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Metals/Electrodeposition of Gold Sodium Thiosulfate


QUESTION: Hello Dr. Poe,
I wondered if you might have some insight on the basics of electrodeposition of a gold sodium thiosulfate lixiviant?
Thanks in advance for any details.

ANSWER: The valences of gold vary and normally are only +3 or +1.  This throws the newbie for a loop when one discovers that gold has many other valences.  Valences 5 through 7 only appear when fluorine is involved. +2 only when sulfur, tellurium or selenium is involved.  Many debate wither +2 is actually a mixture of +1 and +3.  -1, minus one, is rarely known by any but the most experience refiner. It occurs when gold is alloyed with an electropositive metal of considerable strength as sodium, potassium or cesium. It also has been known to happen when gold is dissolved into an alkaline thiosulfate. In the sodium thiosulfate you will have a mixture of gold valences; +1, +3, and -1!  The electroplater will deposit gold as reduction on the negative electrode, but will oxidize the gold (-1) to zero (metallic) at the positive electrode as well!   You can catch the gold particles with an anode bag just underneath the anode (positive electrode).  Voltage is important as is temperature;  sodium gold thiosulfate is basically white , turning light yellow with exposure to the atmosphere.  Small amounts are clear, invisible.  It is also more stable than the chloride or cyanide and must have a higher voltage/amperage to deposit on the electrodes.  Start at four volts and go up to 6 volts if it seems necessary.  Temperature of the plating bath always works best when hot, say around 80C,  Cold will also work, just not as well.  The color of the deposit will depend on concentration, temperature and voltage. It will range from clear flakes (still gold!) to red, blue, brown, reddish brown (cinnamon), yellowish brown, dark brown, gold brown and gold metallic. You will have to work this out yourself, but remember they all melt into gold bars. ---Doctor Poe

---------- FOLLOW-UP ----------

QUESTION: Hello Dr. Poe,
Thank you for your response.  Although I have been able to get some deposition results on our SS Cathode, it has been inconsistent. Probably in part due to running the bath only at 3 volts and ambient bath temp so I will certainly make your suggested changes to see if things might become more consistent and productive.  Not sure given the mechanics of the cell if an anode collection can be done but that is surely to investigate as I resolve other things.
I failed to ask you about a couple of other things.  Is there a sweet spot for the lixiviant relative to PH during leach?  My bath has been around 9 during leach.  However, during plating, the PH spikes up way high.  Does this make sense? I use pool test strips if it is possible the electrochemistry is given a false reading on them??  Nonetheless, can you comment on the ph levels that should be maintained during leach and during electrodeposition?  I have read numerous patents and papers and this subject is all over the place, especially were carbonaceous or sulphide ores are tested but my ore has little preg robbing constituents.
Thanks for any assistance you can provide.
Ken G.

The rise in pH is normal and can be controlled with diluted sulfuric acid or even muriatic acid (HCl). But this also indicates that gold sulfide is dropping off the anode along with free sulfur.  Agitation (constant stirring) will help get you consistency on the cathode. The gold sulfide will redissolve into the higher alkalinity and then proceed to the cathode where the sodium will be released and the gold sulfide will then travel back again to the anode to drop off again. Raise the voltage, keep it hot (quartz immersion heater) and keep it stirred. Add acid sparingly if pH rises to 11. pH 9 is the "sweet spot". Try just adding a bucket under the anode to catch gold and gold sulfides. Every little bit helps, yes?----Doctor Poe


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Christopher Poe


The processing/refining of precious metals including chalcogenic ores like tellurides and selenides.


32 years experience in ore assaying and refining. Presently head chemist and chief refiner of a Philippine metals company.

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