# Sulfuric pulping/sonification



## Topher_osAUrus (May 29, 2017)

This was a discussion initiated back in the "determining the contaminant" thread awhile back. About using sulfuric and agitation for gold powder cleanup.
Seen here :arrow: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=25309&hilit=Sulfuric+sonification&start=30

If you dont want to read through it, here's the highlights on the process
:arrow:


4metals said:


> Lou said:
> 
> 
> > A final sonication with 70% hot sulfuric acid can be used to clean up the gold markedly in purity.
> ...



I didn't want to completely derail that thread, but I haven't had time to make its own until now with my questions on the matter.

Lou used a 70% hot concentrated sulfuric solution with magical agitation by sound. 4metals used an oblique barrel, most likely 55 gallons or something incredibly huge like the size of my garage with a blade mixer that was probably the propeller off a ship from WWII.

Also a noted difference, 4metals used 25% sulfuric acid. 
The solubility of some metals changes substantially in the concentration differences of sulfuric acid. Some things stay in solution when concentrated sulfuric is used, some stay when dilute is used. Is the concentration on one of yours a typo? Or is it no matter, just hot acid and agitation?

The precipitation was done with SO2. The usual culprits are copper, palladium, silver chloride, and I suppose lead sulfate if poorly filtered.

Would a copperas precipitation be the same? Or would is specifically require the dilute pulping leach because of the iron?

Is the traditional washing done firstly? HCl, water, etc? If so, what point is the sulfuric done? Or is it hearty enough to run as a stand alone leach, followed by water to get neutral pH?

Is multiple sulfuric washes done? First with concentrated, then with diluted? Or first diluted then concentrated?

Imagine a small time refiner, what practical way to integrate this method is there? Is a magnetic stirrer "good enough"? Or a good simmer with constant stirring sufficient?

When you say the sonification needs a longer leach, how much longer are we talking here? Until noteable color change in the acid, then decant and add fresh acid? Or decant, add water, and begin the dilute leach?

I am trying a simmer of dilute (maybe 30%) sulfuric on gold powder now. The sulfuric being the only leach after precipitation, the followed by water, drying and melting in a new dish. I don't think I want to try boiling strong sulfuric acid...

If you made it this far, your patience and perseverance is astounding


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## geedigity (May 30, 2017)

Hot concentrated sulfuric acid dissolves metallic copper and silver. Dilute sulfuric acid dissolves iron. Though I am curious to hear the answer. 

Question: I always considered concentrated sulfuric acid to be 98%. Is it still considered concentrated at 70%?


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## anachronism (May 31, 2017)

There's lots of ways to clean up gold. 

Frankly doing it the HCl way works and if you can get that right you can get 4 nines without a problem. Too much is made of alternatives that are either a lot more nasty (take this thread) or a lot more expensive. Maybe nailing the basic process will be enough. It's certainly served me well. 

I'm not saying that I don't use Sulphuric occasionally, I do. BUT and this is a big but- I can get the same results using HCl washes.


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## Topher_osAUrus (May 31, 2017)

anachronism said:


> There's lots of ways to clean up gold.
> 
> Frankly doing it the HCl way works and if you can get that right you can get 4 nines without a problem. Too much is made of alternatives that are either a lot more nasty (take this thread) or a lot more expensive. Maybe nailing the basic process will be enough. It's certainly served me well.
> 
> I'm not saying that I don't use Sulphuric occasionally, I do. BUT and this is a big but- I can get the same results using HCl washes.



I wonder why HCl was not a viable option for 4metals "dirty" gold percentage. And what benefit this pulping gave him over straight HCl.

I agree, sulfuric is a pretty scary thing, especially when hot. But, I consider it another tool I have in my bag, for whenever I need another option.

The 3 grams of gold that I pulped, ended up melting perfectly fine and was nice and clean, but, I cant really use that as a litmus test for success, as it was dropped from a very clean solution.

I use the standard washing procedures, almost religiously, and vary from them rarely. Save the fact I don't use ammonia, unless I have a good bit of AgCl that just wont get cleaned up with the HCl


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## 4metals (May 31, 2017)

The sulfuric pulping is a way of cleaning up gold which was refined directly from karat feedstock without removing the base metals or silver before digesting in aqua regia. This flies in the face of all the rules of refining discussed here to get high purity gold and it is a single pass digestion. This is for the sake of speed, which is often a requirement for commercial refining. If you are removing base metals by inquartation or digesting and precipitating twice, HCl washes may be all you need. 

But this acid is usually green when filtered and the gold is gassed out with SO2. Without using excess nitric if you are using nitric to begin with. The gold drops quickly and the sponge contains traces of silver and copper. The silver from soluble silver chloride, is minimized by icing, and the copper is the result of copper co-precipitating at the end of the gold drop. 

An oblique plating barrel is nothing more than a chemically resistant cement mixer. 


They are used in the electroplating industry. A combination of a mixer agitating the acid and the rotating barrel is all that is needed to solvate the impurities quickly. As Lou pointed out, an ultrasonic cleaner will do the job as well for smaller lots. In this case I was working on larger lots, which would require a large ultrasonic. 

This particular combination allowed alloy to be direct dissolved, filtered, and dropped quickly and the pulping cleaned up the last little bit to assure the high purity. This was done when the "gold rush" was on and karat scrap bars were coming in as fast as they could be had. Turn around was key, and that means speed. Similar lots done today on karat gold will produce the same results but e-scrap gold which may contain other odd-ball metals may require a different leach or agitation method. That is where the analytics come into play.


