Rotary kiln refractory magnesia calcium carbon bricks have many excellent properties and are widely used in refining outside the furnace. However, there are still some problems in the use of magnesia calcium carbon bricks.
High-temperature reaction of magnesium oxide and its resistance to slag penetration
Above 1500°C, the MgO in the Mg-Ca-C refractory matrix readily reacts with the carbon in the matrix as follows. At 1600°C, the reaction becomes more intense, with MgO being reduced to Mg vapor and carbon being oxidized to Co gas, which escapes from the brick body. This causes significant weight loss, a key factor in the failure of Mg-C bricks. MgO+C→Mg(g)+Co(g). Furthermore, during the use of Mg-Ca-C bricks, SiO2 in the slag first reacts with free CaO to form C2S and C3S. These formations increase the slag viscosity, making it difficult for the molten slag to penetrate further into the Mg-Ca-C brick. However, oxidation of the graphite on the brick surface leads to surface decarburization, creating voids that allow the molten slag to penetrate further into the brick body. The MgO in the Mg-Ca-C sand reacts with the CaO-SiO2 slag to form a low-melting phase of MgO-CaO-SiO2. Therefore, MgO is the weak link in the Mg-Ca-C brick’s resistance to slag erosion.
MgO is a relatively weak link in magnesia-calcium carbon refractory materials. Therefore, it is necessary to reduce the MgO content in magnesia-calcium carbon bricks to mitigate the weight loss caused by CaO reacting with carbon at high temperatures. Furthermore, magnesia-calcium sand with a continuous CaO phase should be used to prevent the MgO from directly reacting with the CaO-SiO2 slag.
Generally, when fused magnesia-calcium sand is used as the raw material for magnesia-calcium, the CaO grains form a continuous phase, encapsulating the MgO grains, which protects the MgO. Fused magnesia-calcium sand is denser than sintered magnesia-calcium sand, which significantly enhances the slag penetration resistance of magnesia-calcium bricks. The CaO grains in sintered magnesia-calcium sand are encapsulated by MgO, making them less susceptible to air hydration by water vapor. Fused magnesia-calcium sand, on the other hand, has the disadvantage of readily hydrating CaO. Furthermore, sintered magnesia-calcium sand is more economical than fused magnesia-calcium sand. Therefore, choosing different types of magnesia-calcium sand raw materials has its own advantages and disadvantages.
Adhesives
Phenolic resins are now widely used as binders for carbon-containing refractories due to their high residual carbon content and strong bonding strength. However, for specialized refractory materials like magnesium-calcium-carbon bricks, the presence of CaO makes standard phenolic resins unsuitable. Instead, anhydrous phenolic resins, which do not contain free water, are the only viable alternative. However, anhydrous phenolic resins without free water present more challenges than standard phenolic resins. The loss of free water reduces the fluidity of anhydrous phenolic resins, requiring them to be dispersed in organic solvents such as anhydrous alcohol or ethylene glycol before use, significantly complicating the mixing process. Anhydrous phenolic resins can also undergo a phenomenon of self-hardening over time. The extent of this phenomenon is directly related to the temperature of the resin storage environment and the degree of sealing. This phenomenon, known as aging, directly impacts the resin’s performance and shelf life.
However, anhydrous phenolic resin, like conventional resins, is relatively stable and does not decompose before 250°C. Thermal decomposition begins gradually above 300°C. This inherent characteristic of phenolic resin is virtually unchangeable. However, this inherent thermal decomposition behavior presents another problem in CaO-containing refractory materials. While anhydrous phenolic resin does not contain free water, as a macromolecular organic compound, it inevitably contains some bound water and hydrogen-containing functional groups. Thermal decomposition of anhydrous phenolic resin inevitably produces water vapor. Thermodynamic calculations indicate that CaO hydration persists up to 510°C, leading to a more serious problem: CaO hydration caused by the self-decomposition of anhydrous phenolic resin in Mg-Ca-C refractories.
Therefore, verifying and studying the impact of the decomposition behavior of anhydrous phenolic resin on the performance of Mg-Ca-C bricks is crucial, and finding effective methods to mitigate this issue is crucial.
Waterproofing of free CaO
Although CaO, as an alkaline material, possesses advantages such as purifying molten steel, excellent slag resistance, and thermal stability, its main drawback is that it absorbs water from the air and hydrates. Its hydration proceeds according to the following chemical reaction:
CaO + H₂O → Ca(OH)₂ + (16 × 4.18 kJ)
CaO hydration remains a recognized challenge, prompting extensive research and numerous methods for making CaO water-repellent. These methods range from surface coating of calcium-containing products, such as phosphoric acid treatment and wax impregnation; sintering, which promotes CaO grain growth and forms more stable grains; and admixtures, such as the introduction of oxides such as ZO₂, which react with CaO to form compounds with improved hydration properties.
However, in resin-bonded Mg-Ca-C refractories, due to the rapid decomposition of the resin below 510°C, CaO can hydrate before this temperature.
Carbon enrichment of molten steel
With the increasing demand for clean steel, low-carbon steel, and ultra-low-carbon steel, the high carbon content (10%-20%) of traditional carbon-containing refractories has increased the likelihood of them adding carbon to the molten steel during use, a growing concern. Consequently, their carbon content has made it difficult to use traditional carbon-containing refractory materials under clean steelmaking conditions. Furthermore, from an energy-saving and environmental perspective, the increased energy consumption and waste of graphite resources caused by the excessive thermal conductivity of high-carbon refractory materials are driving a trend toward low-carbon and carbon-free refractory materials.
Since magnesia-calcium carbon bricks also contain a certain amount of carbon, similar to magnesia-calcium carbon bricks, they also have the potential to add carbon to the molten steel. Therefore, controlling the carbon content is crucial. The carbon content should generally be controlled between 2% and 6%. This is primarily due to concerns about adding carbon to the molten steel and reducing the service life of the magnesia-calcium carbon bricks due to decarburization. Furthermore, the higher the amount of graphite added, the lower the compressive strength and bulk density of magnesia-calcium products. Generally, carbon-free magnesia-calcium products have higher strength than those containing carbon. Henan Zhengkuang Machinery has established partnerships with numerous companies in the industry, specializing in the harmless treatment and comprehensive recycling of various solid and hazardous wastes. We offer clients comprehensive project construction services, including design, manufacturing, installation, and commissioning. We have earned the trust of our clients with our service and quality. We welcome inquiries from both new and existing customers.