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Thread: Degassing confusion

  1. #21
    This is the most recent so I'll throw my question here instead of a new thread. I searched the forum but didn't find an answer.

    Where does hydrogen come from so that it is then absorbed my the aluminum? is it from the air from humidity?
    I found that trying to find what I need and then make it work with what I have, is more trouble than designing what I want and doing it.
    me

    "Quick decisions are unsafe decisions."
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  2. #22
    Senior Member machinemaker's Avatar
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    moisture is an issue, but we forget that one of the by products of burning most fuels is water vapor or hydrogen. This is why I push the issue of watching and controlling furnace atmosphere. In my opinion the best solution to not having to deal with gas and dross is to follow good melting practice and not create the problem in the first place. Take the time to clean your scrap before melting, melt in a controlled atmosphere by controlling how your burner is working, pre heat scrap to get rid of moisture ( make sure you are not putting water vapor onto your preheated scrap in the exhaust of your furnace), melt fast and do not hold metal at a hot temperature too long ( melt fast and pour, every time you re-melt you are adding gas to the metal),do not agitate the melt or splash the metal when pouring, in general learn all you can and do what you can to stop the problem before it starts. we had a 10000 lb holding furnace that I kept hot and ready to pour 24/7 and if we did not cast for a while the metal would bubble as we poured from holding metal for days or a week at a time in the furnace. It was a tilt furnace that poured into a trough or launder and the metal looked like when you pour a coke into a glass.

    now a story: when I worked for alumax we had a customer who manufactured aluminum cookware and they thought that the gassier the metal the better their casting success and wanted to know how to make the al gassier. the solution was to toss a potato into the pot before each pour and violently stirring. So if you like the smell of burning potatoes give it a try to put extra gas in the melt
    Kent
    There is beauty, power and excitement in simple old technology!

  3. #23
    Thanks Kent, it appears I did forgot that burning carbohydrogen based fuels might produce hydrogen subproducts, where did I left my brain?! :P
    I found that trying to find what I need and then make it work with what I have, is more trouble than designing what I want and doing it.
    me

    "Quick decisions are unsafe decisions."
    Sophocles

  4. #24
    I intend to use a melted down 50/50 mixture of NaCl and KCl as an aluminium fluxing agent - I made a nice ingot of it the other day. I also read that chlorine allows you to degas. Does that mean that the fluxing salt also acts as a degasser, since there might be chlorine coming from it? Or is this a seperate act completely, even though chlorine happens to be present in the salt?

    Something I have been wondering about is how much flux to use per unit of aluminum. I heard about a teaspoon per melt, but the crucible size can vary quite a bit. Can anyone give a more accurate indication, or is it not that much of an exact science?

  5. #25
    I have barely done anything but I found that when I add salt (table salt, no iodine) the aluminum likes to seep through my sand, but when I don't add salt it doesn't happen. Solely based on my petty pours and babie knowledge, I would compare the salt in aluminum to soap in water. I added an ounce of salt to 1 cup (melted) of 50:50 Al:Cu.
    Furnace Rebuilds: 2. Furnace Lights: 3. Successful Casts: 3. Failed Casts: 1.

  6. #26
    Administrator Site Admin Anon's Avatar
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    It's not a bad comparison: the method of action is different (fluxes are not surfactants) but they do both act as wetting agents. The flux reduces surface tension in aluminum by removing the layer of solid oxide that forms on exposure to air, and acts as a wetting agent by removing impurities at the boundary. Soap reduces surface and interfacial tension in almost the opposite way: by forming an interface at the surface, with one side of the molecule attracted to nonpolar stuff like oils, and the other side of the molecule attracted to polar stuff like water.

    Anyway: The amount of flux varies depending on what you want to do. For general casting in aluminum (from clean scrap or ingots), I'd advise using it sparingly or not at all. The reason is, it promotes gas in the metal by breaking up that oxide layer, and allowing water vapor to dissolve (water vapor is soluble in molten salt, just like solid salt is soluble in liquid water) and dissociate into hydrogen and oxygen. That also answers the degassing question above: salt flux doesn't help to degas, you need chlorine gas, not a chloride compound. Or you can do an inert degassing (safer because you don't have to deal with toxic chlorine gas, but slower because it's removing the gas through agitation and surface area alone, not with a chemical reaction as well) with dry nitrogen, argon, SF6, etc.

    "Sparingly" fluxing is probably a 1/4 to 1/2 tsp of flux in a 5 pound melt. As you use it you'll get a sense of when more flux will help: if you skim your metal and you're removing a bunch of thick lumpy stuff, add a little flux and most of the aluminum will return to the melt, leaving a more powdery dross. If you get lots of flux it will slosh around as a liquid on top of the melt (it's more fluid than water, and typically ends up black due to dross and contamination, but will be clear or nearly clear if it's very clean). You can add some sand to soak that up if you're having trouble skimming it off--if you only add a little the dross will soak it up just fine.

    If you're doing something like remelting a bunch of dross (there's quite a lot of metal still in it, but I don't bother with it--whether it's worth it to you or not, you can decide) then you may want a bunch of flux, enough to completely cover the melt in molten salt. This will make a very gassy pour, so I'd pour into ingots when doing something like this unless you have a well-established degassing process.
    The process of turning stumbling blocks into stepping stones can at times require the use of a large sledgehammer.

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  7. #27
    I have heard about degassing with nitrogen base tablets. I am not sure how effective it is, and what compound it uses.

  8. #28
    Hello and sorry for my bad english :cry:

    My idea for degassing Aluminium is Pool-Chlor-pellet Trichlorisocyanuracid (http://en.wikipedia.org/wiki/Trichloroisocyanuric_acid).
    It decomposes in heat to Chlor. It is easy to buy. I pack it in Aluiminium foil and bring it on the Bottom of the Crucible.
    But dont forget: Chlor is highly Toxic.
    Last edited by PatJ; 09-17-2014 at 10:14 AM.

  9. #29
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    I don't remember exactly (and can't find a reference at the moment) but I think that decomposes to some really bad stuff (cyanide derivatives?), worse than just chlorine gas. I'd recommend instead to use sodium or calcium hypochlorite, which decomposes to chlorine gas and a basic oxide. Of course, chlorine gas is bad enough, so you still need appropriate respiratory protection.
    The process of turning stumbling blocks into stepping stones can at times require the use of a large sledgehammer.

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  10. #30
    Hmmm, cynide derivates could be possible. Not the best idea, but it works
    Last edited by PatJ; 09-17-2014 at 10:15 AM.

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