Sciencemadness Discussion Board - recovery of Pb metal

recovery of Pb metal

Bezaleel - 26-4-2013 at 06:07

I just read in the Melting/cleaning cadmium metal thread that knno4 remarked

Quote:

Melting Cd under KOH gives very good results and avoids oxidation. It also removes impurities (Zn, Pb, etc).

Now, a while back, I dismantled an old car battery, and the electodes consist of what might be described as compressed lead powder, held together by a lead grid.
I tried to melt this in a pan, but couldn't raise the temperature as far as red head. From experience, I know that the highest temperature I can achieve with the setup lies around 450 C.

Is there any salt I can use as a flux in order to melt the lead from the electrodes? I guess the salt should be able to dissolve lead sulphate if it wants to function as a flux.

Ammonium sulphate does dissolve lead sulphate in a watery solution, but is decomposition temperature (235-280 °C) lies below the melting point of lead (328 °C). So testing it for a flux is senseless

Any salts that are suitable?

plante1999 - 26-4-2013 at 06:26

I guess a mix of sodium carbonate, carbon/sugar, and sodium cyanide would make a very nice flux, however you could change the cyanide for iron powder with some success.

Pyro - 26-4-2013 at 07:37

That muck would be mainly PbO. I wouldn't try and recover lead. the lead plates however you can easily melt down. a camping gas stove and an old pot will do fine. I've seen tea lights and sawdust used in lead melting. Be careful of the vapours

blogfast25 - 26-4-2013 at 08:00

Quote: Originally posted by Bezaleel

Any salts that are suitable?

Look for eutectic mixtures of NaCl/KCl/CaCl2. These are often low melting and are excellent solvents for many metal contaminants like oxides etc. They also protect the molten metal against further oxidation.

But with such a low MP, the usual soldering fluxes may well still work.

[Edited on 26-4-2013 by blogfast25]

Bezaleel - 29-4-2013 at 02:12

Thanks for the answers. I will try with a mixture like blogfast mentions. I don't have any cyanides, and neither would like to work with them.

I'd need to look for the composition of soldering flux; I only own it as a solution, and it's quite expensive. NaCl/KCl/CaCl2 hardly cost anything.

plante1999 - 29-4-2013 at 03:52

Yes, but the problem with the ead plate, is that they are already oxidized, you better use a reducing flux like I told you. The objective is not to protect from the atmosphere only, but also to reduce the oxides /sulphate.

blogfast25 - 29-4-2013 at 05:39

Plante: if the amount of oxides is small and superficial, then a molten salt flux should work just fine, cheap and OTC. And who would want to use NaCN just to recover tiny amounts of lead oxide??? Gold, yes, lead, no!

[Edited on 29-4-2013 by blogfast25]

Bezaleel - 29-4-2013 at 06:23

I get your idea, Plante, and I admit that the sulphate content may be 5% - 10% or thereabout. It's not about the recovery of the lead from its sulphate though, but about a way to separate the lead that is in the shell of lead sulphate.

I ran a test trying to just melt the lead out, but that didn't work, as the lead sulphate layer is too thick. Destroying it, mixes everything. Appearantly, the sulphate and the metal stick to one another through adhesion.

Also, melting the lead grid didn't work. So the flux should be able to dissolve the PbSO4 fairly well in order to work in my situation.

Is there any additive to the flux blogfast mentiones, that would increase sulphate solubility or even help reduce the oxides/sulphate?

watson.fawkes - 29-4-2013 at 07:33

Quote: Originally posted by Bezaleel

Is there any additive to the flux blogfast mentiones, that would increase sulphate solubility or even help reduce the oxides/sulphate?

Carbon. Mineral lead sulphate is called anglesite and has been used as a lead ore for primary extraction since ancient times. If you think of this as a smelting operation (albeit a secondary one), you'll see there's a whole class of applicable practice. If you're having trouble melting it, use more heat. Because the melting point of the alloy will raise the vapor pressure of the lead metal itself, use of a cover flux is mandatory.

There's likely also some calcium and aluminum in your battery lead, though not more than 1% total. Just be aware you have dross-forming alloy materials in there.

blogfast25 - 29-4-2013 at 13:25

Carbon: that's the sugar in Plante's suggestion, as it will carbonise completely at smelting temperatures...

