Electrochemistry Question about silver nitrate cell

[mikeinkaty]

I had a total of 168.6 ozt of cemented silver bars that I had made from sterling silver. 24 ozt went into making three batches of electrolyte where each batch used 8 ozt. Two of the batches were used and the 3rd was held in reserve and not used till the very last batch of crystals. This left about 144 ozt to be processed through the two cells.

When I finished the project I had 155.5 ozt of 999+ silver and the 8 ozt still in the 3rd electrolyte solution for a total of 163.5 ozt. This leaves a difference of (168.6-163.5) = 5.1 ozt of missing silver. I would expect to lose around 1.5% in the electrolysis phase and the rest to pot loss and measurement error.

So, the bottom line is that I came close to recovering most of the cemented silver.

But here is what I don't understand. To make the last of the 999 bars I dropped the silver left in the two electrolyte solutions. I melted it and ran it through the 3rd solution to produce the last crystals. The solution most used dropped about 1.5 ozt and the other about 3.5 ozt. One batch had processed about about 100 ozt and the other about 44 ozt. There was no copper oxide buildup on the copper bars when I dropped these two solutions so I'm assuming the pH of the solutions was ok.

The most used solution consumed 6.5 ozt of the silver in solution while using 100 ozt of the silver from the bars. If this is correct, then it would make calculating the amount of silver to disolve for the electrolyte easy to calculate! For each 100 ozt you wish to run through electrolysis, use 6.5+ ozt when making the electrolyte. or roughly 6.5%. I guess this 6.5 ozt was converted to an equal amount of copper nitrate? That would explain why the solutions got more and more blue with each run.

Are my observations here correct??

Mike

 

[butcher]

Mike I have to admit I am confused.

I do not think you would lose much if any silver, although there would be lose of weight as the other metals like copper remained in solutions, some could be temporarily misplaced in filters and left in solution..., some lost in melting, but most of this should show up someplace in your process.

Sterling may have more copper or other metals than marked.

The cemented crystals may have had more copper in the crystals than you suspected (if you used thin copper metal or wire to cement you could even have small pieces of copper).

Cemented powders can have more weight than the metal once melted (moisture or crystals of acids) even if they seem to be dry.


After you try to cement silver out of your spent solutions using copper, you could try to use salt to see if any silver remained.

GSP would be better to answer the more detailed questions, and be able to follow your math better on how much silver would be best to use in the electrolyte, and where loses of weights can occur, as he is experienced in the math and the silver cells, where I am not.

 

[FrugalRefiner]

Are my observations here correct??

Mike

Mike,

I'd say your observations are correct, but your interpretations may be a little bit off.

The most used solution consumed 6.5 ozt of the silver in solution while using 100 ozt of the silver from the bars.

The total amount of metal ions in the electrolyte is limited by the amount of nitrate ions (NO₃^-). The solution doesn't consume the silver. The silver is displaced from the solution by the copper ions as your anodes are dissolved and it ends up in your crystals. Each gram of copper that goes into solution pushes 3.4 grams of silver out of solution as silver crystals.

If this is correct, then it would make calculating the amount of silver to disolve for the electrolyte easy to calculate! For each 100 ozt you wish to run through electrolysis, use 6.5+ ozt when making the electrolyte. or roughly 6.5%.

First, this assumes that your 100 ozt of anodes will always be exactly the same. If you end up with a little more or a little less copper in your cemented silver, or other contaminants like palladium, the balance will change.

Second, you can't run your electrolyte to depletion. If you had only started out with 6.5 ozt of silver in your electrolyte, you wouldn't have been able to process the full 100 ozt. Once your silver content drops below about 15 to 20 grams per liter, or copper content rises over 80 to 100 grams per liter, your crystal quality will suffer.

I guess this 6.5 ozt was converted to an equal amount of copper nitrate? That would explain why the solutions got more and more blue with each run.

The 6.5 ozt of silver was displaced by copper which leaves you with a copper nitrate solution. As more copper goes into solution, the blue color will increase as you observed.

Dave

 

[mikeinkaty]

Thanks FR and Butcher -

I had no idea how the electrolysis processed worked. I thought that if you used 8 ozs to make the solution then when done you could drop out 8 ozs.
I know I was able to make a lot more clean crystals that I had anticipated. But if I used 8 ozs of silver to make each solution and if I processed 100 ozs of 980-990 silver in the first cell, then I must have been pretty close to its limit.

Is there any rule of thumb for calculating how much silver to use in an electrolysis solution? Assuming clean copper dropped silver is what was used to make it and also what is being refined. Would my 6.5% observation be approximately correct for my case?

My choice of the word "consumed" was my non-technical way of saying that it disappeared out of the solution.

Butcher - what would putting salt in the spent solutions do as far as telling if they contained more silver?

Mike

 

[FrugalRefiner]

I don't recall how much solution you had so I can't really say whether your first electrolyte was nearly exhausted or not. You recovered 1.5 ozt by cementation, so you had about 45 grams left in solution. That's nearing the lower limit for 2 liters of electrolyte. Your second solution still had some milage left.

