Gold dissolving in excess Sodium Hydroxide?

[ezhil2146]

Hello,

I went through this patent which talks about recovery of precious metals from process residue/tailings.http://www.google.com/patents/about?id=gmQ6AAAAEBAJ

According to this patent, gold gets dissolved in excess NaOH and gets into the solution.

Can anybody explain the chemistry behind this?

Also how can we precipitate gold out of this solution?

Regards,
Ezhil Kumar

 

[freechemist]

Hello ezhil2146,

First of all: Treating tailings and materials which have already been leached/treated to recover the precious metals they contain according to the patent cited in your post won't make you a rich man. I read through it once and stumbled over a number of 0.0006% of recovered gold from solution, based on the original weight of treated material. That means that 1 ton of original material yields 6 mg of gold, dissolved in an enormous quantity of 10% NaOH, whereas it's actual gold content (before treatment with lye) has been determined to be roughly about 5 to 10 g per ton. Therefor only 0.1% (that means nearly nothing) of the original gold has been dissolved by the sodium hydroxide. The big rest of it lies still in the remaining solids, with the difference, that it now may be much more accessible to further treatment, to recover it (cyanide, thiourea etc.), because sodium hydroxide treatment has altered the properties of the gangue-material (clays, silicates), probably by dissolving some of the alumina and/or silica contained, "liberating" remaining precious metal-values. Every published patent is also a disclosure of what one is doing, and sometimes published in a form which even hides the real essentials of the disclosed invention / procedure.

Regards, freechemist