Extracting copper and nickel from a coin

This was discussed at length in a long-running sciencemadness threat called “Separation of a US nickel” that ran over 11 pages from 2011 to 2018:

http://www.sciencemadness.org/talk/viewthread.php?tid=16811&...

Taking the copper iodide route was discussed, but I am not sure if anyone actually tried it. So I wanted to give it a go.
Starting point was Australian 50c coins, which is said to be made from 75% copper and 25% nickel. One coin weights 15.5g (on my scale) and should thus contain 11.6g copper and 3.9g nickel.

The coin was placed in a 400ml beaker and 70% nitric acid added in stages. See photo. Lots of NO2 fumes! By the end of the day 90g was added which was slight excess. The next morning the coin was completely gone and there was a very dark green-blue solution. See photo.

I wanted to minimize the acid leftover in solution so added 3 x 5c coins, these are small and weighed 8.7g in total. After a few hours with some heating there was no more reaction. The undissolved pieces were removed the weighed at 3.2g. Thus, some 21g of coin went into solution.

I wanted to use just part of the solution so I could try more then once, so I decanted off one third by weighing the total solution and then pouring off until the weight reduced by a third. This was now my working solution and should have contained 5,25g copper and 1.75g nickel. This solution was diluted with 20ml water.

The next step was to try and ppt the copper as copper iodide. 20g potassium iodide was dissolved in 30ml water, this should have been enough to perform a displacement with the copper nitrate as well as the nickel nitrate.

When I previously made copper iodide CuI by reacting CuSO4 and KI I did not use a reducer which resulted in free iodine and a lower yield of CuI (and the product had to be well washed with ethanol to remove all the free copper). This time I use sodium thiosulphate as the reducing agent. Atomistry described such a reaction (with relevant weights) but using copper sulphate, not copper nitrate. I was not sure exactly which other products would form and thus could not work out the stoichiometric amount of sodium thiosulphate to use. I simply used a similar weight ratio for the KI/Na2S2O3 as Atomistry mentioned meaning that with my 20g KI I had to use 28g Na2S2O3. This was dissolved in 40ml water (took a bit of heating to get it dissolved).

Question: Is this more or less the correct amount of Na2S2O3? Woelen previously suggested using sodium metabisulfite Na2S2O5 for this purpose, would it have been a better choice than the Na2S2O3?

The sodium thiosulfate was added to the nitrate solution with stirring. A lime green suspension formed, and I though the experiment was ruined. The KI was then slowly added, and I was happy to see the pale brown color similar to me previous CuI. And happily there was no sign of free iodine. After some stirring the solution was vacuum filtered. The remainder wet was a slightly yellowish white. See photo. The filtrate was light green, which is a color that pointed to nickel. This was the end of the day.

The next morning the (suspected) CuI remainder was washed with ethanol to check for free iodine, but the ethanol wash water stayed completely clear. The product was then tried a few hours on a steam bath until the weight remained the same. The final weight was 15.8g, which was just about exactly what was the theoretical CuI yield from 5.25g of copper. See photo of the CuI from the coin next to the CuI from CuSO4; it looks very similar but the coin CuI is slightly darker. But I saw later it reacts different!

The filtrate that was light green became brown overnight! Filtering brought it back to light green, and showed that the brown suspension was a very small quantity of powder (a fraction of a gram) ; it was so little that it was not even possible to scrape it off the filter paper. I do not know what this compound was, but I saw that if I left the green solution stand more of this brown suspended powder formed.
I then added 4g NaOH dissolved in 10ml of water to the green solution, hoping that any nickel salt will be ppt out as nickel hydroxide. A light green suspension quickly formed, which became darker and more olive green. The solution was vacuum filtered, and the remained washed twice in the filter with water. The wet weight was 19g!

The product was dried in the steam bath, see photo. The weight was checked every few hours. The product kept losing weight until after many hours of drying in the steam bath a final weight of 3.4 gram was reached. The dry product was dark green, see photo.

I think the product is nickel hydroxide Ni(OH)2 but 1.75g nickel should have given only 2.6g hydroxide and I got 3.4, Although it is difficult to be sure when working with such small quantities.

At this point I was reasonably happy I managed to separate the copper and nickel but wanted to do some basic tests on the products.

Suspected nickel hydroxide Ni(OH)2: A small qty was reacted with 70% HNO3 in a test tube. It quickly dissolved to give a green solution which was expected, but at the same time NO2 fumes was seen – I did not think a normal oxide or hydroxide will do that with nitric acid? See photo.

Suspected copper iodide: A series of tests were done with both the coin CuI and the previously made CuSO4 CuI to see if they react the same:
1) With 35% H2O2 (being dropped onto a small qty of product:
CuSO4 derived CuI: Goes black from iodine release, small bubbling,
Coin CuI: Same reaction but more vigorous, to the point where it gets hot enough to start gassing off the iodine.
The test was repeated with the same result, so with H2O2 the coin CuI seems more active.

2) With 95% HsSO4 being dropped on:
CuSO4 derived CuI: Acid slowly goes brown, small bubbles, warm.
Coin CuI: Acid quickly goes black, small bubbles, warm. So again it seems more reactive.

3) With 70% HNO3 being dropped on:
With both iodides the HNO3 turns black and releases NO2, but once again a bit more vigorous with the coin CuI.

4) With 33% HCl being dropped on:
CuSO4 derived CuI: Acid turns yellow, then brown, no smell of chlorine.
Coin derived CuI: Nothing seems to happen! He CuI powder just seems to stay suspended in the HCl without any change to the acid.
I repeated this test with HCl 3 times, same result.

How can this be explained? It must be a copper and iodine compound, but something other than only CuI? I must also mention that after I first filtered off the suspected copper iodide , the filtrate was crudely tested for presence of a iodide by mixing a sample with lead nitrate. No lead iodide ppt formed, which made me wonder what happened to the excess KI.

Quote: Originally posted by Bedlasky

CuI doesn't react with HCl. Your CuI from CuSO4 probably contain some iodine. Na2S2O5 is better because Na2S2O3 slowly decomposes in to sulfur and also forms really strong complex with Cu(I). Thiosulfates dissolve even very insoluble Cu(I) and Ag(I) compounds. Your Ni(OH)2 probably contain some reducing agent, maybe NiS2O3? You can not dehyrate fully Ni(OH)2. It will always contain some water in it's structure. If you want to fully dehydrate it, you must roast it to form NiO.