Refractory Pounding Materials
The basic composition of ordinary refractory batter is similar to that of refractory castables and plastisols, with the difference being that it contains more refractory powder and less binding agent. Soft clay is usually added to increase the viscosity and sinterability of the material. The critical particle size of the refractory aggregate is 10mm, with sizes also available in 5mm. The mass ratio of coarse to fine aggregate is generally between 3:7 and 4:6. A higher proportion of fine aggregate makes it easier to achieve a dense structure. The particle size composition of refractory pounding material generally consists of 60% to 65% refractory aggregates and 35% to 40% refractory powder, resulting in greater accumulation density. The pounded liner also has a larger bulk density.
It should be noted that refractory pounding material is more commonly used in the working layer of steel drums, where it has been found to achieve certain results. Its refractory aggregate and powder typically consist of first-class or special-grade alumina clinker, with the maximum particle size being 10mm. Foliated paraffin grains or powder are added to form alumina paraffin mastication material, while first-class brick magnesia powder or metallurgical magnesia powder is added to form alumina-magnesium mastication material. The powder fineness is typically less than 0.09mm, and more than 90% of the material meets this specification. The binding agent used can be a water glass solution with a modulus of 2.6 and a density of 1.36g/cm³, an aluminum sulfate solution with a density of 1.26g/cm³, or other similar binding agents. The ratio of these components is shown in Table 1.
Practice has shown that the composition of aluminum-magnesium refractory pounding material has a wider application and better effects on the steel drums of Sheng. When the dosage of magnesium sand powder is between 9% and 12%, it can generate sufficient aluminum-magnesium spinel, which provides good slag resistance and a long service life.
After mixing the refractory pounding material, the bulk density of the liner was simulated by pounding with an air pick. Samples were then made using a press. The specimens were naturally cured for 3 days, dried, and tested for performance. The test results are shown in Table 2, with the numbering corresponding to Table 1.
As seen in Table 2, water glass refractory pounding material performs better than aluminum sulfate refractory pounding material. Among the water glass refractory pounding materials, number 2-representing the aluminum-magnesium refractory pounding material-shows the best performance. It has good slag resistance, a high load softening temperature, and high compressive strength at high temperatures. In particular, after burning at 1400°C, its compressive strength reaches 105.9 MPa. However, the line shrinkage after burning is relatively large, reaching 2.21%.
This material can generate aluminum-magnesium spinel at high temperatures, with staggered growth and volume expansion, contributing to the formation of a system of refractory minerals, which enhances its performance. Aluminum paraffin refractory pounding material is characterized by post-burning expansion, with a value of 2.32%. This material does not allow slag to adhere to the liner, making it suitable for use in the lining of steel barrels. However, its high-temperature compressive strength and load softening temperature are lower, which limits its performance in certain applications. Aluminum refractory pounding material, which falls between the two, can also be used as a lining material. This material is suitable for use in ladles as well.
The slag resistance was tested using the crucible method. The chemical composition of the slag is as follows: CaO 41.2%, SiO₂ 10.5%, Fe₂O₃ 10.8%, FeO 18.3%, Al₂O₃ 5.34%, MnO 4.478%, MgO 5.3%, CaF₂ 1.36%, and burnt loss 1.52%. The alkalinity is 3.9. The test results showed that aluminum-magnesium masticated materials are less eroded and penetrated by slag, forming a thin, dense layer that prevents further slag penetration. The slag resistance of aluminum and alumina waxy masticated materials is generally similar, but compared to the aluminum-magnesium masticated materials, the erosion and penetration are approximately twice as large. This indicates that the addition of magnesia powder has a significant effect on aluminum refractory pounding materials.
Specimens formed by pressure using number 2 powder and water glass from Table 1 were fired at 1600°C for petrographic analysis. Polarized light microscope analysis revealed the formation of more aluminum-magnesium spinel, with grains generally ranging from 5 to 7 μm, and a few ranging from 10 to 15 μm. When 2% chromite was added as a mineralizing agent, the development of spinel was improved, with coarser grains generally ranging from 10 to 15 μm and some grains reaching up to 30 μm. The X-ray diffraction spectra of the specimens (Figure 1) also confirmed the formation of aluminum-magnesium spinel at high temperatures, with improved development.
Refractory tamping material is also used in hot work equipment, such as electric furnace tops, industrial frequency induction furnaces, and extra-furnace refining devices. The varieties include phosphoric acid high alumina, corundum-chromium-zirconium, water glass aluminum-zirconium, and magnesium-chromium, among others. These materials generally meet the requirements of the production process, and their service life is also relatively long.
The high-alumina pounding material system consists of bauxite clinker with 88% Al₂O₃ content, used as both aggregate and powder, with added fused corundum powder to enhance the matrix's performance. Suzhou clay is added as a plasticizer. The magnesium-aluminum-chromium pounding material system is formulated with fused magnesium-chromium composite material and aluminum-magnesium spinel materials.
