Sciencemadness Discussion Board

Iron Cell - reaction question

OMG - 7-10-2008 at 19:48

I have setup a crude electrolysis cell to try to plate out iron by dissolving it from an impure anode of copper and iron.
I am using a potassium sulfate and citric acid electrolyte (and water).
So I have what I believe are iron crystals forming on the cathode. But I'm not sure.
I scraped some off, and put them in HCl. The strange thing is that a stinky gas comes off and I'm wondering what it could be. It bubbled a lot right at the start, and smelt, but the crystals didn't dissolve much. I checked it 24hrs later and the solution is pale green (FeCl2?) with a little bit of black still floating around that hasn't been dissolved (yet?).

So I'm assuming its iron that is forming crystals. I'll do some more tests (ie magnet) after I make some more. But I'd like to know what the smelly gas is. There's only so many elements in the reaction, so there doesn't seem much else it could be other than H2S. Or will HCl break up the citrate ion or something?
Any ideas?

or... Does anyone know a way to plate out only 'pure' iron from an anode that contains iron and copper mainly?

not_important - 7-10-2008 at 20:01

Stick piece of iron into a solution of a copper salt, what happens? Reverse that, stick a bit of copper into a solution of an iron salt.

Copper will plate out more easily that iron, dissolve with more difficulty. So you either need to keep the copper from dissolving at the anode, or tie dissolved copper up tightly so that it doesn't plate out.

You do the first by using a low voltage across the cell. You're basically transporting iron from one electrode to the other, this in theory requires almost no voltage to happen. Show the voltage higher and you can dissolve less reactive metals from the anode as well.

As for the results you got, what else might be in your anode metal? What's the ration of copper to iron (and where did you find that odd combination?).

'stinky' is a rather broad term, a bit more precise description might be useful.

OMG - 7-10-2008 at 22:35

So if I use a iron sulfate electrolyte and keep the voltage lower than is required for copper to dissolve, iron should be the only thing being plated. I didn't think it would be that easy.

I made a more significant amount of crystals and put them in HCl, and this time I could definitely smell H2S, so that solves that.

I am experimenting to find a way to separate metals individually from a mixed metal anode which contains precious metals.
(side note: I am also interested in making very pure iron. I read it behaves a lot differently when it is extremely pure, and I am keen to mess around with some)
The reason I am learning this stuff is because I do prospecting and gold panning, etc, and I am collecting pm concentrates from placer gravels. The concentrates are classified by density in the field, I try to only bring back the sands that have an SG of about 7 or higher (my homemade density classifier isn't super accurate). I will be melting these down and casting them into anodes, removing some impurities with flux (and processing the fumes - catch Hg, SO2, etc). So I am experimenting with 'generally' easy ways to extract base metals. Electrolysis seems the most cost efficient way to do this. The metal impurities are mainly copper, iron, and lead.
My idea was to run the anode through cell, pulling out one of the impurities, collecting the anode sludge and melting it back into an anode. And keep doing this until all of the base metals have been removed, then take the resulting anode sludge and process it in aqua regia and precipitate the gold, platinum, etc.
My idea was:
1) NaOH wash while tumbling in ball mill. then rinse the powder.
2) melt into anode,while remove impurities, hg, so2 from sulfides, etc
3) Iron cell - which will now be just a FeSO4 cell I guess,
4) Copper cell - sulfuric based
5) Lead/Silver cell - nitric based (will be parted later) (not too sure what to do about Pd at this stage)
6) then AR the remainder, filter, and selectively precipitate Au, Pt, etc.
I also would like to use this same basic setup to bea able to process e-scrap. (a few more preliminary steps are required though)

Any suggestions whether this would be workable?

not_important - 8-10-2008 at 00:18

you don't even need to start with an iron containing electrolyte, you'll create it as you go if there's some free acid.

Electrowinning/electrorefining usually requires some control of the anode composition as well as the electrolyte. If possible it's better to remove S, Se, Te, P, As, and a few others as much as possible before the electro stage.

I think you'd do better removing the more reactive base metals first. Crushing if needed, air roasting to get oxides, extraction with HCl - only a slight excess, that would get iron and similar metals out. Then a hot NaOH leach, followed by a cool carbonate leach, to remove tin and other metals that have oxides not very soluble in acids.

