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Modern materials in today’s economy require a fast turnaround. Furnace manufacturers and refractory suppliers are working together to shorten the firing cycles wherever possible. Continuous furnaces take care of some of these requirements. However, today’s dynamic markets are forcing some manufacturers to again use discontinuous furnaces.
Increased temperatures and drastically shortened firing cycles require new refractory materials, anchoring systems and furnace designs. A refractory fiber system* introduced approximately eight years ago for small laboratory furnaces has helped end users optimize their manufacturing process or even develop new materials such as technical ceramics. The system can be installed where alumina fiber modules can no longer be used due to very high application temperatures (above 1600°C). The system’s aluminum oxide (or polycrystalline ceramic) fibers have an aluminum oxide (Al2O3) content greater than 72% and are well known for scientific applications and insulation in laboratory furnaces.
However, these fibers aren’t just limited to laboratory applications. Recent developments have enabled these same materials to be successfully used to insulate large industrial equipment for production processes.
Insulation MaterialsTraditionally, ceramic kilns and furnaces have been lined with insulated firebrick (IFB). However, fiber linings can provide several advantages over IFB linings:
Depending on the use of the furnace, the heat storage after firing might be yet another disadvantage of the brick lining. Simply shutting the door can cause the following cycles to heat up too quickly, causing damage to the products being fired.
In short, a fiber-lined furnace can be used in a much more flexible way than any brick-lined furnace. But not all fibers are created equal. Most fiber systems are unable to handle the high heat required for fast firing applications. However, Al2O3 fibers have been developed specifically for this purpose.
Aluminum Oxide Fibers. Ceramic fibers with an Al2O3 content >60% cannot be produced by spinning or blowing from molten material as is the case for standard ceramic (amorphous ceramic fibers or alumino silicate fibers). The reason for this limitation is that rising Al2O3 content increases the surface tension of the melt too high at spinning viscosity.
The production technique for Al2O3 fibers is based on a process in which the fibers are obtained from an aqueous solution consisting of aluminum salt, silica sol and an organic polymer. Fibers obtained from this solution during the first production step still contain inorganic salts and organic materials. The organic compounds are burned, and the inorganic salts disintegrated at temperatures between 500 to 1000°C (932 to 1832°F). Subsequently, the fiber is crystallized at temperatures up to 1400°C (2552°F).
The application of fiber products is mainly restricted by shrinkage. Due to the crystalline structure of the Al2O3 fibers already obtained during the production process, their shrinkage is significantly lower than that of alumino silicate fibers.
Vacuum-Formed Ceramic Fiber Products. Al2O3 fibers by themselves can be used at temperatures up to 1650°C (3002°F), depending on a variety of conditions. To achieve higher application temperatures, these fibers can be used as a basic raw material for vacuum-formed ceramic fiberboards and shapes.
By adding silica, mullite and alumina fillers and binders, the possible application temperature can be increased to 1800°C (3272°F). The fiber shrinkage can be minimized and compensated for by means of mullitization—the reaction between alumina and silica to form mullite, where the density of mullite is lower than the stochiometric density calculated from alumina and silica. The resulting growth is used to compensate for the fiber shrinkage.
Ceramic Coatings. To optimize the fiber lining, Al2O3 fiber coatings can be applied. The coatings will help increase the reflection on the surface of the lining and save energy. At the same time, the coatings put the fibrous material into the radiation shadow, which decreases long-term shrinkage.
Mild chemical attack from off-gassing of sodium, potassium, magnesia, etc., can also be minimized by applying a coating. The coating and alkalis form low-melting eutetics, which create a dense glaze on the hot face. Such self-glazing effects are well known in the porcelain industry.
Some products require a high permeability lining, in which case no coating should be applied. Such examples can be found in the sol-gel ceramic industry, as well in the soft ferrite industry. To minimize the attack on the ceramic fiber, the lining is put under positive pressure from outside. The gases are removed through lined flues. In this way, the lining is cooled and kept clean from the corrosive gases. This technique is used in both electrical and gas fired furnaces.
Anchoring Systems. Only few anchoring systems can withstand the temperatures discussed in this article. In any of these applications, the anchors must be designed to withstand the calculated temperatures, as well as temperatures that might be developed during use due to shrinkage, creep, and thermal expansion and contraction.
Mullite, alumina and SiC anchors are used to mount boards, board modules and composite covers. A modular system has been developed to enable the construction of any size furnace. To achieve temperatures up to 1800°C (3272°F) and withstand very rapid temperature changes, the anchors must be hidden within the insulation.
Refractory Lining DesignThe most common rapid-fire* furnaces around the world operate at temperatures between 1650°C (3002°F) and 1750°C (3182°F) and use vacuum-formed Al2O3 ceramic fiberboards on the hot face. The fiberboards can be high-fired for optimum performance. Small “butcher blocks” (edge grain-adhered strips of boards) provide the highest thermal shock resistance. The back insulation incorporates vacuum-formed boards and blankets properly rated to the heat flow analyses.
