# Formic acid reduction prior to cementation?



## jsargent (Jan 15, 2010)

I ran across a post here a while back suggesting the use of formic acid to reduce silver nitrate solution prior to copper cementation. I'm confused by this. It seems a slight excess of nitric acid is needed to dissolve the copper for the replacement reaction to proceed, so how would the reaction progress if the solution were reduced?


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## Irons (Jan 15, 2010)

I think it was 4Metals that had a post on reducing Silver Nitrate with Sodium Formate. It gives you Silver metal.

I saw that post too, but failed to comment on it earlier.


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## goldsilverpro (Jan 15, 2010)

Here's 4metals' posts on the subject:
http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=1275&hilit=formate

He does no cementation on copper. The silver is dropped directly from the silver nitrate solution, after adjusting the pH on both the silver nitrate solution and the sodium formate solution.


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## Irons (Jan 15, 2010)

goldsilverpro said:


> Here's 4metals' posts on the subject:
> http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=1275&hilit=formate
> 
> He does no cementation on copper. The silver is dropped directly from the silver nitrate solution, after adjusting the pH on both the silver nitrate solution and the sodium formate solution.



Looks like I better start collecting Ants come Spring.


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## jsargent (Jan 15, 2010)

Irons said:


> I think it was 4Metals that had a post on reducing Silver Nitrate with Sodium Formate. It gives you Silver metal.
> 
> I saw that post too, but failed to comment on it earlier.



No it wasn't 4Metals or any of the "regular suspects". I should've made a note of the particular post. Now it's adrift in the data-sea Noxx has created here. 
Anyhow, I'm puzzled by the formic acid reduction thingy, as I mentioned before.


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## Irons (Jan 15, 2010)

jsargent said:


> Irons said:
> 
> 
> > I think it was 4Metals that had a post on reducing Silver Nitrate with Sodium Formate. It gives you Silver metal.
> ...



Try This:

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=1275#p39590


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## Oz (Jan 16, 2010)

Chuckle, it most certainly was 4metals. It sure got my attention as I run a good bit of silver and read that out of nowhere from someone that was making their debut here. 4Metals has become a great asset to the forum.


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## Palladium (Jan 16, 2010)

Irons said:


> Looks like I better start collecting Ants come Spring.




Reckon anybody else got that Irons ? :lol:


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## Oz (Jan 16, 2010)

I had an unnamed member here tell me to go pinch ant butts once upon a time in relation to 4metals reduction method. It was the first I made the connection, but this solution is right up Irons alley. I like how he keeps it basic when possible and it never hurts to have an alternative to the chemical suppliers.


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## Harold_V (Jan 16, 2010)

Oz said:


> 4Metals has become a great asset to the forum.



So much so that I rarely have need to say anything. He is an excellent addition to the forum.

Harold


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## shadybear (Jan 16, 2010)

So how many ant butts does it take to reduce 2 liters of silver nitrate
and does it matter what size the ant butt is? Would big butt ants work better?


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## Irons (Jan 16, 2010)

Palladium said:


> Irons said:
> 
> 
> > Looks like I better start collecting Ants come Spring.
> ...



More than I expected. :lol: :lol:


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## lazersteve (Jan 16, 2010)

Ant's produce formic acid in their stingers. 

Some birds actually irritate ants on purpose to get the ants to spray their feathers down with formic acid for protection from parasites.

Steve


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## markqf1 (Jan 16, 2010)

The birds may like it but the fish will die quickly from it.
After a good hard summer thunderstorm, I've seen alot of fish floating with fireants on their gills.

Mark


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## jsargent (Jan 16, 2010)

Oz said:


> Chuckle, it most certainly was 4metals. It sure got my attention as I run a good bit of silver and read that out of nowhere from someone that was making their debut here. 4Metals has become a great asset to the forum.


Chuckle... no it was not 4Metals. It was AuBro. from this post (and quite an amazing post it was): http://goldrefiningforum.com/phpBB3/viewtopic.php?f=52&t=2259&p=19283&hilit=formic#p19283

What puzzles me is this statement AuBro makes and it puzzles me because I've never heard anyone recommend reduction prior to cementation:

"7) *Before any cementation, ALLWAYS, get rid of any Oxidizing agents (HNO3, HClO). *they will redisolve your valuables and you will be confused and waste precious chemicals. A nice reducing agent is Formic Acid (HCOOH). Once placed in hot oxidizing solution (HNO3) forms Carbon monoxide that goes into the air; leaving no anions in solution. VERY CLEAN. Stop when the solution stops bubling. It is harder to get though and maybe more costly, but worth the investment. Check around. Another One is Urea, however the endpoint for neutralization is harder to see; stops fizzing. If you do not have any of these, evaporate twice until almost dryness. Rehydrate with HCL 2% - 5%."

So tell how ion exchange between copper and silver is supposed to happen if the oxidizing agent (nitric acid) is removed? Unless the solution can saturate with copper, or some metal higher in the electromotive series than silver, one will never recover all the silver. Does AuBro mean to say just add enough reducing agent to LESSEN, but not entirely remove the nitric, in order to reduce the saturation capacity of the solution, or what?


