Excellent performance of combustion-synthesized silicon-nitrogen-oxygen micro powder in improving the overall contracting benefits of iron trough castable

Jiangsu Jingxin New Material Co., Ltd.'s independently developed combustion-synthesized silicon-nitrogen-oxygen micropowder is a key national research and development project of the Ministry of Science and Technology. This product can be widely used in carbonaceous refractory materials, such as iron trough castables, taphole clay, silicon nitride-bonded silicon carbide bricks, and alumina-carbon refractory products. This micropowder possesses the following excellent properties: 1. High thermal conductivity and low coefficient of thermal expansion, stable structure, and excellent thermal shock resistance. 2. High Mohs hardness, exhibiting excellent hardness and wear resistance. 3. Large wetting angle with molten metal and slag, and excellent chemical stability, providing good resistance to molten metal and slag erosion. 4. Strong oxidation resistance; while forming a dense oxide film, the generated products effectively fill pores and hinder oxygen entry.

When applied to iron trough castables, the addition amount is approximately 3%, which can increase the high-temperature flexural strength of the castable by more than 40%, increase the slag erosion resistance index by more than 25%, reduce the oxidation index by more than 30%, and improve thermal shock resistance by more than 35%. This comprehensively improves the overall performance of iron ditching and significantly enhances the overall economic benefits of iron ditching contracting. Field application results from over ten domestic iron ditch refractory material manufacturers show that it can increase iron throughput by 8-25%.

I. Overview of Silicon-Nitrogen-Oxygen Micropowder

The particle size distribution of the micropowder is shown in Figure 1. It can be seen that the micropowder exhibits a multi-peak distribution, with a D50 of 3.05 μm.

Figure 1. Particle size analysis results of silicon nitrogen-oxygen micro powder

The XRD diffraction analysis results of the silicon-nitrogen-oxygen micropowder are shown in Figure 2. It can be seen that the main crystalline phases of the silicon-nitrogen-oxygen micropowder are silicon nitride (α-Si3N4, β-Si3N4) and silicon oxynitride (Si2N2O), as well as trace amounts of Si.

Figure 2. XRD pattern of silicon nitrogen oxide micro powder

The microstructure of silicon-nitrogen-oxygen powder is shown in Figure 3. It can be seen that the silicon-nitrogen-oxygen powder is mainly in the form of rods, short columnar shapes, and irregular granules.

Figure 3. SEM image of silicon-nitrogen-oxygen micro powder.

II. Experimental Design and Sample Preparation

Based on the sample without silicon, nitrogen, and oxygen micropowder, five groups of samples were prepared by adding different mass fractions (1%, 2%, 3%, and 5%) of silicon, nitrogen, and oxygen micropowder. The sample mixes are shown in Table 1.

Table 1 Experimental proportions

III. Experimental Results

3.1 Water Addition

Figure 4. Effect of silicon nitrogen-oxygen micro powder addition amount on water addition amount of sample

As shown in Figure 4, the addition of silicon nitride-oxygen powder helps reduce the water addition of the castable. This is because silicon nitride-oxygen powder is finer than corundum powder, effectively filling the gaps in the system and optimizing the particle size distribution, thus reducing the total water requirement of the castable.

3.2 Cold Strength

As shown in Figure 5, the compressive and flexural strengths reach their maximum values when 3% silicon nitride-oxygen powder is added: the flexural and compressive strengths of the sample fired at 1100℃ increased by 27.9% and 42.3%, respectively; the flexural and compressive strengths of the sample fired at 1500℃ increased by 24.1% and 26.0%, respectively.

Figure 5. Cold flexural strength and compressive strength of specimens after different temperature treatments.

3.3 Thermal Shock Resistance

Figure 6. Flexural strength retention rate of the specimen after thermal shock after firing

With increasing amounts of silicon nitride-oxygen powder, the thermal shock resistance of the samples significantly improved. The flexural strength retention rate after three air-cooling thermal shocks following firing at 1100℃ increased from 68.5% to 99.8%; the strength retention rate after thermal shock following firing at 1500℃ increased from 65.1% to 88.3%.

