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Author Topic: Why nitric and not sulfuric acid for gold recovery?  (Read 1297 times)

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Zolotov

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Why nitric and not sulfuric acid for gold recovery?
« on: March 05, 2017, 02:27:34 PM »

When extracting gold from gold fingers (PCI plated connectors of a PCI card) everybody uses Nitric acid. Why not to use Sulfiric acid? Sulfuric acid is cheaper and it is more stronger. Can someone explain it to me?

TIA
Zolotov
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Arkcon

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #1 on: March 05, 2017, 03:21:00 PM »

Are you sure you've thought this all the way through?

Quote
Sulfuric acid is cheaper and it is more stronger.

I don't think the second part is true.  Why do you think so?  Does "stronger" mean better for the application you have in mind?
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AWK

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #2 on: March 05, 2017, 04:12:32 PM »

Did you check reaction of gold with acids in  texbook?
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Enthalpy

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #3 on: March 06, 2017, 06:36:49 AM »

Most acids don't corrode gold because the redox potential prevents H+ to rip an electron from Au. Check that
https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)
where you see a positive potential (especially gold), the metal stays neutral and the hydrogen ionized. A bit simplified.

Nitric acid has additional properties. Notably, it's a oxidizer.

I persist to think that you don't have by far the minimum knowledge to conduct these operations, which are dangerous.
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Zolotov

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #4 on: March 07, 2017, 06:09:40 AM »

Did you check reaction of gold with acids in  texbook?
You didn't understand the question fully and you start teaching, please read carefully next time. Neither nitric acid dissolve gold. When gold fingers are treated with nitric acid, everything is dissolved except gold. Then you get pure gold. So, my question goes like this: why not replace nitric with sulfuric, if it is cheper. Everybody knows that gold is not dissolved in nitric or sulfuric acid.
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Zolotov

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #5 on: March 07, 2017, 06:12:01 AM »

Most acids don't corrode gold because the redox potential prevents
seems that this forum is full of guys eager to look like they know chemistry, totally misunderstanding what is being asked
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AWK

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #6 on: March 07, 2017, 06:30:52 AM »

You cannot extract gold using nitric acid. Gold reacts only with aqua regia (mixture of HNO3 and HCl). That your knowledge of chemistry is rather poor yet.
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Borek

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #7 on: March 07, 2017, 07:36:36 AM »

You cannot extract gold using nitric acid. Gold reacts only with aqua regia (mixture of HNO3 and HCl). That your knowledge of chemistry is rather poor yet.

That's was completely unnecessary and uncalled for, especially after OP explained what he means.
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Borek

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #8 on: March 07, 2017, 07:42:02 AM »

seems that this forum is full of guys eager to look like they know chemistry, totally misunderstanding what is being asked

To be honest, your question wasn't fully clear and I misunderstood it at first as well, so it is at least partially effect of the way you worded your post.

My bet is that a strong oxidizing acid is better as it will not only dissolve everything sulfuric acid can, but also some of the things like copper, which sulfuric acid wouldn't touch.

Plus, sulfuric acid has a nasty tendency to char organic matter, and what is left is often quite difficult to get rid of.
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AWK

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #9 on: March 07, 2017, 08:45:05 AM »

For extraction (not leaving unchanged) of gold in large scale from electronics selectively mostly cyanide method is used. This methods dissolve gold and silver almost selectively in seconds. Even clinsing with aqua regia in seconds can gives concentrate of gold. The newest selective green methods use organic acids with oxidizer (probably concentrated hydrogen peroxide and some additives). These methods are keep in secret, even are not patented. Using huge amounts of nitric or sulfuric acid for dissolving all metals but gold is a nonsense, even in small scale, and far from cost-effective. 1 kg of pure so called gold finger contains ~5 g of gold.
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Enthalpy

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #10 on: March 07, 2017, 12:04:08 PM »

Oops, I mixed Zolotov up with richtronix, who asked similar questions few days ago.

Do I understand properly now that you want to degrade the epoxy, and perhaps the copper, but not the gold?

Each "Material compatibilty list", for instance that one
http://www2.emersonprocess.com/siteadmincenter/PM%20TopWorx%20Documents/TW_ChemicalCompatibility_chart.pdf
tells about the same: epoxy resists sulphuric acid but not nitric acid.

It's more or less the same with copper, while gold resists both acids.

I'm not so pessimistic about acid costs. In a recent discussion I estimated that an AGP video card contains about 3€ gold. Its part bearing the gold-plated contacts weighs some 3g, so a few 10g acid should suffice, which is affordable.
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Miles

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Re: Why nitric and not sulfuric acid for gold recovery?
« Reply #11 on: March 11, 2017, 05:25:04 AM »

I think that is because nitric acid readily dissolves copper while only hot concentrated sulfuric does so.

Some use ferric chloride which also dissolves copper.
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