Electrochemistry Dissolving large lead bars to recover gold and silver

[DDL]

Hi

My name is David, we are processing iron sulfides and recovering the gold and silver. We have been using lead as a collecter. Our yield we getting is 5 oz of Au and 3.2 oz of Ag per ton. These number come from our assays done on the lead bars by a well known assay office.

Our lead bars are approximately 50 lbs, what we need help with is a good fast way to disslove the lead and recover the metals. The lead will also be recovered to be recycled for extra income.

We want to do this as safe and quick as possible.
Thanks very much

 

[4metals]

One option would be to cupel the lead in these http://www.lmine.com/mm5/merchant.m...uct_Code=16850&Category_Code=magnesite_cupels

Each one will adsorb 30 pounds of lead.

This will recover the PM's quickly but lead recovery may be a challenge. But quite often the real goal is fast cash.

 

[Lino1406]

The concentrations of 5 and 3.2 resp. is in the sulfide or
in the lead? Is this lead process profitable?

 

[Geraldo]

You might not love this, but you could consider electrorefining the lead, leaving the gold and silver behind in a slimes layer while making pure lead for sale.

Google "Betts Lead Refining" or even read the man's book - "Lead Refining by Electrolysis" - Anson Betts, here: http://www.archive.org/details/leadrefiningbyel00bettrich

The basic process: the impure lead bar is your anode. I would bag it in your case (with a polypropylene filter cloth, for example) since you didn't control the lead alloy content and won't have a stable slimes layer. You could use a thin sheet of pure lead as a starter sheet (if you are serious about lead purity) or a stainless sheet (which will contaminate the lead a bit - depends on who buys your lead). The electrolyte is hydroflurosilicic acid, which is basically silica dissolved in HF. HF is very nasty, but H2SiF6 can actually be handled with bare hands without too much discomfort - - I have plenty of times.

For small setups, I have used a rubbermaid container and a 12 V battery charger. We had an ancient rheostat to control the voltage, but you could put some resistors in series to step down the 12 V to under 1 V (our battery charger had a 6 V setting, so less voltage step-down required). H2SiF6 concentration is around 90-100 g/liter. Cell spacing is as close as possible without touching, while leaving space for a thin stick.

In this ghetto case, you will probably not make smooth lead deposits, so someone will have to knock the dendrites down with a stick every 6 hours or so. The system also works better warm, around 30-45 Celsius. If you are serious about lead quality, read Betts book and see about additives like lignin sulfonate (goulac).

The silver and gold will be left in the slimes on the anode (or more precisely, the slimes at the bottom of the bag you put the anode into). If you have bismuth, antimony, arsenic, tin, etc. they will also be in the slimes. Since current efficiency probably isn't a big deal for a small operation, you can run the cell into default, essentially driving as much lead out of the anode as possible (although you have to be careful to still leave enough of an anode to act as an electrode). You can then pull the anode, scrape it, recover the rest of the slimes from the bag, and melt that anode remnant with the NEXT lead block you are going to refine.

It's actually a lot more fun than it sounds, and of course is a very selective way of dissolving lead away from silver and gold.

Good luck!

Best Regards, Gerald

 

[Lino1406]

Same can be said about copper, isn't it
prefered vs. lead?

 

[NoIdea]

Morning all - I need to find the article which describes the use of cornflaked lead + PM's, the cornflakes are but in a iron rotating mesh drum, the anode, the cathode is a lead sheet. The article explains the need for constant movement of the flakes to ensure proper conductivity. The electrolyte is a 1M NaOH solution, no higher.

Only lead and tin are removed

From memory the process was designed to remove lead and tin from e-waste.

Deano

 

[eeTHr]

NoIdea---

I remember a thread on there about that. A couple of people perked up at the selectivity for lead and tin, but then the tread just died out, without much further details. They didn't mention the dilution either, I don't think. So thanks for that.

