Performance Characteristics of Several Different Refractory Ramming Mixes
The basic composition of ordinary refractory ramming mixes is similar to that of refractory castables and plastics, the difference being that they contain more refractory powder and less binder. Soft clay is usually added to increase the material’s plasticity and sintering properties. The critical particle size of the refractory aggregate is 10mm, although 5mm is also used. The mass ratio of coarse to fine aggregate is 3:7 to 4:6. A higher proportion of fine aggregate makes it easier to compact. The particle size distribution of refractory ramming mixes is generally 60%~65% refractory aggregate and 35%~40% refractory powder, which results in a higher bulk density and a higher volume density in the rammed lining.
Ramming Mixtures for Steel Ladles
Refractory ramming mixes are widely used in the working layer of steel ladles and have achieved certain results. Their refractory aggregates and powders are first-grade or extra-grade bauxite clinker, with a maximum particle size of 10mm. When pyrophyllite particles or powder are added, it is called alumina-magnesia ramming mix. When first-grade brick-making magnesia powder or metallurgical magnesia powder is added, it is called alumina-magnesia ramming mix. The powder fineness is more than 90% less than 0.09mm, and a water glass solution with a modulus of 2.6 and a density of 1.36 g/cm³ or an aluminum sulfate solution with a density of 1.26 g/cm³ is used as a binder.
Among ramming mixes used in steel ladles, alumina-magnesia refractory ramming mixes are the most widely used and effective, although their composition varies. When the magnesia powder content is 9%~12%, sufficient alumina-magnesia spinel can be formed, resulting in good slag resistance and a long service life.
After mixing, the refractory ramming mix is tamped to mimic the bulk density of the lining using a pneumatic pick. Samples are prepared on a press. The samples are naturally cured for 3 days, then dried, and their performance is tested.
Main Properties of Ramming Mixes for Steel Ladles
Water glass refractory ramming mixes are better than aluminum sulfate refractory ramming mixes. Among water glass refractory ramming mixes, alumina-magnesia refractory ramming mixes have relatively good performance. They have good slag resistance, high load softening temperature, and high-temperature compressive strength, especially a compressive strength of 105.9 MPa after firing at 1400℃. However, the linear shrinkage after firing is relatively large, reaching 2.21%.
This material can form alumina-magnesia spinel at high temperatures, which grows interlocked and expands in volume. Since it is a refractory mineral, this improves its performance. Alumina-wax stone refractory ramming mixes are characterized by expansion after firing, reaching 2.32%. The lining does not stick to slag during use, making it suitable for steel ladle linings. However, its low high-temperature compressive strength and load softening temperature affect its use. The properties of alumina-magnesia refractory ramming mixes fall between those of alumina and magnesia, and they can also be used in steel ladles.
Slag resistance was tested using the crucible method. The slag chemical composition was: CaO 41.2%, SiO 10.5%, Fe₂O₃ 10.8%, FeO 18.3%, Al₂O₃ 5.34%, MnO 4.478%, MgO 5.3%, CaF₂ 1.36%, and loss on ignition 1.52%. The basicity was 3.9. Test results showed that alumina-magnesia ramming mixes experienced less erosion and penetration by the slag, and formed a thinner, denser layer, preventing further slag penetration. The slag resistance of alumina and alumina-phyllite ramming mixes was basically similar. However, compared to alumina-magnesia ramming mixes, their erosion and penetration were approximately twice as high. This indicates that adding magnesia powder to alumina refractory ramming mixes has a significant effect.
A different set of powder and water glass was used to pressure-form a sample, which was then fired at 1600℃ and subjected to petrographic analysis. Polarizing microscopy revealed the formation of a significant amount of aluminum-magnesium spinel, with grains generally ranging from 5 to 7 μm, and a few from 10 to 15 μm. When 2% chromite was added as a mineralizing agent, the spinel development was better, with coarser grains, generally 10 to 15 μm, and some reaching approximately 30 μm. X-ray diffraction patterns of this sample also confirmed the formation and good development of aluminum-magnesium spinel at high temperature.
Applications of Refractory Ramming Refractories in Other Thermal Equipment
Refractory ramming refractories are also used in thermal equipment such as electric furnace tops, induction furnaces, and ladle refining units. Varieties include high-alumina phosphate and corundum chromium-zirconium, water glass aluminum-zirconium, and magnesia-chromium refractories. They generally meet the requirements of production processes and have a relatively long service life.
High-alumina ramming refractories use bauxite clinker with 88% Al2O3 as aggregate and powder, with a significant amount of fused corundum powder added to improve matrix properties. Suzhou clay is added as a plasticizer. Magnesia-aluminum-chromium ramming refractories are formulated using fused magnesia-chromium composites and materials such as aluminum-magnesium spinel.
