Recovery of gold from thiourea based electropolishing solution

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.

A. Dear prakash,

Thiourea is usually supplied in powder form sourced from specialised manufacturers. A weak solution of ⇦this on eBay or Amazon [affil links] 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 sulfur ⇦this on eBay or Amazon [affil links] .
The sulfur 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.