Formic acid reduction prior to cementation?

[lazersteve]

jsargent,

As an experiment dissolve a few grams of silver nitrate crystals in distilled water and add a copper bus bar.

If you get a reaction without adding nitric acid, you have your answer.

I've always felt like a little free nitric was required for the cementation reaction to work, but I have never had to add any nitric to complete the reaction.

If you are dealing with perfectly clear solutions of silver nitrate then you may be better served producing silver carbonate and decomposing it to silver metal with heat. Lou has made a great post on this process.

Steve

 

[jsargent]

jsargent,

As an experiment dissolve a few grams of silver nitrate crystals in distilled water and add a copper bus bar.

If you get a reaction without adding nitric acid, you have your answer.

I've always felt like a little free nitric was required for the cementation reaction to work, but I have never had to add any nitric to complete the reaction.

If you are dealing with perfectly clear solutions of silver nitrate then you may be better served producing silver carbonate and decomposing it to silver metal with heat. Lou has made a great post on this process.

Steve

Well that's an experiment I'll most likely not ever do since it involves evaporating silver nitrate solution to dryness. Having done that once is enough. So does the reaction proceed or not? If it does, I have to wonder what motivates the copper ions to leave the bar to displace the silver ions from their nitrate embrace, if there's no oxidizer driving the reaction? Anyhow that's all theoretical and just fun to know but my real question is this: When you are doing a silver cementation using copper and at the end of the day the damn thing is sitting there as green as Mother Ireland, what do you do to achieve the lovely blue of the completed reaction? I know sometimes just letting it sit overnight is sufficent, but I am impatient. There's gotta be a way to drive the reaction more efficently. I suspect powdered copper would speed things up, but it's expensive, and copper powder I have produced from reducing copper nitrate solution rapidly oxizes to copper oxide, which is useless for cementation, as far as I can tell. I think the whole problem is the difference in surface areas between the silver that cements and the active surface of the copper bars. I've used sheet copper but that's unworkable as the sheet dissolves away in strips and clumps which contaminate the silver powder. Solid thick bars or fine powder seem to be the only way to go.

 

[peter i]

Well maybe I'm not using the term "saturation" properly. In any case here is what I observe: Let's say we start with a couple gallons of pefectly clear siver nitrate containg 100 ounces of silver in solution. Obviosly the solution is acidic with free nitric acid. Immediately up placing copper bars in the solution, silver begins to cement out. The problem is that the resulting surface area on the silver curds as they fall away from the copper, far exceed the surface area of the copper bars so the silver rapidly re-dissolves. After many hours of this, enough of the free nitric is consumed by the copper to bring to solution to near saturation, i.e. no more copper will dissolve. However, if this point is reached prior to all the silver being cemented out, the reaction will stall and the color of the solution will be emerald green, indicating abundant silver still in solution, rather than the cobalt blue indicating a completed cementation reaction. At this point I always have to add a bit more nitric to complete the process by dissolving more copper to complete the ion exchange. Hence, my original question regarding AuBro's formic acid post.

If that is how you do it, you are doing it wrong.

Always start with less acid than you expect will be needed. Let the reaction run to completion, that is, no more bubbles from the remaining silver even if you heat the beaker.
Pour of the solution and put it somewhere safe (it is metal nitrates (primarily silver-, copper- and zinc- with a trace of nitric acid)

Add fresh acid to the remaining silver, but still not more than what you would expect to be a stoichiometric amount, and let it dissolve.

The last couple of slivers I just save for the next batch. They are to good to throw away, but they are not worth the time it takes to dissolve them separately.

Now you can use whatever method of reduction you prefer, and there is only a little bit of nitric acid left to annoy you.

In this way you save nitric acid and you save reductant, both of which equals money.

 

[Harold_V]

Well that's an experiment I'll most likely not ever do since it involves evaporating silver nitrate solution to dryness.

No, it doesn't. You can grow all the silver nitrate crystals your heart desires simply by dissolving silver so the solution is heavily concentrated. Do it hot, then as it cools, you can actually watch the crystals grow.

Harold

 

[jsargent]

Well that's an experiment I'll most likely not ever do since it involves evaporating silver nitrate solution to dryness.

No, it doesn't. You can grow all the silver nitrate crystals your heart desires simply by dissolving silver so the solution is heavily concentrated. Do it hot, then as it cools, you can actually watch the crystals grow.

Harold

You're right of course. Supersaturation at heat followed by cooling would certainly do the trick.
I have a better handle on the cementation chemistry now. It's not so much that it's pH dependent, as the ions will exchange at the interface of the silver nitrate solution and the solid copper, the reaction being independent of the amount of copper in solution per se and relatively independent of the pH. It's the active process of copper going into solution that affords the opportunity for the metal ions to exchange. A slight excess of nitric seems to facilitate the process if my observations are correct. Too much though and the silver tends to re-dissolve leading to the bothersome green solution and stalled reaction. Thanks.

