Has anyone tried melting copper under molten salt?

[Cryogaijin]

Howdy y'all!

I have over a hundred KG of stripped copper wire I'm wanting to turn into ingots to use in making copper alloys. I'm looking for ways to improve the thermal transfer to the copper from the crucible while blocking oxygen. I've heard of people fluxing with salt (Both table and KCl) and am curious if anyone here has tried it? Did it reduce the amount of dross produced?

I'm melting in a small electric furnace, (Castmaster 3kg with slightly dodgy logic) so there is already low airflow, but I am concerned about the duty cycle of the nichrome heating element. (2 hours at max temp before an hour of cooling.) I figure if the copper doesn't mix with the molten salt, it would provide a superior thermal transfer and would float on top of the copper blocking O2.

Any thoughts? Experience? Anecdotes? Warnings?

 

[cosmetal]

Watching this post with interest and I hope that those with more casting experience than I chime in.

What is your intention with melting your Cu wire into Cu ingots? If trying to densify for shipment to a scrap buyer, you might be lessening the value of your scrap by doing this. Many yards do not buy ingots - they want the wire. Ingots can hide a lot of unknowns within their cores.

Your Castmaster is not a production furnace built for continuous melts and pours. If it has a 2-hour max temp cycle, you are lucky.

As far as dross, I don't have any experience with any salt barrier method. If your copper wire is clean, I think that a constant reducing flame pointed at your melt (while pouring) would be more effective to prevent oxidation of your metal. Copper is considered a noble metal. It should not be producing dross unless contaminated. Copper will oxidize but oxidation is not usually considered to be dross. Dross is contamination usually associated with steel, aluminum, zinc, and lead production that can be skimmed off before a pour.

Have you ever investigated making Shibuichi? Some have used the water casting method while casting copper alone. I haven't tried it (yet) but it is certainly on my "bucket list" both with the classic 75% Cu/25% Ag Shibuichi alloy and with pure Cu alone. The vapor barrier (steam jacket) appears to certainly stop oxygen from getting to the surface of your cast.

Water casting can be very dangerous and I would not recommend trying it until you are experienced and have acquired (and use) the proper PPE.

Peace and health,
James

​

 

[Lino1406]

Positive. Look for US patent "Copper melting under salt"

 

[cosmetal]

Positive. Look for US patent "Copper melting under salt"

"Improved method for melting copper base alloys containing from 2 to 12% aluminum including the steps of covering a molten mass of said copper base alloy with an essentially continuous flux layer of a molten salt consisting essentially of from 10 to 90% by weight of potassium chloride, from 10 to 90% by weight of sodium chloride and less than 5% by weight of other materials, said salt having a melting point of from 660 to 800.degree. C, and adding additional amounts of said alloy through said flux layer."

"2 to 12% aluminum" - I believe his copper wire is common CDA electric wire grades not containing any AL.

Peace and health,
James

 

[voidforged]

Yes you can use salt, although I'm not sure why you wouldn't just use regular flux (borax).

 

[Cryogaijin]

Watching this post with interest and I hope that those with more casting experience than I chime in.

What is your intention with melting your Cu wire into Cu ingots? If trying to densify for shipment to a scrap buyer, you might be lessening the value of your scrap by doing this. Many yards do not buy ingots - they want the wire. Ingots can hide a lot of unknowns within their cores.

I have no interest in selling my scrap. Eventually I'll likely be using everything I produce for either art, nerdity (Minatures, memorabilia, coins, etc), or blacksmithing. At some point I'll be making my GF a bronze axe.

Your Castmaster is not a production furnace built for continuous melts and pours. If it has a 2-hour max temp cycle, you are lucky.

Knew that before I bought it. (I'm an EE, and well aware of the limitations of nichrome heating elements. I've had several hilarious to me but >EXTRODINARILY DANGEROUS< misadventures. I'd anticipated the potential of getting an insulation burn through, and was prepared.) I mentioned my Castmaster has a logic problem, I'm working with them to track it down and hopefully apply the fix, but as such they've sent me a backup element, and when my first fails, I'll be soldering that one directly in. As an EE I'm more comfortable with electric, but eventually I'll be rotating over to propane for the flexibility.

As far as dross, I don't have any experience with any salt barrier method. If your copper wire is clean, I think that a constant reducing flame pointed at your melt (while pouring) would be more effective to prevent oxidation of your metal. Copper is considered a noble metal. It should not be producing dross unless contaminated. Copper will oxidize but oxidation is not usually considered to be dross. Dross is contamination usually associated with steel, aluminum, zinc, and lead production that can be skimmed off before a pour.

I'd really need another set of hands for that. With a salt barrier (Density sub 2g/cm3 and runs as thin and smooth as water) I can pour the salt off to reuse then switch to a different mold to pour the copper into. That's the hypothesis, anyway. The added thermal conductivity of the molten salt should speed the heat transfer to the copper vs copper wire alone, both speeding the melt and halting oxidation. My copper wire ranges from pristine to "How did wire end up this dirty?!?" Most of it is well over 40 years old, some coming from well used jumper cables.

Have you ever investigated making Shibuichi? Some have used the water casting method while casting copper alone. I haven't tried it (yet) but it is certainly on my "bucket list" both with the classic 75% Cu/25% Ag Shibuichi alloy and with pure Cu alone. The vapor barrier (steam jacket) appears to certainly stop oxygen from getting to the surface of your cast.

I have not, yet! At the moment I have over a hundred KG of pure copper wire, so that is what I am "practicing" with. Copper is also my favorite metal for myriad reasons. Once I am consistantly making quality pours, I may consider looking into Ag or Au. As for other casting techniques, haven't researched them yet. PPE is a non-issue, I have an effectively unlimited budget for that, within reason.

Water casting can be very dangerous and I would not recommend trying it until you are experienced and have acquired (and use) the proper PPE.

Peace and health,
James

Thanks for the reply!

 

[Cryogaijin]

Yes you can use salt, although I'm not sure why you wouldn't just use regular flux (borax).

Salt is far cheaper than anhydrous borax, and forms a good liquid barrier. It also transfers heat far better than borax does. Cleanup is also easier. I have Borax for glazing and the like, but I'm not sold on it being the end-all be-all flux. Especially Borax pentahydrate.