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]

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.

Figure 3 - Gas heating of the ceramic foam filter [3]