0 Preface

Refractory castables have the characteristics of easy construction, easy repair, and good integrity. Widely used in tapping trough, ladle lining, electric furnace lining, rotary kiln, etc. They have excellent mechanical properties. However, due to its high wettability to slag and low thermal conductivity, the castable has poor corrosion resistance, slag permeability resistance, and thermal shock stability, especially in the slag line area and the secondary refining area. The situation is more serious, resulting in a shortened lifespan of the liner, which has attracted widespread attention from researchers. For this reason, some researchers have introduced carbon materials with low slag wettability and high thermal conductivity in carbon composite refractory materials into refractory castables to prepare carbon-containing castables, but natural carbon materials such as flake graphite are wetted by water. Poor performance, it is easy to float and disperse unevenly when added to castables, and a large amount of water needs to be added to achieve proper fluidity, resulting in increased porosity of castables and poor performance. In addition, when natural graphite is introduced into carbon-containing castables, due to its high porosity, it is easy to oxidize and produce structural defects at high temperatures, resulting in poor mechanical properties and easy erosion of castables.

Therefore, the researchers proposed to improve the high-temperature performance and water wettability of graphite materials through surface coating modification technology. Ding et al. After Si powder and graphite powder were mixed with different ratios of NaF and NaCl molten salts, β-SiC-coated graphite composite powder was prepared by argon heat treatment, and added to the -SiC-C castable. Partial replacement of spherical asphalt. The results show that the water wettability of the modified graphite is improved, and the oxidation resistance is significantly improved; when the composite powder is added with a mass fraction of 1.5%, the compressive strength and oxidation resistance of the sample are significantly improved. Modified graphite was prepared by doping r- with Ca and applied to -C castables. The results show that the compressive strength of the modified graphite-C castable increases from 21 MPa to 32 MPa after heat treatment at 1773 K. A calcium aluminate coating was prepared on the surface of graphite and added to the -C refractory, and it was found that the creep and spalling resistance of the material were improved. Among them, the surface carbide coating has gradually become a research hotspot due to its good hydrophilicity, excellent oxidation resistance, and similar thermal expansion rate to graphite. At the same time, with the gradual development of natural carbon material resources and the gradual expansion of the scope of application, the importance of preparing low-cost carbon materials has gradually become prominent. Biomass carbon materials prepared from biomass as carbonaceous precursors have the characteristics of wide sources and environmental friendliness, and have attracted the attention of researchers. Using starch as a carbon source, Li et al. prepared carbon microspheres with uniform particle size and good dispersion by hydrothermal method and added them to SiC-C castables instead of 50% asphalt by a mass fraction, which improved the pouring performance. Material fluidity, flexural strength, and compressive strength; Li et al. Using starch as a precursor, graphitized carbon spheres were prepared by hydrothermal carbonization combined with catalytic reactions, and applied to SiC-C castables, and the improvement of mechanical properties and thermal shock stability of castables was tested. It can be seen that the biomass carbon material prepared by the hydrothermal carbonization method can replace graphite to a certain extent and be added to the castable to improve its performance of the castable.

Although the above work has improved the performance of carbon-containing castables to a certain extent, carbon materials are still the main problem affecting the preparation and performance of carbon-containing castables. To further explore the influence of different carbon materials on the preparation and performance of carbon-containing castables, natural graphite was treated respectively in this study, and biomass carbon materials were prepared by hydrothermal catalytic carbonization method using biomass rice husk as raw material. Different carbon materials were compared and studied. The effect of carbon materials on the phase composition, microstructure, oxidation resistance, and erosion resistance of -C castables to obtain -C castables with excellent properties.

1

experiment

1.1 Sample preparation of different carbon materials and -C castables

Natural graphite (purity ≥ 95%, mass fraction) was placed in a sealed sagger, heated to 360 ° C in a muffle furnace, and kept for 2 h to obtain thermally oxidized graphite; graphite and zirconium powder (, purity ≥ 99%, mass fraction) and 95% (mass fraction) NaCl and 5% (mass fraction) NaF (, analytically pure) mixed salt as raw material, according to the molar ratio n (Zr): n (C) = 1:3 mixed with a certain mass After salt mixing, incubate at 900 °C for 3 h. Then the salt in the mixture is dissolved, removed, and dried to obtain ZrC-coated modified graphite; with rice husk () as the precursor, 12.5g rice husk, 1.2g ferric nitrate nonahydrate (analytically pure), and deionized water are used Mix evenly with a magnetic stirrer, add to a reaction kettle at 200°C for 20 h, and then cool naturally. After filtering with a Buchner funnel and rinsing with deionized water until the filtrate was clear, the solid product was dried and kept at 900 °C for 1 h to obtain the biomass carbon material.

