Some advice on removing base metals?

[Platdigger]

"water soluable people"...hmmmm

lol

 

[zorba]

How much does it enter in to the equation where nitric acid is concerned? I was lead to believe that the addition of water provided an extended amount of work by a given lot of acid. It's clear that water should be added to concentrated nitric when silver is dissolved----the question is, does it actually do more work? If so, what, exactly, is the mechanism by which the additional work is provided. Regenerating from the fumes sounds like part of the process. Is there more?

Harold

Harold, im not an expert on these fields. But i am pretty sure that you guys are referring to the passivation process
which mainly occurs when you add really pure nitric to base metals. Therefore, water is added to lower the concentrated value?

 

[Anonymous]

Considering the expense of store bought nitric acid, and the time required to make your own, be sure to dilute the acid to 35% (or even less will work, but slower). The water acts as a medium for the metal nitrates to dissolve into. This is also another reason to add your nitric in controlled doses, and to know how much nitric to use in advance of performing the reaction.

Alright, that cleared my foggy mind up a bit... Apparently, the water also helps as a solvent to the base metals.
So lats say that i have a ~50% nitric which i dilute one part down to 25%, will it still have an effect when dissolving
copper and silver from the already precipitated brown gold sand?

My first post. I have about 9 years industrial experience with gold and base metal refineries.

Seems to be a few gaps in general chemistry here. Let me try to settle the muddy waters a bit.
Some basic chemistry concepts:
1) Inorganic reactions go to equilibrium not completion.
Le Chatelier's principle states that when a system in chemical equilibrium is disturbed by a change of temperature, pressure, or a concentration, the system shifts in equilibrium composition in a way that tends to counteract this change of variable.

For example:
Copper + sulfuric Acid + hydrogen peroxide <----> dissolved Copper + dissolved sulfates + Water
Cu + H2SO4 + H2O2 <-----> Cu+2 + SO4-2 + 2H2O
Basically the reaction will continue until every thing on the left side of <-----> equals the right side. (Equilibrium)
Lets apply Le Chatelier's Principle. If I have a reaction that is in equilibrium then I have copper dissolving and copper precipitating equally. Now if I added more sulfuric acid and hydrogen peroxide, the reaction will be forced to form copper sulfate and water. On the other hand, if I added water or copper sulfate to mixture, the reaction would reverse direction generating solid copper, sulfur acid, and hydrogen peroxide. The reaction is reversible.

My point is, you can't say arbitrarily water will do this or that. It is dependent on the chemical reaction.


2) Solubility Limits
From my experience in industry, the total metals concentration (Copper, Iron, Nickel) in a h2so4-ammonia solution is limited to about 100 g/l at 100F. This an approximate number that is highly dependent on temperature and the metal salt. (ie, Solubility Limits) Water, for the most part, does not dissolve the metals, the acid does. The amount of metal dissolved can limited by the solubility limit. Adding water dilutes the metals concentration, thereby, allowing more metal to dissolve. On the other hand, adding water also dilutes the acid, which will slow the reaction; so, it would be a good idea to monitor acid concentration.
3) Solubility is temperature dependent
The solubility of a metal salt is very dependent on the temperature of the solution. For most reactions, the metals become more soluble with higher temperatures. (This is not always the case however)
I hope this helps.

 

[Anonymous]

As far as separating gold from copper goes, you basically have 3 options:

(1) Selectively dissolve the copper without dissolving the gold. This can be done with nitric acid or by using the muriatic acid/hydrogen peroxide method as developed by Lazersteve. There are tons of very detailed information on the forum for both processes.

(2) Selectively remove the gold without dissolving the copper. This can be done by using the electrolytic sulfuric acid stripper. Here again, there is tons of information on the forum about this.

(3) Dissolve everything and selectively precipitate the gold. This can be done with aqua regia (a combination of muriatic and nitric acids) but, unless the gold makes up a major portion of the material, this is not the way to go, for a number of reasons. With most gold plated electronic materials, such as fingers, this is almost never the way to go. With certain types of old CPUs that contain gold brazes, however, this may be the best method.

Depends on what you are working with, but sulfuric acid and hydrogen peroxide will dissolve copper, not the gold. Another alternative is dissolve everything in aqua regia and use sulfur dioxide to precipitate the copper from solution. Adding HCL will precipitate the silver. Gold can then be electroplated.

