Aluminum Dross Question

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STRR
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Aluminum Dross Question

Post by STRR »

I am considering melting aluminum cans, rather than selling them as scrap. I know there will be a whole lot of dross, as in up to 50% of beginning weight.

My Questions: Dross is aluminum oxide. Will you get more dross if you use a forced air furnace or a natural draft furnace? Will covering the crucible reduce the amount of oxygen getting to the melt and thus reducing dross?

I have yet to build a furnace but am leaning towards propane fired, and building the burner myself. I have been collecting aluminum where and when I can find it. Some of it is "dirty" as in painted, and greasy. I can remove the grease but the paint will be more difficult. I understand the paint will add to the dross problem. I would like to end up with some reasonably good quality ingots for future use or sale.

Thank you all in advance for your advice and input. I need all I can get.

Terry
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Harold_V
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Re: Aluminum Dross Question

Post by Harold_V »

I have little hands-on experience with aluminum, but here's how I see things.

Dross. As you alluded, it's aluminum oxide, although not entirely. In volume it can be returned to producers to be re-processed, for recovery.

Anything you can do to keep oxygen away from the heat would certainly be better than not doing so. Fast melting is probably a good idea. I hesitate to suggest the desired atmosphere (in the furnace), because I simply don't know.

Grease and paint. Death on aluminum quality. Molten aluminum has an affinity for hydrogen, which is the chief cause of porosity in poured aluminum. By melting on very damp days, or by melting anything that has the potential to liberate hydrogen, you risk gas riddled pours. Degassing would be a must. That can be accomplished with chlorine tablets, or by injecting nitrogen just prior to pouring.

If you must melt coated items, they should be cleaned (degreased, with paint burned off) prior to melting, to avoid hydrogen contamination.

You would be best served by not melting such items, just as you'd be best served by not melting cans. You may find you can strike a deal with a scrap yard, whereby you can exchange such items for cast aluminum, or even pistons, which are reputed to be excellent stock for casting. Again, I have no experience in that regard, so do your homework before taking my word for that idea.

Same thing applies to melting alloys that are not intended to be cast. While they will melt, how they pour can be troublesome. That's not to say that it can't be done---it can. Even 6061 will cast, it's just a matter of how well it casts as compared to alloys formulated for casting.

Nothing wrong with building your own furnace. Commercial units tend to be too expensive to justify for the home shop, but they are easily built by those with a little skill and talent. I've built several in my many years, all of which have performed perfectly well, and for melting metals that melt at much greater temperatures than does aluminum. I'd certainly encourage you to build your own.

I used to fire with natural gas, but that is no longer an option for me. The furnace I will build in the near future will be fired by oil, although there's nothing wrong with using propane if that's your choice, especially for aluminum, which demands far less in the way of temperature. I don't particularly care for the dangers of propane, nor its cost, so that won't be my choice.

I'd enjoy seeing how this develops for you. Be certain to post again, providing some updates.

Harold
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DavidF
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Re: Aluminum Dross Question

Post by DavidF »

Melting aluminum cans has never been a thing of choice for casting. They produce alot of dross and when you start to consider the fuel costs involved for melting plus other consumables and the current market price for a good quality ingot of a know alloy, well its better to sell the cans and just buy an ingot. If you do decide to go ahead with melting cans then the best way to go about it is to have a molten heel in the crucible, crush the cans, then push the can under the melt. This will reduce the amount of dross, but be warned, if there is any mositure left in the can it may "POP" and spit molten aluminum at you.
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steamin10
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Re: Aluminum Dross Question

Post by steamin10 »

If you'll listen, I would not consider melting cans, although it is tempting. I did once upon a time early on, and found the resulting metal dismal in quality. Commercial operations do so with bails of crushed cans that fall into a shallow pool of molten metal, in a reducing furnace atmosphere. This pool is constantly draining as new metal melts. They can do that, you have less control, and the paint and plastic liners of the cans present problems in smoke and dross creation for your melted stock.

