Formulation Of Carbonless Corundum-spinel Unfired Ladle Bricks

During the use of unburnt ladle bricks, the hot face undergoes slow sintering, accompanied by a series of chemical reactions. At the same time, the brick is subjected to erosion by components in the slag, leading to the formation of new mineral phases and structural changes within the brick. In contrast, the cold face of the brick experiences minimal physical or structural changes. This difference between the hot and cold sides leads to thermal and structural stresses, resulting in the formation of cracks.

With the repeated thermal cycling of the ladle and continuous infiltration and erosion by slag and molten steel, these cracks continue to expand. The combined effects of slag infiltration, erosion, and thermal shock from molten steel ultimately lead to spalling of the brick, severely affecting the service life of the ladle lining.

The primary method to inhibit spalling is to improve the brick's resistance to thermal shock, slag penetration, and erosion. One approach is to pre-add a portion of spinel (MA) into the matrix of carbon-free unburnt bricks. Magnesium sand and activated alumina micropowder can react with spinel as the nucleus to form an MA–MA composite phase during use. This facilitates uniform and gradual sintering and densification at high temperatures, enhancing strength and forming a microstructure similar to that of sintered magnesia-alumina spinel bricks. The result is improved resistance to slag erosion and spalling.

In addition, to prevent steel and slag from eroding along the brick during use, carbon-free ladle bricks should exhibit a certain expansion rate at high temperatures. This can be achieved by adding appropriate amounts of fused magnesia and Al₂O₃, which react to form magnesia-alumina spinel accompanied by expansion.

Therefore, the developed carbon-free ladle bricks use brown corundum, fused white corundum, and fused magnesia as the main raw materials. Corundum powder, fused spinel, and activated alumina micropowder are also introduced into the matrix. These are co-ground to a certain fineness to produce a self-made activated composite spinel micropowder. The main performance indices of the raw materials used in the test are shown in Table 1.

The binding agents selected were phosphate, SiO₂ micronized powder, lightly fired MgO powder, brine, and sulphite pulp waste.

Test Programme

The formulation design was based on Andreassen's theory of particle packing. Brown corundum, fused white corundum, and fused magnesia were used as aggregates with a critical particle size of 5 mm. Corundum powder, fused magnesia powder, and activated composite spinel micropowder were used as the main fine powders.

The experimental design of the basic formulation (w) is as follows: ≤5 mm brown corundum 50%; white corundum particles (≤3 mm) and fines (≤0.044 mm) totaling 25%; fused magnesia particles (≤2 mm) and fines (≤0.074 mm) totaling 4%; self-made activated composite micropowder 20%; 1% additives; and 4% binding agent.

First, a binding agent selection test was carried out under the conditions of the basic formulation. The binding agents were added in the following forms:

Phosphate

Lightly burned MgO powder

Sulphite pulp waste liquid

SiO₂ micropowder + brine

SiO₂ micropowder + sulphite pulp waste liquid

Next, a test was conducted using the selected binding agent and the basic formulation to evaluate the effect of magnesia addition (mass fractions of 1%, 2%, 3%, 4%, 5%, and 6%), and composite spinel micropowder addition (mass fractions of 0%, 10%, 15%, 20%, and 25%).

Specimen Preparation and Performance Testing

According to the test programme, the raw materials were proportioned and mixed. The granular materials were first mixed uniformly, after which the binding agent was added and mixed for 3–5 minutes. Then, the pre-mixed fine powders were added, and mixing was continued for approximately 18 minutes.

The mixed material was shaped using a 630-ton friction press. The molded specimens were left to air-dry naturally for 24 hours, followed by drying at 200 °C for 12 hours. Subsequently, the specimens underwent heat treatment at 1000 °C for 3 hours and then at 1600 °C for 3 hours.

After heat treatment, the following properties of the specimens under different conditions were tested in accordance with relevant standards:

Bulk density,Apparent porosity,Post-firing linear change rate,Compressive strength,Flexural strength

Thermal shock resistance (characterized by residual flexural strength and strength retention rate after three cycles of water-cooled thermal shock at 1100 °C)

Determination of Binding Agent

Since carbon-free unfired ladle bricks are not subjected to high-temperature firing and only undergo heat treatment at 200 °C for 12 hours, they must retain a certain level of strength during handling, packaging, transportation, and installation. For safety reasons, their room-temperature compressive strength is required to be greater than 30 MPa, making the selection of a suitable binding agent particularly critical.

