Reducing Silver Chloride

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Sylar said:
What have I made, and why didn't it all melt around 950°C to a wonderful glob of relatively pure silver?
It's silver-----you should have used some borax as a flux. Sorry I didn't make mention. You don't need a lot----a sprinkling on the surface will immediately cause the powder and bits of silver you see to become one. If the flux gets quite dirty, add a little more. An excess does no harm. Heat until it is a common mass, then pour to a well blackened mold. You must have the blackened surface, or the silver will solder to the mold. Make sure the mold is iron or steel. Don't use a lower temperature alloy or copper.

You would be best served using a small graphite/clay or silicon carbide crucible. Also, small clay melting dishes are readily available from jewelry supply houses. They are not expensive and will serve very well for the purpose. Fluxing your quartz crucible/dish will likely not be the wisest thing you do.

If your choice becomes either the graphite/clay crucible, or a clay melting dish, be advised that each of them are subject to cracking when they are put to work. If you heat either of them slowly, to expel residual moisture, then put them to work, you will avoid the potential cracking.

Harold
 
Aha, that's on my todo list for tonight then, I have several kgs of borax sitting here doing jack anyway.


I have a SiC crucible, but it doesn't fit my oven, and I have a graphite dish, which I don't like in my oven because is will tend to oxidise, right? So I'm likely using thinwalled clay/porcelain then.
 
Sylar said:
I have a graphite dish, which I don't like in my oven because is will tend to oxidise, right?
I used to own one of those small electric furnaces that use a crucible that is machined from graphite stock. Very handy, but life of the crucible was short. While the cover minimized oxygen exposure, that in and of itself was not enough. The crucible had a short lifespan due to the head burning away. The inner portion of the crucible sustained almost no wear or oxidization---but the crucible was rendered useless because the head was destroyed quickly. As a result, I don't recommend that type of furnace, nor do I recommend a graphite dish of any description. They are an excellent choice in that they help prevent molten silver from absorbing oxygen, but expensive to use. Graphite mixed with clay tends to have a much better lifespan, although anything that you use that is combined with flux will suffer a greatly shortened lifespan. That includes your furnace lining. Truth be told, the clay dishes are the best choice.

So I'm likely using thinwalled clay/porcelain then.
Coors? They're extremely sensitive to thermal shock, so handle with great care. I personally avoided them for the most part.

Do investigate the small clay dishes I recommended, when you get a chance. They're made from a white clay and are available in a few sizes. They even offered white clay crucibles at one time. Don't know if they still do.

You can melt ten ounces of silver in the largest dish. They used to be marketed under the Vigor name. Don't know if they still are----it's been more than 15 years since I bought any. I used to use them for all my melting, including inquartation and the melting of pure gold. For the money, they are the best possible melting dish available, and have a reasonable lifespan. An added benefit is they're very easy to clean when they're coated with dirty flux. If you get involved with one and need that operation, ask on the forum. It has been well discussed previously, but I'm always happy to post on the subject again. Makes it easier for the new guys to learn some of the useful tricks.

Let us know how the silver goes. You should end up with some nice buttons.

Harold
 
Many years ago when I was looking into investment casting of silver I heard several individuals talk about floating carbon (charcoal) on top of their silver in a crucible while in their furnace to create a barrier between the molten silver and any possible oxygen in the furnace. They would use a bar across the lip of the crucible to skim off the carbon when pouring to give themselves a clean pour without the oxidation problems. I should add that I never did this myself but thought it might be useful commentary considering Harold was talking about the problem of molten silver wanting to oxidize.
 
Oz said:
Harold was talking about the problem of molten silver wanting to oxidize.
Actually, it doesn't oxidize, but absorbs nine times its volume in oxygen, expelling it as it solidifies. In a sense, the silver is not effected. It can, however, be the cause of a very rough and irregular surface on cast silver.

The floating carbon prevents the oxygen from entering the molten silver. The addition of even a small amount of copper also controls the oxygen problem-------but that's not a solution for the guy that wants to cast pure silver.

I think Hoke talks about this very subject. Not sure. My copy is not at hand, and I have not looked at the download as of yet. Readers-----what does Hoke say about silver and oxygen?

Harold
 
You can really cut down on how much oxygen the silver absorbs by melting it in a rich furnace atmosphere. Be exceptionally careful doing this--by richening the flame, you produce carbon monoxide. I can vouch that it works quite nicely.


