Refractory: The Definition:
Refractory is basically the definition of crystal form, i.e., polycrystalline, having multiple phases, i.e., polyphase, heterogeneous, inorganic, and non-metallic material which has a high melting point and which can retain its properties at very high temperatures.
Characteristics of Refractory:
• Chemical Composition
• Method of Manufacturing
• Refractoriness
• Fusion Temperature
• Thermal Conductivity
Chemical-based composition: There are 3 basic chemical-based composition:
a) Acidic refractories
b) Basic refractories
c) Neutral refractories
Acidic refractories: In general, acidic refractories are impermeable to acidic materials but easily targeted by basic materials, and are often utilized in acidic conditions with acidic slag. Substances such as silica, alumina, and refractory brick are utilized.
Basic refractories: In areas where slag and atmosphere are important, basic refractories are used. They are stable against alkaline materials, but they may react to acids. Magnesia (MgO) is a typical example of the main raw material. Other examples include dolomite and magnesia-chrome.
Neutral refractories: Neutral refractories are integrated where slag and the atmosphere are either acidic or basic and chemically firm to both acids and bases.Chromium (Cr2O3), alumina (Al2O3), and carbon are typical examples of these materials.
Manufacturing based methods:
1. Dry press process
2. Fused cast
3. Hand molded
4. Formed (chemically bonded, normal or fired)
5. Un-formed (castables, mortars, monolithic-plastic, ramming and gunning mass, dry vibrating cements)
Refractoriness: Refractoriness is the property of a multi-phase refractory to achieve a particular degree of softening at high temperatures without load and is calculated with a PCE test. PCE stands for ‘Pyrometric Cone Equivalent’ which is used by putting several different PCE cones alongside an unknown raw material to assess the “Pyrometric Cone Equivalent” of an unknown raw material. In industrial kilns, pottery kilns, and small hobby kilns, pyrometric cones are used worldwide to track ceramic firings, where the consistent temperature is critical for the final product’s quality being burned. Heat work, the impact of time, and temperature are determined by pyrometric cones.
• Low duty (PCE value 19-28)
• Intermediate duty (PCE 28-30)
• High Duty (PCE 30-33)
• Super Duty (PCE 33-38)
Based on Fusion Temperature: Based on fusion temperature (melting point), refractory materials are divided into three forms.
• The fusion temperature of Normal refractories is between 1580 – 1780 °C (e.g., Fire clay).
• The fusion temperature of the high refractories is between 1780 – 2000 °C (e.g., Chromite).
• The fusion temperature of the Super refractories is less than 2000 °C (e.g., Zirconia).
Based on Thermal Conductivity: Thermal conductivity may identify refractories as either conducting, non-conducting, or insulating. Silicon carbide and Zirconium carbide are examples of conducting refractories, while silica and alumina are examples of non-conducting refractories. Calcium silicate materials, kaolin, and zirconia contain insulating refractories.
Furnace walls use insulating refractories to minimize the rate of heat loss. Because of a high degree of porosity, these refractories have low thermal conductivity, with a desired porous structure of small, uniform pores uniformly distributed across the refractory brick to reduce thermal conductivity. It is possible to further classify insulating refractories into four types:
• Insulating heat-resistant materials with production temperatures less than or equal to 1100 ºC
• Refractory insulating materials with temperatures of application less than or equal to 1400 ºC
• High refractory insulating materials with temperatures of application less than or equal 1700 ºC.
• Ultra-high refractory insulating materials with temperatures of operation is less than or equal to 2000 ºC
Type and application of refractory materials:
For different applications, there are several types of refractories are available such as high alumina refractories, Fireclay refractories, Magnesite refractories, Chromite refractories, Zirconia refractories, silica bricks, Monolithic refractories. Also, bricks of different sizes & shapes are used in diverse application.
Different types of refractories are used depending on the temperatures and the application’s working conditions, such as furnaces, ovens, boilers, kilns, etc.
High alumina refractories: Generally, High Alumina contains more than 45% of alumina. The concentration of alumina ranges from 45% to 95% with a growing percentage of aluminum, high alumina refractory refraction is rising. The melting of a vast variety of metals, shaft blast furnaces, cement furnaces, lime & ceramic furnaces, etc. are few examples that use high refractory alumina.
Fireclay Refractories: Fireclay refractories are mainly hydrated silicates of aluminum 30% – 45% Al2O3, 50% – 85% SiO2, and other minor minerals. Fireclay brick, as it is relatively inexpensive, and refractory brick is the most common type and has significant use in most ovens, regenerators, furnaces, stoves, etc.
Magnesite Refractories: Magnesite refractories, which contain more than 80% of magnesium oxide (MgO), are chemically important materials. They consist of MgCO3 (Magnesite) and Silica naturally present (SiO2). The physical characteristics are generally poor and their high value lies mainly in their resistance to basic slags, particularly to slags rich in limestone and iron. These are the most important refractory community for basic steelmaking processes. Apart from metallurgical furnaces, simple brick is now used in glass tank inspectors and lime and cement stovetops successfully.
Chromite Refractories: Chrome-magnesite content usually contains 15-40 percent Chromia and 40-50 percent sintered magnesia, while at least 60 percent MgO and 8-18 percent Cr2O3 are present in magnesite-chromite refractories. Chrome-magnesite refractories are used for the construction of high-temperature furnace critical paths. These materials can withstand and have elevated refractoriness from corrosive slags and gases.
Zirconia Refractories: Zirconia refractories have a very high room temperature, power that is sustained up to 15000C. It is observed that its thermal conductivity is much lower than that of most other refractories. With liquid metals and molten glasses, zirconia also does not react easily. They are, therefore, useful for metallurgical furnaces and glass furnaces as high-temperature building materials.
Silica bricks: Silica brick has more than 92 percent SiO2 as a refractory material. Quality rocks are raw material. Silica brick possesses outstanding mechanical strength at its present fusion point at temperatures. This compares with many other refractories, for instance, aluminum-silicates, which begin to fuse and creep dramatically below their fusion points at high temperatures. In the glass and steel industry, a wide variety of grades of silica brick are being used.
Monolithic Refractories-Powder form: Monolithic refractories are generally powder form materials that eventually harden to create a solid mass, the name was typically given to all non-shaped refractory products. In many applications including those of industrial furnaces, monolithic refractories replace the conventional type of fired refractories at a much faster rate.
Conclusion:
Refractory is a critical component in the design of any refractory-based application. It can be designed to meet the unique needs of the application. Wisely choose to prevent failure.
