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I guess a better way to ask, if a solution of AR was lacking copper, or another metal that can bounce between oxidation states (Palladium?), would it be conceivable to accidentally form an explosive compound?
Urea is relatively basic compound. So if there is free acid, it eventually do form to some extent. As always with weak bases or acids, reactions are reversible.
Diluted urea nitrate in solution isn´t that extremely bad. Also urea nitrate in it´s pure form, but wet, is somewhat hard to explode. But it certainly can be detonated - with nicely big primer charge. From scraping the edges of the bucket - I do not think that this can be the trigger.

Additionally, with base treatment, nitric acid would be slowly bound to more basic compound - thus eg addition of NaOH would create sodium nitrate and free urea - which will start to decompose in alkaline conditions and liberate ammonia.
Me personally - I don´t use urea in anything regarding refining. I hate the smell of ammonia during workup - and cannot conveniently place drums of waste into the hood, so just sitting in the lab - no way. Also imperfect de-noxing, actual troubles with other things...

When I am doing first drop of gold from eg pins in straight AR, I carefully watch nitric dosing beforehand. Not to have unhealthy excess of nitric. Slurry the SMB in some water to make fluid paste, keep the AR solution hot and with good stirring slowly denox+drop with SMB... If it is first dirty drop, even this is sufficient. Sulfamic is expensive, but better. SMB is dirt cheap, not that nice de-noxer, but it does the job quickly, to acceptable outcome. Waste processing is then more pleasant contrary to urea - smell of SO2 is gone after few additions of slaked lime, as it is the second thing to be neutralized, after excess HCL. From then, just add more to the nicely basic pH, and no acid fumes or other corrosive ballast goes through the pump - so you can use dirt cheap membrane pumps for filtering this, and they last long.
 
I have been asked a few times why I use urea as a reactant in aqua regia leaches after filtration and before precipitating gold.
In Australia most gold is sold through refining companies such as the Perth Mint.
The major contaminant in unrefined gold is usually copper, provided that the copper level is less than 30% there are no penalties for having copper present in the dore.
The major cost elements in aqua regia treatment are chemical inputs and labour.
This means that if you can minimise labour costs you have minimised one of the two greatest costs of treatment.
It also means that employing quick and dirty recovery methods is not only a labour cost saving but does not lead to penalties from the refiners.
If you are not running your product through a refiner you will be looking for a treatment method which gives you a cleaner product at the cheapest cost.
In aqua regia leaching you have costs associated with the leaching stage and the precipitation stage.
In small stage leaching you have the cost of nitric acid, this is minimised by careful staged additions of nitric acid into the leach to ensure that you do not have excessive surplus nitric acid in the leach solution when all of the gold has dissolved.
There are many methods detailed in this forum which will minimise nitric acid additions. These are good methods and are employed by members but will add costs to the processing, mainly by increasing labour costs. These increased labour costs have substantially increased with the latter increased cost of labour in the general labour market.
These increased costs generally are minimal for small scale processors such as members of this forum who do not really price their time into the cost ledger.
In larger operations methods are used to minimise the labour costs as this will more than compensate for any small chemical cost increases due to the cruder processing methods employed.
Generally this means that a quick calculation is made as to the amount of aqua regia needed to leach the amount of material being treated.
The leach is run hot and fast to minimise time and thus labour costs.
You will end up with a leach solution which has a mixture of gold and silver chlorides and nitrates along with base metal chlorides and nitrates and some free nitric acid.
You have two practical options to clean out the free nitric acid.
You can use chemical reactants to destroy the free nitric acid in the solution or you can simmer the solution until the solution fumes change from orange to white.
I have always costed the simmering option as being cheaper than chemical processing but locally you have to compare the costs for the two methods.
It also helps that I usually work in a lab with good extraction systems so I do not have to factor in the cost of installing these systems.
So you now have a liquor which has been denoxxed but which still has base metal nitrates and chlorides present.
If you add a precipitant at this stage you will find that the copper nitrates will reduce from copper three to copper two but only after some of the gold has reduced to gold zero.
The copper three will act as an oxidant to redissolve the finely dispersed gold zero back to gold three in solution.
This sequence will continue until all of the copper has been reduced to copper two.
Unfortunately when the copper has been reduced there will be an excess of nitrate groups in solution, these groups will oxidise some of the copper two back to copper three to continue the roundabout until all of the excess nitrates have been consumed.
The above sequence will cause you to have to add substantially more precipitant than would be needed if the copper was not present.
I am using copper as an example in the above sequence because it is nearly always present in these leaches but any divalent base metal will do the same thing.
The cheapest method I have found to destroy the nitrates is by addition of urea, for me it is cheaper and quicker than adding precipitant. Once again it depends on the local chemical costs and time required for the cheapest method to be used be worked out.
A further advantage gained from the urea addition is that the urea will raise the pH of the liquor.
I usually add urea until the instrumental pH is 1.5, this corresponds to actual pH 2.5.
If precipitant is added at this pH then the gold drop will be much less subject to copper contamination.
Urea does not denox solutions but will destroy complexed nitrates in the solutions, most reactants for free nitric will not destroy these complexed nitrates.
Deano
Just curious here, in what situations is AR suitable/acceptable in mining and ore treatment.
It is usually said it is not used in ores.
I guess that in industrial volumes, Nitric may be cheap enough to actually be a less costly constituent than labour,
but for the majority here it is the opposite.

