Refinind gold with NaClO3 and HCl

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Essigsäure

New member
Joined
Sep 19, 2013
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3
I am a chemist and I made a research for gold refining.

I want use the reaction with NaClO3 (solution saturated) and HCl, but I dont found litterature.

Theoretical the stechiometric reaction is: 2 Au + NaClO3 + 8 HCl ----> 2 HAuCl4 + NaCl + 3 H2O

or: 2 Au + 2 NaClO3 + 9 HCl ----> 2 NaAuCl4 + 3 H2O + 3 HClO

But I am not sure about this reactions.

Somebody can me help with the principal and secondary stechiometrics reactions, and with the operational conditions (Temperarure, Lt/h, ...)
What are the danger of this reaction? This reaction is better than the reaction with Aqua Regia? What are the advantages and the disadvantages?

Thank you
 
Sounds interesting.

I'm not familiar with use of NaClO3 as a starting reagent in gold refining, although it's sometimes mentioned as an intermediate (under certain conditions) when using HCl/NaClO.

There's a little information on the former and a lot on the latter in this forum, including some current threads. Try a search on NaClO3 and NaClO.
 
It can be used.


I prefer to use KClO3/HCl for removing stones because AgCl is much more soluble in KCl solutions.
 
The stoichiometry is almost meaningless in practical application, for it is Cl2 produced by oxidation of the HCl that is doing the work. Cl2 is always used in excess. Obviously, the sweet spot for use is at a temp where the dissolution rate is at its maximum relative to the equilibrium of dissolved Cl2 (too hot and the rate slows at atmospheric pressure). I would suggest 40-60 C at 6-8 M HCl, adding in the chlorate as required to prevent violent foam over and inefficient gas loss.

Chief danger is the chlorine, and Butcher has mentioned potentially dangerous chlorine dioxide (which has never gave me a problem when using this system).
 
As Lou has mentioned, the stoichiometry is not as important as one would imagine. In a perfect world where we were digesting pure metal it would be different and closely following the stoichiometric equations would allow you to conserve on reagents. In a production world where the base metals and the silver content vary, the equations would change to a point it would be maddening to produce a stoichiometric and efficient reaction. As Lou stated it's the Cl2 that is doing the work. When I originally started with this method, I did the math to figure the exact quantities of reagent and did find that the reactions ran in the 65-75% efficiency range as you cannot stop gassing off Cl2 and removing it from the reaction. The efficiency increased as the feed rate of the oxidizer decreased.

The post I referenced above uses atomized alloy in the digestion process, Lou has mentioned using the potassium form in stone removal due to the increased solubility of the silver over the sodium form. I have never tried using chlorate / HCl for stone removal mainly because my experience with the chlorate process has shown me that the reaction proceeds quickly only when the surface area is high. Given Lou's background, I have no doubt it will work if he says it will.

I strive to keep the temperature low in the reaction to the point of dumping volumes of ice into the reaction to cool it. If you look at the solubility curves for silver chloride you will see it is exponentially less soluble in cold acid, I have found that if it gets too hot, the silver is sufficient to lower the purity of the gold when it is dropped. For this reason I have found that, in practice, limiting the solubility of silver in the digestion pays off in higher purity. Lou's logic is perfectly sound for stone removal where the metal cannot be alloyed to keep the silver low. With stone removal you have to digest the alloy as it is presented to you and the skin effect of silver chloride can stop this reaction. So for atomized scrap, the sodium form is cheapest and the silver is not an issue, for stone removal, I would defer to Lou's experience and try the potassium form.
 
I should say that chlorate is only to be used when cylinder delivery of chlorine isn't an option (usually in cities and municipalities). HCl/chlorine is the pre-eminent dissolution method for PGMs, but has a slower rate of dissolution when used on gold. I'm sure there's a mechanistic reason for this but I've not looked into it.

Also, it's not very effective on bulk solids. Like most things we do as refiners, it's a surface area game!
 
Thank you realy much for your answears.

My project preview the use of gold with pt and pd, but without silver.
For this reason I don't have the problem with silver chloride.
For this project I have a glass reactor of 800 lt and a automatical system for the temperature, the pressure and the dosage regulation.
I can use O2 for the oxidation, I dispose of HCl 32% and pure NaClO3 (99.5%).
I have think to use a saturated solution of NaClO3 (50%) for the reaction.
For the temperature, I have found in the literature that the optimale temperature for the dissolution of gold with HCl/Cl2 is 80 C.
We don't use more the Cl2 for security reason (the principal reason is the storage).
And we don't want use more aqua regia for the excesive production of NOx.
For this reason we going to try to use NaClO3 for this reaction.

Thank you really much for yours advices :)

(p.s. Sorry for my english, but this is not my mother language and I have never good learn to write in english)
 
I have always added the chlorate as a powder with aggressive mixing in an open topped (ambient pressure) vessel. I would be careful adding chlorate to a closed reactor with atomized feedstock as you may exceed the pressure limits quickly.

This method will eliminate the NOx from both dissolution and removing it before precipitation of the PGM's. Due to the sodium chlorate, which is in excess in the solution, the Palladium will co-mingle with the Platinum when you add the ammonium chloride to drop the Platinum. But they are easily separated later on.

Good luck with your project, please come back with some feedback after you begin processing.

By the way, welcome to the forum.
 
Hi Essigsäure,

Welcome to the forum, as an other refiner from Switzerland. As I can see, you're working on a big project, apparently involving respectable quantities of gold, platinum and palladium, but no silver. - That's the first point, I am wondering about. The second one is your 800 lt glass reactor, because in my active recovery/refining-times, I was not able, to find such a big one, despite my love for glass as a nearly ideal material for precious metals digestion reactions (high corrosion-resistance, transparency).

About 25 years ago, I probably was in a similar situation, like you now. So let me add a few comments to the answers, you already have got.

Use of a concentrated NaClO3-solution is ok. IMHO it is better, to use a less than saturated solution, because if salt crystals somewhere are formed, they grow there, where we don't want them to be (some kind of Murphys law).

Reaction-temperature: 60o-80oC, at the end of dissolution boiling temperature (outgassing of excessive Cl2). Premature volatilization of elemental chlorine can be suppressed by adding a few volume-percent 48% HBr (azeotropic mixture). Elemental bromine is less volatile and can be held back to a certain extent by letting pass escaping gases through a reflux-condenser. As a bonus, addition of HBr has two more effects. It accelerates the digestion reaction, and, if once present, lowers the solubility of silver significantly.

Lou writes: "Like most things we do as refiners, it's a surface area game!"
It is also a game of different redox-reactions. Oxidizing Br- in the digestion mixture delivers more reactive and less volatile Br2. Pd(II) in the reaction mixture can be further oxidized to Pd(IV), which in turn probably can help to oxidize not yet dissolved metals, to solubilisize them too.

Elimination of aqua regia (NOx) in the digestion-reaction: Did you ever think of using electrical current as the oxidant in the digestion process? This would be the most safe and most clean reactive in your envisaged processing.

Good luck! A feedback on your further processing would be a great pleasure for me.

freechemist
 

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