Bezaleel - 1-5-2013 at 02:37

I understand. I gathered NaCl, KCl, and CaCl2. (MP eutectic = 465 C) Since I coulnd't find an iron crucible, I'll use the nickel crucible I have for melting alkali.

Will mixing and grinding the salts be sufficient to obtain a low melting mixture, or does it remain necessary to heat far above the temperature of the eutectic in order to create the eutectic?

blogfast25 - 1-5-2013 at 05:12

Quote: Originally posted by Bezaleel

Will mixing and grinding the salts be sufficient to obtain a low melting mixture, or does it remain necessary to heat far above the temperature of the eutectic in order to create the eutectic?

You will at least have to heat it to the MP of the lowest melting component, the others should then dissolve into it as more melt forms. A physical mixture at RT is not yet a eutectic mixture. Once you've obtained the eutectic, cast it into something and then break and crush it up when cool.

Have you checked whether Ni is resistant to molten lead? You can have unusual surprises with molten metals: a melt of Cu once burrowed effortlessly through a thick SS crucible of mine!

[Edited on 1-5-2013 by blogfast25]

Pyro - 1-5-2013 at 05:13

just use an old cooking pot. they work fine. I've used them and seen them being used.

blogfast25 - 1-5-2013 at 05:16

Quote: Originally posted by Pyro

just use an old cooking pot. they work fine. I've used them and seen them being used.

Yes, the people that cast their own lead fishing tackle use steel or cast iron kitchenware.

Bezaleel - 1-5-2013 at 06:50

Okay, so it was good to check back before my alkali crucible might have been sacrificed. Thanks

This does mean another delay in the first experiment, unfortunately.

unionised - 1-5-2013 at 07:04

Boiling lead sulphate with a solution of sodium carbonate will convert it to lead carbonate.
That means you don't have to deal with SO2 fumes coming off the mixture when you smelt it.

Heating the carbonate will give the oxide and heating that oxide in the presence of carbon- charcoal, sugar or whatever- should give you lead.
Any smelting can be done in a steel can (the sort that tinned food comes in), but check it's magnetic first. Aluminium isn't so tolerant.

blogfast25 - 1-5-2013 at 09:38

Quote: Originally posted by unionised

[...] but check it's magnetic first. Aluminium isn't so tolerant.

And not all steel is magnetic either...

Bezaleel - 2-5-2013 at 03:56

Yesterday I found a jug for (coffee)milk at the thrift. Purchased it (for only a quarter), as it seemed to be made of SS. Since it's small (about 200 ml content) I could get it orange glowing with my burner.

Eutectic mix of CaCl2, NaCl, and KCl
I put in moisture absorber, which hissed and puffed, and finally melted. I gues the moisture absorber is made of CaCl2.H2O, not dry CaCl2. When heated to orange glow, CaCl2 is about as viscous as water. I added the NaCl and KCl, and the result was a surprisingly non-viscous liquid, thinner than ethanol, and orange glowing. I was happy to see that with this small scale setup, my burner produces enough heat to reach some 800 C (CaCl2 melts at 772 C).

In order to obtain the eutectic, I will need to determine the amount of water in the CaCl2 first. My guess is it's the monohydrate. For the potassium I took lo-salt, that has 66 m% K and 33 m% Na for alkaline metal content. For the eutectic, 1.5 grams of lo-salt, 4.3 grams of NaCl, and 11.30 grams of CaCl2 are required.

Due to the crystal water in the moisture absorber, my mix contained to little Ca, so I added an amount I estimated to make up for the loss of water.

Melting and coalescing lead
In the orange glowing salt mixture, I put a few pieces of the powdery lead electrode from the car battery, which crackled a bit while it melted. I saw some oxides were formed, that rendered the salt thick.

Next I added two spatulas of sugar, that caught fire right away, hoping to reduce the oxides that had thickened the salt. This was a really unfortunate choice, as the sugar became a sort of tar, that made the entire content thick. The salt no longer acted as a covering liquid, but formed heaps shaped like potatoes that have been cooked too long.

I poked around in the mixture with a spoke (made of SS, like the jug), and found that there were some very brilliant beads underneath the salt and sugar tar. These beads looked as if they were yellow glowing, and made of glass, reflecting like a mirror. After observing these beads for a while, I concluded this had to be glowing lead, that had coalesced on the botom of the jug. There was a huge amount of salt-tar-mess though, and it didn't seem viable to recover these lead beads.