While that does consume reagents, your route is pretty energy hungry what with all the anode recasting, and you'll use up fluxes as well. Pottery supply stores will give you cheap carbonate, an alternative to the acid leach would be an air roast followed by fusion with a mix of sodium and ammonium sulfates, Na2SO4 again from pottery supply and recovery while ammonium sulfate is a fertilizer. This should remove many base metals as their sulfates, follow by a couple of boilings with Na2CO3 to convert Pb and similar sulfates to carbonates which hot HCl will extract as chlorides. Or boil with strong NaOH to make plumbates.

If you want a primarily electrolytic, then I'd suggest not going so stepwise. Air roast to remove sulfides (capture SO2, also As, Sb, and a few others), reduce to metals and cast. If your base metals concentration is high enough, the the anode will fully disintegrate when you pull them out. Try to plate the least reactive metal you want to remove, keep the plating bath acid so those metals more electropositive tend to remain in solution. Lead will remain in the anode sludge to some degree if H2SO4 or HCl are the acids,. With HCl, extractions of the sludge with boiling water will remove the PbCl2 (reuse the same water and you both minimise lead loss and avoid pollution problems). If you use sulfuric, then it's the boil with carbonate several times, or trickle Na2CO3 solution through a heated column of sludge; you're driving the PbSO4 <=> "PbCO3" exchange to the right by washing sulfate away. Follow that by acetic acid to dissolve the lead carbonates.

You can work up the electrolyte and cathode for the base metals, the cathode will likely be somewhat of an alloy in this case so you'll need more precisely controlled electrolysis. The sludge will mostly be silver and other P.M.; the nitric acid electrorefining would be the starting point for that; note that if the silver content is too low the anode will not disintegrate, so you may have to add silver and/or some fairly electropositive base metal to the melt before casting.

For the very pure iron, you'll need to start with reasonable pure anodes. The cathode will contain some hydrogen, as you raise the current density the H2 content will increase. There's a range of current density that will give you a rough cathode of iron containing a fair amount of H2 and brittle enough to be fairly readily crushable. Smooth iron will take low current densities and perhaps some additives; it also will require heating in vacuum to remove hydrogen to get malleable iron.

Nerro - 8-10-2008 at 00:44

The H<sub>2</sub>S comes from the presence of sulfides. Sulfides are formed when sulfates are reduced. There is a good chance that you're actually doing just that in your cell.

SO42− + 4 H+ + 2 e− ⇄ SO2(aq) + 2 H2O +0.17
SO2(aq) + 4 H+ + 4 e− ⇄ S(s) + 2 H2O +0.50

watson.fawkes - 8-10-2008 at 05:40

Originally posted by OMG
The reason I am learning this stuff is because I do prospecting and gold panning, etc, and I am collecting pm concentrates from placer gravels.
Ammen's book Recovery and Refining of Precious Metals has a lot of relevant information on this topic, including lead button assaying.

OMG - 8-10-2008 at 17:59

Wow! Thanks for sharing all the knowledge. That gives me something to chew on for a while. My goal is to have something workable by next fall at the latest. Thanks for the book reference. I've got Hokes book and am going through it right now.(Although its not very chemistry oriented)

I think I will be leaning more towards getting as many base metals out before going to maybe just a nitrate cell and then AR the remainder. I don't know the exact composition of metals, other than they are dense, and there is visible gold in them. I am presuming a high enough lead and silver content that chloride electrolysis would get bogged down and passivate itself, which is why I am considering using a nitrate cell. I have a lot to learn, but my main objective is to capture the Au, Ag, and Hg. And make it viable to capture Pt and Pd when present as well as the other dense highly resistant pms when present (even in trace amounts - I would just keep the remaining anode sludge in which they would be present if they were there and process later)
I don't know if I'm trying to do something that is just too hard to do or not. I haven't found reference to any companies that do it, so that may be a bad indication. In any case I still want to learn enough to know why it's not done commercially. I'm enjoying learning chemistry. It's much more interesting than it was in high school!

Do you think it would be possible to use a nitrate cell to get everything (solvable as nitrates) into solution, then using an inert anode and electrowinning each metal back out in stages by switching to a new cathode and increasing the voltage? So I would be going for Pd/Hg first, then Ag/Pb, then Cu, etc. (trying to pull out as much as I put in)
*This is assuming that I use preliminary steps to remove as much base metals as possible before electrolysis.

12AX7 - 8-10-2008 at 18:47

If you want the copper, you can melt it and blow oxygen (or use an oxidizer such as CuO) to eliminate the iron. A fluid flux such as borax should be used. Once the iron is gone, deoxidize and cast.