Sciencemadness Discussion Board

Separation of Lead and Silver?

Intergalactic_Captain - 10-7-2008 at 20:56

On a whim I picked up a bunch (a pound or so, haven't weighed it all out yet) of "Tibetan Silver" jewelry findings on ebay, hoping to pull the silver out for a project. What little information I've been able to find thus far indicates that most of this "Tibetan Silver" floating around is somewhere between 0-20% actual silver content, the bulk of the remainder being lead with some copper mixed in. The current plan is to dissolve it in some poor-man's aqua regia, ie HCl with a dose of NH4NO3 dissolved therein.

I've used this method to successfully purify coin and sterling silver in the past - Both copper and silver dissolve, the copper remaining in solution while the silver falls back out as the chloride. As lead chlorides are insoluble, I'm assuming I'll be left with a mass of mixed silver and lead chlorides and liquid layer of chinese god-knows-what (Not knocking the Chinese, but thery're not particularly well known for their history with Tibet and these came from mainland China).

It's from this point on that I'm unsure of...Checking my tables, the acetate is the only salt that I'd be able to work with crystallization wise with what I have on hand. However, getting to the acetate salt is where I'm stumped...There's probably some stupidly simple way that I'm overlooking, but my brains been running at half-speed of late.

Anyone got a suggestion or a better idea? Assuming there's actually some silver in the stuff I'd like to try my hand at freeing it.

12AX7 - 10-7-2008 at 21:11

As in silver acetate or lead acetate?

With that much lead, I doubt you'll dissolve much of anything with a lot of chloride ions around. HNO3 or a slow time with HOAc + H2O2, methinks.

You might have better luck with a cupel, if you have a furnace to do it in. Oxidize the lead, melt off the PbO and you're left with a pure silver button (assuming it hasn't oxidized as well).

Tim

Formatik - 10-7-2008 at 23:07

Have you looked at the solubilities in various solvents comparing AgCl and PbCl2? E.g.:

PbCl2 is said to: be soluble in KOH + Aq. Easily sol. in NH4NO3 + Aq.

AgCl is also soluble in various organic nitrogenous bases, no doubt liklely under complex formation. Not sure about the PbCl2, but have doubts.

I've added partially darkened AgCl to NH4NO3 solution to see if it would dissolve and it doesn't look like it, so if the PbCl2 dissolves that could be a way of separation. I would do a separate test with PbCl2 alone to make sure, and then try it out.

Jor - 10-7-2008 at 23:45

1. Dissolve in nitric acid, heating is required (watch out for aerosols containing Pb2+, and ofcourse NO2)
2. Plate the silver on copper powder.

Done.

To purify it from remaining copper (I have this procedure from woelen):

1. Dissolve in nitric acid, take about 2mL for every gram --> heating is required.
2. Add aquous NaCl dropwise, untill no more AgCl drops out.
3. Dilute the solution, or silver might be lost as the chloro-complex.
4. boil the solution for a minute, to make the ppt easier to filter.
5. Filter and rinse the AgCl wih water.
6. Add excess aquous NaOH, followed by a reductor. Hydrquinone worked fine for me, but probably other reductors are suitable as well, e.g. ascorbic acid.

Ofcourse if you react all copper powder, this won't be necessary.

Maybe it's also possible to dissolve the lead/silver alloy in nitric, dilute the solution, and add a suitable reductor.

not_important - 11-7-2008 at 00:52

Lead chloride is much more soluble in hot water than silver chloride. Leach the mixed chlorides with boiling water, let it cool and pour it off the PbCl2 crystals, reheat the water and use it again for another extraction. Repeat several times, until the weight of lead chloride is 3/4 or so of what you'd expect for 100% lead as the starting metal.

After that, reduction to the metals followed by cupelation is the simplest off the top of my head. Alternatively the chlorides could be treated with thiosulfate, to that solution add sodium carbonate at 35-40 C to precipitate lead, copper, and a few other impurities, then use Na2S to precipitate Ag2S.

