Sciencemadness Discussion Board

Help Please! Reaction Au+NaClO3+HCl

Essigsäure - 17-9-2013 at 06:41

Hey,

I need help for this reaction.

Theoretical the stechiometrie is :

2 Au + NaClO3 + 8 HCl ---> 2 HAuCl4 + NaCl + 3 H2O

But I don't have found anything about the operative conditions (Temperature, L of NaClO3/h, ...)

And what are the danger of this reaction? What is better and wrost than the reaction with Aqua regia (HCl+HNO3)?

Help Please!!!


woelen - 17-9-2013 at 09:48

It's not a really dangerous reaction. The most dangerous are the fumes. They are very corrosive and toxic, but the same is true for the fumes of aqua regia.

You could start with hydrochloric acid (20%), the gold immersed in the acid, and add solid NaClO3 to that. Stir well, such that the NaClO3 dissolves. The gold will dissolve as well, slowly. You get green fumes, which are mainly Cl2, but also some ClO2 (no need to fear explosion of this, it is too dilute for that). DO THIS REACTION OUTSIDE OR IN A VERY GOOD FUMEHOOD if you value your lungs.

[Edited on 17-9-13 by woelen]

Metacelsus - 17-9-2013 at 10:38

I use hydrochloric acid and sodium chlorate (1:1 molar ratio) sometimes to clean nasty stains off of glassware. It's best if you make a saturated solution of sodium chlorate and then add it to the hydrochloric acid -- at the 1:1 ratio, the chlorate takes a really long time to dissolve. Woelen is right about the fumes; always do it outside. You would be using an 8:1 molar ratio, so the fumes would probably be less, but you should still watch out.

Traveller - 17-9-2013 at 18:22

Unless you really enjoy breathing chlorine gas, why not try this.

Take sodium hypochlorite (NaClO) or sodium chlorate solution (NaClO3), or a mixture of the two, add un-iodized salt (NaCl) to 10% volume of this solution and then SLOWLY begin adding tiny amounts of HCl to this solution. Between additions of HCl, check the pH of the solution with a pH tester; aiming for a pH of 7.5 and NO LOWER.

You will then have a solution of 50/50 hypochlorite/hypochlorous acid, chlorate/chloric acid. The hypochlorous/chloric acids will provide oxidation and the NaCl will provide the chloride ions. This WILL put gold into solution as a chloride WITHOUT gassing you with chlorine gas. This solution also will not put base metals into solution IF your ore is roasted in the presence of oxygen, converting base metals to oxides.

The only down side to this is that HClO and HClO3 are very unstable; wanting nothing more than to lose their oxygen to the atmosphere and become HCl. Your ore and this solution then must be put into an airtight chamber for this process to work and every last bit of air must be excluded from this chamber by solution, allowing any oxidation of these two acids to build up a back pressure that will prevent further oxidation. The chamber must be revolved for 2-3 hours to allow all gold to go into solution. At that time, the pregnant solution is drained and filtered and gold precipitated with sulphur dioxide, sodium metabisulphite, ferrous sulphate or even granular pieces of zinc.

[Edited on 18-9-2013 by Traveller]

woelen - 17-9-2013 at 23:12

Quote:
[...] Your ore and this solution then must be put into an airtight chamber for this process to work and every last bit of air must be excluded from this chamber by solution, allowing any oxidation of these two acids to build up a back pressure that will prevent further oxidation.[...]

Most of the post I understand and it is an interesting thought to try this without all the Cl2 escaping from the solution, but I do not get the idea of having air locked out. You are talking about 'further oxidation'. I do not understand in this context. It is all oxidizing stuff around and you want that. I even think that tightly sealing the container is quite dangerous. What if so much oxygen and/or chlorine is produced that the container explodes due to pressure buildup? I would not feel comfortable with such a 'time bomb' around. So, my advice is to try this method, but not using a tightly sealed container.

Traveller - 18-9-2013 at 06:33

This process CANNOT produce chlorine gas, as the pH is never allowed to go below 5.

Think of chlorinated water in your town's water distribution system. If the pH of the water is anywhere near neutral, a good part of the chlorine in the water, whether it was introduced at the treatment plant as a hypochlorite or chlorine gas, will exist in that pipeline as the oxidizer hypochlorous acid (and hopefully not chloric acid as this is unhealthy). As long as there is only water in the pipeline, and it is under any kind of pressure, the HClO does not oxidize to the atmosphere. However, it is still able to oxidize and kill bacteria, as well as things like iron, manganese and H2S.

The point here is that HClO releasing oxygen in a sealed container is controlled by back pressure, and it does not take very much pressure to stop the release of oxygen. However, once HClO is exposed to atmospheric pressure, as in pouring water from the tap into a glass, the HClO in that water quickly breaks down and becomes HCl, with the O2 released to the atmosphere.

