"Mini Cell" for silver

I realized that the silver I’ve been using for jewellery may contain too much gold to be nice (why throw good money away?) and was not as .999 as I believed it to be (especially when using cementation). Apart from that, the chemistry of the silver cell intrigued me.

I do not need a large capacity, and see no reason to tie up a lot of silver in the cell, so I built a nice little one.

The container is a vase from IKEA with a net volume of approx 700 mL.

The basket is made from Perspex, mainly because it is inert, looks cool and gave me an excuse for playing with chloroform.
In my first basket, I had just drilled holes, but they exposed too little area, and would be blocked by air bubbles whenever the basket was lifted and replaced. I did not have any Perspex rods (the ideal solution) so I cut a number of broad slits in the bottom of the second basket.

The bag is made from non-woven cloth (originally intended for domestic cleaning). My wife volunteered to sew it, once she had seen me try to do it by hand. So she made three in half the time I’d used on the prototype.

Insulation is from a sleeping mat (easy to cut, cheap, stable and I’ve become too old to sleep comfortably on 10 mm of plastic foam).

The power supply is an outdated lab supply, with a resettable 6A fuse.

The positive lead makes contact through a 50 gram fine silver “doorknob” (Thanks to Harold), and the negative is just a fine silver wire running diagonally across the bottom. It is isolated with a piece of PVC insulation when passing the basket.

I run it slow and just compress the silver with a stick until it gets too close to the basket (it takes quite a lot of silver before it does so), and then I empty the cell.

Time to empty the cell for the second time.
The first time yield was approximately 240 grams, I expect a little more this time.

It has been running flawlessly for about a week, and all I’ve done is to compact the crystals with a stick once a day. I’m in no hurry, so I run it at low voltage, growing some nice large crystals (in theory that should improve purity too).

The only thing bothering me is, that the crystals from this cell grow quite a bit larger than the crystals in the cell dedicated to growing large crystal!

Several crystals are needle shaped, 2-3 mm wide and more than 20 mm long.
(They are just so beautiful!)

Looks great, Peter. What's the solution composition - Ag, Cu, HNO₃?

Thanks

The solution is made by dissolving copper-cemented silver in a minimum of 1:1 nitric acid:distilled water. Approx. 1 mL of nitric pr. gram silver. I have not added any copper on purpose; the green colour is from the leftovers from the cementation.*

I’ve been cleaning my stock of (relatively) fine silver, mainly from precipitating as silver chloride after inquartation of gold scrap. The colour did not change going through quite a bit of silver, so I guess it did not contain that much copper after all (I’m very exited to see how much gold can be isolated from the slime!).
A couple of hundred grams of sterling are next, that should add some copper.

*Edit: The concentration is somewhere around 60 grams of pure silver pr. litre, so that’s 30 grams for my 500 mL cell.

peter i said:

A couple of hundred grams of sterling are next, that should add some copper.

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I don't advise running sterling. At the risk of repeating myself, in spite of anything you may read that suggests copper is a requirement in your electrolyte, my experience in making mine from pure silver indicates that it is not a requirement for proper results. The only thing copper does for you is grow soft crystals that can be removed easily. That appears to not be an issue for the type of cell you are running, so it isn't a factor.

Copper lends nothing to the electrolyte that can't be accomplished with pure silver, and cuts short the useful life when it saturates the solution beyond acceptable limits, at which time it co-deposits.

I may have stated, previously, that I started my silver cell with 30 ounces of silver, with an overall volume of approximately two gallons.

My anodes ran about 200 ounces, and were produced primarily from silver that had been cemented with copper. I washed the cement silver well before melting, to minimize the amount of copper that would be included. I never used small pieces of copper that might be left behind, in order to insure that I kept the concentration of copper at a minimum.

By the time I had consumed an anode, the cell was borderline super saturated with copper, which would occasionally start co-depositing along with the silver. Starting with sterling is sure to cut extremely short the life of your electrolyte.

That may not be a problem for you if your objective is to recover traces of other values, and you'll re-use the silver for inquartation, or even alloying for sterling, assuming you can reflect the amount of copper that is co-deposited in your alloying procedure to insure that your end product meets the requirements for being labeled sterling (or coin).

