Different types of thermal insulation and refractory materials have different operating temperatures and performance characteristics, which ultimately determine their applications.
Performance and Applications of Different Thermal Insulation Refractory Bricks
Alumina Hollow Sphere Bricks
Alumina hollow sphere bricks possess excellent thermal insulation properties, along with high operating temperatures, high softening points under load, and high compressive strength. They can be used not only as insulation layers but also in direct contact with flames. Zirconia hollow sphere bricks offer even better insulation performance and higher operating temperatures than alumina hollow sphere bricks. However, zirconia hollow sphere products are expensive, and raw material reserves are limited, generally limiting their use to kilns with specific requirements. Alumina hollow sphere products, on the other hand, are currently the most popular high-temperature, high-strength, lightweight thermal insulation material due to the abundance and cost-effectiveness of alumina raw materials.
Although alumina insulating bricks have a lower operating temperature than hollow sphere products, their bulk density is much lower. Therefore, they have lower thermal conductivity and better thermal insulation performance. Alumina insulating bricks are widely used for insulation in sintering kilns in metallurgy and ceramics industries. They cannot bear heavy loads and should be kept away from environments with strong erosion, abrasion, and corrosion. Based on the alumina content, lightweight alumina-silica insulating materials are mainly divided into high-alumina insulating bricks, mullite insulating bricks, clay insulating fire bricks, and silica insulating bricks.
Because these products have good thermal insulation performance and their operating temperature can be adjusted within a wide range, they are suitable for thermal insulation in various industrial kilns under general operating conditions, without contact with molten metal or slag, without severe thermal shock, without chemical reactions, without corrosion, and without severe mechanical impact or abrasion. They can be in direct contact with flames, but are more often used as an intermediate thermal insulation layer. Examples include thermal insulation for belt sintering machines, pellet roasting furnaces, and steelmaking and rolling heating furnaces in the ferrous metallurgical industry; thermal insulation for aluminum electrolytic cells, aluminum smelting furnaces, zinc electric distillation furnaces, and distillation equipment in non-ferrous metal smelting; thermal insulation for catalytic cracking units, primary and secondary conversion furnaces in ammonia synthesis units, and in waste heat boilers, waste incinerators, carbon roasting kilns, and various types of resistance furnaces. Aluminum-silicon based thermal insulation materials are currently the most widely used thermal insulation materials for thermal kilns.
Insulation Boards
Insulation boards are unburnt products composed of refractory fibers, refractory raw materials, binders, and additives. They are an important type of thermal insulation refractory material. Insulation boards are classified by material into siliceous, magnesia, forsterite, aluminosilicate, and perlite types. These materials are characterized by low thermal conductivity, low density, and good corrosion resistance, and are mainly used in ingot caps and continuous casting tundishes. Therefore, they are respectively called tundish insulation boards and mold insulation boards. Tundish insulation boards are divided into wall panels, base plates, and impact plates, with performance varying depending on the application. When used in tundishes, these boards can reduce tapping temperature, save fuel, and accelerate tundish turnover. Simultaneously, the permanent lining of the tundish can be used for a longer period, reducing refractory material consumption; ingot insulation boards can improve steel yield.
Insulating refractory castables, such as perlite bricks, are widely used in thermal equipment, piping, and accessories. These products can be manufactured using precast block production processes based on the mix proportions of the castable refractory. In the past decade or so, calcium silicate insulation boards (also called thermal insulation boards) have been developed, with a flexural strength of 0.2~0.6MPa, thermal conductivity of 0.05~0.06W/(m·K), bulk density of 0.13~0.23g/cm³, linear shrinkage ≤2% after firing at 1000℃, and a service temperature of 500~1000℃. These boards are widely used in the insulation layers of various kilns and thermal equipment, offering convenient construction, excellent insulation performance, and the ability to reduce lining thickness.
