AR gold refining advice needed

[Anonymous]

So I'm a newbie here, but I'm somewhat familiar with the chemistry, and have read a ton of posts on here. To start off, before anyone tells me to buy Hoke's book, I already have it on order (it should be here next week) and am heading to the library tomorrow to do some more research. I'm just here to see if anyone has simple answers to a couple of questions.

I'm using the basic AR method (1:4 HNO3:HCl), neutralizing the nitric with urea, filtering, boiling, then dropping the Au with SBM (Na2S2O5), filtering again, washing with HCl again, and then smelting. My source material is gold jewelry and scrap, and I'd have to guess as an average it's perhaps 9kt (~35%). Now I've already pulled out quite a bit of gold from this, but I know (really, since I'm working with alloys that I mixed and cast) that there should be at least 1ozt more in the mix, likely more than that from scraps. In this lot I expect there to be (in order of concentration) copper, gold, silver, palladium, and nickel, as well as traces of zinc, aluminum, iron, and platinum. Everything has been grained into small shot. My main goal here is gold recovery, the rest will be sent to a professional refiner after I've accumulated enough of the waste. So here are my problems/questions:

There is still (after being in both AR, then nitric only, then AR again) metal at the bottom of my beaker that refuses to dissolve. Any suggestions?

I suspect one of the reasons I did not get a full recovery was because of the contamination that I discovered halfway through the process, of the air vent I made dripping black (presumably aluminum and iron oxides/chlorides/nitrates that came from the reactions of the chemicals as they left through the vent shaft) into my beaker. While I fixed the source, I've now got contaminated solution, solution in which the gold has somewhat been recovered (but which has more), and undissolved metal scraps (still containing gold). My hope, tell me if this is a good plan or not, is to evaporate all my liquids and put them in with my scrap, and do it all again in one lot. What should I do to start from scratch with all this?

I read the Stannous Chloride post under "General Reaction List," and was wondering if there was a better or different way to determine when you don't have to add any further SBM to precipitate out gold. It is my general understanding (feel free to correct me) that tin compounds are generally highly toxic, and I'd rather not manufacture them (or add any further toxic chemicals) to add to the mix of things. Is there another way to figure out how much SBM to add, without doing molar calculations?

Finally, I'm re-designing and re-making my makeshift fume cabinet, which, while it does it's job, needs to be bigger and better built. Does anyone know of a paint or spray that I can apply to the inside so that it will better resist the acrid environment?

Thanks for any help you guys can provide.

Mike

 

[Harold_V]

There is still (after being in both AR, then nitric only, then AR again) metal at the bottom of my beaker that refuses to dissolve. Any suggestions?

Yep! Start with the inquartation process before you attempt to recover gold from alloys. It's well described in Hoke's book. You can work yourself into a dither by not doing so.

The remnants that are giving you fits are now covered with an impervious layer of silver chloride. You can boil them in ammonium hydroxide, then try again, or you can melt them with silver, dropping the gold content to 25%, then remove all the base metals with nitric before attempting to dissolve the gold. That's what Hoke is going to teach you. It works. I highly recommend you pay attention to that process.

What should I do to start from scratch with all this?

Do you have any zinc? If so, place EVERYTHING in a large container (an ice cream bucket may work fine) and add the zinc. Do it slowly, especially if your solution is highly acidic----it will likely boil over if so. If you have a piece of zinc, add it to the solution, but carefully. If you'd prefer a slower reaction, use a piece of scrap steel. Everything in solution of value will be cemented down, including copper. When the solution tests barren (use stannous chloride), you can then gather all of the solids, including the piece of steel you used. The piece of steel should be boiled in HCl to liberate any traces of values that may be stuck to it. After you've cemented all the values, they should be incinerated to kill the HCl. You can then boil the recovered (cemented) material in dilute nitric, to dissolve copper and silver, leaving behind any gold that may be present. You can also melt everything, using soda ash and borax as a flux. If you choose that route, test the button to see if it will dissolve in nitric. If not, it should be inquarted for processing. Again, Hoke talks about the process.

I read the Stannous Chloride post under "General Reaction List," and was wondering if there was a better or different way to determine when you don't have to add any further SBM to precipitate out gold. It is my general understanding (feel free to correct me) that tin compounds are generally highly toxic, and I'd rather not manufacture them (or add any further toxic chemicals) to add to the mix of things. Is there another way to figure out how much SBM to add, without doing molar calculations?

I'm at a loss to understand why you'd not want to use what is the accepted method for testing for values. The amount of stannous chloride that you create is miniscule---assuming you use good sense in testing. You don't introduce any to your solutions, you take a sample of your solution and apply a single drop of stannous chloride to the sample. I used to do that with a stirring rod and a spot plate. There are other methods, including cotton swabs, but there's not a lot of sense creating more and more things that need to be processed to recover the values. Swabs require incineration, and are not necessary. When a spot plate is used, you simple dump the tests to your stock pot, where any values that may be present are recovered. Again, Hoke will tell you all about that subject.

You can test for gold with ferrous sulfate, but it won't determine the presence of other values, unlike stannous chloride.

Finally, I'm re-designing and re-making my makeshift fume cabinet, which, while it does it's job, needs to be bigger and better built. Does anyone know of a paint or spray that I can apply to the inside so that it will better resist the acrid environment?

There are acid resisting epoxy paints------but for a hood to serve you fully, you should be able to incinerate, not just process with acids. Such a hood should be made from materials that do not burn, and are not metallic (mine was made of asbestos). The call is yours. All depends on how serious you are about refining, and how many things you choose to ignore, thinking they don't matter. Incineration is key to success----

Not trying to give you a bad time, Mike, just trying to make you aware of things. If you need guidance in recovering the values from the dirty solution you mentioned, there's some available for the asking.

Harold

 

[goldsilverpro]

Everything is toxic, if you ingest enough of it, including water. I looked up the LD50 for various inorganic tin compounds and most are in the order of from .1 gm/kg to 2.5 gm/kg. For anhydrous stannous chloride, it is .7 gms/kg, in rats. BTW, the LD50 for SMB is about 1.1 gms/kg - not much higher. Stannous chloride, like many other common chemicals you are exposed to, is toxic, but not frightfully so.

Metallic tin was used to coat the inside of so-called "tin cans" for many years, and is still used to coat the inside of high dollar copper pots and pans. Also, dental amalgam, used for fillings, contains about 25% tin.

Probably the biggest danger of working with many powdered chemicals is in breathing the dust. Just use common sense. Use a dust mask, keep it off your skin, and don't eat it.