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I opened up some lantern batteries, and inside there's $\ce{MnO2}$ powder, carbon rods, and zinc casings, and other stuff that I don't know what it is exactly. I scraped out $\ce{MnO2}$-containing powder, but I would like elemental manganese from it.

How can I obtain manganese from manganese dioxide? How strong a reducing agent do I need? How hot does it need to get? Can I just add charcoal powder and smoke it with an oxyacetylene torch?

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    $\begingroup$ Make sure you use eye protection. I was once cutting up a PP3 since I needed the connector in a hurry and one of the cells- which these days are little steel tubes rather than the old-fashioned wads- went off in my face. $\endgroup$ – Mark Morgan Lloyd Dec 28 '20 at 18:29
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You may heat mixture of $\ce{MnO2}$ with charcoal to higher than $\pu{850 °C},$ in the absence of air. Manganese will be produced in the solid state by reduction of the oxide. But it is sensitive to air oxidation and easily reoxidized. So better use an excess of charcoal. In the industry, rough $\ce{MnO2}$ out of the mine is first reduced to $\ce{MnO}$ by $\ce{CO}$, then leached and reduced into $\ce{Mn}$ metal by a solution of $\ce{Fe^{2+}}$ in the absence of air. Metallic manganese is then dissolved in sulfuric acid and then reduced by electrolysis. See "Manganese metal" over Wikipedia.

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    $\begingroup$ Reduction with charcoal would indeed make sense as the manganese(IV) oxide in the batteries is always already containing graphite for better conductivity. $\endgroup$ – andselisk Dec 28 '20 at 12:03
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    $\begingroup$ @ andselisk : Excellent idea ! $\endgroup$ – Maurice Dec 28 '20 at 17:40
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    $\begingroup$ You can also do the manganese dioxide version of the thermite reaction, aka, Goldschmidt reaction. Just use the manganese dioxide in place of iron oxide. I did this when I was a kid with a basement lab. I used the dried black “manganese dioxide” just like you have. But, if you do this, be really careful: this is an exuberant reaction and safety gear is essential. Also do it outside! I never did this a second time because it was too exciting, if you get my drift. $\endgroup$ – Ed V Dec 28 '20 at 20:35
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    $\begingroup$ @Ed V. You are right. And this thermite reaction words also with the oxide saved after complete extinction of the battery : $\ce{Mn2O3}$. It is a useful way of using a waste substance $\endgroup$ – Maurice Dec 28 '20 at 21:43
  • $\begingroup$ I already upvoted your answer, by the way: that is how I would do it if I had it to do over. ;-) $\endgroup$ – Ed V Dec 28 '20 at 21:47
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Research notes in this ACS article Kinetics of Hydrogen Reduction of Manganese Dioxide that even heating $\ce{MnO2}$ in an atmosphere of hydrogen gas only results in $\ce{MnO}$.

The source further notes that very high temperatures (some 1,600 K) are required for any appreciable formation of the metal.

As such, a more facile approach, simply dissolve the hydrogen created $\ce{MnO}$ (or $\ce{MnO2}$ in concentrated HCl per Wikipedia) in an acid (say sulfuric or possibly acetic) and place the soluble manganese salt in a vessel with a sheet of pure Aluminum (no oxygen exposure).

In time, the $\ce{Mn}$ will be plated out onto the sheet where it can be mechanically removed.

Or, a related alternate path, convert the $\ce{Al}$ sheet into two pieces (electrodes) for electrolysis procedure.

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