Harold, if you're talking about those white, fused silica crucibles that are bone white, then yes indeed, those are premium! They work well for gold, silver, and palladium. Platinum is a bit much for them though.
 
Lou said:
Harold, if you're talking about those white, fused silica crucibles that are bone white, then yes indeed, those are premium! They work well for gold, silver, and palladium. Platinum is a bit much for them though.
No, the dishes I speak of are just clay. They do not have the characteristic straight sides and thick bottom of the ones you reference. Fact is, they look very much like small Coors evaporating dishes, only thicker.

I used the type you're talking about for melting my platinum, what little I melted, anyway. It was important to remove the button immediately upon solidifying, otherwise the melted silica would trap the button in the dish. The problem with this type of melting dish is they don't have a large capacity, unlike the larger sizes of the one I mentioned.

I pickled the recovered button in hydrofluoric acid once removed. I had a small platinum dish that served the purpose. Aside from killing a wart, I can't think of another use for the acid, which I still have.

Harold
 
Success !!!

I have before me now a bead of shiny silvery silverish silver! :p
Weight: 9 grams.

What is the black smoke coming from the hot crucible? Silver vapor?

AgCl + borax = bubbling/foaming + Cl + Ag seems to happen at about 800°C, crystallising out metallic silver until temperature rises enough to melt the produced silver.

What's so special about the borax that it helps consolidate the silver into a metallic state? Can anyone explain the physics of that to me?

By the way, all my AgCl was dry.
The lump I have sound like a brick if tapped and I scrape off the amounts I test with (pretty easy) why does everyone insist to keep it wet?

Other crucibles I found in my stock: Stainless steel.
What are the up and downsides of these?
 
Sylar said:
Success !!!
I have before me now a bead of shiny silvery silverish silver! :p
Weight: 9 grams.
Congratulations! I knew you could do it! :wink:

What is the black smoke coming from the hot crucible? Silver vapor?
I don't have a clue. Unless you are overheating the silver, there shouldn't be any vapor coming from it, but you may be seeing other contaminants burning off. If you failed to wash the cemented silver well, you may be seeing some of the aluminum that was included. GSP or Lou may have an idea what you're seeing.

AgCl + borax = bubbling/foaming + Cl + Ag seems to happen at about 800°C, crystalising out metallic silver until temperature rises enough to melt the produced silver.
I'm short on the chemistry end of things, so I'm at the mercy of what I read, and don't often understand everything even then. In that regard, one of the things I have been lead to believe is that if you heat silver chloride without a reducer, it doesn't convert to elemental silver well, and much of it is lost as vapor, although I have no clue what form it might be.

Regards the above statement, I stand to be corrected, and the subject has come up before, but the evidence at hand, the result of using soda ash in that capacity, is hard to disprove.

I gathered from my research, long ago, that soda ash acts as a reducer. I've been told I'm wrong, but something has to explain the fact that you can heat an old contaminated dish that is heavily coated with borax, then add some soda ash to the mix and end up with metal that wasn't there prior to the addition of the soda ash.

As it mixes with the dirty borax, you start seeing bits of metal appear. Eventually the color of the dirty borax is much lighter, and a nice sized button of metal has appeared. That, to me, indicates that the soda ash is reducing the oxides that were absorbed by the borax. If I'm wrong, I stand to be corrected, but it will take a serious amount of lecturing to make me see it differently, considering I recovered well over 200 ounces of gold from waste materials, along with a like amount of silver, using that process.

What's so special about the borax that it helps consolidate the silver into a metallic state? Can anyone explain the physics of that to me?
Technically? No, I can't. But it does absorb impurities that might otherwise prevent the silver from fusing. It also "lubricates" the silver, allowing it to flow freely. You'll have to trust the input from others that understand chemistry far better than I do if that isn't correct, or isn't enough information. I gave it my best shot.

By the way, all my AgCl was dry.
The lump I have sound like a brick if tapped and I scrape off the amounts I test with (pretty easy) why does everyone insist to keep it wet?
To prevent you having to scrape off what you need! My normal process was to allow it to accumulate in a jar, even if it got well darkened by exposure to light. No big deal, all I was going to do eventually was recover the silver. By keeping it wet you can readily handle the material. I see no advantage to storing it dry----and scraping it to get it in condition to be reduced seems like a lot of wasted effort to me. I say keep it wet, if for no other reason, convenience. Others may have a different viewpoint. Maybe they'll chime in.

