Why add free acid in Ag cell?

Hi guys,

I am running some Ag cells in a refining company, these days I am doing titrations to make sure the free acid(HON3) is in the range of 0.2%-0.9% in the electrolytes.
I noticed that when free acid concentration is lower, the amount of Ag ions are lower, too. In order to keep Ag ions in a good range in the electrolytes, we need to keep adding HNO3 into the system.

I am wondering why do we need free acid in the system?

Thanks!!!

In a perfect world you don't, but when refining sterling in a cell the nitric is consumed because the copper is dissolving in nitric and not putting silver into the system. Plus copper eats up 3x more nitric than silver does. So the free nitric helps keep the silver where you want it so the cell maintains operating efficiency.

How are you titrating the nitric?

4metals said:

In a perfect world you don't, but when refining sterling in a cell the nitric is consumed because the copper is dissolving in nitric and not putting silver into the system. Plus copper eats up 3x more nitric than silver does. So the free nitric helps keep the silver where you want it so the cell maintains operating efficiency.

How are you titrating the nitric?

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Thanks 4metals!
For free acid titration, I use 0.1M NaOH to titrate 5ml electrolyte and use bromophenol blue as indicator. This indicator will change color in pH 3.5-4.0 range from orange to blue.

I also bought indicator phenolphthalein for testing "total acid". The pH range is around 8. But till now, I have not figured out why should I test total acid and what is it used for. Any thought?

Thanks!

Another question I have is, I was told we cannot put too much free acid in the electrolyte or it will dissolve the pure Ag we collect at the bottom of cell. However, why the small amount of free HNO3 only dissolve the anode(impure Ag plate) but not the pure Ag at the bottom? Thanks!

I have seen a lot of silver cell formulas used by large mining operations and the free acid usually ranges from 1% to 5%. The ones using 5% probably do so to keep the silver at a high level. In my experience, most of the excess nitric dissolves crystal, which has a much greater surface area than the anode bars.

I knew a guy that started his cell solution with nitric and distilled water. After awhile, the silver content in the solution was in the right range and the acid was probably about 2 or 3%. He added the HNO3 in increments, over time, to keep the acid low enough (a guess would be a max of 10-12% nitric, by volume) to prevent the muslin anode bags from falling apart. With excess nitric, the crystal dissolves very soon after it's produced. Of course, you don't get any production for awhile.

What type of material are you making your anodes from, and what is the typical assay? I typically run a silver cell by simply titrating the silver routinely and adding nitric acid to bring up the silver into the range where your cell is the most efficient.

I generally do this by making a spreadsheet where you put in the result of your Volhard titration and read off the volume of nitric necessary to dissolve the quantity of silver from the crystal or anodes (usually the crystal is what re-dissolves as it has the most surface area) to bring the cell up to the range you prefer to work in. A cell can be maintained by keeping track of the silver and copper input in the anodes and maintaining the silver levels by nitric additions. Copper will grow to the point where you cannot raise the silver level any more because the copper won't let it into solution so you will need to cut the solution to lower the copper.

4metals said:

What type of material are you making your anodes from, and what is the typical assay? I typically run a silver cell by simply titrating the silver routinely and adding nitric acid to bring up the silver into the range where your cell is the most efficient.

I generally do this by making a spreadsheet where you put in the result of your Volhard titration and read off the volume of nitric necessary to dissolve the quantity of silver from the crystal or anodes (usually the crystal is what re-dissolves as it has the most surface area) to bring the cell up to the range you prefer to work in. A cell can be maintained by keeping track of the silver and copper input in the anodes and maintaining the silver levels by nitric additions. Copper will grow to the point where you cannot raise the silver level any more because the copper won't let it into solution so you will need to cut the solution to lower the copper.

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So I guess I don't need to test the total acid, it does not seem like we need that number for anything. I am using 92-95% silver plates as anode and I am going to control the Ag ion concentration in the electrolyte by adding acid like you said. For the Cu ions, we usually "kill" the cell when Cu concentration is higher than 8-10%.

goldsilverpro said:

I have seen a lot of silver cell formulas used by large mining operations and the free acid usually ranges from 1% to 5%. The ones using 5% probably do so to keep the silver at a high level. In my experience, most of the excess nitric dissolves crystal, which has a much greater surface area than the anode bars.

I knew a guy that started his cell solution with nitric and distilled water. After awhile, the silver content in the solution was in the right range and the acid was probably about 2 or 3%. He added the HNO3 in increments, over time, to keep the acid low enough (a guess would be a max of 10-12% nitric, by volume) to prevent the muslin anode bags from falling apart. With excess nitric, the crystal dissolves very soon after it's produced. Of course, you don't get any production for awhile.

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Thanks GSP, right now I add acid into the electrolyte to make about 2% free acid and after a 12 hours run, the free acid is always drop to about 0.5%. The number looks pretty good from ICP report these days.

Unless you're running a huge operation and the same material day after day, I see no reason to analyze the free acid routinely. Of coursee, situations may arrive where knowing the free acid content is very helpful. When I was involved in maintaining 12, 30 gallon Thum cells, I ran the Ag and Cu daily, but rarely the free acid. In my experience, the free acid sort of self-adjusts and takes care of itself and I rarely worried about it

Other than the film refinery, all the others I worked in were "job shop" refiners. In those, you run anything that's profitable that comes through the door. The material processed through the cells was usually somewhat different every day.