But still, are the cost of Sulfamic in these volumes so much less than Urea?
And what about using a SO2 generator and precipitate with SO2 directly?
As I said, I'm just curious.
 
When I use aqua regia as a gold solvent I always denox with heat to get rid of the free nitric acid. Here it is cheaper than chemical denoxing.
This means that the only source of nitrates is those complexed with base metals.
If you are treating material which is relatively high in copper, which is the most likely base metal to be present, a high level of copper is that which would report > 30% copper to the smelt.
If you are leaching those levels of copper then your processing will not be viable if you are using aqua regia for gold dissolution, in this case you are looking at a cyanide leach, not an acid leach.
If you are using cyanide, I have no idea why you would add urea to the precipitation liquor.
Urea has two specific uses and both are associated with acid leaching using aqua regia.
The first is to chemically remove complexed nitrates from solution and the second is to raise the solution pH to instrumental 1.5, actual 2.5.
in most cases both results can be achieved with the one urea addition to instrumental pH 1.5.
If you are not too concerned about the purity of the gold drop you only need to add enough urea to get rid of the complexed nitrates.
When I started precipitating gold from aqua regia solutions well back in the last century, a quick scan of the literature revealed hazards associated with formation of urea nitrate.
Strangely many operators had been using urea for this purpose, without incident, for many decades previous to my experiences.
I ran a lot of tests on aqua regia solutions where urea had been added prior to adding a precipitant.
I placed solutions on hotplates at up to 500C, took the dried residue and smacked it with a very heavy drop hammer several times.
Absolutely nothing happened.
I also put detonators in bags filled with the residue and got no difference to using a detonator without residue present.
My conclusion was that the level of urea nitrate was extremely low and that it was evenly distributed through the residue.
It was similar to taking the powder from a shotgun shell and stirring it evenly through a bucket of fine sand, the resultant mixture does not present a hazard.
All this is not to say that under all circumstances such residues are totally benign but my conclusion was that under normal usage urea did not cause any concerns.
The only time I have treated ores with aqua regia was on an ore which had very high gold grades and virtually no base metals present and I had a time constraint requiring rapid processing of the coarse gold.
For any normal gold ores the processing cost would be prohibitive and the set up costs and OHS requirements would be immense.
Sulfur dioxide has been used in the past for gold precipitation, my experience has been that no matter what containment circuitry is set up there is always an OHS problem.
Refineries which use sulfur dioxide have very expensive circuitry treating relatively small batches of gold.
Yes it works and is efficient and cheap but it is not for the small scale hobbiest.
Deano
 