Treating the mixed chlorides with saturated salt solution and metallic copper will reduce the AgCl to metallic silver. The mix of metallic copper and silver can be washed free of copper and lead chlorides by first using saturated salt solution, then hot water. Treating of the mixed metals with sulfuric acid can dissolve most of the copper. A final purification then can be done by dissolving them in nitric acid, evaporating to get the solid nitrates, and fusing them at about 250-300 C. The copper nitrate decomposes, after cooling the silver nitrate is dissolved in water and filtered from the copper oxide. The drying and fusing step can be repeated if needed.

One more direct process would be to cast the original metals into electrodes and plate the lead from them onto a cathode using an electrolyte of lead acetate with some free acetic acid, with about 3 volts applied. The silver will not dissolve and falls off the dissolving anode, along with some other alloying metals. Treat that with your nitrate+HCl, wash well, reduce to metal, then melt with a (Na or/and K) borate+carbonate flux, to which about 5% nitrate is slowly stirred in when molten to oxidise most impurities. Or dissolve the anode slime silver in HNO3, boil it a bit, cool and ppt as the chloride.

The last is more of a fussy thing during the electrorefining, but reduces the amount of material needing chemical treatment.

woelen - 11-7-2008 at 02:33

I would go for the route of dissolving the alloy in nitric acid, and then adding chloride. If you don't have nitric acid, then things become somewhat harder and the H2O2/vinegar/patience route should be followed.

Once all of it has dissolved, add a lot of NaOH. Lead is amphoteric and will redissolve again, giving plumbate, the silver remains precipitated as hydrous Ag2O. The hydrous Ag2O most likely is quite impure, but that can be handled in the next step.

Filter the Ag2O from the alkaline solution and add a suitable reductor (e.g. hydroquinone). This reduces the silver, but remains of lead are not reduced. Rinse the now black precipitate with dilute HCl and then rinse with distilled water. Any lead, remaining in the precipitate will be converted to PbCl2. Finally, boil the precipitate with hot distilled water, which dissolves all PbCl2 and then, while still hot, decant the solution and rinse with some hot water. Now you have a pure precipitate of silver metal.

Zinc - 11-7-2008 at 02:50

These are all god ideas but complicated.

Just dissolve your alloy in HNO3 and add copper in block form, not powder (so you can easily separate it from the powdered silver). Silver crystals will form on the surface of the powder and you should periodically scrape them off to expose fresh copper. When done remove the remaining copper block, filter your silver and wash with water. You will get reasonably pure silver.

Intergalactic_Captain - 11-7-2008 at 05:18

Hmm...Lots of good, expensive ideas. Personally, I'm quite fond of nitric acid - However, at the current time, the acid necessary would cost me more than any of the recovered silver would be worth - Maybe when the bottom finally drops out of the dollar it'll be worth it.

not_important and Woelen, could you elaborate on the leaching and plumbate methods? I do have suitable amounts of HCl and lye on hand.

not_important - 11-7-2008 at 07:11

Not much to leaching.

The HCl-nitrate mix dissolves some of the lead and silver chlorides as complexes, when all the metal is dissolved evaporate the mixture to a thin paste to drive off excess HCl as it will interfere with the separation through complex formation. Do this in a fume hood or out of doors with good air circulation as the HCl and chlorine fumes are both bad for you and corrosive to metals.

Add some (*) water to the mixed Pb/Ag chlorides, bring it to near 100 C while stirring. let the Pb/Ag chlorides settle while keeping hot, pour the liquid through a filter and let it cool. Scrape any solids on the filter back onto the precipitate.

When the filtrate solution is cold, lead chloride crystals should have formed. Filter them off, put the filtrate back onto the mixed Pb/Ag chlorides, heat to nearly boiling with stirring for a few minute. Repeat the settle-filter,cool-filter steps until about 3/4 of the original precipitate (going by your numbers on silver content) is gone.

(*) some : 100 cc of water near boiling will dissolve 3 grams of lead chloride. Probably don't want to try to work with more than a couple of liters or so at a time.


