Gold isn't dropping

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orvi

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What is the purpose of boiling solution down to a syrup? I get boiling off excess nitric but boiling down to syrup seems off to me. Just genuinely curious about that one. I don't have alot of experience but of the solutions I have done I've never heated to an actual boiling and a few hrs at near boiling has almost always worked. Of the times I have still actually had a small amount of nitric at that point urea or sulfamic acid quickly fixed it without adding very much volume at all. And I never really dilute solution much with water. I always thought more diluted ment finer gold when dropping? Again I've only done a few batches and just now passing the ounce mark on my stores gold. Almost all from ram. Fingers and bga chips off the ram and the ice chips with legs and the glass chips on the ram.
When you combine nitric and hydrochloric acid, they react together, forming nitrosyl chloride and chlorine:

HNO3 + 3 HCl ---> NOCl + Cl2 + 2 H2O

This is why the fumes comming out of the AR dissolution have golden-yellow colour, contrary to plain nitric on silver, where mainly NO2 is produced and fumes are brown.

It take some time for this reaction to happen. Unreacted nitric acid is very hard to drive off with evaporation, but these decomposition products are gaseous/volatile, so by boiling you accomplish two things:
1. higher temperature = faster decomposition
2. higher temperature = easier driving off gases

Chlorine and NOCl are fairly water soluble, so getting rid of them is slow, and you need to physically remove water as vapor to carry them away.
As you concentrate the solution, content of chlorides and salts rises, and the boiling point also rises = harder for the nitric and its decomposition products to readsorb when once evaporated. And easier to escape from the vessel. That´s why evaporating nearly to syrup is good, but unfortunately lengthy and messy practice.

EDIT: and I forgotten one very important thing. Many metals, not only PGMs, form nitroso-complexes. These complexes hold NOx, easily said. And these could be quite stable. To decompose them, you need to heat up the solution to quite a high temperature to destabilize them and force them to liberate NO molecules, and change them with chloride ligands. This is why boil down to syrup is very helpful - if you denox this way.
You rise the temperature, so disrupting complex´s equilibrium and liberating the ligands (NO) which could then interchange for chlorides more easily.

Huge ammounts of NOCl, chlorine and HCl are evaporated, ruining your fume-hood fan extraordinarily quickly :D and of course, anything around the fume exhaust.
As galenrog said, sulfamic acid always works (you need to heat the solution to get nitric reacting, not only NOCl - this work at ambient temps also), and the cost is not high. Partial boildown of the solution sometimes cannot accomplish full denoxxing, even that you cannot see NOx vapors in covered beaker.
 
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AMS-Pro

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When you combine nitric and hydrochloric acid, they react together, forming nitrosyl chloride and chlorine:

HNO3 + 3 HCl ---> NOCl + Cl2 + 2 H2O

This is why the fumes comming out of the AR dissolution have golden-yellow colour, contrary to plain nitric on silver, where mainly NO2 is produced and fumes are brown.

It take some time for this reaction to happen. Unreacted nitric acid is very hard to drive off with evaporation, but these decomposition products are gaseous/volatile, so by boiling you accomplish two things:
1. higher temperature = faster decomposition
2. higher temperature = easier driving off gases

Chlorine and NOCl are fairly water soluble, so getting rid of them is slow, and you need to physically remove water as vapor to carry them away.
As you concentrate the solution, content of chlorides and salts rises, and the boiling point also rises = harder for the nitric and its decomposition products to readsorb when once evaporated. And easier to escape from the vessel. That´s why evaporating nearly to syrup is good, but unfortunately lengthy and messy practice.

EDIT: and I forgotten one very important thing. Many metals, not only PGMs, form nitroso-complexes. These complexes hold NOx, easily said. And these could be quite stable. To decompose them, you need to heat up the solution to quite a high temperature to destabilize them and force them to liberate NO molecules, and change them with chloride ligands. This is why boil down to syrup is very helpful - if you denox this way.
You rise the temperature, so disrupting complex´s equilibrium and liberating the ligands (NO) which could then interchange for chlorides more easily.