Better to crush and bag your cans and accept the .50 cents or so a lb, and buy crap car wheels, and most any other cast aluminum you can find, and use that. Avoid extrusions, as they have the wrong base content of Silicon that is most important to fluidity, and avoid anything that resembles iron bolts and screws which will destroy fluidity up front, and cannot be removed by any means handy to you. High iron and low silicon, are the two major problems that destroy the working qualities of aluminum for the home shop.

High quality bar aluminum is available, and should be considered for high demand projects. It can be found at foundry metal suppliers in various cities across the US. While not cheap, you may reach that level of quality demand, if you stay in 'Art Casting' long enough.
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STRR
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Re: Aluminum Dross Question

Post by STRR »

Harold, David, & Big Dave,

Thank you for your input. I appreciate your advice, especially you, Big Dave. Your explanation into why extrusions are not desirable will help me a lot. I will take all your advice and sell the cans instead of melting them. I wasn't expecting good quality or quantity aluminum but you all have convinced me it is totally NOT worth the time, money, and effort. Thank You.

One thing you did not touch on is the covering of the crucible to minimize dross. I understand the stirring the melt will increase dross, presumably from introduction of oxygen. Would a crucible lid be worth the time and effort?

Thank you all again, Harold, David, and Big Dave.

Terry
DavidF
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Re: Aluminum Dross Question

Post by DavidF »

One thing you did not touch on is the covering of the crucible to minimize dross. I understand the stirring the melt will increase dross, presumably from introduction of oxygen. Would a crucible lid be worth the time and effort?
If you'll listen, I would not consider melting cans, although it is tempting. I did once upon a time early on, and found the resulting metal dismal in quality. Commercial operations do so with bails of crushed cans that fall into a shallow pool of molten metal, in a reducing furnace atmosphere.
If you do decide to go ahead with melting cans then the best way to go about it is to have a molten heel in the crucible, crush the cans, then push the can under the melt.
You would be best served by not melting such items, just as you'd be best served by not melting cans.
Feel free to experiment, but the ending alloy is not desireable.
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Harold_V
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Re: Aluminum Dross Question

Post by Harold_V »

I purchased crucible covers when I was melting silver, in the hopes that one would help keep oxygen from being absorbed. I also wanted to keep foreign substances from getting in the heat, as the silver was pure, being cast in 100 ounce ingots. Silver has a nasty habit of absorbing nine times its volume of oxygen, just to give it up as it solidifies, as silver does not oxidize easily. The oxygen, upon being expelled, tends to create tiny spires of silver on the surface of the silver, so it goes without saying that it is not desirable to allow it to be absorbed. To that end, it is often recommended to cover the molten surface with carbon (charcoal), which was combine with oxygen to form CO2. I don't know how this would play out for aluminum, however.

Armed with this bit of information, it may well pay to explore options. My experience isn't adequate to provide guidance, aside from knowing that the foundry operated by some friends in Utah used to skim, then introduce nitrogen to degas. It was at this foundry where I learned that there is value in dross, as he was preparing to ship a truck load when I visited, long ago.

Harold

Edit:

I should comment on another experience I've enjoyed, and more than once. When I've attended the GEARS show in Portland, Oregon, I've visited the foundry display, where the guys making nice aluminum castings are pretty much doing "everything" wrong in regards to material selection. I made a point of asking if they were melting 6061, which is not known for its casting qualities. They were. A good way to view this is if you can master pouring alloys that don't pour well, you should have even better success pouring casting alloys.
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Rwilliams
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Re: Aluminum Dross Question

Post by Rwilliams »

The top surface of aluminum cans contains a higher concentration of magnesium which makes that part of the can a stronger alloy. Keeps the pop top from popping in elevated temperatures during shipment. The magnesium in the top makes the top of the can more expensive to manufacture and the reason why back in the early 1990's the tops of all aluminum cans became smaller to reduce costs of manufacture. The recycle places do not pay you for the magnesium value of the cans, just the aluminum value. The magnesium is rumored to make a weaker less desirable casting alloy and you still are not compensated for the magnesium value upon sale to the recycle dealer. With all of the time invested and fuel costs to heat the cans, it seems like a futile adventure in loss of time and money just to melt the cans in a home foundry furnace.
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