White Fused Alumina (WFA) is a critical raw material in the production of many high-performance foam ceramics.
What is White Fused Alumina (WFA)?
First, a quick definition: White Fused Alumina is a synthetic material produced by melting high-purity calcined alumina (Al₂O₃) in an electric arc furnace at temperatures over 2000°C. It is then cooled, crushed, and graded into various grain sizes. Its key characteristics are:
High Purity: Typically >99% Al₂O₃.
High Hardness: 9.0 on the Mohs scale.
Excellent Refractoriness: High melting point (~2050°C).
Good Chemical Inertness: Resistant to acids and alkalis.
Controlled Grain Size: Can be precisely milled and sieved.
Why is WFA Used in Foam Ceramics?
Foam ceramics are highly porous, lightweight structures used primarily as filters (for molten metals) and insulation materials. The requirements for these applications align perfectly with WFA’s properties:
High Temperature Resistance: Foam ceramics used for filtering molten aluminum, iron, or steel must withstand extreme temperatures (700°C – 1600°C) without softening or degrading. WFA’s high refractoriness makes it ideal.
Chemical Inertness: The material must not react with the molten metal being filtered. WFA is extremely stable and will not introduce impurities into the melt.
Thermal Shock Resistance: The sudden thermal gradient when immersing the filter into molten metal can crack inferior materials. The specific formulation of the ceramic slurry (including WFA and binders) is designed to mitigate this, and WFA’s intrinsic properties contribute to a robust structure.
Controlled Porosity & Structure: The foam’s structure (pore size, shape, and openness) is crucial for its filtering efficiency. WFA grains provide the solid, rigid skeleton that maintains this structure under extreme conditions.
High Mechanical Strength: The filter must resist the erosive and mechanical forces of the molten metal flow. WFA’s extreme hardness ensures the filter retains its shape and doesn’t shed particles.
How is WFA Used in the Foam Ceramic Manufacturing Process?
The most common method for making foam ceramics is the replica polymer sponge method. Here’s how WFA fits into the process:
Slurry Preparation: Fine powders of White Fused Alumina (and often other additives like binders, clays, and sintering aids) are mixed with water to form a viscous, creamy slurry. The particle size distribution of the WFA is critical—it must be fine enough to coat the polymer foam evenly but also contain coarser grains to help build a strong structure.
Impregnation: A open-cell polymer foam (e.g., polyurethane) with the desired pore structure (e.g., 10, 20, 30 PPI – Pores Per Inch) is dipped into the slurry. It is repeatedly squeezed and released to ensure the WFA slurry thoroughly coats all the struts of the polymer network.
Excess Removal: The impregnated foam is passed through rollers or centrifuged to remove excess slurry. This is a crucial step to ensure the cells (pores) remain open while the struts (the ceramic walls) are fully coated.
Drying: The coated foam is dried slowly to remove water without cracking the ceramic coating.
Burning Out the Polymer: The dried part is heated to a moderate temperature (~500-800°C) to slowly burn out and remove the polymer foam template. This leaves a fragile, porous ceramic replica.
- High-Temperature Firing (Sintering): The part is then fired at a very high temperature (typically 1450°C – 1650°C). At this stage, the WFA particles sinter—they fuse together at their boundaries without fully melting—forming a strong, monolithic, and highly refractory ceramic skeleton.
Key Specifications of WFA for Foam Ceramics
When sourcing WFA for this application, manufacturers look for:
Purity (% Al₂O₃): ≥ 99.0% is standard. Impurities like soda (Na₂O) can lower the refractoriness and affect sintering.
Grain Size (Mesh/Fineness): Typically ground into fine powders. Common distributions are F240, F280, F320, and even finer (d50 values of 20-50 microns). A mix of grain sizes is often used for optimal packing density and slurry rheology.
Chemistry: Low content of Fe₂O₃, SiO₂, and Na₂O is critical to ensure high-temperature performance and avoid contamination of filtered metals.