Directly bonded magnesia-chrome bricks are MgO-Cr₂O₃ refractory products manufactured by co-grinding high-purity magnesia with low impurity content and chrome concentrate, followed by high-temperature sintering (above 1700°C). Due to the high direct bonding rate of high-temperature mineral phases, they exhibit strong slag resistance, high-temperature strength, and excellent thermal shock resistance. Recombined magnesia-chrome bricks are MgO-Cr₂O₃ refractories manufactured from electrically fused magnesia-chrome sand, formed under high pressure, and fired at 1800°C. Featuring higher direct bonding rates, lower apparent porosity, and greater bulk density, they exhibit superior high-temperature strength and slag resistance compared to directly bonded magnesia-chrome bricks. However, their thermal shock resistance is relatively poor. Key failure characteristics of MgO-Cr₂O₃ refractories in slag line zones of steel refining ladles include: chemical erosion by molten slag, structural spalling caused by slag penetration, and erosion by high-temperature molten steel-slag mixtures. MgO-Cr₂O₃ refractories exhibit moderate resistance to erosion by CaO-SiO₂ system slags with low CaO/SiO₂ ratios (below 2). However, they demonstrate extremely poor erosion resistance against high-temperature CaO-SiO₂ system slags with elevated CaO/SiO₂ ratios, particularly those containing high Fe₂O₃, where the eutectic melting point rapidly decreases.
Enhancing the durability (thermal shock resistance, slag resistance, and erosion resistance) of MgO-Cr₂O₃ bricks used in the slag line region of steel refining ladles is closely related to the properties of secondary spinel within the bricks (formation quantity, size, and distribution). Most domestic and international researchers have confirmed that the formation of secondary spinel within bricks is associated with the raw materials, additives, and manufacturing processes used in brick production:
(1) The quantity of secondary spinel in directly bonded magnesia-chrome bricks increases with the proportion of chromite ore (or Cr₂O₃ content) in the mixture; The amount of secondary spinel in re-combined/semi-re-combined magnesia-chrome bricks increases with the total R₂O₃ content (Cr₂O₃, Al₂O₃, and Fe₂O₃) of the fused magnesia-chrome sand, a decrease in Fe₂O₃ content within R₂O₃, and an increase in Al₂O₃ content.
(2) Secondary spinel formation reaches its maximum when the specific surface area of fine powder in the re-bonded magnesia-chrome brick mixture reaches 5–6 m²/g.
(3) In direct-bonded magnesia-chrome bricks, secondary spinel exhibiting self-crystallization characteristics becomes observable at firing temperatures above 1700°C. Both the size and quantity of secondary spinel increase with further elevation of the firing temperature. At 1800°C, the formation of secondary spinel reaches 6% (volume fraction).
A large body of research findings confirms:
(1) As the amount and size of secondary spinel formation in the bricks increase—for example, when the volume fraction of secondary spinel reaches 6% in direct-bonded magnesia-chrome bricks and 8% in re-bonded magnesia-chrome bricks—the high-temperature flexural strength achieves its maximum value. High-temperature flexural strength serves as a key indicator of MgO-Cr₂O₃ brick wear resistance at elevated temperatures. Since high-temperature wear resistance reflects resistance to erosion by molten steel and slag, direct-bonded and re-bonded (semi-re-bonded) magnesia-chrome bricks with higher secondary spinel formation inevitably exhibit enhanced resistance to erosion by molten steel and slag:
(2) The presence of substantial secondary spinel in re-bonded magnesia-chrome bricks effectively inhibits slag erosion, resulting in the highest slag resistance:
(3) Increasing the fineness of fine powders in the mix (e.g., when the specific surface area of fine powders reaches 5 m²/g) significantly improves the thermal shock resistance of re-bonded magnesia-chrome bricks. In summary, employing optimized raw materials and ultra-high-temperature firing techniques to increase the formation of secondary spinel within the bricks enables the production of direct-bonded and re-bonded (semi-re-bonded) magnesia-chromium bricks with superior comprehensive performance for use in the slag line of refining ladles.
Some countries have used alumina instead of chromite to produce MgO-MgO·Al₂O₃ bricks with excellent thermal shock resistance (Al₂O₃ 30%–40%, MgO 60%–70%). However, bricks containing chromium spinel exhibit superior slag resistance because chromium spinel has lower solubility in silicate melts than alumina spinel. Initial trials of Radex-DB605 bricks in the slag line zone of the ASEA-SKF 150t steel tank, where temperatures fluctuate dramatically, yielded a service life of only 8 cycles. To extend the life of the ASEA-SKF steel ladle slag line (especially the area closest to the electrode), cast magnesia-chromite bricks were tested. Cast magnesia-chromite bricks, represented by “Corhart 104,” are produced by casting a eutectic melt at 2500°C in an electric arc furnace using a mixture of 55% magnesia and 45% chromite ore. After thermal stress relief, they undergo diamond grinding. The phase composition of these cast magnesia-chromite bricks is: periclase and its solid solution 50%, spinel 39%, silicates ≤10%; They feature a dense structure (total porosity <12.0%), with compressive strength reaching 140–165 MPa. The temperature at which 5% deformation occurs under a 0.18 MPa load is as high as 2050°C. However, their poor thermal shock stability and high cost have led to their replacement by high-quality re-bonded magnesia-chromite bricks.
Direct-bonded and re-bonded (semi-re-bonded) magnesia-chrome bricks have the longest service life in the slag line of steel refining ladles and remain an important refractory material for this application today. However, their complex production, cost issues, and the health hazards of hexavalent chromium have driven the development of alternatives.