Harold

“That should add some copper” was meant as a joke.
I’ve never tried to deplete the cell, and I think it could be interesting to see what it looks like when it does.
I am fully aware that running relatively clean silver will give the longest life of the electrolyte. But I’m almost done with my fine silver, and now have a couple of hundred grams of sterling (alloyed from inquartation silver and Swedish silver coins) that I fear could contain as much gold as the value of the silver.

There are to possible ways to deal with it:
Dissolve in nitric, drop with copper, cast ingots and part in the cell.
Or
Take the last run in the cell with sterling, to see how far it will be possible to go before copper starts plating out (and learn a little in the process). If it dies, nothing is lost.
(and 3: save the sterling for later inquartations, and then drop and clean it on that occasion. Buggerit! That’s the logic thing to do…. But then I’ll never know how much gold was in it)

peter i said:

“That should add some copper” was meant as a joke.

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Oofda! I should have known! :lol:

I’ve never tried to deplete the cell, and I think it could be interesting to see what it looks like when it does.

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Of course. That, too, is part of learning, and will provide guidelines for the future.

It's doubtful you'll deplete the cell, however. As long as your filter isn't terribly contaminated, it will continue to pass silver, but copper will begin to deposit as well.

(and 3: save the sterling for later inquartation, and then drop and clean it on that occasion. Buggerit! That’s the logic thing to do…. But then I’ll never know how much gold was in it)

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Curiosity killed the cat, you know!

Actually, I'd likely do the same thing, given equal circumstances. I must remember that some of the folks here are just trying to learn. I have a tendency to look at things from the perspective of "gettin 'er done".

Harold

Just weighed the silver from the second run:

460 grams of very nice shiny crystals

Running it at low voltage made them grow pretty large, several mm across. It was slow but had the nice effect that they were very easy to wash free of silver nitrate (no detectable traces with chloride after washing just three times).
The large crystals also allowed twice the amount of silver in the cell before having to empty it.

I’m running my last silver now (gave up on refining the sterling*) and look forward to working up the slime.

*But I will try some sterling in the micro-cell, just to se what happens

I have run many, many thousands of ounces of sterling through a silver cell. It's less efficient than when running purer silver, but it will still work. It's all in the math. Each ounce of copper dissolved will reduce the silver in the cell 3.4 ounces. When the copper exceeds 12 oz/gal or the silver drops below 4 oz/gal, the final purity will be affected.

The silver can be built up (or, the copper lowered) by dissolving silver in a minimum amount of nitric and replacing some of the solution with it. You can also add some free nitric to the solution. This will dissolve some of the crystal and build the silver up. It takes about 38 mL of nitric to dissolve a tr.oz. of silver.

One amp applied for one hour will, at 100% efficiency, dissolve 4.025 grams of silver or 1.185 grams of copper. The efficiency won't be quite 100%, but it will be pretty close.

Continually do all the math and adjust accordingly.

I have refined my stock of silver, and just wanted to find the concentration of silver in the electrolyte. The electrolyte had not become darker than when it was made, which I interpret as very little copper has been added. (never got to running sterling).

A sample of 10.0 mL was taken, diluted with three times that volume of boiling water and the silver precipitated with 4M HCl.

The precipitate was washed three times with 100 mL of boiling water and dried to constant weight at 130 °C

Mass of AgCl: 1.63g which means a silver content of 123 grams* of silver per litre.


The mini cell grew some rather large and nice crystals, I'll post a few pictures in the gallery.




*"Mass of silver" = "mass of AgCl" X 0.7527

I was asked for some more details, some of it is written elsewhere, but I'll do it anyway:

Peter, I have seen the silver crystals you made in your mini silvercell, I would like to know a bit more about building a cell, what are the bags made of? why do you use them? and what is the plastic for? at what voltage and amperage do you run your power supply unit? can you explain it as if you were teaching a class of newbies? as you can tell I'm pretty new at this. thank you, baja bob

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Remember that the general principle is, that

- the silver is dissolved at "+" and re-deposited at "-"

- elements more noble than silver are not dissolved, or are instantly reduced again should they dissolve, and will end up as a black slime of fine metal particles right next to the dissolving piece of silver.

- elements less noble than silver will dissolve and stay that way. They may try to reduce at "-" but if they do, some silver-ions will be ready to grasp the poor element's electrons, oxidising it back to a soluble ion.
Thus the deposited silver is very pure.

