Optimal Carbon Black Addition For Al₂O₃-SiC-C Refractory Castables: Maximize Comprehensive Performance

Most large and medium-sized blast furnace runners adopt Al₂O₃-SiC-C refractory castables. The most severely damaged part of the main runner is the sidewall area at the iron drop point, which is the interface between blast furnace slag and hot metal. The primary causes of damage include carbon oxidation, slag corrosion, and hot-metal scouring. Studies have shown that the rational selection of the carbon source type and its addition amount can effectively improve the service life of Al₂O₃-SiC-C refractory castables.

Spherical pitch has a low residual carbon content, and flake graphite is difficult to incorporate into castables due to its lamellar structure. In contrast, carbon black, as an amorphous carbon material with a large specific surface area, offers advantages such as high residual carbon content, small particle size, and high reactivity. It is often added to carbon-containing composite refractory products, such as magnesia-carbon bricks and alumina-carbon sliding plates. The incorporation of carbon black can reduce the corrosion and penetration of molten steel into refractories, thereby extending their service life and achieving excellent application results.

Therefore, adding carbon black to Al₂O₃-SiC-C runner castables is expected to enhance their service performance. However, in-depth research on the influence of carbon black on the service mechanisms of runner castables is still lacking. For this reason, N990 carbon black, which has good dispersibility in water, was selected in this study.

This paper mainly investigates the effects of carbon black addition on the room-temperature physical properties, high-temperature flexural strength, and oxidation resistance of Al₂O₃-SiC-C refractory castables, and applies the research results to industrial trials.

Part 1 Experiment

1. Raw Materials

The main raw materials used in the experiment were as follows: fused brown alumina, white fused alumina, 97-grade silicon carbide (97 SiC), silica fume (particle size distribution: D50 < 1.5 μm), spherical pitch (softening point: approximately 110 °C), Secar 71 cement, 98-grade metallic silicon powder (w(Si) = 98.24%), activated alumina micropowder (particle size distribution: D50 < 2 μm), N990 carbon black, and sodium hexametaphosphate. The chemical compositions of the main raw materials are shown in Table 1.

2. Specimen Preparation

Fused brown alumina and white fused alumina were used as aggregates, while white fused alumina, silicon carbide, and alumina served as the matrix, with cement acting as the binder. The carbon black content was gradually increased to mass fractions of 0, 0.5%, 1%, 1.5%, and 2%. The remaining mass was supplemented with alumina powder. The prepared castables were labeled as 1#, 2#, 3#, 4#, and 5#, respectively. The detailed mix proportions of the specimens are shown in Table 2.

The ingredients were batched according to the mix ratios specified in Table 2. The prepared materials were dry-mixed for 60 s, followed by the addition of a fixed water dosage (4.2% by mass, meeting the construction workability requirements) and wet-mixing for an additional 120 s. After thorough mixing, the mixture was vibration-molded on a shaking table to produce bar-shaped specimens with dimensions of 40 mm × 40 mm × 160 mm. The specimens were cured in the molds at room temperature for 24 h before demolding, and then dried in a constant-temperature oven at 110 °C for 24 h. Subsequently, the specimens were fired in an air atmosphere at 1000 °C and 1500 °C, respectively, with a holding time of 3 h at each temperature, and finally cooled naturally to room temperature in the furnace.

3. Performance Testing

Vibrating fluidity:

The vibrating fluidity of the castables was measured using a TZ-345 cement mortar fluidity tester under test conditions of 25 vibrations within 30 s.

Bulk density and apparent porosity:

The bulk density and apparent porosity of the specimens, both in the dried and fired states, were measured in accordance with GB/T 5072-2008 and GB/T 2997-2000, respectively.

Cold crushing strength:

The cold crushing strength of the specimens before and after firing was determined according to YB/T 5201-1993.

High-temperature flexural strength:

The high-temperature flexural strength was tested at 1450 °C with a holding time of 1 h in an air atmosphere, in accordance with GB/T 3002-1982.

Linear change rate after firing:

The dimensional changes of the specimens before and after firing were measured following GB/T 3001-2007, and the linear change rate after firing was calculated accordingly.

Decarburized layer thickness:

The decarburized layer thickness of the specimens was determined in accordance with GB/T 13244-91.

Microstructure observation:

The microstructure of the fracture surfaces of the specimens after high-temperature flexural strength testing was observed using an EVO18 scanning electron microscope (SEM, Zeiss, Germany).

Part 2 Results and Discussion

1. Effect of Carbon Black Addition on the Room-Temperature Physical Properties of Castables

1.1 Effect of Carbon Black Addition on the Fluidity of Castables

At the same water addition level, the variation in the fluidity of the castables with different carbon black contents is shown in Figure 1. As shown in Figure 1, the fluidity of the castables first increases and then decreases with increasing carbon black addition. When the carbon black content exceeds 1%, the change in fluidity is not significant; when the carbon black content exceeds 1.5%, the fluidity gradually deteriorates.

This behavior can be explained by the good dispersibility and fine particle size of carbon black. At low addition levels, carbon black can fill micropores and reduce water demand. However, at higher addition levels, carbon black exists outside the pore structure, increasing the required water content and consequently decreasing the fluidity.

