# Aluminium in alkali. Chemistry help please.



## anachronism (Jul 23, 2017)

Chemistry question for the boffins on here. Help would be appreciated.

I have a large amount of gold plated Aluminium to process. There is no copper barrier so this doesn't lend itself to scratching and leaving in strong Nitric to remove the gold plating. Ideally a CN leach of some description would be perfect however the reaction between Al and NaOH prevents the normal way of pH control.

Are there less reactive compounds that would provide a pH of 9 or higher in a stable manner?

I have a feeling that I'm looking for the impossible, but my research has led me to research papers with some pretty esoteric methods of passivating Al in a base solution and I'd welcome the assistance.

Jon


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## Yggdrasil (Jul 23, 2017)

Strangely, this gave me a minute amount of comfort.
The "guru's" out there are just educated versions of our self. 
They are us, and just like us they do not know everything, just very close to everything, 
regarding refining.
  :wink:


Thanks Jon for being "one of us"


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## Yggdrasil (Jul 23, 2017)

Sorry!
I have no answer for your challenge.
Aluminum are inert in nitric I think, but will that passifiy the gold if the gold in in the top layer?
BR Per-Ove


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## Yggdrasil (Jul 23, 2017)

Or?? 
Maybe I'm hoping to be one of you guys?
In retrospect, this was not written i disrespect but in awe of what we can accomplish here.
BR vPoA


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## FrugalRefiner (Jul 23, 2017)

Have you tried reverse AR? Like, very reverse. Strong nitric and drops of HCl. I never tried it, just thinking it's the same principal as gold on stainless.

Dave


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## Yggdrasil (Jul 23, 2017)

Dave! 
Of course he have, he is one of you guys.
I think he is asking for tips, slightly off the beaten track.
One of these secret things that work if things are within correct limits.
Jon please inform us after, if you can,
BR PoA


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## anachronism (Jul 23, 2017)

Ygg although that's really kind of you, you did hit the nail on the head about one thing in particular- yes I'm looking for something slightly "left field" 8) 

Dave- thanks for the tip, I can do it in reverse AR no problem. As Ygg mentioned I'm looking at the Alkaline option. I'm not frightened of some interesting chemistry to deal with on this one. 

Jon


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## Topher_osAUrus (Jul 23, 2017)

anachronism said:


> Chemistry question for the cognoscenti on here. Help would be appreciated.
> (sounds more elegant...and a lot less like buffoon  )
> I have a large amount of gold plated Aluminium to process. There is no copper barrier so this doesn't lend itself to scratching and leaving in strong Nitric to remove the gold plating. Foolish to ask, but just in case you havent.. -did you try a small piece in nitric?
> Ive had Al goldfill stuff that released the foils in a nitric digest, with consistent agitation the foils ended up floating and the Al stayed on the bottom of the beaker. I do not know for sure if there wad a substrate layer between the base metal and gold clad though. .
> ...



I cant think of the title off top, but I know Göran had a thread on removing gold plating from Al, hopefully he sees this and can share the link. I'll keep looking for it though. I think Dave is on to something though. Reverse AR may be the best bet.


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## Topher_osAUrus (Jul 23, 2017)

https://www.911metallurgist.com/hydrometallurgy/how-maintain-ph-on-leaching-tank/

They mention lime as a pH control?


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## anachronism (Jul 23, 2017)

That's cool Topher but it wasn't what I was looking for because it wasn't specific to Al BUT it led me onto a search term that yielded this so you got me there in a roundabout way- thanks 8) 

"Aluminum and many of its alloys are susceptible to corrosion when exposed to alkaline solutions. This is frequently reported for sodium and potassium hydroxide. On the other hand, the low solubility of lime and calcium hydroxide solutions limits their corrosive action on aluminum materials. High corrosion resistance is observed in ammonium hydroxide solutions for most aluminum alloys"

This MAY be what I was looking for- chemists?


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## Topher_osAUrus (Jul 23, 2017)

Ahh, I misunderstood your initial inquiry.
Im with you now.
8)


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## anachronism (Jul 23, 2017)

No drama Toph, you've opened up a whole host of doors for me there by making that mistake. 8) 8) 

My thanks- now to look at the chemistry of scrubbing Ammonia. Onwards and upwards.

Thanks again.


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## Lou (Jul 23, 2017)

anachronism said:


> That's cool Topher but it wasn't what I was looking for because it wasn't specific to Al BUT it led me onto a search term that yielded this so you got me there in a roundabout way- thanks 8)
> 
> "Aluminum and many of its alloys are susceptible to corrosion when exposed to alkaline solutions. This is frequently reported for sodium and potassium hydroxide. On the other hand, the low solubility of lime and calcium hydroxide solutions limits their corrosive action on aluminum materials. High corrosion resistance is observed in ammonium hydroxide solutions for most aluminum alloys"
> 
> This MAY be what I was looking for- chemists?