You want to reuse the same solution over and over so as to minimise loss of AgCl. However as the amount of PbCl2 remaing goes dow, you start extracting the AgCl as a halide complex, thus the reason for stopping while some PbCl2 remains.

If you extract until about 2/3 the PbCl2 has been extracted, then switch to fresh water containing a small amount of sodium or potassium nitrate, you could then continue the extraction process until PbCl2 crystals stopped forming when you cool the extraction liquid; at that point the mixed chlorides will be mostly silver chloride.

You can then use Woelen's method to complete the separation. Filter off and air dry the Ag/Pb chlorides,absolute dryness is not desired - they can be slightly moist. Weight them, then add to enough strong NaOH solution to give the following

2AgCl + 2NaOH => Ag2O + 2NaCl
PbCl2 + 2NaOH => PbO + 2NaCl
PbO + NaOH + H2O => Na(+) + [Pb(OH)3](-) plumbite ion

with excess NaOH remaining.

Heat the mixture for some minutes, stirring well. Filter, transfer the precipitate back into the beaker and cover with 2N NaOH, heat and stir for a few minutes. Filter, wash precipitate with water, then reduce it. There was an earlier thread that mentioned using sugars as reducing agents for silver, which is much cheaper than hydroquinone, and continue per the rest of Woelen's suggestion (HCl,...)


If you had a Soxhlet extraction apparatus you could use it to remove PbCl2 from AgCl, although you might have to remove the extracted lead chloride from the boiling flask ever so often.

ShadowWarrior4444 - 11-7-2008 at 13:42

Provided that electricity is cheap where you are, I am very fond of electrochemical alloy separation.

A bath of one part 32% sulfuric acid, one part denatured alcohol with the application of 1.4 volts should cause metallic silver to form at the cathode in a branching pattern, as well as metallic copper to form as a fine powder. The lead I believe will drop out at the anode.

This is a fairly simple separation method, there are doubtless others that would be equally or more effective and most likely lower cost than any purely chemical technique.

[Edited on 7-11-2008 by ShadowWarrior4444]

S.C. Wack - 11-7-2008 at 15:08

If you don't have much excess HNO3 present, add only enough HCl to precipitate everything, and dilute with water, the amount of complexed silver should be pretty low, yes?

Solubility of PbCl2 in boiling water: 33.4 g/L
AgCl: 21 mg/L

Dissolving the AgCl in conc NH4OH and adding a dilute solution of formate, dithionite, borohydride, etc. will precipitate the silver quantitatively.

Intergalactic_Captain - 11-7-2008 at 16:43

I think I'm going with the leaching method, will start a small amount tomorrow to see if there's actually any silver in it before committing the lot...They're crosses, not of particularly good quality, but something seems a bit sacriligeous about wasting them for nothing.

Anyone got a good link on the complexation issues before I go off searching? As for reduction, I remember the old post - That's where I got the idea for the coin/sterling separation - I can verify that the corn syrup method works. Funny thing though - I accidentally left the beaker for about a month, by the time I got back to it the water was gone and I was left with a layer of silver powder under syrup (I used waaaayyy too much apparently, just kinda threw some in to see if it really worked)...Much to my surprise, there was white and green mold growing on top of it! After all these years of hearing the wonders of colloidal silver, I was taken aback - That is, until I realized that I had powdered silver, not the colloid.

not_important - 11-7-2008 at 17:15

Quote:
Originally posted by ShadowWarrior4444
Provided that electricity is cheap where you are, I am very fond of electrochemical alloy separation.

A bath of one part 32% sulfuric acid, one part denatured alcohol with the application of 1.4 volts should cause metallic silver to form at the cathode in a branching pattern, as well as metallic copper to form as a fine powder. The lead I believe will drop out at the anode.
...
[Edited on 7-11-2008 by ShadowWarrior4444]


The problem is that lead is the majority metal, and that with a sulfate electrolyte you'll have to keep adding sulphuric acid. If you use an acetate bath and plate lead, the silver remains behind and little make-up acid is needed.

Plating silver would work OK if it made up at least 2/3 of the alloy.

Note that all of these proposals would need changing if there is much tin or antimony in the starting alloy.