Huge ammounts of NOCl, chlorine and HCl are evaporated, ruining your fume-hood fan extraordinarily quickly :D and of course, anything around the fume exhaust.
As galenrog said, sulfamic acid always works (you need to heat the solution to get nitric reacting, not only NOCl - this work at ambient temps also), and the cost is not high. Partial boildown of the solution sometimes cannot accomplish full denoxxing, even that you cannot see NOx vapors in covered beaker.
Well, I guess here's my question then - If original poster had already evaporated or "boiled" everything to a syrup what part of this process could've have allowed the gold particulates to just "suspend in the solution"? That's why i was wondering, if he had maybe dissolved the gold back into the Aqua regia, and then used Ferrous Chloride to precipitate the gold (like he was going after the platinum), it would have allowed it to settle instead of suspend for whatever reason it did in his original syrup. I'm aware that using Ferrous chloride or Iron(II) chloride would contaminate the gold, but he would've at least been able to recover it, and refine the impurities out of it from that stage with a different process, and get past whatever his initial problem may have been. Without knowing what the cause was, that didn't allow the gold to settle in his syrup, could this route have been a viable solution for him to be able to recover the gold, as it is a simple/common precipitant, when nothing else seemed to work for the original poster?
 
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orvi

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Well, I guess here's my question then - If original poster had already evaporated or "boiled" everything to a syrup what part of this process could've have allowed the gold particulates to just "suspend in the solution"? That's why i was wondering, if he had maybe dissolved the gold back into the Aqua regia, and then used Ferrous Chloride to precipitate the gold (like he was going after the platinum), it would have allowed it to settle instead of suspend for whatever reason it did in his original syrup. I'm aware that using Ferrous chloride or Iron(II) chloride would contaminate the gold, but he would've at least been able to recover it, and refine the impurities out of it from that stage with a different process, and get past whatever his initial problem may have been? Without knowing what the cause was, that didn't allow the gold to settle in his syrup, could this route have been a viable solution for him to be able to recover the gold, as it is a simple/common precipitant, when nothing else seemed to work for the original poster?
Very small, non settling particles occur mainly on two occasions. First is that there is very very little gold, and it just dont clump together and stay suspended in the liquid for very very long time.
Second is that the solution was not clear, eg there was some precipitate, opalescence/milkiness, or worse - some kind of dissolved polymeric substance which could form micelles in the liquid, or blocking the particles of gold to agglomerate. Notorious for this are things like tin hydrated oxides or metastannic acid fine dispersions.
Doing silver contacts, there would always be solder containing tin. Nitric will create metastannic acid, and if this is not filtered clean, attempted precipitation on copper would create rather fine particles of silver, which do not clump together or grow bigger (sadly, my personal experience also).

If your material contained some polymers which could be attacked by acids, or resins... They could be liquified to the some extent, and stay in solution in the form of short-length segments. And acting similarly as cotton soaked in water :D blocking effective diffusion in the liquid.
And when effective diffusion is blocked... Many nucleation sites are available = greater number of individual growing particles = logically they will be smaller in size, because there is limited gold in solution. And voila, semi-colloidal suspension is born :)

There is simply that many factors that could be significant, you can experience this even if you think everything was done right. Golden rule is: dirtier and lower-grade the material is, probability of difficulties arising from the process also rise :p mainly with pulling the gold out of the complicated matrix and getting it precipitated in some workable form.
 

FrugalRefiner

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That's why i was wondering, if he had maybe dissolved the gold back into the Aqua regia, and then used Ferrous Chloride to precipitate the gold (like he was going after the platinum), it would have allowed it to settle instead of suspend for whatever reason it did in his original syrup. I'm aware that using Ferrous chloride or Iron(II) chloride would contaminate the gold, but he would've at least been able to recover it, and refine the impurities out of it from that stage with a different process, and get past whatever his initial problem may have been. Without knowing what the cause was, that didn't allow the gold to settle in his syrup, could this route have been a viable solution for him to be able to recover the gold, as it is a simple/common precipitant, when nothing else seemed to work for the original poster?
AMS, do you mean ferrous chloride, or ferrous sulfate?

