Molten aluminium filtration
Molten aluminium filtration process
Aluminium alloys have big tendency toward oxidation. Therefore, they often contain contaminants that are harmful to physical, mechanical, electrical and aesthetic properties of aluminium products. The most common of these contaminants are non-metallics inclusions. A primary means of removing such solid contaminants is aluminium melt filtration.
The filtration process consists of passing the molten aluminium through a porous device, such as a filter. The filter captures the inclusions contained in the flowing metal. The filter material must not be destroyed by molten metal. So, most filter media are various ceramic materials.
Inclusions in aluminium
The table in Figure 1 shows the main types of solid-phase inclusions in molten aluminium alloys and their characteristics [1]. These inclusions have:
- densities from lighter to heavier than the molten aluminium itself
- a range of sizes and shapes from globular (oxides and borides) to stringer-like (oxides and spinels).
Figure 1 – The main types of solid-phase inclusions
in molten aluminium alloys and their characteristics [1]
Molten aluminium filtration basics
Two types of filtrations
Two types of filtrations occur when filtering particles from a flowing liquid aluminium stream:
- cake filtration
- depth filtration.
Cake filtration
- Inclusion removal occurs by mechanical entrapment of inclusions on the surface of the filter.
- Particles lager than 30 micrometers mostly are captured by this method.
- The particles agglomerate to form a filter cake on the filter surface. This cake further help filtering of inclusions.
- Ceramic foam filters work largely in this manner .
Depth filtration
- This filtering mode prevails in bed filtration.
- Depth filtration is capable of removing particles much smaller than 30 micrometers.
- Molten aluminium flows through a very winding path in a deep bed filter (Figure 2).
Figure 1 – 3D and 2D images of ceramic foam [2]
Figure 2 – A bed filter used for in-line processing [1]
Molten aluminium filtration efficiency
Filtration efficiency depends on several factors [1]:
- The nature of the molten metal:
- the amount of inclusions
- the size of inclusions
- the shape of inclusions
- the distribution of inclusions
- The dynamic conditions:
- Lower molten metal flow rates result in greater filtration efficiency.
- Greater filter surface area increases filtration efficiency.
- The proper balance between cake and depth filtrations.
- Greater length or depth of filter dimension gives greater filtration efficiency.
Figure 3 compares the efficiencies of ceramic foam and bed filter.
Figure 3 – Comparision of filtration efficiency of
a bed filter and a ceramic foam filter
as a function of melt velocity [1]
Types of filters
Fiberglass filters
- Provide moderate filtration, particularly of larger particles and inclusions exceeding 100 micrometers.
- Fiberglass cloth are materials which commonly used for aluminium filtration (Figures 4 and 5).
Bed filters
- Compose of tabular aluminium oxide.
- Quite common in casting of mill products.
- Make sense where up to 2000 tons of a single alloy must be filtered in-line between holding furnace and casting station (see Figure 2).
Bonded particle filters
- Find greater use in foundry and die casting operations.
- Consist of a refractory grain (either Al2O3 or SiC) bonded together to form rigid structure (Figure 6).
- Often used vertically to separate melting or holding furnace hearths from the dip-out well (Figure 7).
- Can be refreshed between uses by back-flushing to remove much of the accumulated filter cake [1].
- Up to 500 tons of throughput has consistently been achieved before filter replacement is necessary, depending on initial metal cleanliness [1].
Ceramic foam filters
- The most common types of filters used in aluminium casting.
- Produced by slurry coating a ceramic cellular foam after drying and firing to burn out the original foam. A ceramic foam filter is a ceramic replica of the original organic foam structure (see Figure 1).
- Suitable ceramics for these filters are alumina, zirconia, mullite, chromic oxide [1].
- These filters are about 75% porous. Pore sizes are measured by pores by lineal inch (ppi).
- Used in flat, platelike form (Figures 8 and 9). Plate sizes vary from 305 by 305 vv (12 by 12 in.) to 585 by 585 mm (23 by 23 in.). The size depends on flow rate (up to 680 kg/min.) b casting size (up to 34,000 kg).
- Smaller filters are used in the die casting and foundry industries (Figure 10).
Cartridge filters
- Provide a great of surface area and have very fine pore size (Figure 11).
- High priming head of metal may be required before flowthrough begins (up to 0,3 m).
- Flow rate is not very high.
- Filtration efficiency is up to 95% and more for particles less than 5 micrometers.
Figure 4 – Formed, rigidized filters made from woven fibreglass.
Used in permanent mould and sand casting applications [2]
Figure 5 – Rigidized fibreglass sprue filters
used in low-pressure casting aluminium foundries [2]
Figure 6 – Pyrotek bonded particle filter [2]
Figure 7 – A vertical bonded particle filter used for holding furnace filtration
for die casting operation [1]
Figure 8 – Typical ceramic foam filters [2]
Figure 9 – The principle of operation of the ceramic foam filter in the filter box [3]
Figure 10 – Foundry ceramic foam filters [2]
Figure 11 – A foundry filters placement [1]
Figure 12 – A cartridge filter [1]
Sources:
- Molten Aluminum Processing and Casting / Aluminum and Aluminum Alloys – ASM Speciality Handbook //Ed. J.R. Davis – 1996
- https://www.pyrotek.com – Pyrotek – 2024
- Molten Metal Processing / R. Otsuka // Handbook of Aluminum: Vol. 1. Physical Metallurgy and Processes – 2003