Tektron Gold Cyanide solution

[rexhavoc]

Hi all,
Rummaging around in the fume hood found half an ancient bottle of Tektron (Au-1l, Class 6 UN1588 is all it says) that I'm told once upon a time was used for electroplate repair of gold PCB contacts.
No documentation seems to have survived, same for an online presence of the manufacturer.

I'm way out of my element here - can someone point me to a simple method to precipitate, or plate out, the Au before disposal?

thanks,
rex

 

[nickvc]

My advice place the solution in a beaker heat add zinc dust and stir to precipitate the gold, once you have the gold precipitated decant the solution for safe disposal, you can remove any excess zinc using nitric, this needs doing in a good fume hood.

 

[rexhavoc]

Thanks Nick, I like the sounds of that.

Regards,
rex

 

[goldandsilver123]

The "recipe" that we use is:

Add 25 g of sodium hydroxide for every liter of solution
then add 12g of zinc dust with lots of stirring for every gram of gold that you expect to recover.

Stir for 15 minutes then let it settle for 12 hours.

This recipe is in large excess, but works.

Remember to wash the zinc with water, many many times to remove the cyanide. Otherwise Hydrogen Cyanide will form when you add acid.

Edit:

You can plate it on steel wool (cathode, negative), using a stainless steel (anode, positive) container, with around 4 V. This is my preferred method.

 

[rexhavoc]

There's the rub - how much Au could one reasonably expect per volume unit of a commercial plating solution, in order to approximate the required Zn?
Gold plated steel wool - fit for a king's kitchen

 

[upcyclist]

There's the rub - how much Au could one reasonably expect per volume unit of a commercial plating solution, in order to approximate the required Zn?
Gold plated steel wool - fit for a king's kitchen

You're right, you don't know--it all depends on how much it's been depleted. After you make sure you've raised the pH, add the zinc incrementally. When the zinc stops cementing out gold, the powders forming at the bottom will shift from brown (gold) to grey (unreacted zinc).

 

[goldandsilver123]

For decorative gold plate there's around 4 g/L, for technical application it's usually higher, but that's when the solution is brand new.

It's important to add sodium hydroxide to help dissolve the zinc.

In basic pH (around 10) zinc will form Zn(OH)2 that is solid, and it's the way the ppt zinc in waste treatment (after cyanide destruction), all the unprecipitated zinc will be held by cyanide in this pH.

In strongly basic pH the this precipitate will dissolve to form [Zn(OH)4]2− and help the gold precipitation be faster and precipitate the gold close to quantitatively.

Zn(OH)2 + 2OH- -> [Zn(OH)4]2−

 

[nickvc]

A test I use is to place a small piece of aluminium foil into the solution after you think the gold has precipitated, if there's still gold in the solution it turns the foil a tan colour, it may not be a perfect test but it works for me and I rarely leave much gold in the solution.

 

[upcyclist]

A test I use is to place a small piece of aluminium foil into the solution after you think the gold has precipitated, if there's still gold in the solution it turns the foil a tan colour, it may not be a perfect test but it works for me and I rarely leave much gold in the solution.

That's a handy trick--thanks!