The aluminum-zirconium masticated material system consists of bauxite clinker with 85% Al₂O₃ content as the refractory aggregate and powder, with the addition of 64% zircon powder (ZrO₂) and coke clay. The magnesium-chromium refractory masticated material system consists of 91% MgO brick magnesium sand as the aggregate and powder (55% and 15%, respectively), with 47% Cr₂O₃ chromite aggregate and powder (15%), and a water glass solution as a binder, at 4% to 5%.
High-strength magnesium refractory pounding material is made from electrofused magnesium sand, with 97% MgO as the aggregate and powder. The critical particle size is 5mm, with a four-level ingredient ratio of bone powder material at 7:3 to 6:4. Phosphate is used as the binding agent, in an amount of 2% to 4%, with the addition of composite metal powder. When making samples, the molding pressure is selected according to the specified bulk density. After the specimen has cured, its performance is tested. The results are listed in Table 3. From the table, it is clear that the performance of this refractory pounding material is superior, with the highest post-burning compressive strength of 123.2 MPa, the highest load softening temperature exceeding 1700°C, and the lowest apparent porosity of 13%.
In high-alumina batter, mixing corundum powder to improve the grade of the matrix can enhance its performance. When zircon powder is doped into high-alumina refractory battering material, the presence of zircon, which contains impurities, lowers the decomposition temperature from 1670°C to 1540°C. It decomposes into ZrO₂ and SiO₂. At high temperatures, ZrO₂ forms oblique zircon, while SiO₂ combines with Al₂O₃ to form mullite. These two refractory minerals, along with corundum or mullite, interlace and symbiotically strengthen the organizational structure, improving the material's strength and erosion resistance.
At the same time, due to the volume effect, this helps compensate for material shrinkage and enhances resistance to spalling. In high-alumina or magnesium refractory pounding materials, doping with chromite can generate chrome corundum or magnesium-chromium spinel at high temperatures. Even if these two minerals are not generated, the corundum, magnesium sand, and chromite form a skeleton, which, when filled with a silicate phase, also creates a better organizational structure and bonding phase that improves performance. However, the amount of chromite should not be excessive, as it will reduce the load softening temperature and strength of the refractory pounding material.
Tests show that in magnesium-chromium refractory pounding material, the chromite content should generally not exceed 30%, with an optimal range of 10% to 20%, as shown in Figure 2. This is mainly due to the impurities introduced by excessive chromite. Additionally, chromite is added in the form of aggregate, and pre-synthesized magnesium-chromium sand fines should be incorporated into the matrix to improve the sintering degree and mitigate volume expansion. To enhance the performance of magnesium-chromium refractory pounding material, a composite bonding agent made from sodium hexametaphosphate and phosphate ester should be used. Furthermore, metal aluminum powder and metal iron powder should be mixed in to improve medium-temperature strength.
Figure 3 shows the relationship between the phosphate dosage and the strength of the magnesium batter. From the figure, it can be seen that the compressive strength after drying and firing at 1000°C increases with the increase in the phosphate binder dosage, and the optimal dosage is around 3%.
In high-alumina refractory pounding material, bluestone minerals can also be mixed in. Through high-temperature decomposition, these minerals form mullite and produce a volume expansion effect, which helps offset some of the shrinkage of the pounding material, thereby improving its performance. The grade of the bluestone minerals has a significant impact on the performance of the refractory pounding material, so it should be doped with its concentrate. The typical dosage is generally between 15% and 35%.
The refractory aggregate and powder are made from secondary bauxite clinker, with the maximum particle size of the aggregate being 5mm. The mass ratio of coarse to fine aggregates is 1:1. Suzhou mud is used as a plasticizer, and a water glass solution with a specific gravity of 1.38 is used as the binder. The main properties of this type of refractory pounding material are shown in Table 4.
As seen in the table, after doping with the bluestone minerals, there is no significant effect on the strength of the refractory pounding material or its load softening temperature. However, the post-burning linear change shifts from contraction to expansion, indicating the expansion effect of this type of material.
It should be noted that, as an expanding material, bluestone has the best effect. After firing at 1400°C, the linear change shifts from -0.4% to +1.6%.
In addition, there are refractory pounding materials such as alumina cement-based high-alumina and corundum materials, magnesite cement-based magnesium refractory materials, and phosphate or phosphate-bonded refractory materials, all of which have also been applied. To facilitate usage, refractory factories mix the pounding material with a preservative and uniformly wet-mix it. Afterward, it is sealed in plastic bags, where it can be stored for 3 to 6 months. During this period, the material retains its plasticity, allowing for pounding construction without significant degradation in performance. This type of pounding material is also known as plastic refractory pounding material.