EE THr
Silver Member
I thought I would toss this out here because I haven't seen it mentioned on this board yet.
This might help those who are experimenting with the recovery of tiny gold particles from black sands.
I saw this mentioned on http://goldrefiningforum.com/phpBB3/ and have sometimes experienced this myself.
The color of gold particles vary, for two different reasons. One is because of impurities. The other is because of particle size.
When refining gold, in the common home-refining methods, you end up with a tan powder which is not shiny. It's what you might call flat tan color when it is around .999+ in purity. When it is melted, it then takes on the appearance of the usual nice, yellow, shiny, gold.
If there are impurities still in it, it will be darker in color.
But sometimes, depending on what else is in the dissolved solution, it will precipitate into very tiny particles, and these will also be darker in color, even if the gold is .999+ in purity. This will be darker, the smaller the gold particles are---all the way to black!
So, if you wind up with very fine black powder in with your small-mesh gold, it might not be fine black sand!
This could account for some of the "disappearing" gold, that shows up in fire assays of black sands, but evades recovery.
I haven't looked at these black gold particles under a microscope, so I'm not sure if they would be identifiable as other than the standard black sands, or not. But I do know that they dissolve in Aqua Regia, and being so small they dissolve very fast. [Edit added: And the solution test strong for gold. Also, when re-refined in an uncontaminated solution, they will precipitate as the larger, tan, powder.]
These show up in home refining when a precipitation has settled, then when the remaining solution is poured off and it is let set for a week or two, a small amount of this really tiny black gold will settle on the bottom of the solution container. It then looks like a thin, dark, film on the bottom. But if you disturb it, you can see that it is powdery in form.
In addition, colloidal gold can be entirely different colors. There are several specialized precipitators which are used to form colloidal gold. One which everyone is familiar with is stannous chloride, when testing a solution for the presence of dissolved gold. Stannous chloride precipitates purple colloidal gold, and when the solution has a high percentage of gold, the precipitate will appear very dark or even appear to be black. And that's what the purple "stain" from the stannous chloride is---collodial gold.
Other specialized precipitators will produce different colors, but the chemicals to do so are somewhat exotic and expensive. The different colors are due to the different lengths of the colloidal chains. There are some pictures on the Internet, if you search for them with "colloidal gold colors." Here is an example, at the bottom of this page: http://www.webexhibits.org/causesofcolor/9.html
But what may be of the biggest concern is the non-colloidal, tiny black particles of gold that may be in with the common black sands.
This might help those who are experimenting with the recovery of tiny gold particles from black sands.
I saw this mentioned on http://goldrefiningforum.com/phpBB3/ and have sometimes experienced this myself.
The color of gold particles vary, for two different reasons. One is because of impurities. The other is because of particle size.
When refining gold, in the common home-refining methods, you end up with a tan powder which is not shiny. It's what you might call flat tan color when it is around .999+ in purity. When it is melted, it then takes on the appearance of the usual nice, yellow, shiny, gold.
If there are impurities still in it, it will be darker in color.
But sometimes, depending on what else is in the dissolved solution, it will precipitate into very tiny particles, and these will also be darker in color, even if the gold is .999+ in purity. This will be darker, the smaller the gold particles are---all the way to black!
So, if you wind up with very fine black powder in with your small-mesh gold, it might not be fine black sand!
This could account for some of the "disappearing" gold, that shows up in fire assays of black sands, but evades recovery.
I haven't looked at these black gold particles under a microscope, so I'm not sure if they would be identifiable as other than the standard black sands, or not. But I do know that they dissolve in Aqua Regia, and being so small they dissolve very fast. [Edit added: And the solution test strong for gold. Also, when re-refined in an uncontaminated solution, they will precipitate as the larger, tan, powder.]
These show up in home refining when a precipitation has settled, then when the remaining solution is poured off and it is let set for a week or two, a small amount of this really tiny black gold will settle on the bottom of the solution container. It then looks like a thin, dark, film on the bottom. But if you disturb it, you can see that it is powdery in form.
In addition, colloidal gold can be entirely different colors. There are several specialized precipitators which are used to form colloidal gold. One which everyone is familiar with is stannous chloride, when testing a solution for the presence of dissolved gold. Stannous chloride precipitates purple colloidal gold, and when the solution has a high percentage of gold, the precipitate will appear very dark or even appear to be black. And that's what the purple "stain" from the stannous chloride is---collodial gold.
Other specialized precipitators will produce different colors, but the chemicals to do so are somewhat exotic and expensive. The different colors are due to the different lengths of the colloidal chains. There are some pictures on the Internet, if you search for them with "colloidal gold colors." Here is an example, at the bottom of this page: http://www.webexhibits.org/causesofcolor/9.html
But what may be of the biggest concern is the non-colloidal, tiny black particles of gold that may be in with the common black sands.
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