Order of Precipitation?

[Nickolas]

Hi Guys,
I've been reading for a bit, and a question came to mind.
Is there an order of precipitation when dealing with an AR
soup of silver, gold, platinum and palladium ?
In the past, I've only dealt with recovering gold.
Thanks

P.S. This is my first time here and I'm sorry if I've over
stepped.

 

[butcher]

You will not have much silver in aqua regia, it forms an insoluble silver chloride AgCl, very small amounts of AgCl2 from concentrated high chloride solutions will precipitate as AgCl once the solution is diluted.

As far as an order of precipitation of the other metals it would be a choice that the circumstances and the person refining would dictate.

 

[AshesoftheUniverse]

I think Nickloas is alluding to the reduction potential at which the metals in solution would revert to the 'zero' state without other manipulations.

 

[butcher]

I think Nickloas is alluding to the reduction potential at which the metals in solution would revert to the 'zero' state without other manipulations.


I still must not understand the question?
AshesoftheUniverse
Where do metal ions revert back to zero states without being manipulated, or given back electrons, or complexed without adding a reducing agent or a complexing agent, or electrical current?


I cannot see where the reduction potential of the metal or the reactivity series of the metal would be used to determine in which order one would choose to reduce metal or complex it from solution a solution of aqua regia.

 

[AshesoftheUniverse]

For example, we boil-off the Nitric Acid in order to reduce the Oxidation Potential of the solution in order to precipitate Gold with Sodium Bisulphite. This is a standard operation that many here are familiar with. Now, if we take the same solution and allow the Oxidation Potential to find it's own way and monitor the Oxidation Potential of the solution, we will see, that the metals contained in the solution begin to precipitate out of solution as the Oxidation Potential begins to drop, ignoring supersaturation effects for the moment, we will see that the transition from a Salt to the reduced Metal , occur at well defined REDOX potentials for a given Element species. This can also occur when the Metal species in solution are of a negative potential, such as in Cyanide or other similar pathways.

 

[butcher]

For example, we boil-off the Nitric Acid in order to reduce the Oxidation Potential of the solution in order to precipitate Gold with Sodium Bisulphite. This is a standard operation that many here are familiar with.

You should never boil off nitric acid, a slow evaporation process with heat...

With evaporation or a chemical reaction we can remove the oxidizer (nitric) from solution, thus removing the oxidizing agent in the solution that will allow us to precipitate gold later with a reducing agent, but the removal of nitric this has no effect on the oxidation or reduction potential of the gold ions in solution or other metals, the gold can stay in this state of oxidation without reduction or a-gain in electrons to be able to form gold atoms, and the gold or other metals will not precipitate, yes we have changed the oxidation potential of the solution but not that of the metals or the ions in solution...

Now you stated in your example that the refiner has chosen to precipitate gold first using sodium bisulfite, where here he has made a choice to precipitate gold before other PGM metals, this does not have much to do with the reduction potential of the metals, he could have chosen to precipitate or complex any on of the PGM'S before reducing gold, even if those metals have a slightly higher reduction potential as the gold has...


Now, if we take the same solution and allow the Oxidation Potential to find it's own way and monitor the Oxidation Potential of the solution, we will see, that the metals contained in the solution begin to precipitate out of solution as the Oxidation Potential begins to drop, ignoring supersaturation effects for the moment, we will see that the transition from a Salt to the reduced Metal, occur at well-defined REDOX potentials for a given Element species. This can also occur when the Metal species in solution are of negative potentials, such as in Cyanide or other similar pathways.

The statement above may be true if you were using electrolysis or some other chemical or a displacement reaction like cementing with zinc to remove metals from cyanide or other solution...

I cannot see where it has any relativity to the discussion, the metals will not precipitate on their own to become metal, so we will not see that the transition from a Salt to the reduced Metal of the platinum group metals or a complex of those metals, without the use of a reducing or complexing agent...

 

[AshesoftheUniverse]

I think you miss the point. Each Metal species in solution has a range of REDOX potentials in which it will precipitate the Metal. As you transition between potentials, the metals will precipitate in a fixed order, assuming there is not some contaminant adjusting the behavior of the precipitation.
Those of you familiar with commercial or more advanced setups will recognize the use of REDOX measurements to fine-tune the precipitation.

 

[butcher]

We apparently are discussing apples and oranges.