1.2 Effect of Carbon Black Addition on Bulk Density and Apparent Porosity of Castables

The variations in bulk density and apparent porosity of the specimens after heat treatment at different temperatures with varying carbon black contents are shown in Figure 2. As shown in Figure 2(a), the bulk density of the castables first increases and then decreases after heat treatment at 110 °C for 24 h, 1000 °C for 3 h, and 1500 °C for 3 h. Figure 2(b) shows that, after low-, medium-, and high-temperature treatments, the apparent porosity of the specimens first decreases and then increases, reaching a minimum when the carbon black content is 1.5%.

This behavior can be explained as follows. On one hand, carbon black has a bulk density of 1.85 g/cm³, which partially replaces the higher-density white fused alumina micropowder, resulting in a reduction in bulk density. On the other hand, with increasing carbon black content, it fills the pores, densifies the structure, and promotes the sintering of the specimens, thereby increasing bulk density. However, excessive carbon black addition reduces the fluidity of the castables and decreases structural compactness. Consequently, after heat treatment, the bulk density decreases while the apparent porosity increases.

1.3 Effect of Carbon Black Addition on the Linear Change Rate of Castables after Heat Treatment at Different Temperatures

The variation in the linear change rate of specimens fired at 1000 °C and 1500 °C with different carbon black contents is shown in Figure 3. The linear change rates are positive after firing at both 1000 °C for 3 h and 1500 °C for 3 h, indicating that the specimens expand under both conditions.

With increasing carbon black content, the linear expansion of specimens fired at 1000 °C increases gradually. This may be due to the in-situ reaction between carbon (C) and silicon (Si) occurring at 800–1200 °C, which generates primary silicon carbide (SiC) crystals and causes expansion. As more carbon black is added, additional primary SiC crystals are produced, resulting in greater expansion.

For specimens fired at 1500 °C, the linear expansion rate first increases and then decreases, reaching a maximum at a carbon black content of 1.5%. In the range of 0–1.5% carbon black, the increase in linear expansion may be attributed to both the formation of SiC and the mullitization reaction between Al₂O₃ and SiO₂. In the range of 1.5–2% carbon black, the decrease in linear expansion may be related to the reduced structural compactness of the specimens.

1.4 Effect of Carbon Black Addition on Cold Crushing Strength of Castables

The variation in the cold crushing strength at room temperature of specimens after heat treatment at different temperatures with varying carbon black content is shown in Figure 4. The cold crushing strength of specimens dried at 110 °C shows little change with increasing carbon black content. For specimens fired at 1000 °C and 1500 °C, the cold crushing strength first increases and then decreases, reaching a maximum at a carbon black content of 1.5%.

The strength of the specimens is influenced by multiple factors, including density, phase composition, and microstructure. The variation in room-temperature strength with carbon black addition may be attributed to the amorphous nature and large specific surface area of carbon black. At an appropriate addition level, carbon black can be well dispersed in the castables, fill the pores, and promote densification and sintering.

2. Effect of Carbon Black Addition on High-Temperature Flexural Strength of Castables

The effect of carbon black addition on the high-temperature flexural strength of the castables is shown in Figure 5. As illustrated, the flexural strength of the specimens increases with increasing carbon black content; however, the strength gain becomes negligible when the carbon black content exceeds 1.5%.

In addition to the formation of well-developed mullite crystalline phases, a large number of silicon carbide (SiC) whiskers are also observed in the specimens. Since carbon serves as the primary reactant in the formation of SiC, the generation of β-SiC whiskers in the pores and the enhanced mullitization at high temperatures are closely associated with the addition of carbon black. Both the mullite crystals and SiC whiskers contribute significantly to the improvement of the high-temperature flexural strength of the castables.

3. Effect of Carbon Black Addition on Oxidation Resistance of Specimens

The oxidation behavior of the specimen cross-sections (40 mm × 40 mm) after heat treatment at 1000 °C for 3 h and 1500 °C for 3 h showed that the extent of oxidation at 1500 °C was lower than that at 1000 °C. This is mainly because a dense oxide film forms on the specimen surface at the higher temperature, which mitigates the oxidation process. The thickness of the decarburized layer decreased gradually with increasing carbon black content, thereby improving the oxidation resistance of the specimens. Specifically, specimens with 1.5% and 2% carbon black exhibited comparable oxidation resistance.

This behavior is primarily attributed to the increasing density of the specimen structure with carbon black addition, which slows down oxidation by oxygen. However, when the carbon black content reached 2%, the fluidity of the castables deteriorated, the apparent porosity increased, and the oxidation rate accelerated. As a result, the oxidation resistance showed no significant improvement compared with that of specimens containing 1.5% carbon black.

Based on the comprehensive performance analysis, the optimal addition amount of N990 carbon black in the Al₂O₃-SiC-C blast furnace runner castables was determined to be 1.5%.

Part 3 Industrial Test and Application

Based on the laboratory results, a formulation containing 1.5% carbon black was applied in an industrial trial at an iron and steel company in Tangshan. The blast furnace has a volume of 1080 m³, with the main runner extending 13.5 m to the tapping pit and the slag runner measuring 9 m in length. A total of 62 tons of castables were used, of which 43 tons were applied to the main runner. The castables remained in service until the next relining, achieving a service life of 78 days with a total iron output of approximately 160,000 tons, demonstrating favorable application performance.

Part 4 Conclusions

The incorporation of N990 carbon black improved the room-temperature physical properties of the Al₂O₃-SiC-C castables at low addition levels, while excessive addition led to a slight decrease. High-temperature flexural strength and oxidation resistance were enhanced, and the castables exhibited optimal comprehensive performance at a carbon black content of 1.5%.

The runner castables produced based on this formulation achieved satisfactory results in industrial trials.