To Inquart or Not to Inquart

[RickRag]

Hi guys,

I've cemented out the PGM's with copper and am filtering them now. The solution was negative for values using SnCl. Suprisingly, I did get quite a lot of metal using the copper bus bar. Because there were base metals in the solution, did some of them cement out along with the values?

In retrospect, I discovered the hard way that all the nitric had not been eliminated in that refinement. I had a lot of brown fumes upon addition of SMB and needed a lot of SMB. Eventually I got the solution to look like the pictures included and the dark precipitate settled leaving the pale solution shown.

Accordingly, the remaining AR containing the bulk of the dissolved dental scrap was treated with sulfamic acid to deNOxx what nitric was left. I did not have a gold button in the solution to ascertain how much Sulfamic was necessary but I thought somone suggested adding it until no more fizzing was observed. I did that and filtered the remaining solution. Upon addition of SMB there was quite a reaction that confirmed that I still had not removed all of the nitrates, as before. So I added additional Sulfamic until there was no more fizzing in the heated solution. I then proceeded with additional SMB to drop the gold. I got a lot of precipitate that appears to be a light tan, however the amount of SMB added seems to be more than what I should expect based on the provious refinement. In the pictures of previous refinement, you can see the pale color of the depleted solution containin the PGM's. The solution I am now attempting to drop the gold from is still quite a bit darker and doesnt seem to be getting any lighter even though I've added what I expect to be sufficient SMB for the gold expected. Each addition of SMB brings a reaction that fizzes and but no brown fumes. I dont wan to complicate things by adding too much SMB as others have indicated it will precipitate out other base matals as well, (and may have already have done so) and those are certainly present in the solution i'm dealing with here. How can I tell if I have added enough SMB? Because there are PGM's left in solution, a positive SnCl test is not definitive. Is the color of a positive SnCl test sufficiently distinct to determine if all the gold has been dropped but not the PGM's? Or should i take a small sample of what Im working with and add SMB in sufficient amounts to see if I get the pale solution I got previously? This would keep from adulterating the remainder of the batch. Any guidance would be appreciated.

Rick

 

[g_axelsson]

Rick, SMB added to an acidic solution with HCl only will fizz a lot, giving off colorless SO₂. That is normal.
As you have added a lot of SMB a stannous test probably gives a brown false positive.

If a solution is properly denoxed you only need to add enough SMB so a stannous test gives a negative test for gold. (No purple.) If you add SMB and get brown fumes then you probably have nitric left in your solution. It is the gold that is redissolved that gives off the brown NO_x gas. When the nitric is used up the gold precipitates and stays out of solution. A stannous test is the proper way to see if there is any gold left in solution.

My experience with PGM powder is that it is a lot more voluminous than gold powder so it might look like more than it really is.

Göran

 

[anachronism]

After posting that last reply, I realized my main reference, Hoke’s book, was noticeably deficient on precipitating gold with SMB and deNOxxing solutions using urea, formic acid and sulfamic acid. I also found the answer to my question about excess SMB in the presence of nitrates. I also discovered that my solutions did in fact have an excess of HNO3, thus requiring elimination to avoid using unnecessary amounts of SMB. Because I’m doing this outside I’m reluctant to use evaporation due to the nearby structures exposed to acid fumes. A chemical deNOxxing is thus preferred. I’ve seen some info on formic and sulfamic acid use but nothing quantitative. Urea use seems to have mixed opinions but the forum members with experience seem to avoid its use. Does anyone have a reference for the use of sulfamic or formic acids? Or urea if my specific method warrants its use? thanks again for all the help.

Rick

Hoke's book is deficient in many ways more than that. It was written nearly 100 years ago and whilst there are some amazing parts for testing of metals and some other things the developments in technology and the difference in products that people try to refine from make it a good "side read" as opposed to a main textbook.

It's also been misinterpreted because the lady was frankly brilliant and alluded to some serious chemistry that many people missed.

