Refining Gold With Acid?

There is a very good reason why gold is classified as a nobel metal- aqua regia, mercury, and cyanide compounds are the only chemicals that will touch it. Aqua regia is definitely NOT for home experimentation.

I've chemically refined gold using aqua regia and it's not too complicated, but you need the acids and you really should have a fume hood to protect yourself from the fumes. You can also use concentrated hydrochloric acid plus sodium chlorate solution in combination to generate a corrosive chlorine mixture that dissolves gold, palladium, and platinum.

If your gold is in very small pieces you can leach it with nitric acid alone; this will dissolve silver, copper, and other base metals without touching the gold. I don't expect this would work very well with solid gold jewelry but you could try it. Just be careful.

Here's a 50-gram nugget of 24-carat gold I prepared using the aqua regia approach.

Nope. Nitric acid will nicely clean a piece of gold jewelry but will not attack the gold at all. Aqua regia (royal water) attacks gold with concentrated hydrochloric acid in an oxidizing medium (concentrated nitric acid). Both are needed for the gold to be dissolved. Plus... what happens then? Well, you have gold dissolved in an acidic aqueous medium. You could probably neutralize the acids with baking soda and water but the gold itself might not precipitate out of the solution as a gold dust like material that could easily be filtered off, dried, and recovered. It could well come out as a gooey sludge of some kind that you would have a lot of trouble handling. Such a sludge might be recoverable via high temperature roasting to drive off the water and perhaps other impurities but the vapor coming off of there could be toxic or at the very least quite irritating. A chemist would do this in a muffle furnace inside a working hood. A ceramic crucible would be needed to hold the gold slurry / sludge.

I agree with the others who suggest selling this to someone who does professional gold refining.

The Incas had an interesting way increasing the purity of gold in the surface layer of a gold object.

If the object was say 50% gold copper alloy, the piece was immersed in an acid to leach out the copper, leaving the gold behind. The surface was then beaten to create the finished surface.

The alloyed gold is stronger and harder than pure gold, but the surface has a rich gold layer.


Which brings me to an interesting question:
can gold be refined by just using hydrochloric acid only?

For this situation, the gold would have to be reduced to a fine gold dust, before being immersed in a hydrochloric acid solution only. As I understand it, all chlorides are soluble in water.

After the reaction, the whole lot would be allowed to stand for some time, then the copper chloride solution decanted off, leaving a heavy gold sludge.

This sludge could then be filtered, several times, the gold being retained in the filter. Perhaps some sort of aqueous electrolytic process could follow, to further refine the gold.


I am definitely NO chemical engineer, it just seems reasonably logical to me.


Comments?

Well, the gold itself will not dissolve in HCl but most of the inorganic impurities that are contaminating the gold might very well dissolve into the HCl. If you are thinking about a multi-stage extraction process to remove the impurities, that would probably work to some extent. Not sure how pure the gold would become from this. It is doubtful that any of the platinum group metals would be soluble in HCl, so if these are part of the "impurities", then another purification method would likely be needed.



This could be a considerable physical problem. Gold is quite soft as metals go, so it would be difficult to reduce it to a dust-like material. If a substance can be dissolved and then rapidly precipitated, a very fine dust-like material often results. Just as often, the rapidity of the precipitation occludes impurities within the particles of the material of interest. Not sure if gold could be ground to a powder without it smearing and flattening all over the grinding equipment, though.



With perhaps a few exotic exceptions, this is true. Not all elements form chlorides, though. Gold and platinum do not form compounds very easily at all. In many cases, the compounds that can be formed from them tend to be complexes with various other atoms or molecules. This is how cyanide dissolves gold... as an aqueous sodium cyano complex, like this:

4 Au + 8 NaCN + O2 + 2 H2O â†' 4 Na[Au(CN)2] + 4 NaOH

While this is an effective gold leaching process, it is also extremely dangerous. Cyanide compounds are extremely toxic and can form airborne HCN gas if the solution pH is allowed to drop below about 10. The wastes from this process are also quite toxic and much chemical treatment is needed to neutralize their toxicity.



I have filtered any number of sludges and every single one of them was a pain in the neck. They filter VERY slowly in most cases and vacuum is needed to filter them at all. The filter medium has to be sized correctly for that particular sludge or the pores in the filter will either plug very quickly or allow the bulk of the sludge to pass through it. This can sometimes be avoided via cooking the sludge in a muffle furnace for several hours at 600-700C and then doing additional purification steps on the resulting dry solid.

I'm not a metallurgist or a mining engineer, so do not know a lot about the refining of gold. I can help a bit with the chemistry of some processes, though. The Internet is a great resource for learning about the various commercial processes for gold and silver recovery.

Like posting 16mb pictures and cleaning coins, home gold refining strikes me as one of those "just because you can do it, doesn't mean you should do it" things.

Silver chloride is one famous chloride salt that will not dissolve in water. So if your raw gold contains silver (something that I understand is common in Australian gold) then HCl won't pull that out. Platinum-group metals won't come out, either, which are also likely to be trace components of native gold.