Sulphur content of the electrodes
Finally, I added a few larger pieces of one of the electrodes, that consist mainly of a fine grid with what seems to be compressed powdered lead (this is really brittle, and crumbles as you break it). These pieces dissolved/melted/reacted, and to my surprise, only a very small amount of sulphur dioxide evolved. There was some smoke though.

After all had cooled down, I found I had obtained a dark grey solid, that would not at all dissolve in water. Apparently, there is too much lead, lead oxides, and other suff in the salt. The water turned brown on longer standing, probably due to remnants of the sugar.

I will have another go soon, and see whether I can recover some lead and separate it from the oxides without the use of sugar. Maybe I should try charcoal instead of sugar, as that won't become a tar. Twice the amount of salt wouldn't hurt either.

blogfast25 - 2-5-2013 at 07:59

Why not start from the assumption that it is CaCl2.H2O and work from there? The error will be quite marginal is it isn’t and you’ll still be quite close to a eutectic composition. Your CaCl2 desiccant is probably contaminated with alkali metal chlorides anyway.

Yes, molten chlorides are really thin liquids and they also tend to get everywhere because of vapour pressure.

The sugar may be optional, I’d have a go without it: molten salts are generally speaking good solvents for other salts. Fine charcoal could be an option. Or else: carbonise the sugar separately, before adding to the melt.

watson.fawkes - 2-5-2013 at 10:33

Quote: Originally posted by blogfast25

The sugar may be optional, I’d have a go without it: molten salts are generally speaking good solvents for other salts. Fine charcoal could be an option. Or else: carbonise the sugar separately, before adding to the melt.

Sugar seems like a bad idea. Carbon is what you want, not something-with-carbon; I cannot recall reading much anything about smelting with carbon-substitutes. The reason, insofar as I've been able to tell, is that the active reducing agent is mostly (though not entirely) carbon monoxide, CO, and you get a gas-liquid or gas-solid interaction with higher reaction rates. The CO converts to CO2, reducing the target compound by removing oxygen. Then, since the gas is also in contact with the carbon source, there's also the reaction CO2 + C <--> 2 CO, which regenerates the CO. (The full thermodynamics of that equation are surprisingly complicated at smelting temperatures.) The problem with carbonaceous-but-not-carbon compounds is that you don't regenerate CO at anything like the same rate.

Bezaleel - 3-5-2013 at 07:05

H2S development from electrode lead in molten salt with sugar
Yesterday, when I tried to clean out the little jug, I noticed that H2S is being formed at the highest temperature I can reach (which is where PbSO4 decomposes? -- SO2 is released). I have the impression that this is another consequence of the addition of a hydrocarbon, instead of just carbon. So take care if you repeat this experiment.

Further experiments that I may take on this weekend will be:
* melting of only the lead electrode, to see whether it does release SO2 (as has been claimed in another thread)
* check of unionised's claim that the sulphate could be replaced by carbonate by a sodiumcarbonate solution
* melting the electrode under a salt mix without sugar, to see how much lead will coalesce

I am left with a question:
The salt/lead mix I got out of the jug is dark grey, similar to the colour that old pieces of lead have. What causes this dark grey colour? I cannot think of any lead salt with this colour. That this would be small amounts of PbS, seems strange to me, as the electrodes have a very light colour, and SO2 is released from the melt.

watson.fawkes - 3-5-2013 at 07:28

Quote: Originally posted by Bezaleel

Yesterday, when I tried to clean out the little jug, I noticed that H2S is being formed at the highest temperature I can reach (which is where PbSO4 decomposes? -- SO2 is released). I have the impression that this is another consequence of the addition of a hydrocarbon, instead of just carbon.
[...]
The salt/lead mix I got out of the jug is dark grey, similar to the colour that old pieces of lead have. What causes this dark grey colour? I cannot think of any lead salt with this colour.

I agree about the H2S. I had considered talking about excess H as doing nothing useful, but put no thought into it.

