B(a)P
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Separating zinc and manganese sulfate
I have been meaning to try out some manganese chemistry for some time, but had not stubbled on a convenient source. I recently noticed a product in my
local hardware store that is 50/50 manganese sulfate and zinc sulfate.
After spending yesterday evening pondering an approach to separate these two I thought I would pose the question here as I am sure others will have
better ideas than mine.
I have not been able to come up with a salt that would lead to a significant difference in solubility between zinc and manganese.
The first approach I thought I could take, which is tenuous at best, would be to dissolve some of the product in water then slowly add ethanol. Some
sources say zinc sulfate is slightly soluble in ethanol, but all say manganese sulfate is not. When I start to get some precipitate I could then test
the solid to see if it contains any zinc sulfate, if not continue the process until I detect some zinc sulfate in the solid, at which point I should
have a yield (likely small) of crude manganese sulfate that could be recrystallised to get fairly pure manganese sulfate.
The other thought I had (possibly more tenuous) was converting the sulfates to carbonates then heating them to about 150C. The zinc carbonate will
decompose at this temperature. I will then presumably be left with a mess of manganese carbonate and zinc oxide and likely other things. I can then
dissolve the zinc oxide/manganese carbonate in sodium hydroxide solution to form soluble sodium zincate and hopefully have the manganese carbonate
left as a solid.
I would appreciate any feedback on my ideas and any ideas you may have for the separation of these two compounds. Thanks in advance!
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Texium
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Why bother with converting to carbonates and calcining when you could just treat the sulfate solution with concentrated NaOH to precipitate the
manganese and keep the zinc in solution?
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B(a)P
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Wouldn't the zinc also precipitate out as hydroxide?
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Texium
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The zinc hydroxide will redissolve to form water soluble zincates if the pH is high enough. Might take a couple rounds of leaching the precipitate
with NaOH to get all the zinc out. Not sure.
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Texium
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Thread Moved
26-4-2022 at 17:53 |
Lion850
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B(a)P
"I have been meaning to try out some manganese chemistry for some time, but had not stubbled on a convenient source"
Not sure what you consider a convenient source, but manganese sulphate is readily available on eBay (and from some garden supplies) and I'm sure from
Science Essentials in Brisbane you will be able to get a range of manganese salts. I also have excess of manganese sulphate in case you want to come
round and pick some up 
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B(a)P
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Thanks Lion850, since doing my research into the separation of these two compounds i have been inundated with google adds for readily available
sources of zinc and manganese sulfate. It was a bit of an impulse purchase after seeing it on the shelf. Given I have already made the purchase I will
give the separation a go.
I will start with the suggestion made by Texium. I also found a related video by NurdRage on selective precipitation that will give me somewhere to
start.
https://www.youtube.com/watch?v=BLJgBSrhZI8
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clearly_not_atara
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I'm not sure how reliable the dissolution of zincates is. Plus, manganese could be oxidized and dissolve as some variation of manganate. It seems like
it might be easier to just treat the solution with an excess of hydrogen peroxide, which will oxidize the Mn present to MnO2, and won't affect ZnSO4
at all.
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B(a)P
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Thanks clearly_not_atara, this seems like the best approach so far. I was just reading into oxidation of Mn2+ ions using peroxide and the solution
will need to be basic for this to occur. Trying to basify the solution may cause hydroxide to precipitate though?
I also just read that manganese sulfate is paramagnetic, so maybe the separation will be as simple as a mechanical separation using a magnet!
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violet sin
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Effect of manganese, magnesium, sodium and potassium sulphates on zinc electrowinning from synthetic acid sulphate electrolytes
D.J.MacKinnonJ.M.Brannen
Hydrometallurgy
Volume 27, Issue 1, July 1991, Pages 99-111
Attachment: mackinnon1991.pdf (1.6MB)
This file has been downloaded 291 times
"The roast-leach-electrolysis process for the production of zinc metal employs a zinc sulphate electrolyte which may contain substantial
concentrations of manganese, magnesium, sodium and potassium sulphates. These species are generally thought to be inert with respect to zinc
electrowinning..."
