined microcrystalline graphite in the market.

Experimental

Microcrystalline graphite of 200 mesh was used in this study, and its chemical and ash compositions are listed respectively in Tables 1 and 2.

Table 1

Chemical composition of microcrystalline graphite/wt-%

C	Ash	Water content	S
87·92	9·52	0·43	0·16

Table 2

Chemical composition of ash in microcrystalline graphite/wt-%

SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	Na2O	TiO2	S	P
54·48	23·83	9·93	2·81	1·93	2·85	0·74	1·24	0·32	0·24

The microstructure analysis on microcrystalline graphite was performed with energy dispersive analysis and SEM (XL30TMP Philips, Phoenix EDAS). The mineral composition of microcrystalline graphite was investigated by X-ray diffraction (XRD) (X'Pert PRO PW3040 Philips), and the analysis of high temperature properties of microcrystalline graphite was performed with differential thermal analysis–thermogravimetric thermal analysis (NETZSCH STA449). After that, the properties of microcrystalline graphite containing MgO–C refractories were investigated and compared with the flake graphite containing MgO–C refractories. Then, constitutes of the oxides in the ash of microcrystalline graphite under different temperatures were calculated with the Equilib module of FactSage. Fused MgO (96·63 wt-%MgO), flake graphite (97·52 wt-%C, 100 meshes), Al powder (98·5 wt-%Al, 180 meshes), pitch powder and resin were used in the experiment. The batch compositions of the samples are shown in Table 3.

Table 3

Batch compositions of samples/wt-%

	Fused MgO/mm
Number	5–3	3–1	1–0	180 meshes	Microcrystalline graphite	Flake graphite	Pitch powder	Al powder
A	28	25	15	19·3	9·7	…	1·5	1·5
B	28	25	15	18·2	4·8	6	1·5	1·5
C	28	25	15	17	…	12	1·5	1·5
D	28	25	15	17	4	8	1·5	1·5
E	28	25	15	17	2	10	1·5	1·5

The thermal shock resistance, oxidation resistance, hot bending strength and thermal expansion of samples were investigated in this experiment. In order to protect MgO–C samples from oxidation in the thermal shock resistance experiment, the thermal shock resistance test was carried out via heating–natural cooling process; the samples were buried in sagger with coke powder and heated under 1100°C for 1 h and then cooled to room temperature by natural air cooling, and the samples were subjected to three thermal shock cycles. The residual bending strength of samples was tested after thermal shock resistance experiment. The oxidation resistance test was carried out under 1100°C for 2 h; then, the oxidation percentage (sectional surface area of oxidation part divided by original sectional surface area of the sample) of the samples was calculated. The hot bending strength was performed under 1400°C.

Results and discussion

Characterisation of microcrystalline graphite

The result of XRD analysis on microcrystalline graphite is shown in Fig. 1. The main mineral in microcrystalline graphite is graphite; the others are silicate, mainly smectites and possibly containing little kaolinite and diaspore. No quartz detected in the microcrystalline graphite by XRD is due to probably its poor crystalline degree. It can be seen from Tables 1 and 2 that the carbon content of microcrystalline graphite is nearly 88 wt-%; the S content is quite lower and would give less effect on the properties of refractories. The compositions of ash are mainly SiO₂, Al₂O₃ and Fe₂O₃; the content of CaO, MgO, K₂O and Na₂O is little. Considering the additional amount of graphite used in MgO–C refractories in practice, the K₂O and Na₂O introduced in refractories via microcrystalline graphite would be less, so they would only have little effect on the properties of the refractories.

1

X-ray diffraction result of microcrystalline graphite

Figure 2 and Table 4 show the SEM and energy dispersive analysis on microcrystalline graphite respectively. Obviously, the particle size of microcrystalline is quiet small, and most of them are aggregates with polyhedron shape. The impurities are located among the microcrystalline graphite particles or on the surface of particles. It can be found from SEM that microcrystalline graphite particle are in loose condition with high porosity.

2

a magnification × 500; b magnification × 5000Scanning electron microscopy analysis on microcrystalline graphite

Table 4

Element analysis of microcrystalline graphite

Point	Element	C	O	Al	Si	Ti	K
1	wt-%	87·17	7·35	1·88	2·90	…	0·7
	at-%	91·78	5·81	0·88	1·30	…	0·23
2	wt-%	58·04	6·57	…	4·79	30·60	…
	at-%	79·84	6·79	…	2·82	10·56	…

The impurities in microcrystalline graphite would result in low melting phase at high temperature and decrease the mechanical properties and corrosion resistance of refractories.^18,25 On the other hand, the reasonable amount of low melting phase could be helpful for the thermal shock resistance and would not decrease the high temperature properties of refractories when the content of microcrystalline graphite can be controlled in a certain range during refractory manufacturing.^26–28

Figure 3 shows the results of differential thermal analysis–thermogravimetric analysis on microcrystalline graphite (DC means degree centigrade). The starting oxidation temperature of microcrystalline graphite is 640°C, and the oxidation peak temperature is ∼770°C. The corresponding temperature of flake graphite in this study are 650 and 890°C respectively.

