blogfast25 - 6-4-2008 at 09:35
I've isolated MnO<sub>2</sub> many times from battery dry electrolyte by dissolving the MnO<sub>2</sub> in strong HCl to
MnCl<sub>2</sub> and reoxidising the Mn (+II) to Mn (+IV), as described in the relevant thread on this forum.
Now I've been experiencing problems with a pottery grade MnO2 that contains about 20 - 25 w% iron oxide(s), expressed as
Fe<sub>2</sub>O<sub>3</sub>. I want to eliminate the iron.
I'm wondering if the Mn and Fe could be separated by other means, as the HCl route is messy and generates lots of Cl<sub>2</sub>.
I have 51 w% H<sub>2</sub>SO<sub>4</sub> and am wondering whether some selective leaching would allow the Fe to dissolve as
sulfate, leaving the MnO<sub>2</sub> behind, without need to reoxidise or re-precipitate.
Anyone have experience or ideas about this? 
woelen - 6-4-2008 at 13:01
I think you will have a hard time doing this selective leaching. Problem is that the particles, even if they are very small, only react at their
surface. You might get some Fe out from the surface, but the Fe, deeper inside the particles is not reached at all. What ever method you use, you need
to either melt, or completely dissolve the material, in order to get all Fe in solution (or melt).
An added problem with potteries grade material is that it most likely is calcined. This makes the material less reactive. I have some potteries Fe2O3
and this stuff is amazingly inert. Most likely your MnO2/Fe2O3 materia forms some mixed salt, with iron and manganese at regular places in a crystal
lattice, and then you can speak of a (possibly non-stoichiometric) compound, and not simply a mix. First row transition metals are most noteworthy
capable of forming all kinds of mixed oxides of indeterminate stoichiometry.
You could try to get the material in solution with your H2SO4, by adding Na2SO3 or NaHSO3. The sulphur dioxide, formed in this reaction will reduce
the MnO2 to Mn(2+) and I expect it to reduce the Fe(3+) to Fe(2+) and then the iron and the manganese go into solution. In this way you have a mix
with Mn(2+) and Fe(2+). Adding an oxidizer like Na2S2O8 will precipitate out MnO2, even in somewhat acidic solutions, and Fe(3+) will remain in
solution. Unfortunately using H2O2 as oxidizer does not work. In acidic solution, H2O2 works as reductor for MnO2 and besides that, it is decomposed
catalytically very easily with Mn around.
[Edited on 6-4-08 by woelen]
mining method
chemrox - 6-4-2008 at 17:57
Here's an old mining method where the Fe is precipitated and the Mn stays in solution. No experience with it so don't know about how messy it may be.
Maybe you could get the material in solution treat it with dry ice, precip the iron carbonate hit it with acid and recover the oxide
[Edited on 6-4-2008 by chemrox]
Attachment: Fe from Mn by carbonate.pdf (780kB)
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blogfast25 - 7-4-2008 at 05:51
Woelen:
Agreed on the inertia of pottery grade Fe2O3: mine is also remarkably resistant to dissolution in HCl, it appears to dissolve only in part.
But the pottery grade MnO2 did dissolve easily in 32 w% HCl, even when cold, similarly to battery electrolyte and the Fe present dissolved readily as
well.
I was also thinking of trying to part reduce the MnO2 by prolonged high temperature glowing with an excess of carbon, this should reduce it to Mn3O4,
perhaps even MnO.