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"Recovery of gold from thiourea based electropolishing solution"

March 18, 2010

Q. Dear sir,
we do the electropolishing process with thiourea based solution; we get good results but the gold recovery portion is the difficult one since the salt where we add it will get crystal after a week so; this will create a confusion for the gold recovery.
How can I do the process to recover the removed gold?

Best regards,

Prabu Abi
Research work - Hosur, Tamil Nadu, India

April 22, 2010

A. I Bhupesh, gold refining officer in jewellery company. I suggest you to add zinc in it if it is acidic or it contains cyanide then add NaOH and zinc. Keep whole solution for whole night. Then filter it wash it properly on buckner funnel. Dry that residue. Then dissolve it in nitric acid to remove base metals. You will see impure gold residue below the solution. Then refine it by aqua regia.

Bhupesh Mulik
jewellery- Mumbai

May 19, 2010

Q. Hi,
Mr. Prabu.

Can you please tell us about the electropolishing process with thiourea based solution; what are the chemical used and what is the process. I am looking for this process for long time. Please let me know I will be very much thankful to you.

Kamlesh Gupta
- Dubai, UAE

July 29, 2015

Q. Dear Bhupeshji,


Greetings to you,
1. Can you please share as to what is THIOUREA solution?
2. Is it made of Cyanide or any other chemicals?
3. Can it be used instead of Sodium Cyanide which is used for Stripping of 9 to 18 kt White gold / Yellow gold? If yes, what is the procedure?

Awaiting your reply,
Best regards,

- navi mumbai,maharashtra,india

July 30, 2015

A. Dear prakash,

Thiourea is usually supplied in powder form sourced from specialised manufacturers. A weak solution of thiourea is prepared, and the first stage is the oxidation of thiourea to form formamidine disulfide:
2CS(NH2)2 + 2Fe3+ = C2S2(NH)2(NH2)2+ 2Fe2++ 2H+

The role of the formamidine is to oxidise the gold to form a gold-thiourea complex:
2Au + C2S2(NH)2(NH2)2 + 2CS(NH2)2 + 2H+ = 2Au(CS(NH2)2) 2+

Importantly, "formamidine acts as an oxidiser as well as a complexing agent, supplying about 50% of the ligands to the complexation" and due to this thiourea leaching of gold is faster than cyanide leaching [43]. The overall equation for thiourea leaching is:
2Au + 4CS(NH2)2 + 2Fe3+ = 2Au(CS(NH2) 2++ 2Fe2+

To drive the equation to the right, thiourea must be present in excess, and ?the ratio of complexing and oxidising agents must be carefully adjusted? to avoid excessive oxidation of the thiourea solution and consequent excessive use of reagents [43].
In a final step, the formamidine breaks down irreversibly to cyanamide and elemental sulphur.
The sulphur is a potential problem to the success of the thiourea method, for it forms a fine grained sticky coating which can inhibit the leaching of gold.

bhupesh mulik
cac admixture - mumbai,india

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