It is always interesting to see the same questions being asked at regular intervals, one which still surprises me is the removal of base metals using acids.
It is as if the use of alkalis has never been considered.
Lead, tin and zinc are all soluble in caustic solution the usual caustic working level is around 20%, heat enhances the reaction.
If someone is wanting to use one of these as a collector metal, with caustic leaching you have the option of using whichever suits your purpose best rather than worrying over how to acid leach the metal away after use.
If you are just wanting to dissolve tin from scrap this presents a simple method, a rinse after leaching gets you material ready for precious metal leaching.
You can easily electrowin the base metals back from the caustic liquor if this is a viable method for you.
Deano
 
Lead, tin and zinc are all soluble in caustic solution

You can easily electrowin the base metals back from the caustic liquor if this is a viable method for you.

Hello Deano - & as always good info/reading

I have a question concerning the electro winning of tin from caustic solution

Tin whisker growth - as you know a lot of my refining was the refining of E-waste so I used a relatively fair amount of HCl to remove tin to get rid of it before going after the PMs

I was dissolving enough tin to make it worth while recovering it from the leach solutions as I had a market for the tin

So - when I first decided to start recovering the tin from the HCl leach I attempted doing so with a winning cell with the logic that it would also give me back HCl to be reused to dissolve more tin

The problem I ran into was the rapid tin whisker growth which would cause the whiskers to grow between anode & cathode & short out (shut down) the cell --- the whisker growth was rapid enough that it required frequent checking on the cell to knock down the whiskers & especially more so as the tin deposit built up on the cathode - enough so that it was actually a distraction from preforming other refining tasks while running the cell - unlike a silver cell or copper cell where I could turn it on & walk away to do other refining while the silver cell ran

As a result I abandoned the idea of electro winning tin & instead used zinc to cement the tin from the leach

So the question is - how is whisker growth effected in electro winning from a caustic solution --- more - less - or same as winning from a acid solution

Kurt
 
Back last century when I was doing this type of work I used a cell with a rotating cylinder electrode.
This cell type was developed to maintain high current efficiency but had the unexpected bonus of minimising whisker growth from base metals.
These cells were available commercially but I have lost all references to the manufacturers name etc.
My recollection is that whisker growth in alkaline cells was related to both metal levels in solution and caustic levels and that it was bad enough to cause me to go for the rotating cylinder electrode type cell.
 
Boys sorry, i'm not reading a single word more from top post of this page, I really need to know that any solution i might make won't be able to explode/detonate as long as it is liquid. I didn't really worry about this unless talking nitric but it is probably the moment i do need to know.

I also were unaware that it could happen with hcl, I don't know exactly how, or maybe i have a vague idea but I did learn (and thank you all that i did) that it can happen......

Now, ofc there are combinations and combinations but i would love the certainity that it cannot happen while it still liquid/with water.
 
Boys sorry, i'm not reading a single word more from top post of this page, I really need to know that any solution i might make won't be able to explode/detonate as long as it is liquid. I didn't really worry about this unless talking nitric but it is probably the moment i do need to know.

I also were unaware that it could happen with hcl, I don't know exactly how, or maybe i have a vague idea but I did learn (and thank you all that i did) that it can happen......

Now, ofc there are combinations and combinations but i would love the certainity that it cannot happen while it still liquid/with water.
What are you talking about?
No mention on explosive compounds here, at least not the last year.
You need to use the reply button on the post you refer to, so other people can see what you mean.
 
Back last century when I was doing this type of work I used a cell with a rotating cylinder electrode.
This cell type was developed to maintain high current efficiency but had the unexpected bonus of minimising whisker growth from base metals.
These cells were available commercially but I have lost all references to the manufacturers name etc.
My recollection is that whisker growth in alkaline cells was related to both metal levels in solution and caustic levels and that it was bad enough to cause me to go for the rotating cylinder electrode type cell.

