For ladle refining operations, magnesia-chrome refractories remain the most erosion-resistant refractories. The problem with these refractories is that it’s difficult to reconcile erosion resistance and thermal shock resistance. Specifically, traditional direct-bonded magnesia chrome bricks with high erosion resistance often have low thermal shock resistance; conversely, high thermal shock resistance often results in lower erosion resistance.
Performance Characteristics of Magnesia Chrome Bricks
The performance of directly bonded magnesia-chrome bricks includes the relationship between Cr2O3 content and slag resistance, and the relationship between Cr2O3 content and critical temperature difference. The critical temperature difference is the temperature difference at which the flexural strength of a sample reaches zero after undergoing thermal shock tests with various temperature differences. Comparison shows that as the Cr2O3 content increases, the erosion resistance of directly bonded magnesia chrome bricks improves, but its thermal shock resistance decreases.
Because the operating conditions of magnesia-chrome bricks vary in different refining units and different parts of the same unit, their optimal service life also varies. Therefore, only when their critical temperature difference equals the maximum permissible temperature change, and refractory materials are selected empirically to suit the operating environment, can the service life of the material be improved.
In directly bonded magnesia chrome bricks, MgO and chromite are the main components, but their properties are different. Because the coefficient of expansion of chromite is smaller than that of MgO, cracks are generated around the chromite after firing, which can prevent the propagation of cracks, i.e., it has the effect of interrupting cracks within the chromite. To improve the thermal shock resistance of directly bonded MgO-Cr2O3 bricks, an effective method is to increase the amount of chromite. However, this also has negative effects, as chromite inevitably contains impurities such as SiO2, which increases the amount of these impurities in the magnesia-chrome bricks. This hinders the implementation of ultra-high temperature firing techniques used to improve corrosion resistance, resulting in a decrease in the corrosion resistance of this type of refractory material.
To achieve both corrosion resistance and thermal shock resistance in directly bonded magnesia chrome bricks, there are two approaches:
- (1) Controlling the particle size distribution of the raw materials and reducing the critical particle size.
- (2) Selecting special additives.
How to obtain directly bonded magnesia chrome bricks that combine thermal shock resistance and corrosion resistance?
To obtain directly bonded magnesia-chrome bricks that combine thermal shock resistance and corrosion resistance, directly bonded magnesia chrome bricks were produced according to the required batching method for the experiment. The characteristics of this type of directly bonded magnesia-chrome brick are as follows:
- (1) With the increase of particle size greater than 1 mm, its apparent porosity also increases.
- (2) When the content of particles greater than 1 mm decreases to 30 wt%, its flexural strength increases.
- (3) With the decrease of particle content greater than 1 mm, the corrosion resistance increases. Specifically, when the content of particles greater than 1 mm is reduced to 5 wt%~0 wt%, the corrosion resistance is about 20% higher than when the content of particles greater than 1 mm is 34 wt%.
- (4) Directly bonded magnesia chrome bricks with a particle content of 34 wt%~5 wt% greater than 1 mm do not peel off after 25 cycles of repeated heating and air cooling.
Microstructural studies revealed that when the content of particles larger than 1 mm was below 5 wt%, the R₂O₃ in the chromite diffused almost entirely to the center of the MgO particles due to the use of MgO particles smaller than 1 mm. Therefore, secondary spinel was formed throughout the MgO particles, a key reason for their excellent erosion resistance.
Further microstructural observations showed that repeated heating and cooling resulted in the formation of microcracks around several regions containing MgO particles approximately 0.5 mm in diameter. These microcracks were cut off at chromite particles larger than 1 mm, thus preventing the formation of larger cracks. In other words, the presence of a small amount of particles larger than 1 mm in the magnesia-chrome bricks directly inhibited crack propagation.
Based on the above discussion, the following conclusions can be drawn:
In directly bonded magnesia-chrome bricks, reducing the content of particles larger than 1 mm to improve corrosion resistance, while controlling the content of these particles within the range of 8 wt% to 4 wt%, allows for the formation of a network of fine cracks during heating and cooling, thus inhibiting the propagation of larger cracks and improving the thermal shock resistance of this type of magnesia chrome brick. Clearly, this type of directly bonded magnesia-chrome brick possesses both corrosion resistance and thermal shock resistance.
Adding special additives to magnesia-chrome bricks can also yield products with both corrosion resistance and thermal shock stability.
The problem of improving thermal shock resistance by adding special substances with a lower thermal expansion rate than magnesia chrome bricks, resulting in microcracks, can be addressed.
Because the effect of adding this special additive to magnesia-chrome bricks is mainly to reduce the thermal expansion rate of the matrix, a magnesia-chrome brick with inconsistent thermal expansion rates between the particles and the matrix (improper thermal bonding) is obtained. Microstructural studies revealed that cracks are generated between this relatively low thermal expansion rate matrix and the relatively high thermal expansion rate of the coarse particles. The formation of these cracks leads to residual stress in the matrix near the coarse particles, thus increasing the energy value (K2R/E) required for the formation of the fracture surface in magnesia chrome bricks. A higher K2R/E value is beneficial to the material’s thermal shock resistance.
However, magnesia-chrome bricks with a large amount of special additives have a greater number of cracks present after firing, resulting in a reduced effect on inhibiting crack propagation.
Directly bonded magnesia chrome bricks exhibit the highest thermal stability when the energy required to form the fracture surface increases almost linearly with the crack propagation rate. Adding CaCO3 (12-10%), Cr2O3 (1wt%-20wt%), or Al2O3 (1wt%-20wt%) powder with a particle size of 0.1-2.0 mm to MgO-Cr2O3 bricks not only improves their corrosion resistance but also enhances their thermal shock resistance. This allows for the production of directly bonded MgO-Cr2O3 bricks with both high corrosion resistance and thermal shock resistance.
Comparison of Thermal Shock and Erosion Resistance of Different Magnesia-chrome Bricks
Based on different raw materials and production processes, magnesia chrome bricks commonly used in the copper smelting industry can be divided into:
- 1) Ordinary silicate-bonded magnesia-chrome bricks;
- 2) Directly bonded magnesia-chrome bricks;
- 3) Rebonded magnesia-chrome bricks;
- 4) Semi-rebonded magnesia-chrome bricks;
- 5) Co-fired magnesia-chrome bricks;
- 6) Fused cast magnesia-chrome bricks.
These six types of magnesia chrome bricks exhibit different thermal shock and erosion resistance properties. In terms of thermal shock resistance, silicate magnesia-chrome refractories are the best, directly bonded magnesia-chrome bricks are slightly inferior, and electrofused rebonded magnesia-chrome bricks have slightly poorer thermal shock resistance. Fully synthetic magnesia-chrome bricks perform moderately well in terms of impact resistance, slightly inferior to directly bonded magnesia-chrome bricks but superior to electrofused rebonded magnesia chrome bricks.
The erosion results of copper-nickel converter slag on various magnesia-chrome refractories were studied using the rotary slag resistance test method. Depending on the degree of direct bonding, the slag resistance of magnesia-chrome bricks decreases as the degree of direct bonding decreases. Excellent slag resistance and certain thermal shock resistance make fused cast magnesia chrome bricks a promising material for applications. In current pyrometallurgical copper smelting processes, magnesia-chrome bricks are widely used due to their superior slag resistance. In critical areas of converter blowing, electrofused rebonded magnesia-chrome bricks or high-chromium-content direct-bonded magnesia-chrome bricks are generally used. In areas with strong thermal shock, such as converter blowing, direct-bonded magnesia chrome bricks with better thermal shock resistance are generally used.
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