Improving Thermal Insulation Of Magnesia Dry Mix To Reduce Refractory Consumption Per Ton Of Steel

At present, the main refractory material used for the working lining of continuous casting tundishes is magnesia dry vibratable mix. This dry mix features good resistance to corrosion by high-iron and basic molten slags, a long service life, no contamination of molten steel, convenient installation, and easy removal and turnaround of the tundish. It has been gradually adopted for tundish working linings, showing satisfactory performance and promising application prospects.

However, magnesia dry mixes generally have a high bulk density and exhibit the following disadvantages:

High thermal conductivity leads to rapid heat loss through the tundish lining, causing a fast temperature drop of molten steel and high energy consumption.

High density results in increased refractory consumption per ton of steel, wasting mineral resources and raising costs for enterprises.

Magnesium oxide has a large thermal expansion coefficient, poor thermal shock resistance, a tendency to absorb moisture and hydrate, and low resistance to thermal and structural spalling-all of which significantly shorten the service life of the refractory.

In addition, current tundish dry mixes are mainly bonded with phenolic resin. During heating, as the tundish temperature rises, the phenolic resin gradually cures to form a carbon network, providing sufficient strength to the dry mix. Nevertheless, phenolic-bonded dry mixes can cause carbon pickup in molten steel, which seriously affects the quality of clean steel. Meanwhile, the cured resin decomposes at 200°C–800°C, releasing gases such as CO₂, CO, CH₄, H₂, and H₂O. Free phenol and the generated gases produce irritating fumes that are harmful to the health of on-site workers. Therefore, existing refractories for tundish working linings can no longer meet the requirements for cleanliness and thermal insulation.

This paper focuses on the research and development of a resource-saving, lightweight, and environmentally friendly tundish dry mix with low apparent bulk density. It not only reduces refractory consumption per ton of steel and improves thermal insulation, but also uses an environmentally friendly binder that emits no harmful or irritating gases, thereby offering excellent environmental performance.

Experiments

1.1 Raw Materials and Experimental Scheme

The main raw materials used in this experiment and their chemical compositions are listed in Table 1.

Sintered magnesia, forsterite, and magnesite were used as the main raw materials (their physical and chemical indexes are shown in Table 1). The particle sizes of 5–1 mm, ≤1 mm, and 200 mesh were blended in a fixed ratio, with binder, additives, and a small amount of paper fiber added. The experimental formulation is given in Table 2.

1.2 Test Procedure and Property Measurement

According to the designed formulation, a special iron cup was used as the container, and its mass (m₁) was measured. The well-mixed materials of each group were placed into a bulk density tester. The iron cup was positioned at the bottom, filled with the materials, and the surface was leveled to achieve natural loose packing. The total mass (m₂) was then measured. The sample mass was calculated as m = m₂ − m₁, and the bulk density was calculated as m / V (where V is the volume of the iron cup).

The well-mixed materials were further blended in a mixer and then manually rammed into triple cement specimen molds with dimensions of 160 mm × 40 mm × 40 mm. For the cuboid specimens, the materials were added and compacted layer by layer until the mold was completely filled. The rammed specimens, together with the molds, were dried in an oven at 200 ℃ for 3 h, then cooled and demolded.

The bulk density, cold crushing strength, and linear change after firing of the specimens were measured after heat treatment at 200 ℃ × 3 h and 1500 ℃ × 3 h, respectively, to evaluate the low-temperature bonding behavior and sintering performance. In addition, the prepared dry mix was manually rammed into disc molds with a diameter of 180 mm and a thickness of 20 mm, cured at 200 ℃ for 3 h, and then demolded. The thermal conductivity was measured at different temperatures.

Results and Discussion

2.1 Bulk Density and Apparent Porosity / Volume Density

Figure 1 shows the bulk density and volume density of the dry mix.

