Building large electrolytic cell for silver

[pljohnson]

Ive searched and searched to no avail after watchin lazersteves video on Silver Refining, but theres too much junk in my search results. Im a newbie and am looking to set up a larger thum cell, but have no clue as to what im doing. Any advice or links would be appreciated. And thanks in advance for your patience.

 

[goldsilverpro]

You should provide a lot more information to get meaningful answers. What type of silver do you plan on purifying? How much of it do you want to run, how often? Do you have a furnace? Etc., etc.

 

[pljohnson]

I plan on purifying .925 sterling or .90 coins. I want to be able to put in more than just a few ounces at a time. Maybe 6-16oz at a time and be able to purify at least a lb a day. What I dont want to do is have to do small amounts over and over. I have a melting furnace on the way and nitric acid, thats about it for now. Waiting to see all the other items i will need besides molds and such. Im guessing Ill need 2 crucibles, one for sterling and the other for pure silver. Thanks for your time.

 

[nickvc]

GSP built and ran plenty of large silver cells and his book may be the bible your looking for it's for sale here on the forum, I think it's available through Lasersteves site.
It's also on the bottom of his posts.

 

[goldsilverpro]

My book tells how to operate a silver cell but it doesn't give exact plans for building one. Besides operation, I think it gave overall dimensions and a sketch. The sketch is also here.

http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=39&t=5195&p=44531&hilit=thum#p44531

Although I have run lots of sterling through silver cells, it requires constant attention, more knowledge, more tricks, and many solution changes. The copper builds up so rapidly in the solution, it's hard to keep up with it. Every oz of copper dissolved reduces the silver in the solution 3.4 oz. You would be constantly making up fresh replacement solution. For this reason, if possible, it is always best to upgrade the silver to about 98-99% first. The most common way to do this is to first dissolve the sterling in nitric and cement it out with copper.

A large standard 30 gallon Thum cell will process 500-600 oz in 24 hours or about 175 oz in 8 hours. To do your 15 oz/day in 8 hours, a small cell of 3-5 gallons should be adequate.

 

[pljohnson]

Thanks so much for your time and great replys. A couple last questions if you dont mind. I see in lazersteve video that he uses a vauum pump to filter out solution is this necessary. Can I not just place in a filter and rinse thorougly multiple times, changing the filter if needed. I need to know a larger way of getting the solution out of the pure chrystals. Also do I need borax to line my 2 kilo electric melting furnace(I read that I should have bought my own, too late) and do I need sodium ash? Any more advice would be great. Also, does your book give advice and casting so that theres not tons of oxygen getting soaked into my bars?

 

[goldsilverpro]

Thanks so much for your time and great replys. A couple last questions if you dont mind. I see in lazersteve video that he uses a vauum pump to filter out solution is this necessary. Can I not just place in a filter and rinse thorougly multiple times, changing the filter if needed. I need to know a larger way of getting the solution out of the pure chrystals. Also do I need borax to line my 2 kilo electric melting furnace(I read that I should have bought my own, too late) and do I need sodium ash? Any more advice would be great. Also, does your book give advice and casting so that theres not tons of oxygen getting soaked into my bars?

You can use a plastic filter funnel and filter paper. If your crystal is dense, as it should be, it will filter and rinse quickly. You can easily test the rinse water exiting the tip of the filter funnel by collecting a little in a beaker or a test tube and adding some ammonia to it. In good lighting, put the beaker in front of a white piece of paper and notice the color of the solution. If it is blue, at all, keep rinsing. You can use distilled water for the last couple of rinses.

As you probably know, we don't care much for those electric melters. To make the crucibles last longer, try to avoid fluxing. I would try just drying the clean crystal and then melting it with no flux. You may have a few BBs hung up in the crucible, but you can get that next time. Don't use that crucible for anything but pure silver.

I don't think I dealt much with the oxygen problem in the book. One thing that makes it worse is to get the melt too hot. People on the forum have come up with various ways to solve it. I solved it by not pouring open face ingots. Instead, I used a book mold. That way, all of the gassing roughness is formed on the sprue, which is cut off of the bar after cooling. Someone on the forum posted photos of a book mold they had made from an open graphite mold.

