chornedsnorkack
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Salt cementation reaction
It´s a classic reaction, invented by Lydians in ancient times of 7th...6th century BC, and the source of money of the world.
What happens?
The basic reaction is
Ag + NaCl + ??? -> AgCl + ???
How can the reaction ever go ahead?
I don´t expect Ag, let alone a small impurity of Ag in Au, to displace metal Na.
So you need oxidant. What is it?
Where does Na go?
Important basis for it is differential reactivity of Au and Ag. Au does form chlorides at low temperature under influence of aqua regia or chlorine,
but those decompose to metal on heating to about 300 Celsius. AgCl melts at 450 and endures over 1000 Celsius without melting.
So what are the ??? reagents and products above? What is the temperature range?
Pt forms PtCl2 that melts at 581 Celsius. When gold contains appreciable amounts of Pt alongside Ag, does Pt partition into the chloride
phase or remain in metal?
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Texium
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From a quick google of the subject, it appears that this process was not typically carried out using only salt. As with nearly all ancient chemistry
from before there was a good understanding of elements and compounds, there are many variations, and most of them involve very complex mixtures.
Typically these gold parting mixtures included salt alongside urine or a soil especially rich in nitrates and/or ammonium salts. This of course would
lead to a cyclic process of the silver being leached out of the mixture by formation of silver nitrate or a silver ammine complex, and the solubilized
silver being precipitated as AgCl. Then it's roasted at high temperatures to recover (substantially more) pure molten gold and vapors of AgCl which
are deposited and further refined to pure silver metal.
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chornedsnorkack
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In this
https://chemistry.stackexchange.com/questions/55734/how-were...
Berthelot is quoted as referring to "calcined red vitriol" as an ingredient.
Is it Fe2O3? Or Fe2(SO4)3?
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Fulmen
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Did you even try to research this? from Wikipedia:
Hoover and Hoover[16] explains the process thus: under heating salt (sodium chloride, NaCl) decomposes in the presence of silica and alumina (from the
brick dust or clay) to produce hydrochloric acid and also some chlorine. This reacts with the silver to produce silver chloride (AgCl).
We're not banging rocks together here. We know how to put a man back together.
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chornedsnorkack
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Quote: Originally posted by Fulmen  | Did you even try to research this? from Wikipedia:
Hoover and Hoover[16] explains the process thus: under heating salt (sodium chloride, NaCl) decomposes in the presence of silica and alumina (from the
brick dust or clay) to produce hydrochloric acid and also some chlorine. This reacts with the silver to produce silver chloride (AgCl).
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Yes, I tried just that! And the Wikipedia made absolutely no sense. Because silica and alumina are not oxidants and do not contain hydrogen. The
reaction
4NaCl+SiO2->4HCl+???
is flagrantly impossible because no H atoms on the left side.
Reaction
4 NaCl+SiO2-> 2Cl2+???
also absurd. No oxidant on the left strong enough to produce an oxidant as strong as elementary chlorine.
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Boffis
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It is a curious and well known phenomina that salt attacks silver, that's why silver dining services used to have a glass liner inside the salt
cellars. I have often wonder about how this reaction occurs and its precise chemistry. It is worth noting that AgCl is not decomposed by aqueous
alkalis and therefore I suspect that the atmospheric oxygen and water are important reactant and the reaction occurs alone the lines of:
2Ag + 2NaCl + H2O + O -> 2NaOH + 2AgCl
The highly insoluble nature of the silver chloride driving the reaction by removing the silver ions. The NaOH then absorbs CO2 or some other acid
moiety preventing the reversal of the reaction. This reaction occurs spontaneously at room temperature probably using water gathered from the air by
the hydroscopic salt meaning that the salt grains become covered with a layer of saturated salt solution.
I don't know if this reaction works at high temperatures but I suspect that something similar will. Given that in ancient times most ores mined were
from the oxide zone and rich in Fe3+ and Mn4+ as well as silica I can imagine that a reaction such as:
2Ag + 2NaCl + Fe2O3 + 2SiO2 -> Na2SiO3 + 2AgCl + Fe2SiO4
would occur. The iron silicate is called fayalite and, from personal experience, is a very common component of ancient slags. I once worked on a
project to recovery copper and precious metal from ancient slags in Turkey that were more than 3500yrs old and consisted mainly of fayalite and glass
in which occurred tiny prills of copper alloy.
I am not sure how this would help the ancient but maybe they could leach the AgCl out of the matt with lime and urine (ammonia) and then cement the
silver onto copper. Or perhaps molten silver chloride is simply immiscible with the silicate melt and separates out.
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chornedsnorkack
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Yes. If "calcined red vitriol" were Fe2(SO4)3 then the reactions could go:
Fe2(SO4)3+6NaCl->3Na2SO4+2FeCl3
FeCl3 melts and then boils slightly over 300 Celsius
And is reasonably strong oxidant to attack Ag:
FeCl3+Ag->FeCl2+AgCl
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