Refractory materials play a key role in the design of industrial furnace linings. These materials can withstand temperatures above 1580°C. When selecting materials, it’s important to understand their properties and the furnace’s operating conditions. With the right calculations, you can achieve both technical reliability and cost efficiency in your design.
Refractory materials come in many forms and can be grouped into two main categories based on manufacturing methods: shaped refractories and unshaped refractories. Below is a simple explanation of both, provided by Zhengzhou Firebird New Material Co., Ltd.
These are pre-formed blocks or bricks with fixed shapes. If their thermal conductivity at 350°C is less than 0.70W/(m·K), they are called insulating refractories. Shaped products can be made using different forming methods: fired bricks, unfired bricks, or fused cast bricks. Based on their chemical composition, they are divided into:
Acidic bricks:
Silica bricks (SiO₂ content above 93%) and lightweight silica bricks.
Fireclay bricks with SiO₂ over 50% (the rest mainly Al₂O₃), sometimes called semi-acidic.
Neutral bricks:
High alumina bricks (slightly acidic) and chrome bricks (slightly basic).
Basic bricks:
Bricks made mainly from MgO and CaO, such as magnesia bricks, magnesia-alumina bricks, magnesia-chrome bricks, and dolomite bricks.
Special bricks:
High-performance materials with excellent heat resistance, thermal shock resistance, and strength at high temperatures. Examples include zircon bricks, silicon carbide bricks, alumina-silicon carbide bricks, and high-temperature refractory fibers.
Fireclay Brick
These are mixtures of refractory aggregates, binders, and additives. Some are ready to use, while others need to be mixed with liquids on-site. They are divided into dense and insulating types. Based on the hardening method and type of binder, they include:
Ceramic bonded – Harden by sintering during heating.
Hydraulic bonded – Set at room temperature via hydration.
Chemical bonded – Harden at low or room temperature through chemical reactions.
Organic bonded – Harden at room or slightly elevated temperatures.
Unshaped materials can also be grouped by usage:
Refractory ramming mix: Can be used directly or mixed with liquid and applied by ramming. It hardens under heat.
Refractory plastic: Delivered in soft lumps, shaped by ramming, vibration, or pressing, then hardens upon heating.
Refractory castable: Supplied as dry mix, used after adding water or other liquids. Applied by casting, vibrating, or tamping, and hardens without heat.
Refractory spraying mix: Specially made for pneumatic or mechanical spraying. Hardening methods follow one of the types above.
Refractory mortar: Applied with trowels or used to fill joints by dipping or pouring. It can be hydraulic, ceramic, or chemical bonded.
Refractory coating: Made of fine aggregates and binders, with higher liquid content than mortar. Applied manually or by spraying tools.
Based on the main raw materials or functional ingredients, unshaped refractories are further named, such as high alumina plastic, low-cement clay castable, or ultra-low-cement alumina castable.
Nano Thermal Insulation Sealing Mix — Furnace Roof Insulation
Smaller footprint and no need for shaping and firing processes—lower energy use and cost.
Fewer types of special-shaped bricks required—easier inventory and construction.
Better lining integrity—higher strength and airtightness with lower heat loss.
With metal or ceramic anchors and fibers, they offer excellent thermal shock and mechanical resistance and are easy to repair.
Easier to store, transport, and apply using machines. However, strict construction control is necessary to maintain performance.
In some cases, unshaped materials can be pre-cast into blocks for use.
Dense shaped products and dense unshaped materials are mainly used as the hot face layer in high-temperature furnaces. Insulating bricks and lightweight castables are typically used in the middle layer or as insulation layers in medium- and low-temperature furnaces. Most furnace linings use a multi-layer structure, with the innermost layer focused on heat resistance and the outer layers on insulation.
Using composite linings helps reduce heat and energy loss, speeds up heating/cooling, improves furnace efficiency, and reduces the overall furnace weight. It also simplifies structure and increases productivity.
Refractory materials should be evaluated based on:
Mechanical properties – e.g., compressive strength, flexural strength.
Chemical resistance – e.g., slag resistance, atmosphere compatibility.
Physical properties – e.g., bulk density, refractoriness, softening under load, thermal expansion, thermal shock resistance, thermal conductivity, specific heat capacity.
Choose materials based on a balanced evaluation of all needed properties. Final selection should also consider budget and expected return on investment. Some performance factors, like service life, are hard to predict, so experience plays a big role in choosing the right material.
The trend is moving toward more diverse, high-performance, and energy-saving materials. With advancements in production technology, refractory fibers are now being used not only for insulation but also for high-temperature working layers.
The goal is to select materials tailored to the furnace type and working zones, improve design by using multi-layer linings, and match the service life of each layer. When several options are available, choose the one that offers the best cost-performance ratio.
