Potassium Ferricyanide Leach

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hello,

in cyanide zinc-out instruction you can read, you have to put in some zinc powder and zinc metal sheet also to avoid redesolving of precipitated Au again.

best regards, frank!
 
Cyanide leaching requires an oxidator, often atmospheric oxygen dissolved in the leach is enough. When the solution goes to the cementation step in mines the oxygen is removed first by passing it through a vacuum chamber. That stops the remaining cyanide to dissolve the gold again.

If there is oxygen and excess cyanide in the solution when going to the cementation step the redissolving of gold should use up the oxygen after a while. It will also use up the excess cyanide.

If there is no free cyanide left the gold is replaced by the zinc in the gold-cyanide complex and that stops it from dissolving any gold.

It is similar to cementing out gold from aqua regia, until the AR runs out of nitric some gold will dissolve again but it will cement out by the copper.

Göran
 
Thanks Göran, that is pretty much my understanding too. With oxygen removed, the cyanide will no longer be able to dissolve gold and is replaced in solution by your donor metal.

Is there any disadvantage to Aluminium instead of Zinc for the cementation ?
 
I have been doing some research and found a couple of interesting articles.

Young 1993, "CYANIDE REMEDIATION: CURRENT AND PAST TECHNOLOGIES" https://www.engg.ksu.edu/HSRC/95Proceed/young.pdf
Under Photolysis they discuss the splitting of ferric and ferrous hexacyanide complexes by light. In it they state that a wavelength of circa 420 nm or shorter is needed to split off CN- ions. That is a lot longer than 254 nm that you use. Your lamp should have the wavelength to split off CN-.

Another interesting article : "Sunlight-Induced Photochemistry of Aqueous Solutions of Hexacyanoferrate(II) and -(III) Ions", http://pubs.acs.org/doi/abs/10.1021/es00046a016

One thing they talked about in the articles was that UV has the ability to break down cyanide all the way down until it really is non toxic. Maybe the shorter wavelength in your lamp broke down the released cyanide as fast as it was produced. By the way, 254 nm is really short and should NOT be used for tanning. It doesn't give a tan but it damages or kills the cells of the skin.
Another factor is that 254 nm is absorbed by water a lot quicker than 400 nm, so it is only the top layer of liquid that is affected by the shorter UV. 400 nm is actually within the visible part of light, at the deep blue side.

I found a third article that was a bit off topic but well worth the read. It is a run through of a number of different methods to destroy cyanide waste. "CYANIDE DESTRUCTION" http://www.sgs.com/-/media/global/documents/flyers-and-leaflets/sgs-min-wa017-cyanide-destruction-en-11.pdf

Göran
 
g_axelsson said:
I have been doing some research and found a couple of interesting articles.
Click to expand...

Awesome articles/links Goran --- much more detailed then anything I found in my research - THANKS for posting them 8) :!:

Could you maybe add those links to the thread I started on CN destruction/disposal so we have as much as possible info related to CN waste treatment in one spot :?: :!:

One thing they talked about in the articles was that UV has the ability to break down cyanide all the way down until it really is non toxic. Maybe the shorter wavelength in your lamp broke down the released cyanide as fast as it was produced.

Göran
Click to expand...

Correct & the point I was making in my last post & again "thanks" for confirming that as a "possibility"

Kurt
 
hello,

very interesting, thank you a lot!

o.k. and thanks about information about wavelenght!

but at the moment outdorrs the sunlight is MUCH, MUCH to weak here...unfortunately!

Please give me some information about a improvised electrolytic "precipitation" of dissolved gold from cyanidic (or ferri/Ferro) sollutions.

Is stainless sheet as anode and cathode o.k., how much volt and how much amps per cm²?

is this method preferential for very low concentrations of Au like in these test? (with long running times) or is zincing a better option?

is a waterjet pump stron enough to get out the O2 from the sollution by vacuum
or is a stronger vacuum needed?

best regards!
 
So... I bought some ferricyanide on ebay to test the leach. Mixed a gram with 200 ml water and some NaOH to get the pH to 11. I didn't measured it, just calculated the amount needed and added it.

I had some partially plated pins that I split down the middle and added half of it to the ferricyanide leach. Then I put it outside to do it's magic. After a few days without any visible change I added a dab of hydrogen peroxide.

A few days into the leach the weather turned really cold and the leach froze almost solid. Now I took it inside and measured the pH and it was around 9, not as high as I would have liked... it seems like somehow the pH had dropped during the leach. As I now had it indoors I wanted to be on the safe side and added quite a lot of NaOH. After that the crystal clear golden solution started to get murky.

Over the following days the murkiness got more pronounced and a layer of orange brown sludge started to precipitate on the bottom. At the same time it looked like the plating started to disappear.

Now it's been sitting for two weeks in the window and it looked like all the plating had been dissolved. The sludge on the bottom looks like rust and the pH is down at 10 again. I'm pretty sure that carbon dioxide isn't responsible for the consumption of NaOH, the leach was covered in double layers of saran wrap so the gas exchange should be minimal.

I think that the iron cyanide complexes splits up from light and the iron is precipitated as iron hydroxide, freeing up the cyanide and driving the reaction towards more free cyanide that can react with the gold.

The leached pins did have some gold left inside the contact surfaces so I will run it in a fresh leach for some more time. Other than that it seems to be working just fine although slow. Next step is to recover the gold from the cyanide solution. I'm going to try with electrolysis.

The pregnant leach is crystal clear with the rust at the bottom. Decanting or filtering can probably solve that easily enough. The color of the leach is paler than it was when I started, at least that is what I remember.

This was a first test so next time I will take more notes. It took a lot longer than I first suspected so some details were lost in the fog of time... :mrgreen:

Here is the money shot of the unleached pins, the pregnant leach and the leached pins.
Ferricyanide-1.jpg

Göran
 
Elektrikis said:
kernels said:
anachronism said:
Hi Kernels

You must make sure you buy FerrO not FerrI. They are similar however it's much much harder (nigh on unworkable)to use UV to liberate the CN- ions with Ferricyanide.

Jon
Click to expand...

Bugger, not exactly what I was hoping to hear. I will keep hunting to see if I can source either ferro- or Sodium Cyanide then.
Click to expand...


Hi!

the reactions between ferri + koh and H2O2= ferro + o2

If you made anhydrous ferro.(white) Disolv in water and macke free KCN and no need UV.
Click to expand...
Please Elektrikis how do I make an anhydrous ferrocyanide (white)
 
There's a book called "General Chemistry" by Linus Pauling that contains all kinds of interesting things. A lot of the reactions discussed on this forum are also in this book.
 

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