# SO2 Sulfur Dioxide Gas Generator



## kadriver (Jan 30, 2013)

Hello:

I am trying to learn how to produce some clean SO2 gas for precipitating pure gold from an aqueous solution of goldIII chloride.

This reaction can also be produced by adding sodium metabisulfite to the goldIII chloride solution.

But it is thought that the clean gas, bubbled through the goldIII chloride produces a cleaner precipitation with less drag-down of other metals (if present in the goldIII chloride solution).

I can get the gas from my gas supplier, but they want a rental fee on the container of $0.50 per day.

I have all the equipment needed to make the gas.

Can I use sodium metabisulfite (Na2S2O5) mixed with hydrochloric acid to make some clean SO2 gas suitable for clean precipitation of gold from a solution of goldIII chloride?

Should sulfuric acid be used instead of hydrochloric acid?

Should the gas be scrubbed or cleaned through a bubbler prior to being used to precipitate the gold?

Any help here would be greatly appreciated

kadriver


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## butcher (Jan 30, 2013)

Here are a few ways to generate sulfur dioxide gas:

You can just burn sulfur (without air) to make sulfur dioxide gas SO2; a generator could be made from pluming pipe, or some other metal container.

S8 +8O2 -->8SO2 (g)

High heating sodium metabisulfite (sulfur is cheaper)
Na2S2O5 --HEAT --> Na2O + 2SO2 (g)

Dripping HCl acid onto a powder of sodium bisulfite:

NaHSO3 + HCl --> + NaCl + H2O SO2 (g)

Concentrated sulfuric acid heated with copper will generate SO2 gas 
Popular science had an article for making the generator.

http://books.google.com/books?id=fCYDAAAAMBAJ&pg=PA52&lpg=PA52&dq=sulfur+dioxide+gas+generator&source=bl&ots=2LUTmrzZQI&sig=gvlQ7Zb4pfClMvHBg4-3Rkm3o-8&hl=en&sa=X&ei=rXkJUfSuJOrjigKyvoG4Bg&ved=0CGIQ6AEwCA#v=onepage&q=sulfur%20dioxide%20gas%20generator&f=false


Dripping HCl onto sodium metabisulfite:

Na2S2O5 + 2HCl --> NaCl + H2O + 2SO2 (g)

Sulfuric acid dripped into sodium metabisulfite:

Na2S2O5 + H2SO4 --> Na2SO4 + H2O = 2SO2 (g)

:lol: we would need pictures of your decision


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## kadriver (Jan 31, 2013)

Great info Butcher - thank you.

I am planning to use sodium metabisulfite and sulfuric acid.

I'll add a little water to make a slurry of SMB, then drip H2SO4 through a separatory funnel.

I'll try and make a video as I go, but being my first experiment with the gas, this could prove to be challenging.

Maybe I'll do a trial run with no video first, then when I get the process down, I'll redissolve the gold and do it again, making a nice video of the entire process.

Thanks again Butcher, you have always been a tremendous help to me, and to this forum.

Kadriver


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## Irons2 (Jan 31, 2013)

butcher said:


> Here are a few ways to generate sulfur dioxide gas:
> 
> You can just burn sulfur (without air) to make sulfur dioxide gas SO2; a generator could be made from pluming pipe, or some other metal container.
> 
> ...



One needs a supply of Oxygen if Air is not used. 8)

Here's a great video of Sulfur burning in pure Oxygen:

http://www.youtube.com/watch?v=V1sQO91UvFI


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## butcher (Jan 31, 2013)

Thanks Irons,

Not sure how I done that (without air), then had all of the oxygen in the reaction, I may have been holding my breath too long sometimes I believe my brain is starved of oxygen.


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## kadriver (Feb 2, 2013)

I did it with SO2 gas.

I made the gas using a slurry of 100 grams of Na2S2O5 and dripping 50% sulfuric acid on it in a gas generator.

I bubbled the gas through some concentrated sulfuric acid to dry the gas (not that it was necessary).

The gold precipitated out perfectly - I now have a new precipitant in my arsenal.

Here is a video of the entire experiment complete with a mishap (gas delivery tube glogged with powdered gold).

kadriver

[youtube]http://www.youtube.com/watch?v=1Usv2Lh10nQ[/youtube]

Edited one time to add a parenthesis


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## mjgraham (Feb 2, 2013)

I like it, thanks for the video.


