# Help removing Selenium from Gold



## mikemrtwin (Jan 30, 2022)

Hello, I have refined a bunch of gold jewelry. Towards the end of the traditional refining I found it to be heavily contaminated with selenium. About 0.455%

I was suggested to use sodium hydroxide to keep the selenium in solution while the gold precipitated. Reading more into the topic it states that it has to be done at boiling temperatures but that statement meant that it will drop out as sodium selenide. I did experiment with it and I noticed that the gold would drop out first prior to hitting a pH 2. I did two experiments with oxalic acid and sodium hydroxide and just sodium hydroxide. At the end of the oxalic and sodium hydroxide test pink residue was coming out. I'm assuming this is sodium selenide and the remaining oxalic acid dropping out again. 

I still find that the gold sample is testing high in selenium. During the testing the reaction between the gold bearing solution and sodium hydroxide was very violent. Can anyone clarify on the proper procedure? Would anyone be interested in refining the selenium out for a small portion of the gold and let me know there procedure?


----------



## nickvc (Jan 31, 2022)

I fail to understand where the selenium came from if you are only refining karat scrap, is it possible that what you assume to be selenium is something else ?


----------



## mikemrtwin (Jan 31, 2022)

Yes I highly believe that it came from a chemical that I used in the process. Like sulfuric acid is a carrier of selenium, sodium Metabisulfite, for examples. Purchased from bad source. Let me know, thanks


----------



## Elemental (Jan 31, 2022)

@mikemrtwin

Depending on your volume of gold you are currently working with, I personally would switch from chemical to electro-refining. Build a cell and plate the gold out, leaving the selenium behind. I'd imagine it would be similar to the silver cells that most of use to purify to fine silver. You'd need to do a little bit of research about the best voltage to use and cathode (perhaps just a piece of 24k gold wire). I'm not a gold refiner yet, but I'm sure some of our more experienced members will chime in as well.

Good luck with it and keep us posted.

Elemental


----------



## mikemrtwin (Jan 31, 2022)

Thanks, yes I did also build an electrolytic cell and research the temperatures and voltage requirements. I got it to plate but it carried the selenium with it. I used a flattened 1 g gold bar to plate on. I'm assuming that it didn't help that my solution was also contaminated with selenium from the beginning. When I tested it all the other impurities that were known to exist in the sample refining we're eliminated so I know that the cell did work to some extent. 

The gold came from a 10 oz refining batch. Hopefully someone will see this post that worked with selenium removal


----------



## Elemental (Jan 31, 2022)

Selenium contaminated Ag & Pd


Hello everyone I have been trying to process this metal to get rid of selenium but I think everything went in vain. We tried with various fluxes such as sodium nitrate, potassium permanganate, wood chunks, caustic soda independently. Direct Nitric acid dissolution followed by AgCl precipitation...




goldrefiningforum.com


----------



## nickvc (Jan 31, 2022)

You could try different chemicals such as HCL with additions of either peroxide or chlorine to dissolve the gold and different precipitants such as copperas or ascorbic acid, another thought try cementing the gold out on copper , small trials would be advised.


----------



## butcher (Jan 31, 2022)

In industrial electrolytic copper refining, the anode sludge can contain gold, silver, lead, copper, selenium, tellurium, as well as several other metallic elements and their salts.

The anode slime or sludge is washed in water to help remove the bulk of the soluble salts.
Dried and roasted, held for a period of time around 700 degrees F, to drive off acids and salts and volatile components as much as possible or convert them to oxides or another form.

Selenium upon fusion can sublimate (be driven off as a gas which can then cool and reform as a liquid in another cooler portion of a furnace, eventually crystalizing to the surface walls of the furnace. this idea is used to test for selenium in a test tube heating the crystal and then checking the cooler portion of the glass tube for the selenium sublimate, I may have some terms of the chemistry wrong in my explanation here.
roasting the anode sludge first to fuse the mass, crushing and repeating if needed, till you can bring the roasted powders up to a glowing red heat (we do not wish to melt them), then we stir to give the red hot glowing powders good exposure to the oxygen in the air, helping to drive off volatiles and gases, then to oxidize the other metals left involved in these powders.

These powders are then leached in diluted sulfuric acid, to help remove metals like copper.
Much of the remaining base metals, selenium, tellurium, are removed in a smelting process, or in a flux melt process before being cast into silver-dore anodes...


----------



## Elemental (Jan 31, 2022)

Pardon my ignorance, but could you just completely refine the gold like normal with Aqua Regia and SMB and leave the Selenium as a salt in solution? I'm finding information that the selenium makes a complex with the gold. Can anyone share the chemical reaction(s) taking place here for the selenium?


