Molten aluminium processing

When discussing the quality of aluminum melt, we mean the degree to which an aluminium alloy is free of the following contaminants: alkalis (sodium, calcium, and lithium), non-metallic inclusions, and dissolved hydrogen.

Figure 1

Video: Molten aluminium processing

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The general sequence of molten aluminium processing steps is shown in Figure 2. It consists of crucible pre-treatment upstream of the furnace, furnace processing, and in-line treatment – degassing and filtration. Additionally, the transfer of the liquid metal between each processing step must be given particular attention.

Figure 2

It is need to note that the molten aluminium supplied to the cast house comes from two distinctly different sources: smelter electrolysis and remelt/recycled operations. The impurities presentin the metal supplied  from these sources are also different and can affect the metal treatment strategy that is used. Remelted metal is normally associated with higher levels of hydrogen, calcium, and hard oxide inclusions that are formed during high temperature scrap melting process. On the other hand, smelter metal is associated with higher levels of sodioum, aluminium carbide inclusions, as well as non-metallic inclusions from the addition of large quantities  of alloying elements.

Figure 3

Figure 4 summarizes the low impurity levels that must be achieved for various products. Compared to the incoming impurity levels, highly efficient impurity removal is required during metal processing. Note that according to this figure, not all extruded aluminum profiles require filtration.

Figure 4

The table in Figure 5 shows the main methods of processing aluminum melt. These methods include crucible, launder and furnace treatments, such as:

• settling;
• gas purging;
• chlorination;
• filtration;
• addition of primary aluminium;
• addition of aluminium alloys.

Figure 5

The principle of gas purging in a crucible, a launder or a furnace is shown in Figure 6. The efficiency of purging increases with minimization of the size of gas bubbles, thus increasing the interfacial gas-metal contact area.

Figure 6

The efficiency of purging liquid aluminum using various methods (lances, porous plugs and high-speed rotors)  compared to simple furnace settling is shown in Figure 7.

Figure 7

Filtration is considered an essential metal processing step. The object of filtration is to improve the metal quality to the degree required for demanding products and applications. Main types of filters employed in the aluminium industry are a deep bed filter, a ceramic foam filter (CFF), and a cartridge filter. The working principle of a ceramic foam filter is shown in Figure 8.

Figure 8

Sources:

1. A Technical Perspective on Molten Aluminum Processing / Peter Waite // Light Metals 2004.
2. Melt-Treatment of Aluminium – Ways to a High Performance Metal / Friedrich, B.; Krone, K.; Kräutlein, C – ME Process Metallurgy and Metal Recycling
   Aachen University of Technology, Germany, 2004