# Potassium Ferricyanide Leach



## kernels (Feb 22, 2017)

Hi guys,

I'm at the point now where I've successfully recovered around 3 ounces of gold from E-waste over the past year and a half. The vast majority of this has been from ICs and plated fingers.

I am very interested in moving on towards recovery from plated connectors / pins, once again, it will be small scale.

I have ordered some Potassium Ferricyanide as it seems to be reasonably easy to access because of it's use in photography. Sodium Cyanide and Potassium Ferrocyanide are comparatively difficult to get hold of. 

I have a few questions to get me onto the right track. My suggested process is as follows:

1. Dissolve some Potassium Hydroxide (Caustic Soda) in water until pH is 11.5
2. Dissolve some Potassium Ferricyanide in the solution. Is 2g/L about the right concentration ?
3. Add a small amount of oxidizer to the solution - Hydrogen Peroxide. 
4. Place some gold plated items in the solution.
5. Place the beaker in a safe location in sunlight - Why ? What does the UV do ?
6. Keep agitating the solution regularly until all gold is dissolved.
7. Acidify (with HCl) a single drop of the pregnant solution in my fume hood to test for Au with Stannous Chloride.
8. Remove excess oxygen from the solution - I have a vacuum pump, but have also seen it done with yeast.
9. Add pieces of Aluminium foil to the solution to cement out the gold
10. Test again as per step 7 to confirm all gold has been cemented.
11. Wash all cyanide solution traces out of the resulting powder and add wash water to the main solution.
12. Kill the cyanide in the main solution with household bleach. - How do I know it has all been destroyed?
13. How is the left-over cyanide waste disposed of safely ?

I would very much appreciate any answers to the above questions or pointers if I've skipped any steps.

Thanks!!


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## anachronism (Feb 22, 2017)

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


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## kernels (Feb 23, 2017)

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



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.


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## Elektrikis (Feb 23, 2017)

kernels said:


> anachronism said:
> 
> 
> > Hi Kernels
> ...




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.


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## frank-20011 (Feb 24, 2017)

hello,

@kernels:

i would interchange step 1 with 2 and instead 3 you can better use aeration by an aquarium air compressor (same with "real" cyanide leaching).
so you have your oxidiser (there are many papers which discribe the oxidising AND mixing of cyanide leaching sollutions, espeacially in mining leaching operations, by bubbling in of compressed air or O2) and so you have no problems with your step 8.
in step 8 i have used yeast but...yeast  ...i think about MnO2 but i don't know, what is the best but you have to destroy and remove H2O2 or O2.
UV light (sunlight) breaks the ferrocyanide and "frees" the CN-molecule so you get your active ingredient: cyanide.

today i make my next little test: 1g of potassiumferrocyanide in 100ml of aqua dest at 80°C, i put in 8 (really only 8) different pices (and types) of newer connector pins in the sollution and placed it under a 2x18W UV flourescent bulb for 1hr.
after that i let it stand (without the lights) for 5 more hours but only a weak bleaching of the platings, no real deplating even at these high concentration.

it was almost my last try with ferrocyanides and i will only make one real last try at the summer outdoors but i do not know why sunlight should work and why (in my case today) artificial and intensive UV light does not work.
the UV lamp i have used today was such a rubbish of a "face tanning shop" for the use at home.

best regards...frank!


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## FrugalRefiner (Feb 24, 2017)

frank-20011 said:


> in step 8 i have used yeast but...yeast  ...i think about MnO2 but i don't know, what is the best but you have to destroy and remove H2O2 or O2.


I've read that applying vacuum to the solution will also remove the O2. I have no personal experience.



> the UV lamp i have used today was such a rubbish of a "face tanning shop" for the use at home.


I wonder if that might be the problem. There are different frequencies and strengths of UV light. 

Dave


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## frank-20011 (Feb 25, 2017)

hello,

i haven't a good vacuum pump and i do not know how deep pressure should rise for deaeration!
the same problem with yeast-a really DIYer methode and i don't know if it work's, how long the reaction should take place and so on. i let stand it over night.
cody used this method successfully.

(https://www.youtube.com/watch?v=HhuwO8AjM7k, start at 3:00)

@ frugal refiner: as i've wrote: i will repeat the test in spring or better summer but the artificial UV source is much more powerfull than the UV by sunlight, you need a shorter time to get skin demages like burnings than in the sun and it would surprise me if you need the mixture of different wavelenghts like present in the sunlight.

energy is the codewort, energy make your skin get red and energy smashed the molecules BUT: no evidence for these thougst!


best regards, frank!