Other crucibles I found in my stock: Stainless steel.
What are the up and downsides of these?
For melting?

BIG MISTAKE!

Molten metals are strong solvents of other metals. That means that what you melt in a stainless vessel is likely to dissolve a portion, or all of the vessel used for melting. If you were to flux, you'd also solder the lot to the vessel. Assuming it was heated long enough, you'd dissolve the wall to the point where you'd spring a leak, losing your values. You'd also contaminate the values because of the dissolution of the vessel. Makes no sense to purify a metal, only to contaminate it with something different when melting.

You can dissolve gold in molten lead and never get near gold's melting point. Same applies to other elements. Do all your melting in non-metallic vessels. Even if they don't dissolve, they tend to solder to the vessel.

Harold
 
Erm, you shouldn't just melt silver chloride by itself?! That is quite hazardous to your health and it just wastes your silver! You'd have better luck adding it to NaOH, filtering the Ag2O and melting that; that works a treat. Just fusing silver chloride with borax won't do much. Some of the silver disproportionates to silver and chlorine gas (so small you won't see the yellow), but the rest of it goes up as that black smoke, because it has a vapour pressure. Bad idea.

Your best bet is to try sodium carbonate, charcoal, and silver chloride. The sodium carbonate doesn't act as reducer in the traditional sense, because carbon's oxidation state remains the same. Instead, it serves to soak up the chloride off the silver, which forms table salt, which melts and volatilizes. There are some free electrons floating about that hit the Ag+ and reduce it; this can come from the chloride (silver is strong enough an oxidizer at those temperatures to oxidize chloride to chlorine). Anyway, really bizarre things happen at those temperatures. Pyrochemistry is never easy to define.

You can also use some copper or iron pieces to cement your silver chloride to silver metal. I prefer the use of NaOH because it will leave fine silver that isn't contaminated after melting (because the sodium volatilizes off).


Lou
 
Here's 2 patents for the reduction of silver chloride to silver metal, both by Kunda - 4306902 and 4388109. They require blending anhydrous sodium carbonate intimately (requires grinding together - he does it dry in a blender) with silver chloride and heating to a temperature of between 1058 F and 1157 F. If held within these limits, there is little, or no, attack on the crucible. Silver powder (finely divided), mixed with sodium chloride results. The sodium chloride is finally leached out with hot water. In practice, for 143 grams of silver chloride, 106 grams of anhydrous sodium carbonate is needed - 1 mole Na2CO3/1 mole AgCl. In the second patent, it took one hour to completely convert, at the required temperature.

I attended an IPMI refining seminar in 1980. One of the speakers gave a talk on silver chloride conversion and he felt that the Kunda method was the best. Both Harold and I have the same IPMI book that contains the papers from that seminar.

I have never attempted this method, but I have always felt that it sounded viable. The temperature range is critical. So is the intimate blending of the 2 chemicals.
 
Ah, so the black vapor was actually AgCl ... makes sense.

I tried with the Na2CO3 as well and that DOES produce far better results! Mind you I'm working on lab scale, yesterday's output was 30 grams silver.

I just figured that, while adding AgCl to the crucible and Cl2 coming out of it that I must be getting full conversion to Ag. When pouring the molten mass in a carbon dish I immediately saw it was no metal at all. So the chlorine I saw/smelt must have been "free chlorine" in the AgCl. I also read that there exist many forms of AgCl with all slightly varying chlorine contents, so perhaps +1000°C reverts them all to basic AgCl?

Anyway, I placed the still hot black mass in the crucible again on high heat and slowly started adding Na2CO3. It wasn't quite anhydrous, but there wasn't enough crystal water to make for much problems. Bubbling and foaming resulted with some close calls on the crucible level, but steady adding and careful control made out to a nice yield anyway.
The still unconverted AgCl was separated from the metal and saved for the next conversion run.

Meanwhile I have a bigger batch of AgCl under low concentration HCl (5-10%) with a piece of Al standing somewhere it can't do much harm. Next test on wet conversion will use some sulfuric acid and iron, since I'm fresh out of HCl.
What I still had was barely enough to neutralize the ammonia in which I tried dissolving some AgCl, only to come to the conclusion that 10% ammonia solution can't really dissolve much AgCl weight per weight.