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## jsargent (Jan 16, 2010)

> Try This:
> 
> http://goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=1275#p39590



No it wasn't that post it was this one: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=52&t=2259&p=19283&hilit=formic#p19283 (see item #7)

Though I must say the post by 4Metals on using formate INSTEAD of copper cementation may be superior still. Takes a long damn time to saturate with copper a silver nitrate solution containing 100 ounces of silver, especially when using scrap bus bars. Gets expensive too with copper being up in price.


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## Harold_V (Jan 17, 2010)

jsargent said:


> Takes a long damn time to saturate with copper a silver nitrate solution containing 100 ounces of silver, especially when using scrap bus bars. Gets expensive too with copper being up in price.


I don't have a problem with your comments about the price of copper, but I'm not sure you have it right with the point of saturation. There is no relationship between the percentage of copper in solution and the cementing of silver. There is an electron exchange that results in silver being cemented, and copper being dissolved. Take note that you can recover silver instantly with copper, from a solution of nothing but silver nitrate, with no dissolved copper present. 

You can recover 100 ounces very quickly. Simply present a large surface area to the solution. The greater the area, the faster the cementation. Use more, or larger, pieces of copper. 

Harold


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## Oz (Jan 17, 2010)

jsargent said:


> So tell how ion exchange between copper and silver is supposed to happen if the oxidizing agent (nitric acid) is removed?


This is the point with 4metals process, by removing the nitric you already have elemental silver and the copper cementation is not used.

I have not seen the thread you are referencing in a long time. I was trying to give credit where it was due as I thought you were referring to what 4metals had first posted here for precipitating silver from nitric.

In reading the post you are talking about it seems (at least to me) that formic acid was being suggested for the removal of oxidizing agents from AR prior to cementation on copper. It goes without saying that there would be no silver in solution with AR having formed a silver chloride precipitate if it was present. 

If you wished to use formic acid for precipitating silver from a nitric solution I would follow 4metals direction as it makes little sense to start with formic then change to copper cementation part of the way through. 

Hopefully you see where the confusion came from as no ill will was intended. At least we are all on the same page now.


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## jsargent (Jan 17, 2010)

Oz said:


> jsargent said:
> 
> 
> > So tell how ion exchange between copper and silver is supposed to happen if the oxidizing agent (nitric acid) is removed?
> ...


 I believe you're right Oz... the method posted by AuBro is probably in reference to AR solutions and cementing out gold, not silver. Totally agree about 4Metal's method being a sensible method to try. Thanks!


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## jsargent (Jan 17, 2010)

Harold_V said:


> jsargent said:
> 
> 
> > Takes a long damn time to saturate with copper a silver nitrate solution containing 100 ounces of silver, especially when using scrap bus bars. Gets expensive too with copper being up in price.
> ...


 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.


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## lazersteve (Jan 17, 2010)

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


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## jsargent (Jan 17, 2010)

lazersteve said:


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


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.


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## peter i (Jan 18, 2010)

jsargent said:


> 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.


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## Harold_V (Jan 19, 2010)

jsargent said:


> 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


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## jsargent (Jan 21, 2010)

Harold_V said:


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


 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.


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## Lou (Jan 21, 2010)

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.


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## jsargent (Jan 22, 2010)

Lou said:


> 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-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.


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## Oz (Jan 22, 2010)

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!


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## jsargent (Jan 22, 2010)

Oz said:


> 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.


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## skippy (Jan 29, 2010)

Lou said:


> 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?


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## Lou (Jan 29, 2010)

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.


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## jsargent (Jan 30, 2010)

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.


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## 4metals (Jan 30, 2010)

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.


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## Oz (Jan 30, 2010)

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


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## jsargent (Jan 30, 2010)

4metals said:


> 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.


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## jsargent (Jan 30, 2010)

Oz said:


> 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.


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## qst42know (Jan 30, 2010)

4metals said:


> 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


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## 4metals (Jan 30, 2010)

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.


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## qst42know (Jan 31, 2010)

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.


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## jsargent (Feb 1, 2010)

4metals said:


> 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?


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## jsargent (Feb 1, 2010)

4metals said:


> 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.



Passivation was indeed the problem. Kept things stirred today and the cementation proceeded without a hitch. Gracias!

Next step is to build your nifty little airlift stirrer.


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## goldsilverpro (Feb 1, 2010)

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



I'm not 4metals, but I'll tell you what we did with it. Most big places I worked in had the waste picked up. About 5-6 years ago, in Texas, a company picked up our waste acid for $.85/gallon and cyanide waste for $1.25/gallon - not at the same time!!. Of course, we had big volume - maybe 1200-2000 gal/month, or more. If I remember right, the company was Univar, a division of VWR. About the same time, the same company charged about $7/gallon in rural Missouri for acid waste. I guess the price depends a lot on where you are. Also, in Texas, we kept it in large storage tanks and the guy was there for less than an hour. In Missouri, it was in about 50 drums and they also hauled the empty drums away - it took them all day. In California, for years, an oil company picked up our acid waste for free. They supposedly, for some reason, flushed oil wells with it.


