Gold Precipitation and pH

[Mark Orenstein]

Greetngs all,

1) I recently read that when precipitating gold from solution with SMB the pH should be slightly acidic to almost neutral. In videos of people performing this process I don't remember anybody testing the pH of aqua regia solution before adding the SMB. Also. why would you test it if it's made up entirely of two strong acids? It's obviously going to be highly acidic. But again I do not remember anyone adjusting the pH before applying the SMB. I am probably forgetting something vital but I am still a bit perplexed as to what I am missing.

2) Also, I have a aqua regia solution with dissolved gold that needs to be precipitated however my first attempt did not go well probably because there was still nitric acid in solution. I suspect that there is too much SMB contained in the solution too. What should I do to recover the gold? Thanks very much for any advice!!!

 

[Shark]

Keeping AR acidic comes from using heat to remove the excess nitric. HCl is a gas dissolved into water and will come out with heat. Heat was used quite often in old methods due to the over use of nitric. As the solution was reduced additions of HCl was used to keep the solution acidic and help remove the nitric as well as any Nox trapped in it. SMB needs the HCl PH wise as well to aid precipitation.

 

[Deano]

In gold precipitation, metabisulfite is used as a source for generating sulfur dioxide gas in the gold chloride solution, the sulfur dioxide is the reductant which precipitates the gold as reduced metal.
You need acid conditions for generation of the sulfur dioxide, the acid conditions are supplied by the hydrochloric acid in the gold chloride solution.
Usually the gold chloride solution will have a pH <1 after any excess nitric acid is removed from the solution by simmering the aqua regia solution after all the gold has been dissolved and any residual solids are removed by filtration.
Simmering is carried out until the fumes from the solution change colour from brown to white.
The filtration step is only required if high levels of solids are present.
These solids include ore residues and some organics such as those left after treating e-waste, these solids are capable of adsorbing gold chloride from solution and the level of adsorption is proportional to contact time.
After the simmering step the addition of metabisulfite will cause precipitation of the gold chloride as gold metal.
You will also get co-precipitation of some base metals as coatings on the gold.
This co-precipitation can be minimised by adjusting the pH to instrumental 1.5 which is actually pH 2.5.
Use urea to do this adjustment as it also destroys any base metal nitrates in the solution and prevents nitrate re-dissolution of the precipitated gold.
If you have large amounts of base metals, usually copper, in the solution you will need to add large amounts of urea to get to the required pH.
Make sure that you use technical grade urea as agricultural grade urea has high levels of binders which will cause you grief.
The urea is fairly cheap compared with the cost of gold losses, purchase in 20 kg bags.
Deano

 

[Shark]

Use urea to do this adjustment as it also destroys any base metal nitrates in the solution and prevents nitrate re-dissolution of the precipitated gold.

Thanks Deano, this is a question I had been looking into for sometime. I have seen this occur occasionally and had become curious if the use of urea could solve the problem but haven't had it happen lately to test it yet

 

[Mark Orenstein]

Thanks Shark and Deano, both answers were excellent!