When selecting refractory materials, besides the price, the following factors must be considered: long service life, good insulation performance, easy and quick installation, and fast maintenance.
Unshaped refractories, also known as monolithic refractories, are composed of loose aggregates and fine powders, and they don't require firing or shaping before use. Their composition and properties can be adjusted flexibly according to needs, such as the type and amount of binder, additives, and the construction method (casting, ramming, spraying, etc.). This adaptability allows the development of larger, more complex, and integrated structures in refractory materials, earning them the title of the second-generation refractory materials.
Monolithic refractories are a crucial base material in the application of high-temperature kiln linings. Refractory castables, a vital type of unshaped refractory, are characterized by short supply cycles and the ability to form on-site without being pre-fired. They can be used to create seamless linings, also known as monolithic refractory materials. High-alumina castables, low-cement high-alumina castables, steel fiber-reinforced wear-resistant castables, and corundum castables are widely used in the linings of cement kilns and other thermal equipment.
1. Types of Unshaped Refractories
1.1 Grouting Material
Grouting material, also known as casting material, is mixed with water to form a material with good fluidity. After proper curing, it can be used following a specific baking process. The aggregates can include alumina, corundum, or alkaline refractory materials, while the binders can be calcium aluminate cement, sodium silicate, ethyl silicate, or phosphate. Grouting material is often used with metal or ceramic anchors and can be reinforced with stainless steel fibers to enhance resistance to mechanical vibration and thermal shock. It is commonly used in the linings of heat treatment furnaces, ore roasting furnaces, and various high-temperature smelting channels.
1.2 Plastic Refractory
Plastic refractories are mud-like or clay-like materials that deform easily under applied force without cracking and retain their shape after the stress is removed. They are made from materials like semi-silica, clay, high-alumina, zircon, and carbon, with the addition of plasticizers like high-plasticity clay or other additives to enhance plasticity. Plastic refractories are mainly used for building integrated furnace linings and are often combined with metal or ceramic anchors.
1.3 Gunning Mix
Gunning mix is a refractory mixture applied using a gunning machine. It can be categorized into wet gunning, semi-dry gunning, and flame gunning methods. Wet gunning uses compressed air to spray a slurry with 20-40% refractory powder, creating a fine mist with high adhesion. Semi-dry gunning adds water at the nozzle to moisten the refractory powder, allowing for thicker layer application. Flame gunning involves projecting the mix into a flame, where it partially melts and adheres to the lining.
1.4 Refractory Coating
Refractory coating is applied to refractory brick linings, and its composition varies depending on the application. It can be formulated as a paste or slurry and typically uses binders like phosphate or sodium silicate, with additives to enhance application properties. It is mainly used as a protective layer or for repairing damaged areas of thermal equipment linings.
1.5 Ramming Mix
Ramming mix is a low-plasticity or non-plastic loose refractory material. The mix can include silica, clay, high-alumina, corundum, zircon, silicon carbide, carbon, or magnesium. Ramming mixes are compacted using forceful ramming, resulting in low porosity and high density, making them suitable for furnace linings and containers holding molten metals.
1.6 Shotcreting Material
Shotcreting material is a semi-dry mix applied using a shotcreting machine to form linings, often used in steel ladles. Materials include silica, clay, high-alumina, and zircon.
2. Applications of Unshaped Refractories
2.1 Prefabricated Blocks of Castable
Prefabricated blocks of castable are treated at low temperatures, making them low-carbon and green refractory materials. They improve the performance of refractory linings, reduce material consumption, and provide stable and reliable performance.
2.2 Application in Blast Furnaces
Blast furnaces are key equipment in iron smelting. Unshaped refractories like high-alumina cement and phosphoric high-alumina castable are used in small blast furnaces, while resin-bonded alumina-carbon bricks and SiC castables are used in large furnaces.
2.3 Application of Refractory Materials in Steel Ladles
The increase in tapping temperature and the extended dwell time of molten steel in ladles have led to significant changes in the refractory materials used in ladles. Steel ladles, which previously predominantly used shaped refractory materials, are now largely replaced by unshaped refractories. The use of unshaped materials in ladles not only reduces labor but also facilitates automated construction and drying in factories, improving overall cost-effectiveness. When the side walls of a ladle are lined with unshaped materials, the labor involved in maintenance can be reduced by 40%. If the entire interior lining of the ladle is made of unshaped materials, the labor reduction can reach 70%. Due to their good resistance to corrosion, minimal structural spalling, long service life, and ability to improve steel quality, Al2O3-spinel castables have become the primary refractory material used in ladles. However, their usage is limited by temperature and the dwell time of molten steel. Al2O3-MgO castables developed by Japan's Kawasaki Company offer improved material strength and slag penetration resistance, with a service life 20% longer than Al2O3-spinel castables. Currently, magnesium-based refractories and alumina-magnesia-carbon refractories are being trialed in ladles, showing initial positive results.
2.4 Application of Unshaped Refractories in Non-Ferrous Metallurgy
Reflective Furnaces
In aluminum refining, reflective furnaces are major consumers of refractory materials. The furnace crucible has transitioned from brick masonry to a monolithic structure. Refractory materials in contact with the molten metal are typically acid-salt-bonded plastics, phosphate-bonded bricks, or low-cement castables. The refractory materials for the hot face of the lower sidewalls are usually the same as those for the crucible, while insulating materials may include lightweight insulating castables, insulating clay bricks, or ceramic fibers. Furnace roofs generally use high-quality rammed materials or burnt castables. Furnace doors are usually made with dense castables or lightweight castables, or a mixture of both.
In electrolytic aluminum refining, unshaped refractories are mainly used in aluminum holding vessels. These refractories must withstand aluminum melt erosion, rapid thermal cycling, and provide good thermal insulation. Non-working linings of aluminum holding vessels typically use lightweight castables or lightweight high-alumina bricks. If the aluminum melt is to be held for an extended period, insulating castables are used to prevent the melt from solidifying during transport. To reduce the weight of the vessel and enhance insulation, non-working linings now use alumina hollow sphere refractory castables. Working linings are usually built with high-alumina bricks with low SiO2 content, and some vessels now use corundum-based refractory castables.
In copper refining, due to the strong corrosiveness of CuO as a component of slag, alkaline refractory materials are widely used in the refining furnaces. For example, parts of reflective furnaces, converters, flash smelting furnaces, and rotary anode furnaces all use alkaline magnesia-chrome bricks or MgO-based alkaline ramming materials, especially for maintenance and repair. In the refining stage, where slag volume and temperature are lower, castables, plastics, spray materials, and high-alumina bricks are used as alumina-silicate refractory materials.