Think of a bottle of Coke with the cap on. No CO2 is visible bubbling to the air gap in the neck. This is because CO2 dissolves in solution at 25 psi, which will be the pressure in the neck. As soon as you "pop" the cap off, pressure in the neck returns to 14.7 psi, back pressure is removed and the CO2 begins bubbling to the surface.

It is not even necessary to use HCl in this process. The NaCl will provide all of the chloride ions. The pH of the NaClO can be lowered in a much safer manner by using vinegar (acetic acid).

Before the introduction of cyanide leaching around 1900, this process, and other variations of it, was the most used method for recovering gold from mining ores and was only abandoned because cyanide was cheaper and much easier to use.

You can try this without using a sealed container but, the oxidizers will likely take the path of least resistance and give up their oxygen to the atmosphere instead of going to work oxidizing gold.

Essigsäure - 18-9-2013 at 07:28

Ok, but my question is now:

This is the principaly reaction:

2 Au + NaClO3 + 8 HCl ---> 2 HAuCl4 + NaCl + 3 H2O

And wich are the secondary reaction? Can somebody me write all the secondary (or the principaly) stechiometric reaction?

Traveller - 18-9-2013 at 08:24


8NaClO + 2Au + 6HC2H3O2 --> 2NaAuCl4 + 6NaC2H3O2 + 3H2O + O2

Sodium Tetrachloroaurate or "Sodium Gold Chloride"

Traveller - 18-9-2013 at 09:15

Precipitation of gold:

6SO2 + 6H2O + 12NaC2H3O2 + 2NaAuCl4 -->
12HC2H3O2 + 3Na2S2O6 + 8NaCl + 2Au

woelen - 18-9-2013 at 09:57

@Traveller: I agree with all your reactions as described above and will not comment on those, there is only one thing which I do not agree upon and that is your remark about the HClO in the water pipes of the municipal water network. The big difference is that in that network, the amount of HClO is VERY low, compared to what is used for dissolving gold.

Another thing is that the comparison with CO2 from a soft drink bottle is not a very good one. The CO2 in such a bottle already is present as CO2 and it is simply that lower pressure reduces solubility of CO2 in water. With oxygen from HClO in water, the back-pressure of already existing oxygen does not reverse the reaction. Hypochlorite and hypochlorous acid which have decomposed to chloride or HCl and have given off their oxygen as O2 cannot be converted back to hypochlorite or hypochlorous acid. The reaction is not a simple equilibrium, it is one way! I am quite sure that if this process was used in the past (and I certainly believe it was), then there was some mechanism in the plants which allowed venting of pressurized gas (which indeed most likely was fairly pure oxygen with only a small fraction of chlorine).

Traveller - 18-9-2013 at 10:36

Quote: Originally posted by woelen  
@Traveller: I agree with all your reactions as described above and will not comment on those, there is only one thing which I do not agree upon and that is your remark about the HClO in the water pipes of the municipal water network. The big difference is that in that network, the amount of HClO is VERY low, compared to what is used for dissolving gold.

Another thing is that the comparison with CO2 from a soft drink bottle is not a very good one. The CO2 in such a bottle already is present as CO2 and it is simply that lower pressure reduces solubility of CO2 in water. With oxygen from HClO in water, the back-pressure of already existing oxygen does not reverse the reaction. Hypochlorite and hypochlorous acid which have decomposed to chloride or HCl and have given off their oxygen as O2 cannot be converted back to hypochlorite or hypochlorous acid. The reaction is not a simple equilibrium, it is one way! I am quite sure that if this process was used in the past (and I certainly believe it was), then there was some mechanism in the plants which allowed venting of pressurized gas (which indeed most likely was fairly pure oxygen with only a small fraction of chlorine).


Believe it or not, I am actually the operator of a small water system serving a town of 500 people.

You are mistaken about HClO. The percentage of HClO in water does not make a difference in how it degrades to HCl. A higher percentage of HClO simply means there are more HClO molecules. All that will happen, if the levels of HClO in water are elevated, is that the release of oxygen from the HClO molecules will be greater and the back pressure achieved much quicker, thus retarding and stopping the release of oxygen from the HClO molecule.

I do not know if, in your last paragraph, you were referring to a water treatment plant, or an old mining operation. In either case, NO venting of oxygen ever took place. Here is an article from the 1890's, describing a process similar to the one I described. In this process, however, they are making hypochlorite/hypochlorous acid through electrolysis of brine. Interestingly, the gold chloride is returned to the electrolysis cell, gold is deposited at the cathode, and the "chlorine solution" regenerated through electrolysis and sent back to the leaching chamber.

http://www.miningandmetallurgy.com/metallurgy/electrolytic-p...

[Edited on 18-9-2013 by Traveller]

[Edited on 18-9-2013 by Traveller]

Traveller - 18-9-2013 at 10:39

BTW, if you are interested, I can prove that a strong solution of HClO, kept in a sealed container, will not "blow up" or lose any great amount of oxygen.