It can be seen from Figure 1 that after replacing phenolic resin powder with glucose, the bulk density of the dry mix changes only slightly, while the volume density after heating at 200 ℃ for 3 h decreases slightly. When the glucose addition is 6 wt.%, the volume density is reduced by approximately 5%. However, with the addition of magnesite and forsterite, both the bulk density and the volume density after heating at 200 ℃ for 3 h change little. With the introduction of paper fiber, the bulk density and the volume density (after 200 ℃ × 3 h) decrease significantly. At paper fiber additions of 0.1 wt.%, 0.2 wt.%, and 0.3 wt.%, the bulk density is reduced by 2.7%, 5.4%, and 8.1%, respectively, while the volume density is reduced by 1.3%, 2.6%, and 5.7%, respectively. Comparing Sample G1 and Sample G12 (with glucose as the binder, 40 wt.% forsterite, and 0.3 wt.% paper fiber), the bulk density and volume density of Sample G12 are both reduced by about 9%. The reduction in bulk density and volume density enables lightweight design of the material, lowers refractory consumption per ton of steel in the tundish, and reduces refractory costs for enterprises. It can also be observed from Figure 1 that the volume density after heating at 1500 ℃ for 3 h shows a decreasing trend with increasing magnesite addition.

Figure 1 also shows that the introduction of low-volume-density magnesite and forsterite has little effect on the bulk density and the volume density after heating at 200 ℃ for 3 h. This is attributed to the fact that the volume density of magnesite and forsterite is about 7.9% lower than that of dead-burned magnesia. At a 40 wt.% addition level, the theoretical reduction in volume density is about 3.1%. However, volume density is affected by many factors, such as particle shape, which depends on the structural characteristics and crushing method of the raw materials, leading to differences in particle morphology and size. Therefore, the effect on the volume density of the dry mix is not significant in this study and requires further investigation.

Using glucose as a binder has a noticeable effect on reducing the bulk density and volume density of the dry mix, which is mainly attributed to differences in particle size: glucose particles are coarser than phenolic resin powder, whereas fine resin powder can fill pores more uniformly. It can also be seen from Figure 1 that the volume density after heating at 1500 ℃ for 3 h follows a trend similar to that after 200 ℃ × 3 h. The volume density of Samples G-5 to G-8 (with magnesite) after heating at 1500 ℃ for 3 h decreases significantly and shows an approximately linear decline with increasing magnesite addition. This is because magnesite begins to decompose at about 400 ℃ during heating, releasing CO₂ gas; the reaction becomes intense at 550–650 ℃ and is completed at around 1000 ℃, forming light-burned MgO with a loose texture, high porosity, and low volume density, thereby reducing the volume density of the dry mix after heating at 1500 ℃ for 3 h.

2.2 Physical Properties at Room Temperature

Figure 2 shows the effects of different raw materials on the physical properties of the dry mix. Compared with phenolic resin, specimens using glucose as the binder exhibit lower cold crushing strength (CCS) after heat treatment at 200 ℃ for 3 h and 1500 ℃ for 3 h. The CCS of the specimens gradually increases with increasing glucose addition. At a glucose addition of 6 wt.%, although the CCS of the bonded specimens is still lower than that of specimens bonded with phenolic resin, it can fully meet the demolding requirements for on-site application.

Phenolic resin forms a strong three-dimensional network structure after low-temperature heat treatment. Glucose has a melting point of 146 ℃ and undergoes cross-linking and condensation reactions during curing at 200 ℃ to form a three-dimensional network structure. In addition, water generated from the decomposition of glucose during heating can chemically react with magnesia, contributing to the strength development of the dry mix. Glucose is non-toxic and harmless and does not release irritating gases such as phenol and formaldehyde during heating. With the advantages of being environmentally friendly and low-cost, glucose is a promising green binder to replace phenolic resin.

