Refractories

SPECIAL REPORT/REFRACTORIES:High-Temperature Fiber Products

March 1, 2011
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Fiber-based insulation products offer superior thermal efficiency for high-temperature kilns.


Since its invention and commercialization by The Carborundum Co. in the 1950s, ceramic fiber insulation has been used in many industrial applications. Over the years, extensive research and development has resulted in the introduction of different fiber chemistries and product forms into the refractory market. Examples of advances include soluble fiber and high-temperature polycrystalline mullite fiber materials.

These technical developments have expanded the market for fiber-based products into applications where more traditional refractory products (such as bricks or castables) have historically been used. In many cases, fiber-based insulation is now the industry standard for high-temperature thermal management.

Custom shapes and designs are commonly engineered for high-temperature applications.

Benefits

By nature, fiber-based refractory products have a very low density compared to traditional refractory products. This characteristic results in reduced energy consumption when the materials are used in high-temperature industrial applications such as batch-type kilns.

Fiber-based refractory materials decrease the energy wasted due to refractory heat loss and heat storage. Typically expressed as btu/sf-hr, heat loss is the amount of energy per unit of time that is lost through the kiln walls and roof. Heat storage is the amount of energy required to bring the kiln to equilibrium during heat-up. Since heat storage does not include a time component, it is typically expressed in terms of btu/sf.

Additional benefits of a fiber-based refractory lining system include:
  • Reduced weight, which requires less steel furnace support structures
  • Faster heat-up and cool-down times, which result in potentially decreased firing times
  • Immunity to thermal shock, thereby reducing required maintenance


Manufacturing

Traditional ceramic or soluble fiber products are manufactured through a high-temperature furnacing process during which oxide powders are blended and melted. A molten stream of this blend is then fiberized, either through a blowing or spinning process. The end product is fiberized material, as well as very small, glasslike beads called shot particles that are a manufacturing byproduct.

The bulk fiber material is then processed into different product forms, such as blankets, modules, papers, boards, textiles and wet mixes. The maximum temperature use limit of fiber products manufactured via this method is approximately 2600°F (1426°C).

Another class of refractory fiber products includes those manufactured via a sol-gel process. For this process, liquid raw materials are batched and chilled. Fibers are produced when the liquid-saturated solution is processed through spinning cups. These fibers are then collected, dried and calcined to chemically stabilize the mullite fiber chemistry.

The basic composition of these fibers is alumina and silica; alumina percentages range from 72-95% (the chemistry range for mullite materials). The high alumina content of these fiber products allows them to be used in potentially harmful fluxing applications where products with higher silica concentrations may fail. Since the fiberization process occurs at ambient temperatures, virtually no shot is associated with polycrystalline mullite fibers, which minimizes potential product contamination.

The maximum temperature use limit of fiber products manufactured via the sol-gel method is approximately 3000°F (1650°C). The higher temperature capabilities of these fibers make them well-suited for many processes within the industrial ceramic market, which generally uses higher operating temperatures compared to other industries.

Polycrystalline mullite fibers can be processed and fabricated into a variety of product forms.

Polycrystalline Mullite Fibers

Similar to furnaced fibers, polycrystalline mullite fibers are additionally processed into various product forms, including blankets, modules, and vacuum-formed boards and shapes. Blankets are used as consumable wraps or covers for high-temperature applications and as a component for module fabrication. Modules are typically used to insulate large, batch-type kilns that process and sinter a variety of ceramic products.

When used in high-temperature tunnel kiln applications, polycrystalline mullite fiber modules can help achieve a low-mass kiln car design. Vacuum-formed boards and shapes are manufactured through a wet process by adding organic and inorganic binders. These products are commonly used as insulation for smaller, laboratory-type furnaces and as setter tiles to support product as it is thermally processed through a batch or tunnel kiln.

Multiple Options

Fiber-based insulation is an attractive option for many ceramic industry refractory users who need to evaluate their heat management requirements. Polycrystalline mullite fiber products extend the temperature use limit to such a point that end users can recognize the thermal performance benefits of a low-density, fiber-based product that is able to withstand the high operating temperatures common to the ceramic industry.

For more information, contact Unifrax I LLC at 2351 Whirlpool St., Niagara Falls, NY 14305; call (716) 278-3800; e-mail info@unifrax.com; or visit www.unifrax.com.

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