Aluminum-zirconium ramming refractories use bauxite clinker with 85% Al2O3 as refractory aggregate and powder, with zircon powder (64% ZiO2) and coke clay added. The mix proportions of magnesia-chromium refractory ramming mixes are as follows: 55% MgO (91%) brick-making magnesia aggregate and 15% powder; 15% Cr2O3 (47%) chromite aggregate and 15% powder; and 4%–5% water glass solution.
High-strength magnesia refractory ramming mixes use fused magnesia (97%) as aggregate. The critical particle size is 5 mm, with a four-stage batching process and an aggregate-to-powder ratio between 7:3 and 6:4. Phosphate is used as a binder at 2%–4%, along with composite metal powder. During sample preparation, the molding pressure is selected according to the specified bulk density. Performance is tested after the samples have cured. This type of refractory ramming mix exhibits good performance, with a maximum compressive strength of 123.2 MPa after firing, a maximum softening temperature exceeding 1700℃ under load, and a minimum apparent porosity of 13%.
Adding corundum powder to high-alumina ramming mixes to improve matrix grade enhances their performance. In high-alumina refractory ramming mixes with zircon powder, the decomposition temperature of the zircon decreases from 1670℃ to 1540℃ due to impurities, and it decomposes into ZrO2 and SiO2. At high temperatures, the former forms clinoptilolite, and the latter combines with Al2O3 to form mullite. These two refractory minerals, intermingled with corundum or mullite, strengthen the microstructure, improving strength and erosion resistance.
Simultaneously, due to the volume effect, material shrinkage is compensated, enhancing resistance to spalling. Adding chromite to high-alumina or magnesia refractory ramming mixes can form chromium corundum or magnesia-chromium spinel at high temperatures. Even without the formation of these two minerals, a framework of corundum, magnesia, and chromite, filled with silicate phases, can still create a good microstructure and bonding phase, contributing to improved performance. However, the amount of chromite should not be too much; otherwise, it will reduce the load softening temperature and strength of the refractory ramming mix.
The Influence of Chromite Dosage on Magnesia-Chrome Refractories
Experiments show that the amount of chromite in magnesia-chrome refractory ramming mixes should generally not exceed 30%, with a suitable dosage of 10-20%. This is mainly due to the excessive impurities introduced by chromite. Simultaneously, chromite should be added in aggregate form, and pre-synthesized magnesia-chrome sand powder should be added to the matrix to improve sintering degree and reduce volume expansion. To improve the performance of magnesia-chrome refractory ramming mixes, a composite binder of sodium hexametaphosphate and phosphate ester should be used, along with the addition of metallic aluminum powder and metallic iron powder to improve intermediate-temperature strength.
The Relationship between Phosphate Dosage and Strength in Magnesia Ramming Mixes
With the increase of phosphate binder dosage, the compressive strength after drying and firing at 1000℃ also increases, with the optimal dosage being approximately 3%.
Kyanite Group Minerals
In high-alumina refractory ramming mixes, kyanite group minerals can be added. Through decomposition at high temperatures and the formation of mullite, a volume expansion effect is generated, offsetting some of the volume shrinkage of the ramming mix and improving its performance. The grade of kyanite group minerals has a significant impact on the performance of refractory ramming mixes; therefore, they should be added as concentrates, typically at a dosage of 15% to 35%. Refractory aggregates and powders are made from secondary bauxite clinker, with a maximum aggregate particle size of 5mm and a coarse to fine aggregate mass ratio of 1:1. Suzhou clay is used as a plasticizer, and water glass solution with a specific gravity of 1.38 is used as a binder. The main properties of this type of refractory ramming mix are as follows: Adding kyanite group minerals has no significant effect on the strength and load softening temperature of the refractory ramming mix; however, the linear change after firing changes from linear shrinkage to linear expansion, demonstrating the expanding agent effect of this type of material.
As an expansion agent, kyanite showed the best effect, with its linear change increasing from -0.4% to +1.6% after firing at 1400℃.
In addition, aluminate cement high-alumina and corundum refractory ramming mixes, periclase cement magnesia refractory ramming mixes, and phosphoric acid or phosphate refractory ramming mixes have also been used. To facilitate user application, refractory plants add preservatives to the refractory ramming mixes, wet-mix them thoroughly, and then seal them in plastic bags. They can be stored for 3-6 months while still retaining plasticity and being suitable for ramming construction, with no significant decrease in performance. This type of ramming mix is also called plastic refractory ramming mix.