3

Differential thermal analysis–thermogravimetric result of microcrystalline graphite

Properties of microcrystalline graphite containing MgO–C refractories

Oxidation resistance

The oxidation resistance of MgO–C refractories that contain microcrystalline graphite, flake graphite and their mixes is depicted in Fig. 4. The oxidation resistance of MgO–C sample that used flake graphite only is the best; the oxidation resistance of MgO–C sample that used microcrystalline graphite only is the worst. The oxidation resistance of MgO–C samples that used mixed graphite in which microcrystalline graphite content is no more than 4 wt-% is close to that of the MgO–C sample with flake graphite only.

4

Oxidation resistances of samples

The result shows that the oxidation resistance of samples decreased with the increment of microcrystalline graphite content. Thus, the following experiments were carried out based on compounding D, E and C to investigate the related properties of microcrystalline graphite containing samples.

Thermal shock resistance

The thermal shock resistance of samples was measured, as presented in Fig. 5. With the increment of microcrystalline graphite, the maintaining rate of bending strength is increased, which means that the addition of microcrystalline graphite is good for the thermal shock resistance in this study.

5

Maintaining rate of bending strength after thermal shock resistance

The above results can be attributed to the different impurity levels between the two types of graphite used. The contents of impurities in microcrystalline graphite are much more than that of flake graphite, and the porosity of microcrystalline graphite is higher (the porosity of samples with different kinds of graphite is shown in Fig. 6). The microcrystalline graphite containing MgO–C sample is easier to absorb the stress than samples with flake graphite only during thermal shock cycle.

6

Apparent porosity of samples

Hot bending strength

Figure 7 depicts the result of the hot bending strength of samples. The hot bending strength of the MgO–C sample that contains flake graphite only is the maximum, and the more the microcrystalline graphite in the sample, the lower its hot bending strength.

7

Hot bending strength of samples

Expansion rate of samples

The relationship between expansion rate of the samples and heating temperature is shown in Fig. 8 (MT stands for sample C, and HMT stands for sample D). It can be seen from the result that, with the increment of temperature, the expansion rate of samples is increased, but the expansion rate of the sample with 4 wt-% microcrystalline graphite and 8 wt-% flake graphite is decreased after 1380°C. Yet, the expansion rate of sample with flake graphite only is continuously increasing.

8

Relationship between expansion rate of sample and temperature

With the increase in temperature, potential reactions would occur between carbon in microcrystalline graphite and oxides (e.g. Fe₂O₃, SiO₂) in ash, resulting in the oxidation of carbon as indicated by reactions (1) and (2) (1) (2) When ΔG^θ is equal to 0, T₁ (thermodynamically starting reaction temperature for reaction (1)) is 930·5 K, and T₂ (thermodynamically starting reaction temperature for reaction (2)) is 1918 K. The relationship between the Gibbs free energy of reaction and temperature is shown in Fig. 9. Fe₂O₃ will react with carbon under the experimental condition, thus will decrease the oxidation resistance of the sample containing microcrystalline graphite (Fig. 4), and SiO₂ does not react with carbon under the experimental temperature. However, in the practice, SiO₂ may react with carbon under higher temperature.^29–31

9

Relationship between the Gibbs free energy of reaction and temperature

Constitutes of the oxides system in ash under different temperatures were calculated by FactSage in order to study the influence of microcrystalline graphite on the properties of MgO–C refractories. The simulation result shown in Fig. 10 reveals that liquid amounts (slag) increase with temperature. When microcrystalline graphite is introduced to MgO–C refractories, the formation of low melting phase at high temperature can offset partially volume expansion, and the expansion rate of sample containing micrcocrystalline graphite deceases (Fig. 8). At the same time, the low melting phase can absorb thermal stress during thermal shock cycle and result in the improvement of thermal shock resistance of the sample with microcrystalline graphite (Fig. 5). However, excessive liquid phase decreases the mechanical properties of samples under high temperature (Fig. 7). If the content of microcrystalline graphite in refractories can be kept in a reasonable range, this could be helpful for the thermal shock resistance and would not decrease the high temperature properties of refractories sharply. The proper addition amount of microcrystalline graphite in MgO–C refractories should be no more than 4 wt-% according to this study.

10

Relationship between constitute and temperature of ash in microcrystalline graphite

Conclusions

• 1

The oxidation peak temperature of 87 grade microcrystalline graphite is ∼770°C; the microcrystalline graphite particles are in loose condition with high porosity.

2

2 The major chemical compositions of ash in microcrystalline graphite are SiO₂, Al₂O₃ and Fe₂O₃; the content of CaO, MgO, TiO₂, K₂O and Na₂O is little. FactSage results show that those oxides will generate a large amount of liquid phase at high temperature, and with the increment of temperature, the content of liquid phase increases. It can be seen from the thermodynamic calculation that the oxidation–reduction reaction would take place between carbon in microcrystalline graphite and some oxides in ash, resulting in oxidation of carbon.

3

Microcrystalline graphite does influence the properties of MgO–C refractories. It would decrease the high bending strength, oxidation resistance and expansion rate of the sample; on the other hand, the addition of microcrystalline graphite does help to the improvement of thermal shock resistance of the sample. It is indicated that the proper addition amount of microcrystalline graphite in MgO–C refractories should be no more than 4 wt-% according to this study.