3.4 High-Temperature Flexural Strength

Figure 7. Effect of silicon-nitrogen-oxygen micropowder addition on high-temperature flexural strength

The samples treated at 1100℃ showed the same trend as those treated at 1500℃. After treatment at 1100℃, the high-temperature flexural strength without silicon nitride-oxygen powder was 2.68 MPa. When the amount of silicon nitride-oxygen powder added was 3%, the high-temperature flexural strength reached 5.39 MPa, an increase of 101.1%. The high-temperature flexural strength of the samples after firing at 1500℃ was still the highest for the 3% powder addition.

3.5 Antioxidant Properties

Figure 8. Effect of silicon-nitrogen-oxygen micropowder addition amount on antioxidant properties

As shown in the figure, the addition of silicon-nitrogen-oxygen powder significantly improves the antioxidant properties of the sample. The decarburized layer area is minimized when the addition amount is 3%~5%.

3.6 Slag Erosion Resistance

Figure 9. Effect of silicon nitrogen-oxygen micro powder addition amount on slag resistance performance.

As shown in the figure above, the addition of silicon-nitrogen-oxygen powder significantly improves the slag erosion resistance of the sample. Without the addition of silicon-nitrogen-oxygen powder, almost all the slag reacts or penetrates into the sample. When the addition amount is 3%, the remaining slag is at its maximum, and the erosion layer and penetration layer areas are also minimized. When the addition amount is 5%, the erosion layer and penetration layer areas increase compared to 3%.

IV. Analysis and Discussion

4.1 XRD Phase Analysis

XRD phase analysis was performed on the matrix portion of the sample treated at 1500℃ (Figure 10). The figure clearly shows the following significant changes in phases with the increase of silicon-nitrogen-oxygen powder: (1) O'-Syron phase gradually increases; (2) Mullite phase gradually increases; (3) Correspondingly, alumina phase gradually decreases.

Figure 10. Effect of adding silicon nitrogen oxide micropowder on the phase change of the matrix.

4.2 Microstructure Analysis

Figure 11 shows SEM images of the samples after calcination at 1100℃. It can be seen that due to the relatively low calcination temperature, the sintering of the matrix portion of sample SN0 is not obvious, resulting in relatively poor bonding between materials and scattered large pores in the matrix portion. Compared to sample SN0, sample SN3 is more dense, with significantly fewer large pores in the matrix portion. This is partly due to the addition of silicon nitride powder filling the internal pores of the sample, resulting in better matrix density; secondly, the addition of silicon nitride powder promotes the sintering reaction, initiating earlier and more vigorously. The expansion and products generated by the reaction can fill some of the pores, thereby inhibiting oxygen entry and improving the material's density and strength.

Figure 11. SEM image of the sample after firing at 1100 °C.

The samples treated at 1500℃ were analyzed using scanning electron microscopy. Figure 12 shows that without the addition of silicon nitride powder, only a small amount of fine granular mullite exists in the matrix. With the addition of 3%, obvious columnar mullite phases are visible in the matrix. The SN3 sample contained distinct tubular and whisker-like O'-Serone phases.

Figure 12. SEM images of SN0 and SN3 samples after treatment at 1500℃.

The aforementioned in-situ generated whiskers and columnar crystals fill the matrix of the sample, providing a good pinning effect and bridging between phases. This significantly strengthens the sample structure, improves its strength and toughness, and consequently, markedly enhances its high-temperature flexural strength, thermal shock resistance, slag erosion resistance, and permeability.

V. Conclusions

1. Silica-nitrogen-oxygen micropowder can significantly improve the high-temperature flexural strength, thermal shock resistance, and cold strength of iron trough castables.

2. Silica-nitrogen-oxygen micropowder can greatly improve the oxidation resistance and slag erosion permeability resistance of castables.

3. The introduction of silica-nitrogen-oxygen micropowder can increase the amount of iron in the blast furnace tapping channel by 8-25%.