Dissolve gold into cyanide solution

[mutalashi]

I have very fine 50 kilos ash of gold bearing computer mother boards' very well burnt chips, and turned into a very very fine powder in the ball mill. This ash contains 75 grams of gold. I prepared a cyanide solution of 2% concentration in 150 itres of water and put gold bearing ash into a leaching tank which very well rotated the whole stuff for 24 hours. The leach tank is a rotating tank which rotates the ash very well. The atmospheric air was incorporated into the tank from the bottom of the tank. The temperature of the tank was kept at 50 degree celcius all the gold particles were in contact with the cyanide solution. And once the dissolution of gold into cyanide solution was tried without heating the water. But even then the gold did not dissolve into cyanide solution. Please help and throw light on the factors that did not let the gold dissolve into cyanide solution? We sent the cyanide samples twice to the lab. The report showed there was no gold in the solution.

 

[nickvc]

Without knowing what else is in the ash it's hard to give you an answer as several things could be inhibiting the dissolution of your gold or other elements may be been dissolved and consuming the cyanide before it can effectively dissolve the gold or even something is precipitating the gold from the solution.

 

[mutalashi]

Without knowing what else is in the ash it's hard to give you an answer as several things could be inhibiting the dissolution of your gold or other elements may be been dissolved and consuming the cyanide before it can effectively dissolve the gold or even something is precipitating the gold from the solution.

Thanks for your cooperation. These chips contain silicon, germanium, little copper oxide and little bit of palladium, silver, and pins made of copper and nickel. The copper and nickle pins are seperated from the ash through the seive. I have checked it very carefully. Nothing precipitates the gold. Because gold is in the form of small gold balls. 1 kilo of chips contain 1.5 grams of gold. And when we check the qty. of gold by panning the ash, qty. of gold remains the same.

 

[Lou]

Ash, huh?

Well,


1. CN is not good for large particles of gold, those may take weeks to dissolve
2. You will have copper and other materials that will compete for CN ligand
3. When you say ash, and coming from this process, I presume there is much carbonaceous material. If there is much carbon, then it will be ''preg robbing'' so the gold is ad/absorbed onto the carbon. You've made a Carbon-in-pulp situation.

Also, you're free CN concentration is probably 10X too high.

 

[Deano]

Lou has pretty well covered the things you have as possibilities for gold losses in cyanide solution.

The most likely is that the gold cyanide has been adsorbed onto the ash particles, it takes an experienced operator to carry out an ashing so that there are no carbon particles present at the end.

Assuming that you have loaded the gold cyanide onto the ash particles you have two options.

The first is to redo the ashing step overnight at 600 to 650^oC making sure that the ash layer is no more than 1 cm thick to start with. Needs to be done in large pyrex or similar dishes. Depending on the fineness of the ash particles you may need to turn over the ash layer a couple of times to make sure that oxidation is complete.
The gold can then be brought back into solution by leaching the re-ashed residue with a 1 gram per litre cyanide solution.

The second option is to mix the pulp with some coarse activated carbon which has been screened at least at 1 mm and washed to remove fines. This carbon should have been tumbled in a bottle roll or similar to generate fines for removal, otherwise you will make these fines in the contact step and lose gold to these fines.
Stir the pulp and carbon mixture with only enough vigour to suspend the coarse carbon in the pulp for 15 to 30 minutes you must have at least 1 gram per litre cyanide in the pulp and a carbon level of around 5 grams per litre.
Now screen the carbon from the pulp, dry the carbon and ash overnight as above.
The gold can be gotten from the ash residue by leaching with cyanide or aqua regia.

The above is an equilibration contact, you will need to repeat the contact and subsequent steps until the gold recovered per stage is not worth the cost of processing.

Best if the initial ashing step is done properly in the first place.

Deano