 

[Rhodium]

Another alternative is dissolve everything in aqua regia and use sulfur dioxide to precipitate the copper from solution. Adding HCL will precipitate the silver. Gold can then be electroplated.

So2 will drop gold from ar.

 

[Anonymous]

So2 will drop gold from ar.

Oops, add urea to eliminate nitric acids, need to add the HCl to drop the silver first...then SO2

 

[Oz]

Another alternative is dissolve everything in aqua regia and use sulfur dioxide to precipitate the copper from solution

I think the point Rhodium may have been making is that SO2 will precipitate gold from a chloride solution preferentially over the copper you stated. Yes it will also precipitate copper if used in excess, but what little copper is there is what proper washing technique is for.

 

[Anonymous]

Another alternative is dissolve everything in aqua regia and use sulfur dioxide to precipitate the copper from solution

I think the point Rhodium may have been making is that SO2 will precipitate gold from a chloride solution preferentially over the copper you stated. Yes it will also precipitate copper if used in excess, but what little copper is there is what proper washing technique is for.

I stand corrected, was 15 years ago, sorry I a little flamboyant. The gold drops. We used to wash then dissolve the gold at that point and then plate. Gives very high quality gold.

 

[Oz]

After you precipitated your gold with SO2 and you had finishing washing it you mention you re-dissolved it and then plate. I am assuming you were making a plating solution from your refined gold. Do you care to walk through the steps you followed to make your plating solution?

 

[Anonymous]

For example:
Copper + sulfuric Acid + hydrogen peroxide <----> dissolved Copper + dissolved sulfates + Water
Cu + H2SO4 + H2O2 <-----> Cu+2 + SO4-2 + 2H2O
Basically the reaction will continue until every thing on the left side of <-----> equals the right side. (Equilibrium)
Lets apply Le Chatelier's Principle. If I have a reaction that is in equilibrium then I have copper dissolving and copper precipitating equally. Now if I added more sulfuric acid and hydrogen peroxide, the reaction will be forced to form copper sulfate and water. On the other hand, if I added water or copper sulfate to mixture, the reaction would reverse direction generating solid copper, sulfur acid, and hydrogen peroxide. The reaction is reversible.

My point is, you can't say arbitrarily water will do this or that. It is dependent on the chemical reaction.


Hi,
Are you sure about the above? Adding water to a copper sulfate solution has never forced copper out of solution for me, nor has adding copper sulfate forced out elemental copper.

From my understanding all of the components, copper, sulfuric acid, and hydrogen peroxide are in a higher energy state before the reaction than they are after the reaction because there is heat energy generated by the reaction that is transferred to the environment.

I also think regeneration of H2O2 is also a limiting factor as it is hard to form and breaks down in the presence of metal ions.

Jim

 

[Anonymous]

After you precipitated your gold with SO2 and you had finishing washing it you mention you re-dissolved it and then plate. I am assuming you were making a plating solution from your refined gold. Do you care to walk through the steps you followed to make your plating solution?

I am getting some backlash from the establishment in this forum, probably deserved. Guess I was a bit vague, not 100% technically correct. Anyway, there are many ways to skin a cat.

The replating process I was referring to is called electrorefining and if this topic is of interest look up the Wohlwill process. I am aware of at least one gold refinery using it. Basically you make the powdered gold the anode and replate the gold to the cathode. Impurities will tend to remain at the anode or under it as slimes. The electrowinning solution can be a weak acid (HCL) or cyanide/caustic solution. Basically run the cell at about 2-3 volts. Amps will depend on the amount of surface area (number of cathodes/anodes) you are working with, the electrolyte, and the amount of gold plated.

Depending on gold content and impurities present at start, you can get 99.999% pure.
Design thoughts
1)Use cathodes/anodes with large surface area. This makes the process quicker, but will increase amp usage. (follows Faraday's law of electrolysis)
2)There are different ways to create the anode. You can melt the gold and cast it into shape, melt the gold and place a stainless steel hook into it, or make a basket to hold the gold powder.
3)Use of HCL prevents silver from plating since it forms AgCl and falls to bottom of cell; not sure about other PGM's though. You may be able to optimize voltage to lower pgm content if you have it.

