There are three basic methods in commercial use for removing impurities from aluminum melts. The aluminum foundry often uses argon, nitrogen, or chlorine as the degassing agent.
The aluminum foundry degassing uses inert gases (argon and nitrogen) that can effectively remove hydrogen at least to a certain extent, but is relatively ineffective in removing active material impurities such as sodium, calcium, lithium, and excess magnesium.
Argon and nitrogen are effective in aluminum foundry degassing, but they can only remove a certain amount of absolute hydrogen. Another problem with industrial-grade argon or nitrogen is that these gases may contain moisture and oxygen, which can form hydrogen and aluminum oxide. Ultra-high purity gas avoids this problem. Neither nitrogen nor argon have a significant effect on the oxides or particles present in the melt.
Argon is an inert gas, it does not react with the melt, so hydrogen is removed by diffusing into the argon bubbles. The particles are removed by a purification mechanism. Except for possible mechanical stirring, argon has no effect on the elements. In principle, nitrogen has the same effect as argon. However, it is generally believed that in similar situations, when argon or nitrogen is used, dehydrogenation is faster. Similarly, when using argon, the absolute value of hydrogen remaining in the melt will be slightly lower than when using nitrogen.
The aluminum foundry degassing uses chlorine, which is added to the melt quickly reacts with AlCl3, which is gaseous at a temperature above about 374 degrees Fahrenheit, so the melt is flushed by the upwardly rising AlCl3 bubbles. Chlorine is very effective for hydrogen removed by diffusion, because the partial pressure of hydrogen in AlCl3 is practically zero. However, the stoichiometric excess chlorine gas is discharged from the melt in the form of pure chlorine, which brings safety and environmental problems. In addition, the reaction between chlorine and aluminum is quite slow compared to other metals.
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