Yes that will work.


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## Lou (Jul 23, 2017)

Scrub ammonia with sodium bisulfate solution.


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## anachronism (Jul 23, 2017)

Thanks old friend. 

 

Jon


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## nickvc (Jul 23, 2017)

You can use lye to dissolve it.


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## Lou (Jul 23, 2017)

Do not use lye for ammonia gas!

Lye itself will strip the pieces but the solution will not be a fun one to filter!

Jon, in this case a high ORP (or making the pieces to be stripped anodic) is the way to go


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## anachronism (Jul 23, 2017)

Lou said:


> Jon, in this case a high ORP (or making the pieces to be stripped anodic) is the way to go



Yeah I've found a lot of articles on that exact topic funnily enough- you know I think I do have a way to make the base material anodic. Thanks!


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## Platdigger (Jul 23, 2017)

I think what Lou meant by "making the pieces to be stripped anodic" is merely to have them connected to the positive terminal.


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## g_axelsson (Jul 23, 2017)

Topher_osAUrus said:


> I cant think of the title off top, but I know Göran had a thread on removing gold plating from Al, hopefully he sees this and can share the link. I'll keep looking for it though. I think Dave is on to something though. Reverse AR may be the best bet.


http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=19177&start=20#p194541

I used reverse AR for the gold directly plated on aluminium. Today I would probably use the sulfuric cell.

Göran


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## anachronism (Jul 24, 2017)

Platdigger said:


> I think what Lou meant by "making the pieces to be stripped anodic" is merely to have them connected to the positive terminal.



Yep I'm good with that Ralph. I was meaning that I've got a simple way to make a large quantity anodic.


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## Platdigger (Jul 24, 2017)

I must have read your post wrong. For some reason I thought you said "base metal" not "base material".
My bad.
Oh, by the way Anachronism, it's not Ralph, but that's ok.


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## anachronism (Jul 24, 2017)

Platdigger said:


> I must have read your post wrong. For some reason I thought you said "base metal" not "base material".
> My bad.
> Oh, by the way Anachronism, it's not Ralph, but that's ok.




Yeah my apologies I read your post "on the fly" as I was leaving this morning!


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## goldsilverpro (Jul 24, 2017)

For gold on an aluminum base, nickel or no nickel, I have always used the electrolytic concentrated sulfuric stripper (so-called reverse plating). Works great but, as always, you should keep the current low enough to not exceed a solution temp of about 100F-105F. I have mentioned this several other times on the forum. I have even stripped fairly thick gold coatings (say, 3-5 microns) without affecting the Al. However, there are lots of different Al alloys out there and I haven't tried them all. From my experience, though, I would expect them all to work.


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## anachronism (Jul 24, 2017)

Thanks Chris.


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## Geo (Jul 26, 2017)

That's a head scratcher. Are you sure there's not a copper or nickle strike beneath the gold layer? I only ask because I have never seen gold directly on aluminium before. I normally don't recommend it but a prolong leech in dilute HCl may bring the gold off in large sheets without dissolving the whole piece.


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## anachronism (Jul 26, 2017)

Geo said:


> That's a head scratcher. Are you sure there's not a copper or nickle strike beneath the gold layer? I only ask because I have never seen gold directly on aluminium before. I normally don't recommend it but a prolong leech in dilute HCl may bring the gold off in large sheets without dissolving the whole piece.



Thanks for the valued input Geo I appreciate it. The thing is, I'm looking at tonnes of product per month and prolonged soaks don't cut the mustard when it's got to be processed and out the door. 

Jon


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## g_axelsson (Jul 26, 2017)

Geo said:


> That's a head scratcher. Are you sure there's not a copper or nickle strike beneath the gold layer? I only ask because I have never seen gold directly on aluminium before. I normally don't recommend it but a prolong leech in dilute HCl may bring the gold off in large sheets without dissolving the whole piece.


Here is one example of gold directly on aluminium.
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=19177&start=20#p194541

Göran


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## Geo (Jul 26, 2017)

g_axelsson said:


> Geo said:
> 
> 
> > That's a head scratcher. Are you sure there's not a copper or nickle strike beneath the gold layer? I only ask because I have never seen gold directly on aluminium before. I normally don't recommend it but a prolong leech in dilute HCl may bring the gold off in large sheets without dissolving the whole piece.
> ...



Very nice. I suppose there will be examples of just about any combination and configurations of stuff like this.


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