High-Temperature Insulating Wools: Classification (part 1)
Back in the 1980s when I first started working in refractories, an old refractory foreman at Bethlehem Steel told me, “There are no bad refractories; you just put them in the wrong spot.” Now all these years later, I’ve come to realize he was right on both counts. There are no bad refractories, and 50 isn’t old.
Every refractory material has both technological and economic advantages and disadvantages relative to the specific application. For the correct selection, it is important for the plant operator, the kiln/furnace supplier and the refractory supplier to work in partnership to achieve the optimum technical and economic solution. The solution presented is based on using high-temperature insulating wool (HTIW) products, which boast significant advantages compared to traditional refractory products on investment cost, operating expenses, reliability, overall efficiency and the fast availability of the equipment following relining or maintenance and repair work.
What is high-temperature insulating wool (HTIW)? HTIW in the form of alumino-silicate fiber (ASW; Rath ALSITRA) and mullite polycrystalline fiber (PCW; Rath ALTRA® 72) provide excellent chemical, physical and thermomechanical properties and belong in this group of high-temperature insulating wools along with alkaline-earth silicate fiber (AES). For our North American colleagues, refractory ceramic fiber (RCF) and polycrystalline fiber are called HTIWool in most of the world.
As a result of increased requirements on industrial furnaces with application temperatures above 900°C (1652°F), the use of HTIW has increased greatly in the last decade. Much of this demand is where the insulation is exposed to high thermomechanical (temperature shock), mechanical or chemical stresses. Considering this general demand, special ultra-lightweight products made of HTIW with their outstanding thermal, thermomechanical and chemical properties are particularly suitable for application in modern industrial furnaces.
The advantages of these materials are obvious.
- Optimized specific energy consumption (with up to 50% energy savings compared to conventional dense/heavy linings)
- Increase in the overall efficiency of high-temperature furnaces
- Reduction of greenhouse-gas emissions
- Excellent chemical stability
- Outstanding thermomechanical properties (e.g., almost unlimited thermal shock resistance)
This paper presents an overview and examples of the use of HTIW products in thermal-process engineering in the steel industry. Examples include:
- Module lining (combined systems) in forging furnaces
- Ultra-lightweight burner blocks
- Ultra-lightweight insulation for water-cooled rollers in roller-hearth furnaces (e.g., continuous for casting/thin casting compact-strip production – CSP/thin-slab casting)
Refractory Materials and Products of HTIW
Refractory materials can be classified into four main groups in accordance with Fig. 1:
- Dense, shaped refractory products
- Unshaped refractory materials (monolithic)
- Functional refractory products
- Heat-insulating materials
The main group of heat-insulating materials includes HTIW products, heat-insulating and insulating refractory bricks and other heat-insulating materials (e.g., calcium-silicate and microporous materials, etc.).
An overview of the HTIW products is also shown in Fig. 1. The range of products formed from these ultra-lightweight materials extends from wool through mats/blankets and modules to vacuum-formed products in the form of boards, functional products and shaped components.
HTIWs used as raw materials for the above-mentioned ultra-lightweight refractories are part of the group of inorganic, man-made mineral wools. An overview of HTIWs is shown in Fig. 2.
The products made from alumino-silicate wool (ALSITRA) and polycrystalline wool (ALTRA) are undoubtedly most important in the group of HTIW for thermal-processing installations and industrial furnace construction. Thanks particularly to their outstanding technical properties, these products are now indispensable for a wide range of industrial applications in the temperature range of 600-1800°C (1110-3270°F).
Products made of AES, another form of high-temperature insulation wool, exhibit lower chemical resistance and are more prone to recrystallization, thereby limiting their potential application in thermal-process engineering. The main application for these AES materials is in the domestic appliance industry and in industrial processes for temperatures to a maximum of 900°C.
Alumino-silicate wool (ALSITRA) and the AES wools are produced in a melting process and a subsequent blowing or spinning process. Crystallization, shrinkage and sintering processes limit the application temperature of these products to below 1300°C (2370°F).
In contrast, HTIWs on the basis of alumina (PCW, e.g., ALTRA 72) are produced with sol-gel technology and heat-treated at temperatures up to 1400°C (2550°F) during the production process. The materials produced in this way have classification and application temperatures up to 1650°C (3000°F). Application-specific and optimized delivery forms (vacuum-formed components ALTRAFORM) ensure the suitability of these products up to temperatures of 1800°C.
Table 1 lists the most important physical and chemical properties for the evaluation of HTIWs. The classification temperature of HTIWs is defined as the temperature at which a permanent linear change (shrinkage) of 4% is not exceeded after 24-hour heat treatment in an electrically heated laboratory furnace in a neutral atmosphere.
The actual maximum application temperatures of amorphous HTIW (ASW and AES wool) are generally at least 150-200°C (safety allowance) below this classification temperature. This is because, in contrast to the determination of the classification temperature in ideal, neutral-firing conditions with a relatively short exposure (24 hours), the products used in the field are not only exposed to high temperatures but to additional chemical and physical stresses that often deviate far from ideal conditions and therefore limit the application temperature.
In contrast, products made of polycrystalline high-temperature wool (PCW; e.g., ALTRA) can be used without a safety allowance up to the actual classification temperature, even in industrial applications.
Besides the typical chemical compositions and the resulting classification temperatures, the actual application temperatures under process conditions, chemical resistance to acids and bases and apparent density are important for the use of the materials in industrial furnaces. These conditions differ widely in the field of thermal-process engineering applications in which HTIW is used.
The most suitable materials, and especially the most appropriate high-temperature insulation wool for the respective application, can be selected based on the specifications in TRGS 619 (Technical Rules for Hazardous Substances). This technical guideline is a valuable aid to all involved with thermal-processing installations – plant operators, furnace and refractories suppliers – in the selection of a suitable refractory material.
At the same time, TRGS 619 provides an excellent possibility for the documentation of the furnace lining concept for regulatory agencies and/or for those responsible for occupational health and safety and environmental protection.
The competent user and furnace supplier will, in consideration of this directive, opt for the use of HTIW as refractory lining material for a thermal-processing plant providing this material proves technically suitable in an objective analysis.
Figure 3 shows, in simplified and clear form, the possible temperature ranges for the application of HTIW products, with indication of the frequency of application in the respective temperature window.
With regard to the enormous technical developments in the construction of industrial furnaces and the energy savings that have only been made possible thanks to the application of high-temperature insulation wool products in different industrial sectors, thermal insulation for progressive companies with state-of-the-art processes and temperatures exceeding 900°C would be unthinkable without these materials.[7,8]
In part 2, we will continue our discussion of the benefits of HTIW and cover some specific applications such as burner blocks and roller-hearth furnaces.
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- VDI-Richtllinie 3469 ”Emissionsminde - rung – Herstellung und Verarbeitung von faserhaltigen Materialien – Hochtempe – raturwollen,” Blatt 5, 03/2007
- DIN EN 1094
- Technische Regel für Gefahrenstoffe 619, ”Substitution für Produkte aus Alumini-umsilikatwolle (TRGS 619),” Ausgabe February 2007
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