Aqua Regia re-use

[Blakey]

Hello All,
I'm a geologist and with permission I was allowed to take some quartz breccia from one of our mines to experiment with gold recovery. Assay results show the ore to have approximately 120g/t Au of which is mostly free gold. I put the ore into the crusher at work and then a mill to reduce it to a very fine flour. I then tried panning it which takes a very long time so I built a Miller Table/wash table. This works well but still takes a long time and some gold is lost. So now I am going the chemical route and using aqua regia to obtain the gold. I made up a test run using a small beaker that I filled up to the 75ml mark with the ore then added the AR solution to the beaker for a final volume of 200ml with a 3:1 ratio of HCl to Nitric. My question is after the gold is dissolved and I filter off the gold bearing solution can I re-use this to treat another batch? I read that the AR can hold a lot of gold in solution, more than I have in my ore. Will it lose it's effectiveness over prolonged use/ heating?
Cheers

 

[butcher]

As soon as you mix aqua regia it begins a reaction (even without metals involved) gases start forming and begin to gas off from solution, (or react with metals) before gassing off. Heat will drive the reactions faster.

With your ore the aqua regia may still have free HCl and nitric, which could be used on a second or third batch.
but you need to keep several things in mind.

Heat can make the acid more aggressive or work faster but heat also drive off the gases that are doing the work.

Ore has other metals (or salts of metals) involved, Aqua regia can attack both gold and other metals, other metals can also replace gold from solution.

So if the solution loaded with gold and then used on a fresh batch of ore you could load it with more gold chlorides, but also there comes a point where the gold can be cemented back out of solution by the process of adding to a fresh batch of ore.

Base metals will go into solution easier than gold (pretty much related to their reactivity as a metal, more reactive metals will dissolve easier and faster than the less reactive metal Gold).
base metal or metals higher in the reactivity series replace less reactive metals from solution, copper will replace gold from solution, iron will replace copper from solution, zinc will replace iron...
zinc will replace gold copper and iron and any other metal below it in the reactivity series of metals.

Aqua regia begins decomposing as soon as it is made (granted the decomposition is relatively slow with out metals), and can be very aggressive with fine metals... mixing and storing it is just a bad idea.

Never premix aqua regia and store it in closed containers the gases can build up making a potential acid bomb.

Any time you work with ore, proper crushing and roasting can greatly improve the ore for a leaching process, it will take many salts of metals and oxidize them,making them easier to dissolve and not react as counterproductive with your leaching process, again the metal oxides will dissolve easier than gold, and can and will replace gold from the leach solution.

Ore can produce some very deadly gases (in the roasting process), and in the leaching process arsenic is one of many poisons which can be produced from working with ores.

The gold can be both in free gold and gold compounds to refractory ores resistant to a leaching process.
Quartz veins which produce gold, the gold is normally more free gold than gold compounds.
these same veins can hold other metals also Iron compounds being normally the greatest, but many other metal compounds can also be involved.
The gold and other metals have seen hot acids at high pressures before, so there are also salts of those acids and metals in your ore to deal with now in your leaching process, here is where crushing and proper roasting may prove the most important step in your leaching process.

Free gold on quartz can be extracted with aqua regia (aqua regia on most gold ores would not only be foolish but a waste of time and money).

I would first separated as much gold from the quartz as possibly, with mechanical means, panning gold wash tables... Then try a test batch with your leach on a sample of the tailings, to see how much gold you can recover or if it is actually even worth it.

If your loosing gold panning, or on tables... You may just need to make improvements in your techniques or equipment of the mechanical processes, this could very well lead to recovering more gold than you could get in your leaching process...

There is much more to this but I believe this covers some of the important parts.

 

[Deano]

Go to a commercial lab and ask them how they carry out aqua regis digests on ore.

They will tell you very quickly that they will never attempt to carry out the type of test you are interested in.

The reason is that the ore-aqua regis mix is heated during the digestion stage with associated loss of nitrous component both as fumes and via chemical reaction.

Good luck attempting to filter this pulp with a paper filter if the nitric has not been completely removed from the solution.

If the nitric has been completely removed via a simmer until the fumes change from brown to white you will have, after filtration, a solution with totally unknown levels of various metals.

To convert this to aqua regia again you will need to add nitric acid to the original ratio and attempt to forget that you will have lost some of the hydrochloric acid and you do not know how much hydrochloric acid you need to add to replace this loss.

