Filtration of liquid aluminum
Molten aluminum in a melting furnace before further processing - casting into ingots or castings - almost always requires filtration. Filtration methods depend on the casting technology used or the method of processing aluminum melt. generally considered, that it is possible and beneficial only when working with large enough volumes of metal. However, filters can be used for large industries., and in small foundries, including for filtering metal during casting of each individual cast.
Ceramic foam filters
In aluminum casting, ceramic foam filters are most commonly used.. Usually they are square plates with side to 178 to 660 and thick 50 mm (figure 1). They are made by introducing polymeric cellular foam into a liquid ceramic pulp. Then this ceramic coated foam is dried and fired., to remove (burn out) the polymer base. The result is a ceramic replica (imprint) of the original organic foam structure [1].
The main ceramic materials for this technology are alumina, zirconium dioxide and silicon carbide [2, 3]. Three-dimensional and two-dimensional images of ceramic foam are shown in the figure. 2.
Figure 1 - Typical ceramic foam filters [2]
Figure 2 - 3D and 2D images of ceramic foam [1]
A convenient characteristic of ceramic foam structure is the number of pores per linear inch (ppi). Industrial ceramic foam filters usually have brands from 10 to 80 ppi [1, 2]:
- Filter c 10 ppi has a minimum cell size 3800 m, and the maximum cell size is 5100 m.
- Filter with 50 ppi has a mesh size 1000-1150 m.
- Filter with 80 ppi has a mesh size 600-700 m.
Filter dimensions depend on its expected performance., the duration of casting and the location of the filter in the filter unit. The choice of the filter grade (ppi value) is determined by the characteristics of the aluminum alloy, casting temperature, degree of contamination of the melt and the required level of purity of the metal at the exit.
Gas or electric heating
Filter well before casting, to ensure its normal operation. After casting, the filter can be reused, if constant heating is provided (figure 3). If the metal hardens in the filter, then for the next casting it must be changed to a new one.
Figure 3 - Gas heating of the ceramic foam filter [3]
Ceramic foam filters in the foundry industry
Typical industrial use of a ceramic foam filter in a filter box, which is installed in the metal transfer line from the melting furnace to the dispensing furnace or casting unit, shown in the figures 4 and 5.
Figure 4 - The principle of operation of the ceramic foam filter in the filter box [1]
Figure 5 - Industrial filter box with ceramic foam filter [2].
Above is an electric heated lid.
In the gutters
At large enterprises, aluminum is filtered during continuous or semi-continuous processing from several tons to hundreds of tons of liquid aluminum. This requires large area filters., which are installed in the system for transferring the melt from the melting or holding furnace to the casting equipment (Figure 6).
Figure 6 - The use of ceramic filters in the casting trough [3]
Filter Filling Bowl
When pouring molten metal, a simple thermally insulated pouring bowl system with filter can be used (Figure 7). This filter is for filtering. 100-500 kg of molten aluminum. If the filter is designed for only one casting, then the pouring bowl itself can be used for several dozen pouring operations [3].
Figure 7 - Filling bowl with filter.
Top left - electrically heated lid [3]
Partition filter baffles
Filter baffles in transfer furnaces are usually the most efficient way to apply filtering to casting workshops.. The filter is installed in the chamber of the transfer furnace in the form of a partition, which separates the compartment for the intake of the melt from the rest of the furnace volume (figure 8). In the case of crucible furnaces, this filter baffle is U-shaped. An optimally selected filter can filter out several hundred tons of molten metal over its life. Its replacement is carried out during the scheduled maintenance of the refractory lining of the furnace [3].
Figure 8 - Filter baffle in the dispensing oven [3].
The heated lid is visible on the left.
Gating system filtration
The best results when filtering liquid aluminum are achieved then, when the filter is placed directly in the gate system, which serves for casting a cast product (figure 9). These filters are disposable..
Figure 9 - Disposable ceramic foam filters in the gating system [1]
Gating Filtration
Direct casting through a sprue bowl, in which the filter is installed, may apply, for example, for gravity sand casting. This method applies circular filters (Figure 10).
Figure 10 - Disposable ceramic filters in the sprue bowl [3]
Filtration on injection molding machines
In the case of low pressure casting technology, the gating system is a metal or ceramic pipe (Figure 11). Instead of a disposable filter mesh, a special round ceramic foam filter is installed in it (Figure 12).
Figure 11 - Low pressure casting machine
with disposable ceramic filter [3]
Figure 12 - Round disposable ceramic filters
for low pressure die casting machines [3]
Sources:
- Molten Metal Processing / R. Otsuka // Handbook of Aluminum, Vol. 1 – 2003
- https://www.pyrotek.com – Pyrotek Corporation - 2020
- Specifics of aluminium alloys filtration - LANIK company - 2020
