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What are the types of thermal insulation and refractory materials?

Release time:

2026-05-06 09:32

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Lightweight refractory materials (also known as insulating or thermal-insulating refractories) are characterized by high porosity, low bulk density (typically less than 1.3 g/cm³), and low thermal conductivity. They are primarily used as insulation layers in industrial kilns and thermal-process equipment to reduce heat loss, conserve fuel, lighten the furnace structure, and enhance production efficiency.
Its main characteristics are excellent thermal insulation, but poor slag resistance, mechanical strength, and wear resistance; therefore, it should not be in direct contact with molten slag or liquid metal, nor should it be used in areas subject to severe abrasion or heavy loading.
1. Classification of Lightweight Refractory Materials
Lightweight refractory materials can be classified in various ways; the following are common:
(1) Classification by bulk density
• Lightweight brick: bulk density 0.3–1.3 g/cm³.
• Ultra-lightweight brick: bulk density less than 0.3 g/cm³.
(2) Classification by operating temperature
• Low-temperature insulation material: service temperature 600–900°C.
• Medium-temperature insulation materials: service temperature 900–1200°C.
• High-temperature insulation materials: service temperature exceeding 1200°C.
(3) Classification by Product Shape/Form
• Shaped refractory products: lightweight refractory bricks (such as clay-based, high-alumina, silica, mullite, and alumina-based lightweight bricks).
• Unshaped materials: lightweight refractory castables (lightweight insulating castables), lightweight refractory concretes, etc.
• Other: refractory fibers (such as ceramic fibers), fiber products, calcium silicate boards, microporous insulation materials, alumina hollow sphere products, and the like.
(4) Classification by Main Chemical Composition/Raw Materials
• Clay-based lightweight bricks (Al₂O₃ content: 30%–48%): These are relatively common, produced in large quantities, and typically used at temperatures of 1200–1400°C.
• High-alumina lightweight bricks (Al₂O₃ ≥48%): exhibit high strength and excellent thermal stability, with a service temperature range of 1350–1500°C (higher temperatures can be achieved with ultra-high-purity grades).
• Siliceous lightweight brick (SiO₂ ≥91%): high load-softening temperature, service temperature up to 1550°C, excellent thermal shock resistance, and slight expansion at high temperatures.
• Mullite lightweight bricks: excellent thermal shock resistance, suitable for medium- to high-temperature insulation.
• Other: diatomite bricks (typically used at temperatures ≤1100°C), alumina hollow-sphere bricks, corundum-based bricks, calcium silicate boards, and others.
In terms of manufacturing processes, lightweighting is typically achieved by introducing a large number of pores through methods such as the burn-out additive method (using combustible materials like sawdust), the foam method, chemical methods, or the molten-slag injection method.
2. Main Applications of Lightweight Refractory Materials
Lightweight refractory materials are primarily used as thermal insulation (backing) layers, rather than as working layers that are in direct contact with flames or process materials. Their core function is:
• Reduce heat storage and heat loss in the kiln (resulting in fuel savings of 24% to 45%).
• Achieve rapid heating/cooling to improve equipment efficiency.
• Reduce the weight of the furnace body, simplify the structure, and improve the working environment.
Typical application areas:
• Various industrial kilns: thermal insulation layers or lining backings for ceramic kilns, glass kilns, metallurgical furnaces (including heating furnaces and annealing furnaces), cement rotary kilns, petrochemical cracking furnaces, and others.
• High-temperature pipelines and equipment: thermal insulation for high-temperature flue ducts, hot-air ducts, boilers, and incinerators.
• Other thermal equipment: insulation structures for electric furnaces, induction furnaces, soaking furnaces, and other applications where weight reduction or surface-temperature lowering is required.
• Unshaped materials: Lightweight castables are commonly used for monolithic cast insulation layers in complex-shaped sections, offering convenient construction.
Precautions:
• Not suitable for direct contact with molten slag, glass melt, liquid metal, or areas subject to severe abrasion.
• Low mechanical strength; not suitable for use as a load-bearing structure.
• Selection must take into account the operating temperature, chemical atmosphere (acidic or alkaline), and thermal shock conditions. For example, silica lightweight bricks are suitable for acidic atmospheres, while high-alumina/mullite bricks are better suited to higher temperatures and severe thermal-shock environments.
3. Brief Overview of Development Trends
Modern lightweight refractories are advancing toward lower thermal conductivity (through microporosity), higher strength, composite formulations (such as fiber reinforcement and nano-modification), and green, energy-saving solutions, including the optimization of lightweight aggregates (e.g., expanded clay, hollow glass microspheres, and hollow spheres) and the application of novel unshaped refractory materials.