The authoritative public forum
  for Metal Finishing since 1989
-----

Gold Bombing Process and Equipment

I am interested in information about Gold Bombing using Cyanide and Hydrogen Peroxide.

Any direction to the appropriate information would be helpful.

Steven E. Cope
IB Goodman Co. Jewelry Manufacturing - Newport, Kentucky, USA
2005

---

2005

Recovery of gold after aqua regia process.

Importance - during the aqua regia process some refiners do not know that AGCL masks the gold alloy metal which has AG content below 12%.

During the refining some AU metal alloy may be maskened by AGCL which we can not see visually because the colour of AGCL (gray)matches the silica from jewellery dust.

So there is possibility of remaining some AU , depends on the caratage.if any factory, Making only 9k or 10k white gold jewellery then there is less recovery of gold from refining waste compared with 14k or 18k working.

So our aim to reduce the losses by increasing the recovery from dust & avoid the mistakes of chemical reaction happens during the refining.

After the refining the waste material by aqua regia, these waste contains acidic medium.before going to cyanide leaching, we have to remove the acid because it creates a strong exothermic reaction & evolves HCN gas which is deadly poisonous.

The refined waste from aqua regia process must be ignited again in muffle furnace & make a fine powder by grinding.

This fine powder taken into a large plastic bucket.if the diameter of bucket is widen then it is very effective for process.add perfect quantity of water into the dust to make a slurry of dust .now add NAOH to maintain the ph in between 8-10.means slurry should be basic in nature.

Now its time to add nacn in proper proportion depends on quantity of au present into the dust. cyanide gets complexed with the AG & AU.but the rate of leaching is very slow ,so it requires to fastness the reaction so we use an oxidising agent instead of atmospheric oxygen in the leaching of AU.example-permagnates, persulphates,peroxide, dichromates,

ferrocyanides , H2O2, CAO2 etc..

I always prefer H2O2 or CAO2.H2O2 is most suitable oxidising agent for replacing atm.oxygen because only water & oxygen (no toxic products)are formed in decomposition of H2O2.

CAO2 is also more suitable ,H2O2 starts reacted quickly. refiners should be aware of one point that while adding H2O2 in proper proportion add sloely into the cn-leaching dust.while added H2O2 always slowly stir the leaching solution, otherwise recovery will affects.strirring plays important role. Always stir slowly because the reaction is bumping & exothermic.keep this solution for max. 24 hrs. after 24 hrs., filter the solution ,wash the residue fully & collect the filter solution in large plastic bucket & now add more NAOH & zinc in proper proportion as per the gold & silver content.

Keep this solution about 4 hrs., now decant or filter the solution, the residue must be washed fully it should not contain CN because CN is volatile. while burning the residue , gold may evaporated with CN.

After burning the residue ,remaining ashes you can melt or treat burn material to nitric acid to dissolve large amount of ag.when ag dissolves in nitric acid we can see brownish coloured gold powder in solution.fiter the AGSO4 solution,treat the residue by aqua regia process to get pure gold.

Note - according to my experience, I recovered approx. 500 gm of silver & 25 gm of gold in 8 kg of refined dust through this method.

Limitations-

dust v/s water -0.8% to 1.15%

NAOH v/s water-0.025%to 0.030%

NACN v/s dust-0.045% to 0.078%

H2O2 v/s dust-0.030% to 0.06%

Bhupesh C. Mulik
Core Jewellery Ltd - India