Hello my name is Cory and I am currently working on recovering gold from gold nanoparticle waste. The waste usually comes in centrifuge tubes and is dissolved in aqua regia. From there, it is filtered and reduced with sodium-borohydride (NaBH4). Thus comes my question, I have read on here that zinc flour has been used to precipitate gold along with other bases and I was wondering what other suggestions anyone had. NaBH4 has worked well but I am trying to optimize the process as best I can in attempt to find an even better reducing agent. Thanks for your responses.
Gold Recovery from gold nanoparticle waste
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If the material is nanomollecular in size I would use an acid/bleach(sodium hypochlorite) leach instead of the AR leach,and precipitate with SMB(sodium metabisulphite) or Copperas(ferrous sulfate).
You would most likely have the best results though with sulfur dioxide gas,trickled into your pregnant solution,but that is just my oppinion.
You would most likely have the best results though with sulfur dioxide gas,trickled into your pregnant solution,but that is just my oppinion.
Thanks for your response. Would you dissolve the SMB and Copperas in water before adding it to the Sodium hypochlorite? Currently, I use about 15ml of aqua regia to dissolve the nanoparticle waste in the 45ml centrifuge tubes. I just add it and shake the tube and the solid is completely removed from the sides. Would you use the same amount of sodium hypochlorite?
Personally, I would stick with the aqua regia, since it's probably faster. Sodium borohydride (SBH) is expensive but, since it sounds like you're working in tiny amounts, why fix it if it ain't broke? The only problem with SBH is that it will also drop most other metals.
If there are other metals involved, you might consider a more selective reducing agent for the gold, such as SMB or sodium sulfite (I wouldn't use SO2 because, with these small amounts of gold, it is overkill). To use these, though, you should try to keep the nitric in the AR at a low enough level so that you don't end up with a lot of excess nitric. This is because the SMB or sodium sulfite will react with the excess nitric before it drops the gold and the ratio of SMB or sodium sulfite needed to the excess nitric present is quite high. The AR doesn't have to be 3:1 or 4:1 to dissolve gold. About any combination of HCl and nitric will work. In practice, it would take about 1 ml of nitric, in combination with whatever amount of HCl you need to make up the 15 ml, to dissolve a gram of gold. If this is too slow, use just a little more nitric. If you don't have much excess nitric at all, it will take about a gram of SMB or sodium sulfite to drop a gram of gold. You can use these reductants as a solid or you can dissolve them in a little water. Test with stannous chloride solution to make sure all the gold has precipitated. Try not to use too much of the reductant. Play with it.
If there are other metals involved, you might consider a more selective reducing agent for the gold, such as SMB or sodium sulfite (I wouldn't use SO2 because, with these small amounts of gold, it is overkill). To use these, though, you should try to keep the nitric in the AR at a low enough level so that you don't end up with a lot of excess nitric. This is because the SMB or sodium sulfite will react with the excess nitric before it drops the gold and the ratio of SMB or sodium sulfite needed to the excess nitric present is quite high. The AR doesn't have to be 3:1 or 4:1 to dissolve gold. About any combination of HCl and nitric will work. In practice, it would take about 1 ml of nitric, in combination with whatever amount of HCl you need to make up the 15 ml, to dissolve a gram of gold. If this is too slow, use just a little more nitric. If you don't have much excess nitric at all, it will take about a gram of SMB or sodium sulfite to drop a gram of gold. You can use these reductants as a solid or you can dissolve them in a little water. Test with stannous chloride solution to make sure all the gold has precipitated. Try not to use too much of the reductant. Play with it.
Goldsilverpro,
There have not been many metals involved (sometimes iron) but the main problem we are having is that there are organics dropping with the gold. The gold np waste has Pc-4 and polyethylene glycol as part of the complex as well. We have been heating the reduced waste to remove the organics and it has worked, but for waste that contains larger amounts of organics, we have not had much luck. One suggestion was to add chloroform to the reduced waste to remove organics, but when trying to do green chemistry you tend to want to stay away from chloroform. Would SMB or sodium sulfite help with dropping only the gold and no organics? Thanks a lot for your help.
There have not been many metals involved (sometimes iron) but the main problem we are having is that there are organics dropping with the gold. The gold np waste has Pc-4 and polyethylene glycol as part of the complex as well. We have been heating the reduced waste to remove the organics and it has worked, but for waste that contains larger amounts of organics, we have not had much luck. One suggestion was to add chloroform to the reduced waste to remove organics, but when trying to do green chemistry you tend to want to stay away from chloroform. Would SMB or sodium sulfite help with dropping only the gold and no organics? Thanks a lot for your help.
crusinek said:
It's been about 25 years since I've use SBH and, even then, I just played with it. If I remember right, in liquid form, it contains a lot of NaOH. If so, maybe that has something to do with the organics dropping. The use of a different reductant may work. You'll just have to try it.
Another thought. You may be able to get rid of the organics by first shaking it or letting it soak overnight (probably better to not disturb the gold) with some concentrated sulfuric. You should end up with a bunch of very finely divided carbon in the sulfuric from the organics breakdown. You could pour the sulfuric/carbon off and possibly reuse the sulfuric. Then rinse the tube out carefully a few times before using the AR. Who knows, it might work.
