potassium ferrocyanide

[Geo]

I'm going to try an experiment. I've been thinking about the action of sunlight on the ferrocyanide. If I make a saturated solution of ferrocyanide at PH 11 and set it in the sun for a few days, will the cyanide/iron bond break naturally? If it does, iron will settle out of solution leaving cyanide in solution. There may not need to be any metal involved to make the cyanide. If PH is adjusted with potassium carbonate, it should produce potassium cyanide in solution. Is this correct or am I missing something?

 

[FrugalRefiner]

As I understand it, the sunlight (or UV light of the correct wavelength) will break the bond and you will have a cyanide solution. But I don't believe the iron will settle out. It will remain in solution. I also believe the reaction is reversible, and when the light is removed, the ions will recombine into ferrocyanide ions. That's assuming potassium hydroxide is used to adjust the pH. I'm not sure what will happen if you use potassium carbonate.

I hope Deano will see this thread because I know he understands the reaction fully.

Dave

 

[Yggdrasil]

This is how I did understood it in Deanos previous writings on the topic.
I may be wrong though, so I hope some of the masters chime in on this :wink:

 

[Deano]

The reaction of sunlight or uv light on ferri or ferro cyanide complexes in solution is fully reversible, this means that as fast as the cyanide complexes are split off, some of the previously split off complexes will rejoin with the ferro or ferri complexes to reform the original complete complexes.

If ferro cyanide is used then the ferro part of the original complex will stay happily in solution at neutral pH and be available for recombination with the cyano complexes.

If ferri cyanide is used then the ferri part of the original complex will very slowly form ferric hydroxide and precipitate out of solution at pH 7.

The precipitation is not truly total as a very small fraction of the ferri ions will remain in solution. If the uv stimulus is stopped then this small amount of ferri ions in solution will recombine with the cyano ions to preform the original ferri cyanide complex.

When this happens some of the precipitated ferric hydroxide will solubilise as ferric ions in solution and be available for recombination with the cyano ions. This sequence will continue until all of the ferricyanide complexes have reformed.

Thus at neutral pH if the uv stimulus is continued, not taking into account the small level of cyanide loss from the solution by volatilisation as HCN gas, the ferro and cyanide complexes will rapidly reform the original ferro cyanide complexes which will rapidly resplit ad infinitum.

Similarly the ferri cyanide complexes will split into ferri and cyanide ions but due to the ferri tending to precipitate out as ferric hydroxide there will be a lesser recombination rate and the % of cyanide ions in solution will be higher than for the ferro cyanide.

The above only applies to solutions at or near pH 7.

The problem with pH 7 solutions is that the cyanide ions in solution tend to prefer being in the form of HCN gas, some of which will volatilise and be a health danger while the rest of the HCN gas lessens the % of cyanide ions available for reformation of the original ferro and ferri cyanide complexes so all of the above has the reaction rates impacted.

So all of the above is the basic reactions of these complexes but the question is how do you push all of the reactions to one side so that you just have the maximum level of free cyanide for leaching.

This is done for ferri cyanide by raising the pH to 11 at which pH the cyanide is at a maximum in the form of cyanide ions and the resolubilisation of the ferric ions from the ferric hydroxide is minimised.

Ferro cyanide will have the same cyanide ions/HCN gas in solution ratio as for ferri cyanide at pH 11 but it has the problem of the ferro complexes still being stable and soluble until, for practical purposes, pH 12.

There are two options for the ferro cyanide leaching, the first is to raise the pH above 12 to minimise the level of ferro complexes in solution, the second is to add an oxidiser to the solution at pH 11 so that the ferro complexes are converted to ferri complexes which will then precipitate out as ferric hydroxide.

The cheapest way to do the ferrous to ferric conversion is by bubbling air through the solution so that the oxygen in the air can act as an oxidiser.

Either way you will end up with a liquor containing ferric hydroxide. Ferric hydroxide is a strong adsorbent of gold cyanide complexes in solution. The degree of adsorption is much greater if the adsorption can occur as the ferric hydroxide is being formed but it is still high with previously formed ferric hydroxide.

This means that for the best results you should form your insoluble ferric hydroxide at pH 11 by either starting with ferri cyanide or by starting with ferro cyanide and bubbling air through the solution until all of the ferro complexes have converted to ferri complexes.

You then stand the solution away from uv so that the ferric hydroxide can separate out by precipitation and just the clear solution containing cyanide ions
is used for gold leaching.

If you are not too worried about gold losses on the ferric hydroxide then you can either skip or minimise the clean up of the ferric hydroxide stage.

pH adjustments are usually done with lime water or caustic solution, the first will give you calcium cyanide solution and the second will give you sodium cyanide solution.