I believe Lazersteve starts with about 100 g. per liter, Harold used a little more, some use a little less.

I understand I was being nit picky about the difference between consumed and displaced. My comment was for others who may read this thread in the future.

I'll answer your question to butcher as well. If you add salt (NaCl) to a solution containing silver (Ag), you will immediatly see a white, cloudy/curdy precipitate of AgCl.

Dave

 

[butcher]

If you have a nitrate solution, in this case copper nitrate and you added a chloride like HCl or NaCl salt. if silver was is in solution you will see a white cloud form, if enough silver you would get a silver chloride precipitant form, you can use this with a small sample for testing purpose or could remove the silver from solution with this method.



Cementing on copper is the other choice if for some reason you did not want to form silver chloride, or you wished to keep the copper nitrate for some other use.

You could use either, but as the spent electrolyte is pretty loaded with copper nitrate, I was suggesting checking for silver there or cementing it out or precipitate it with HCl or salt (salt is cheap).

The metals do not just dissolve at the anode and move over to the cathode to form a crystal or plate out.

The metal atom at the anode loses electrons this metal goes into solution joined by a nitrate ion to form silver nitrate in solution.
Then at the same time way over at the other side of the cell, around the cathode there is a solution of silver nitrate the silver gains an electron and is reduced to metal (growth of crystal in this case) this frees the NO3 ion in solution to join with another silver atom over at the anode.

So besides the copper that uses two NO3 per Cu to form Cu(NO3)2, verses silver that uses one NO3 to form AgNO3, the metal cations stay in solution, and well as the nitrate anions.

Kind of like one ball thrown into the circle or jar knocks another ball out of the circle or jar, but the jar still contains the same content of balls, now we do have different sizes of balls so one bigger copper ball thrown in knocks more of the smaller silver balls out, but metals are not added or removed, more like just replaced.

Although slight amounts can be due to evaporation, concentration and other factors but then we are nitpicking.

As one silver goes in, another silver comes out on the other side of the cell, but if one copper goes in two silver come out.
By weight if one gram of copper goes in 3.4grams of silver come out.

So if we started with a jar full of metal balls we will end with a jar full of metal balls, in this case you just happen to have more of the bigger copper balls in your jar with probably very little silver balls.

Read GSP's posts on making and maintaining the electrolyte, he goes into great detail, on how why and what, sometimes his math gets way out there, but most of the time I can follow him on the simple stuff, at least as far as I have needed up to this point, but sometimes I think I would need to hire a good math teacher to interpret the complicated stuff to a language I could understand

 

[mikeinkaty]

I don't recall how much solution you had so I can't really say whether your first electrolyte was nearly exhausted or not. You recovered 1.5 ozt by cementation, so you had about 45 grams left in solution. That's nearing the lower limit for 2 liters of electrolyte. Your second solution still had some milage left.

I believe Lazersteve starts with about 100 g. per liter, Harold used a little more, some use a little less.

I understand I was being nit picky about the difference between consumed and displaced. My comment was for others who may read this thread in the future.

I'll answer your question to butcher as well. If you add salt (NaCl) to a solution containing silver (Ag), you will immediatly see a white, cloudy/curdy precipitate of AgCl.

Dave

Dave - I used 8 ozt in 3.5 liters, or 71 grams per liter. That cell processed around 100 ozt of cemented silver before I took it down and recovered about 45 grams of silver. It was very dark blue when I tood it down. It started out as almost clear with a tint of blue. One thing I noticed on a few batches of crystals in that cell is that the crystals recovered weighed more than the bar being processed by a small amount. That's what got me started in thinking that some of the silver must be coming out of the solution. I noticed this on 4 bars that I had disolved and dropped twice so they may have been higher silver content. I had done these bars twice before I knew I was going on ahead with Electrolysis. Had I known, I would have not processed them twice.

Mike

 

[mikeinkaty]

Cementing on copper is the other choice if for some reason you did not want to form silver chloride, or you wished to keep the copper nitrate for some other use.

You could use either, but as the spent electrolyte is pretty loaded with copper nitrate, I was suggesting checking for silver there or cementing it out or precipitate it with HCl or salt (salt is cheap).

Butcher - I still have those two electrolyte solutions. I'll go test a small amount with salt. But then, If I do see a lot of white percipitate, how do I process that? I though white percipitate at this stage meant Sodium Nitrate??

Also, I'm going to check to see if there are any balls in there!

Mike

 

[FrugalRefiner]

Mike,

You may have pushed that first batch of electrolyte a little too far. 45 grams of silver left in 3.5 liters of solution = 12.86 grams per liter. That's a little below the recommended minimum level. Once the level of silver gets too low you will start to codeposit copper. That's why you can't run the electrolyte until the silver is depleted.

Butcher's Balls

Dave

 

[mikeinkaty]

Butcher -

After reading your note I went to the shop and picked out a gallon jug of rince water that was pretty clear with no color to speak of. I added salt water and low and behold a white cloud boiled up. I recovered maybe 2 cubic cc's then did the NAOH / syrup thing. The NAOH made it turn black and the syrup made it turn dark brown. I filtered it and dried it and actuall recovered 3 grams of silver. Polished the little button up and it polished like silver and looks like silver.

