As a bonus, addition of HBr has two more effects. Elemental bromine is less volatile and can be held back to a certain extent by letting pass escaping gases through a reflux-condenser. Premature volatilization of elemental chlorine can be suppressed by adding a few volume-percent 48% HBr (azeotropic mixture). Reaction-temperature: 60 o-80 oC, at the end of dissolution boiling temperature (outgassing of excessive Cl 2). 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). Use of a concentrated NaClO 3-solution is ok. So let me add a few comments to the answers, you already have got. 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).Ībout 25 years ago, I probably was in a similar situation, like you now. That's the first point, I am wondering about. As I can see, you're working on a big project, apparently involving respectable quantities of gold, platinum and palladium, but no silver. Welcome to the forum, as an other refiner from Switzerland. 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.
Nox installer stops at 99.5% skin#
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. Lou's logic is perfectly sound for stone removal where the metal cannot be alloyed to keep the silver low. For this reason I have found that, in practice, limiting the solubility of silver in the digestion pays off in higher purity. 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. I strive to keep the temperature low in the reaction to the point of dumping volumes of ice into the reaction to cool it.
Given Lou's background, I have no doubt it will work if he says it will. 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. 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. The efficiency increased as the feed rate of the oxidizer decreased. 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. As Lou stated it's the Cl2 that is doing the work. 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. 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. As Lou has mentioned, the stoichiometry is not as important as one would imagine.
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