Sciencemadness Discussion Board

Remove Ag from Cu

uber luminal - 25-8-2005 at 15:29

I have copper bridges, which I collected from old electrical breakers and swtich bases, that used silver for the contacts.

I have several hundred of such bridges and parts that range in mass from 150g to 300g. (try to aproximate their size through density or something)

The silver contacts are about 3x2 cm by 0.5 mm thick. Given that I have several hundred of these, and both the copper and silver are of great use to me, I would like to isolate the two, with the least cost.

Attempt #1. Initialy I looked at using an oxy/fuel torch to heat melt the silver off. This works. its just mechanically bonded to the surface. After enough heating... mind you how thermaly conductive copper and silver are... the silver runs off, into a pool of water and solidifies for collection.

While this method works, it is counter productive. The time I have to stand there, and remove the the silver costs money. The gas and oxygen I use up, also costs. This is not a good way.

attempt #2. Ferric Chloride. I dissolved Fe3O4 into HCl to make FeCl3 (and FeCl2) and set it into a vacuum oven for awhile to evaporate extra H2O. I suspended a few test pieces into the condensed solution, such that the silver was under the surface. I had hoped to take advantage of something like this... FeCl3 +Cu --> FeCl2 +CuCl, CuCl + FeCl3 --> FeCl2 + CuCl2.

and leave the silver behind, or droping off into the bottom of the solution. Results were... yes, silver had dropped to the bottom of the solution. Not as much silver was collected as the initial estimated volume of silver. Not only that, but the piece of copper that was pulled from the solution, (was clearly eteched about 2-3 mm on all sides) and instead of being super clean copper (white), or mostly clean copper(orangy/yellow), it was milky white. kind of the color of silver... as to what happened here, I dont know. I thought it might have been a CuZn alloy, but confirmed that it was pure copper. (the silver that was collected, was also mostly pure silver(96%). I am currious as to what the deal with that is.

But Anyway, I thought I would come and ask you guys before I started another idea.

Keep in mind, that I am looking for low cost method, to work for several hundred of these things. I also want to preserve the a good portion of the copper(but not all of it), and as much of the silver as I can get.

any ideas?

uber luminal - 25-8-2005 at 15:41

I should probably ammend, that the piece of silver found in the bottom was a solid. however there was a dense white paste found with it. (denser than the rest of the solution anyway) adding the paste to water and mixed, forms yellow water. Silver Chloride?

12AX7 - 25-8-2005 at 15:56

Can you electroplate the copper through HCl?


Fleaker - 25-8-2005 at 16:11

It would be nice to have even more details, namely the % of silver and copper in the material.

Well, if I was in your position, this is what I would do:

Melt all the material in a graphite crucible with a teaspoon of borax, and sodium carbonate or charcoal to deoxidize. Skim and cast this material (at about 1135C/2075F) into a rectangular mold (I use a very rich MAPP flame to blacken the mold so it doesn't stick.) Now you have your anode which should be 95%+ Cufor it to be efficient. An electrolyte I've found to work well is 255 g/L copper sulfate, 75 g/L sulfuric acid (or about 42 mL of 18M). Run the bath about 38C/100F with current density of 70 A/ft^2 at a voltage of 7V.
Remember to use direct current. The cathode should be made of pure copper sheet around which the copper will deposit.

This method works well because you are working with just copper and silver and it is very easy to do. I'm making the assumption that you have a furnace that can get 1200C. I prefer to electrolytically purify silver rather than do it via nitric acid (outlined below) because it uses less reagents and is less noxious (the NOx fumes from dissolving silver are not exactly healthy :-( ).

The sludge that you get on the bottom of the cell should be your silver. Take that silver mud and boil it in sulfuric acid, then melt it, using the same crucible, and cast a silver anode. Then you can electrochemically purify that with a silver nitrate/nitric acid soln as the electrolyte.

