Electrochemistry What happens to your silver?

[mikeinkaty]

OK, I make a 140 gm/liter (18.8 ozs/gal) silver concentration using 3.5 liters of water/nitric acid and 16 ozs of 990 silver. I use that electrolyte in a cell to process 100 ozs of 990 silver bars. At that time I calculate the Ag concentration of the electrolyte to be about 110 gm/liter (14.7 ozs/gal). With that, I am able to recover about 97, maybe 98, ounces of 9999 silver crystals.

So I put 116 ozs of 990 silver into the system and I get back 97-98 ounces of 9999 silver. But, I still have about 13 ounces of silver in the electrolyte solution for a total of about 110 ounces of silver (97+13=110).

What happened to the missing 6 ounces? Is it converted into vapor or some other form of chemical still in the solution?

Are my calculations correct in showing that the Ag concentration in solution in the electrolyte continually gets smaller as more silver is processed? Are my calculations approximately correct?

If correct, then one can expect about a 5.5% silver loss, at best, when doing electrolysis?

Mike

 

[Palladium]

That's just not possible. Something is wrong with your numbers. If you start with true ( That's the question ) 99% silver then running it through the cell should produce a theoretical 1% loss. How do you know it was 99% or did you assume that based on the fact cemented silver is suppose to be 99% ?

 

[mikeinkaty]

That was hypothetical. Doesn't matter what the % is.

But, lets say that you start with 980. That means that copper (usually) is 2%. So, if you process 100 ounces of 980 stuff, you are putting about 2 ounces of copper into solution. And, according to what I think I have read here, that 2 ounces of copper would push 2 x 3.4 or 6.8 ounces of silver out of the solution - hence, the silver concentration drops. If that's the case, then where does that silver go?

I never have done an electrolysis run where the wt of the crystals at completion was greater than what the anode bar was at startup.

Did I intrepret Gsp wrong here: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=86&t=17188#p174062 His number of 3.4 does pretty well argee with my results when I dropped what silver was left in my electrolyte couple of weeks ago.

Mike

 

[Lino1406]

I think you need to calculate on weight basis,
not on volume basis. For that you need to have the
density of each solution you mentioned

 

[nickvc]

Mike I hope I can help you make sense of this but be aware I'm no chemist. When you run a silver cell you start with your electrolyte holding say 100 grams a litre of silver as the cell starts to work the silver deposits onto the cathode and as silver in the anode dissolves into the electrolyte copper does to. The silver concentration of the cell will slowly drop as the copper replaces it, remember copper needs more nitric per gram than silver does to be dissolved, so the same volume of electrolyte will slowly be more and more copper and less silver, think of cementing your sterling with copper, how much copper will replace the silver in solution certainly not the same weight but much less. You can refresh your electrolyte by adding more nitric to dissolve more silver but eventually you will need to replace it unless you have access to inhouse assays.

 

[Palladium]

As has been said, the silver cell is not a batch operation. It is made to run continuously with addition of more anode bars and the addition of more silver nitrate to make up for the copper dissolved.

It seems you are basing your answers on " I got this much in a batch therefore these numbers should apply". I can assure you that you will never loose a gram of silver in that cell if it's operated right. Not to say you can't misplace it. What goes into that cell comes out of that cell. The numbers don't lie. The procedure may lie to you but not the numbers. Keep feeding that cell and then take average numbers and you will start to find you aren't loosing anything when you run a continuous batch process. You basing your numbers on one batch. If it goes in it comes out somewhere. Find the fault.

 

[mikeinkaty]

Mike I hope I can help you make sense of this but be aware I'm no chemist. When you run a silver cell you start with your electrolyte holding say 100 grams a litre of silver as the cell starts to work the silver deposits onto the cathode and as silver in the anode dissolves into the electrolyte copper does to. The silver concentration of the cell will slowly drop as the copper replaces it, remember copper needs more nitric per gram than silver does to be dissolved, so the same volume of electrolyte will slowly be more and more copper and less silver, think of cementing your sterling with copper, how much copper will replace the silver in solution certainly not the same weight but much less. You can refresh your electrolyte by adding more nitric to dissolve more silver but eventually you will need to replace it unless you have access to inhouse assays.

Nick - I understand that. But you say "The silver concentration of the cell will slowly drop as the copper replaces it". My question is - where does that silver go?
Mike

 

[mikeinkaty]

I guess maybe it is that the Silver concentration in the solution is dropping while the copper concentration is rising. In other words, the ratio of silver to copper is changing. But, the volume of silver in the solution is staying the same?

But, what about the amateur refiner that might put a cell together for a 100 ounce run then does not do that again for 6 months or more. Will they be able to recover all the silver they originally put into the electrolyte?

Mike

 

[nickvc]

Mike that's why I used cementation as an example? If you have a kilo of silver in solution and only put 150 grams of copper in you then have a copper and silver solution but the 150 grams will cement out around 1/2 kilo of silver, the same happens in your cell the volume hasn't changed but the silver concentration slowly drops as it plates out and copper replaces it, the nitric can only hold so much metal in solution and less copper than silver, it's why silver is used for inquartation, it takes less acid.
You are part right but no the silver volume drops as the nitric takes on more copper, you don't lose the silver from the cell it's still there, when you have no more silver to refine simply cement your solution with copper and keep for the next time you have material to process.

 

[Palladium]

But, what about the amateur refiner that might put a cell together for a 100 ounce run then does not do that again for 6 months or more. Will they be able to recover all the silver they originally put into the electrolyte?

Mike

Save it for the next setup. I pour my solution into a two-liter Mt Dew bottle and store.

