Sciencemadness Discussion Board

Dissolving Gold with Chlorine gas

kadriver - 6-3-2013 at 12:00

Neat video with a simple chlorine gas generator used to dissolve gold powder.

http://www.youtube.com/watch?v=Iwvt_QHeb7o

kadriver

blogfast25 - 6-3-2013 at 13:50

Nice demo, not a very practical way to dissolve gold.

kadriver - 6-3-2013 at 17:57

You said it - took a long time and very inefficient.

but, this is a preferred method to completely eliminate nitrosol compounds from the process (nitrates tend to bring down other metals when they are present).

The 2 grams of powdered gold used in this experiment was produced by first dissolving the gold in aqua regia, then precipitating with sodium metabisulfite (a sulfur based precipitant)

This produced gold powder in the range of 99.90% purity.

To get to the level of extreme purity that I was seeking, a trade off had to be offered - the inefficient (and impractical) use of chlorine gas.

by using two different dissolution methods and two different precipitants, the resulting gold should be close to five nines (99.999%) in purity.

Plus I had never performed this operation before and it was great fun!

kadriver

[Edited on 7-3-2013 by kadriver]

Traveller - 7-3-2013 at 09:34

I believe a leach made from iodine/potassium iodide at a pH of 4-5 will also give you .999 fine gold.

12AX7 - 9-3-2013 at 09:37

"Hmm, I want gold chloride. Well, I've got gold, and I've got chlorine..." What could be more logical?

Looks like it would benefit greatly from a bubbler: it looks like every single bubble of chlorine broke the surface and passed into the fume hood without reacting, only the brief surface contact contributing any into solution. Very fine bubbles, and a long test tube or graduated cylinder, would increase absorption, and probably reaction rate as well. Plus some sort of agitation to keep the gold suspended, which I'm sure would be a bit of a tall order, but if you can get a magnetic stir bar in the bottom of a cylinder, it would probably help, even if it doesn't fully disperse the gold.

Tim

AndersHoveland - 9-3-2013 at 14:36

Quote: Originally posted by kadriver  
took a long time and very inefficient.

The gold would have dissolved much faster if some sodium chloride or hydrochloric acid was added into the solution also. The same strategy is also used to dissolve PbCl2 during ore processing.

It's obvious why if we look at the reaction equation:

2 Au + 3 Cl2 + 2 Cl- --> 2 AuCl4-

If no additional chloride ions are added, the chloride has to come from the equilibrium of the elemental chlorine with water, which is not very efficient.
Quote: Originally posted by kadriver  
then precipitating with sodium metabisulfite (a sulfur based precipitant)

I prefer to reduce my gold solutions using hydrogen peroxide, it will not leave any residue. Many people are not familiar with the fact that H2O2 can also fuction as a reducing agent in many reactions.

[Edited on 9-3-2013 by AndersHoveland]

Traveller - 9-3-2013 at 22:18

Anders

Would that explain the gold leaching method I found online that used HOCl at pH 7 with NaCl added to it? Is HCl considered to be a chloride as well?

Could you explain the whole chloride thing a little more in depth for the science challenged such as myself?

Bob

garage chemist - 10-3-2013 at 03:53

A mixture of HCl and NaClO3 dissolves gold very rapidly upon warming, faster than full strength aqua regia.
This also gives a solution free from nitric oxides. Residual chlorate can be destroyed by adding excess HCl and boiling until the chlorine smell is gone, which only takes a few minutes.

Traveller - 29-3-2013 at 20:25

Quote: Originally posted by AndersHoveland  
Quote: Originally posted by kadriver  
took a long time and very inefficient.

The gold would have dissolved much faster if some sodium chloride or hydrochloric acid was added into the solution also. The same strategy is also used to dissolve PbCl2 during ore processing.

It's obvious why if we look at the reaction equation:

2 Au + 3 Cl2 + 2 Cl- --> 2 AuCl4-

If no additional chloride ions are added, the chloride has to come from the equilibrium of the elemental chlorine with water, which is not very efficient.
Quote: Originally posted by kadriver  
then precipitating with sodium metabisulfite (a sulfur based precipitant)

I prefer to reduce my gold solutions using hydrogen peroxide, it will not leave any residue. Many people are not familiar with the fact that H2O2 can also fuction as a reducing agent in many reactions.

[Edited on 9-3-2013 by AndersHoveland]


If he is bubbling chlorine gas through water, wouldn't he be making HOCl and HCl? Would it be necessary to add additional HCl? Or is this what you mean when you say the equlibrium of elemental chlorine in water?

[Edited on 30-3-2013 by Traveller]

kadriver - 26-4-2013 at 21:58

Quote: Originally posted by garage chemist  
A mixture of HCl and NaClO3 dissolves gold very rapidly upon warming, faster than full strength aqua regia.
This also gives a solution free from nitric oxides. Residual chlorate can be destroyed by adding excess HCl and boiling until the chlorine smell is gone, which only takes a few minutes.


I have both of these chemicals available.

I must give this a try because I have never heard of it.

One way to rid the nitric from gold that has been dissolved in aqua regia is through evaporation.

The dissolved gold is slowly evaporated to a syrup, then HCl is added to drive off the nitric oxides.

You can watch the red gas escape upon addition of the HCl.

Sometimes the evaporation can be carried out too long and a crust forms with bits of gold metal.

To get the bits of gold metal back into solution, a few crystals of NaCLO3 are added after rehydration with HCl.

The addition of NaCLO3 causes chlorine gas to be formed and this is what re-dissolves the small bits of metalic gold.

But I have never used HCl and NaCLO3 to completely dissolve the gold - no nitrates to deal with is a big plus in refining gold.

If nitrates are present during precipitation of the metalic gold, then others metals may precipitate out also, contaminating the fine gold.

This is the main reason (so I have been told) for eliminating the nitrosol compounds before precipitating the gold with sulfur based precipitants.

Thanks for the tip garage chemist!

kadriver