Technology and Performance of Magnesia-Chrome Bricks for Ladle Slag Line
Direct-bonded magnesia-chrome bricks are MgO–Cr₂O₃ refractory products made from high-purity magnesia with low impurity content and chromite concentrate through co-grinding and firing at high temperatures (above 1700 ℃). Owing to the high direct bonding rate of high-temperature mineral phases, they exhibit excellent slag resistance, high-temperature strength, and good thermal shock resistance.
Re-bonded magnesia-chrome bricks are MgO–Cr₂O₃ refractory products produced using fused magnesia-chrome sand as the raw material, followed by high-pressure forming and firing at 1800 ℃. With a higher direct bonding rate, lower apparent porosity, and higher bulk density, re-bonded magnesia-chrome bricks exhibit superior high-temperature strength and slag erosion resistance compared with direct-bonded bricks; however, their thermal shock resistance is relatively poor.
The main damage characteristics of MgO–Cr₂O₃ refractories applied at the slag line of steel refining ladles include chemical erosion by molten slag, structural spalling caused by slag penetration, and scouring by high-temperature molten steel and slag. MgO–Cr₂O₃ refractories possess a certain resistance to CaO–SiO₂ slags with a low CaO/SiO₂ ratio (less than 2). However, for CaO–SiO₂ slags with a high CaO/SiO₂ ratio at high temperatures, especially those with high Fe₂O₃ content, the eutectic temperature drops sharply, resulting in very poor erosion resistance.
Improvement in the durability (thermal shock resistance, slag resistance, and scouring resistance) of MgO–Cr₂O₃ bricks used at the slag line of steel refining ladles is closely related to the properties (formation amount, size, and distribution) of secondary spinel within the bricks. Numerous researchers, both domestic and international, have confirmed that the formation of secondary spinel in bricks is associated with raw materials, additives, and brick-making processes, as described below.
The amount of secondary spinel in direct-bonded magnesia-chrome bricks increases with an increase in the proportion of chromite (or Cr₂O₃ content) in the batch. For re-bonded or semi-re-bonded magnesia-chrome bricks, the quantity of secondary spinel increases with higher total R₂O₃ (Cr₂O₃, Al₂O₃, and Fe₂O₃) content in fused magnesia-chrome sand, a reduction in Fe₂O₃ content, and an increase in Al₂O₃ content within R₂O₃.
The formation amount of secondary spinel reaches a maximum when the specific surface area of fine powder in the batch of re-bonded magnesia-chrome bricks is 5–6 m²/g.
When the firing temperature of direct-bonded magnesia-chrome bricks exceeds 1700 ℃, secondary spinel with self-crystallization characteristics can be observed. Both the size and quantity of secondary spinel increase with further elevation of the firing temperature. When the firing temperature reaches 1800 ℃, the formation amount of secondary spinel reaches 6% (by volume).
A large number of research results have confirmed the following:
(1) With increases in both the formation amount and size of secondary spinel in the bricks, the high-temperature modulus of rupture (HMOR) reaches a maximum value. For example, this peak performance is achieved 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 modulus of rupture is a key indicator for evaluating the high-temperature wear resistance of MgO–Cr₂O₃ bricks, while high-temperature wear resistance is an important metric reflecting resistance to scouring by high-temperature molten steel and slag. Therefore, direct-bonded, re-bonded (and semi-re-bonded) magnesia-chrome bricks with higher secondary spinel content will inevitably exhibit enhanced resistance to scouring by high-temperature molten steel and slag.
(2) Re-bonded magnesia-chrome bricks contain a large amount of secondary spinel, which inhibits slag erosion and thus provides the best slag resistance among such products.
(3) Improving the fineness of fine powder in the batch (for example, when the specific surface area of the fine powder reaches 5 m²/g) can significantly enhance the thermal shock resistance of re-bonded magnesia-chrome bricks.
In summary, by adopting optimized brick-making processes-such as appropriate raw material selection and ultra-high-temperature firing-to increase the formation amount of secondary spinel within the bricks, direct-bonded, re-bonded (and semi-re-bonded) magnesia-chrome bricks with excellent comprehensive performance for application at the slag line of steel refining ladles can be produced. The typical properties of several direct-bonded magnesia-chrome bricks manufactured by a certain company are shown in Table 1. The typical properties of re-bonded (and semi-re-bonded) magnesia-chrome bricks available both domestically and internationally are shown in Table 2.
In some countries, alumina was once used to replace chromite in the manufacture of MgO–MgO·Al₂O₃ bricks (containing 30%–40% Al₂O₃ and 60%–70% MgO) with excellent thermal shock stability. However, bricks containing chrome spinel exhibit superior slag resistance because the solubility of chrome spinel in silicate melts is lower than that of alumina spinel.
Initially, Radex-DB605 bricks were trialed at the slag line of 150-ton ASEA-SKF steel ladles, where temperatures fluctuate drastically, but their service life was only eight heats. To extend the service life of the slag line of such ASEA-SKF ladles-especially the area closest to the electrodes-fused-cast magnesia-chrome bricks were tested. Fused-cast magnesia-chrome bricks, represented by Corhart 104, are produced by melting a mixture of 55% magnesia and 45% chromite in an electric arc furnace at 2500 ℃ to form a eutectic melt, followed by casting, thermal stress relief, and final machining with diamond tools.
The phase composition of such fused-cast magnesia-chrome bricks is as follows: periclase and its solid solution account for approximately 50%, spinel accounts for 39%, and silicates do not exceed 10%. These bricks feature a dense structure (total porosity < 12.0%), an ultra-high cold crushing strength of 140–165 MPa, and a refractoriness under load (5% deformation under a load of 0.18 MPa) of up to 2050 ℃. Nevertheless, due to their poor thermal shock stability and high cost, they have been replaced by high-quality re-bonded magnesia-chrome bricks.
Direct-bonded and re-bonded (or semi-re-bonded) magnesia-chrome bricks have dominated slag line applications in steel refining ladles for a long time and remain an important refractory option for this position to this day. However, the complex production processes, high costs of these magnesia-chrome bricks, and the hazards of hexavalent chromium to human health have driven the development of alternative materials.