FeSO4 reaction

Hi,

Yesterday I precipitated gold from an aqua regia solution (roughly 100g/L Au and contaminated with some copper) using a saturated FeSO4 solution. Initially on adding the FeSO4 the solution took on the usual murky yellow-green-brown colour as gold dropped out but suddenly it turned very dark, almost black. I left the solution to settle out overnight in a loosely capped erlenmeyer flask: Today the solution was still very dark and I could see many small gas bubbles coming up from the solution. When the solution was filtered and exposed to the air the dark colour gradually vanished into a much lighter olive-green.

I decided to do a test with the spent solution and added a small amount of FeSO4 and the dark colour immediately came back. After a few minutes the dark colour started to fade from the top. There was also gas bubbles rising up from the bottom. After perhaps five minutes the colour was back to olive-green and there was a very small amount of black residue on the bottom. I have never seen this reaction before and can't really understand what is going on. Does anyone have an idea?

Spent solution:
View attachment 1

Added FeSO4:
View attachment 1

After a couple of minutes>


After five minutes>

Hi there.

At what points in the process did you Stannous test and what were the results? I know the answers to your questions however if you follow it though logically you'll benefit more.

anachronism said:

Hi there.

At what points in the process did you Stannous test and what were the results?

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As Jon said ?

to clarify ---

Initially on adding the FeSO4 the solution took on the usual murky yellow-green-brown colour as gold dropped out

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Did you test with stannous at this point - if so what where the results

Today the solution was still very dark and I could see many small gas bubbles coming up from the solution

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did you test with stannous again at this point - if so what where the results

I decided to do a test with the spent solution

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was this a stannous test

and added a small amount of FeSO4 and the dark colour immediately came back. After a few minutes the dark colour started to fade from the top.

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did you test with stannous again at this point - if so what where the results

There was also gas bubbles rising up from the bottom. After perhaps five minutes the colour was back to olive-green and there was a very small amount of black residue on the bottom.

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did you test with stannous again at this point - if so what where the results

Kurt

Your first two images are the same, this can cause a problem trying to diagnose your situation. While I think I know where Jon is going with his line of thought, and I agree with Kurtack's questioning as well, I would like to inject another concept. Since you have FeSO4 on hand, and it seems to be in the form of copper sulfate/iron sulfate. Why not at least test your solution with it before saturating the entire volume? This could be done while waiting for some stannous chloride.

I'm sorry but I can't post a reply. All I get is "Forbidden. Message seems to be spam."

FigiMon said:

I'm sorry but I can't post a reply. All I get is "Forbidden. Message seems to be spam."

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But, you just posted a reply... :?

What is the difference between what you were wanting to reply and the post you just made?

Dave

Yes, it's really weird. This is what I intended to post:

First of all, apologies for not replying earlier but I've been travelling and had no way to check in before now. Also, sorry I messed up the images.

Tests with SnCl were done before precipitation (very positive for gold - almost black) and after precipitation (negative - no gold left).

The thing I can't understand is the reaction I got where the gold-bearing aqua regia suddenly turned so dark on addition of the FeSO4. I have never seen this before. Usually I add about an equal volume of saturated ferrous sulfate to the gold solution (which I generally try to keep at a concentration of 100g/L solution) and the colour gradually goes from orange to murky yellow-brown as the gold comes out of solution and stays there. This time it flipped to a third state where most (all?) the gold had dropped but the solution went completely black or at least very very dark.

I was initially worried that some of the gold was left in solution and this was the cause of the reaction. However I have cleaned and thoroughly dried the precipitated gold and massed it. I got 74.8 g vs the expected 74.1 g. Also, as noted above, the solution tests negative with SnCl.

FigiMon said:

Tests with SnCl were done before precipitation (very positive for gold - almost black) and after precipitation (negative - no gold left).

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I'm really sorry but that doesn't answer what you've been asked by both myself and Kurt.

Can you provide that information please so we can best help you?