Deano

 

[Geo]

Great. Thank you Deano. That is a great help. I did try an experiment with gold plated pins. I started the leach with ferrocyanide at PH 11 with a concentration of 4g per liter. I bubbled air through a glass pipette placed near the bottom of the beaker. I bubbled air continuously for three days, night and day. On the third day the pins looked relatively gold free. I checked the PH and removed the solution from the pins. After rinsing with distilled water adjusted to PH 11, I ended with two liters of solution. I checked the PH again and loosely capped the flask. Since then, the liquid, which was cloudy, cleared and a rusty looking solid has settled out. I imagine it is the iron complex. Inspecting the pins showed that all of the gold did not strip. The majority had stripped but there was still a lot of visible gold. I added dilute chlorine bleach to the pins and all the equipment in a large plastic tub. In the container with the pins, I noticed a dark, nearly black frothy solid form. I filtered the mass into a filter and repeated the wash but no more solids formed. Can I repeat the leach of the pins after boiling the pins in distilled water to destroy the hypochlorite?

 

[Deano]

Cyanide is a relatively slow leach and requires substantial movement of the leach liquor through the material being leached in order to get leaching finished in a reasonable time frame.

I have no idea why you added the hypochlorite to your leach, hypochlorite will destroy cyanide per se and also the existing cyano complexes.

You are most likely forming a copper hydroxide complex which will contain much of the previously cyanided gold.

If you want, for whatever reason, to add hypochlorite to your solution at least recover the gold from the solution with carbon before doing the addition.

If you boil the pins with clean water you will remove the hypochlorite and can then repeat the cyanide leach.

Life is much easier if you use better agitation in the leach/pin contact.

You do not have to add high levels of oxygen through the water by bubbling, you only need enough to keep the dissolved oxygen level around 7 ppm.

Deano

 

[Geo]

Thank you. I did not add the hypochlorite to the solution. I added it to the pins after they were removed so that I could handle them easier. All of the equipment such as beakers, hose, pipettes, funnels and anything else was washed in hypochlorite. The solution is being stored in a flask. I was planning on evaporating the solution while keeping the PH at 11. Once the solution is evaporated to less than 1000ml's, I'll add zinc powder to see if anything drops. The black frothy mass came from the rinse of the pins. I adjusted the PH with potassium carbonate. I'll take some pictures and add them tomorrow to help explain.

 

[Deano]

Cyanide is just a poisonous chemical, it does not have any magical properties, it will only cause you problems if you fail to observe the basic operating conditions.

It will rinse off in water the same as any other chemical, if you have done your rinse properly then your pins will be free of cyanide after rinsing.

If you have any concerns then get a meter with a cyanide probe so that you can get exact readings on cyanide in solution levels.

pH control of cyanide solutions is usually done using lime or caustic, carbonates can cause unwanted side effects depending on your water quality.

If you have stripped gold from the pins with the leach then you will have gold in solution and it will come out on zinc, you will only be able to see it if there is enough gold in solution in the first place.

Keep in mind that there will be some redissolution of the precipitated gold, your idea of concentrating the gold in solution level before adding the zinc will help minimise this redissolution.

Deano

 

[Geo]

Thank you. That is very helpful. I really appreciate it. This started as a project to make cyanide gold plating solution. I haven't tried that yet. I feel it is best to learn one part at a time. I have available gold from electronics. I have been studying the basics of plating. There are complete plating kits but it is not the same. I'm am in the process of making a work station with a dedicated fume hood and scrubber just to work with small batches of cyanide using the the process to make my own cyanide leach from ferrocyanide. Ferrocyanide is relatively cheap for me.

If I may ask, without lowering PH by careful additions of diluted HCl, is there any base or process to remove free cyanide from solution?

 

[Deano]

There are two simple options to remove free cyanide from solution.

You can oxidise the cyanide with hypochlorite or strong peroxide, keeping in mind that these options will also oxidise complexed cyanide.

The other way is to consume the free cyanide by complexing it with anything which will form a soluble cyano complex if you wish to keep the product in solution or to complex the free cyanide with anything which will form an insoluble cyano complex if you wish to separate the product from the solution.

The cheapest complexant for soluble cyano complexes is copper, for insoluble cyano complexes most metallic ferri and ferro cyanide complexes are insoluble.

Deano

 

[Geo]

So add iron oxide? Or iron metal? I believe the iron would be better because it will not plate out the same way the gold will if at all.

Thank you for the help.

 

[Deano]

There are no easy ways to form an insoluble ferrocyanide complex as a copper form.

The free cyanide must first be complexed with a soluble ferrous salt at a pH < 9. It makes sense to do this in the absence of uv light so that you are not splitting the ferrocyanide salt back into free cyanide and ferrous salt.

The resultant solution is then contacted with a soluble copper salt so that the insoluble copper ferro cyanide can form.

If you want to form a copper ferricyanide complex you first form the ferro form and allow it to oxidise while wet to the ferri form.

Adding iron metal will not do anything to help you.

Deano

 

[Geo]

Thank you very much. I will do some more small batches and see if I understand all I have read.