Talk:Gold parting

Salt cementation

The chemical explanation given cannot be correct. Sodium chloride does not decompose even when melted. It certainly cannot decompose spontaneously to sodium metal and chlorine gas, except at very much higher temperatures.

It is much more likely that the oxidising agent is oxygen from the air. One could write a balanced chemical equation such as

4Ag + O₂ + 4H⁺ → 4Ag⁺ +2H₂O

This would resemble the reaction used in gold cyanidation, in which the crucial oxidation of the metal is done by oxygen. The other components in the cementation process help by removing the silver ions that are formed once silver has been oxidised; this makes the oxidation possible as silver is not oxidised by air alone.

I don't know if this suggestion is correct and I haven't the time to check out the literature. Sorry about that. I normally prefer to make positive contributions, but in this case my contribution is negative, at least for the moment. Petergans (talk) 14:28, 1 February 2010 (UTC)

Hey, I've looked through my notes and I think I can answer your query. The equation,(Au + Ag) + NaCl → AgCl (g) + Au, was by Bayley (2008:143)(bibliography given on main page). This equation was based on the description of the salt cementation process given in Hoover and Hoover (1950) and which is described by Theophilus in that book. I agree that the dissociatation of common salt, NaCl, is not an easy process to accomplish, but it is possible under the right conditions. The salt cementation process as described by Bayley (2008:143; 1990:20)and Dodwell (1971:52) which are both based on the description by Theophilus have both been tested by experiment as is described by Notton (1974:54) which I will briefly describe here. The ingredients he used were a gold alloy (copper, silver (37.5% gold)), salt and brick dust. The alloy was beaten into thin sheets to increase surface area. The gold alloy sheets, common salt and brick dust were put into an earthenware pot with a lid and then heated for several hours at 800 degrees C. Notton stopped heating when 'no more salt fumes were evolved'. The resultant refined alloy contained 93% gold. Therefore the process with the ingredients described above in the conditions described does work. Bayley in his equation does his best to describe what is the most likely reaction process, that the sodium chloride decomposes at high temperature, which is probably aided by the present of silica and alumina (in the brick dust) which may act as a catalyst. The chlorine then acts on the silver (maybe as hydrochloric acid if water is present? or as free chlorine) to make silver chloride and the gold is left untouched (Bayley 1990:20). As a further point Notton tried salt cementation without brick dust, and heating at 800degrees C for 5 days and this also worked to refine the gold, but it took much longer. Hope this answers your query. Decomposing salt is not easy but it does seem to be the process occuring in the parting of gold and silver. Matthew Phelps (talk) 13:45, 26 February 2010 (UTC)

Thank you for this extensive reply. I have no doubt that the process works but unfortunately I think that there is a fundamental flaw in the argument. that the sodium chloride decomposes at high temperature, which is probably aided by the present of silica and alumina (in the brick dust) which may act as a catalyst. The point is, a catalyst cannot change the equilibrium compostion of a reaction. All a catalyst can do is to speed up an existing reaction. It cannot cause NaCl to decompose more than it would do in the absence of a catalyst. I can make the following observations.

• The temperature of 800C is very close to the melting point of NaCl - 801C. The presence of other substances may cause the NaCl to melt by depressing the melting point. Indeed, once some silver has been oxidised the Ag⁺ ion may dissolve in molten NaCl as AgCl. The brick dust may also function in this way.
• The reaction Ag +NaCl → AgCl + Na is intrinsically unlikely because sodium is much more electropositive than silver. This reaction is implied by the quoted equation:(Au + Ag) + NaCl → AgCl (g) + Au because the Na cannot just disappear.
• A balanced equation that does not require water to be present is : 4Ag + O₂ → 2Ag₂O. As I said before, this reaction does not occur with silver alone. What I meant by the effect of the other substance was that a further reaction such as Ag₂O +3NaCl → AgCl + Na[AgCl₂] + Na₂O could help to lower the total reaction energy. This is where the analogy with gold cyanidation comes in. In that case the formation of a cyano complex of gold allows oxidation of gold to take place. I am suggesting that the formation of a chloro complex may allow oxidation of silver. This is analogous to the tarnishing of silver at room temperature in the presence of sulphur containing compounds due to the formation of the species Ag₂S in which the silver has been oxidised by air.
• The mention of "salt fumes" is mysterious. It does suggest to me that moisture is somehow implicated.

It's a shame that all this is speculative, because it would be quite easy to show experimentally whether or not oxygen is required for the process to work. Petergans (talk) 17:41, 26 February 2010 (UTC)

The equilibrium isn't at issue, because the AgCl formed volatilises and escapes. 122.148.225.114 (talk) 17:59, 16 July 2015 (UTC)

Thanks for talking me through it again, I think you convinced me! I'm beginning to see what you mean, especially about the 'missing' sodium in the first equation and the problems of silver being able to 'steal' the chlorine from the much more reactive sodium. I'm at uni (not doing chemistry though!) so I think I'm going to have to make some enquiries in the chemistry department. And also read more details of gold cyanidation. And if it's still not clear what's going on, maybe, as you mentioned, I could enquire into running it as an experiment with and without oxygen if they let me. I'll let you know how I get on! Matthew Phelps (talk) 16:59, 1 March 2010 (UTC)

As you are not a chemist, you may find it helpful to break the gold cyanidtion reaction into its components.

1. oxidation of gold: Au - e^- → Au⁺
2. reduction of oxygen: O₂ +4e^- → 2O^2-
3. complexation of gold: Au⁺ +2 CN^- → [Au(CN)₂]^-
4. removal of oxide: O^2- + H₂O → 2 OH^-

The final balanced equation is obtained by first balancing the number of electrons in the oxidation and reduction and then also balancing the masses of the other reagents. With silver and chloride the same reactions could apply, with chloride in place of cyanide, except for stage 4, which will be 2Na⁺ + O^2- → Na₂O or something like that. Actually, sodium metal burns in air to form the peroxide Na₂O₂, so maybe that is a reaction product. It would be easy to check for peroxide in the reaction mixture. Formation of peroxide would significantly reduce the amount of energy involved in reducing oxygen as only two electrons are gained..

• O₂ +2 e^- → O₂^2-

Petergans (talk) 11:02, 3 March 2010 (UTC)

Schiston

This could be casein. Being composed of proteins perhaps it had the same role as adding ammonia and nitrates.

Although this connection is just as likely spurious and the "schist" just means parting and could in the case of casein just refer to the parting of the milk solids from the whey, and the "stone called schiston" could be an entirely different substance. 122.148.225.114 (talk) 18:10, 16 July 2015 (UTC)

References