Designing a DIY silver cell

[Shark]

Metal gears would be best for long term use, I would think.

 

[Palladium]

One of the best designs i ever built was a rotating cathode disk with a stainless scraper. 6 anodes in close proximity to one disk with two sides. Hands down!
Stainless disk on a stainless all thread rod. Stainless nuts. Stainless steel scraper blades. Self cleaning. Stainless BBQ grill rotisserie drive. Plastic holding tank. I just wish i had got to keep it!
This was my mock up for the idea...........

 

[Shark]

I watched that close for a long time waiting to see how it turned out. I also didn’t want to suggest someone else’s idea, but glad you brought it up.

 

[Palladium]

I watched that close for a long time waiting to see how it turned out. I also didn’t want to suggest someone else’s idea, but glad you brought it up.

The turn out part was after this video the client cut me a check! After that came a NDA.
The proud owner of it i can say is a forum member. If they chose to post it so be it.

 

[Mudville9]

Having recently retired from a rather large precious metals refinery, I feel more at ease in sharing general information developed over the years. Keep in mind that large scale refining is very different from what appears typical sized systems found here. We had the advantage of a fully staffed lab, PLC control of pretty much all aspects of refining, a very competent engineering department etc.
Our Balbac-Thum cells were mothballed 40 years ago in favour of Möbius cells. Nothing can compete with Möbius cells for efficiency, but depending on the design, they can still take appreciable sweat and manpower to operate.
I would be happy to answer any questions on silver (electrolytic or chemical) or gold refining (electrolytic, chemical or chlorination), barring some proprietary information. Yes, we used Wohlwill cells for refining. When you reach a certain level of production, they really are quite efficient. The added bonus is their ability to gather PGM's very efficiently.
As for silver electrolytic cells, I will share a couple of misconceptions I see bandied around in various forums and papers.
The following only applies if you are looking to refine to 99.9% purity. If you want 99.99% purity, or you need to meet ASTM specs, then much greater care of the system is required.
1) Voltage - Many people have definitive limits on what the voltage should be. With well designed Möbius cells, voltage can run much higher that considered "normal". Running at 6 volts per cell is entirely acceptable and I have run much higher during tests.
2) Amperage - Similar to voltage, most literature sets amperage too low. Yes, Balbach-Thum cells generally operate at a low amps per square foot of cathode, but Möbius cells can be run much higher. Sorry for the following units, but you can convert to whatever you wish. Assuming the gold in the silver anode is less than 5%, amperage is typically set to an upper limit of 1000 amps per square meter of cathode area. I have run tests at 2000 amps per square meter and easily achieved 99.9%. Just don't run out of feed during the run!
3) These high current and voltages do raise their own problems. Contacts need to be maintained squeaky clean to prevent overheating, acidification of the electrolyte can be a concern and electrolyte circulation through the cells is required.
4) Copper in solution - Since large refineries accept pretty much all grades of incoming feed, an appreciable amount of material to the silver cells was high copper feed. We would run everything greater than 85% silver through the cells and the copper levels would climb quite rapidly at times. Running up to 120 g/L copper still produced 99.9% purity with 500-600 ppm copper. Perfectly acceptable.

If there are particular questions people have, let me know. I only pop in here once in a while, but I will check back.

 

[4metals]

Contacts need to be maintained squeaky clean to prevent overheating.

This is a problem I see often, solution getting hot. It is always an issue of the electrical
contact and always creeps back in to the picture. I always try to get clients to apply dielectric paint once good connections are made or at least dielectric grease to maintain good contacts.

What, if anything, have you seen used to maintain the contact integrity especially when cranking up the power?

 

[Mudville9]

Again, not likely possible with small scale refining, but a thin (1mm) gold contact brazed to the copper busbars at each connection point works wonders

 

[4metals]

Thanks for that tip. For a medium to large producer, the labor is the same if you manually harvest 300 or 1500 ounces from each cell and despite the cost of gold invested to improve the contact efficiency, the benefits in production without increased labor would eventually justify it.

Never heard a silver refiner complain about having to cast more anodes to keep up with cell production.

 

[Shark]

I always try to get clients to apply dielectric paint once good connections are made

Nice, I have never even heard of dielectric paint, but I have seen many situations where it would have been a big help.

 

[Shark]

acidification of the electrolyte can be a concern

Would this occur while replenishing the electrolyte or does it occur from the high voltages and amperage's in some way?

 

[Mudville9]

Thanks for that tip. For a medium to large producer, the labor is the same if you manually harvest 300 or 1500 ounces from each cell and despite the cost of gold invested to improve the contact efficiency, the benefits in production without increased labor would eventually justify it.

Never heard a silver refiner complain about having to cast more anodes to keep up with cell production.