I have no explanation for the gas bubbles, but the color could be explained by the copper contamination.
Fe²⁺ + Cu²⁺ -> Fe³⁺ + Cu⁺
Cu⁺ is quite dark in solution while Cu²⁺ is emerald green. Oxygen from the air oxidizes the Cu⁺ into Cu²⁺ and makes the solution pale at the top after standing for a while.

Jon, you told me a month ago, add the green stuff, when the solution turns dark the gold is out. :wink:

FigiMon, if I'm correct then the solution should go pale after a while. You could add hydrogen peroxide to test my theory, this should clear it up quite fast.

Göran

Thanks for the replies.

@anachronism, @Kurt
I tested with stannous twice, before dropping the gold (adding FeSO4) and after the gold had dropped out (when the solution had turned black but was clearing up towards the olive-green seen in the last picture). I did not test at intermediate steps. The first test was positive, the second negative.

@Göran
That makes perfect sense. I have seen that very dark colour in other experiments when reducing Cu2+ with Cu metal. It just didn't occur to me that FeSO4 would reduce the copper ions. Also I have never really had significant amounts of copper left in solution before precipitating the gold, hence haven't seen the reaction in this context. Thank you. By the way, do you think that the tiny amount of black sediment in my test with the spent solution could be copper reduced all the way to metal?

Oh, and just to avoid confusion, the copper contamination came from the silver used to inquart the starting 18K gold. The inquarted gold wasn't quite fully digested in HNO3 before the aqua regia. The copper did not come from the FeSO4

Thanks! Apologies I missed a line in your post.

If your solution went black after a further addition of Ferrous Sulphate, and dropped some more powder then it's very likely to be more gold. What can happen if you haven't completely denoxxed is that as you are settling overnight some of the gold is redissolved back in to the solution. It can also happen with an extremely dirty solution, but given the colour of your pictures, they don't look too filthy. The bubbling was a good giveaway. If you looked at the top of the solution you might have even seen a line where the bubbling started.

You've highlighted a really good thing here- that you need to test your solutions at more points than people initially think. Double checkign the tests too, is never a bad thing.

By the way- good catch- you avoided throwing gold away and that's always a win.

Jon.

No, I don't think copperas could reduce copper all the way back to pure metal. It is often recommended for use in dirty solutions and if it would precipitate copper it would have been mentioned before.

Without testing the black powder I would not hazard a guess on what it is and how it ended up there.

Göran

g_axelsson said:

No, I don't think copperas could reduce copper all the way back to pure metal. It is often recommended for use in dirty solutions and if it would precipitate copper it would have been mentioned before.

Without testing the black powder I would not hazard a guess on what it is and how it ended up there.

Göran

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If you're adding Ferrous sulphate to a gold bearing solution and you get black powder- the chances are extremely high that you have in fact got gold mate. I'd be happy betting a buck on that. 8) 8)

Brilliant, although it cannot be more than a tenth of a gram. Thanks for your replies, appreciated.

anachronism said:

g_axelsson said:

No, I don't think copperas could reduce copper all the way back to pure metal. It is often recommended for use in dirty solutions and if it would precipitate copper it would have been mentioned before.

Without testing the black powder I would not hazard a guess on what it is and how it ended up there.

Göran

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If you're adding Ferrous sulphate to a gold bearing solution and you get black powder- the chances are extremely high that you have in fact got gold mate. I'd be happy betting a buck on that. 8) 8)

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An ore was smelt , get sludge from copper refining cell, process with nitric and sulfuric.
Later on use Aqua regia, denox nitric and the color of Aqua regia was this
Use feso4 and it makes solution darkish
Let leave it for a night, next day the solution color was back to as it is and a yello salt percipitaed,
Any ideas what's this

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There are many sulfate salts that are insoluble in water, lead sulfate for example.

You didn't test your aqua regia solution with stannous to see if you had any precious metals at all?

Göran