 

[Lou]

Silver is not going to redissolve even in the presence of free nitric acid (the protons of which are requisite to the quick reduction) as long as copper is present.

I can have a gallon of nitric acid with 1 gram of silver and 10 grams of copper and at no point will the silver dissolve until the copper is consumed. This is the same concept behind using magnesium or zinc as sacrificial anodes.
EDIT: Oz PM'd me with an objection: he said that in an unstirred solution, he has seen silver crystals fall off the copper and then dissolve in solution. The only way this should be possible is if the solution is not homogeneous. If it is not stirred, the high concentrations of copper ion would prevent silver from cementing. Any silver dissolving while copper is still present would be a local phenomenon in an unstirred, diffusion-controlled system.

I suppose I should have said that any alloy of silver and copper will have the copper dissolve preferentially to the silver.

The green color that you see is not from silver redissolving, nor is it even indicative of a stalled reaction--this solution you see any time copper ions are in an acidic, oxidizing milieu. Try dissolving copper in conc. HCl with some hydrogen peroxide added--it too will be emerald green like nickelous nitrate. Now add copper powder to it (a reducing agent). You will get a white-gray precipitate of cuprous chloride and the solution will turn to that pretty blue.

 

[jsargent]

Silver is not going to redissolve even in the presence of free nitric acid (the protons of which are requisite to the quick reduction) as long as copper is present.

I can have a gallon of nitric acid with 1 gram of silver and 10 grams of copper and at no point will the silver dissolve until the copper is consumed. This is the same concept behind using magnesium or zinc as sacrificial anodes.
EDIT: Oz PM'd me with an objection: he said that in an unstirred solution, he has seen silver crystals fall off the copper and then dissolve in solution. The only way this should be possible is if the solution is not homogeneous. If it is not stirred, the high concentrations of copper ion would prevent silver from cementing. Any silver dissolving while copper is still present would be a local phenomenon in an unstirred, diffusion-controlled system.

I suppose I should have said that any alloy of silver and copper will have the copper dissolve preferentially to the silver.

The green color that you see is not from silver redissolving, nor is it even indicative of a stalled reaction--this solution you see any time copper ions are in an acidic, oxidizing millieu. Try dissolving copper in conc. HCl with some hydrogen peroxide added--it too will be emerald green like nickelous nitrate. Now add copper powder to it (a reducing agent). You will get a white-gray precipitate of cuprous chloride and the solution will turn to that pretty blue.

Lou-Thanks for clearing up the nature of the green solution. Oz is exactly right about the silver re-dissolving in an unstirred solution. I've seen it many times as I'm doing my silver digests in heated but unstirred 5 gallon buckets. The solution starts out on the acidic side due to it being solution from digest of inquarted silver. I think I'll use some sacrificial silver on the clear silver nitrate solution after the gold residue is removed to eat up any excess nitric acid, rather than allowing the copper to get eaten up by the free nitric. At least that way I can recover nearly 100% of the silver as opposed to copper which just ends up as rather useless copper nitrate.
Only thing I'm not clear on is why, if the solution turns green, it tends to stay that way and the cementation reaction apparently stalls? But, if I add just a touch of nitric, it sets things in motion again and the reaction completes. Seems counterintuitive.
Thanks again to you, Harold and Oz for the help.

 

[Oz]

Jsargent,

If you have a silver nitrate solution with excess nitric adding extra silver to consume the nitric is the way to go as peter i pointed out above. It is better still not to use too much nitric in the first place however, as nitric can be pricey. By consuming your extra nitric before cementation you will no longer have the problem of your silver redissolving or your reaction stalling in my experience. Not having excess nitric will also mean that you will only need 1 gram of copper for every 3.4 grams of silver you need to cement.

Have fun, be safe!

 

[jsargent]

Jsargent,

If you have a silver nitrate solution with excess nitric adding extra silver to consume the nitric is the way to go as peter i pointed out above. It is better still not to use too much nitric in the first place however, as nitric can be pricey. By consuming your extra nitric before cementation you will no longer have the problem of your silver redissolving or your reaction stalling in my experience. Not having excess nitric will also mean that you will only need 1 gram of copper for every 3.4 grams of silver you need to cement.

Have fun, be safe!

Absolutely. I forgot to thank Peter i too and the rest of those who took the time to answer.

The Devil's in the details as they say. It's one thing to say "Just put a copper bar in your silver nitrate and the silver will cement out." and quite another to do it exactly the right way so as to minimize problems and maximize profit.

 

[skippy]

Silver is not going to redissolve even in the presence of free nitric acid (the protons of which are requisite to the quick reduction) as long as copper is present.

I can have a gallon of nitric acid with 1 gram of silver and 10 grams of copper and at no point will the silver dissolve until the copper is consumed. This is the same concept behind using magnesium or zinc as sacrificial anodes.
EDIT: Oz PM'd me with an objection: he said that in an unstirred solution, he has seen silver crystals fall off the copper and then dissolve in solution. The only way this should be possible is if the solution is not homogeneous. If it is not stirred, the high concentrations of copper ion would prevent silver from cementing. Any silver dissolving while copper is still present would be a local phenomenon in an unstirred, diffusion-controlled system.