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## butcher (Feb 2, 2013)

Kadriver, 

I love it, I also like the lab glass setup.

That is one way that too much SMB would not be used to precipitate the gold, you could not use too much SO2 gas, and the SO2 gas is not adding more impurity to your gold, maybe members who like to use way too much SMB could use this to keep from adding too much SMB.

I agree there would not be much reason to dry the sulfur dioxide gas with the concentrate sulfuric in your second flask, which could make the setup a littler simpler.

I also wonder if you started a good gas flow first before putting the gas bubbler tube down into the gold chloride solution, if it would help to keep solution out of the tube so your bubbler would not get stopped up with gold precipitate in the end of the tube?

You are making me want to get some more lab glass (and actually use it).

Thank you for doing the video it was great, very well done.


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## kadriver (Feb 2, 2013)

Thanks to both or you.

I would recommend gassing the gold chloride with SO2 to anyone.

The tube got plugged up for the exact reason you cited Butcher.

I assembled the apparatus completely before I started the acid drip and gas flow.

I have made other gases, but never SO2

When I started the drip the first time the gas flow was erratic, even halting a couple of times due to insufficient acid drip and poor reaction between the acid and the Na2S2O5 in the gas generator. 

I was dripping the sulfuric acid into the dry Na2S2O5 in the gas generator, the acid would slide off the side of the pile of chemical and form a pool, but not much reaction (this was later fixed by making a slurry with a little distilled water).

I need some sort of equalizing tube between the funnel and the gas generator - maybe another piece of glassware and another experiment.

When the gas flow stopped, the gold chloride solution was allowed to enter the tip of the gas delivery tube in the bubbler.

Since it was in direct contact with the SO2 gas inside the tiny tube, gold began to precipitate inside the tube.

That is how the tip got plugged up.

I cleaned it out by disassembling the bubbler, inverting the tube and back flushing the little bits of gold that had formed into a clean beaker.

Its not in the video, but I recharged the gas generator by added much more Na2S2O5 than the first go around (100 grams vs 50 grams).

I also filled the funnel with 50/50 dilute sulfuric acid (which turned out to be unnecessary). Better to have too much and not need it than the other way around!

Plus I added about 10 or 15 ml distilled water to the sodium metabisulfite in the gas generator to make a slurry the second time around.

The first time around I dripped the acid on dry sodium metabisulfite - I think this dry configuration contributed to the erratic flow of gas, and to the formation of gold powder in the tip of the gas delivery tube in the bubbler.

It was very exhilerating to watch the gold precipitate from the bubbling gas.

bubbling the gas through the sulfuric acid in the small flask between the gas generator and the bubbler turned out to be a good gas flow indicator.

The experiment was a success, I just wanted to demonstrate to myself what the reaction would look like, how long it would take, and to get the experience of doing it.

I still want to see if my gas supplier can get me a small bottle of the gas - it would be much simpler if I could just turn a valve to get the gas to flow.

But building and runnung the apparatus was a blast!

Thank you.

kadriver


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## lazersteve (Feb 4, 2013)

Great video Kevin! 

Have you tried the SO2 gas on a dirty pregnant solution, say one containing copper chloride or iron chloride as are common when refining cpu scrap? 

As a side note to your wash routine:

I have found that ammonia is not required and in fact is undesirable in your waste stream. Silver chloride can be removed easily and completely *before precipitation* with proper cooling, dilution, and filtration of the pregnant solution. For these reasons I no longer use any ammonia or it's compounds in my recovery and refining work. I consider ammonia a hazardous contamination in my refining cycle. I made this decision when I was filtering a solution containing ammonia and the entire filter spontaneously burst into flames. 

I also don't understand how people are adding too much SMB when precipitating gold. I precipitate numerous times a day with dry SMB and never have problems with excess SMB even with the dirtiest of solutions. I stir vigorously and add the dry SMB without weighing or measuring. When I smell SO2 I quit adding. The only thing I can figure is that they are using way too much nitric acid and not killing the excess with Sulfamic acid prior to precipitation. 

I've generated a variety of gases myself and it's very fun, but have found solid forms the the same gases to be much safer and 'user friendly'. 

