Serious Education ... plus the most fun you can have in metal finishing.
Selective precipitation of gold from aqua regia, leaving PGM in solution
February 25, 2010
Q. I am an analytical chemist for a recycler. I am new to this area and have a question regarding a selective precipitation of gold from an anolyte solution of 10% by volume aqua regia. The solution also contains PGM. The gold is used to inquart the sample so the PGM materials are soluble. I would like to be able to recover, and reuse the gold without contamination from the other materials. Can anyone offer any insight to how to accomplish this gold selective recovery?
Thanks so much in advance.
Analyst - Hebron, Ohio
March 6, 2010
A. Ferrous Sulfate is commonly used to precipitate gold without precipitating more than traces of the platinum group metals. However, this can only be done by first removing any free nitric acid still present in the aqua regia solution. This is usually done by first doing a triple evaporation process on the aqua regia solution. Details of all this can be obtained on the goldrefiningforum.com website.Chris Owen
- Nevada, Missouri, USA
March 12, 2010
Q. Thanks Chris.
But I thought that using ferrous sulfate will also precipitate out any Pd along with the Au. I was hoping for a straight up gold selective precipitating agent.
Can you, or anybody else, please clarify? I may be mistaken on the Pd precipitation.
Again many thanks to all for any assistance.
- Hebron, Ohio, USA
March 13, 2010
A. You try oxalic acid. If you are trying to know the purity of gold sample, you prefer fire assay system. Pd will precipitate by ferrous not ferrous sulphate. Using ferrous, reduction takes place. OK, consider that FeSO4 contaminate your gold. I suggest that you wash the ppt by hot water to remove acid in it. check by pH paper. Then you add nitric acid to dissolve your ferrous and Pd in it.
I hope you will understand the process.
jewellery - Mumbai, India
March 16, 2010
A. Ferrous sulfate won't drop either Pt or Pd along with the gold. About the only contaminate in the gold will be a little iron from the ferrous sulfate. If you have Pt or Pd, it is common to first use ferrous sulfate. Then re-dissolve in aqua regia and drop a second time with a sulfite such as sodium metabisulfite, sodium sulfite, or SO2. If you do this right, you should end up with at least 999.5 Fine gold.
If you use urea to eliminate nitric, however, you could drag down some Pt or Pd when using any precipitant. Either use no excess nitric in the aqua regia or correctly evaporate the aqua regia to eliminate any excess nitric for best results.
An excellent reference for the use of ferrous sulfate in the presence of Pt or Pd is C.M.Hoke's, "Refining Precious Metal Wastes" [link is to info about book at Amazon] It has been re-printed and is available from several places on the internet - IPMI, legend-reno.com, Ganoskin, etc. If they still sell it, I think Ganoskin has the best price. This 360 page book is the bible for refining jewelry scrap.
- Nevada, Missouri
February 2, 2012
Q. Hi, over the past few years, I have been recovering gold and silver from ores that contain these metals using aqua regia. I am familiar with CW Amen's book as well as CM Hoke. I now have ore concentrates that have gold, silver, and platinum group metals in it and can't find any detailed information on the correct and complete procedure for precipation of all metals when using Aqua Regia. Any help would be appreciated.Ken Sheppard
- Joshua Tree, California, USA
January 4, 2013
I want to know 2 things if you can help me please?
I dissolved gold ore in aqua regia (AR) and then I used urea until the nitric acid was neutralized. The AR was heated at about 75°. First tried to drop silver with salt and nothing happened. I then used oxalic acid to try and drop the gold but nothing happend for 4 hours. I then switched the heat off and went to sleep. The next morning I went to do a bit of cleaning, and I see that the pyrex dish is full of crystals. Can you tell me what the crystals are and why did no silver or gold drop?
The second thing that I want to know is: Can I elecroplate the gold out of AR?
Or maybe electroplate the gold out of the gold ore by itself?
hobby - Johannesburg, South Africa
May 10, 2016
A. Hi Peter, and others interested in precipitating gold. This is something I have done and, in my opinion, hydroquinone is the best reagent for precipitating gold from a hydrochloric acid solution, but once again there must be no nitric acid present.
Another way of precipitating gold is with iron shavings or zinc dust. The iron or zinc does into solution and the gold coats the zinc or reports as a brown powder which is easy to melt into a globule of gold using an oxy-acetylene flame.
There was a small gold mine in Zimbabwe where gold was collected by ordinary dish scrubbing iron wool. Very simple chemistry. the Fe is more electro-negative than the Au so the gold is displaced from solution by Fe. The precipitated gold powder/ dust is mixed with a flux, usually Borax= sodium carbonate and a little silica and melted in a carbon or silicon carbide crucible. At the setup in Zimbabwe the gold was poured into small slabs which were used as sacrificial anodes in an electro-winning tank. the pure electroplated gold was collected on graphite cathodes. I found that iron shavings or zinc dust will displace gold from an AR (Aqua Regia) solution without the HNO3 first being removed. Boiling off the last trace of nitric acid is very time consuming.
Hope that helps.
Have a good day.
- Uvongo ,KZN, South Africa