[Mennig]

Newbie here, if I'm in the wrong hole just kick me where I need to go.

I have a quantity of Aluminum Silver alloy and I wish to recover the Ag.  From research on the product I have determined the it is a 96% Al and 4% Ag alloy. The current material is in the form of radio tower cable boxes ranging in weight from 5-6# to maybe 15# each.

Intially I will reduce bulk buy grinding the boxes into a powder, maybe 100-200 mesh.

i am considering using Na OH(aq) to reduce the Al. 2Al(s)+2Na(aq)+6H20(1)--->2NaAl(OH)4(aq)+2ha(g).

Please bear with me, it's been 50 yrs since my last chemistry class!

Now I believe (??) the Ag(s) should be non reactive with NaOH and end up as a slurry in the bottom of my tank.  At this point I am intending to decant and filter my liquid.  H20 Rinse and  Refilter.

This is where I get confused.  Would my Ag be an OH? or could I just take this filtrate disolve it in HNO3-H20, precipitate the Ag as AgCl and cement.  Any assistance would be greatly appreciated.

[Arkcon]

Aluminum hydroxide forms a soluble complex with an excess of hydroxide.  Silver doesn't react at all.  So, conceptually, you just need to add lots of NaOH.  But you do have to think practically.  How much alloy do you have?  How much space does it take up?  How much NaOH solution would it take to submerge it all?  How much filtering and rinsing will you need?  And where will you sent the much greater volume of rinse water?  How much silver slurry will you get?  Is it a tiny amount that you could blow away with a sneeze, or a very large amount.  Will you try to melt it into a lump?  Do you have a furnace meant for that.  If this alloy is used for something, it may be more valuable as is.

[Mennig]

Thanks for your input.  I have approx. 4000# so it's worth a little trouble.  At 4% that makes nearly 160# of silver.  I do have a smelter already set up in an industrial operation, so no problem there.  One more question however.  I would like to recover the Al in a form that could be marketed,  any suggestions?

[Borek]

I have approx. 4000#

Around 2 tonnes, right? Wow.

You will need something like 10-12 tonnes of NaOH, that's a whole truck.

Somehow I doubt you will be able to do it without breaking zillions of OSHA and environmental regulations.

I would like to recover the Al in a form that could be marketed,  any suggestions?

Al or Ag?

[Mennig]

The major value is in the Ag but I don't want to waste all the Al either.  Maybe I could go about this in another way in which I haven't considered?

It could be smelted but I have know idea how I would separate the two?

[vmelkon]

You are going to end up with some aluminum compound. I guess one of the biggest uses is as Al2O3 for sandpapers.

For silver, at least it is easy to turn its compound back to the metal.

[Mennig]

Maybe I'm really going about this in the wrong way?  Maybe I should be considering electrochemical means.  I know electrolytic copper refinning is a relatively straight forward operation. The electrical potential between Al (-1.71v) and Ag (+0.80v) is even greater than that in copper refining.

Idea:  What if I used a Silver Nitrate electrolyte with my bulk Al/Ag scrap box hanging on the Anode (+), "I would assume I would want to bag it " to contain my Al slime. For the cathode (-) I would use a pure silver cathode sheet.  I have no idea at this moment what my electrolytic concentration would have to be or the proper voltage or amperage but this looks like it should work?  Then I would have the two elements back to their original forms, the Al in my anode bag and the Ag plated onto my Cathode that should be near 999 and with what looks like very little waste products at the end. Suggestion?

[vmelkon]

The Al will oxidize and probably form the hydroxide. Al is a very reactive metal (almost in the same family as sodium, potassium, magnesium).

[Mennig]

After I put up the last post I got to thinking about OH buildup, maybe I could use it to my advantage.  Since the amount of Ag in this alloy is so small maybe I could use a half cell type of arrangement.  One cell would be my .999 silver anode in a nitric acid electrolyte and the other would be my Al alloy as the cathode in an Al OH electrolyte, put a salt bridge between them and let them do all the work.  Won't even need any outside current.  Will it work?

[Mennig]

OOPS!  Just thought about that last comment for a minute.  The bridge will only let electrons cross not ions.  If I just use a standard stripping cell with with silver nitrate as the electrolyte, .999 cathode and alloy as anode, won't the OH buildup cause a H problem? Not as to release of H(g) but as to interference with ion transfer? 

[Arkcon]

Theoretically, exposing two metals of greatly differing electronegativity should cause the more active on to rapidly corrode away.  Maybe in a moderate saline environment over a long period of time.  However, some alloys are resistant to corrosion, for example, stainless steel.  I doubt 5% silver makes the alloy very corrosion resistant, but still, the properties may surprise you.

[Mennig]

Well of course I would want this separation to take place within a reasonable period of time.  Which takes me back to the idea of a salt bridge with two difference cells.  My mind is unsure if ION transfer is capable in the case of a salt bridge.  If it is capable then I would just need to think of some electrolyte to use in my anode to allow my alloy dissolve without excessive OH formation.  Then again maybe I am creating a problem that isn't there.  Would OH formation be a problem in a half cell application?

[Mennig]

I am still persuing the separation of a 96% Al, 4% Ag alloy to economically recover the Ag.  Possible separation by electrolytic stripping.  First consider my alloy as the anode in the stripping cell. Second consider pure Al as my cathode.  Understanding the intense reaction of Al to acidic attack it would be necessary to make a very dilute HCL solution as my electrolyte. My assumtions are as follows. (1) With a very dilute solution of HCL the initial reaction of the Al cathode and alloy annode would not be dramatic, infact it would take some time for the reaction to show any signs of activity do the the surface oxidation of both annode and cathode. (2) The introduction of current should increase the decomposition of the alloy and produce travel of Al ions to the cathode.  (3) Ag and excessive Al in the form of AlOH or AlO3 should should accumulate as slimes in the bottom of my tank. (4) H offgassing.
Comments/Suggestons

[Arkcon]

I would suggest you not use HCl.  HCl reacts with silver ions to produce insoluble silver chloride.  I'd suggest a dilute nitric solution, even sulfuric acid produces a (somewhat stable) silver compound silver sulfate.

[Mennig]

but couldn't I just cement out the silver from the silver chloride in the presents heat and Fe?  My reasoning for using HCL instead of nitric is to not have to deal with nitric oxide and wouldn't HCL as the electrolyte allow for a more complete transfer of Al to my cathode?  This way not only would I be recovering the Ag but I would be retaining a pure, or nearly so, Al?