Refractory raw materials are the foundation of all refractory products, regardless of technological advancements. The quality and performance of refractories are directly influenced by the raw materials used. The evolution of modern refractories relies heavily on these materials. For instance, magnesia-alumina spinel bricks, iron-alumina spinel bricks, and magnesia-iron spinel bricks depend on magnesia-alumina spinel, iron-alumina spinel, and magnesia-iron spinel as their core ingredients. Similarly, high-alumina bricks like anti-spalling bricks and silicon-carbide bricks rely on zircon, silicon carbide, andalusite, and kyanite. Almost all high-end unshaped refractories require micro-powders such as silica fume and alpha-alumina, along with various chemical additives. Understanding these raw materials is essential for producing high-quality refractories or developing innovative solutions.
Magnesia-alumina spinel is a refractory raw material known for its high melting point, low thermal expansion coefficient, low thermal conductivity, excellent thermal shock resistance, and strong corrosion resistance. It is classified into sintered magnesia-alumina spinel and fused magnesia-alumina spinel based on the synthesis process.
Magnesia-alumina spinel offers superior corrosion resistance, abrasion resistance, and thermal stability. Its primary applications include replacing magnesia-chrome sand in spinel bricks for cement rotary kilns, eliminating chromium pollution while providing excellent anti-spalling properties. It is also used in ladle castables to enhance the corrosion resistance of steel lining, making it a key material in steelmaking refractories. High-quality pre-synthesized spinel provides a new raw material for producing high-purity unshaped and shaped refractories.
Zircon sand is valued for its low thermal expansion, high thermal conductivity, and strong chemical stability. It is widely used in refractories, including zirconia-based materials like zircon corundum bricks and zirconia refractory fibers.
Zircon sand is used to produce zircon bricks for glass kilns, ladle bricks, ramming mixes, and castables. When added to other materials, it enhances their properties. For example, adding zircon to synthetic cordierite broadens its sintering range without affecting thermal shock resistance. In high-alumina bricks, zircon improves thermal shock resistance, making it ideal for anti-spalling high-alumina bricks. It is also a source for extracting zirconium dioxide (ZrO2).
Silicon carbide is renowned for its high hardness, electrical conductivity, high-temperature resistance, and strength. It is widely used in abrasives, heating elements, refractories, and structural ceramics. Silicon carbide is available in black and green varieties, with black silicon carbide being more common in refractories.
Silicon carbide has an incongruent melting point of 2760°C at atmospheric pressure and a flexural strength of up to 625 MPa. Its excellent thermal shock resistance is due to its moderate thermal expansion coefficient (4.7×10^-6 K^-1) and high thermal conductivity (84 W/(m·K)). These properties make it a preferred material for manufacturing ceramics and refractories, particularly silicon carbide bricks.
Andalusite and kyanite belong to the kyanite group of minerals, along with sillimanite, collectively known as the “three stones.” High-purity kyanite, andalusite, and sillimanite are used in aluminum-silicon alloys for aerospace and marine applications or in ceramic components like spark plugs, thermocouple sheaths, and crucibles. They are also essential in refractories.
– Kyanite: Due to its high expansion rate, small amounts of kyanite can compensate for the shrinkage of refractories at high temperatures. Its low transformation temperature makes it suitable for refractories that shrink at lower temperatures, making it ideal for mid-to-low-grade refractory additives.
– Andalusite: Andalusite exhibits low thermal expansion, but larger quantities are needed to compensate for shrinkage. Increasing its content improves the refractory’s creep resistance and thermal shock resistance, though at a higher cost. Andalusite is thus better suited for mid-to-high-grade refractories.
Silica fume is a byproduct collected from the exhaust gases during the production of metallic silicon or ferrosilicon alloys. In an electric arc furnace, quartz is melted into SiO2 liquid, which is then reduced by carbon to form silicon. Some SiO2 is reduced to SiO gas, which escapes with the exhaust, oxidizes to SiO2 in the air, and condenses into glassy microspheres with an average particle size of less than 1 μm, captured by dust collection systems.
Silica fume is also collected during the production of desilicated zirconium from zircon. It contains 90-98% SiO2, a surface area of 15-30 m²/g, a median particle size of 0.3-0.5 μm, and a pH of 5.3-7.6. Imported silica fume is purer and more acidic, while domestic silica fume contains trace amounts of MgO and R2O, making it slightly alkaline. Although refractory castables made with imported silica fume exhibit excellent flowability, they come at a higher cost.
The advancement of refractories hinges on these five essential raw materials: magnesia-alumina spinelin, zircon, silicon carbide, andalusite, kyanite, and silica fume. Their unique properties enable the development of high-performance refractories tailored to diverse industrial applications. By leveraging these materials, manufacturers can innovate and optimize refractory solutions, ensuring enhanced performance, durability, and cost-efficiency in industries such as steelmaking, cement production, and glass manufacturing.
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