The dark grey color may be from sulfide, but a surface layer from your conditions would also contain mixed oxides. The color has to do with the crystal habit of the surface layer, which is far from a bulk crystal where "ordinary" colors are seen. The question I'd have is whether the material under the surface has the sheen of bulk lead. If it doesn't, it means that you don't have particularly pure lead yet, which I could certainly believe.

Bezaleel - 3-5-2013 at 07:56

Well, the material underneath the surface cannot be seen. When inside the crucible, everything is orange glowing. When cold, I am left with an opaque dark grey mass, which on breaking proves to be virtually homogenous. No sheen of metal there. So in this case, the dark grey colour is all through the substance.

Of course, it could be that it consists of microscopic lead crystals with a dark grey outer layer. Note that the dark grey mass is insoluble. The volume ratio of salt and lead that went into the jug is about 1 salt : 4 lead. I recovered about 1 gram of shining lead. The remaining (estimated) 100 grams have formed the dark grey mixture. Which is surprisingly hard, gives the impression of a ceramic.

blogfast25 - 3-5-2013 at 08:11

Highly unusual. I'd expect something that works for a reactive metal like aluminium to at least work also for a less reactive one like lead. Instead the flux seems to be chewing up the metal, forming what sounds like a glassy material. Is that stuff brittle by any chance?

kmno4 - 3-5-2013 at 08:46

There are simple and cheap wet methods for converting Pb salts (generally - any salts) to Pb.
You can use Pb/PbSO4/PbO... mixtures, all is converted to Pb.

Original idea from book:
Take two Zn plates, make paste from PbSO4 and H2O and place it between Zn plates. Immerse all in NaCl solution and wait. PbSO4 will change to Pb sponge, Zn react to ZnSO4. It works, but can take many days to complete reaction.

Modifications:
use cut iron sheet (not powder - hard to remove), may be Zn coated, iron wire, it does not matter. Take bottle, put iron, Pb salt (or mixture), water and some acetic acid (citric, sulfuric, etc). Stop the bottle and shake it from time to time. End of reaction can be indicated by immersing a piece of clean iron for some time (in slurry) - surface of iron should not be covered by Pb sponge.
Remove remaining iron, decant, wash, dry, melt Pb. Do not store prepared dry Pb for a long time, because it slowly covers with oxide.

I tested Al sheets but I got hard to remove "Al(OH)3" hard particles. I think it is question of bad conditions (just tested it only to see if it works at all). Maybe NaOH instead of acidic solution would be better ? Not tested, but if someone wants, worth trying.

Important : Pb in car battery always contains some Sb, even few %.

ScienceSquirrel - 3-5-2013 at 08:57

Quote: Originally posted by kmno4

There are simple and cheap wet methods for converting Pb salts (generally - any salts) to Pb.
You can use Pb/PbSO4/PbO... mixtures, all is converted to Pb.

Original idea from book:
Take two Zn plates, make paste from PbSO4 and H2O and place it between Zn plates. Immerse all in NaCl solution and wait. PbSO4 will change to Pb sponge, Zn react to ZnSO4. It works, but can take many days to complete reaction.

Modifications:
use cut iron sheet (not powder - hard to remove), may be Zn coated, iron wire, it does not matter. Take bottle, put iron, Pb salt (or mixture), water and some acetic acid (citric, sulfuric, etc). Stop the bottle and shake it from time to time. End of reaction can be indicated by immersing a piece of clean iron for some time (in slurry) - surface of iron should not be covered by Pb sponge.
Remove remaining iron, decant, wash, dry, melt Pb. Do not store prepared dry Pb for a long time, because it slowly covers with oxide.

I tested Al sheets but I got hard to remove "Al(OH)3" hard particles. I think it is question of bad conditions (just tested it only to see if it works at all). Maybe NaOH instead of acidic solution would be better ? Not tested, but if someone wants, worth trying.

Important : Pb in car battery always contains some Sb, even few %.

Quite a lot of 'lead' is actually a lead antimony alloy as the antimony make it harder eg bullets, shot, wheel balancing weights, etc.

watson.fawkes - 3-5-2013 at 09:18

Quote: Originally posted by Bezaleel

When cold, I am left with an opaque dark grey mass, which on breaking proves to be virtually homogenous. No sheen of metal there. So in this case, the dark grey colour is all through the substance.

Of course, it could be that it consists of microscopic lead crystals with a dark grey outer layer. Note that the dark grey mass is insoluble.