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## 4metals (Feb 1, 2010)

The shops I do work for run the entire spectrum. Some treat it and discharge to sewer. Others drop out the values and do the iron thing, melt the copper and use it for low grade refiners bars. They then drop the iron with mag ox and have it carted as a hazardous waste. The water is discharged.

The there are those who cement the values and have the waste acid hauled away. For a small shop, hold and haul is simple. The shops that do complete treatment need instrumentation for analysis and it gets complicated. The high salt content of the treated water makes it difficult to analyze via AA and even ICP has problems when aspirating all that chloride. 

With all you have to do to comply with the EPA today, hold and haul is the best bet. When you tell the EPA you're treating they crawl up your butt and stay there. Balanced quantities of acid in and waste out make them happy and they leave you alone.


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## jsargent (Feb 2, 2010)

goldsilverpro said:


> > 4metals I was wondering if you could tell me what the refiners you deal with typically do with their copper nitrate waste?
> 
> 
> 
> I'm not 4metals, but I'll tell you what we did with it. Most big places I worked in had the waste picked up. About 5-6 years ago, in Texas, a company picked up our waste acid for $.85/gallon and cyanide waste for $1.25/gallon - not at the same time!!. Of course, we had big volume - maybe 1200-2000 gal/month, or more. If I remember right, the company was Univar, a division of VWR. About the same time, the same company charged about $7/gallon in rural Missouri for acid waste. I guess the price depends a lot on where you are. Also, in Texas, we kept it in large storage tanks and the guy was there for less than an hour. In Missouri, it was in about 50 drums and they also hauled the empty drums away - it took them all day. In California, for years, an oil company picked up our acid waste for free. They supposedly, for some reason, flushed oil wells with it.


Thanks GSP. I buy my nitric acid at Univar so it seems only fitting I should pay them to haul it off when I'm done with it. LOL Now when I can afford the deposit on a 55 gallon stainless steel drum, then nitric costs go waaay down. They want over $800 now for the deposit. Dang.


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## jsargent (Feb 2, 2010)

4metals said:


> The shops I do work for run the entire spectrum. Some treat it and discharge to sewer. Others drop out the values and do the iron thing, melt the copper and use it for low grade refiners bars. They then drop the iron with mag ox and have it carted as a hazardous waste. The water is discharged.
> 
> The there are those who cement the values and have the waste acid hauled away. For a small shop, hold and haul is simple. The shops that do complete treatment need instrumentation for analysis and it gets complicated. The high salt content of the treated water makes it difficult to analyze via AA and even ICP has problems when aspirating all that chloride.
> 
> With all you have to do to comply with the EPA today, hold and haul is the best bet. When you tell the EPA you're treating they crawl up your butt and stay there. Balanced quantities of acid in and waste out make them happy and they leave you alone.


Hold and haul it is... I have an aversion to bureaucrats crawling up my butt.


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## g_axelsson (Apr 14, 2014)

skippy said:


> Lou said:
> 
> 
> > 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.
> ...





Lou said:


> 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.


It isn't often I have to disagree with Lou but in this case I have to.

Firstly, copper would not protect silver in a nitric acid solution unless it is in electrical contact. The protection of iron ships with zinc is dependent on contact and larger structures have several sacrificial anodes to give a good protection.

Secondly, I agree with Lou in "Zinc is more active than iron, so it should preferentially dissolve.", what I'm not agreeing with is that almost no silver would deposit on the iron.
I would expect zinc in contact with iron in an acidic environment would form a battery with the zinc being the negative electrode and iron the positive. The positive silver ions would cement on the iron electrode and deposit there, giving iron a positive potential compared to the zinc. If there is no electrical contact between the zinc and the iron vessel both zinc and iron would cement silver.
... and stuff is never as simple as you first believe, especially in electrochemistry. After further thoughts I realized that the potential of the zinc and iron would be almost the same so the silver would cement both on the iron and the zinc although it's only the zinc that goes into solution. Then I remembered that dissimilar metals in contact gets a voltage difference, the Volta potential and after some research I also found out that it is very hard to calculate as it depends on surface conditions of the two metals. So right now I don't know if either zinc or iron will get all the cemented silver but most probably it will cement on both, polarization effects on the surface will probably average out any finer details of the potential differences.

Oh, I just realized there is a good argument for silver cementing on the iron.
- When cementing silver on copper it soon gets covered in a fine silver moss. Any new silver deposits on the more noble silver, not on the surface of the copper.
- In the same way silver would deposit on the more noble iron (relative to zinc). Anyhow, after a few seconds it actually deposits on the fresh silver surface, not on the zinc or iron, turning this discussion into a moot point. :mrgreen: 

Göran


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