I2/KI gold leaching with ultrasonic cleaner

[orvi]

Hi

I2/KI leaching method is well known, but for gold plated copper scrap not particulary well suited because of quick depletion of iodine by reaction with copper and also reprecipitation of gold on naked copper surface. It was one of my first experiences with refinig. Unexperienced in this area, I witnessed both of these bad side reactions happening, slowly building the white deposit of CuI on the bottom of the beaker Times changed, i sticked to AR for the long time.

But whole process with I2/KI is very easy, straightforward, iodine could be recycled... I was thinking about it a lot. Reasons are, that compared to dissolving stuff in AR, leaching with iodine is much more elegant, it is quick when more concentrated lixiviant is used, and most notably, sh*tton of liquid metal waste is not produced. Processing half to one kilo batches of pins with AR started to be serious pain, mainly in terms of waste management, not mentioning polluting atmosphere with nitrogen oxides.
Sulfuric stripping and cyanide approach are well established, but have their own risks/hazards and inconvenience issues. Somewhat i am reluctant to cyanide, for obvious reasons, and also obtaining cyanide in that quantity... There will be some legal issues. Sulfuric cell will be the option of choice(for me) from these two approaches. But back to iodine.

Being aware of the side reactions and thus low efficacy on copper based material, i was thinking about this:

1. do the classical stripping, I2/KI in water, observe the pins, when goldplate is gone, swirl the beaker around and pour/rinse the liquid out of it. There will be lots of precipitate from copper iodide and also reprecipitated gold/junk.
2. cover the etched pins with water and place the beaker to the ultrasonic bath/cleaner for several minutes. As the gold dissolves from the material, it reprecipitates on the surface of the copper, but hence the dark colour of the pins, i assume this deposition is in the form of some aggregates which dont stick to the copper very strongly. Ultrasound could possibly shake things enough so the fine gold adhered to the surface fall of the pins and can be rinsed out in form of mud/sludge.

That is theory in my head. Now the reality
Do you think the ultrasound could clean the gold from the surface of copper ? I dont exactly know how strongly the reprecipitated gold adhere to the surface of copper. I read here on the forum and also personally experienced the PGM cementation on copper. This isnt going very well (without stirring/bubbling), as the platinum metals tend to bond somewhat strongly to the copper surface.
I am pretty much certain that somebody out there tried cementation of gold with copper, maybe this observation could place this idea correctly to fiction/possible category

Thank you for your thoughts and responses
orvi

P.S.: there is other possible way of stripping gold from ores/escrap, working with N-bromosuccinimide (acting as oxidant) and pyridine (acting as ligand). folks had some interesting results, up to 90% recovery rate from scrap/ore. nearly neutral pH (8,2). this would be interesting, pyridine and NBS are relatively cheap chemicals (can´t beat the cyanide tho), altough recycling would be nearly impossible. in the scientific world awesome, in the real world maybe troublesome. but certainly interesting to read

 

[orvi]

Well, i was too curious, so i did a small experiment
Starting with 250g of mid-grade pins (gold-nickel-Sn10Cu alloy), prepared leaching solution (40g I2/160 g KI/400 ml H2O according to patent) and give it a go.
Leaching was quick, after maybe 3 minutes, solution start etching the copper alloy really quickly, gold was gone from the surface, so i just poured off the liquid. Pins were immediately covered with white layer of CuI.
Then i washed them with water, covered them with another portion of water and put them to ultrasonic cleaner for 5-7 minutes. Bulk of the reprecipitated stuff and CuI drossed off them, exposing etched metal surface.
Poured off the sludge from the pins, rinse well with water...
Filtered the insoluble CuI on Buchner, wash the cake with water till completely white. Gold was dropped with sodium sulfite - nice effect, very good dense precipitate. Whole process took about 40 minutes, including the preparation of the leaching solution.

From the point what i wanna test out - if the reprecipitated gold can be recovered by action of ultrasound: kind of.
Measuring the pins directly after the I2/KI leach and washing, XRF shown somwhere around 0,7 % Au on the surface of the pins (average).
After the ultrasound treatment, XRF measured around 0,3% Au (average).

I was hoping that it will improve things much more, but still, it kinda works. Observing the whole process, washing the pins with water in the beaker is tedious, because the precipitate tend to end up locked between the material and is not washed off from the bottom of the beaker easily. Placing the treated pins into the kitchen strainer and sift the dross of them could help to get last bits of liberated particles off

 

[kosmow]

Aqua Ammonia will attack the copper and help clean up all the copper in the mixture. The copper turns into a blue liquid that can be poured into a plastic barrel and for recovery I just used aluminum foil but Zinc may work better for the outer cation that it needs to drop out of the solution until the liquid turns clear with the copper settled in the bottom.
The Ammonia is a good liquid fertilizer but it's main drawback is the fumes and getting it on your skin or eyes. Aqua Ammonia is easy to obtain here from the local chemical supply company and its not terribly expensive.

 

[orvi]

Aqua Ammonia will attack the copper and help clean up all the copper in the mixture. The copper turns into a blue liquid that can be poured into a plastic barrel and for recovery I just used aluminum foil but Zinc may work better for the outer cation that it needs to drop out of the solution until the liquid turns clear with the copper settled in the bottom.
The Ammonia is a good liquid fertilizer but it's main drawback is the fumes and getting it on your skin or eyes. Aqua Ammonia is easy to obtain here from the local chemical supply company and its not terribly expensive.

I just needed to dross off loose gold which reprecipitated on the surface of bronze pins. Copper iodide is of very bad solubility in aqueous solutions, so it is more convenient to just filter it. Of course, if it is a good quantity of iodine, it would be wise to recover iodine from it.