Table 2 shows the effect of various binding agents on the performance of carbon-free ladle bricks. It can be seen that, except for phosphate and sulphite pulp waste-whose combined use fails to meet the required compressive strength-all other combinations produced specimens that meet the strength requirement.

However, due to the short shelf life of lightly burned MgO powder, its reactivity is difficult to control, and quality assurance is challenging. Additionally, specimens using SiO₂ powder + brine as the binding agent exhibited significantly better performance than those using SiO₂ powder + sulphite pulp waste liquid.

Therefore, SiO₂ powder combined with brine was ultimately selected as the binding agent for the development of carbon-free ladle bricks.

The Effect of Adding Magnesium Sand on the Post-Firing Linear Change Rate of Carbonless Steel-Clad Bricks

The finer the particle size of magnesium sand, the faster the reaction rate of spinel formation with Al₂O₃; conversely, coarser particles react more slowly. At the same time, increasing the critical size of magnesium sand increases the depth of slag penetration into the material while gradually reducing the degree of corrosion. Therefore, adding a certain amount of coarse magnesium sand particles helps avoid the rapid generation of spinel, thereby reducing reaction-induced expansion. This is beneficial in preventing high-temperature cracking and spalling of unburnt ladle bricks during use.

To avoid intense expansion and late-stage over-sintering during service, magnesia sand is applied in both particle and fine powder forms. Figure 1 shows the linear change rate of carbonless steel-clad bricks with different amounts of electrofused magnesium sand after sintering at 1600 °C for 3 hours, with the amount of activated composite spinel powder kept constant.

As seen in Figure 1, when magnesium sand was added at a mass fraction of 3%, a slight expansion occurred in the carbonless ladle bricks. However, when the mass fraction exceeded 4%, the post-firing linear expansion increased significantly. It is well known that for ladle bricks, excessive post-firing linear contraction results in large brick gaps, causing steel leakage, while excessive post-firing expansion generates high stress, which easily leads to structural spalling.

Therefore, the appropriate mass fraction of magnesium sand to be added in this test is 4%.

Figure 2 shows the effect of adding reactive composite spinel powder on the thermal shock resistance of the specimens. The MgO content was kept constant by adjusting the addition of magnesia and corundum fines.

As seen in Figure 2, the thermal shock resistance of the specimens initially improves and then deteriorates with increasing amounts of active composite spinel powder. Similar to the specimen strength, thermal shock resistance improves when the mass fraction of composite micropowder is ≤ 20%, but significantly decreases when the mass fraction reaches 25%. The best thermal shock resistance is observed at mass fractions of 15% and 20%.

This behavior is because the continuous spinel phase formed in the matrix at high temperatures fills the spaces between particles. With a moderate amount of composite micropowder, an appropriate amount of spinel phase and microcracks are generated, which effectively buffer thermal stress and improve thermal shock resistance. However, excessive addition of composite micropowder produces too much in-situ spinel at high temperature, causing significant volume expansion. This leads to loosening of the matrix structure and a decrease in bonding strength, thereby reducing the material's thermal shock resistance.

Therefore, the optimal mass fraction of active composite spinel micropowder in this test is 20%.

Comparison of the Properties of Carbonless Ladle Bricks with Those of Magnesium-Aluminium-Carbon Unburnt Bricks Currently Used in Steel Ladles

Table 3 shows the physical properties of the developed carbon-free unfired ladle bricks compared to the existing magnesium-aluminium-carbon unfired bricks currently used in ladles.

As can be seen from Table 3, the developed carbonless ladle bricks exhibit slight expansion after treatment at 1000 °C, with increased strength. This indicates that MgO begins to react with Al₂O₃ at 1000 °C to form spinel, which enhances the matrix bonding strength and helps resist thermal spalling caused by thermal stresses. After treatment at 1600 °C, the carbonless bricks show reduced apparent porosity, indicating the formation of a dense and uniform structure. This improvement enhances resistance to slag penetration and reduces structural spalling.