I'm very familiar with ferrous sulfate (copperas) being used to reduce gold from an AR solution, but I've not heard about using ferrous chloride.

Dave
 

AMS-Pro

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AMS, do you mean ferrous chloride, or ferrous sulfate?

I'm very familiar with ferrous sulfate (copperas) being used to reduce gold from an AR solution, but I've not heard about using ferrous chloride.

Dave
Hello Dave, how are you enjoying this Ohio weather?
As far as specifics go, I believe both chemical classifications ( yours, and mine ) could be used, or are technically right with how we're referencing their use in this post.
The FERROUS ion that moves to FERRIC. It becomes more oxidized as it picks up Cl from the Gold Trichloride. An excess of Ferrous or any precipitant is usually a good idea. A drawback with Ferrous is it co-precips with Gold as Ferric Hydoxide and has to be washed out.

Ferrrous Chloride is FeCl2 or Ferrous Sulfate (Copperas) is FeSO4. This does the work of precipitation. Ferric ion (FeCl3) is the oxidized result and is now useless and in the way.

Edit* I am using the above as reference to answer your question.

FeCl2 replacement reaction
And this referenced post, that the quote came from, in its entirety is an interesting, short read.
 
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AMS-Pro

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Very small, non settling particles occur mainly on two occasions. First is that there is very very little gold, and it just dont clump together and stay suspended in the liquid for very very long time.
Second is that the solution was not clear, eg there was some precipitate, opalescence/milkiness, or worse - some kind of dissolved polymeric substance which could form micelles in the liquid, or blocking the particles of gold to agglomerate. Notorious for this are things like tin hydrated oxides or metastannic acid fine dispersions.
Doing silver contacts, there would always be solder containing tin. Nitric will create metastannic acid, and if this is not filtered clean, attempted precipitation on copper would create rather fine particles of silver, which do not clump together or grow bigger (sadly, my personal experience also).

If your material contained some polymers which could be attacked by acids, or resins... They could be liquified to the some extent, and stay in solution in the form of short-length segments. And acting similarly as cotton soaked in water :D blocking effective diffusion in the liquid.
And when effective diffusion is blocked... Many nucleation sites are available = greater number of individual growing particles = logically they will be smaller in size, because there is limited gold in solution. And voila, semi-colloidal suspension is born :)

There is simply that many factors that could be significant, you can experience this even if you think everything was done right. Golden rule is: dirtier and lower-grade the material is, probability of difficulties arising from the process also rise :p mainly with pulling the gold out of the complicated matrix and getting it precipitated in some workable form.
I'm content with your explanation of how the gold particles could suspend in his syrup. Ironically, your explanation is eerily similar to a concern I have with running material that I have access to, and a lot of (gaylords full). My solution to it is more of a eco-goldex, electrolytic based process I'm currently working on/developing, that however is for a different post, at a different time. Thank you for your response.
 
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Porque você disse que "Dissolveu ouro em água régia, ferveu tudo a um xarope para garantir que todo o nítrico fosse evaporado". Se você fez isso em um béquer alto com um vidro de relógio, deve ter levado muito tempo, pois a maioria dos vapores produzidos se condensaria no vidro de relógio e pingaria de volta no béquer repetidamente. Teria sido mais rápido evaporá-lo a uma temperatura mais baixa em um béquer curto sem o vidro
Em
Sou grata por estar sempre aprendendo com você. obrigado

Ícone Verificado pela comunidade



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Primeiro, você não quer ferver suas soluções para reduzir seu volume. Você quer evaporá-los. Quando você ferver, você perderá um pouco do seu ouro. A evaporação é mais lenta, porque é feita a temperaturas inferiores às de ebulição, mas mantém todo o seu ouro no recipiente.

Estou movendo sua postagem para a seção Ajuda necessária, pois você está pedindo ajuda.

Nesta seção você encontrará outro tópico que trata de um problema semelhante; Ouro demorando muito para assentar após o terceiro ciclo de lavagem com água .

Dave
 
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