I just watched this (https://youtu.be/jLX1-tNnvEo?t=897)(14:57) and the guy in the video used vacuum and argon gas to remove any oxygen from the melter but I don’t understand why is the argon used if you are already vacuuming all the air out anyway?
In: Chemistry
Ok, so there’s two things to talk about here.
If you don’t want oxygen, you can try to just pull a vacuum. But pulling a really good vacuum is difficult and impractical for anything but really expensive systems, so you still end up with some oxygen. And in a lot of cases, some oxygen is still bad. You want nearly no oxygen.
Flushing with argon is a fairly good way to do that. You pull a vacuum, refill with argon. Now the leftover oxygen is mixed with the argon. So you pull the vacuum again. You’re left with less oxygen. Repeat a few times. I personally do it three times in my work, it serves to bring it down to an acceptable level for my needs. So I could just now pull a final vacuum, BUT!
The second thing. You might NOT want vacuum. Say, when you’re melting things. Especially, as it were, using a plasma which actually requires gas in-between.
The one property of vacuum is that it’s an absence of stuff. And nature… Well, abhors a vacuum. So if you have something in vacuum and and can turn to gas, it will. Especially hot liquids.
You might be familiar with a fact that the boiling point of water drops as pressure drops, say on top of a mountain. Well, it’s the same for everything else. Even metals. You can start getting a metal vapour coming off liquid hot metal. And unless you literally aim to do that, it’s usually undesirable. You usually want something close to atmospheric pressure.
On the other hand, that’s exactly the point of vacuum drying.
Plus, another issue is sealing. If you have vacuum, stuff from the outside REALLY wants to come in. Well, what you can do is very slightly overpressurise the inside instead. That way, instead of air coming in, some argon leaks out. But you can just top it up. And that way, you can get away with a system that isn’t perfectly sealed, which is cheaper and more tolerant of errors.
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