I wonder why it isn't a more popular subject with escrappers? It seems like it would be really handy for items with solder on them, especially the old stuff with lead.

 

[NoIdea]

NoIdea---

I remember a thread on there about that. A couple of people perked up at the selectivity for lead and tin, but then the tread just died out, without much further details. They didn't mention the dilution either, I don't think. So thanks for that.

I wonder why it isn't a more popular subject with escrappers? It seems like it would be really handy for items with solder on them, especially the old stuff with lead.

I wondered the same thing, I am going to try this approach using the iron screen/mesh found in CRT's for the anode, i have a 155.45g nugget that has a density of 14.1g/cm3, made using lead as a collector. Skimming the oxide layer, heating, skimming over and over. :roll:

Pleased with the result though and discovered a few things along the way.

Deano

 

[eeTHr]

NoIdea---

I got a little excited when I wrote my last post. I don't think the NaOH was used as an electrolyte, or at least it wasn't mentioned in the person's post. I thought it was just used as a dissolving solution.

Maybe this thread isn't getting read by many, because of it's topic title.

I was going to start a new topic, just on the selective dissolving of lead and tin, but NaOH is so darn nasty, I wouldn't want to give any impulsive beginners the idea that it's something that can be done without all the safety precautions.

 

[NoIdea]

Hi eeTHr - found the link

click here

Deano

 

[eeTHr]

NoIdea---

Thanks. Here is the thread on NaOH dissolving lead and tin.

I forgot that it said to add peroxide.


Here's another thread on it. Lots of questions, but no real answers.

An interesting link about other properties of NaOH, some may be helpful in refining.

 

[goldsilverpro]

You might not love this, but you could consider electrorefining the lead, leaving the gold and silver behind in a slimes layer while making pure lead for sale.

Google "Betts Lead Refining" or even read the man's book - "Lead Refining by Electrolysis" - Anson Betts, here: http://www.archive.org/details/leadrefiningbyel00bettrich

The basic process: the impure lead bar is your anode. I would bag it in your case (with a polypropylene filter cloth, for example) since you didn't control the lead alloy content and won't have a stable slimes layer. You could use a thin sheet of pure lead as a starter sheet (if you are serious about lead purity) or a stainless sheet (which will contaminate the lead a bit - depends on who buys your lead). The electrolyte is hydroflurosilicic acid, which is basically silica dissolved in HF. HF is very nasty, but H2SiF6 can actually be handled with bare hands without too much discomfort - - I have plenty of times.

For small setups, I have used a rubbermaid container and a 12 V battery charger. We had an ancient rheostat to control the voltage, but you could put some resistors in series to step down the 12 V to under 1 V (our battery charger had a 6 V setting, so less voltage step-down required). H2SiF6 concentration is around 90-100 g/liter. Cell spacing is as close as possible without touching, while leaving space for a thin stick.

In this ghetto case, you will probably not make smooth lead deposits, so someone will have to knock the dendrites down with a stick every 6 hours or so. The system also works better warm, around 30-45 Celsius. If you are serious about lead quality, read Betts book and see about additives like lignin sulfonate (goulac).

The silver and gold will be left in the slimes on the anode (or more precisely, the slimes at the bottom of the bag you put the anode into). If you have bismuth, antimony, arsenic, tin, etc. they will also be in the slimes. Since current efficiency probably isn't a big deal for a small operation, you can run the cell into default, essentially driving as much lead out of the anode as possible (although you have to be careful to still leave enough of an anode to act as an electrode). You can then pull the anode, scrape it, recover the rest of the slimes from the bag, and melt that anode remnant with the NEXT lead block you are going to refine.

It's actually a lot more fun than it sounds, and of course is a very selective way of dissolving lead away from silver and gold.

Good luck!