I'd have to guess that you've got significant lead (II) chloride, which is poorly soluble in water. I'd have to guess that this chloride is also more soluble in lead than in the alkali chloride flux, given your result. If that's true, the lead chloride would have partitioned into the lead.

I recommended a cover flux, but I didn't put any consideration into what it ought to be. I should have recommended you look up what's already known to work; my apologies. Fluxes take some experience to understand, and I might have pointed out that a bit of research was warranted.

I looked into a bit just now. Borax and other borates are used as covers for cleaning lead when casting. Lime and limestone are both used in recovery smelting. So it seems that you want a flux based on oxides, carbonates, and borates. Lime can help suppress some of the SO2 formation by forming CaSO4. Borax will lower the melting temperature. These are all glass-forming materials, I'll note, which leads me to think that soda ash (principally Na2CO3) might be useful as well.

You've got a new problem, however, in that you've got to get out the chloride. It'll likely need to come off as either Cl2 or HCl, so that it gasses out rather than redissolves. If it doesn't come off as Cl2 (try that first), you'll need to add a hydrogen reducer of some form. For cleaning lead, folks are using hydrocarbons such as paraffin wax or even motor oil. They flame off fairly soon after addition, so beware.

And don't take me on faith. I did a bit of reading, but you probably should do your own at this point.

blogfast25 - 3-5-2013 at 13:37

Quote: Originally posted by kmno4

Important : Pb in car battery always contains some Sb, even few %.

I had a lot of trouble dissolving battery lead in nitric acid (70 %), which I believe is due to these hardeners (Ca seems also to be used). Pure (reagent) lead dissolved much more easily. Very frustrating...

AJKOER - 3-5-2013 at 16:10

Quote: Originally posted by kmno4

I agree with Kmno4 on the use of wet methods as the thought of lead metal fumes is a safety issue for me.

This reference "Level Course in Chemistry", by J. G. R. Briggs, page 275, link: http://books.google.com/books?id=449tWjfDgVUC&pg=PA275&a... notes that PbO dissolves in hot aqueous NaOH (or in hot dilute HNO3):

PbO + 2 NaOH --> Na2PbO2 + H2O

Actually, I have dissolved Pb only the slow way with Acetic acid and H2O2, so I cannot testify as to the speed of this reaction.

Then, per the authors comment on page 276, if so desired, the lead salt can be oxidised to a brown precipitate of PbO2 by an alkaline H2O2 or alkaline hypochlorite (done this).

Bezaleel - 3-5-2013 at 16:11

Okay, well, that's a lot of comments

. Let me post two pictures of the 'ingot' I cast in my effort to clean the jug. The piece of material pictured weighs 20 g. Total amount of lead electrode used was 50 g, not 100 as estimated earlier. Total amount of flux was around 20 g, based on hydrated CaCl2 (hydrate still unknown, not tested).

Left: on top of the 'ingot', a few small spheres of lead can be seen.
Right: bottom of the 'ingot' (has a metallic sheen here and there, but cannot be seen on picture).

Closer inspectation reveals that the bottom contains very impure lead. The top seems to be a hard still undetermined mixture.

Before starting to test with completely different types of lead regaining experiments, my next step should be redoing the experiment with more and clean flux.

A second step could be the use of borax. Lime and sodiumcarbonate melt at 825 C (aragonite) and 850 C, which both lie above the temperatures I can reach.

I know that both antimony and calcium are probably present in the lead. These experiment are of exploratory nature, so to speak. A real nice workup of lead from these electrode plates is still far away.

Edit: I dissolved pieced of a Pb electrode in 70% HNO3, but lower concentrations (40-50%) worked better for me. There was a large amount of gunk coming off that plate though, which was difficult to separate. I managed to do this through a fine fritte, but nothing would then work to clean the fritte, neither acids, nor lye, nor ammonium sulphate, which is capable of dissolving PbSO4.

I decided that I'd like to have reasonably pure lead ingots, and work their purity up when needed.

[Edited on 4-5-2013 by Bezaleel]

12AX7 - 3-5-2013 at 21:25

The mass should be a composite matrix of salts, amorphous carbon (I can't imagine the sugar left anything else behind, it should be fully pyrolyzed), and lead in varying oxidation states.