Lou, I'm wondering if your discussed principal of silver not redissolving will extend to something I was curious about. Say you have a noble metal solution in an iron container, if you were to put a zinc block into the container, would the noble metal precipitate solely on the surface of the zinc rather than the iron container?

 

[Lou]

In theory, yes. In a perfect, idealized situation, yes. For the most part, yes. Zinc is more active than iron, so it should preferentially dissolve.

You guys all ask the hard questions.

 

[jsargent]

Ok, Lou, Oz or anyone who has dealt with this problem, I'm all ears.

Here's the deal: I've run a little experiment whereby I inquarted 10 ounces of .999 silver to one ounce of .999 gold, dissolved the silver with a 30% solution of nitric acid, and collected 100% of the gold as the nitric acid insoluble residue. So far so good....

I used a minimum amount of nitric and at the end of the digest I sacrificed part of an additional ounce of silver to consume any residual nitric.

The clear silver nitrate solution was then about pH 4.5 to 5.0

Placed two copper bus bars in the solution and as one would expect, silver began to drop nicely and continued to drop for several hours.

THEN, as happens every time I do this, the solution turns green, the reaction appears to slow down or stop, and some of the silver appears to go back into solution. The pH is unchanged. The copper bars appear to be passivated by some sort of dark residue, not the bright color of clean copper.

The bars are 1 inch wide x 3/16 inch thick electrical copper which I assume are as pure as one can commonly get.

The solution is at room temperature and unstirred, though I give it a manual stir periodically.

So what's happening? The green solution will now sit for days and never turn the blue of a completed cementation reaction.

If I add a touch of nitric to the green solution, it will turn blue, and the reaction seems to proceed, but that seems counterintuitive, as all of you have mentioned that cementaion reactions are not dependent on pH or liberation of copper ions, which makes sense.

What doesn't make sense is why this happens without fail and what I can do about it.

 

[4metals]

I think you are passivating the copper by lack of movement of the solution. I usually have the shops I set up build one of these to hold the bars and move the solution. Makes taking them out easier too.

 

[Oz]

Have you taken a small sample of your green solution and tested for silver that is uncemented with a drop of HCl?

 

[jsargent]

I think you are passivating the copper by lack of movement of the solution. I usually have the shops I set up build one of these to hold the bars and move the solution. Makes taking them out easier too.

Great design! I used similar airlift systems in my fish farm. I love it.

 

[jsargent]

Have you taken a small sample of your green solution and tested for silver that is uncemented with a drop of HCl?

No, but I will. Good idea.

What I have done is taken the green solution, put a clean copper bar in it, added a little nitric and watched the solution turn blue and silver cement out in considerable quantity.

Maybe 4metals air lift stirrer will help.

 

[qst42know]

I think you are passivating the copper by lack of movement of the solution. I usually have the shops I set up build one of these to hold the bars and move the solution. Makes taking them out easier too.

I didn't see any indication of holes at the top?

Rather than rising and falling within the same tube wouldn't the solution flow better if you had an opening at the top allowing the bubbles and solution to flow out. Like an under gravel aquarium filter.

http://images.google.com/imgres?imgurl=http://ecology.pnl.gov/Rivers/Aquarium/filter.jpg&imgrefurl=http://ecology.pnl.gov/Rivers/Aquarium/Aqua.html&usg=__ngJcSdW8MtaydyEMGj-2F8y0tQ0=&h=446&w=500&sz=125&hl=en&start=4&um=1&itbs=1&tbnid=v75cPbA4QxsRoM:&tbnh=116&tbnw=130&prev=/images%3Fq%3Dundergravel%2Bfilter%26hl%3Den%26rlz%3D1T4ADBF_enUS241US242%26sa%3DN%26um%3D1

 

[4metals]

The refiners I deal with cement out their waste when it is a certain volume in their holding tanks, we just make it short enough so the liquid flows out the top. Holes would do the same thing unless you stand the thing on the bottom and the holes are above the acid. The air stream also helps to keep the precious metals from accumulating on the copper, they coat it but usually break off from the air flow. The pipe is removed and after settling for a few hours the acid can be decanted.

 

[qst42know]

That makes sense, I just assumed the hand grip holes were above the solution level.

I suppose the hand grip holes could be above the solution level if you added a PVC tee at the top.

 

[jsargent]

The refiners I deal with cement out their waste when it is a certain volume in their holding tanks, we just make it short enough so the liquid flows out the top. Holes would do the same thing unless you stand the thing on the bottom and the holes are above the acid. The air stream also helps to keep the precious metals from accumulating on the copper, they coat it but usually break off from the air flow. The pipe is removed and after settling for a few hours the acid can be decanted.

4metals I was wondering if you could tell me what the refiners you deal with typically do with their copper nitrate waste?