There are many types of antioxidants for magnesia carbon ...

Magnesia-carbon bricks are alkaline refractory material made of magnesia and carbon materials, using asphalt or resin as binders. They combine the high refractoriness of magnesia refractories with the superior slag and molten steel resistance of carbon refractories. Due to these properties, magnesia-carbon bricks are extensively used in harsh environments of the steel industry, such as converter furnace mouths, electric furnace slag lines, and ladle working layers, where high temperature, severe mechanical abrasion, and intense slag corrosion prevail.

However, the carbon component in magnesia-carbon bricks is prone to oxidation at high temperatures and in oxidizing atmospheres. This oxidation leads to structural deterioration, erosion, peeling, and ultimately, failure of the brick. To counteract this, various antioxidants are added, such as boron carbide, metal aluminum powder, metal silicon powder, and their mixtures, to enhance the bricks' quality by inhibiting carbon oxidation.

Large crystal fused magnesia (particle sizes ranging from 5-3mm to <0.088mm) was used as the aggregate. Flake graphite, high-temperature asphalt (particle size <0.088mm), liquid phenolic resin, boron carbide, metal aluminum powder, and metal silicon powder were added. The precise chemical compositions are detailed in Table 1. The raw materials were mixed uniformly in a specific order before being press-molded to create standard samples labeled MT1 to MT5.

Bulk Density, Apparent Porosity, and Compressive Strength Test

Samples were dried at 110°C for 6 hours, then further dried at 200°C for 12 hours. Tests for bulk density, apparent porosity, and compressive strength at room temperature were conducted according to national standards. Results showed that adding boron carbide, metal aluminum powder, and metal silicon powder altered sample densities. The volume density decreased, reaching its lowest with metal silicon powder alone, but increased when metal aluminum and silicon powder were combined.

The decrease in volume density corresponded with increased apparent porosity and decreased compressive strength at room temperature. The reduced density and weakened solid-solid bonds within the sample caused by antioxidants resulted in this trend.

High Temperature Flexural Strength Test

Samples were heated to a specific temperature and held for 30 minutes before testing high-temperature flexural strength, following national standards. The results indicated that the flexural strength generally increased with the addition of antioxidants. Adding boron carbide reduced internal oxygen partial pressure, forming a protective liquid B2O3 film that slowed oxidation. The formation of trimagnesium borate further sealed pores and improved strength.

When heated, metal aluminum powder reacted with carbon and CO, creating high melting point substances like Al4C3, Al2O3, and MA, leading to volume expansion and improved strength. Metal silicon powder produced SiO2 at 600°C, which blocked pores and enhanced oxidation resistance. Together, metal aluminum and silicon powders resulted in a denser, stronger brick.

Heating Permanent Linear Change Rate Test

Samples were heated and maintained at specific conditions to test their heating permanent linear change rate. Results showed irregular trends with antioxidant addition. Boron carbide oxidation produced liquid phase B2O3 and trimagnesium borate, promoting sintering and potentially causing slight expansion due to the higher expansion coefficient of magnesia used in samples. Addition of metal aluminum powder promoted volume expansion due to the formation of secondary spinel and other high refractory substances. Silicon metal powder addition led to sintering from SiO2 formation and slight expansion from M2S formation.

Anti-Slag Performance Test

A static crucible method was applied to test slag erosion resistance. Steel slag was placed in a crucible sample, heated to a specific temperature, maintained for 3 hours, cooled, and measured for hole expansion. Results indicated that antioxidant addition decreased slag corrosion resistance, except for MT4 with silicon powder alone. Aluminum powder broadened the reaction temperature range, sealing pores and maintaining similar performance to samples without antioxidants.

In summary, adding antioxidants generally reduced bulk density and compressive strength of magnesia-carbon bricks. Combining metal aluminum and silicon powders yielded the highest high-temperature flexural strength. Boron carbide and silicon powder caused beneficial slight expansion, enhancing high-temperature stability. The best comprehensive performance was seen with antioxidant metal silicon powder.

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