I agree with watson, my understanding of smelting is that CO is required. With no access to catalytic oxygen, nor solubility for gas*, the oxides just kind of sit there. I doubt PbO has too much solubility in chlorides (an addition of fluorides would probably change that, however), so reaction would then be limited by solubility of CO and PbO, and solid-state diffusion (on the lucky particles that are touching). Otherwise, if you continue to raise the temperature (until PbO melts, at which point the chlorides will be evaporating nicely, too), you'll get good reaction rates (I will note that PbO is corrosive towards Fe, even though the Pb metal sloughs off without dissolving iron or its oxide).

*Hydrogen and steam appear to be pretty soluble in salt, which is unfortunate when it comes to aluminum, which loves to om nom nom hydrogen gas when molten. Of course, the gas comes out when it freezes, mushrooming and ruining castings. The traditional solution is bubbling either inert gas (remove H2 by dilution) or chlorine (consume it as HCl, much more reactive, faster).

I don't see any way PbCl2 would form. Well, maybe not entirely. I suppose there's an outside chance that the double displacement CaCl2 + PbO <--> CaO + PbCl2 could occur. PbCl2 isn't the most stable salt out there (though more stable than, say, CuCl2), but CaO (at least in bulk form) has comparable binding energy to MgO, which as we know, is a powerful endpoint in certain reactions.

The salt should fall apart, slowly but surely, with an extended soak in water. Bash it up beforehand to make smaller pieces. I have found that, when using a large excess of chlorides to clean aluminum slag, the oxides form a tough composite which does, however, dissolve in water, releasing the oxides within. Test for pH: if it's basic, some PbCl2 may've formed...and since disproportionated...hmm...never mind.

Next time, try heaping lead into the container and heating it. Then add charcoal on top. I'm guessing you'll start with a mushy powdery mess, of various lead-related colors (until it begins glowing too bright to see its own color). The charcoal will start out cold, then start sparking to life on top. Stir it in. You should start to see metal droplets appearing. Let it cook for a while, stirring occasionally. Eventually, you should notice the liquid level underneath has risen, while the amount of crap on top has decreased. If you want to coalesce better, you can melt some salt on top (but use pre-melted salt ingots, don't introduce moist raw chemicals to a hot melt!), then pour everything off. Borax and silica based slags are traditional, of course, but the powder will have more surface area for reduction, so don't go nuts (otherwise, think about trying to reduce metal out of a lump of lead crystal glass!).

Tim

Bezaleel - 6-5-2013 at 08:35

Thinking about what may have been formed, two things came to mind, that might explain the insolubility of the product: CaSO4, and CaPbO2/CaPbO3. Mixed with PbCl2, metallic lead and carbon, I could imagine a hard solid may form.

watson.fawkes - 6-5-2013 at 10:42

Quote: Originally posted by Bezaleel

A second step could be the use of borax. Lime and sodiumcarbonate melt at 825 C (aragonite) and 850 C, which both lie above the temperatures I can reach.

All these oxides melt together, with a wide variety of eutectics with locally minimal melting points. Don't take the isolated melting points as anything like determinative. I pulled up a couple of eutectic mixtures from a ceramics site:
Lead Oxide and Boric Oxide Eutectic 493 °C
Lead Oxide, Boric Oxide and Silica Eutectic 485 °C
The site isn't comprehensive, unfortunately, but it is indicative.

Also, remember that high-melting oxide combinations in this context are also called "slag" or "dross".

Bezaleel - 6-5-2013 at 15:07

I just tried to clean the "crucible", but after pouring out all that could be poured out, and melting new salt, and repeating this twice, the result was still a grey mass. This looks far from promising.

When I cleaned the "crucible" afterward, I also found it now has a small hole in it, so appaerently the mixeture has eaten itself a way through the stainless steel. I decided that for now, this will end my activities in melting the battery lead under salt mixtures.

The last post on page 1 of kmno4, mentiones a different procedure, based on the difference in oxidation potential of Pb and Zn. Maybe this will be a better starting point. The result from that method will still need melting and work up, but maybe it will remove much of the lead compounds. Supposed it will yield a mixture of Pb an Pb oxides, then melting it together under borax could provide a good route to a relatively pure metal, as watson.fawkes has mentioned.

[Edited on 6-5-2013 by Bezaleel]