Classification and Application of Monolithic Refractory Materials in Various Industries

Classification and Application of Monolithic Refractory Materials in Various Industries

Besides price, the following factors should be considered when selecting refractory materials: long service life; good thermal insulation; simpler construction methods and faster construction speed; and quicker maintenance.

Unshaped refractories, also known as bulk refractories, consist of bulk particles and fine powders and require neither firing nor shaping before use. They allow for flexible modification of the material’s composition and processing, such as the composition and particle size; the type and amount of binder; the selection and adjustment of admixtures (e.g., plasticizers, hardening accelerators, retarders, water-reducing agents); and the diversification of construction methods (pouring, tamping, spraying, casting, plastic application, etc.). This has led to a significant advancement in the development of refractory brick shapes towards larger, more irregular, and monolithic structures, earning them the title of second-generation refractories.

Unshaped refractories are one of the essential basic materials in the application of refractory lining technology in high-temperature kiln industries. Refractory castables are an important type of monolithic refractories. Their key characteristics include short delivery times, no limitation on equipment shape, and the ability to be used directly after on-site molding and baking of a loose mixture with suitable anchors without pre-calcination. Refractory castables can be used to create seamless linings, also known as monolithic refractories. High-alumina castables, high-alumina low-cement castables, steel fiber abrasion-resistant castables, and corundum castables, among other monolithic refractories, have been widely used in the lining design of cement kilns and have served as refractory layers in various parts of thermal equipment for many years.

Classification of Unshaped Refractory Materials

1.1 Grouting Material

A material with good fluidity after being mixed with water, also known as castable. After molding, it needs proper curing to allow it to set and harden, and can be used after baking according to a certain system. Grouting materials use aluminosilicate clinker, corundum materials, or basic refractory clinker as aggregates; lightweight castable materials use expanded perlite, vermiculite, ceramsite, and alumina hollow spheres as aggregates. Binders include calcium aluminate cement, water glass, ethyl silicate, polyaluminum chloride, clay, or phosphates. Admixtures are determined according to the application, and their function is to improve workability and enhance physical and chemical properties.

Construction and molding methods for grouting materials include vibration method, pump injection method, pressure injection method, and spraying method. When using grouting materials as integral linings, they are often used in conjunction with metal or ceramic anchors. Adding stainless steel fiber reinforcement can further improve its resistance to mechanical vibration and thermal shock. Grouting materials are used as linings for various heat treatment furnaces, ore roasting furnaces, catalytic cracking furnaces, and conversion furnaces. They are also used as linings for smelting furnaces and high-temperature melt channels, such as lead-zinc melting furnaces, tin baths, salt bath furnaces, steel or iron tapping troughs, steel ladles, and suction nozzles for vacuum circulation degassing devices for molten steel.

1.2 Plastics 

Plastics are clay or clay blanks with plasticity. They easily deform without cracking when subjected to appropriate external force; after the stress is relieved, they no longer deform. Plastic materials include semi-siliceous, clayey, high-alumina, zircon, and carbonaceous materials, as well as lightweight plastics. Plastics must contain plasticizers, which are mostly highly plastic clays. Plasticizers can also be used to improve the plasticity of this clay. Plasticizers include carboxymethyl cellulose, dextrin, and lignin sulfonates. The binders used in plastics include plastic clay, phosphoric acid, aluminum dihydrogen phosphate, and aluminum sulfate. Alumina-based refractory materials with added phosphoric acid or phosphate binders will react chemically with alumina during storage, forming insoluble aluminum orthophosphate, which hardens the slurry. Therefore, preservatives such as oxalic acid, citric acid, and acetylacetone must be added.

Construction methods generally employ tamping or vibration. When using refractory materials to construct monolithic furnace linings, metal or ceramic anchors must be used. Refractory materials are used as linings for thermal equipment such as soaking furnaces, heating furnaces, and boilers, and are also used to wrap water-cooled pipes in heating furnaces.

1.3 Spraying Material 

A refractory mixture used for spraying or applying using a spraying machine. According to the spraying method, it can be divided into wet spraying (or slurry spraying), semi-dry spraying, and flame spraying. Wet spraying uses compressed air to spray a slurry containing 20-40% refractory powder, achieving high dispersion, high adhesion, and allowing for relatively uniform thin-layer spraying. Semi-dry spraying involves wetting the refractory powder, which is then sprayed by compressed air, with water added at the nozzle. The water content is 11-14%, resulting in a lower adhesion rate, but allowing for thicker layers. Pyroblasting, a type of dry spraying, involves delivering the spraying material into the flame of a fuel-oxygen spray gun. The material partially melts and adheres to the brick lining within the flame.

Spraying materials include aluminosilicate, aluminosilicate-zirconium, magnesia, magnesia-calcium, and magnesia-chromium compounds. Binders used include sodium silicate, phosphates, polyphosphates, bitumen, and resins. To improve adhesion, additives such as clay, bentonite, and lime are added. To ensure good sintering, sintering aids such as serpentine, pure olivine, lime, refractory clay, and iron oxide are also added.

1.4 Refractory Coatings 

Materials applied to refractory brick linings. Depending on the application requirements and construction methods, refractory coatings are formulated into paste or slurry forms. The binders used vary depending on the material; for example, alkaline coatings for continuous casting tundishes use phosphates, polyphosphates, and magnesium sulfate; high-alumina coatings use clay, aluminum dihydrogen phosphate, aluminum chromium phosphate, and water glass. Plasticizers and other admixtures are generally added to improve the coating’s spreadability. Coatings are mainly used as protective coatings for the linings of various thermal equipment or for repairing localized damage to brick linings.