Since this type of application stretches the limits of the insulation and anchoring system, a well-designed layout is required. The layout must account for thermal expansion, shrinkage and growth, as well as atmospheric effects. Special gas atmospheres, especially if reducing, can limit the maximum service temperature.
In general, the furnaces are insulated with several layers that are selected based upon the individual thermal profile. This allows for the most cost-effective selection of materials. Rapid-fire creates extreme temperature gradients throughout the insulation layers. Therefore, the insulation segments should be rather small to minimize thermal and mechanical stresses.
The Al2O3 fiberboard composite is a modular design, which allows the original equipment manufacturer (OEM) to design the furnace to the end user’s requirements. The system uses vacuum-formed ceramic fiber material, both mullite- and alumina-bonded. These materials can be adjusted in chemistry, application temperature and pre-firing to meet specific requirements.
Pre-shaped insulation blocks, tongue and groove boards and hanging composite covers reduce the design work for the OEM. At the same time, the installation can be done in a fraction of the time it takes to build a brick furnace. The furnace can be used right after installation without a long curing time.
Furnace ApplicationsThe Al2O3 fiberboard composite can be used in almost any application. However, the main end users are manufacturers that produce small parts that can tolerate rapid-fire conditions and fast cycles.
The Al2O3 fiberboard composite is also used for double covers where the inner roof can withstand cold face temperatures up to 1400°C (2552°F). This technique uses the same basic ideas as double-arch constructions, but the fiber properties generate a much more dynamic system.
MaintenanceTo create the longest life possible, the fiber lining should be maintained properly. After two years of service, the micro-cracks in the lining should be filled with a mastic and all surfaces should be re-coated with the ceramic fiber coating. This will keep the micro-cracks from opening and will help keep the lining in good shape for years to come. A maintenance job on a furnace containing 13 m3 of fiber lining takes about five hours and is a very cost-effective way of keeping the furnace operating in good condition for many years.
A Flexible AlternativeAl2O3 fiber linings have been used in laboratory R&D applications for years. Now they’re finding new applications. With the development of vacuum-formed fiberboards and protective coatings, Al2O3 fiber products have become a more flexible, energy-efficient alternative to IFB linings in high-temperature, fast firing industrial kilns and furnaces.
For More InformationFor more information about Al2O3 fiber linings, contact Rath Performance Fibers, 501 Silverside Rd., Suite 131, Wilmington, DE 19809; (302) 793-0282; fax (302) 793-0289; firstname.lastname@example.org.
SIDEBAR: A Demonstration InstallationA 10 cubic meter (~353 cubic foot) 1750°C (3182°F) furnace was installed in early 1998 at the main works of Rath Austria (see photo). The purpose of this unit was twofold: 1) to produce insulating firebricks (IFBs) up to grade 34 and special bricks for waste incineration, and 2) to demonstrate high-temperature refractory materials.
The furnace features heavy duty castable in the kiln cars, IFB 34 lining in the sidewalls and Al2O3 fiber materials in back wall and roof. Even though this furnace will be used for many different types of firings, all furnace sections have been designed for uniform furnace temperature.
This application was the first in which the Al2O3 fiberboard composite was installed in such a high volume—as well as gas-fired—furnace.
Results After Two Years
Parts of the brick wall moved more than planned and caused a 1⁄2-in. step in the left sidewall within the first few firings, then stabilized. One burner block has been slightly pushed out of its firing angle. However, both effects are superficial and do not minimize performance.
The most critical part of every furnace is usually the roof. In enlarging the Al2O3 fiberboard composite lining from laboratory size to 10 cubic meters, some lessons still had to be learned. The roof segments installed in this furnace are 14 x 14 in. The photo above shows the hot face of such a segment with a few micro-cracks in it. Reducing the segment size to 10 x 10 in. eliminated this issue in later units.
The fiber back wall also shows cracks and gaps, which, in most cases, split the tongue and groove boards in half. In newer furnaces, the wall system has been optimized by using semi-size tongue and groove boards, forcing all of the thermal expansion and contraction into the expansion gaps.
The color of the refractory lining is dictated by chromium released by firing bricks for waste incineration applications. Even in those areas where the fiber turned pink, there is no increased shrinkage compared to the white areas in the furnace (see photo).
The burner, which has been pushed out of its original blasting angle, has turned slightly toward the fiber back wall. This caused local surface melting and spider cracking. Even so, this surface did not negatively affect the performance of the lining.
The performance of the fiber lining over a two year period convinced the last remaining skeptics that fiber linings can be successfully used in industrial furnaces between 1600°C (2912°F) and 1800°C (3272°F).