Alternatively, you can just dissolve it all in nitric acid, drop it out as AgCl, filter and wash, then add to conc. ammonia, decant, reprecipitate, boil with alkali hydroxide and then reduce to metallic silver with glucose/dextrose powder (solution should be boiling).

The_Davster - 25-8-2005 at 16:29

Dissolve the whole alloy in nitric acid. Try not to use too much of an excess. Add a large length of coiled copper wire to the resultant solution. Let sit until solution tests free of Ag+(test a sample with a small ammount of NaCl for AgCl precipitation). Remove the copper wire from solution and scrape off the silver crystals into the solution. Filter and wash the silver crystals. Keep the copper(II) nitrate filtrate solution and do whatever you want with it. You end up with copper(II) nitrate and silver metal.

Tried and works.

Fleaker - 25-8-2005 at 16:46

Sure it works, but if he had the dozens of gallons of Nitric Acid to dissolve hundreds of them I doubt he'd have asked in the first place. If he has say, 200 of these, and an average mass is 225 grams, then that is 45000 grams, or 45kg of the material. I typically use 'about' 120 mL of 70% nitric acid to dissolve 30g of 92% Ag. If he went to dissolve it all in nitric he'd need over 40 gallons to dissolve it all.

If that's what he wants to do, then I have a source of nitric acid for bulk purchase ($33 for 60L of 70%)

IMO it's far easier, and far far cheaper to do it electrochemically.

uber luminal - 25-8-2005 at 17:04

ooo. these are nice idea's.

I think there might be confusion with this being an alloy.
Its just high purity copper with probably brazed high% silver solder to the tops. It really is only mechanically bonded, not an alloy... er... figurativly anyway. Such that I could also use liquid nitrogen to embrittle the boundaries and just bust the silver off.

So I dont think that melting the whole thing, or tossing the whole in into acid would be cost conservative. I suppose I could just submerge the areas with silver on them eh?

I do have an electric furnace that will reach 1100C. I cold probably borrow time on a box furnace that reaches 1700C though. or I have a VAM, but really I dont think, mixing the two metals would be productive.

Fleaker - 25-8-2005 at 17:18

I do it all the time (melting copper and silver) and it is productive if done correctly. If you could tell me, or better yet send me a sample to see how much silver is in it, I might buy all of it and process it myself. :)

I have no idea as to whether or not you have access to liquid nitrogen at reasonable prices or the proper gloves/equipment required to handle the cryogen. If you've ever worked with it, then I'm sure you know that you had better have gallons of it to embrittle all of those pieces.

12AX7 - 25-8-2005 at 18:10

If it's just silver soldered on, stuff them into the electric furnace (with a reducing atmosphere if possible) and set it for a temp that melts the stuff off. Set the pieces edgewise on a ceramic grate (I'm sure you can find someone to make a piece like this if you don't have one or can't make one yourself) and let the silver and silver solder drip into a ceramic tray below (get one of these too :P).

At worst, the metals will diffuse and dissolve and you'll get ~28%wt Cu melting off at 785C or 72%wt Cu at 965C (depending on how much of the molten silver drips off before it dissolves Cu).


BromicAcid - 25-8-2005 at 18:56

I think I posted this somewhere before and a member of this board, cyclonite4, tried this before and didn't get promising results but here it is from:

Descriptive Inorganic Chemistry 3rd edition (Reoff Rayner-Canham and Tina Overton):
Even though copper is a relatively unreactive metal, it is attacked by concentrated acids. In particular, copper reacts with boiling hydrochloric acid to give a colorless solution and hydrogen gas. The reaction is particularly surprising because hydrochloric acid is not a strong oxidizing acid. The copper (I) ion formed in the oxidation is rapidly complexed by the chloride ion to form the colorless dichlorocuprate (I) ion, CuCl2-. It is this second equilibrium that lies far to the right and "drives" the first step:


When the solution is poured into air-free distilled water, copper (I) chloride precipitates as a white solid

And being that hydrochloric acid is dirt cheap this may be a way to remove the copper from your silver to recover just that alone and in the process make some copper (I) chloride.