Refractory fiber is a highly efficient thermal insulation and refractory material. It possesses the softness and high strength of ordinary fibers, allowing it to be processed into various products such as ceramic fiber boards, ceramic fiber modules, ceramic fiber cloth, and ceramic fiber blankets. It also possesses properties not found in ordinary fibers, such as partial oxidation resistance, corrosion resistance, and high-temperature resistance, compensating for the brittleness of ordinary refractory materials. Furthermore, refractory fiber has a low bulk density, typically 0.1~0.2 g/cm³, which is 1/20~1/10 of that of ordinary clay bricks and 1/10~1/5 of that of lightweight clay insulating fire bricks. Therefore, using refractory fiber to replace refractory bricks, standard firebrick bricks can reduce the weight of the furnace body and the thickness of the furnace walls. For example, using fiber lining in a heating furnace can reduce the weight of the original heavy furnace lining by 80% and the furnace wall thickness by 50%. In addition, refractory fiber has a low specific heat, only 1/72 of that of ordinary refractory bricks and 1/42 of that of lightweight clay insulating fire bricks.
Therefore, refractory fibers are frequently used as linings in intermittent kilns, significantly reducing heat loss, saving fuel, and increasing heating rate. Refractory fiber products are increasingly widely used in industrial kilns, achieving significant energy-saving effects. For continuously operating kilns, using refractory fiber products can save 3% to 10% of energy. For intermittently operating thermal equipment, the energy savings from fiber refractory materials are even more pronounced, reaching 10% to 30% or higher. The method of lining refractory fibers has a significant impact on their thermal insulation effect. When using a stacked lining, the fiber direction is perpendicular to the heat flow direction, increasing radiant heat and, under the same conditions, increasing thermal conductivity by 20% to 30% compared to a layered lining. When used as a layered lining, the fiber direction is aligned with the heat flow direction, and the thermal conductivity mainly depends on the product’s thermal conductivity.
Refractory fibers are applied to industrial furnaces in two forms: fiber-lined linings and full-fiber linings. Refractory fiber-lined linings have three application methods: internal insulation, where fibers are pasted or inlaid onto the hot surface of the furnace wall; external insulation, where fibers are pasted or anchored onto the cold surface of the furnace wall; and internal insulation, where fibers are placed in the middle of the furnace wall. This method is rarely used because it has many drawbacks. Refractory fiber linings are generally made of blankets, 20-50mm thick. The use of full-fiber linings can significantly reduce energy consumption, improve furnace thermal operation, save steel, reduce furnace weight, extend furnace life, improve the working environment, and ensure production safety. However, the initial cost is relatively high. There are two traditional structural forms for all-fiber furnace linings: layered and stacked. In current applications, a combination of these two structural forms is typically used, employing a composite lining that combines layered and stacked methods, secured with anchor plates, arranged in an alternating pattern, and riveted together. This type of all-fiber furnace lining maintains the tightness of the layered method while utilizing the strong corrosion resistance of the stacked method, achieving excellent energy-saving effects. It extends the furnace’s lifespan and facilitates enhanced production.
Thermal insulation refractory brick materials can only function effectively when their maximum allowable operating temperature is not exceeded. Therefore, the selection of thermal insulation refractory materials should be based on the furnace’s energy-saving objectives and requirements, structural characteristics, working conditions, and service life requirements. For example, in a copper reverberatory furnace, the non-working layer can be constructed using high-strength lightweight refractory clay bricks, standard firebricks or refractory fibers. The inner lining, due to slag erosion and intense flame impact, can only be constructed with heavy refractory bricks, not lightweight porous thermal insulation materials. The application of thermal insulation refractory material layers on the outer surface or hot side of furnace walls results in different insulation effects. For continuously operating furnaces, external insulation is generally more reasonable, involving the use of dense refractory bricks in the working lining. Using lightweight bricks in the non-working layers significantly reduces heat loss, achieving energy savings of over 10%, while ensuring the strength and stability of the furnace walls. For intermittently operating kilns, while external insulation reduces heat loss, the increase in heat storage loss sometimes exceeds the heat dissipation loss. Therefore, internal insulation is often used to reduce heat storage loss. Some non-smelting furnaces even have their walls entirely constructed of lightweight bricks, standard firebrick bricks, resulting in significant energy savings.
Precautions for Use
Based on the performance and characteristics of thermal insulation refractory materials, the following issues should be carefully considered during use:
(1) Thermal insulation refractory materials have poor slag resistance and cannot be in direct contact with high-temperature melt or high-speed furnace dust.
(2) Thermal insulation refractory materials experience significant shrinkage upon reheating; the long-term service temperature should be at least 50-100℃ lower than their firing temperature.
(3) Thermal insulation refractory products have poor wear resistance and cannot be used in areas with frequent opening and intense vibration.
(4) Thermal insulation refractory products have low mechanical strength and cannot be used in load-bearing areas.