 

[pljohnson]

Another question if you dont mind, I need to power my thum cell, I plan on doing a 5 gallon setup to start with and get familiar with silver refining via electrolisys. Ive searched on rectifiers and variacs in the forum, but none seem to answer what IM looking for as a guy who knows nothing about electricity. The only thing I do have in my shop is a 12 volt 40 amp car charger. Ive got money to invest to buy what is needed, is there something I can buy to hook to the car charger to lower the voltage, or what can I buy to just plug into 110v socket and turn into proper voltage for 5 gallon thum. Also, I wouldnt mind investing in a 60amp or more rectifier for the long run if it will still work on a 5 gallon setup. I think this power supply help is the last I need before I can set up the cell, but if you have any other tips or knowledge of what else I need to buy, it would be appreciated. Maybe I can send a tip your way

 

[Harold_V]

The cell I built held about two gallons of electrolyte, and was more than adequate for processing anodes that weighed up to 200 troy ounces. If memory serves, they were about 9" square. I used a rectifier that was capable of up to 25 amps, and never came close to using its capacity. As you alluded, I used a Variac to control the output. However, early on, I made the choice to operate my cell at reduced capacity, to insure that it didn't require attention in knocking down the crystals frequently. I allowed it to run 'round the clock, so it required getting up a couple times each night to insure it operated properly. It grew dense crystals until the level of copper became excessive, at which time it grew long, spindly threads, which would quickly short the cell. At that point I replaced my electrolyte.

I can see no good reason for you to create a large cell unless you intend to transition to running larger volumes. I advise you to keep in mind the fact that you must have a considerable amount of pure silver tied up in electrolyte---in my case it was 30 ounces. I shudder to think of the volume you'd require in building a 5 gallon capacity cell, although you would benefit in having the ability to part more silver before you'd have to address the excessive copper that will build up.

Consider the idea of making a small cell, to which you could add another small cell, assuming you find the need. A large cell will dictate that you operate only when you have a large volume to process, whereas a small cell will permit operation with just a few ounces.

Through personal experience, I know that adding copper to the electrolyte is not a requirement. Not doing so results in the silver initially plating out in a rather tough sheet, often difficult to remove from the cell, but that changes quickly as the copper content rises. You can safely assume that the addition of copper insures crystals that are easily removed, however, so if shorter production life from any given batch of electrolyte isn't an issue, you may wish to start with traces of copper.

Depending on the nature of your battery charger, it may or may not serve your purpose. If you have the funds, you may wish to explore the purchase of a plating power supply. Small units, lare enough to serve your needs, are not all that expensive. Do a search on the net for rectifiers and/or plating power supplies.

Harold

 

[rusty]

Short write up from Modern Casting magazine, Sept 10/ 2010 - Pouring Precious Metals.

http://content.yudu.com/A1p3q9/ModernCastingSept10/resources/21.htm

 

[pljohnson]

Thanks again for your replies. I found a 25 amp plating rectifier on Ebay I am about to purchase. I think ill stick to a 2 gallon cell as you suggested for now...So far the only materials I have on the way are a melting furnace(I know, not the best idea), 2 graphite crucibles, steel ingot for anodes, graphie ingot for casting, nitric acid, rectifier and ILl get the 30oz of Ag on Monday. As far as supplies that I cant pick up locally to finish the process from begginning to end casting, are there any more items I need to order? I dont have the ability to build an all steel tank and will probably just design a plastic tank with a steel sheeting as Gold Pro used.

 

[Harold_V]

and will probably just design a plastic tank with a steel sheeting as Gold Pro used.

Nothing wrong with that idea. For the cathode, use a sheet of either 316 or 304 stainless steel. It can sit on the bottom of the vessel, which can be something as simple as a 5 gallon bucket. No need to get things real complicated.

The one feature I designed in my tank was to have a place to sit the anode basket while I was scraping down the silver crystals. My tank was tapered to each side. One side accommodated the anode basket, while the crystals that were scraped were piled on the opposite side for storage until I emptied the cell. Do keep that in mind. You don't want to move the basket off the cell unless you have addressed the electrolyte that will continue to drip from the bag.

While I'm not promoting my design, here's a picture so you'll better understand what I was trying to describe.

