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
