# Best method of the fast precipitation maximum percent of PGM from Aqua Regia solution



## Ultrax (Apr 24, 2022)

Hello guys,


Please, advise the best method to precipitate a maximum percent of PGM from Aqua Regia solution.

Now I work with rich sulfide ore concentrate with PGM at about 800 gr/ton (mainly Pt, Ag, Au).

I have to use Aqua Regia (AR) 30% as a dissolver because of its high oxidizing potential compared to other methods like direct chlorination or cyanides. Other methods cannot dissolve all PGM from concentrate even after doubled dissolution.

After filtering, I usually evaporate the AR solution into syrup to remove NOxes and dilute it with the same quantity of distilled water.

So I need to precipitate all PGM metals from the AR solution with the maximum possible percentage.

Suppose I can use these methods for further treatment of the solution: 

A) Chemical precipitation 
(I've tried step-by-step starting from Pt precipitation with ammonium chloride, then Au precipitation with hydrazine, and finally cementing Ag with copper)
By the way, I don't know why but from diluted AR solution, AgCl always precipitates only partially - the rest of silver (Ag) is cementing with copper). I don't know, may be pH is still low (about 2) to dropping out AgCl. Frankly, I don't like this multistep method  It's too long for me.

B) Zinc cementation

C) Electrolysis with graphite anode and lead cathode

D) Total solution evaporation to dry powder and smelting and cupellation.

What methods will fit better for my purposes?

I mean with minimal efforts with a maximum % of PGM extracted.
I want to emphasize that I need to precipitate all PGM (not Au only) because I plan to clean and separate Pt/Au/Ag metals later. 

Please advise,
Thanks a lot,


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## FrugalRefiner (Apr 24, 2022)

Zinc will cement just about everything in the solution. Cementing on copper would cement all the precious metals without all the other junk. See When In Doubt, Cement It Out.

Dave


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## Ultrax (Apr 25, 2022)

Thanks, Dave,

Zinc is ok, but it requires further acid-step to remove possible iron (Fe) and unreacted zinc (Zn) traces. 
If we use copper, a film of copper chloride on the surface of the copper sheet might significantly slow down the cementation process. 
I guess it requires a couple of days.

What will precipitate in the electrolysis process with lead cathode and graphite anode?
Should it be just the PGM group? Or does it depend just on voltage despite the type of cathode? 
I try to avoid lingering filtering and further acid dissolution of dropped-out Fe(OH)3... My ore concentrate contains 2-5% of iron and iron oxides.

Roma,


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## Lino1406 (Apr 26, 2022)

For information when we have FeCl3 in solution it may dissolve some AgCl


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## nickvc (Apr 27, 2022)

Try fine copper powder after precipitating the gold out which will only precipitate the values left, use good agitation and test so you do not over use the copper, once done you can dissolve it in minimal AR to make recovery of the platinum easier.


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## Ultrax (Apr 27, 2022)

Lino1406 said:


> For information when we have FeCl3 in solution it may dissolve some AgCl


Thanks! I suspected that base metal chloride complexes in AR solution will interfere with the precipitation of silver chloride (AgCl).


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## Ultrax (Apr 27, 2022)

My task is just to choose a most effective one-step method for mass processing of ore concentrate and maximize extraction of all PGM from Aqua Regia solution. 
I can do it well with several stages of processing, but it's just a long time.
And it goes without saying that the waste solution is processed additionally (1-2 weeks) with iron plates to cement traces of all the rest metals.

By the way, does anyone destroy the nitric acid (HNO3) in Aqua Regia with ethyl alcohol(C2H5OH)? 
Yes, I know that this reaction is unstable and uncontrollable and requires very careful execution, 
but how effective is ethyl alcohol in destroying nitric acid than, for example, urea?


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## Stibnut (Apr 27, 2022)

The preferred de-NOxing chemical on this site is sulfamic acid, which decomposes nitric acid to nitrous oxide without generating potentially explosive compounds like urea nitrate or ethyl nitrate. Ethanol could also produce silver fulminate if silver is present. Even if there might be some conditions where ethanol won't produce dangerous compounds, there's no reason to risk it when sulfamic acid is cheap, safe, and effective.


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## nickvc (Apr 27, 2022)

I feel you are looking for a process that does not exist or one I do not know of, all recovery processes tend to be just that so dissolving all your values in AR has to viewed as a recovery process with all your values in the solution, if you did as I suggested and precipitated your gold first and then used copper powder to cement out the platinum you would be able to produce a concentrated platinum solution which would be easier to precipitate and recover a higher proportion of it, I gather getting it all is not easy.