Accurately weigh the fine powder raw materials according to the formula shown in Table 1 and pre-mix for 3 minutes, then add the aggregate and mix for 3 minutes. Add the mixed materials into the mixing pot, dry mix for 30s, then add water and wet mix for 3min. According to the platform method, take the water addition at a flow rate of 170- as the standard, and vibrate the castable into a strip sample of 25mm×25mm× and a cylindrical sample with a diameter of 25mm.  50 mm × 50 mm and 70 mm × 70 mm × 70 mm crucible samples, the aperture is Φ30 mm × 40 mm. Release the mold after curing at room temperature for 24 hours, and cure at room temperature for 24 hours after demoulding. The samples were then placed in an oven and dried at 110 °C for 24 hours. Then, the sample was put into a CSL high-temperature box furnace for heat treatment in a carbon-buried atmosphere, and the temperature was raised to 1400 °C at a rate of 5 °C/min, kept for 3 h, and then cooled naturally.

Table 1 Sample formula composition

1.2 Testing and Characterization

The phase composition and microstructure of the samples were analyzed by an X-ray diffractometer (X'Pert Pro MPD) equipped with an energy dispersive spectrometer and a field emission scanning electron microscope (Nova 400 Nano SEM). Apparent porosity and bulk density are tested according to GB/T 2997-2000, and room temperature flexural strength and room temperature compressive strength are tested according to GB/T 3001-2008 and GB/T 5072-2008. The heat-treated Φ50 mm×H50 mm cylindrical sample was placed in a CSL high-temperature box furnace and kept at 1100 °C for 3 h in an air atmosphere to test the oxidation resistance of the sample. After the sample is cooled in the furnace, the sample is cut transversely, and a digital camera is used to take photos of the cross-section of the sample after oxidation. The static slag resistance test method was used to detect the slag erosion resistance of the samples. Put 30 g of slag (see Table 2 for the chemical composition of the slag) into the crucible sample and place it in a CSL high-temperature box furnace at a temperature of 1400 °C in an air atmosphere. After 3 h of heat preservation, it was naturally cooled. Cut the crucible sample along the height direction, and observe the erosion of the sample by the slag.

Table 2 Chemical composition of slag

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Results and discussion

2.1 Composition and structure of different carbon materials

Figure 1 shows the XRD spectra of different carbon materials. It can be seen that the diffraction peaks of graphite are located at 2θ=26.61°, 43.46°, and 54.81°; compared with thermally oxidized graphite, only ZrC appears in the XRD spectrum of the modified heat-treated sample. and graphite diffraction peaks, indicating that the synthesized modified graphite composite powder sample reacted completely. Diffraction peaks of amorphous graphite and SiO2 also exist in the prepared biomass carbon materials, which are also the main element composition in rice husks, indicating that rice husks are transformed into composite powders rich in carbon and silicon dioxide after carbonization by hydrothermal reaction.

Fig.1 XRD spectra of different carbon materials

2.2 Effect of different carbon materials on the physical properties of samples

The amount of water added to each group of samples under the fluidity conditions that meet the construction requirements is shown in Table 1. It can be seen that the amount of water added to the ZrC-coated modified graphite castable is the least, indicating that the performance of the modified water-wet graphite is improved; adding biomass The amount of water added to the carbon castable samples was slightly lower than that of the thermally oxidized graphite-added castable samples. The apparent porosity of the three groups of castable samples is shown in Fig. 2. Compared with the samples added with thermally oxidized graphite and biomass carbon materials, the samples added with ZrC-coated flake graphite had lower porosity due to less water added. Apparent porosity and high bulk density. However, the bulk density of the samples added with flake graphite and biomass carbon was lower. It shows that the addition of ZrC-modified graphite improves the compactness of the castable, and the biomass carbon material prepared by the hydrothermal method also has good water wettability, but because there may be more micropores in the castable, the castable The greater the water content, the greater the dosage, the greater the porosity and the lower the bulk density of the sample after drying and high-temperature treatment.

Figure 2 - Bulk Density and Apparent Porosity of Castable C

Figure 3 shows the room temperature flexural strength and compressive strength of the -C castable samples with different carbon materials added. It can be seen that the flexural strength and compressive strength of the castable sample added with thermally oxidized graphite are the lowest, which are 7.5 MPa and 18.5 MPa, respectively; The compressive strengths were 8.0 MPa and 24.50 MPa, respectively, but the mechanical properties of the castables did not improve significantly. This may be due to the addition of ZrC-coated flake graphite castable samples during heat treatment, because part of ZrC is oxidized to form ZrO2, and ZrO2 will undergo phase transformation to generate volume expansion to fill the pores inside the castable, thereby generating microcracks inside. Excessive expansion is likely to cause damage to the material structure. The flexural strength and compressive strength of samples added with biomass carbon materials were the highest, which were 70% and 115% higher than those of graphite-added samples, respectively. Due to the good fluidity of the biomass carbon material, it can be evenly dispersed into the interior and pores of the material, and the SiO2 and residual oxides in it can promote the firing of the sample during the high-temperature heat treatment process. As a result, the mechanical properties of the castable samples were significantly improved.