----------



## orvi (Jan 31, 2022)

I personally never experience selenium as problem during refining, but considering it´s high affinity to gold and mercury forming stable selenides... It could be tricky to remove. 
In highly acidic enviroment, selenides could break up, forming hydrogen selenide. But simultaneously, when gold is dropped from solution containing selenium, it could adsorb/bind/coordinate with gold very strongly (similarly as sulfur, but stronger - wider atomic radius). Meaning you cannot wash it off easily like you do wash copper with boiling HCl.
*When finding persistent impurity in general, best practice is to change the method of processing as Elemental suggested - to circumvent the pathway the impurity is known to follow the gold.*
Also as mentioned - other precipitants could help either avoid contamination (using pharma quality vitamin C powder assure you that no selenium is present for example) or change the way selenium could bind to the gold = changing some surface physical-chemistry and similar stuff  it could make significant differences in outcome.

One thing that comes to my mind:
When you drop the gold powder with whatever precipitant, I think here you have the best chance to eliminate the impurity. Gold precipitate has wide surface area, so you can try various leaching techniques to eliminate the possible selenium contamination either in physically adsorbed form (like Se as element) or chemically bond form (like some gold selenide etc). Advantage here is that you can try numerous formulas/leachants/acids/bases without affecting gold in any significant way - trying like 5 things in row, while measuring the content of selenium in resulting gold powder without gold being lost.
First, I would try* boiling the gold powder with nitric acid.* Selenium could be oxidized with nitric acid to selenic acid - which could be possibly leached out of the powder this way. 
Many selenium compounds like selenium oxide could be dissolved into strongly basic solutions due to amphoteric properties - so next, I will try boil it with some 10-20% sodium hydroxide. Filter/decant and measure the changes in the powder composition.
Finally, i would try sequence of acidic wash with basic wash. I will boil the powder with concentrated hydrochloric acid for extended period of time (like hour or so), then decant and wash the precipitate. Next, I will apply strong base like aqueous 10-20 % sodium hydroxide, also for some time.
Idea behind is: if the compound you are fighting is gold selenide (hypothetically), acid could break the selenide itself when strong, but after this, liberated hydrogen selenide could re-adsorb on the surface of the gold due to high affinity. If this (hypothetically) happens, you can on turn with strong base neutralize the hydrogen selenide and wash it off the gold.
But if the selenium is present in some "exposed" form, either in the form of elemental Se or some gold selenide-like compound, it will be destroyed with nitric acid with high probability. 

I should add that these advices are just my guesses, based on my knowledge, not real experience.

Wishing luck with solving this problem


----------



## Platdigger (Jan 31, 2022)

I believe Butcher is refering to a book I have.
From reading that book it's not easy.


----------



## FrugalRefiner (Jan 31, 2022)

I believe Lou has talked about problems with selenium. If I recall, he mentioned that SMB will drop selenium with the gold. Do a search for "selenium" with Lou as the author.

Dave


----------



## Deano (Jan 31, 2022)

Under acid conditions selenium will be present as selenic acid which is a gold complexant. The gold selenate bond is stronger than the gold chloride bond so stronger precipitants are required to drop clean gold, this becomes an expensive proposition.

It is possible to stage raise the solution pH to around 11 to precipitate out all dissolved metals from chloride solutions and then to redissolve just the gold with glycine. You cannot use cyanide for this step as it will complex with selenium.

Deano


----------



## Lino1406 (Feb 1, 2022)

As Orvi said, nitric acid will remove selenium


----------



## orvi (Feb 1, 2022)

Deano said:


> Under acid conditions selenium will be present as selenic acid which is a gold complexant. The gold selenate bond is stronger than the gold chloride bond so stronger precipitants are required to drop clean gold, this becomes an expensive proposition.
> 
> It is possible to stage raise the solution pH to around 11 to precipitate out all dissolved metals from chloride solutions and then to redissolve just the gold with glycine. You cannot use cyanide for this step as it will complex with selenium.
> 
> Deano


I find some more info:
_"Hot, concentrated selenic acid reacts with gold, forming a reddish-yellow solution of gold(III) selenate:[9]

2 Au + 6 H2SeO4 → Au2(SeO4)3 + 3 H2SeO3 + 3 H2O"_

Just found on wikipedia. More digging into references, I found JACS article from 1902 (good old days) issuing the dissolution of gold in selenic acid. Author state that fairly concentrated selenic acid is needed, and high temperature (230° - don´t know if °C or °F, not specified) to dissolve the gold leaf. From the original article:

_"The selenate has been obtained as very small yellow crystals. It is soluble in hot concentrated selenic acid, forming a reddish-yellow solution, and separates on cooling. It is insoluble in water, and can be completely separated from selenic acid by dilution, when it appears as a yellow precipitate. It is soluble in
sulphuric acid and in nitric acid. Hydrochloric acid decomposes the salt with evolution of chlorine, forming auric chloride and selenious acid. It is decomposed by heat below redness, metallic gold being obtained."_

Also, nitric acid is described to oxidize the selenium just to Se(IV) oxidation state, so in theory, no selenic acid will be present.


----------