Figure 2 also shows that when the glucose addition further increases to 7 wt.%, the increase in CCS of the dry mix becomes insignificant, while the linear change rate after heat treatment at 1500 ℃ for 3 h increases noticeably. Therefore, the recommended addition level of glucose is 6 wt.%.

With increasing magnesite addition, the cold crushing strength of the specimens after heating at 200 ℃ for 3 h changes little, whereas the crushing strength after heat treatment at 1500 ℃ for 3 h shows a gradual decreasing trend, and the linear change rate follows the same pattern. This is mainly attributed to gas release during the decomposition of magnesite upon heating, resulting in a loose structure and high porosity.

As the paper fiber addition increases, the crushing strength of the specimens after heat treatment at both 200 ℃ for 3 h and 1500 ℃ for 3 h decreases. This is because the introduction of paper fiber hinders close packing of the material, weakens the bonding effect, and leaves pores inside the material after volatilization during heating, thereby degrading the physical properties. When the paper fiber addition is 0.3 wt%, the room-temperature physical properties still meet the on-site demolding requirements. In addition, the shrinkage and combustion of paper fiber during heating form micro-pores and channels inside the dry mix, which facilitate the rapid discharge of gases from binder decomposition, relieve internal stress, and reduce the risk of working lining collapse.

As shown in Figure 1(b), the linear change rate after heat treatment at 1500 ℃ for 3 h gradually increases with increasing magnesite addition. This is caused by gas evolution during magnesite decomposition, the resulting loose structure, and sintering shrinkage at high temperatures.

For specimens with forsterite addition, the linear change rate after heat treatment at 1500 ℃ for 3 h is significantly reduced. This is attributed to the further reaction of MgO with SiO₂ to form a forsterite phase; its micro-expansion fills pores and reduces the linear change rate, while also improving the slag penetration resistance of the material.

2.3 Thermal Conductivity

A comparison of the thermal conductivity of specimens G-1, G-3, G-9, and G-12 at different temperatures is shown in Figure 3. The results indicate that the thermal conductivity decreases at all test temperatures.

Compared with G-1, the thermal conductivity of G-3 decreases significantly at 600 ℃, 800 ℃, and 1000 ℃. This is because phenolic resin forms a carbon network after carbonization, which enhances thermal conductivity, whereas G-3 uses glucose as the binder with much lower residual carbon, thereby reducing thermal conductivity.

Compared with specimen G-3, the thermal conductivity of specimen G-9 is further reduced, which is attributed to the lower thermal conductivity of forsterite sand relative to magnesia (6.7 W·m⁻¹·K⁻¹ at 1000 ℃).

Specimen G-12 exhibits a lower volume density due to the addition of paper fiber. Meanwhile, fine pores are formed during heating, increasing the porosity. This reduces the formation of low-melting phases, effectively prevents their aggregation, hinders sintering, and thus decreases thermal conductivity.

Lower thermal conductivity is beneficial in two ways. On one hand, it helps form a large temperature gradient from the hot face to the cold face of the working lining, enabling gradual sintering from the working lining to the permanent lining and forming a dense structure on the hot face. This prevents the penetration of molten slag into the permanent lining through cracks. On the other hand, lower thermal conductivity reduces the heat dissipation rate of the tundish lining, improves thermal insulation performance, lowers the shell temperature, slows the temperature drop of molten steel during service, and saves a substantial amount of heat energy.

Conclusions

Using non-toxic and harmless glucose instead of phenolic resin as the binder provides sufficient demolding strength for the material and releases no irritating gases such as phenol and formaldehyde during heating, demonstrating the advantages of environmental friendliness and low cost.

The addition of paper fiber can reduce the bulk density, volume density, and thermal conductivity of the dry mix, enabling a lightweight design of the material and contributing to energy conservation and reduced consumption.

Using glucose as the binder and introducing forsterite and paper fiber both help reduce thermal conductivity, slow down the heat dissipation rate of the tundish lining, and thus improve its thermal insulation performance.