Fig.3 The room temperature compressive strength and flexural strength of castable C

2.3 Effects of different carbon materials on the phase composition of samples

Figure 4 is the XRD spectrum of aluminum-carbon castable samples prepared by adding different carbon sources. From the spectra, we can see that the main phases of the samples after heat treatment at 1400 °C are corundum, carbon, anorthite, spinel, etc. The S2 added with ZrC-coated flake graphite contains ZrO2 surface coating generated by ZrC oxidation. The S3 sample added with biomass carbon materials has an obvious peak at 2θ=26.6°, which corresponds to the (002) crystal plane of graphite (ICDD: 01-075-2078), indicating that the degree of graphitization of carbon materials increases after heat treatment.  The results of XRD patterns show that the phase composition of the three groups of samples is the same, indicating that the addition of different carbon sources has little effect on the phase composition of the -C castable samples.

Fig.4 XRD spectra of three groups of castable samples

2.4 Effect of different carbon materials on the microstructure of samples

Figure 5 is an SEM photo of the fracture of the castable sample with thermally oxidized graphite, ZrC-coated flake graphite, and biomass carbon material added respectively. It can be seen from Figures 5(a) and (b) that the internal pores of the sample added with thermally oxidized graphite are not tightly bound due to the presence of a large amount of water, and silicon carbide whiskers are generated in the pores to hurt the sample. The performance improvement is small. As shown in Figure 5(c) and (d), the complete ZrC-coated flake graphite can be seen in the S2 sample, and the coating layer on the graphite surface is found to be slightly peeled off when the magnification is higher; the addition of the S2 sample The amount of water is significantly less than that of S1, and the porosity in the sample is also reduced after drying and heat treatment. The side shows that the ZrC coating improves the water wettability of graphite and improves the strength of the material to a certain extent. Influence. For samples added with biomass carbon materials, as shown in Figure 5(e), (f), combined with energy spectrum analysis (Figure 5(g), (h)), it is found that the product after hydrothermal carbonization has good water absorption wettability and dispersibility. At the same time, the carbon microspheres in the sample improved the fluidity of the castable, which can effectively reduce the amount of water added. In addition, due to the high reactivity of the carbon in the sample after hydrothermal carbonization, the generated silicon carbide whiskers react with the matrix at high temperature, forming a ceramic bonding phase inside the sample, which promotes the connection between particles and improving the test performance. performance. such mechanical properties.

Fig.5 SEM photos and energy spectra (g, h) of thermal oxide graphite (a, b), ZrC coated flake graphite (c, d), biomass carbon material (e, f) castable fracture

2.5 Effects of different carbon materials on the oxidation resistance and slag erosion resistance of samples

Fig. 6 is a photograph of the cross-section of the -C castable sample after the oxidation test at 1100 °C. The white area outside the sample is the oxidized part, and the gray-black area in the center is the unoxidized area. It can be found intuitively that S3 has the largest oxidation area, followed by S1 and S2. It shows that the ZrC coating layer on the graphite surface prevents the further oxidation of the interaction between graphite and oxygen, reduces the emission of CO to a certain extent, and delays the oxidation degree of the castable sample; there is a microporous structure inside the high-temperature heat treatment, which increases the air ingress Passage of the sample, resulting in reduced oxidation performance.

Fig.6 Cross-sectional photos of samples after heat treatment in an oxidizing atmosphere

The cross-sectional photos of the three groups of samples after the slag erosion test are shown in Figure 7. It can be seen that the crucible morphology of all the samples remains intact. Due to the low slag wettability of graphite, the slag in the crucible sample added with thermal oxide graphite and ZrC-modified graphite is in a loose state, and the wettability between the slag and the sample is poor, and the sample has good slag erosion resistance. On the contrary, the slag plate in the crucible sample added with biomass carbon material is blocky, and the contact surface between the slag and the sample is relatively large, which is mainly because the biomass carbon material promotes the connection between the particles inside the castable. heat treatment.  , to prevent the infiltration of slag; on the other hand, SiO2 in the biomass carbon material reacts with the slag after contacting the residual oxide in the rice husk precursor, which increases the viscosity of the slag and prevents further penetration and erosion of the slag, so The sample has good resistance to slag erosion.

Fig.7 Photograph of the cross-section of the crucible sample after 1400℃ slag corrosion for 3 h

3

In summary

(1) The aluminum-carbon castable that introduces thermal oxide graphite meets the construction requirements, and the amount of water added is high. Although there are interwoven whiskers in the residual pores inside the samples, the mechanical properties of the samples are still poor. The addition of biomass carbon materials can significantly improve the mechanical properties of the castable, and its flexural strength and compressive strength are 70% and 115% higher than graphite, respectively.

(2) Adding ZrC-modified graphite can improve the oxidation resistance of castables. The slag erosion resistance of aluminum-carbon castables prepared with biomass carbon materials as carbon sources is comparable to that of thermal oxide graphite, and both have better corrosion resistance. Performance.

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