Knowledge and experience where gathered, with some nice shiny Ag as reward! Life is good 8)
 
Lou,

I read somewhere, recently, that the reaction between the silver chloride and sodium carbonate, at elevated temperatures, is not a true reduction reaction, even though the silver is obviously reduced. I have looked for the source of that information, but can't find it. Here's the equation:

4AgCl + 2Na2CO3 = 4Ag + 4NaCl + 2CO2 + O2

I don't remember exactly what they said, but I think they said it was an exchange reaction, after the CO3 was broken down by the heat.

What is your take on this, Lou?
 
Harold_V said:
Technically? No, I can't. But it does absorb impurities that might otherwise prevent the silver from fusing. It also "lubricates" the silver, allowing it to flow freely. You'll have to trust the input from others that understand chemistry far better than I do if that isn't correct, or isn't enough information. I gave it my best shot.

Harold

Oh, but you could!

Borax is a flux, lowering the surface tension allowing the silver to flow (flux means flow).

It's also a rather efficient solvent for many metal oxides, removing them and forming a glassy compound that's low melting and immiscible with the silver.


@Sylar:
The small nodules (for lack of a better word) on your silver is a result of oxygen liberating from the solidifying silver, making the silver "foam".
 
Looks explainable ...

4AgCl + 2Na2CO3 = 4Ag + 4NaCl + 2CO2 + O2

So what happens at about 1000°C:
Na2CO3 can't exist at such high temperatures:
Na2CO3 => Na2O + CO2
CO2 escapes the reaction at a fairly early stage, starting decomposition at about 400°C. So this is what remains of the equation:

4AgCl + 2Na2O => 4Ag + 4NaCl + O2
The O2 probably gets trapped in the Ag, and the NaCl start to vaporize off ... getting rid of it all with take some time because boiling point is 1413°C according to the msds I found.


Thanks for explanation on borax Peter. I often forget about the solvent properties at such high temperatures. My thermocouple was the first victim of that. It's about 2 cm shorter from stirring the melt with it :roll: Thank god the silver produced a good solder connection or my temp reading would have been useless until I got a new one. On the downside, my silver is now alloyed with some stainless steel and a bit of Ni and Cr. I guess I'll get rid of those in the series of experiments utilizing electrical current.
 
goldsilverpro said:
Lou,

I read somewhere, recently, that the reaction between the silver chloride and sodium carbonate, at elevated temperatures, is not a true reduction reaction, even though the silver is obviously reduced. I have looked for the source of that information, but can't find it. Here's the equation:

4AgCl + 2Na2CO3 = 4Ag + 4NaCl + 2CO2 + O2

I don't remember exactly what they said, but I think they said it was an exchange reaction, after the CO3 was broken down by the heat.

What is your take on this, Lou?

GSP,

Here's how Loewen describes the details of the reaction:

4AgCl + 2Na2CO3 = 2Ag2CO3 + 4NaCl

and via thermal decomposition

2Ag2CO3 = 2Ag2O + CO2

and finally through further thermal decomposition

2Ag2O = 4Ag + O2

Steve
 
Either way is fine by me, as long as I get some silver in return for my efforts :)

I have taken a liking to the Fe and H2SO4 process for the reduction of AgCl, so everything will be treated this way. Strange thing though, when I stir, I get brown gasses (NOx) bubbling up from the reaction :?:

What could be responsable for this? Residual Ag(NO3)2 decomposing?
 
Steve, that makes much better sense and that is a reduction reaction.

I should've remembered that silver carbonate decomposes immediately to Ag2O. Ag2CO3 is an intermediate and will be transient, the silver oxide less so. The slowest step is forming carbonate from the silver chloride. This reaction is an excellent example of thermodynamics driving a reaction that normally wouldn't occur but for the high temperature. All of the products but the silver are volatile, two being gases, and one being darn close.

Still, it would be naive to assume that even that nice, proper equation is strictly what is happening.

Also, the silver oxide trick is apparently a very old one indeed. I have great success with it any time I do it, but researching into it, it was known by chemists 500 years ago! Still very convenient.
 
If the tarnished layer is not too thick, then there might be a more simple solution available for you to clean the crystals. The proposed method is to electrochemically exchange the sulphur between the silver and another metal.
This video should explain the principle http://www.youtube.com/watch?v=vGCx9HZwYBo :idea: , it works quite well, except for the surface appearance, it will not usually restore the polish.
 
A way to do this that produces no harsh vapors, and little or no hazardous waste is to mix the AgCl with sodium hydroxide (caustic soda) and use Hydrogen Peroxide (H2O2) to obtain the silver. The output (if using very clean AgCl is Ag, NaCl and water.
 
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