I thought it is you that missed the point.

I was speaking of refining the precious metals, where we selectively reduce them from a solution of aqua regia as metal or as a complex with the use of different reducing agents...


What are the oranges about?

Nickolas, Sorry if we are getting a bit off-topic with our discussion, I think you will find Hoke's book very helpful.

 

[AshesoftheUniverse]

Actually, we are discussing the same thing. We use reducing agents to adjust the Oxidation Potential to allow the Metal to precipitate.
The original question is if there is an order in which the Metal precipitates. If you understand what I was trying to describe was to say "Yes!" in simpler terms.

 

[butcher]

No, we are not discussing the same thing,

Ok yes if we were discussing the use of an inert anode under controlled electrolysis.
Or a chemical reduction reaction with the use of Sodium Hydroxide...

Maybe you can explain what you are talking about, using a specific procedure...

 

[butcher]

AshesoftheUniverse,
You are discussing oranges, I was speaking of apples, both are fruit, but they are not the same.

An ORP measurement (oxidation-reduction-potential) is basically a millivoltmeter which uses the solution as an electrolyte to give a millivolt reading, giving us information of the (electrolytic cells) potential (voltage) which is an indication of the chemical substance's tendency to oxidize or reduce (some other chemical substance or metal...
It will only tell us half the story, or give us clues to a half-reaction... Giving us a voltage measurement or potential of a half-reaction which is based on 1 molar concentration (using Nernst equation) to gain some understanding of the known chemical substances involved in the chemical substances of the solution being tested...

Very similar to a pH meter in construction and use, which is also used similarly in testing a solution or electrolyte, these two meters are helpful to gain information and are often used together along with other tools and indicators.

Cyanide leaching we may wish to keep an eye on pH, temperature and the ORP...
In corrosion protection say in steam boilers, and other industrial equipment... we may keep an eye on the ORP, pH... along with other chemical tests...
A thiosulfate leaching process...
Sulfide or hydroxide precipitation of wastewater or other solution...
Trying to decide if the solution would dissolve zinc and not attack aluminum...
If the solution was chlorine or chlorides or hypochlorites...
Along with the color of the solution and other tests (or chemical tests) to determine what valance the iron in solution may be in...

In refining of gold and platinum group metals using aqua regia we are working with apples, it is different from where we may be working with oranges using pourbaix diagrams of solutions, pH meters, temperature, concentration, and a single reducing agent, and attempting to precipitate metals or groups of metals salts, sulfides, or hydroxides from a solution...

 

[AshesoftheUniverse]

Actually, An ORP Meter is very similar to a pH meter. The one I have been using for many Years has both capabilities, by just changing electrodes.
The environmental authorities are very fond of them as well, so I recommend getting to know how to use one.
Besides that, changing your way of thinking about how you go about "getting the Gold" might take you out of your comfort zone.
I think there are more people qualified to assume that responsibility than I.
Have a nice Day.

 

[anachronism]

Actually, An ORP Meter is very similar to a pH meter. The one I have been using for many Years has both capabilities, by just changing electrodes.
The environmental authorities are very fond of them as well, so I recommend getting to know how to use one.
Besides that, changing your way of thinking about how you go about "getting the Gold" might take you out of your comfort zone.
I think there are more people qualified to assume that responsibility than I.
Have a nice Day.

I may have misread your intentions however it appears that now you're just being patronising. Don't load the gun then waltz off into the sunset without following it up. If you've got something constructive to add then let's have it.

 

[butcher]

The order of oxidation of metals in a recovery or refining process is chosen many times not only by the refiner but also by the material or metals involved, their reactions to different acids or even the same acid at different temperatures, or concentrations, or different oxidation states...

We try to separate the metals as much as possible before we even put them into solution, and try to get them back out of solution as pure as possible...

we really do not have a certain order of oxidizing these metals (loss of electrons), or a certain order of reducing (gaining electrons) these Ions back to the metal, or to a complex of the metal in a fixed order.

The Platinum group, in general, is harder to oxidize, this can aid in recovery of these metals.
Rarely do we deal with the platinum group, and when we do, letting it follow our silver, or even using copper cementing it from solution in a stockpot may be a better option than trying to reduce it and precipitate it as a complex...

Again the material... and the options may help a refiner choose an order of precipitation.
Hoke's book will help the refiner understand the options, and gain a better understanding of the material...