Fleaker - 11-7-2008 at 17:24

How pure do you want the silver? If you just want it 99,5% or better this is what I do:

Dissolve the material in 35% nitric acid.
Add sodium chloride to it. Dilute with warm water. Add more salt, look for more precipitate.
When all is precipitated you're left with a greenish blue solution. The precipitate should be rinsed once with very hot water and then boiled with water and filtered hot. After that, dissolve it in medium strength ammonia (shaking helps). It will go slightly blue in colour. Filter. Repeat until no more precipitate complexes in ammonia. To break the complex, add HCl and you have have pure AgCl. Or you can do as S.C. Wack suggested and reduce it with those reducing agents, all of which (save the formate) are somewhat expensive.

Normally I just precipitate as the oxide/hydroxide with base. Then heat that and the oxide disproportionates and it will melt to clean silver. If the silver is pure, it will have a rough surface upon cooling. If you feel like waiting a bit, you can also cement the silver out with zinc, copper, or iron (keep aluminum out of it!) and a 3M sulfuric acid solution.


If you had a lot of it, it might be worth it to use the Parkes' process. Dissolve away the zinc with HCl and then process the silver, no worries about the lead contamination.

not_important - 11-7-2008 at 20:54

Quote:
Originally posted by Fleaker
How pure do you want the silver? If you just want it 99,5% or better this is what I do:

Dissolve the material in 35% nitric acid.


Note that zie has posted
Quote:
Personally, I'm quite fond of nitric acid - However, at the current time, the acid necessary would cost me more than any of the recovered silver would be worth.


so you start off telling zir to do exactly what zie said zie didn't want to do.
Quote:


If you had a lot of it, it might be worth it to use the Parkes' process. Dissolve away the zinc with HCl and then process the silver, no worries about the lead contamination.


Not true, see books such as Schnabel's Handbook of Metallurgy which discuss Parkes' process and other in some detail, or Wysor's Metallurgy. The zinc scum formed on top of the lead contains several percent lead, generally in excess of the amount of silver, plus additional lead as entrained metal and oxide. The process is for lead containing a percent or less of silver, and even then lead with silver content in the upper end of that range take several extracts with zinc to get most of the silver.

Fleaker - 12-7-2008 at 09:53

You learn something new everyday! Thanks not_important; I always thought lead and zinc were practically not miscible. I also didn't know that it was used solely for silver reclamation rather than gold. Thank you for the book recommendations by the way.


As for him not wanting to use nitric, why not? Making nitric yourself ought to cost less than $20/gallon for concentrated non-fuming acid. Or he could buy it for $4.50/gal or whatever it costs from an industrial supplier, the caveat being that he needs to buy a 15 gallon stainless container that has a deposit.

I'm interested in a few more details on the electrolysis with lead acetate/acetic acid electrolyte.

not_important - 13-7-2008 at 06:25

Schnabel's Handbook of Metallurgy mentions the lead electrowinning, that's where I'd start.

As for why not use HNO3, no idea except it was stated to be too expensive and perhaps they've not the apparatus for making their own. Buying can be difficult, especially in some arreas or if you are younger and/or not a business of the proper sort.

JohnWW - 13-7-2008 at 17:38

Noting that cyanide solutions are used for dissolving Ag and Au out of crushed ores, I wonder if they might be of some use in their extraction from such alloys also containing Pb and Cu, although Cu is also likely to dissolve as a cyanide anionic complex.

not_important - 14-7-2008 at 03:06

There can be problems extracting minor components out of alloys, glasses, or microcrystalline masses; the major, non-soluble component(s) shield the minor one against solution.

In some of the old wet assay methods, a known quantity of silver was added to the lead/precious-metal alloy before cupelation. The resulting precious metal button was then treated with nitric acid to dissolve silver, gold remaining as a powder, the solution having its silver determined and the added silver subtracted out. The additional silver was to insure that the alloy button had enough silver that all of it would be dissolved away.

So given the fairly low expected silver content mentioned in the first post, it might be difficult to selectively extract it out of the alloy.