1.5 Ramming Mixtures 

A type of loose refractory material with very low or no plasticity. Materials include siliceous, clayey, high-alumina, corundum, zircon, silicon carbide, carbonaceous, and magnesia-based materials. Depending on the material and application conditions, ramming mixes can use inorganic or organic binders similar to those used for castable refractory mixes, such as water-soluble dextrin, carboxymethyl cellulose, lignin, sulfonates, and polyvinyl alcohol; or water-resistant and thermoplastic materials like paraffin wax, asphalt, tar, phenolic resin, and atactic polypropylene.

Ramming mixes are applied using forced ramming, resulting in lower porosity and higher density. Therefore, among monolithic refractories, ramming mixes are particularly suitable for lining furnaces and various containers holding molten metal, such as hearths of open-hearth and electric furnaces, linings of various induction furnaces, blast furnace tapping channels, and steel ladles.

1.6 Projectile Mix 

A semi-dry mortar material projected using a projector for lining. Primarily used for constructing integral steel ladle linings. Materials include siliceous, waxy, clayey, high-alumina, and zircon-based materials. High-siliceous and high-alumina projectile mixes are most commonly used.

Applications of Monolithic Refractory Materials

2.1 Characteristics of Castable Precast Blocks

Castable precast blocks require only low-temperature heat treatment, making them low-carbon and environmentally friendly refractory materials. They represent a unique technology in refractory lining. They can improve the performance of refractory linings, reduce refractory material consumption, and offer stable quality and reliable performance. We know that adding steel fibers to castables aims to improve their mechanical properties, inhibit crack initiation, or limit crack propagation when they do form.

2.2 Application of Unshaped Refractory Materials in Blast Furnaces

Blast furnaces are crucial equipment in ironmaking. Small blast furnaces were previously constructed using precast blocks of high-alumina cement and high-alumina phosphate refractory castables, but now resin-bonded alumina-carbon unburned bricks are commonly used. Large blast furnaces use SiC castables for their water-cooled walls, and refractory castables and silicon nitride fillers for the furnace bottom and surrounding brick joints. A growing trend in refractory materials for furnace walls is the use of castables. The No. 2 blast furnace at the Fukuyama plant of Nippon Steel Pipe Co., Ltd. used a high-density castable of 50% Al2O3 and 45% SiO2 in the upper part of the furnace body; after six years of use, spalling was minimal.

2.3 Application of Unshaped Refractory Materials in Steel Ladles

The increase in tapping temperature and the extended residence time of molten steel in the ladle have led to significant changes in refractory materials used in steel ladles. Steel ladles were previously constructed using shaped refractories… The main body of the ladle, now replaced by monolithic refractories, saves labor and enables automated factory construction and drying, improving overall economic efficiency. When the ladle sidewalls are monolithic, maintenance work can be reduced by 40%; when the ladle lining is entirely monolithic, the reduction is 70%. Due to its advantages such as good corrosion resistance, minimal structural spalling, long service life, and ability to improve steel quality, Al2O3-spinel castable has become the main refractory material for ladles. However, its use is limited by temperature and molten steel residence time. The Al2O3-MgO castable developed by Kawasaki Corporation of Japan has improved material strength and slag penetration resistance, and its service life is 20% longer than that of Al2O3-spinel castable. Currently, trials of magnesia-based and alumina-magnesia-carbon-based refractory castables on ladles are showing initial success.

2.4 Application of Unshaped Refractory Materials in Nonferrous Metallurgy

Aluminum reverberatory furnaces are major consumers of refractory materials in aluminum plants. The refractory materials used in the furnace hearth have evolved from brickwork to integral structures. Refractory materials in contact with the hot metal of the furnace hearth are typically acid-base bonded plastics, phosphate-bonded bricks, and low-cement castables. The composition of the hot-face refractory materials for the lower sidewalls is usually the same as that of the furnace hearth. Insulation materials can be lightweight insulating castables, insulating clay bricks, or ceramic fibers, etc.; the furnace roof material is usually high-quality refractory materials such as rammed earth and sintered refractories; the furnace door is usually made of dense castables or lightweight castables, or a mixture of both.

In electrolytic aluminum smelting, unshaped refractory materials are mainly used in aluminum ladles. The refractory materials used for aluminum containers must be resistant to the erosion of molten aluminum, withstand rapid heating and cooling, and have good thermal insulation properties. The non-working lining of aluminum containers typically uses lightweight castables or lightweight high-alumina bricks. If the molten aluminum in the container is to be stored for a long period, insulating castables are usually used to prevent solidification during transport. Currently, to reduce the weight of the container and enhance insulation, alumina hollow spherical refractory castables are used for the non-working lining. The working lining is generally constructed using high-alumina bricks with low SiO2 content; some containers now use corundum refractory castables.

In the copper smelting industry, because CuO, as a component of slag, is highly corrosive, basic refractory materials are widely used in the roughing furnace. For example, basic magnesia-chrome bricks or basic ramming mixes based on MgO are used in various parts of the reverberatory furnace, converter, flash furnace, and rotary anode furnace, especially during furnace maintenance and repair. In the refining section, since the slag volume and temperature are lower, aluminosilicate refractory materials such as castables, plastics, sprayables, and high-alumina bricks can be used.