Melting, casting, cleaning, weighing, assaying, cleaning crucibles, can be surprisingly costly

 

[Mudville9]

Would this occur while replenishing the electrolyte or does it occur from the high voltages and amperage's in some way?

Not from replenishing the electrolyte, but from daily running of the cells. It depends on the elements contained. High amounts of gold is probably the worst culprit.

 

[Mudville9]

This is a problem I see often, solution getting hot. It is always an issue of the electrical
contact and always creeps back in to the picture. I always try to get clients to apply dielectric paint once good connections are made or at least dielectric grease to maintain good contacts.

What, if anything, have you seen used to maintain the contact integrity especially when cranking up the power?

Poor electrical contacts start fires.
My guess is about 85% of a cells resistance is related to the electrolyte, which heats the solution. High current density likely requires cooling of the solutions.

 

[4metals]

Melting, casting, cleaning, weighing, assaying, cleaning crucibles, can be surprisingly costly

Yes but also standard operations for a melt shop. The one thing is the Moebius always requires remelting of anode stubs. That can account for 20% of the anode production.

Assaying is usually done on large melts as a composite because the bars are all settled and belong to the refiner so a large melt coupled with a stacked mold makes many anodes and fewer assays.

Care has to be taken not to count the anode stubs as new feed into the system. Especially if they are not logged out as remelt stubs. I had a client do that once, the accountants didn’t know what was logged in and not logged out because it was a remelt and cast not new feed. They thought they had massive losses when they really had poor accounting.

 

[FrugalRefiner]

A Moebius cell can usually output slightly higher purity.

I’d like to see some background to that statement. May be true, I haven’t seen it.

Well, I've spent the last week looking back through my books, and I haven't found it. As I remember, the logic was that with a Thum cell you have crystals growing on top of crystals at all different angles, and in some places they can entrain small amounts of electrolyte with whatever contamination it might carry. Because of this overlapping structure, they are also harder to rinse completely clean. With a Moebius cell, the crystals are constantly being knocked down before this can happen, and they are easier to rinse.

Mudville9, do you have any feedback on this? By the way, it's nice to see you posting again!

Dave

 

[Mudville9]

Yes but also standard operations for a melt shop. The one thing is the Moebius always requires remelting of anode stubs. That can account for 20% of the anode production.

Assaying is usually done on large melts as a composite because the bars are all settled and belong to the refiner so a large melt coupled with a stacked mold makes many anodes and fewer assays.

Care has to be taken not to count the anode stubs as new feed into the system. Especially if they are not logged out as remelt stubs. I had a client do that once, the accountants didn’t know what was logged in and not logged out because it was a remelt and cast not new feed. They thought they had massive losses when they really had poor accounting.

20% stubbage or remelt rate should be at the very top end of the spectrum. It is very much up to the philosophy of the refiner, but with a small amount of extra care and work, stubbage can be reduced to as little as 2.5%. I liked going with the lower remelt rate since all stubs were washed free of any adhering gold slimes and dried before recasting to anodes. Smaller stubs made this washing much easier and faster.

 

[Mudville9]

Well, I've spent the last week looking back through my books, and I haven't found it. As I remember, the logic was that with a Thum cell you have crystals growing on top of crystals at all different angles, and in some places they can entrain small amounts of electrolyte with whatever contamination it might carry. Because of this overlapping structure, they are also harder to rinse completely clean. With a Moebius cell, the crystals are constantly being knocked down before this can happen, and they are easier to rinse.

Mudville9, do you have any feedback on this? By the way, it's nice to see you posting again!

Dave

Thanks, nice to be back.
Retirement frees me from disclosing information that I felt uncomfortable doing while employed. Thum cells produced the most beautiful crystals and I actually found them easier to wash than crystal from Möbius cells. Unless we were producing high purity silver for specific purposes, I wanted the resulting silver crystals to be as close to 99.9% as possible, no free give-away here
Both set of cells can produce in excess of 99.997% purity, so not sure one is better than the other in that respect.

 

[orvi]

Do you have experience on how high palladium content could be in the anodes, and after electrorefining, what proportion will end up in electrolyte and what proportion goes to the slimes and "fine" silver ?

I have one feed material that averages around 1% Pd and I try to gain some insight on how it could be effectively refined to obtain pure silver and palladium "something" in the end

 

[snoman701]

Do you have experience on how high palladium content could be in the anodes, and after electrorefining, what proportion will end up in electrolyte and what proportion goes to the slimes and "fine" silver ?

I have one feed material that averages around 1% Pd and I try to gain some insight on how it could be effectively refined to obtain pure silver and palladium "something" in the end

Do a literature search. There is a paper that tells you where to keep your maximum electrolyte Pd PPM for minimal dissolution of the Pd anode.

 

[snoman701]

Challenges in the Electrolytic Refining of Silver—Influencing the Co-deposition Through Parameter Control​

Search for that on google