Best Regards, Gerald

Here are some ways of recovering gold and silver from lead:
(1) Dissolving the lead in hot weak nitric acid. The silver will also dissolve, but it could be cemented with copper. This will generate lots of waste solution and the lead could be difficult to recover.
(2) Cupel the lead as suggested by 4metals.
(3) Melt and use the Parkes process - collect the Au/Ag with zinc. This was for silver but I assume the gold would also be collected. A similar process published by the US Bureau of Mines used aluminum instead of zinc. The BoM process was mainly for removing the PMs and copper from contaminated Pb/Sn wave solder used in the printed circuit industry.
(4) Melt, cast bars, and dissolve and plate out the lead. The Au and Ag would be caught in the anode bag.

Electrolytic systems are my forte' and I do love the idea of electrorefining PM bearing lead. I downloaded the Betts book and will look it over when I get the chance. With the right electrolyte composition and, if the solution contamination could be kept to a minimum (when the contaminants reach a certain detrimental level, however, most can be removed by selective treatments) , the solution could be used over and over. This would reduce waste solutions, considerably, and would allow the lead to be accumulated, melted, and sold.

Being an old plater, I read Gerald's post with interest. Lead is deposited (mainly) from solutions containing either fluoboric, fluosilicic, sulfamic, or (maybe) acetic acids. For the electrorefining of lead containing silver, however, since the silver will co-deposit from the fluoborate system, the fluosilicate or sulfamate systems would be the best electrolyte choices. In these, the Au and Ag would end up in the bagged anode sludge.

The fluosilicic acid (same as the acid used in the Betts system as discussed by Gerald - this acid has many different names) would work great, but it seems to be more dangerous than Gerald thinks it is. However, many things we use are dangerous. With the proper knowledge, setup, and controls, the dangers of most any system can be reduced or nearly eliminated. The best info I could find on the dangers and properties of this acid is in the following link. Although the article is concerned mainly with the use of this acid to produce fluoridation of drinking water, it provides much general information. I would suggest that anyone thinking about using this acid read this article. Please note that, unlike sulfamic acid, fluosilicic acid will etch glass similarly to HF.

http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPDF/Fluorosilicates.pdf

Here's a general search for the Betts process. It seems that it is the standard method for working lead from sulfide deposits.

https://encrypted.google.com/search?hl=&q=betts+process+lead&sourceid=navclient-ff&rlz=1B3MOZA_enUS408US409&ie=UTF-8&aq=1&oq=Betts+process&lr=all

Were I to experiment with this, especially for use on a small scale, I would definitely attempt to use the sulfamate system, instead of the the fluosilicate. Sulfamic acid is cheap. I recently bought a 50# bag from Univar for $.58/pound and it's available on eBay for about $4/#. Also, sulfamic acid is much safer than the fluosilicic acid. In both systems, if tin is present in the lead, it will co-deposit with the lead.

In order to produce a non-spongy, sound cathode deposit, whether using the fluosilicate or the sulfamate system, a certain amount of lead must be dissolved in the solution before any electrolysis takes place. If you don't start with any lead in the solution, you will spend a lot of time screwing around with the crappy cathode deposit. In this link, notice that the starting solution in the Betts process contains 70g/l of lead, as lead fluosilicate. Also, in the literature, there are additives that will smooth out the deposit even more.

http://www.metsoc.org/virtualtour/processes/zinc-lead/lead.asp

According to the literature (the definitive book, Modern Electroplating, by Lowenheim), the usual operating conditions for a lead sulfamate plating solution are:

Lead - 110 to 165 g/l (as lead sulfamate)
Free sulfamic acid - enough to lower the pH to 1.5
Temp - 24 to 50C (75 to 122F)
Cathode current density - 0.5 to 4 A/dm² (4.6 to 37 A/ft²)

If the pH is too low or the temp is too high, the solution can decompose. The pH of 1.5 (using glass electrodes) is maintained by sulfamic acid (to lower) or ammonia (to raise). In this solution, both the anode and cathode efficiencies are 100%.