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## Ultrax (Apr 27, 2022)

nickvc said:


> I feel you are looking for a process that does not exist or one I do not know of, all recovery processes tend to be just that so dissolving all your values in AR has to viewed as a recovery process with all your values in the solution, if you did as I suggested and precipitated your gold first and then used copper powder to cement out the platinum you would be able to produce a concentrated platinum solution which would be easier to precipitate and recover a higher proportion of it, I gather getting it all is not easy.


Why copper powder better than zinc powder? I understand that the position of copper in the electrochemical series excludes iron from dropping out of AR solution, but copper as cementator works for a long time - several days and requires strong agitation. Zinc might be much faster decision, even with necessity to remove traces of iron from PGM precipitate in sulfuric acid later.

If try to restore metals separately, it is easier to restore platinum (Pt) first - with ammonium chloride, and then gold (Au)with hydrazine (N2H4) or another preferable reducing agent.
And, at the end cement the traces of silver(Ag) with copper or iron plates.

The problem is, that I need the simplest possible extraction of all PGM metals (except iron, desirable) with a quick one-step processing of the concentrate in Aqua Regia solution. 
When all the MPG metals are extracted, of course they will be redissolved in Aqua Regia again and separated sequentially, each with its own reducing agent. But it is second stage of my work.

Another option is to evaporate the solution of Aqua Regia to dryness under fume hood, heating the precipitate up to 800C, smelting with lead and cupellation in magnesium crucible. But it is also quite long process. 

Therefore, I hesitate in choosing between cementation and electrolysis with lead cathode.


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## Ultrax (Apr 27, 2022)

Stibnut said:


> The preferred de-NOxing chemical on this site is sulfamic acid, which decomposes nitric acid to nitrous oxide without generating potentially explosive compounds like urea nitrate or ethyl nitrate. Ethanol could also produce silver fulminate if silver is present. Even if there might be some conditions where ethanol won't produce dangerous compounds, there's no reason to risk it when sulfamic acid is cheap, safe, and effective.


Thanks, you are absolutely right, but I use very few nitric acid in AR 1:5 and less. So in most cases, I have a minimal quantity of unreacted HNO3, which is not enough to produce potentially dangerous nitro compounds.
But I agree that is not a good idea.

Ethyl alcohol was an idea to replace the process of the 3-times evaporating the AR solution and do not create additional urea-nitric or sulfuric compounds to the solution.

I always thought that nitric acid + sulfamic acid should produce sulfuric acid as a result, not NOxes? Am I wrong?


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## Stibnut (Apr 27, 2022)

Ultrax said:


> I always thought that nitric acid + sulfamic acid should produce sulfuric acid as a result, not NOxes? Am I wrong?


You're right, it also produces sulfuric acid. Some of us see as a benefit because this precipitates any lead that might have gotten in as lead sulfate.

NH3SO3 + HNO3 --> N2O + H2SO4 + H2O


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## orvi (Apr 28, 2022)

Ultrax said:


> My task is just to choose a most effective one-step method for mass processing of ore concentrate and maximize extraction of all PGM from Aqua Regia solution.
> I can do it well with several stages of processing, but it's just a long time.
> And it goes without saying that the waste solution is processed additionally (1-2 weeks) with iron plates to cement traces of all the rest metals.
> 
> ...


Not everything is that easy as it may sound. Sometimes super easy and "straightforward" procedures tend to produce more mess and in the end they are not that "straightforward" as they may be.

Copper cementation is the easiest one. Require few days of constant agitation of the solution (see Dave´s post When in doubt, cement it out), but you get practically only values (PGMs, AuAg) with bit of copper that tend to stick to the particles of values a bit. Only disadvantage here is time I think, and from your responses you sound like you have the time as one of major factors. You can speed up the reaction by using copper powder (big surface area) as it was suggested.

