Refractory materials are a crucial component of industrial furnace lining design. Their refractoriness is typically specified to be above 1580°C. During design, it's important to understand the refractory material's characteristics and the lining's operational requirements, and perform quantitative calculations to ensure the finished product is technologically advanced and economically sound.

Material Classification

Refractory materials come in a wide variety of types, which can be divided into shaped refractory products and unshaped refractory materials based on the manufacturing process. Helu Energy Saving technicians can help you understand the concepts and differences between these two types of materials:

Shaped Refractory Products

Shaped refractory products with a specific shape, whose thermal conductivity at 350°C is less than 0.70 W/(m/K), are called insulating refractory products. Shaped refractory products are categorized by the forming process as fired bricks, unfired bricks, and fused cast bricks. According to chemical properties and mineral composition, they are divided into:

(1) Acidic bricks: refers to silica bricks and lightweight silica bricks with SiO2 content of more than 93% and clay refractory bricks with SiO2 content of more than 50% (the rest is Al2O3 and other minerals). The last type is also called semi-acidic bricks.

(2) Neutral bricks: generally refers to high-alumina bricks (slightly acidic) and chrome bricks (slightly alkaline);

(3) Alkaline bricks: magnesia bricks, magnesia-alumina bricks, magnesia-chrome bricks and dolomite bricks with MgO and CaO as the main components.

(4) Special bricks: materials with special properties such as high melting point, good chemical stability and thermal shock resistance, and high high temperature strength. Usually include pure oxides, non-oxides and metal ceramics, such as zircon bricks, silicon carbide bricks, brown corundum-silicon carbide bricks, etc. High-temperature refractory fibers also belong to this category.

Amorphous refractory materials

Refractory materials made of a mixture of refractory aggregates, binders and additives. Some are directly applied in the delivered state, while others are applied after being mixed with certain liquids. Amorphous refractory materials are of two types: dense and heat-insulating. According to the hardening process and the nature of the binder, they are divided into four categories: ceramic bonding (hardening due to sintering during the calcination process), hydraulic bonding (hardening by condensation at room temperature and hydration reaction), chemical bonding (hardening by chemical reaction rather than hydration reaction at room temperature or lower than the temperature of ceramic bonding), and organic bonding (bonding or hardening at room temperature or slightly higher temperature). Amorphous refractory materials can also be divided into three categories according to the type of use: integral construction materials and repair materials, masonry and joint materials, and refractory coating materials. 1. Integral construction materials and repair materials include: (1) Refractory ramming materials. They can be applied directly or mixed with certain liquids and applied by ramming. Most of them are hardened by heating above room temperature. (2) Refractory plastics. Delivered as a soft, plastic billet or irregularly shaped mass, ready for direct use, applied by ramming, vibration, pressing or extrusion, and hardened during calcination. (3) Refractory castables. Delivered as dry material, applied after adding water or mixing with other liquids, applied by pouring, vibration, ramming, and compacting when necessary, and can solidify and harden without heating. (4) Refractory spray coating. A specially prepared mixture for application by pneumatic tools or mechanical spraying, with hardening characteristics that can be classified into one of the first three types. 2. Masonry and jointing materials, namely refractory slurry. Applied with a trowel or similar tool, it can also be used for grouting or dipping blocks, and is divided into three types: water-hardened bond, ceramic bond, and chemical bond. 3. Refractory coatings are a mixture of fine aggregate and binder. Various binders can be used, and the water (or other liquid) content is generally higher than that of refractory slurry. Applied by hand, pneumatic tools, or mechanical spraying. On the basis of the above classification method, the mixture is further classified according to the properties of the main components and (or) the aggregates that determine the characteristics of the mixture and named accordingly, such as high-alumina refractory plastics, clay-based low-cement refractory castables and corundum-based ultra-low-cement refractory castables. Compared with shaped refractory products, the advantages of amorphous refractory materials are: (1) The factory occupies a small area, does not require molding and firing processes, consumes very little energy, and has low investment and operating costs. (2) The variety and quantity of special-shaped bricks can be significantly reduced, which is convenient for material preparation and construction. (3) The furnace lining has good integrity, improves strength and airtightness, and reduces heat loss. (4) After being equipped with metal or ceramic anchors or reinforced with stainless steel fibers, it can improve the resistance to mechanical vibration and thermal shock, and can also locally repair damaged parts. (5) It is easy to store and transport, which is conducive to the mechanization of furnace construction. However, construction management must be strict, otherwise it will affect the physical and chemical properties and life of the furnace lining. Sometimes, amorphous refractory materials are made into prefabricated components for use. Material selection Refractory products and dense amorphous refractory materials are mainly used for the refractory layer of high-temperature furnace linings. Insulating refractory products and lightweight castables are generally used between the refractory layer and the insulation layer of high-temperature furnace linings, or directly as the insulation layer of medium and low-temperature furnace linings, in addition to being used for the refractory layer of medium-temperature furnace linings. It is rare to use a single refractory material for furnace linings. The most common is a composite lining: the innermost layer is mainly for refractory performance, and the remaining layers are mainly for insulation performance. Sometimes diatomaceous earth refractory bricks are also included in the list of insulating refractory products. The insulation layer not only reduces heat dissipation loss and heat storage loss, but also increases the furnace heating and cooling speed, as well as the furnace efficiency, operating rate and output. In addition, it simplifies the furnace structure and reduces the furnace weight. The performance of refractory materials has three aspects: (1) Mechanical properties. Such as compressive strength and flexural strength. (2) Chemical properties. Such as slag resistance and reaction with controlled atmosphere. (3) Physical properties. Such as bulk density, refractoriness, load softening temperature, thermal expansion coefficient, reburning line change rate, thermal shock stability, thermal conductivity and specific heat capacity. According to the purpose of refractory materials, start from the comprehensive comparison of various required properties, select several refractory materials respectively, and finally confirm them based on investment and economic benefits. Since some important indicators (such as service life) are difficult to quantify, they can only be arranged and compared according to priority when selecting materials. Therefore, design experience and usage experience still play a considerable role in material selection. The development trend of furnace refractory materials is variety diversification, material quality improvement and energy saving. With the development of production technology, in addition to the refractory insulation layer of the furnace lining, high-temperature resistant refractory fiber products are gradually being promoted for the refractory layer of the furnace lining. The goal of material selection is to select the appropriate refractory material type according to the type of furnace and different parts of use, and at the same time improve the furnace structure design and construct a multi-layer composite furnace lining so that the layers of materials in different temperature zones fail at close to the same time, thereby extending the overall service life of the furnace lining. When different materials are available for application under the same usage conditions, materials with good economic benefits should be used first.