Here's a great site for those of you that want to generate your own gases on a small scale using syringes, baggies, and tubing:

Microscale Gas Chemistry

Steve


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## kadriver (Feb 14, 2013)

Steve, 

I just recently learned that ammonia would dissolve silver chloride.

I have heard that it is dangerous and even explosive when mixed with certain other compounds.

I would have had to change my shorts if I seen a filter paper burst into flames like that.

And you are 100% on the button with removal of AgCl through cooling - I had a gold chloride solution that was crystal clear. I had to leave abruptly and when I returned the next day there was a fine layer of silver chloride deposited on the bottom. It was very cold that night and my shop is in the garage.

It made a believer out of me with respect to cooling to remove silver chloride.

I will heed your warning with regard to ammonia - thanks for the heads-up.

I have not tried SO2 except this one time. I am trying to get my gas supplier to allow me to fill a converted O2 bottle with SO2.

They want $0.50 per day rental fee to use one of their bottles.

Thanks Steve.

kadriver


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## lazersteve (Feb 15, 2013)

Kevin,

I was pretty shocked by the filter incident myself. I was doing a coarse filtration to separate ammonium hydroxide dissolved silver chloride using several layers of 15" coffee filters. The solution drained through and the sunlight ignited the filter paper. The whole thing burnt away in a matter of seconds, melting the large (5 gallon bucket size) plastic colander... very scary indeed!

Steve


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## kadriver (Feb 16, 2013)

I have never dissolved a large quantity (say a half liter) of silver chloride, only small amounts doing aquaintance experiments.

But I can see where this would be a useful refining technique if I get a bunch of junk mixed in with my silver chloride.

If I did use ammonium hydroxide to dissolve silver chloride, making sure to keep the filter paper rinsed with fresh water during the filtering operation, and do it in a fumehood, I think it could be done safely.

I can see where ammonia boils are unnecessary if the silver chloride is properly removed with chilling before precipitation.

Thanks - Kevin


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## lazersteve (Feb 17, 2013)

I keep all the solids left after incineration of the filters from each GF batch until I have a few liters of the burnt material. Once I save up a load of silver chloride laced junk and gold all mixed together, I use diluted acid and a zinc bar to convert the silver chloride back into metallic silver. Once the chloride is all converted, I vacuum filter, rinse well, and treat the solids with diluted nitric to remove the silver. This leaves the gold powder, rhodium foils, and junk in the filter mash ready to be leached with AR. It never ceases to amaze me how much gold gets trapped in the silver chloride left overs from gold filled processing. As an added bonus, you will find Rh foils, loose stones, and the occasional piece of platinum or karat gold scrap in the mix.


Steve


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## kadriver (Feb 19, 2013)

lazersteve said:


> I keep all the solids left after incineration of the filters from each GF batch until I have a few liters of the burnt material. Once I save up a load of silver chloride laced junk and gold all mixed together, I use diluted acid and a zinc bar to convert the silver chloride back into metallic silver. Once the chloride is all converted, I vacuum filter, rinse well, and treat the solids with diluted nitric to remove the silver. This leaves the gold powder, rhodium foils, and junk in the filter mash ready to be leached with AR. It never ceases to amaze me how much gold gets trapped in the silver chloride left overs from gold filled processing. As an added bonus, you will find Rh foils, loose stones, and the occasional piece of platinum or karat gold scrap in the mix.
> 
> 
> Steve



This is a great tip, thank Steve


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## lazersteve (Feb 19, 2013)

Kevin,

The best part of the outlined process (besides the gold and rhodium!) is that you don't add any ammonia to your waste stream. I also do not use ammonium chloride to precipitate my PGMs either. Potassium chloride works fine and no pesky ammonia to deal with.

Steve


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## Shark (Dec 2, 2014)

Thanks everyone for the post. I knew I had seen it before, but it took nearly a week of digging to find again. I hope to try this in the next few weeks if all goes well, and was missing the little bits of wisdom I re-found here.


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## Shark (Dec 31, 2014)

lazersteve said:


> Great video Kevin!
> 
> Have you tried the SO2 gas on a dirty pregnant solution, say one containing copper chloride or iron chloride as are common when refining cpu scrap?
> 
> ...



That web sight has some very good information. I missed it the first time, but am more than happy to have kept reading this thread over and over. Thanks for such a great link.


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