Hope this helps

 

[Harold_V]

The electrowinning solution can be a weak acid (HCL) or cyanide/caustic solution.

Depending on your objective, cyanide may not work. It is not selective, and will transfer silver as well as gold. I expect it would also co-deposit copper. It is beyond reason to expect fine gold using it as an electrolyte unless you start with fine gold, and have no contaminants in the electrolyte. It can be an excellent recovery process, but not worth a damn for refining.

Harold

 

[Oz]

I am getting some backlash from the establishment in this forum, probably deserved. Guess I was a bit vague, not 100% technically correct. Anyway, there are many ways to skin a cat.

I appreciate you taking the time to reply to my inquiry. I had the impression from your profile that you were are metallurgical engineer for some years predominately in precious metals. I had hoped that you could have outlined an exact procedure that you used to follow in your course of work, it had the potential of being a variation that had not been discussed on the forum prior given your background. As you stated, there is more than one way to accomplish the same thing, and I am always willing to hear of a method that is different from others.

No one here is trying to give you grief, but there are members here with considerable knowledge and they strive for accuracy. If someone posts something it will be scrutinized and corrected if necessary. If it is new, novel, or not understood, clarification will be asked for. This is what sets this forum apart from others. It is a great place to be if one wishes to learn refining.

 

[goldsilverpro]

Pony Express,

Harold is right in that the other coinage metals, silver and copper, will co-deposit with the gold in a cyanide system.

Search for Wohlwill cell on this forum and you will find it's been discussed quite a few times. The usage of the cell originally patented by Emil Wohlwill is quite well known. However, due to the tremendous costs of setting it up properly (about an oz of Au/liter), I doubt if anyone on the forum is using it except, perhaps, those members who are presently dealing with very large quantities. Also, the best thing to use for bussing is pure silver and the best cathodes to use are pure gold sheet. I'm sure it could be jury-rigged a bit, but one shouldn't stray too far from the basic setup. I am also curious how you would efficiently set up an anode basket to hold gold powder. Instead of providing us with a technical analysis through such things as Le Chatelier's Principle, perhaps you could give us a more practical step-by-step on exactly how you set things up. It would be much more valuable. Like plating, when you get down to the nitty-gritty of it, refining is almost totally a practical endeavor.

There's about half a dozen of us on the forum I could name that have a combined total of about 200 years of professional refining experience. Three are moderators. Interestingly enough, I'm sure that all of us would admit to having learned many things since being on the forum. I certainly have and I spent 40 years in the refining business. One of the most enticing things about precious metals is that one never knows it all. When I first came on the forum, I thought I knew it all. It didn't take long to get my comeuppance.

Chris

 

[Anonymous]

More great comments. I really hope I am not upsetting anyone. I realize there is a lot of expertise here, and I came to the forum looking for expertise since I was think of doing a start up. Sorry this post is sliding from Advice on Removing Base metals.

When I first posted, I had not read Hoke's book, but I have read or skimmed most of it now. I understand the background of the forum better.

Now to address some issues brought up. Cyanide vs HCL. It comes back to selecting the correct method from your starting point. You will be disappointed with the Wohlwill process if you are not at least 95% gold to start with. As mentioned before, platinum group metals may not separate either. Yes, plating onto purified gold and casting the anodes totally with gold will give a higher end product, but it will also increase inventory. Comes back to objectives, what is the starting point? What is the desired end point? If you want the ultra pure, you need gold cathode/anode; otherwise, save some bucks plating on SS cathodes.

I agree, the cyanide method will transfer silver, and I have seen copper. I brought up the cyanide method since the original post was about base metals. The scenario I had used cyanide, we had a lot of base metals, ie, gold plated on steel wool. Starting with a product that was probably 50% iron, replating in this instance replaced the retorting step (oxidizing the steel), and eliminated a whole bunch of smelting. Difference in purity went from 87% (retort/smelted) to 90-92% (replate/smelted) of gold. If you want more documentation, look up the Mesquite Mine, CA. The process has been documented.

Some interest in Le Chatelier's principle in above comments. I do not have time at the moment to add more comments; however, since this is a fundamental chemistry concept that anyone in the refining business should understand, I will make separate post at a later date.

Cheers