If you now attempt to use this semi reconstituted aqua regis on a second batch of ore you are hoping that the capacity of the aqua regis to both solubilise and retain gold in solution has not been impinged upon by the metals in solution from the first leach.

If you follow the normal protocol of carrying out separate digests for separate samples you will avoid the above concerns and will also have data on the grades of these samples which will be required to do statistical analysis to establish not only a head grade but how many fliers there are, high grade ores are notorious for spottiness of gold values.

Deano

 

[Blakey]

Thanks for the responses thus far.
I should probably clarify that my attempts to extract the gold are for fun only and not in anyway related to actual practices at my mine. As I said I am a geo and not a metallurgist so this is a good learning experience for me. Unfortunately I can't visit a commercial lab as the nearest one is some 600+ kilometres away......hooray for the internet.
The ore is from an orogenic system so is extremely base metal poor but does contain a significant amount (3-5%) stibnite or antimony trisulphide which I am guessing will complicate the gold recovery. The plan was to add sodium metabisulfite to precipitate out the gold. While high grade ore zones can be spotty, I have spent the last 2 years drilling this resource and I am very sure of the grade I have. It will be interesting to see how much I can extract vs what the total amount of gold is available with a home garage setup (I do the rxn in open air with appropriate PPE gear). I will keep you posted with the recovery results when I'm done.
Try as I may, I could not get the Miller table to hold all the gold as the ore has been milled to less than 75 microns. Also it would seem that due to the milling action some gold is 'rolled' into little tubes that promptly wash down my table. This is why I am trying the chemical route now. Unfortunately I have no access to cyanide so I went with AR.
From your advice Butcher, it sounds like I might need to do a few trials to see if reusing it is possible/viable. I would treat it all at once but I don't have an appropriate reaction vessel large enough so several smaller batches it is.
If you have any other ideas or suggestions on how to recover the gold i'm all ears.

 

[Blakey]

So after a bit of reading I am thinking that the AR will dissolve stibnite and adding the sodium metabisulfite will precipitate both the gold and antimony. This means I will have a gold bead after melting that has a relatively high amount of antimony impurity.
This will greatly impact my recovery rate calculations

 

[Roywhip]

It feels like it's just a question of what you use to drop your gold out of solution , if your using SMB to drop the gold , not only is the SMB a very volatile companion for the situation ( the smell alone can kill you) but it's also partially an acid so every time you reuse the aqua regia you would be dissolving and precipitating the gold at the same time and might throw out valuable material AND getting frustrated that out stannous test wasn't proving useful because you were precipitating out the gold as you went, if you use Zinc powder to precipitate all your gold out through displacement , then not only would you be able to reuse your aqua regia ( possible a 1/4 addition of original use) but the gold wouldn't drop out right away like it will if you still have SMB in your aqua regia from the last use.

 

[Geo]

I missed this one when posted. 120g of gold per ton of ore? And they let you walk out of the mine with some big chunks to play with? 120 grams of gold per ton. And here I am thinking 1 ozt per ton was a magical number. 4 ozt per ton. That's just quite unbelievable. A ton of ore will fit easily in the bed of a regular size pickup truck with room to spare. You can move a ton of crushed gravel with a shovel in less than an hour.

 

[7CHEESE8]

:lol:

I missed this one when posted. 120g of gold per ton of ore? And they let you walk out of the mine with some big chunks to play with? 120 grams of gold per ton. And here I am thinking 1 ozt per ton was a magical number. 4 ozt per ton. That's just quite unbelievable. A ton of ore will fit easily in the bed of a regular size pickup truck with room to spare. You can move a ton of crushed gravel with a shovel in less than an hour.

 

[Joshuabarrreett42042]

So best to use Zinc well so how can I get palldum out of my ewaste

 

[FrugalRefiner]

When in doubt, cement it out.

Dave

 

[Palladium]

When in doubt, cement it out.

Dave

I'm stealing this saying! :mrgreen:

 

[alexxx]

When in doubt, cement it out.

Dave

I'm stealing this saying! :mrgreen:

Me too...

 

[kurtak]

So best to use Zinc well so how can I get palldum out of my ewaste

This makes no sense at all

best to use zinc ? --- For What - details of what you are asking please

so how can I get palldum out of my ewaste ? --- out of what e-waste - not all e-waste has Pd - in fact MOST e-waste has "little" to NO Pd

Kurt

 

[FrugalRefiner]

I've given this cursory advice numerous times, but you gentlemen have inspired me to elaborate. So I have created a new post with a little more detail at When In Doubt, Cement It Out.