 

[Geo]

I have found that when using a chemical precipitant, it's better to precipitate from a concentrated solution. When cementing precious metals on a higher reactive metal, it is better to work with a dilute solution. There are two easy ways that I know of to prepare the solution and they both are different ways of achieving the same thing, adjusting the PH value of the solution.
One way is to adjust the PH by adding a base to the solution. Sodium bicarbonate is an example.
The second way is to dilute the solution with distilled water until the correct PH is reached.
Personally, I like dilution best. The less reactive metal, in this case Pd, will not easily stay in a metallic form in a solution that has a PH of 1 or less. The reason for putting a large chunk of copper as apposed to stranded wire is because the copper is sacrificed to the acid. The small strands will break apart and contaminate the cemented Pd.
I would like to suggest you use a different metal to cement the Pd onto, aluminum. Aluminum will cement almost all of the metal that can be cemented from the solution. Bear in mind that the recovered metal will be impure. Now that you have all the metal in a small container, you can dissolve ALL the metal in the smallest amount of acid needed and have a concentrated solution to work with. Now a chemical precipitant for Pd will work much better. I believe Sam has a video that has a good demonstration of the very process.

 

[RickRag]

Thanks Goran, et. al, for your input.

I have had to divide up the bulk solution due to the size of my vessels. I now have two 1 liter quantities in two vessels. I added more SMB to one of them as described in the above post and that vessel now contains approximatley 1.75 L. That is what prompted me to post the questions above. Thank you all for your answers. In order to avoid adding more SMB to the two solutions, I took a small amount (25 ml) from the more dilute solution (1.75L) and added more SMB to see if anything dropped out. Nothing did, however after an hour or so a clear crystalline solid appearred in the bottom of the beaker, which I presume to be SMB that has precipitated due to its high concentration/lowered temperature. The stain on the left is from that small sample that had the crystalline solid. Is this likely a false positive due to the excess SMB that simply precipitated. No dark precipitate was apparent in the small sample taken. The other two stains are from the two remaining bulk containers, one more dilute but higher SMB content than the other. The one at the top is from the more dilute but higher SMB content solution than the one on the bottom. Can someone help intrepret these for me? I'm trying to determine if I have dropped all of the gold. Both solutions have the remaining base metals as well as palladium and platinum in solution.

 

[Geo]

The brown stain is a false positive. The green in the top test is palladium and the purple stain on the bottom looks like gold. Place a solid piece of copper in the solution and heat it. It will force the SO2 out of the solution and deplete the solution of nitric acid. It will also cement any precious metals. It will most likely take a few days for any cemented metal to settle completely if the solution is dirty.

 

[RickRag]

Thanks Geo for your insight.

As I am trying to determine if I have dropped all of the gold from my AR solutions containing PGM's, I have read numerous sources. The methods I have come across simply say to add SMB to the deNOxxed, filtered, diluted AR solution until no more gold drops. The solutions I am working with are fairly dark and It is difficult to see if any precipitant is still forming upon each addition of SMB. Should I dilute these solutions until they are transparent enough to be able to see any flocculant should it form? Or is there some other method that is more objective?

In posting this question above so far, no one has replied with a specific method for that.

Based on Geo's reply, is it standard practice to intrepret SnCl tests for the answer?

To further complicate things, and on another note, I have read GSP's compilation of GRF posts etc. that he sells for those interested in further reading. Very informative, thanks Chris. I have read his posts about dropping gold using sulfites, more specifically SMB, and I have concluded that, in his opinion, SMB is not completely selective in dropping gold from AR sloutions containing gold and PGM's. He indicates that ferrous sulfate (copperas) is the only precipitant that is selective for gold only. I have not read that elsewhere on the forum but don't deny that I haven't done an exhaustive search either.

Is this true? Is gold dropped with SMB from AR in the presence of PGM's typically contaminated with Pd and Pt? If so to what extent? What does one do to further separate these PM's? is refining a second time typical? Further clarification on this subject would be helpful or a reference where I can find more information would be appreciated.

Thanks again for everyones input.

Rick

 

[g_axelsson]

As I wrote above... use stannous to test for dissolved gold. Works perfect even in solutions heavy with base metals, as you have seen with your own test.