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## g_axelsson (Feb 25, 2017)

When it comes to energy in light, the wavelength is the most important factor. Shorter wavelength have higher energy and a photon can drive a chemical reaction. If the wavelength is too long nothing will happen.
Photons doesn't cooperate, they always work one by one.

Göran


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## kernels (Feb 26, 2017)

Thanks everyone for your contributions, I will run a few experiments with H2O2 and also air bubbler to see if I can first get gold into solution, then experiment to try and get it back out.

I did manage to find a source for ferro, but will still try the ferri just to see for myself that it doesn't work.

At the moment we are in summer here and the UV in New Zealand is very high (burn time for skin is measured in minutes), so I will definitely come back with some results once I have something working.


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## frank-20011 (Feb 26, 2017)

hello,

"the wavelength is the most important factor"

that's exactly what i try to express: under the arteficial light (that i've used) you need only a few minutes to get a skin burn and so there is more energy in the emmissed wavelenght than in the sunlight.

"At the moment we are in summer here and the UV in New Zealand is very high"

very comfortable situation  

i have to wait for that what we call "summer".

best regards!


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## g_axelsson (Feb 26, 2017)

frank-20011 said:


> hello,
> 
> "the wavelength is the most important factor"
> 
> that's exactly what i try to express: under the arteficial light (that i've used) you need only a few minutes to get a skin burn and so there is more energy in the emmissed wavelenght than in the sunlight.


What is the wavelength then? Effect have nothing to do with it.

Göran


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## frank-20011 (Feb 27, 2017)

hello,

sorry, only a misunderstanding, i agree absolutely with you:

"When it comes to energy in light, the wavelength is the most important factor"

and so my conclusion of it was: in sunlight i need 30min to get a skin burn (only a example, in the summer, at the moment i can stay a week in sunlight and nothing happens to my skin), under my UV lamp i need only 15mins and so, i only thought: more energy by my lamp, more and faster reaction takes place.

that's my explenation to ME: my lamp hurt the skin an so it should be enough to break ferri into CN.

enough, best regards, frank!


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## g_axelsson (Feb 27, 2017)

That is the problem. If it is a lamp used to tan it is long wave UV while for example a lamp used in a water filter to kill bacteria is using short wave UV. Long wave UV doesn't have the killing effect that short wave light have.

If you have a tanning lamp with a high effect it will work faster than a tanning lamp with lower effect or even the sun. But if it lacks the wavelength a higher effect lamp will not work just as a weaker lamp with the same wavelength. Not working but faster doesn't mean it will work.

Sunlight contains both short wave and long wave UV light.

I don't know what wavelength of the UV light is needed to get the effect described above so this discussion could be moot.

Göran


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## kurtak (Feb 27, 2017)

Keep in mind that though UV is "needed" to break the iron bond of ferrocyanide in order to make the CN available to dissolve gold - but - UV is also a destroyer of CN - & if H2O2 is used as an oxidizer the ability of UV to destroy CN is increased

I can only assume the more intense the UV energy (shorter wave length) the more destruction of CN by the UV

In my research on the destruction of CN I found two methods that use UV to speed up the destruction of CN

1) is using H202 in combination with UV - the UV (bulb) wave length being used is 254 nm - which is the same wave length use in tanning bulbs

2) uses ozone in combination with UV - with the same 254 nm wave tanning bulb

Any one of those elements (H2O2/ozone/UV) cause destruction of CN "by them selves" - but the destruction is VERY slow when it is just one of those elements 

However - when high energy (short wave) UV is used in combination ether ozone - OR - H2O2 - the speed of CN destruction is increased

It should also be noted that the closer the UV bulb - the more UV energy absorbed by the solution - the further away the UV bulb the less energy absorb by the solution 

In other words - it "appears" - that UV used with ferrocyanide can be a double edged sword (needed to break the Fe bond & free the CN --- but also - if over done - result in destruction of the CN you are trying to free) 

frank-20011 posted



> and placed it under a 2x18W UV flourescent bulb for 1hr.
> 
> but only a weak bleaching of the platings, no real deplating even at these high concentration.
> 
> ...