" The concentrations of zinc and acid, as well as the electrolyte feed rate, to the cell were adjusted so that the concentrations of zinc and acid in
the electrowinning cell were maintained at 50 and 150 g/ 1, respectively,.."
I looks like the current efficiency is still in the 90's+ with high concentrations of additional sulfates. With the starting mix at 50g/l of Zn +
acid, and contaminations like 20g/l Na, 10g/l Mn, 40g/l Mg = CE 93.9%. ( Table 4)
.. so if you acidified it and plated out the zinc, could be pretty clean besides electrode choice contamination. Worst case you muck it up trying
other routes and you have a back up way to get the zinc out?
I found this oddly specific by title whilst searching. Playing with metals in sol. is something I miss, and still try to read about. Hope it helps.
-VS
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clearly_not_atara
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Quote: Originally posted by B(a)P  | Thanks clearly_not_atara, this seems like the best approach so far. I was just reading into oxidation of Mn2+ ions using peroxide and the solution
will need to be basic for this to occur. Trying to basify the solution may cause hydroxide to precipitate though?
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Solutions are not merely "basic", "acidic", they have a range of about 10^14 in the concentration of protons and hydroxide ions. This paper says that
MnO2 is produced by rxn with H2O2 around pH 6:
https://www.icevirtuallibrary.com/doi/10.1680/jenes.15.00009
Strongly acidic solutions may suppress the formation of MnO2, but I don't think that this happens above about pH 2 or so. On the plus side, it's
pretty easy to just test and check. Hydrogen peroxide is cheap and its decomposition leaves no residue.
| Quote: | I also just read that manganese sulfate is paramagnetic, so maybe the separation will be as simple as a mechanical separation using a magnet!
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If you had mixed powders of pure crystals, and a centrifuge, this could work well. Unfortunately, once the
salts are stuck together, magnetic forces won't be strong enough to separate them.
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Boffis
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Hi B(a)P; It may be easier than any of the above methods because Mn an Zn sulphates do not readily form solid solutions and their sulphates have very
different solubility characteristics.
I would suggest trying some small scale experiments along the following lines: Weigh out a small sub-sample of your salt mixture accurately, say
20.000g. Take a small volume of hot (c 80 C) water, say 10ml, add one or two drops of dilute sulphuric acid to prevent hydrolysis of the salts and
then add the mixed salts until the mixture appears saturated. If the mixture is cloudy, heat to near boiling and filter hot, allow to cool slowly. If
I am right the zinc sulphate which has a steep solubility/temperature relationship will crystallise out while the Mn sulphate which has an almost flat
solubility curve will remain in solution. Evaporating down the residual solution until crystals start to appear, dilute just enough or heat to a
higher temperature to make the crystals disappear and cool. Any crystals that form with still be Zn dominant with 7H2O. Evaporating further will
eventually cause the MnSO4.H2O (monohydrate) to crystallise from the hot solution. When the crystals slurry is getting thick, filter hot to recover
the mainly Mn sulphate. And so on, some experimentation may be required but I think something along these lines will work.
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B(a)P
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I finally got around to attempting the separation of the manganese and zinc.
Physical separation was first tried using a strong magnet and as clearly_not_atara suggested this does not work, no part of the powdered mixture was
collected by the magnet.
The oxidation of Mn2+ to Mn4+ to form MnO2 using peroxide was tried (again as suggested by clearly_not_atara). This
did work, but not in the pH range suggested in the paper. Firstly a solution of the mixed powder in distilled water was made. The pH was measured at 6
using litmus paper. An excess of 50% peroxide was slowly added. The solution went from clear a pinkish colour to just clear. A small amount of sodium
hydroxide was slowly and carefully added causing a strong reaction at the surface of the solution (use caution when adding hydroxide to peroxide).
Substantial gasses were released and a dark precipitate formed, which quickly disappeared. It wasn't until the pH got to about 7.5 that the
precipitate started to persist. This is a good way to remove the manganese, but the zinc remained in solution now contaminated with sodium ions.
The electrolysis suggested by violet sin will be attempted soon.
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