Harold

 

[pljohnson]

Well ive decided to go with about a 2 gallon cell set up. Right now Im finishing up filtering my cemented silver. Man this takes too long. Im thinking about just melting my .925 into anodes next time instead of purifying them to 98-99% pure like I am now. I know it will contaminate the electrolyte faster, and ill have to change it out, but surely that is faster than having to go through all this trouble to just get it down to 99% pure first. Any suggestions to speed this up. Im anxious to get it melted. ALso, what will happen if there is still a small amount of nitrate solution left in the cemented silver, will I have a huge problem when melting the cemented silver?

 

[goldsilverpro]

You might skip the cementation and the silver cell and try 4metals formate method. I haven't tried it but, if 4metals says it works, it works.

http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=1275&p=39590&hilit=formate#p39590

 

[Harold_V]

Any suggestions to speed this up.

Filtering the solution?

Why?

Because the resulting silver is relatively dense, all you must do is stir the cement silver, to liberate any trapped silver nitrate, which will then be cemented as silver metal. At this point, you should still have metallic copper present in the solution. When you are assured that you have recovered all of the silver, remove the copper, stir well, then allow the cement silver to settle. When it has (it takes just minutes), siphon off the solution, and add water for a further rinse. Repeat until you rinse water comes out clear of color. At that point, if you have a Buchner, get the silver in the funnel and tamp down firmly. Once you have rinsed the recovered silver well, it filters perfectly well. Note that my procedure was to use tap water. I was not concerned about traces of silver nitrate being converted to silver chloride. YMMV.

When you get no more water from the Buchner, invert the funnel over an evaporating dish, discharging the tightly packed cement silver to the evaporating dish for forced drying. Heat over a low flame until you see traces of brown gas coming off the silver. At that point you know it's free of water, and traces of trapped nitric acid are coming off. The dried powder can now be melted. I used a gas fired crucible furnace. I'd tamp the cement silver with the butt of a pestle, compacting it and preventing it from being blown from the crucible.

To avoid losing some of the finely divided silver to the flux, apply a light covering of borax once the silver has started to melt, not before, and DO NOT mix flux with the silver. That pretty much assures you'll lose some.

The added flux will pick up oxides that may remain from the cementing process. The melted silver is best poured to a cone mold, where the slag is easily separated from the silver button.

It was my policy to allow all of the decanted copper solution to settle, in case traces of silver were carried over in the siphoning operation. Also, should there still be traces of silver nitrate present, I routinely added rock salt to the solution, stirring until it was dissolved. Any values that may have been eliminated will be collected by this method.

After settling for a day, the now barren copper nitrate solution was siphoned and placed in a recovery container, where scrap steel recovered the dissolved copper.

Harold

 

[Alentia]

Through personal experience, I know that adding copper to the electrolyte is not a requirement. Not doing so results in the silver initially plating out in a rather tough sheet, often difficult to remove from the cell, but that changes quickly as the copper content rises. You can safely assume that the addition of copper insures crystals that are easily removed, however, so if shorter production life from any given batch of electrolyte isn't an issue, you may wish to start with traces of copper.
Harold

Does it worth while to make electrolyte from Sterling Silver, with 91% Silver 9 % Copper? Will other impurities from Sterling in the electrolyte will get on a way?

 

[goldsilverpro]

Unless the sterling is really old, I would imagine it's just silver and copper. Just make sure there's no solder or other metals or materials attached to it - for example, one piece sterling spoons and forks would be fine but knife handles I would be leery of. The 9% copper won't cause any problems. In fact, most people, myself included, like to start with some copper in the solution and 1/10 as much copper in solution as you have silver sounds fine.

 

[Alentia]

Thank you, GSP!

Another thought of utilizing what I have. Using multiple SS knife blades welded together and covering iron part with epoxy? Will electrolyte react with epoxy in any way?

 

[Geraldo]

@Alentia,

It is a lot easier to scrape silver dendrites off a single sheet than a bunch of knives like you have pictured. Most commercial Thum-Balbach cells have large, smooth bottom cathodes (with tapered sides) to make it a little easier to scrape the dendrites.

As for epoxy, depending on type, it will probably be ok. Ideally, a novolac type epoxy is preferable - some of the cheaper (think dollar-store) epoxies are easily embrittled.

Best Regards, Geraldo

 

[qst42know]

If you are having trouble finding a suitable small piece of stainless try the hardware store for a stainless putty knife, or the second hand store for a spatula without holes or slots.