Then in another post Frank posted



> the artificial UV source is much more powerfull than the UV by sunlight



So - as much as UV is needed to break the Fe bond & free the CN of the ferrocyanide - it is also possible you are destroying the CN by using a high intensity "tanning" bulb

That would be especially true if you are placing the UV bulb close (&/or "very" close) to the CN leach solution - & even more so true if you are using H2O2 as an oxidizer in the leach solution

So - if you are using H2O2 - change over to using an aquarium air bubbler "with" the bubbler stone - & ether move your "tanning" bulb farther away - or change over to another type UV bulb (like the ones used in aquariums for lizard's) to see if that improves the leaching with ferrocyanide

in other words - you want enough UV to break the Fe bond & free the CN --- but not so much that it destroys the CN as fast as the Fe bond is broke 

Kurt


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## frank-20011 (Feb 27, 2017)

hello

and thankyou...a lot of true objections and inspirations.

at first i am waiting excitedly for the results of kernels tests with a real nice intensive sunlight and later i will repeat a leaching test at the summer...

best regards!


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## kernels (Feb 27, 2017)

Some Results from my first test this morning . . . 

I dissolved 0.5g of Ferricyanide (yes Ferri) in 200mL of water and added a tiny amount of Sodium Hyddroxide to get the pH up towards 11

I then placed a few gold plated SMA connectors in the solution and placed the beaker in direct sunlight. I didn't add the air bubbler, but stirred the solution every 20 minutes or so for a few hours.

Result - SMA connectors were fully deplated in about 2 hours, I then added some connectors (from 80's mainframe board) which look to be thick plating. They have been in for an hour or so when this picture was taken and are starting to lighten up.

First picture shows the solution shortly after placing in the sun



A few of the deplated connectors after a few hours, looking good



The other connector after an hour or so



Next step will be trying to get the gold back out of solution, but the concentration will be so low that it might be difficult to know if it is cementing out or not.


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## kernels (Feb 27, 2017)

Here is a before-and-after of the SMA connectors


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## frank-20011 (Feb 28, 2017)

hello,

crazy....crazy, really nice!

with ferrO you need maybe only 10 minutes?

i think you can't get it out by zn or al (steel wool) because it's to less, to less to get it back from the precipitating metal in visible amounts but you can make a detection before and after the out-zincing.

maybe get out these very small amounts of Au by an improvised electrolytic cell? two sheets of stainless?
is it possible? 



best regards, frank!


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## anachronism (Feb 28, 2017)

Hi Kernels

Thank you for sharing that. I've learned something there!

Always good to learn.

Jon


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## kernels (Feb 28, 2017)

Another update, the trial connector was also successfully deplated and the cementing on strips of aluminium foil appears to be working (very slowly). The foil strips definitely show the brown of gold cementing out. I took this picture yesterday evening, I will get another picture tonight when I get home from work. 

I am a bit confused about the cementing, if there is still free cyanide, won't the cyanide just re-dissolve any gold that cements out ?


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## frank-20011 (Feb 28, 2017)

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!


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## g_axelsson (Feb 28, 2017)

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


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## kernels (Feb 28, 2017)

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 ?


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## anachronism (Mar 1, 2017)

Yeah it's cheaper Kernels.  

Jon


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## g_axelsson (Mar 1, 2017)

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


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## kurtak (Mar 1, 2017)

g_axelsson said:


> I have been doing some research and found a couple of interesting articles.



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



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

Kurt


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## frank-20011 (Mar 1, 2017)

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!


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## g_axelsson (Apr 8, 2017)

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.



Göran


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## kernels (Apr 8, 2017)

Excellent! Please let us know your results with recovering the gold with electrolysis!


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## sethby (Feb 5, 2020)

Elektrikis said:


> kernels said:
> 
> 
> > anachronism said:
> ...


Please Elektrikis how do I make an anhydrous ferrocyanide (white)


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## anachronism (Feb 5, 2020)

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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## jimdoc (Feb 5, 2020)

https://www.pdfdrive.com/general-chemistry-e157973572.html


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## anachronism (Feb 5, 2020)

Thanks Jim you're a gent.


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## armahema (Jan 17, 2022)

Recovery of Gold from Ore with Potassium Ferrocyanide Solution under UV Light​








Recovery of Gold from Ore with Potassium Ferrocyanide Solution under UV Light


In this study, potassium ferrocyanide, a nontoxic cyanide precursor in dark and diffuse reflection environment, was applied as reagent for the leaching of gold. The free cyanide ions could gradually release from potassium ferrocyanide solution under the ultraviolet light. Orthogonal leaching...




www.mdpi.com


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