Yes, I do the Moebius cell almost everyday, get about 1500oz silver flakes(crystals) per cell per day. the yield right now is around 78-90% with a average of 85%, give or take. I understand the yield could be as high as 95%, that's why I am trying to figure out what part of cell we could do better to improve the yield. I started with density of electrolyte(1.1-1.7g/L), temperature(90-120F), volts(2-3V) and amperes(1000-1100A), now free acid. I am also going to test anode bag liner pore size and mixing or agitation after free acid.
With the help of testing the free acid, I could control the Ag ion concentration in the electrolyte by adding different amount of new Nitric acid, and keep it in a 5%-8% range, that's about 60-96g/L. Before I take over the moebius cells, the final Ag ion concentration when we "killing " the cell could as high as 13-15%(156-180g/L), consider we need to treat these electrolyte as waste, it's not worth it.

Please let me know if there is anything else I should do to improve the Ag yield in final Ag crystal.

Thanks!!

What is the composition of the anodes? What % silver?

goldsilverpro said:

What is the composition of the anodes? What % silver?

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mostly 95%, once in a while 92% Ag. No Pd, no Pt, very limited gold. And some Cu and Zn.
No element higher than 0.5% other than Ag, Cu, Zn.

What range do you keep the silver concentration at? I prefer to stay between 40 and 60 grams per liter. Do you have a picture of the cell? How are you knocking down crystals and what is your anode cathode spacing?

What range do you keep the silver concentration at?
60-96g/L

Do you have a picture of the cell?

I don't have a photo now but it's very similar to this one.

How are you knocking down crystals
Between every cathode and anode we have a set up rod to knock down crystals for like 5-10 times/min

what is your anode cathode spacing?
The space is about 12-15cm

Is it an Italimpianti cell? The clamps look like it.

When you add everything up, including the loss of silver production when some of the crystal is re-dissolved when you add nitric, I think the main reason for lower efficiency is that the purity of the anode bars are fairly low. The base metal percentage is from 5 to 8%, which is considerable, Don't forget that each gram of Cu or Zn entering the solution can, in essence, force out about 3.4 grams of silver because the amount of NO3 is finite. NOTE: Is it a coincidence that while your efficiency if 15% less than 100%, the amount of silver eliminated (in essence) is 5% x 3.4 = 17% (very close to 15%)? A way to recover this silver is to add more nitric, which will dissolve more silver. Also, there is always some resistance in the system which lowers the effective amperage down a bit.

The most efficient silver cell, one that both dissolves and plates out nearly (but not quite) 4.025 g of Ag/amp-hour would be one where the anode bars are pure silver. But even then, you will never have a 100% efficient cell. I think an average of 85% efficiency is excellent, considering the very lo-grade material you're feeding into the cell. If you want a higher cell efficiency, semi-purify the silver first by dissolving it in nitric and cementing it out with copper bars. Also, since you're getting rid of most of the base metals, the solution will require much less messing with and will last longer. With a silver cell, though, you always have to pay the piper, in one way or another. Everything always balances.

Six of one, half dozen of another. I'm full of cliches today.

4metals said:

Is it an Italimpianti cell? The clamps look like it.

photo.jpg

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It's this one:

goldsilverpro said:

When you add everything up, including the loss of silver production when some of the crystal is re-dissolved when you add nitric, I think the main reason for lower efficiency is that the purity of the anode bars is fairly low. The base metal percentage is from 5 to 8%, which is considerable, Don't forget that each gram of Cu or Zn entering the solution can, in essence, force out about 3.4 grams of silver because the amount of NO3- is finite. A way to recover this silver is to add more nitric, which will dissolve more silver. Also, there is always some resistance in the system which lowers the effective amperage down a bit.

The most efficient silver cell, one that both dissolves and plates out nearly (but not quite) 4.025 g of Ag/amp-hour would be one where the anode bars are pure silver. But even then, you will never have a 100% efficient cell. I think an average of 85% efficiency is excellent, considering the very lo-grade material you're feeding into the cell. If you want a higher cell efficiency, semi-purify the silver first by dissolving it in nitric and cementing it out with copper bars. Also, since you're getting rid of most of the base metals, the solution will require much less messing with and will last longer. With a silver cell, though, you always have to pay the piper, in one way or another.

Six of one, half dozen of another. I'm full of cliches today.

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LOL. Thx GSP! I like the method you mentioned--semi-purify the silver first. I actually already did the experiment a couple of week ago in lab, the silver I collected is about 99%. But, to scale it up, it takes a lot of work and a lot of money. It may take some time to get it done.

goldsilverpro said:

.... Also, there is always some resistance in the system which lowers the effective amperage down a bit.
.

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He mentioned anode to cathode separation of 12-15cm. (Or in our stupid system of measure : 4.7-5.9inches)

Im small fish, so my cell doesn't compare... But, is this not the most likely reason for increased resistance, and lower efficiency?
-so long as the silver is kept up in solution above the lower threshold anyways?

Also, I haven't seen it mentioned at all really in regards to silver cells, but how much of an effect does cathodic polarization have? And is small additions of nitric over an extended amount of time a means to alleviate it? Or is it desirable in this form of electrolysis to prevent hard deposits (like ammesemetria [or whatever his name was] mentioned in his now censored thread)?

I cannot find anything on the internet for Strom Tek but the cell pictured doesn't appear to have the volume to produce 1500 ounces per day, or maybe it's just the angle in the picture. How many liters is the cell? I see the scraped crystals fall into the cone section and you open a valve to drain out the crystals, into that dolly I assume?

Is there one valve or one valve, a reservoir, and a second valve? A single valve would release a torrent of silver nitrate so I assume the second valve just isn't visible. This is the kind of 2 valve trap I am familiar with. The top valve is open and the silver falls into the reservoir (with a little port hole for a visual) when you harvest the crystals, you close the top valve and open the bottom so the silver crystals drop out into your collection vessel. The only potential problem with the system is if you do not harvest the silver and the reservoir fills so the top valve cannot shut off the flow you are in trouble. I have only come across this issue on long weekends, so the power is reduced to produce less.



Or for a smaller system