One way to remove PGM:s from gold is to do an inquartation. When I melted my first batch of 50g refined gold I might have had palladium as a contaminant, after inquartation the gold came out perfectly. The obvious palladium in the inquartation (the green color of copper + palladium) came from the silver I used, the button came out at almost the same weight after re-refining.
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=61&t=19840

There are two ways palladium or other metals might contaminate your gold.
1. It is precipitated by the chemicals you are using to precipitate the gold. Different chemicals have different drawbacks. A re-refining of the powders with another precipitant usually works well and is a standard way to make ultra-pure gold.
2. Mechanical, part of the liquid you precipitate from is mechanically trapped inside the gold powder. Proper washing can remove a lot of the contaminants. Boiling in HCl and then water several times can work wonders. Read Harolds advice. http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=325&p=2620#p2620

If you drop gold from a solution so heavy in base metals that you can't see the gold powder I would recommend that you do a second refining of the powder after a quick wash. It's faster to redissolve the powder, filter and precipitate the gold again than trying to wash it clean. The dirty gold from the first drop will be slow to settle, but the second time you drop the gold it will behave a lot better. Use ferrous sulfate to drop the gold from the dirty solution and SMB the second time, then you can visually judge how dirty the first drop was, it shouldn't leave more than a very pale solution. If the solution is colorless then the gold is good and with a little bit of cleaning following Harry's advice you should have a very good product.

Oh... and use a clean melting dish and a cleaned torch when melting so you don't make it dirty at the end. :wink:

Göran

 

[Geo]

When I am precipitating gold from a dirty solution or a solution that contains PGM's, I use copperas (ferrous sulfate) as it does not precipitate PGM's the way SMB can. Even though SO2 is selective for gold, it is not exclusive for precipitating gold. It can also precipitate copper, silver, Pd and a couple other elements in very small amounts. Copperas is much more selective because it works on a different principle. The iron in one solution replaces the gold in the other solution. No SO2 is produced. You can achieve very pure gold by precipitating with copperas first from a dirty solution and then redissolving the precipitate and dropping the gold with SMB.

As a side note, after the gold is removed from solution, the solution can be evaporated slowly until crystals begin to form. The higher reactive metals will crystallize first and the less reactive metal salts will crystallize last. This is a way of removing some of the troublesome metals. Reduce the volume until a small amount of liquid remains. Decant this to a clean vessel and test with stannous chloride. The resulting solution should be colored but clear. The test will be easy to see. Rinse the crystals with the smallest amount of chilled water that is needed to rinse the crystals.

 

[g_axelsson]

As a side note, after the gold is removed from solution, the solution can be evaporated slowly until crystals begin to form. The higher reactive metals will crystallize first and the less reactive metal salts will crystallize last. This is a way of removing some of the troublesome metals. Reduce the volume until a small amount of liquid remains. Decant this to a clean vessel and test with stannous chloride. The resulting solution should be colored but clear. The test will be easy to see. Rinse the crystals with the smallest amount of chilled water that is needed to rinse the crystals.

That is not correct, it is the solubility factor combined with the concentration of cation and anion that decides which salt crystallizes first.

A couple of extreme examples are:
- Lead chloride crystallizes at lower concentration than gold chloride.
- Silver chloride crystallizes at lower concentrations than lead chloride.

The solubility of a substance is measured in the solubility konstant, Ksp.
By looking on the article on Wikipedia you can get an idea of how messy solubility calculations can be.
https://en.wikipedia.org/wiki/Solubility_equilibrium#Dissolution_with_dissociation
And this is a simplification by itself too. Just look at how copper(I) chloride or silver chloride becomes more soluble in concentrated chloride solutions or how amphoteric compounds vary in solubility depending on pH.

This is a complex subject, enough complex that books can and have been written about it many times.

Göran

 

[Geo]

Thank you Goran. I certainly am not talking about absolutes and am referencing my own observations.
Troublesome elements such as silver and lead would normally be removed before you get to the stage of precipitating gold. My chemistry knowledge is very limited to say the least and only comment about what I've seen or have been taught by people who know better, such as yourself. In my own experience, I have found that you can remove some of the unwanted metal salts by careful evaporation and rinsing the resulting crystals. I can say that without removing lead early in the process, right after dissolving the material, lead will follow gold all the way through to the end of the process.

 

[g_axelsson]

If you want you could refine various dissolved metals just by letting the solution crystallize, the metal with highest concentration / lowest solubility would form crystals first. After a while there would be several different forms of crystals and you can sort it just by the look of it.

I have tried it with silver and copper nitrate after dissolving a bit of sterling. The silver nitrate forms colorless crystals while the copper nitrate forms beautiful deep blue crystals.
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=37&t=26647
The third picture in the thread shows the risk of mechanical entrapment, if you have a gold solution and create crystals which you remove, some of the gold solution might be lost with the crystals.

It's a slow process but it works really well if you are careful.

Göran