The main problem is how to obtain the lead sulfamate (or lead fluosilicate) in order to make up the solution. You can purchase lead sulfamate (or, fluosilicate) but, it may be expensive - for a large scale, though, this would be the best way to go. A cheaper way would be to dissolve either lead hydroxide, lead carbonate, or lead oxide (litharge) in a solution of fairly hot, fairly saturated, sulfamic acid or fluosilicic acid. To make these lead compounds, I would think that one could use sodium hydroxide or sodium carbonate to precipitate these (or a combination of these compounds) from a solution of pure lead dissolved in nitric acid. I might mention that strong nitric won't work well for dissolving lead, as lead nitrate crystals will soon coat the lead and prevent further dissolution. In my experience, a hot solution of 7 parts distilled water and 1 part nitric will dissolve the lead and prevent crystallization.

For economic purposes, the use of acetic acid (vinegar) might be worth a shot also, using similar logic as in the above discussion. That would certainly be the cheapest, but I have no idea as to what the parameters would be. I did find some discussion of this on this sciencemadness thread.

http://www.sciencemadness.org/talk/viewthread.php?tid=10814#pid130783

 

[notch]

I would use some old Lead-Acid Battery plates to make up the solution. It's cheap and can be found most anywhere. The Lead Oxide in the plates would dissolve quickly.

 

[goldsilverpro]

I would use some old Lead-Acid Battery plates to make up the solution. It's cheap and can be found most anywhere. The Lead Oxide in the plates would dissolve quickly.

Excellent idea, especially for a small hobbyist setup! However, with a serious setup, I most always suggest going the full route and using the proper chemicals. This eliminates any unforeseen problems and gets you off to a good start.

In my last post, I kind of ran things together and didn't really separate the systems that I would recommend for the 2 different types of users. To summarize.

For a serious, larger scale, continuous operation. For a trouble-free operation, use the Betts fluosilicate system, exactly as the literature dictates, using lead fluosilicate and fluosilicic acid to make up the solution. This has been the standard system for about 100 years, all the bugs have been worked out, and there's tons of info available. Learn all you can about the safety involved when working with these chemicals and install the proper equipment. Don't cut corners. I would suggest getting some analytical equipment to run frequent analyses of the lead content of the solution and make the needed adjustments. Although the stated efficiencies are 100%, that will not usually be the case. For example, the undissolved slimes on the anode can reduce the anode efficiency. Since the deposit comes from the solution and the lead in the solution comes from the anodes dissolving, any reduction in the anode efficiency will eventually deplete the solution of lead.

For the hobbyist or small scale operators, I would probably play around with the other solutions I mentioned. I think most any of them would work, once you find the right parameters. To put these together and to get the right mix, some math ability would certainly help. And, they are safer than the fluosilicates. To experiment, first use about 1 liter beaker quantities and never change more than one variable at a time.

 

[HAuCl4]

Parkes process as noted by GSP is the way to go for speed and simplicity, imho. 8)

The lead stays as metal, and the good stuff sticks to the zinc. It is a good idea to bring the temperature of the lead to some 1200 C, and then let it cool slowly to 1000 C. Copper and other base metals magically float to the surface (no cupelling) and can be skimmed as a dross. Then you add the zinc for the parkes process to collect the goodies.

 

[goldsilverpro]

Parkes process as noted by GSP is the way to go for speed and simplicity, imho. 8)

The lead stays as metal, and the good stuff sticks to the zinc. It is a good idea to bring the temperature of the lead to some 1200 C, and then let it cool slowly to 1000 C. Copper and other base metals magically float to the surface (no cupelling) and can be skimmed as a dross. Then you add the zinc for the parkes process to collect the goodies.

Here's a photo of the Parkes process in operation. Doesn't look like much fun to me. I wonder what the average life-span of the guys doing the skimming is?

http://www.daviddarling.info/encyclopedia/S/silver.html

Other things I don't like about it:
(1) 1200C lead!!! Are you sure that's right? If so, lots of bad fumes that must be contained.
(2) The skimmings always contain some lead. This is most always cupelled.
(3) After cupelling the skimmings, the zinc is distilled off. Bad fumes. More containment equipment. You might be able to dissolve the zinc with something like weak H2SO4 without dissolving any silver but, maybe not, since the silver and zinc are alloyed.