Now I need to go back and study the actual procedure with the right proportions of stuff. I did rince the AgCl.

Mike

 

[mikeinkaty]

Mike,

You may have pushed that first batch of electrolyte a little too far. 45 grams of silver left in 3.5 liters of solution = 12.86 grams per liter. That's a little below the recommended minimum level. Once the level of silver gets too low you will start to codeposit copper. That's why you can't run the electrolyte until the silver is depleted.

Butcher's Balls

Dave

THanks dave -
Next time I'll shoot for processing 100 ounces but will add about 12 oz to the solution. Now that I know what's going on I know it will not be lost.

Mike

 

[goldsilverpro]

After cementing, the purity will be around 99%, depending on how well you rinsed.

If the silver was 99% after cementing, that would mean that 144 x .01 = 1.44 oz of copper entered the solution. Therefore, 1.44 x 3.4 = 4.9 oz of silver was displaced from the solution. That's very close to the 5.1 oz you said you lost.

Some silver always ends up in the anode slimes caught in the anode bag.

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.

For good silver crystal purity, it is best to not let the silver in solution to drop below about 3-4 oz/gal. or the copper to be above 9 or 10 oz/gal.

Under perfect conditions (100% efficiency at both electrodes), you would deposit about 4g of silver per amp-hour and dissolve the equivalent at the anode. Unless the silver in solution is too low, the cathode efficiency will be near 100%. However, the anode efficiency will start declining, due to such things as the anode getting smaller and the anode being encased in anode slimes. In other words, the electrode efficiencies will be out of balance and more silver will deposit than is dissolved. This will slowly deplete the silver in solution.

 

[mikeinkaty]

After cementing, the purity will be around 99%, depending on how well you rinsed.

If the silver was 99% after cementing, that would mean that 144 x .01 = 1.44 oz of copper entered the solution. Therefore, 1.44 x 3.4 = 4.9 oz of silver was displaced from the solution. That's very close to the 5.1 oz you said you lost.

Some silver always ends up in the anode slimes caught in the anode bag.

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.

For good silver crystal purity, it is best to not let the silver in solution to drop below about 3-4 oz/gal. or the copper to be above 9 or 10 oz/gal.

Under perfect conditions (100% efficiency at both electrodes), you would deposit about 4g of silver per amp-hour and dissolve the equivalent at the anode. Unless the silver in solution is too low, the cathode efficiency will be near 100%. However, the anode efficiency will start declining, due to such things as the anode getting smaller and the anode being encased in anode slimes. In other words, the electrode efficiencies will be out of balance and more silver will deposit than is dissolved. This will slowly deplete the silver in solution.

Thanks Gsp!
I recovered 5 ozt total from the two solutions and I had disolved 8 ozt in each of the two. So, IF I recovered all the silver, then there would have been 11 ozt pushed out of the solutions, total. But, I'm beginning to think I did not recover it all. I will be in the shop today putting salt in the waste vessels to see if anything else shows up. If an appreciable quantity does show than that begs the question - why didn't the copper do the job? I did stir it well while the copper was there.

When I recovered that 5 ozt that was dropped with copper I carefully melted it into one bar then put it in the unused electrolyte. I recovered like 4.9 ozt in crystals. So, I know the process works. I just gotta do it right!

Mike

 

[Palladium]

How long did you leave the copper in there and how much solution did you have? How much surface area for the copper to react with? There are trillions on ions zinging there way around in a solution. Some of those are silver, some copper, water, and such forth. It can take awhile for the process of cementing to interact with all these ions so you can recover your silver. They have a lot of traveling to do. I won't even start with the kinetics involved.

Here's a little experiment i decided to do awhile back just to answer some questions for myself. I drop several oz's of gold from a solution and decided to see how long it took for the chemical reaction of the gold dropping to take place to completion.
I filtered my solution the gold dropped from in the following increments. 30 min, 1 Hr, 2 hours, 4 hours, 12 hr, 24 hr. What i noticed was that gold is constantly precipitating from the solution as chemical reactions take place to conclusion.

 

[mikeinkaty]

I did recover almost another ounce (28 grams) from the 1st electrolyte solution using the salt/naoh/syrup method. That was new for me. I have 20 more gallons of spent solution from all my batches and will try the salt method on those. I guess there is a formula somewhere for calculating the quantities of the 3 additives. ( I know there is - I just need to go search for it). I added salt to one 5 gallon container still inside my shop and there was no color change at all. That was a container that I had left copper in for a week and that had already been re-processed. I so expected it to turn white that it took me by suprise when it didn't.

Palladium, using your suggestion, sort-of, I think in the future I will take the copper out when I notice no more reaction but hold the 'almost' spent solutions for a final drop later in 5 gallon buckets. I sort of did that on this first project but only as an after-thought. This will keep my work-flow progressing by not having to wait days for the last atoms to drop.

I'm also going to go to larger batches and do the dropping in larger buckets. I got this neat little siphon pump couple of week ago that makes this practical now. $5 at Ace Hardware.

Mike