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Effect of silicon nitride on the slag resistance properties of corundum blast furnace gunning materials

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Abstract: Using tabular corundum as raw material, pure calcium aluminate cement and SiO2 micro powder as binding agent, a crucible sample of corundum blast furnace gunning material was made, and then the blast furnace slag corrosion resistance test was conducted in a reducing atmosphere at 1550℃ for 3 hours. . The effect of silicon nitride addition (0, 5% and 10% respectively) on the slag corrosion resistance of corundum blast furnace gunning materials was studied. Through microscopic analysis of the corrosion samples, the slag corrosion mechanism was obtained. The results show that when silicon nitride is not added to the corundum gunning material, slag can easily corrode the gunning material. However, adding 5% and 10% silicon nitride improves the slag permeability resistance of the gunning material. It greatly improves its slag resistance; silicon nitride has a network structure in the gunning material matrix, which is also the main reason for improving the gunning material’s resistance to slag erosion.

Keywords: silicon nitride, slag resistance, gunning material, blast furnace

For a long time, domestic and foreign iron-making workers and researchers in related professions have explored and researched the factors that affect the life of blast furnaces in many aspects (such as blast furnace design, blast furnace raw materials, blast furnace operations, cooling systems, construction, refractory materials, etc.) . Promising results have been achieved and the service life of the blast furnace has been continuously improved. Among them, refractory materials have a great impact on the life of the blast furnace. Nowadays, maintaining the furnace lining through blast furnace gunning is one of the important means to improve the life of the blast furnace. The blast furnace gunning material is affected by the construction method, resulting in a small volume density and high porosity of the gunning layer. Compared with the masonry lining, it is more susceptible to erosion and damage by blast furnace slag. Therefore, for the gunning materials used below the blast furnace shaft, it is very important to have good slag resistance. For this reason, silicon nitride materials are introduced in order to improve the slag resistance of the new blast furnace gunning materials. This work studied the effect of silicon nitride on the slag resistance performance of corundum blast furnace gunning materials and its erosion mechanism through static slag resistance tests.

Test method

The main raw materials used in the test are tabular corundum and Si3 N4 powder, with pure calcium aluminate cement and SiO2 micro powder as the binding agent. The sample proportions are shown in Table 1.

Table 1 Sample proportion (w) %

Sample Notabular corundumAlumina micropowderSiO2 micropowderPure aluminic acidCalcium cement Si3N4 powder
5 ~3 mm3 ~2 mm2 ~1 mm1 ~0. 2 mm< 0. 2 mm
T1102020141556100
T2102020141056105
T3102020145561010

Add an appropriate amount of water to the test materials in different proportions, stir and mix, then pound into the iron mold, and demould after 24 hours. Put the sample block into water at about 20℃ for 24 hours, take it out and place it in a ventilated and dry place, and dry it naturally for 24 hours. Then put the sample block in an oven at 110°C to dry for 24 hours, take it out, and cool naturally. Mechanically drill holes into the specimen to form the crucible. Add 100 g of blast furnace slag to each slag-resistant crucible sample. In order to make the test closer to the actual usage conditions of the blast furnace, the crucible sample was placed in a sagger, buried with graphite powder, then sealed, heat treated in a high temperature furnace at 1550°C for 3 hours, and taken out after cooling. The slag used in the test is the blast furnace slag of Angang Iron and Steel Plant. The chemical composition (w) of the slag is: CaO42.14%, MgO7.03%, SiO240.06%, Al2O3 6.88%, FeO0.53%, S 0.72%. The eroded specimens were sectioned longitudinally for appearance and microstructural analysis.

Results and discussion

Appearance analysis of corrosion samples

Figure 1 shows the cross-sectional appearance of the sample after erosion. It can be seen from the figure that when silicon nitride is not added, the slag penetration of the sample is relatively serious, with obvious erosion zones and reaction layers. There is very little slag remaining in the crucible, and most of the slag penetrates into the sample. When the addition amount of silicon nitride is 5%, the slag penetrates the sample slightly. When the addition amount of silicon nitride is 10%, the bottom of the sample is intact, the slag hardly wets the bottom and inner wall of the sample, and there is a large amount of slag in the crucible.

Figure 1 Appearance of the sample after erosion

It can be seen that as the addition amount of silicon nitride increases, the material’s resistance to slag penetration increases. This is because silicon nitride is not oxidized in a reducing atmosphere, and the wetting angle between silicon nitride and slag at high temperature is 110° to 130°, which is much larger than the wetting angle between corundum refractory material and molten slag. This makes it difficult for blast furnace slag to penetrate into the pores of the material, thereby improving the material’s ability to resist slag erosion.

Microstructure analysis and corrosion mechanism analysis

Microscopic analysis of the T1, T2, and T3 etched samples was performed using SEM. Figure 2 is the SEM image of the T1 sample after erosion. It can be seen from the figure that there is no obvious boundary between the erosion layer and the transition layer of the sample, and the entire sample has a loose structure and many holes. Although some small particles in the picture have smooth edges and have not been eroded, the slag has spread to the entire sample. The erosion situation can be known from the Ca element surface distribution map. The slag penetrates into the interior of the sample through the matrix of the sample, and the interior of the large particles has been corroded by the slag. The CaO in the slag reacted with the matrix part of the gunning material, and a new mineral phase was generated on the transition layer. According to thermodynamic analysis, it can be concluded that the generated mineral phase may be an aluminum-silicon-calcium series low-melting point compound. The diffusion of Fe element is the same as that of Ca element, so the low melt formed after FeO and CaO in the slag react with the aluminum and silicon raw materials in the gunning material quickly melts into the slag. At the same time, the slag penetrates into the gunning material layer along the loose interface and pores, and further reacts with the aggregate and matrix to cause erosion.

Figure 2 SEM image and element surface distribution map of T1 sample after slag erosion

Figure 3 is the SEM image of the T2 sample after erosion. It can be seen from the image that there is a very obvious interface between the slag zone and the transition zone. It can be seen from the element distribution map that the Ca element distribution gradually decreases from the slag zone to the transition zone, indicating that the slag penetration is less. The silicon nitride in the raw material is distributed in the matrix, which plays a role in hindering the invasion of slag. The distribution of silicon nitride in the matrix has a network structure, which is also the main reason for the improvement in penetration resistance.

Figure 3 SEM image and element surface distribution map of T2 sample after slag erosion

Figure 4 shows the morphology of the T3 sample after erosion. It can be seen from the figure that there is no transition zone, and the interface between the slag and the gunning material is very obvious. Due to the addition of 10% silicon nitride powder, the slag could not enter the matrix. Compared with the morphology of the T1 sample after erosion, it shows that the addition of silicon nitride plays a role in resisting slag erosion.

Figure 4 SEM image and element surface distribution map of T3 sample after slag erosion

Conclusion

When silicon nitride is not added to the corundum gunning material, the blast furnace slag can easily corrode the gunning material. However, silicon nitride is added to the gunning material. The slag resistance permeability of the gunning material is improved and its slag resistance ability is greatly improved. Silicon nitride has a network structure in the sample matrix, which is the main reason for improving the slag resistance of the gunning material. As the amount of silicon nitride added increases, the ability to resist slag penetration increases. The appropriate addition amount of silicon nitride powder is more than 5%.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

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