Dissolving Gold with Chlorine Gas from a Gas Generator

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kadriver

kadriver

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Joined
Oct 25, 2010
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Here is another experiemnt I did to demonstrate the use of chlorine gas to dissolve gold powder.

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

The gold powder (about 2 grams) was from a previous refining using aqua regia to dissolve the inquarted gold, then sodium metabisulfite to precipitate the pure gold.

After dissolving with chlorine, I precipitated the 2 grams of gold using oxalic acid - the precipitation is not shown in this video.

This gas is no joke deadly and I don't see any way of doing this procedure without the use of a fume hood.

Even in the hood I still got a few wafts of the chlorine as I did the experiment.

kadriver
 
The down side to chlorine gas in HCl is it will only work if the gold scrap has a huge surface area. You used powder and a few pieces were still stubborn and didn't want to dissolve quickly. This just won't work with granulated karat scrap, it takes too long.

The answer is atomizing the scrap to get huge surface area and just adding powdered sodium chlorate to the acid to release chlorine gas and dissolve the gold. Big plus is no nitric to get rid of. Still need a hood!
 
Another great video Kevin.

A few suggestions:

1. Get a 3 neck flask for the gas generator, one for liquids in, one for solids in and one for gas out. I have several of these flask and find them very handy. You can buy 24/40 jointed valves to control the gas out line and a simple glass stopper for the solids input port.

2. Put a little heat to the reaction flask. 40 C would be plenty. Too much heat drives the chlorine out, too little and your reaction is slow.

3. Add stirring to the reaction flask. A simple magnetic stirring hotplate works wonders. The secret to dissolving gold with HCl-Cl is stirring.

4. Don't dry out your gold powder if you are going to redissolve it with chlorine gas and HCl. Transfer the moist gold powder directly from your buchner or wash beaker into the gas apparatus reaction flask.

Steve
 
Great info Steve, thanks for sharing that.

I have a hot plate stirrer and I should have used it.

I did not mention it in the video (I should have) but I did add low heat to the beaker to try and help the gold dissolve.

I just received a brand new organic chemistry set from SGV Scientific.

It is a 30 piece set and has a 3-neck flask.

http://www.ebay.com/itm/NEW-Advanced-Organic-Chemistry-Lab-Glassware-Kit-24-40-30-Pcs-the-PREMIUM-Series-/270962763485?pt=LH_DefaultDomain_0&hash=item3f16a3a6dd

I'll need a gas-out adapter to connect the deliver hose to the 3-neck flask.

Cool stuff!

kadriver
 
4metals,

I agree, trying to dissolve anything other than the powder would take way too long.

I must try the sodium chlorate in HCl for my next experiment.

How does one go about atomizing the gold?

kadriver
 
Just some thoughts from my rambling mind,

The chlorine is not staying in the acid solution long to dissolve the gold, bubbling through so fast.

Maybe a taller reaction vessel like a very large test tube with magnet stirring to keeping gold powder up in solution, so the gases have to be traveling further though solution to escape, smaller and more bubbles for better surface area.

could you chill HCl saturate it with chlorine gas, before adding the gold, then warm it up slowly with gold in solution and the generator bubbling more chlorine in as the solution warms up.

As we know heat can speed the reaction but is a double edged sword because heat also forces chlorine out of solution faster.

Maybe some experimenting with temperature.

When we dissolve gold powders in HCl and bleach NaCLO, the chlorine is formed in solution with the acidic HCl and the slightly basic bleach, also the oxygen in the bleach may play some part in helping to act as an oxidize in solution, the bleach also also lower pH of our acid (which may also form some HOCl in solution), this may be beneficial to the reaction helping to keep chlorine in solution longer to oxidize the gold, (the sodium from the hypochlorite [NaOH and NaClO] forms salts with the HCl/NaClO leach which can a problem if trying to concentrate solution to remove excess chlorine or concentrating a PGM solution).

With HCl/NaClO (Clorox) or 4HCl/HNO3 (aqua regia) we also have oxygen also reacting in the formula's.


I wonder how possibly, dilution, pH adjustment or even adding a little Hydrogen peroxide to add oxygen may help to keep chlorine in solution longer, or to react with gold.
 
Years ago I worked in an analytical lab and had to measure total cyanide from mixed cyanide solutions, to do this you enclosed the sample in a flask and added HCl in one side of a flask and allowed HCN to bubble out the other side. The HCN bubbled through a sodium hydroxide solution was converted to Sodium cyanide making the analysis less complicated. The gas bubbles went into a spiral bubble trap which allowed the bubbles to enter the bottom and spiral up the grooves in the vessel allowing a very long contact time and complete dissolution of the gas into the caustic liquid.

Now forget about cyanide, that same trap used to increase contact time would work for chlorine gas in the acid as well. I googled a few descriptions of the piece and came up with some amazing weird results but not what I was looking for, if I can remember what it was called I'll post a picture.

But I think this is essentially what Butcher is talking about.
 
There are many thought provoking ideas here. Carefully controlling temperature to see what temp promotes dissolution of the gold. Adjusting and monitoring pH to get the best range to dissolve the gold with Cl2 gas.

I am accumulating the apparatus that will enable me to do some of these experiments.

I would really like to find an inexpensive pH monitor that would enable me to continuously monitor the pH of a boiling solution - such as oxalic acid, or hot chloroauric acid solutions.

I knew that having the gold powder laying on the bottom of the beaker like that was not an efficient way to do this experiment. The correct glassware to bring the gold powder and the Cl2 gas together is probably available, just have to hunt for it.

Maybe something like this mineral oil bubbler. It could be tilted at an angle on a hot plate stirrer with a tiny stir bar. Instead of mineral oil, the HCl and gold powder could be placed in the tube, sealed and then the Cl2 gas bubbled through.

kadriver
 

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This works best on PGM blacks. Weird thing is, it seems to go faster on Pt/Pd than Au?


Pressure is the name of the game!
 
Kevin, Lou has given you the answer pressure,he may not say much but I've learnt to listen carefully when he speaks.
As to atomising the gold this was discussed some years back here on the forum,it's basically molten gold poured through a doughnut of high pressure water jets into a large collection vessel, if i remember correctly its then processed through a controlled form of the chlorox method, it's the method many large commercial refiners use now as its quick and can be used to process huge volumes and all values are available within 24 hours and no nasty nitrous fumes or nitric solutions to contend with, its replaced the Miller process. The downside is the cost of the atomising equipment for the small refiner but I'm sure someone will crack that here on the forum at some stage with all the various experts in so many different walks of life represented, we just need to work on it.
 
Chlorine gas dissolves quite easily in water and becomes hypochlorous acid and hydrochloric acid, if the pH is above 5. In this case, what is not dissolving is bubbling to the surface as a gas. Also, from what I've been able to research, not enough HCl is made to seriously alter the pH from neutral, if that is where you begin.

The same solution of hypochlorous acid can also be made much more easily and safely from a strong bleach solution (calcium or sodium hypochlorite) by gently lowering the pH with HCl, in small increments, to just above neutral. If sodium or calcium chloride is added, it will put gold into solution, without the addition of any further HCl than what is used to bring the hypochlorite solution to just above neutral. However, the solution must be kept under pressure during the leaching process or the unstable HOCl will oxidize to the atmosphere and leave HCl behind, seriously lowering the pH, and potentially releasing chlorine gas. Aside from Cl2 gas being a nuisance, the unwanted lowering of the pH of this chlorine solution will, once a certain acidity is reached, allow this solution to break up any iron oxides present. Once iron is not tied up as an oxide, it will go into solution instead of the desired gold.
 

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