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Author Topic: Why do some metal sulfides generate acid when they oxidize, but not all?  (Read 2664 times)

Offline Hugo_P

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Hello! Does anyone have an idea why metal sulfides having a metal/sulfide ratio <1 produce (sulfuric) acid when they are oxidized by oxygen (for example FeS2, MoS2), while metal sulfides having a metal/sulfide ratio >= 1 don´t generate any acid during the same oxidation? In the case of the acid producing sulfides, water always takes part in the reaction (thus supplying hydrogen) but that isn´t the case for the other sulfides.


 

Offline damocles

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Well, yes! It is fairly obvious from the stoichiometry, although the chemistry of FeS2 and MoS2 is a little different:

FeS + 2 O2 ==> FeSO4
PbS + 2 O2 ==> PbSO4

but

FeS2 + 5/2 O2 + H2O ==> FeSO4 + H2SO4

The case of MoS2 is somewhat more complicated.

MoS2 + 61/2 O2 + 4 H2O ==> MoO3.2H2O + 2 H2SO4
« Last Edit: 31/05/2013 02:13:32 by damocles »
 

Offline damocles

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Here is the overall picture: Monovalent and divalent sulfides ( M2S and MS) are either ionic or network compounds. They will roast to produce the corresponding ionic sulfates (M2+SO42– or M2+SO42–.

Trivalent sulfides mostly do not exist; I suspect that when they do they will be exceptions to the rule.

Substances with formula MS2 or higher are all network compounds. They can be
• equivalent to FeS2 if they are effectively polysulfides of a metal with a stable +1 or +2 oxidation state, reacting to form an ionic sulfate and sulfuric acid, or
• sulfides of metals with a higher oxidation state -- like MoS2 -- which will normally react to produce a network oxide and sulfuric acid.
 

Offline SorryDnoodle

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Can this method be used to produce concentrated sulfuric acid? Specifically using iron sulphate?
 

Offline damocles

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Can this method be used to produce concentrated sulfuric acid? Specifically using iron sulphate?
FeS2 is Iron(II) disulfide -- pyrite or marcasite.
It certainly could be used to produce concentrated sulfuric acid.

Even monovalent or divalent sulfide minerals -- CuS, CuFeS2, PbS, if they are roasted in the open air, will produce CuO, Fe2O3, Pb3O4 etc., along with SO2 and a small amount of H2SO4, which will be emitted as gases that damage the environment.

If the effluents are collected, purified, and passed over a V2O5 catalyst you can obtain SO3, which is the material you need for making sulfuric acid. Collection of the effluents also reduces the damaging effects on the environment.

http://en.wikipedia.org/wiki/Contact_process
« Last Edit: 01/06/2013 04:07:25 by damocles »
 

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