Bags and basket
The idea is to
- hold the dissolving silver suspended in the solution at a suitable distance from the depositing silver. It's quite a job, because the environment at "+" is rather aggressive, and as the bars dissolve, they crumble and change shape. The basket should shield the silver as little as possible, and of course be chemically inert.
- Catch the slime! This is partly silver, but more importantly: GOLD and PGMs. The bag must be chemically resistant, must filter out the small particles and must be able to burn when you are finished and want to isolate the good stuff.
- catch the anode slime to avoid contaminating the refined silver.

Power
I use as little as 0.7 V DC but others use a couple of volts.
The amount of silver moved is a direct function of the current:

goldsilverpro said:

One amp applied for one hour will, at 100% efficiency, dissolve 4.025 grams of silver or 1.185 grams of copper. The efficiency won't be quite 100%, but it will be pretty close.

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Silver atoms give up an electron at "+" to become soluble Ag⁺ ions, and at "-" the ions receive an electron, and deposits as metallic silver.
The more electrons moving (aka "current") the more silver is moved.

The current is a result of Ohm's law: U=R*I where the distance from "+" to "-" area of electrodes and concentration of ions in the electrolyte determines "R" and the power supply determines U.

Crank up the voltage, and the current will rise, but too high a current will give a strandy growth that shorts out the cell.
A better way to increase the current is to use a more concentrated electrolyte, a larger area, and then the temperature will also rise (further raising the current, because the ion mobility increases at higher temperature)

I have plenty of time and want large crystals, so 0.7 Volts are fine for me, and that gives me ½ an Ampere, depositing 2 grams of silver per hour.

If I had little time and large amounts of silver, I would redesign the cell radically, but for my purpose, it's just what I need.

Hope this helped.
Regards
Peter

Peter, thank you for your response, it answered a lot of questions, it did raise several others for me, I could not tell from the photos what the cross bars were made of, were those clips that hold the plastic box what were they made of, you wrote that the basket was perspex, is that a type of plastic and if you built it, what did you glue it together with, I did not understand the reference to chloroform at all, some dimensions would help, sorry for all the questions, I'm just not familiar with a lot of the terminology. thanks again, Baja Bob

Perspex is a plastic

http://en.wikipedia.org/wiki/Acrylic_glass said:

Poly(methyl methacrylate) (PMMA) or poly(methyl 2-methylpropenoate) is the synthetic polymer of methyl methacrylate. This thermoplastic and transparent plastic is sold by the tradenames Plexiglas, Limacryl, R-Cast, Perspex, Plazcryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast, Oroglass and Lucite and is commonly called acrylic glass or simply acrylic.

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It cuts relatively easy, can be heated and bent, and glueing pieces together is done by making a good fit, placing a couple of drops of chloroform at the seam (where it is drawn along the seam by capillary action). After a couple of hours the pieces are welded together, as were they cast in one piece.

A polyethylene box with slots in the bottom could be used too, but perspex is fun, sexy and good looking.

The basket hangs on the edge of the glass on two wooden rods (Kebab skewers) through holes in the end plates.
Wood is bad! It sucks up electrolyte and silver nitrate crystals start growing on it. ASAP I'll fit two pieces of perspex instead.

The best scale bar I can give is the one pint plastic beer glass, and that the volume of electrolyte is about ½ litre.
I did not work with any specific dimensions myself, but just grabbed a vase in the cupboard with the approximately right volume.

The clips are alligator clips for the electrical connection, one is attached to a lump of silver in the basket and the other to the wire at the bottom where the silver deposits.

Hi Peter
Can you explain this a bit more.

What is Doorknob
It seems like this is the dirty silver

You are quite right.

It is simply a lump of the "allmost fine silver" that is being refined, that I've cast into a flat cylinder. The contact to it is a bit of .999 silver wire. I draw a bead at the end of it, and simply cast it directly into the lump.

The mold is the "inner steel ring" from a ball bearing laying flat on a piece of 10mm steel plate. A coating of oil or soot ensures that the metal will not stick.
An easy way to make pleasing lumps of metal.
(When casting sterling, the reducing effect of the oil will give the "golden hue" as seen)

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