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## anachronism (Jun 1, 2017)

4metals said:


> An oblique plating barrel is nothing more than a chemically resistant cement mixer.



Haha you have NO idea how many arguments Nick and I have about cement mixers. That line put a smile on my face 4metals. Nick is not going to let me hear the end of it. :lol: :lol:


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## nickvc (Jun 1, 2017)

Ooh the R VEC is been discussed again  
For those wanting to know the initials were worked out so Jon didn't have to say concrete mixer, they stand for
Rotary Value Extractor Concentator, it sounds better to Jon, we argued the merits until the last job together.


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## Lou (Jun 1, 2017)

Generally speaking, the wash method I suggested with HCl will work sufficient for the purposes of base metal contaminants. I do not advocate the use of ammonia for silver chloride removal whatsoever. The only benefit ammonia has is a colorometric indication of the copper ion content of the gold sponge--if ammonia gives no blue coloration, the gold can be reasonably assured to be free of copper down to a few tens of ppm.

The sulfuric pulping is usually used on gold that is grosquely contaminated with Pd, AgCl, and/or CuCl2. It is not perfect.

Sonication was just a means of getting more agitation and faster speed than tumbling or magnetic stirring. It is highly effective for dissolving 14K in aqua regia and parting alloys in nitric.

Regards,


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## 4metals (Jun 1, 2017)

> grosquely contaminated



Even the spell checker choked on this one!


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## FrugalRefiner (Jun 1, 2017)

covefefe!


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## g_axelsson (Jun 1, 2017)

FrugalRefiner said:


> covefefe!


 Please, use a spellchecker, it spells covfefe, everyone knows that! :mrgreen: 

Göran


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## Lou (Jun 1, 2017)

Hahah.

Post car accident I seem to be crossing wires...and words!! 

Maybe I am thinking about gold so grossly contaminated it is grotesque.


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## Topher_osAUrus (Jun 2, 2017)

Thank you guys, I appreciate the insight.

So, from what i gather, the sulfuric was a viable option because of the contaminants contained. And, I assume it was chosen because doing a hot aggressive leach with HCl (with those contaminants, and parameters) would have ended up redissolving some of the gold, correct?

I would, however, like to know still, if it is 75% acid that is best, or 25%.

I have searched for solubility of silver chloride in sulfuric acid, but to no avail. This website seemed promising, but the excerpt left much to be desired.
:arrow: http://pubs.acs.org/doi/abs/10.1021/j100895a044?journalCode=jpchax#/doi/abs/10.1021/j100895a044?journalCode=jpchax

I remember Göran telling me a method to circumvent that, but I cannot recall how precisely.

It seems though, that concentrated is best.

A question that doesn't matter(except for satisfaction of curiosity), does the agcl convert to silver sulfate, or is it a double complex silver chloride ion?

Geedigity, I looked in my CRC book, but I couldn't find a direct answer as to "what IS concentrated sulfuric acid[what%]" 
I think it is high 90s though, but am probably mistaken.

Anyways, thanks again 4metals, and Lou.


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## 4metals (Jun 2, 2017)

One of the simplest ways to limit silver carry over is to add ice to the acid before you filter it. The solubility of silver in cold acid is significantly lower than the solubility of silver chloride in warm acid. So a decent grade of filter paper (coffee grade won't cut it) and you will catch a lot of that silver that comes out of the colder acid in the filtration step and eliminate a lot of it from your gold sponge. When I set up someone new to refining one of the first things I tell them to be on the lookout for is a commercial ice maker on the auction sites. 

We all know that most of the silver chloride remains on your filter paper anyway, that is because silver chloride concentration is never all that high in warm aqua regia to begin with. But it is that slight solubility that causes the contamination because if it is in solution it passes right through the filter. Years ago, I remember filtering acid and knowing the solution was well filtered, the paper was a good tight grade and the paper sealed to the Buchner funnel well. When the gold was dropped and a sample was assayed on an ICP, the main contaminant was consistently silver. Then, one evening we filtered about 20 gallons of pregnant aqua regia and it was left over night in the vacuum flasks to be precipitated in the morning. When we came in the next day, every one of the 5 gallon filter flasks had a coating of white silver chloride on the bottom of the flasks. I went around the corner to the deli and got bags of ice and came back to chill and refilter the acid. That is when I started icing every batch of acid before filtration, and the result, verified by our ICP assays, showed a marked decrease in silver. So we got an ice maker and it became part of the SOP.


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## Topher_osAUrus (Jun 2, 2017)

I generally ice my solutions before precipitation when I am about to filter. I only asked because I was curious about the "upper limit" for how dirty the powder can be for sulfuric to clean it up right.

Mainly because I would like to adopt fully the "first drop is always recovery drop" and neglect the dilution pre precipitation (to save on waste space).
So the practicality of a sulfuric leach came to mind.

If i could just add the smb without diluting, use sulfuric to clean it up best I can, redissolve and get out any traces of AgCl with icing on a 50mL solution instead of a 500mL solution.

The more I ponder it though, the more I believe it is just best to do it right the first time. Dilute it, filter it clear, drop it, clean it up the traditional way. Reusing those HCl washes to feed the first dissolve when recovering gold. 
The wheel IS already round, right?

I do like the idea of having this leach as an option, should I ever need it though. I appreciate you sharing your experiences with us. 

I cannot thank you guys enough for all you do here.


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