Magnesia-Chrome Bricks For Ladle Slag Line: Manufacturing Process & Performance Properties

Direct-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 firing at temperatures above 1700 °C. Owing to the high degree of direct bonding between high-temperature mineral phases, they exhibit strong slag resistance, high hot strength, and excellent thermal shock resistance.

Rebonded magnesia-chrome bricks are MgO–Cr₂O₃ refractory products made from fused magnesia-chrome clinker as the raw material, formed under high pressure and fired at 1800 °C. Featuring an even higher degree of direct bonding, low apparent porosity, and high bulk density, rebonded magnesia-chrome bricks demonstrate superior hot strength and slag corrosion resistance compared with direct-bonded grades. However, rebonded magnesia-chrome bricks have relatively poor thermal shock resistance.

The main degradation characteristics of MgO–Cr₂O₃ refractories used in the slag line of refining ladles include chemical corrosion by molten slag, structural spalling caused by slag penetration, and erosion by high-temperature molten steel and slag.

MgO–Cr₂O₃ refractories show a certain resistance to CaO–SiO₂-based slags with a low CaO/SiO₂ ratio (below 2). However, when exposed to CaO–SiO₂ slags with a high CaO/SiO₂ ratio at elevated temperatures-especially those with high Fe₂O₃ content-their corrosion resistance deteriorates significantly, as the eutectic temperature decreases sharply.

Improving the durability (thermal shock resistance, slag resistance, and erosion resistance) of MgO–Cr₂O₃ bricks used in refining ladle slag lines is closely related to the characteristics of secondary spinel in the bricks, including its content, grain size, and distribution.

Most researchers worldwide have confirmed that the formation of secondary spinel is associated with raw materials, additives, and manufacturing processes:

(1) In direct-bonded magnesia-chrome bricks, the amount of secondary spinel increases with a higher proportion of chrome ore (or Cr₂O₃ content) in the batch. In rebonded or semi-rebonded magnesia-chrome bricks, the secondary spinel content increases with increasing total R₂O₃ (Cr₂O₃, Al₂O₃, and Fe₂O₃) in the fused magnesia-chrome clinker, a lower Fe₂O₃ fraction, and a higher Al₂O₃ fraction within R₂O₃.

(2) The maximum formation of secondary spinel occurs when the specific surface area of fine powders in rebonded magnesia-chrome brick batches reaches 5–6 m²/g.

(3) Secondary spinel with autogenous crystalline characteristics can be observed in direct-bonded magnesia-chrome bricks fired above 1700 °C. Both the size and quantity of secondary spinel increase with further increases in firing temperature, and its volume fraction reaches 6% at a firing temperature of 1800 °C.

Numerous studies have confirmed that:

(1) As the content and grain size of secondary spinel in the bricks increase, the hot modulus of rupture reaches its maximum value-for example, when the volume fraction of secondary spinel is 6% in direct-bonded magnesia-chrome bricks and 8% in rebonded magnesia-chrome bricks. The hot modulus of rupture is an important indicator for evaluating the high-temperature abrasion resistance of MgO–Cr₂O₃ bricks, and this resistance in turn reflects their ability to withstand erosion by high-temperature molten steel and slag. Therefore, direct-bonded, rebonded, and semi-rebonded magnesia-chrome bricks with higher secondary spinel content exhibit improved resistance to erosion by high-temperature molten steel and slag.

(2) The large amount of secondary spinel present in rebonded magnesia-chrome bricks hinders slag penetration and corrosion, resulting in superior slag resistance.

(3) Increasing the fineness of fine powders in the batch-for instance, when the specific surface area of the fine powders reaches 5 m²/g-significantly improves the thermal shock resistance of rebonded magnesia-chrome bricks.

In summary, by adopting optimized raw materials and ultra-high-temperature firing processes to promote the formation of secondary spinel in the bricks, direct-bonded, rebonded, and semi-rebonded magnesia-chrome bricks with excellent overall performance for use in the slag line of refining ladles can be produced.

The typical properties of several direct-bonded magnesia-chrome bricks from one company are listed in Table 1. The typical properties of rebonded (semi-rebonded) magnesia-chrome bricks from domestic and international sources are presented in Table 2.

Certain countries have used alumina instead of chromite to produce MgO–MgO·Al₂O₃ bricks with excellent thermal shock resistance (containing 30%–40% Al₂O₃ and 60%–70% MgO). However, bricks containing chrome spinel exhibit superior slag resistance, since chrome spinel has lower solubility in silicate melts than alumina spinel.

Radex-DB605 bricks were initially tested in the slag line of a 150 t ASEA-SKF ladle under severe temperature fluctuations, achieving a service life of only 8 heats. To extend the service life of the slag line in this ASEA-SKF ladle, especially in the area near the electrodes, fused-cast magnesia-chrome bricks were subsequently tested.

Represented by Corhart 104, fused-cast magnesia-chrome bricks are produced by casting a eutectic melt consisting of 55% magnesia and 45% chromite in an electric arc furnace at 2500 °C. After thermal stress relief, the bricks are finished by diamond cutting and grinding.

The phase composition of such fused-cast magnesia-chrome bricks is as follows: periclase and its solid solution (50%), spinel (39%), and a silicate phase (no more than 10%). The bricks feature a dense structure (total porosity < 12.0%), a compressive strength as high as 140–165 MPa, and a temperature of 2050 °C corresponding to 5% deformation under a load of 0.18 MPa. Nevertheless, due to their poor thermal shock resistance and high cost, they were replaced by high-grade rebonded magnesia-chrome bricks.

Direct-bonded, rebonded, and semi-rebonded magnesia-chrome bricks have been used in the slag lines of refining ladles for a long time and remain an important refractory option for such applications. However, the complex production process, relatively high cost, and the health hazards associated with hexavalent chromium have driven the development of alternative materials.