Zinc precipitation of values is also quite straightforward, if you do it right. You need very good stirring and slow and steady addition of zinc powder to vigorously stirred solution. Solution will heat up by the reaction, and if you have active HCL in the juice, it will eat up any iron what also precipitate with zinc. Also, PGMs could cement on it.
Platinum is sometimes stubborn to cement, and I do not know exactly why... It takes ages for Pt to precipitate from AR solution - sometimes.
I have experience with this method just regarding PtPdRh mixture, without copper, containing ton of chromium, nickel, cobalt and iron.
Never add DMG beforehand (for Pd removal) of precipitating other PGMs, otherwise you will reduce Pt(IV) to Pt(II) with any reducing agent used like Zn or Cu, which will immediately form a complex with DMG and stay locked in it 

Long alkyl chain primary amines like octylamine and nonylamine could be also used for precipitation of PGMs from aqueous HCL solutions (mainly Pt). I am not sure about efficiency and I doubt it would be close to 99,9%  but it is as simple as pouring octylamine in, stirring vigorously for 1 hour and filtering. Bad thing is octylamine can be recovered, but very painfully, it stinks and precipitate is very voluminous and sticky due to static electricity interaction 

For de-noxing, it is wise to left the solution hot for some time just sitting and evaporating. The intention is not to evaporate the large volumes of liquid, but let the heat and HCL to break up the HNO3 down to Cl2 and NOCl. These are gasses, which easily escapes the solution when hot, and thus you need to use much much less denoxxing agent in the end. Certainly go with sulfamic acid. It isn´t that expensive and could be purchased in large quantities - it is used for de-scaling of coffee machines etc. Ethanol will inevitably produce acetaldehyde (which is toxic and has boiling point of 20°C) and ethyl nitrate (which is also toxic, explosive and overall something you certainly do not want).

You need to choose priorities and go from there. My only question is: why are you leaching the ore with AR ? Is smelting to the dore uneconomical ? Or you do not have equipment ?

I hope you are safe out there.
Слава Україні!


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## Ultrax (Apr 28, 2022)

orvi said:


> Not everything is that easy as it may sound. Sometimes super easy and "straightforward" procedures tend to produce more mess and in the end they are not that "straightforward" as they may be.
> 
> Copper cementation is the easiest one. Require few days of constant agitation of the solution (see Dave´s post When in doubt, cement it out), but you get practically only values (PGMs, AuAg) with bit of copper that tend to stick to the particles of values a bit. Only disadvantage here is time I think, and from your responses you sound like you have the time as one of major factors. You can speed up the reaction by using copper powder (big surface area) as it was suggested.
> 
> ...


I appreciate your support of my lovely Ukraine, thank you so much. Unfortunately, we have to work under Russian rockets' assaults. And we will do our best.

Thank you for your advice,
About your question - we have tried many different options for our concentrate: chlorination, thiosulfate leaching, and cyanides.
These methods have one problem - low specific solution capacity for platinum group metals (usually milligrams per liter). When you work with ore with a low concentration of PGM but in huge volumes - you can use anything mentioned above successfully. 

But we have a very rich sulfide concentrate of more than 800 gr/ton.
And therefore have two options only: try to use a stronger dissolver to avoid losses or use direct smelting in an electric arc ore-thermal furnace with metal-collector like lead. 
For economical reasons, we cannot use a powerful (100Kwt) arc furnace now, in smaller furnace process is unprofitable.

So we've decided to try AR (as for now), because 1 liter of concentrated AR can dissolve and hold more than 150 gr of PGM, and in a relatively small volume of solution (200-500 L), an acceptable yield of metals can be obtained. That's why I'm trying to select the way to more fast extraction of PGM from the solution. 

I understand that it is difficult to work with such a volume of Aqua Regia, so I ask questions about optimizing the process. So far, (A)cementation and possibly (B)electrolysis are the favorites. 
I wonder if it is possible to avoid the necessity of the destruction of nitric acid in AR at all (from chemical point of view), might be at the expense of greater consumption of cementing metals like copper or zinc?


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## Ultrax (Apr 28, 2022)

nickvc said:


> I feel you are looking for a process that does not exist or one I do not know of, all recovery processes tend to be just that so dissolving all your values in AR has to viewed as a recovery process with all your values in the solution, if you did as I suggested and precipitated your gold first and then used copper powder to cement out the platinum you would be able to produce a concentrated platinum solution which would be easier to precipitate and recover a higher proportion of it, I gather getting it all is not easy.


Thank you! I will try your advice.


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## orvi (Apr 29, 2022)

Ultrax said:


> I appreciate your support of my lovely Ukraine, thank you so much. Unfortunately, we have to work under Russian rockets' assaults. And we will do our best.
> 
> Thank you for your advice,
> About your question - we have tried many different options for our concentrate: chlorination, thiosulfate leaching, and cyanides.
> ...


Thanks for an explanation. I see your reasons.

