RogueRose
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Registered: 16-6-2014
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Degassing moulten metals (especially Al, Zn & maybe copper) - how does it actually work?
I've seen such a wide range of "salts" used to degas aluminum before it's poured, and I've also seen a tone of fluxes used that are supposed to do
this to a degree but more to keep the oxidation separated from the good metal. I recently saw a person adding NaCl wrapped in a piece of aluminum
foil into a crucible and then pushing it down to the bottom and stirring.
What I have a difficult time with is understanding is the none of the common salts seem to have a BP or decomp temp anywhere near the working temp of
molten Al (~1300-1500F), so does the chloride just say in the metal along with the sodium, potassium or whatever is added?
I see that the chlorine is supposed to react with the H2 bubbles but I still have a hard time understanding how this way removes the H2? Does it for
HCl and leave the Na/K behind or does that come out in the slag? Also would MgCl2 be a better option if you didn't mind having a little Mg in your
mix (though the MP is doable , the MP seems way to high unless there is something else happening here).
I also noticed that there are some places using argon and nitrogen to degas the liquid.
I'm not asking for what is the best to use, I'm wondering if any of you know the science if this works and if there are better options, things that
can actually improve the metal quality (making an alloy for example, or adding trace metals by this method, similar to how carbon is added to steel)
[Edited on 8-1-2020 by RogueRose]
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Junk_Enginerd
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Registered: 26-5-2019
Location: Sweden
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I think the majority of fluxes work through two major mechanisms.
One is covering, which borax is popular for. It melts, wets the metal surface and thereby stops oxygen from reaching the metal.
The other one, which is what I believe you're asking about, has to do with solubility. Metal oxides are often soluble in salts. Think about good old
soda glass for example. Silica is the key component of glass, but its melting point is very impractical. To lower the melting point, soda(sodium
oxide, usually added as sodium carbonate) is used to dissolve the silica. I think the glass analogy holds up pretty good with metals and flux. Metal
oxides usually have extremely high melting points, but they can be dissolved in an appropriate salt. This won't remove any impurities in itself, but
it will be gathered into one coherent liquid that is more easily removed.
I'm sort of out on a limb here and not 100% sure, so take it with a grain of salt, but it makes sense to me.
So that's oxides and solid jmpurities. When it comes to gasses I'm less sure, but most common dissolved gas is hydrogen which is very reactive, so I
assume that adding something oxidizing, but selectively oxidizing to hydrogen rather than the metal, it can be converted to for example H2O or HCl and
boiled off. I don't think you're gonna convert NaCl to metallic sodium via simple thermal reduction though. Ohhh wait... Maybe what you get is HCl,
removing hydrogen, and then sodium oxide, which steals oxygen from the atmosphere or even better the metal you're melting.
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