All in all, a very dangerous system that requires too many separate operations with much safety equipment required. It's not as simple as you say and I think it would be much more expensive to properly set up.

Give me an electrolytic system any day. Much, much safer, cleaner, and, to me, much, much simpler. The lead melts at about 328C - maybe a little higher (or, it might be lower) with the contamination in it. With the fluosilicates, some fume control would be needed.

 

[mikesmadness]

Over the last several years, while preparing for retirement and stashing anything I could come across that contained gold, I stumbled upon a lead bar covered in gold. It is thin in places but thick (up to 1/8" ) on one end. A stripping cell comes to mind as one way to recover the goodies. Does anyone have any other options? How much larger than the bar should the stripping cell be?

 

[HAuCl4]

As usual it depends on scale. If you have only a few kilos of lead, forget about it. If you process a lot then I think it's the way to go. All drossing, etc is automatic and no fumes escape. That scheme/photo is likely 200 years old...hehe. Ammen has a good description, but no photos. I saw one system with 2 sealed crucibles and a tundish in between. Very little fumes. Very fast to process. With proper equipment everything is easy and simple, with bad equipment...try hammering a nail on concrete with no hammer, etc.

My only beefs with electrolytic systems is that they take too long, and require constant attention.

 

[goldsilverpro]

As usual it depends on scale. If you have only a few kilos of lead, forget about it. If you process a lot then I think it's the way to go. All drossing, etc is automatic and no fumes escape. That scheme/photo is likely 200 years old...hehe. Ammen has a good description, but no photos. I saw one system with 2 sealed crucibles and a tundish in between. Very little fumes. Very fast to process. With proper equipment everything is easy and simple, with bad equipment...try hammering a nail on concrete with no hammer, etc.

My only beefs with electrolytic systems is that they take too long, and require constant attention.

Different strokes for different folks.

I truly squirm when thinking about even the remote possibility of there being any vaporized lead or zinc in the air. Without spending the big bucks in this system to handle it, that possibility could very well become a constant reality.

Have you actually used the Parkes system, from start to finish?

 

[goldsilverpro]

Over the last several years, while preparing for retirement and stashing anything I could come across that contained gold, I stumbled upon a lead bar covered in gold. It is thin in places but thick (up to 1/8" ) on one end. A stripping cell comes to mind as one way to recover the goodies. Does anyone have any other options? How much larger than the bar should the stripping cell be?

Are you saying that the bar is 1/8" thick in places or the gold is 1/8" thick in places and thin in others? If the latter, how do you know that? Also, if the latter, that doesn't make much sense at all. If plated, the gold would be thin and fairly uniform over the entire bar. I may be wrong, but I see no way the gold (high melting point) could be cast around the lead (very low melting point). At the high melting point of gold, at 1/8" thick gold, the metals would surely alloy and the bar would look terrible. Also, for what practical reason would the maker of the bar want to apply gold in such an extreme variation of thickness? None, that I can think of. It's not impossible but very unlikely, unless, for some extremely weird reason, the guy maybe placed only the end of the bar in the solution and plated it steady for two days or more - that's about how long it would take to plate 1/8" thick gold (3125 microns). What is considered to be heavy gold eletroplate is only about 2.5 microns thick.

A number of years ago, there were many gold plated lead bars sold as paperweights. The markings on them were made to resemble old bars from the gold rush days. I saw many of them. The plating on them was very thin and the value probably didn't exceed $.25 worth of gold per square inch, at today's market.

If you want to remove the gold, a concentrated sulfuric stripping cell would work best, since it wouldn't appreciably attack the lead. I wouldn't expect there to be much gold, but I've been wrong before.