Formic acid reduction prior to cementation?

[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.

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.

 

[goldsilverpro]

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.

 

[4metals]

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.

 

[jsargent]

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.

 

[jsargent]

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.

 

[g_axelsson]

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?

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