Ceramic Foam Filter
A Molten Aluminium Filtration Box is a refractory-lined chamber integrated into the molten metal flow path (typically between the furnace and the casting unit). Its core component is a Ceramic Foam Filter (CFF), a porous ceramic plate designed to physically trap non-metallic inclusions (impurities) as molten aluminium is poured through it.
This system is a critical and standard piece of technology in modern aluminium foundries and casthouses for producing high-quality, defect-free cast products.
Purpose and Why it's Necessary
Molten aluminium always contains solid impurities called inclusions. These include:
- Oxides: (Al₂O₃) from surface oxidation during melting and transfer.
- Spinel: Complex oxides often containing magnesium.
- Carbides: From linings or tools.
- Refractory particles: Erosion from furnace linings and troughs.
- Clusters: Agglomerations of these particles.
If not removed, these inclusions lead to:
- Poor surface finish.
- Reduced mechanical properties (strength, ductility).
- Porosity and defects in final products.
- Catastrophic failure in high-integrity applications like aerospace or automotive parts.
The CFF box is the most efficient and widely used method to remove these inclusions.
Key Components of the Filtration System
A typical system consists of three main parts:
1. The Filter Box (or Filter Casse):
- A robust steel box heavily lined with refractory cement or pre-cast refractory panels.
- Designed to hold molten aluminium and withstand thermal shock and chemical corrosion.
- Has an inlet well (where metal enters) and an outlet well (where filtered metal exits).
- Often includes a heating system (gas or electric) to prevent metal freeze-up.
2. The Ceramic Foam Filter (CFF):
The heart of the system. It is a three-dimensional, open-cell foam structure made from materials like:
- Alumina (Al₂O₃)
- Zirconia (ZrO₂)
- Silicon Carbide (SiC)
It is characterized by its Pores Per Inch (PPI):
- 10-20 PPI: Coarse filtration for roughing applications.
- 30-50 PPI: The most common range for general casting (e.g., extrusion billets, foundry ingots).
- 50-80 PPI: Fine filtration for high-quality products like aerospace sheet, foil, and critical castings.
The filters are brittle and must be handled carefully before use.
3. The Filter Gasket and Sealing System:
- A high-temperature ceramic paper gasket is placed between the filter and the box ledge.
- A hold-down beam or refractory clamp is used to press the filter down onto this gasket, creating a seal that forces all the molten metal to flow through the filter, not around it.
How It Works: The Filtration Process
The process is primarily deep bed filtration, involving two main mechanisms:
1. Mechanical Sieving:
Large inclusions and clusters are physically blocked on the surface of the filter because they are larger than the filter's pore openings. This is the initial stage.
2. Depth Filtration:
As a cake of inclusions builds up on the surface, it itself becomes a filter layer.
Smaller inclusions are trapped within the complex three-dimensional network of the filter's cells through mechanisms of:
- Interception: Particles touch and adhere to the filter walls.
- Brownian Diffusion: Very fine particles drift and collide with the cell walls.
- Gravitational Sedimentation: Particles settle onto cell walls.
- Inertial Impaction: Particles cannot follow the tortuous flow path and impact the ceramic material.
This combination provides extremely high filtration efficiency, often >95%.
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