Process and performance of magnesia-chrome bricks used in ladle slag line

Directly bonded magnesia-chrome bricks are MgO-Cr2O3 refractory products made by grinding high-purity magnesia sand and chrome concentrate with low impurity content and firing at high temperature (above 1700℃). Due to the high direct bonding rate of high-temperature mineral phases, they have strong slag resistance, high-temperature strength and excellent thermal shock resistance; re-bonded magnesia-chrome bricks are MgO-Cr2O3 refractory products made by high-pressure molding and firing at 1800℃ using fused magnesia-chrome sand as raw materials. Due to the higher direct bonding rate, low apparent porosity and high volume density, re-bonded magnesia-chrome bricks have higher high-temperature strength and slag erosion resistance than direct-bonded magnesia-chrome bricks. However, the thermal shock resistance of re-bonded magnesia-chrome bricks is poor. The main characteristics of the damage of MgO-Cr2O3 refractory materials used in the slag line of refined steel tanks are: chemical erosion of slag, structural spalling caused by slag penetration and erosion of high-temperature molten steel slag. MgO-Cr2O3 refractory materials have a certain corrosion resistance for CaO-SiO2 slag with low CaO/SiO2 ratio (less than 2), but for CaO-SiO2 slag with high CaO/SiO2 ratio at high temperature, especially when the Fe2O3 content is high, the low eutectic temperature drops rapidly and the corrosion resistance is very poor.

Improving the durability (thermal shock resistance, slag resistance and erosion resistance) of MgO-Cr2O3 bricks used in the slag line of refined steel tanks is related to the properties of secondary spinel in the bricks (production amount, size and distribution). Most researchers at home and abroad have confirmed that the generation of secondary spinel in bricks is related to brick-making raw materials, admixtures and brick-making processes:

(1) The amount of secondary spinel in directly bonded magnesia-chrome bricks increases with the increase of the proportion of chromium ore (or Cr2O3 content) in the batch; the amount of secondary spinel in rebonded/semi-rebonded magnesia-chrome bricks increases with the increase of the total amount of R2O3 (Cr2O3, Al2O3 and Fe2O3) in fused magnesia-chrome sand, the decrease of Fe2O3 content in R2O3 and the increase of Al2O3 content.

(2) When the specific surface area of fine powder in the batch of rebonded magnesia-chrome bricks reaches 5~6m2/g, the amount of secondary spinel generated is the largest.

(3) When the firing temperature of directly bonded magnesia-chrome bricks is above 1700℃, secondary spinel with self-crystallization characteristics can be observed; the size and amount of secondary spinel increase with the further increase of firing temperature. When the firing temperature is increased to 1800℃, the amount of secondary spinel generated reaches 6% (volume fraction).

Numerous research results confirm that:

(1) With the increase of secondary spinel generation and size in the brick, for example, when the volume fraction of secondary spinel in direct bonded magnesia-chrome brick reaches 6% and the volume fraction of secondary spinel in rebonded magnesia-chrome brick reaches 8%, the high temperature flexural strength reaches the highest value. High temperature flexural strength is an important indicator to measure the high temperature wear resistance of MgO-Cr2O3 brick, and high temperature wear resistance can reflect the important indicator of resistance to high temperature molten steel and slag erosion. Therefore, the high temperature molten steel and slag erosion resistance of direct bonded and rebonded (semi-rebonded) magnesia-chrome bricks with a large amount of secondary spinel generation will inevitably improve:

(2) There are a large number of secondary spinels in rebonded magnesia-chrome bricks that prevent slag erosion, so the slag resistance is the highest:

(3) Increasing the fineness of fine powder in the ingredients (for example, when the specific surface area of fine powder reaches 5m2/g), the thermal shock resistance of rebonded magnesia-chrome bricks is significantly improved. In short, by adopting the brick-making process of selecting raw materials and ultra-high temperature firing to increase the secondary spinel production in the brick, the direct-bonded and re-bonded (semi-re-bonded) magnesia-chrome bricks with high comprehensive performance for refined steel tank slag line can be obtained. Typical performance of several direct-bonded magnesia-chrome bricks of a certain company

Some countries have used alumina instead of chromite to make MgO-MgO·Al2O3 bricks (Al2O330%~40%, MgO60%~70%) with good thermal shock stability. However, bricks containing chromium spinel have strong slag resistance because the solubility of chromium spinel in silicate melt is lower than that of aluminum spinel. Initially, Radex-DB605 bricks were tried in the slag line of ASEA-SKF150t steel tank where the temperature fluctuated violently, and the service life was only 8 times. In order to extend the life of the ASEA-SKF steel tank slag line (especially the part closest to the electrode), fused-cast magnesia-chrome bricks were tried. The fused-cast magnesia-chrome bricks represented by "corhart104" are made of a mixed raw material of 55% magnesia and 45% chromite, which is cast from a eutectic melt at 2500℃ in an electric arc furnace, released thermal stress, and finally cut and polished with diamond. The phase composition of this fused-cast magnesia-chrome brick is: 50% periclase and its solid solution, 39% spinel, and no more than 10% silicate; it has a dense structure (total porosity <12.0%), a compressive strength of up to 140~165MPa, and a temperature of 5% deformation under a load of 0.18MPa of up to 2050℃, but it has poor thermal shock stability and is expensive, so it has been replaced by high-quality rebonded magnesia-chrome bricks.

Directly bonded and rebonded (semi-rebonded) magnesia-chrome bricks have occupied the slag line of refined steel tanks for the longest time, and are still an important refractory material available for the slag line of refined steel tanks today. However, the complex production, price issues, and the harm of hexavalent chromium to human health of these magnesia-chrome bricks have forced people to develop substitutes.

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