Yes, in the end you will end up spending the cementation. Theoretically you do not need to de-nox, but you will de-nox anyway and inevitably with zinc or copper for cementing. It will go back and forth, nitric will continually redissolve the cement, which then in turn re-precipitates on the metal for cementation... Etc 

It is possible to use zinc for this, but if you have lots of nitric in the solution, it become wasteful. So in the end of leaching, I will opt for de-noxing step while the solution is still hot. Then proceed with copper cementation. If it is possible limit access of the air by covering the vessel or tank - oxygen in combination with HCl will dissolve copper unnecessarily without any positive effect for PGM cementation.


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## Ultrax (Apr 29, 2022)

orvi said:


> Thanks for an explanation. I see your reasons.
> 
> Yes, in the end you will end up spending the cementation. Theoretically you do not need to de-nox, but you will de-nox anyway and inevitably with zinc or copper for cementing. It will go back and forth, nitric will continually redissolve the cement, which then in turn re-precipitates on the metal for cementation... Etc
> 
> It is possible to use zinc for this, but if you have lots of nitric in the solution, it become wasteful. So in the end of leaching, I will opt for de-noxing step while the solution is still hot. Then proceed with copper cementation. If it is possible limit access of the air by covering the vessel or tank - oxygen in combination with HCl will dissolve copper unnecessarily without any positive effect for PGM cementation.


Thanks, I'm tending towards cementing by copper  Maybe you know how to calculate the amount of sulfamic acid to neutralize the remaining HNO3 in pregnant AR solution? Or is its excess not critical and its amount can be calculated from the initial amount of nitric acid in Aqua Regia?


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## nickvc (Apr 29, 2022)

Be aware that adding sulphamic will create sulphuric acid which can be troublesome when using paper filters, add it slowly when your solution is at least warm and if you get no reaction after an addition you should be good to go.


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## Ultrax (Apr 29, 2022)

nickvc said:


> Be aware that adding sulphamic will create sulphuric acid which can be troublesome when using paper filters, add it slowly when your solution is at least warm and if you get no reaction after an addition you should be good to go.


No reaction: Do you mean just absence of bubbles of NO|NO2?


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## orvi (Apr 29, 2022)

Ultrax said:


> No reaction: Do you mean just absence of bubbles of NO|NO2?


If you do not get a reaction, that means remains of nitric aren´t present. That is fine, unless you know you leached everything from the solution. Because if you do not have nitric in there, you aren´t leaching PGMs. Nitric decompose over time in hot AR, and byproducts are chlorine, NOCl and NO2 - that means your oxidizer just fizz away without effect slowly. Be aware of this fact. You need to add sulfamic acid to at least warm solution, not cold. During de-noxing with sulfamic, nitrogen and nitrous oxide are evolved, so be careful.


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## orvi (Apr 29, 2022)

Ultrax said:


> Thanks, I'm tending towards cementing by copper  Maybe you know how to calculate the amount of sulfamic acid to neutralize the remaining HNO3 in pregnant AR solution? Or is its excess not critical and its amount can be calculated from the initial amount of nitric acid in Aqua Regia?


The ammount of de-noxing agent isn´t possible to calculate, if you do not know how much nitric you have still in the juice. Theoretically, you can titrate small sample and then go with whole vat... But I will do it steadily to the point where reaction stop. It takes some time for sulfamic to dissolve in AR solution, so add slowly. And stir it well.


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## Ultrax (Apr 29, 2022)

orvi said:


> If you do not get a reaction, that means remains of nitric aren´t present. That is fine, unless you know you leached everything from the solution. Because if you do not have nitric in there, you aren´t leaching PGMs. Nitric decompose over time in hot AR, and byproducts are chlorine, NOCl and NO2 - that means your oxidizer just fizz away without effect slowly. Be aware of this fact. You need to add sulfamic acid to at least warm solution, not cold. During de-noxing with sulfamic, nitrogen and nitrous oxide are evolved, so be careful.


Does the rate of decomposition of nitric acid in aqua regia depend on temperature? What is the optimal temperature for leaching of PGM? Thanks for the help!


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## orvi (Apr 29, 2022)

Ultrax said:


> Does the rate of decomposition of nitric acid in aqua regia depend on temperature? What is the optimal temperature for leaching of PGM? Thanks for the help!


Yes, of course. Higher the temperature, quicker the decomposition. When AR is cold, gaseous active decomposition products like chlorine and NOCl are more soluble in it. When heated, decomposition is quicker and also the gasses escape the hot solution quicker.

This is a very broad question. I am not an expert in leaching PGMs, so I can only tell you - higher the temperature, quicker the dissolution. But it comes with cost of replenishing "dead" nitric acid. You have sulfidic ore... that is very different from metallic PGMs.
Maybe some PGM guys out there could give you an answer.


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