Washing cement silver with Sulphuric acid to rinse out copper.

[Amol Gupta]

I hope this thread to be a single point for understanding for the effect of washing cement silver with Sulphuric acid.

People have had mixed results when washing cement silver with dilute(10%) sulphuric acid.
Some report of the silver goes into the solution as silver sulphate while others not. I'm pretty sure there is some proper way of doing this.

Here is my understanding of the same.

To start of we have cement silver(in solid state) and copper nitrate(in aqueous state).
This is my starting point.

I decant the copper nitrate and rinse my cement silver until until I have colorless water coming out.(Hereonto referred to as step 1).

It is at this people use a 10% sulphuric acid and boil it for an hour or so, which in most cases yields a blue solution containing CuSO4.

Wash of the cement silver again till the clor of the rinsed solution is colorless.

This is the procedure that is followed predominantly and here is my understanding of the same.

H2SO4 + Cu(NO3)2 = CuSO4 + 2HNO3

There are two possible places where I can loose my silver
1. Silver reacting with sulphuric acid and going into the solution as silver sulphate.
2. Silver reacting with released nitric acid and forming silver nitrate.

The test for silver in the solution is pretty simple adding HCL to the solution will precipitate the silver as silver chloride.

Now here is my understanding of things of how this thing should be carried out.

1. If I add sulphuric acid to my cement silver before carrying out step 1, the cement silver will contain at this point contains a lot of copper nitrate which when reacting with sulphuric acid will release a lot of nitric acid which will consume my pure silver and carry it along with my the solution.
2. The sulphuric acid needs to be very dilute(10% in most cases) in order to only attack the copper in the precipitate and not the silver. Also the nitric acid this released during the process will be very dilute not dissolving the the silver.

I'd really appreciate a discussion on the same because people have had varying results carrying out the process.

 

[4metals]

As discussions have been hammering out here on the forum for some time now, if you do your step 1 properly, you will not leave copper in with your Silver when you cement it to begin with. The main trick is to leave some Silver in the nitrate form when you drop with copper. Best results are attained with good mixing (because a layer of Silver metal sitting on the bottom of the vessel can trap solution under it leaving it holding Silver and trapping solution.). By good mixing it means mechanically plowing the dropped Silver metal. The copper you use to drop the Silver also matters. To start, a heavy pure copper slab can do the lions share of the work, as you get to the end, switch to finely divided copper powder. Remember the mechanics of cementation, which is contact with the copper metal and the Silver ion is required to drive the reaction.

Silver in solution is easy to test with a few drops of Hydrochloric Acid in water forming a cloud indicating Silver Chloride. Stop adding the copper powder, which is added SLOWLY, when you have a very faint indication of Silver Chloride in your test.

Pump off the solution at this point and add more copper powder to get the rest of the Silver to drop. It will not be as clean as the first dropped Silver so it needs to be processed with the next batch.

If you do as listed above, and rinse well until the solution is clear you should have removed all of the copper because it will not be in metallic form, only water soluble copper nitrate.

If you have rinsed well, the nitric acid level will be almost non existent and you will not have to worry about sulfuric acid creating nitric acid if you still choose to do a 10% sulfuric acid leach. Keep it warm, not hot, and mix the water and acid well before exposing it to Silver metal. Dilute sulfuric should not be an issue but concentrated sulfuric (as in pouring it in where the Silver and water is soaking and mixing in situ) will form Silver sulfate. So mix your dilute sulfuric before adding to the Silver. And if you mix the sulfuric and water it should heat up enough that extra heat will not be needed.

Also, proper mixing at the sulfuric step is needed to assure contact with the metallic Silver you are trying to clean.

If you get the first steps right I do not think the second step will be necessary. But you can do smaller batches and sample the Silver after the first process and again after the sulfuric leach process and send both to a lab to instrumental analysis for copper to see if the second step is even needed.

 

[Martijn]

To add to 4metals reply:
Some tips for rinsing, it may be obvious; but testing is done with small amounts of solution in a small beaker or testtube with small amounts of reagent.

Test the solution for silver with HCl as said, and test for Copper nitrate with a soda ash or Lye solution and look for a blue color.
Once these washes come out clear, and you cemented as 4metals said, you should have pretty pure silver.

To get an idea of copper residue in your silver, if you can't send it to a lab, dissolve a sample of silver in nitric and see if any color comes out. Silver nitrate is crystal clear, any copper will show blue.

The last bit of silver can also be cemented out on a solid copper bar, it will leave less copper(-powder) in solution.

You don't have to wash what you did not put in there. Sterling and 'dirty' cemented silver can be used for inquarting gold.