Fleaker - 25-8-2005 at 19:15

Excellent Bromic Acid! I'd quite forgotten that method as alternative means to parting it. It requires very concentrated HCl IIRC and is *very* irritating from the fumes. Copper is also soluble in hot sulfuric, but HCl is definitely cheaper, at least to the home chemist.

Nonetheless, I still maintain that electrolysis is the cheapest method to process that quantity.

neutrino - 25-8-2005 at 19:31

>>Copper is also soluble in hot sulfuric

So is silver. Hot, concentrated sulfuric acid is a strong enough oxidizer to dissolve silver, among other things.

Archimede - 26-8-2005 at 17:02

This is an interesting topic. I just went in my garage and broke an old breaker (squareD 20A) with a hammer. I found a few parts that look like a 'silvery' metal, non shiny tho and non frail and just slightly malleable (meaning that when I cut it with a diagonal cutter, the small pieces don't fly around). Question:

How can I determine which part is made of silver? Is there a simple test to do?

I recall a guy that was getting silver from the liquid used to develop camera films using electrolisys. The pieces I cut didnt seem to have a core made of copper, but even if it does, I still think that there have to be a way to getting it thru electrolisys while recycling the solution. Would the method described above work for me?

thanks in advance


uber luminal - 27-8-2005 at 06:12

I may take a picture of a few samples on monday. I thought about it more, and really, liquid nitro will not work. mainly because the two metals thermal expansion coef. are about the same, and they are both ductile, even at those temps.

I thought about just putting them in a furnace at 1000C, and letting the silver drip off as well, but i have this funny feeling that... no, I know copper and silver will diffuse, and that the surface tension of these two metals are frustrating.

Another idea was to place cut off as close as possible, the silver parts, and place it into a furnace at 600C, and jet a high oxygen mix into the furnace. The copper would oxidize more readily, where as the silver will not. However, the scaly black copper oxide, slows any further oxidation considerably, but not completely. It would probably take a few days to work. This of course falls into the... too much work catagory, with the cutting, and eating electricity for several days.

Another idea had, would be to find a grinding material which is harder than silver (not hard), but bound at a high yeild strength. I would use said grinding material, to grind off the silver and collect the dust formed, pop it into the arc melter, and viola.

at this point, I think I will investigate the boiling hydrochloric and the elctrochemical seperation.

Keep idea's coming if you have them. Thanks :)

S.C. Wack - 27-8-2005 at 08:39

Ag2SO4 is much less soluble in water than the CuSO4, however. Excess H2SO4 can be boiled off...or it can be neutralized with NaOH, the sludge washed, and converted back to the sulfates, I would think. The Ag2SO4 can then be dissolved in a lot of water and reduced with Cu, or whatever.

hinz - 8-6-2006 at 13:33

I've a problem with the recovery of silver from silver cutlery.
I've fîrst cutted the cutlery in small pieces and put it into a beaker with appr. 25% H2SO4. then I electrolysed the stuff with a PC-power supply. But the shitty cutlery contained far to much copper and I often had to drain the electrolyte into another beaker and wash the copper sulfate with hot water out of the cutlery and later crystallise the CuSO4, put the electrolyte back into the beaker and continue with the electrolysis.

The problem is that I have to do that, because the Cu2+ which gets reduced at the kathode won't stay at the kathode, instead it(Cu) falls off. The SO42- forms aggain CuSO4 with the Cu oxidised at the anode.
The fallen off copper is very fine and will come in contact with the anode so I electrolyse always the same copper.

How can I inhibit the copper from falling off the kathode or is it possible to put a small beaker under the kathode so that the copper falls into it. Maybe there are some other ideas how to get the copper out of the electrolyte

I'm not an inorganic chemist but some silver salts are quite useful for me. eg. AgF for the nucleophilic substitution of an halogen by F- or AgNO3 for some nitrations

The_Davster - 8-6-2006 at 14:41

Try putting a 'bag' around the cathode. I had decent results with antimony this way using a bag made of a paper towel.