Effect Of Silicon Metal Powder On Oxidation Resistance Of Al₂O₃-SiC-C Taphole Castables

To improve the oxidation resistance of taphole channel castables, in addition to increasing their density, antioxidants must also be added. Silicon metal powder, also known as industrial silicon powder, reacts with carbon at high temperatures to form silicon carbide. It is preferentially oxidized over carbon, forming a protective layer and thereby slowing down the oxidation of the castable. This paper investigates the effect of silicon metal powder addition on the properties of the Al₂O₃-SiC-C taphole mix, especially its oxidation resistance.

Experimental Method

1. Raw Materials

The main raw materials used in the experiment include fused brown corundum (Al₂O₃: 95.82%), silicon carbide, α-Al₂O₃ micropowder (D₅₀ = 1.08 μm), white corundum powder (particle size = 0.044 mm), silica fume (SiO₂: 96.98%), spherical pitch, and pure calcium aluminate cement.

2. Sample Preparation

The raw materials were accurately weighed according to the proportions shown in Table 1, dry-mixed in a mixer for 5 min, and then wet-mixed with water for 3 min until a uniform mixture was obtained. The mixture was vibrated and formed on a vibrating table into bar specimens measuring 40 mm × 40 mm × 160 mm and cube specimens measuring 70 mm × 70 mm × 70 mm (with a central hole 30 mm in diameter and 30 mm in depth to serve as a crucible).

After curing at room temperature for 24 h, the specimens were demolded and dried at 110 °C for 24 h. The bar specimens were heated to 1000 °C and 1450 °C, respectively, held for 3 h, and fired in an electric furnace. The crucible specimens were filled with 25 g of blast furnace slag from a steel plant, and a static crucible slag resistance test was conducted at 1450 °C for 3 h in an electric furnace.

Chemical composition of blast furnace slag (%): Al₂O₃: 17.56, Fe₂O₃: 0.71, CaO: 40.16, MgO: 8.24, SiO₂: 30.06.

3. Sample Testing

The apparent porosity, bulk density, cold modulus of rupture (CMOR), and linear change rate of the specimens were tested in accordance with YB/T 5200-1993 (2007). After firing at 1450 °C for 3 h, the cross-sections of the specimens showed a uniform color and could not be clearly distinguished. Therefore, to compare oxidation resistance, specimens fired at 1000 °C for 3 h were used for oxidation resistance analysis. After measuring the modulus of rupture of the specimens fired at 1000 °C for 3 h, their cross-sections were used for oxidation resistance evaluation.

The slag-resistant specimens were cut open along the center, and the slag penetration depth and corrosion were measured and analyzed using a vernier caliper and other methods.

Results and Discussion

1. Effect of Silicon Metal Powder on Physical Properties

The effects of silicon metal powder addition on the linear change rate, bulk density, and porosity of the taphole channel specimens fired at 1000 °C for 3 h are shown in Figure 1.

It can be seen from Figure 1 that when the addition amount of silicon metal powder is 1%–4%, the linear change of the specimens shows a gradually decreasing trend, reaching a minimum at 4%, and then increasing thereafter. The bulk density increases gradually when the silicon metal powder addition is 1%–2%, reaching a maximum at 2%, then decreases and remains almost unchanged at 3%–5%. In contrast, the apparent porosity shows a continuous decreasing trend with the increase in silicon metal powder content.

This is because, with the increase in silicon metal powder addition, more SiO₂ is generated by the reaction between Si and O₂, which reduces the linear change. The reaction products block the pores, resulting in a gradual decrease in apparent porosity.

2. Effect of Silicon Metal Powder on Strength

The effect of silicon metal powder addition on the flexural strength of the specimens is shown in Figure 2. It can be seen from Figure 2 that after heat treatment at 110 °C × 24 h and 1450 °C × 3 h, the flexural strength of the specimens gradually increases with the increase in silicon metal powder addition.

3. Effect of Silicon Metal Powder on Oxidation Resistance

The effect of silicon metal powder on the oxidation resistance of the taphole channel specimens is shown in Figure 3.

According to the cross-sectional photos in Figure 3, the darker area in the center represents the unoxidized or slightly oxidized carbon portion. The area of this central region gradually increases with the increase in silicon metal powder addition, indicating that the oxidation resistance of the specimens treated at 1000 °C for 3 h improves within the experimental range as the silicon metal powder addition increases.

The reaction mechanism of silicon metal powder as an antioxidant is as follows:

During the firing process of the specimens, carbon is first oxidized to CO(g). The CO then reacts with metallic Si to form SiO(g), while CO is reduced back to C.

Therefore, with an increase in silicon metal powder addition, more carbon is reduced in the specimens within the same period of time, and more SiO₂ is simultaneously produced. SiO₂ acts as a liquid phase that can block pores and improve the densification of the material.

Thus, from the cross-sectional observation, the larger the dark central area observed with increasing silicon metal powder addition, the better the oxidation resistance.

4. Effect of Silicon Metal Powder on Slag Corrosion Resistance

The effect of silicon metal powder addition on the slag corrosion resistance of the taphole channel specimens is shown in Figure 4.

By measuring the most severely eroded positions on the sides of the specimens using a vernier caliper, it was found that when the silicon metal powder content in the specimens ranges from 1% to 5%, the side erosion thicknesses are 3, 2, 2, 3, and 4 mm, respectively. This indicates that the slag corrosion resistance of the specimens is optimal when the silicon metal powder addition is 2% or 3%. Considering the high cost of metallic silicon and the overall cost-performance of the material, the optimal addition amount of silicon metal powder is determined to be 2%.

The addition of silicon metal powder to the taphole mix produces two effects. On one hand, the oxidation resistance of the specimens improves with increasing silicon metal powder content, and the slag resistance is also enhanced. On the other hand, as the silicon metal powder content increases, the Al₂O₃ content in the matrix of the specimens decreases, leading to a relative decline in slag resistance.

When the addition amount is 1% and 2%, the first effect dominates, resulting in improved slag resistance.

When the addition amount is 2% and 3%, the two effects tend to balance, and the slag resistance remains essentially unchanged.

When the addition amount is 3%, 4%, or 5%, the second effect outweighs the first, resulting in decreased slag resistance.

Therefore, considering all factors, the appropriate addition amount of silicon metal powder is 2%.

Conclusions

Adding silicon metal powder to Al₂O₃-SiC-C taphole channel castables and subjecting them to heat treatment leads to a gradual decrease in linear change, an increase in bulk density, a gradual decrease in apparent porosity, and a gradual increase in flexural strength as the silicon metal powder content increases.

When the silicon metal powder content increases from 1% to 5%, the oxidation resistance of the specimens treated at 1000 °C for 3 h improves with increasing silicon metal powder addition.

After the slag resistance test at 1450 °C for 3 h, the slag corrosion resistance of the specimens is optimal when the silicon metal powder content is 2% or 3%.

Based on comprehensive consideration, the optimal addition amount of silicon metal powder in Al₂O₃-SiC-C taphole channel castables is 2%.