 

[Amol Gupta]

Cementing silver in a staged manner is very appealing, I'm curious as to the copper pieces one uses to cement.

I have used pieces of copper that I had lying around(which developed a black coating this is copper oxide which is my best guess) when I use this copper for cementing I guess the copper oxide falls down and is a contaminent reducing the purity of the silver recovered.

 

[4metals]

Cementing silver in a staged manner is very appealing, I'm curious as to the copper pieces one uses to cement.

Atomized fine mesh copper is the best for this process. Like this. Using it routinely you would be better off building a small atomizer and making your own. There are threads on the forum about that. It will pay for itself as there is a large markup on atomized copper powder.

 

[Amol Gupta]

Atomized fine mesh copper is the best for this process. Like this. Using it routinely you would be better off building a small atomizer and making your own. There are threads on the forum about that. It will pay for itself as there is a large markup on atomized copper powder.

I'm curious as to the copper pieces one uses to cement

The statement was with respect to the lines that follow, I was wondering if the copper pieces is used to cement have a coating of copper oxide will it contaminate the cement silver or the copper oxide will be consumed by the acid...?

 

[4metals]

It all depends on IF it is copper oxide. If it is the following will happen;
CuO + 2HNO3 → Cu(NO3)2 + H2O. If the discoloration is something else that folks have lying around in a refinery what happens next is anyones guess. Just like everything else when trying to make high purity metals, garbage in, garbage out.

 

[Lou]

I personally prefer the chloride process for silver recovery and converting the chloride to cement silver with iron chunks in a tumbler with sulfuric acid.

Then I prefer to wash with water until neutral then put a rinse with a borax nitre solution (this solution gets reused many times), dry and melt. That seems to give silver of high purity by fluxing in situ.

 

[anarxi]

Every snipe praises its own swamp.

via chloride, reduction hydrochloric acid and aluminum.
washing with water and sodium hydroxide..

 

[anachronism]

Every snipe praises its own swamp.

via chloride, reduction hydrochloric acid and aluminum.
washing with water and sodium hydroxide..

Haha it certainly does. Silver chloride for me too, reduced with sulphuric and zinc.

 

[Amol Gupta]

Haha it certainly does. Silver chloride for me too, reduced with sulphuric and zinc.

Can you recover your nitric once you convert the silver to silver chloride....?

 

[anachronism]

Can you recover your nitric once you convert the silver to silver chloride....?

No it's not possible.

 

[Amol Gupta]

No it's not possible.

If I cement it with copper there are ways I can get back my nitric acid

 

[Yggdrasil]

No it's not possible.

I think that is not neccessary completely true.
If you use HCl slightly below full conversion.
The Nitric will be slightly diluted but still usable for more Silver.
I think GSP (Chris) had an interesting post on this.
One eventually reach a point where it is too dilute though.

 

[anachronism]

If I cement it with copper there are ways I can get back my nitric acid

Fair enough. In honesty it's not something I'm very experienced with as I've not had a need to do so.

 

[anachronism]

I think that is not neccessary completely true.
If you use HCl slightly below full conversion.
The Nitric will be slightly diluted but still usable for more Silver.
I think GSP (Chris) had an interesting post on this.
One eventually reach a point where it is too dilute though.

Thanks Ygg.

 

[Yggdrasil]

If I cement it with copper there are ways I can get back my nitric acid

Not without distillation I think.
Converting the Copper Nitrate to Copper Sulfate.

 

[Amol Gupta]

Not without distillation I think.
Converting the Copper Nitrate to Copper Sulfate.

You are right about that, but I get my nitric back which is pretty useful to me...

 

[Yggdrasil]

You are right about that, but I get my nitric back which is pretty useful to me...

These methods all have their quirks.
The method with HCl was outlined by GSP as the only time he created Silver Chloride by intention.
The rest of the time he cemented on Copper.
The HCl method recreates the Nitric but dilutes it.
The Salt method creates Sodium Nitrate but do not dilute the remaining Nitric.
The Cementing creates Copper Nitrate.

All these methods has a path to reclaim the Nitric but with some effort though.

 

[Amol Gupta]

These methods all have their quirks.
The method with HCl was outlined by GSP as the only time he created Silver Chloride by intention.
The rest of the time he cemented on Copper.
The HCl method recreates the Nitric but dilutes it.
The Salt method creates Sodium Nitrate but do not dilute the remaining Nitric.
The Cementing creates Copper Nitrate.

All these methods has a path to reclaim the Nitric but with some effort though.

Recovering through copper is something I know and I'd rather stick to that, as the saying goes 'There are multiple ways to skin a cat'