cyanide and ferricyanide wastes

[100tific]

hello,
I have been reading something on the forum about cyanide and ferricyanide waste used in chemical electropolishing, or in gold extraction from minerals, but I have not found any post where it is written in detail how to dispose of this waste safely.

I spent some time researching how to do it. First, those that contain free cyanide, which are blue in color. It seems that using the hydrogen peroxide method and a copper catalyst works, but I have a problem with ferricyanides, and that is that they interfere with the potentiometric titration with silver nitrate. That is, I cannot measure them.
In order to destroy that free cyanide, I must first break down the ferricyanide into free cyanide and iron. For this I am using persulfate and UV light.
I have carried out cyclic voltammetry tests on ferricyanide to verify that I can effectively eliminate it with persulfate and UV light, but although I can lower its concentration after 10 hours of exposure to light, I cannot completely eliminate it.
Does anyone have experience with this? Could you give me some advice?
Thanks

 

[anachronism]

The blue you're referring to is also known as Prussian Blue and it's used as a die. Isn't that Ferric Ferricyanide that's come from whatever they were leaching? I've not come across free cyanide in a solution presenting as a blue colour myself.

 

[Deano]

If you have a blue colour in solution it is, as Jon said, most likely Prussian Blue, it is certainly not from dissolved cyanide on its own.
Ferrous and ferri cyanide complexes will split into free cyanide salts in solution and the corresponding ferro and ferri salts.
The cheapest and easiest way to do this is to have a slowly stirred solution of these salts out in bright sunlight.
UV light does not penetrate to any great depth in water so you need to keep presenting fresh solution at the liquid surface for the uv to react with.
In order to drive the reaction to completion you can add caustic to prevent reversion to the original salts when the solution is being stirred.
Under alkaline conditions the ferri salts will precipitate from solution as ferric hydroxide and thus will not be available to recombine with the cyanide to form the original salts.
The formation of the cyanide and ferro and ferri salts is an equilibrium reaction and there is not a lot of sense just playing around with this equilibrium as the salts pass in and out of the sphere of influence of the uv at the liquid surface.
The dissolved oxygen in the solution will oxidise ferro complexes to ferri complexes, the reaction can be sped up by peroxide addition.
Any organic will be degraded by a combination of uv and an oxidation agent such as dissolved oxygen.
Chemical oxidants such as peroxide and hypochlorite will speed up the rate of oxidation but are more expensive than dissolved oxygen.
This is why large scale cyanide tailings dams are often remediated to remove free cyanide by spraying the liquid into the air during daylight hours.
Deano

 

[100tific]

i have a blue color in solution due to Cu2+. this wastes are produced by polishing gold pieces using KCN, and H2O2 . The waste have AuCN i recover later the Au using Zn, and my solution have now Zn(CN)2, CN- and other ions metals in solution. this solution have no ions Fe, but this is not the problem.

There are another procedure. the jeweler use a solution of K3[Fe(CN)6] and KCN and apply a potencial. this can polish the surface of the piece dissolving its first layer and producen AuCN too or some complex of ferri and gold. i recover this gold using Zn too, and in the waste i have [Fe(CN)6]3-, CN- and others ions metals.

I know how to do a mesurement of CN- in the first matrix. But when i have ferro/ferri cyanates in solution i cant measure it in a valoration with AgNO3 because it produces some interferences.
I want to know how to break the bond between Fe and CN- in ferricyanides with the objetive of measure the CN- in the solution, for later, destroy it using H2O2 or another method

 

[anachronism]

As Dean mentioned there's a difference in the solution CN- content between straight metallic cyanides and the others. A straight KCN solution disassociates to K+ and CN- completely. The pH of the solution determines how much of that CN- remains as CN- and how much of it forms HCN. Incidentally the dangers occur when the HCN portion of the free cyanide exceeds the amount that can remain dissolved in the solution. The others do not completely disassociate, instead they reach an equilibrium state with a much lower CN- content. I may be incorrect however I believe Ferri will be the lowest CN- concentration out of the two. As such I don't think it's necessarily the Silver is completely to blame for the issue you are experiencing.

I'm pretty certain Dean will correct me if I have somehow got that incorrect but that's my understanding of the chemistry.