 

[mikeinkaty]

Mike that's why I used cementation as an example? If you have a kilo of silver in solution and only put 150 grams of copper in you then have a copper and silver solution but the 150 grams will cement out around 1/2 kilo of silver, the same happens in your cell the volume hasn't changed but the silver concentration slowly drops as it plates out and copper replaces it, the nitric can only hold so much metal in solution and less copper than silver, it's why silver is used for inquartation, it takes less acid.
You are part right but no the silver volume drops as the nitric takes on more copper, you don't lose the silver from the cell it's still there, when you have no more silver to refine simply cement your solution with copper and keep for the next time you have material to process.

Nick -

I did cement two batches of electrolyte and only recovered about 1/3 the amount of silver that I had originally put in. But, the increased copper in the solution may have caused problems with the cementing process. I did not check the Ph of the solution cause then I did not have a Ph meter. Anyway, that got me to thinking that Ag was being lost somewhere and thinking about the economy of electrolysis. After only recovering 1/3rd I put copper back in and left it there for a week and nothing more dropped out. The solution was a very dark blue indicating increased copper buildup possibly indicating that my anodes were less than 990.

With this last batch of sterling I have been doing the ammonia test on the cement rince water starting with about the 3rd or 4th rince. I have noticed that after the rince water is clear it still takes 3 or 4 more rinces to get an ammonia test that does not turn blue. My anode bars now appear to be higher quality, i.e. less copper. I always did the ammonia test when rincing the crystals, but just started doing that on the cement. My anode bars would occasionally show a tint of copper somewhere on them but the crystals have always been clean and of good color.

Mike

 

[amesametrita]

OK, I make a 140 gm/liter (18.8 ozs/gal) silver concentration using 3.5 liters of water/nitric acid and 16 ozs of 990 silver. I use that electrolyte in a cell to process 100 ozs of 990 silver bars. At that time I calculate the Ag concentration of the electrolyte to be about 110 gm/liter (14.7 ozs/gal).

You dissolved 16 toz = 497.5 g in 3.5 litre.
Solution is 140.75g/l Ag+, 1.42g/l Cu
You refined 100 toz = 3110.35g = 3079.25g Ag, 31.10g Cu

 

[mikeinkaty]

Thanks amesametrita! So, the answer is --- It goes into the crystals! Your numbers agree exactly with my spreadsheet. The spreadsheet just didn't say where all that missing silver went.

couple more questions.
1) what makes the Ph get out of whack and what can one do about it if it does?
2) If AgNo3 solution was added occasionally to keep the silver concentration up, then a cell like that should be able to process about 800 ounces of 990 before the copper concentration gets to large. What do you do then?

An observation - If the silver being processed is 980 then the electrolyte will be good for only 400 ozs before the copper concentration gets to large. If processing 925, then the Cu concentration would become to large at about 100 ounces of metal processed. But, you'd have to pump up the AgNo3 at 40 ozs processed with about 10 more ounces of silver. But, all that silver is being recovered in the crystals. When you hit the 100 oz point then you'd have to make new electrolyte. I'm assuming copper metal would drop the remaining silver at that time?

Would running 925 bars through the cell be worth it to not have to go through the dissolve stage, rincing the cement, and pouring the anode bars? I don't think one would save any nitric acid but save by not having to do the messy job of dealing with the cement. Does the dissolve stage remove any metals that would contaminate the crystals?

I'm not planning on running 925 just trying to understand the process.

By the way, my security camera guarding my RV up in OKLA caught a mountain lion snooping around last night!

Thanks
Mike

 

[amesametrita]

Thanks amesametrita!

You are welcome.

 

[Palladium]

1) If you work against the clock, drop with glucose or sodium formate.

:arrow:
An easy method of producing 4 nines silver even if base metals are in solution is to reduce the solution with sodium formate as follows:

1 Filter solution to remove all ppt's
2 Raise pH to 1.5 using liquid caustic raise the temp to 130 degrees F
3 Reduce the silver by adding a solution made of saturated sodium formate which has been reduced to a pH of 4 using formic acid (filter any precipitate)
4 The solution will turn brown from the reduction of the silver and will turn a bright blue (if the silver nitrate was from sterling) when the reaction is completed.
5 Test for completeness of the reaction by sampling the solution on a glass rod and dripping a drop into chlorinated tap water. White ppt means more silver is present. Add more formate solution until test yields clear drops into chlorinated tap water.

Needless to say no chlorides in the make up chemistry water Works extremely well on sterling silver scrap digested in 50% Nitric/Distilled water. The starting pH is critical as is the pH of the formate reducing solution.

 

[mikeinkaty]

1) what makes the Ph get out of whack and what can one do about it if it does?
http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=50&t=17215
Pay attention to Part.6

So, I could go to the wife's pantry and dump in a little Cream of Tarter?

From Wikipedia: Potassium bitartrate, also known as potassium hydrogen tartrate, has formula KC4H5O6, is a byproduct of winemaking. In cooking it is known as cream of tartar. It is the potassium acid salt of tartaric acid, a carboxylic acid.

Mike

 

[amesametrita]

Never tried without a buffer.
Probably it works.

 

[mikeinkaty]

I just measured the Ph of my electrolyte. 3.5L with 8 oz silver that just this morning i had added 300ml of distilled water. After processing 40 ounces of 990 it showed a Ph of 0.40. So, looks like it has a way to go yet.

I tested a small sample of cream of tarter in 20 ml of distilled water and it read between 3.5 and 3.55.

I just put my last 100 grams of sterling in nitric. Got 12 ozs of 990 bars waiting on the cell. I may show a little profit with this batch.

Mike

 

[amesametrita]

Otherwise enjoy.