What Material Is Used for Furnace Lining?

Refractory materials are a critical component in the design of industrial furnace linings. They are typically defined as materials with a refractory temperature above 1580°C. When designing furnace linings, it is essential to understand the characteristics of refractory materials and the service requirements of the lining. Quantitative calculations should be conducted to ensure the final design is both technically advanced and economically efficient.

Classification of Refractory Materials

Refractory materials come in a wide variety of types and can be classified based on manufacturing process into shaped refractory products and monolithic (unshaped) refractory materials. Firebird’s technical experts provide the following explanation of the concepts and differences between these two categories:

1. Shaped Refractory Products

These are refractory materials that have a defined shape. If their thermal conductivity at 350°C is lower than 0.70 W/(m·K), they are classified as insulating refractory products. Based on forming technology, shaped refractories can be divided into fired bricks, unfired bricks, and electro-fused cast bricks. According to chemical characteristics and mineral composition, they are further classified as:

• Acidic bricks: Include silica bricks and lightweight silica bricks with SiO₂ content over 93%, and fireclay bricks with SiO₂ content above 50% (the rest mainly Al₂O₃), often referred to as semi-acidic bricks.
• Neutral bricks: Typically high alumina bricks (slightly acidic) and chrome bricks (slightly basic).
• Basic bricks: Include magnesia bricks, magnesia-alumina bricks, magnesia-chrome bricks, and dolomite bricks, primarily composed of MgO and CaO.
• Special refractory bricks: Designed for high melting point, excellent chemical stability, thermal shock resistance, and high-temperature strength. This group includes pure oxides, non-oxides, and cermet products, such as zircon bricks, silicon carbide bricks, and alumina-silicon carbide bricks. High-temperature refractory fibers also belong to this category.

2. Monolithic (Unshaped) Refractory Materials

These are mixtures of refractory aggregates, binders, and additives. Some are supplied ready to use, while others require mixing with liquids prior to installation. Monolithic refractories can be either dense or insulating. They are classified by hardening mechanism and binder type into:

• Ceramic-bonded: Harden by sintering during firing.
• Hydraulic-bonded: Harden at ambient temperature through hydration reactions.
• Chemical-bonded: Harden at room or low temperatures via chemical reactions (excluding hydration).
• Organic-bonded: Harden through physical bonding or slight heating.

Based on application, monolithic refractories are categorized into construction and repair materials, refractory mortars, and refractory coatings.

1) Construction and Repair Materials:

• Refractory ramming mixes: Supplied dry or semi-wet, installed by ramming, and hardened under heat.
• Refractory plastics: Delivered in plastic form, installed by pressing or ramming, hardened during firing.
• Refractory castables: Mixed with liquids on-site, installed by casting or tamping, harden at room temperature.
• Refractory gunning mixes: Applied using spraying equipment, bonding type varies.

2) Refractory Mortars:

Used for bricklaying and joint filling. Types include hydraulic, ceramic, and chemical bonding.

3) Refractory Coatings:

Mixtures of fine aggregates and binders with high liquid content. Applied manually or by spraying. Named according to aggregate type, e.g., high-alumina plastic, low-cement clay castable, ultra-low cement alumina castable.

Advantages of Monolithic Refractories:

• Smaller plant footprint, no shaping/firing needed, low energy consumption.
• Fewer special bricks, simpler inventory and construction.
• Better lining integrity, strength, and gas tightness.
• Enhanced vibration and thermal shock resistance with metal/ceramic anchors or fiber reinforcement.
• Easy storage, transport, and suitability for mechanized construction.

Note: Strict construction management is required. Some monolithic refractories are used as precast blocks.

3. Material Selection

Dense refractory products and dense castables are used in the refractory layer of high-temperature furnaces. Insulating bricks and lightweight castables are used in medium-temperature furnaces or as insulation between refractory and insulating layers in high-temperature furnaces. Most furnace linings are composite linings, where the inner layer focuses on refractoriness and outer layers on insulation.

Insulation layers reduce heat loss, improve heating/cooling rates, increase efficiency and output, and simplify design and structure.

Key Properties of Refractory Materials

Refractory materials exhibit three categories of properties:

• Mechanical: Compressive strength, flexural strength.
• Chemical: Slag resistance, reaction to furnace atmosphere.
• Physical: Bulk density, refractoriness, softening temperature, thermal expansion, permanent linear change, thermal shock resistance, thermal conductivity, specific heat capacity.

Selection is based on comprehensive performance comparison. Some indicators (e.g., service life) depend on practical experience. Design and usage history play a key role in prioritizing materials.

Trends and Goals in Furnace Lining Material Selection

Furnace refractory development is shifting toward diversified types, higher quality, and energy-saving performance. With advancements in production, refractory fibers are now used not only in insulation layers but also in refractory working layers.

The goal of selection is to match the material type to the furnace area, optimize structure, and build multi-layer composite linings so all materials wear out simultaneously. Among available options, the most cost-effective solution should be chosen.

Partner with Firebird for Your Refractory Material Needs

At Firebird, we specialize in providing high-quality refractory materials tailored to meet the specific needs of your industrial furnace applications. Whether you’re looking for shaped refractories, monolithic materials, or advanced insulating solutions, our team of experts is here to help you select the perfect materials for optimal performance and cost efficiency. Reach out to us today to discuss your project and discover how Firebird can enhance the reliability and longevity of your furnace linings.