Thanks for the reply Deano - yes - back when I was doing this (refining) I did do a bit of research on cells for winning tin & found a couple videos with the cells set up mechanically to deal with the whisker growth

One had a rotating drum for the cathode & if I remember right it had a bar with springs that kept the bar held against the drum but also allowed to move out/away from the drum as the tin built up on the drum which kept the tin whisker knocked down

The other one used a plate for the cathode & then had a roller that was timed to roll over the plate every so often to knock down the whiskers

At that time my primary focus was of course refining PMs so was not inclined to spend a lot of time/money on building/buying cells for base metal recovery - though I played with them a bit to see if they were worth the investment in my operation

At the time I had LARGE volumes of iron, copper, aluminum & zinc etc. as by products to the large volume of E-waste (& other scrap) I handled

So - for me it was easier to utilize those by products for my waste solutions --- iron to cement copper & zinc to cement tin

The only reason I went after the tin was I had a guy that did bronze & brass casting so I had a market for the tin & as well I sold him copper & zinc

That of course is the short story :D;);)

Kurt
 
Boys sorry, i'm not reading a single word more from top post of this page, I really need to know that any solution i might make won't be able to explode/detonate as long as it is liquid. I didn't really worry about this unless talking nitric but it is probably the moment i do need to know.

I also were unaware that it could happen with hcl, I don't know exactly how, or maybe i have a vague idea but I did learn (and thank you all that i did) that it can happen......

Now, ofc there are combinations and combinations but i would love the certainity that it cannot happen while it still liquid/with water.
A lot of the Urea/nitrate solutions don't become explosive as is. Let me further quantify that explosive qualities are further broken down into what is termed in the industry by their "precariousness" to explode. Some can be detonated by the least amount of friction, up to the point of requiring the minimum booster requirements, to detonate at a high percentage of times. This being said, we as a forum do not discuss the refining to the point of these compounds, as they could be miss used by others for the wrong reasons. If you are interested in what you are making, a book called "The Chemistry of Powders And Explosives", will give you much knowledge about the chemistry, precariousness tests used, and history of these compounds. For liability reasons, this information will not be discussed on this forum.
Quantity of material is a big factor. If you suspect you have made a reactive compound, convert it to smaller individual quantities, and remove to a safe area until you figure out what you have. Leave in a water filled solution, with a cover to keep from drying out. About all the help I will give.
 
One had a rotating drum for the cathode & if I remember right it had a bar with springs that kept the bar held against the drum but also allowed to move out/away from the drum as the tin built up on the drum which kept the tin whisker knocked down
I have been infatuated with rotating drum cells since I first started playing with copper cells. I thought I had that under control but here it is again back to researching these cells. :)
 
Here is a link to an open access paper on a new leach type for gold and palladium.
The large reserve of oxidant will make precipitation recovery not viable in a process format.
I do not know if activated carbon would extract just the precious metals under the leach conditions specified but recovery would need to be either by carbon or electrowinning.
This is not a leach for ores but may have a place in E-waste processing.

https://reader.elsevier.com/reader/...egion=us-east-1&originCreation=20230501025713

Deano
 
Here is a further new leach for gold, from a quick read it appears to have promise for gold in e-waste.
What the effect of other metals is I have no idea.
Keep in mind that DMSO will act as a transfer agent to penetrate the skin, precautions must be taken.
"Experimenting With Dimethyl Sulfoxide To Leach Gold From A Columbian Artisinal Gold Ore"
https://doi.org/10.3390/met13111855
 
Thank you for not laying the hammer down, i appreciate the heads up
Sorry for not welcoming you in a proper way.
I'll try again :)
Welcome to us

Here is our forum rules and other important links for your studies:

We ask our new members to do 3 things.
1. Read C.M. Hokes book on refining jewelers scrap, it gives an easy introduction to the most important chemistry regarding refining.
It is free here on the forum: Screen Readable Copy of Hoke's Book
2. Then read the safety section of the forum: Safety
3. And then read about "Dealing with waste" in the forum: Dealing with Waste

Suggested reading: The Library

https://goldrefiningforum.com/threads/gold-refining-forum-rules.31182/
 

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