Sciencemadness Discussion Board - Manganese dioxide drying fail - what the heck?

Manganese dioxide drying fail - what the heck?

Diachrynic - 5-11-2017 at 01:11

So I decided to extract some MnO₂ from d-cell zinc-carbon batteries.

I dumped the MnO₂ into a plastic cup and washed it with distilled water. Then I filtered off the MnO₂.

The washing should have get rid of the electrolyte.

Then I transferred the black paste into a small stainless steel pot and heated it a bit in a warm water bath (I don't have a hotplate yet.)

Everything looked fine. After a bit of water had evaporated, I took the pot out and let it sit to let the remaining water evaporate on its own (or until a have a hotplate).

When I came back to it this morning, I was shocked!

The MnO₂/C-paste had turned the stainless steel pot into a sieve - what the crap? It ate holes in the metal, which looked rusty, with some green stuff, hydroxides I suppose.

What went wrong? Is stainless steel just unable to withstand wet manganese dioxide? Is this a sign my washing wasn't good?

[Edited on 5-11-2017 by Diachrynic]

unionised - 5-11-2017 at 01:27

" Is this a sign my washing wasn't good?"
Probably.

A plausible explanation would be that you didn't remove all the ammonium chloride electrolyte.
On heating that gave HCl.
That reacted with the pot directly, or it reacted with the MnO2 to give Cl2- which would react with stainless steel.

j_sum1 - 5-11-2017 at 01:47

I have done this exact same thing. Effective washing of battery paste is really hard to do. And filtering the gunk takes ages. The electrolyte may be zinc chloride or ammonium chloride or both. Either way, the presence of chlorides in an acid environment combined with the air-excluding properties of the gunk will cause rapid pitting of stainless steel.

Not that it directly pertains to your question but you ought to know that the paste is about 30% graphite powder. This stuff is messy.

Diachrynic - 5-11-2017 at 02:03

Maybe a washing with NaHCO₃ could reduce the level of acidity? Or just mooore washings? Either way, it is not a nice thing to work with.

I know about the carbon content of this stuff, but I don't really care. I wanted some for experiments with oxygen, i.e. decomposition of H₂O₂; the carbon content probably doesn't matter there.

It looks awful though, the thought never crossed my mind that this crap could be that corrosive towards stainless steel.

j_sum1 - 5-11-2017 at 02:28

The thing with the carbon is it holds the moisture. You are better to keep it in glass and avoid putting it in stainless altogether.
The green you saw was chromium from the SS -- either oxide or hydroxide.

I would keep NaHCO3 out of the equation. Typically your next step is to react with HCl, H2SO4, oxalic acid or SO2 -- all acidic. So there seems little point in attempting to neutralise at this stage.

Diachrynic - 5-11-2017 at 02:43

Oh well.

The next time more washing and drying in glassware. Thank you for your advice.

Since the stainless steel pot is now damaged beyond the point of usefulness, I'm going to clean it and then use it as a source of stainless steel for making dichromates >:D

macckone - 5-11-2017 at 09:50

Better method of purifying this stuff is to just add hydrochloric acid right off
the bat, and dissolve out the zinc and manganese chlorides. Do this
OUTSIDE as it can release chlorine gas. But you need GLASS because zinc
chloride will eat most metals (Hastelloy C is resistant).

Then do a fractional crystallization, zinc chloride is much more soluble.
The easiest method for doing the fractional crystallization is to boil off as
much water as you can until you have a soup then add just enough boiling
water to dissolve everything. Then cool to 0C, this should give a nice
crop of manganese chloride crystals. Some will stay dissolved. Alternatively
you can warm slowly with vacuum to remove water. Air drying is out
of the question as zinc chloride is very hygroscopic.

Once you have the chloride, converting it to an oxide is easy. Add sodium
carbonate solution to a manganese chloride solution and instant chemical
snow. Any ammonium chloride carried over will stay in solution as the
carbonate is soluble.

Heat the manganese carbonate to yield the oxide.

macckone - 5-11-2017 at 10:53

Here is a video of an alternative process that doesn't use the fractional crystallization which can be messy.
https://www.youtube.com/watch?v=BCmTzZa6ncY

j_sum1 - 5-11-2017 at 15:04

Quote: Originally posted by macckone

Better method of purifying this stuff is to just add hydrochloric acid right off
the bat, and dissolve out the zinc and manganese chlorides. Do this
OUTSIDE as it can release chlorine gas. But you need GLASS because zinc
chloride will eat most metals (Hastelloy C is resistant).

Then do a fractional crystallization, zinc chloride is much more soluble.
The easiest method for doing the fractional crystallization is to boil off as
much water as you can until you have a soup then add just enough boiling
water to dissolve everything. Then cool to 0C, this should give a nice
crop of manganese chloride crystals. Some will stay dissolved. Alternatively
you can warm slowly with vacuum to remove water. Air drying is out
of the question as zinc chloride is very hygroscopic.

Once you have the chloride, converting it to an oxide is easy. Add sodium
carbonate solution to a manganese chloride solution and instant chemical
snow. Any ammonium chloride carried over will stay in solution as the
carbonate is soluble.

Heat the manganese carbonate to yield the oxide.

Nice procedure. I think I know of four variants now. But recall, if you are after a pure Mn compound then you generally have iron contaminant. You may need to take steps to remove that.

MrHomeScientist - 6-11-2017 at 14:27

I would think reacting with HCl would yield copious amounts of chlorine, and you'd need a lot of acid to get it to run to completion. Plus with all the carbon it'll be impossible to tell when it's done with the manganese!

Not that I have a better idea, just pointing out what I see. NurdRage did a video or two on battery paste purification too.

j_sum1 - 6-11-2017 at 14:43

Nurdrage gives three methods:
1. Reacting with strong acid, filtering, precipitating with NaOH. Then there was a clever trick of dividing into two portions, dissolving one and then neutralising with the other portion of Mn(OH)2 to achieve a solution without iron contamination. He used sulfuric acid. I have done it with HCl and, yes, a lot of Cl2 is produced. (This method was originally posted here by peach and is based on an industrial method according to a conversation I had with blogfast.)
2. Reacting with gaseous SO2, filtering and crystallising
3. Reacting with a combination of sulfuric acid and oxalic acid

The sodium carbonate precipitation method posted by Mackone seems like a good idea also but I can't see it eliminating iron contaminant.