Reducing ZnO to Zn: Thermal vs. Electrowinning

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Discussion Overview

The discussion revolves around the methods of reducing zinc oxide (ZnO) to zinc (Zn), specifically comparing thermal reduction in the presence of carbon and electrowinning processes. Participants explore the feasibility and economics of direct electrolysis of ZnO in various contexts, including potential applications in battery technology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants describe the thermal reduction of ZnO using carbon and the electrowinning process involving sulfuric acid and electrolysis as the two main commercial methods for producing Zn.
  • One participant questions the economic viability of directly electrolyzing ZnO in the presence of an electrolyte, suggesting that this method could be a potential alternative.
  • Another participant notes that zinc oxide is nearly insoluble in water or non-acidic electrolytes, which may impact the feasibility of direct electrolysis.
  • There is a proposal that electrolysis of melted ZnO at high temperatures could yield Zn, drawing a comparison to aluminum production from alumina using a flux.
  • A participant reformulates the original question, suggesting that applying an electric potential to a Zn anode oxidized to ZnO in an alkaline electrolyte should facilitate the reduction of ZnO without requiring high temperatures.
  • Mathematical calculations are presented regarding the charge required for the electrolysis of Zn, indicating a potential cost-effective method for Zn production.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility and economic implications of various methods for reducing ZnO to Zn. There is no consensus on the effectiveness or practicality of direct electrolysis of ZnO compared to established methods.

Contextual Notes

Participants acknowledge limitations related to the solubility of ZnO in electrolytes and the economic factors influencing the choice of reduction methods. The discussion includes unresolved questions about the efficiency and purity of products from different processes.

Who May Find This Useful

This discussion may be of interest to those studying chemical engineering, materials science, or electrochemistry, particularly in the context of metal extraction and battery technology.

mheslep
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The two commercial methods of reducing ZnO to Zn appear to be
Thermal, in the presence of C:
2 ZnO + C → 2 Zn + CO2
and electrowinning:
ZnO + H2SO4 → ZnSO4 + H2O (followed by electrolysis on the ZnSO4)

But what is the effect of straight electrolysis on ZnO in the presence of some electrolyte to begin with? That is, doesn't electrolysis directly reduce ZnO (as in a charging a depleted Zn-Air battery), and if that's the case why isn't ZnO electrolysis an economic way of producing Zn from ore?
 
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zinc oxide is nearly insoluble in water or non-acidic electrolyte.

I'm not sure about the economics of the two processes you reference. I would imagine the electrowinning process produces very pure material and the carbon reduction would produce less pure product.
 
Probably is possible to get Zn from electrolysis of melted ZnO at high temperatures using a flux, as the way Al is produced from Al2O3 using Na3AlF6 as flux. In the case of Al there is quite no other way (you don't get Al from the oxide reducing it with C and you don't get Al from electrolysis of acqueous solutions of its salts); in the case of Zn the two processes you described are certainly much more simple and cheap, so I presume it's for this reason they are used.
 
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Probably my OP question is ill phrased. Let me try again.

Given a Zn anode, who's surface is largely oxidized to ZnO, and the ZnO is in contact with some alkaline electrolyte, should not the application of an electric potential to the electrode force reduction of the ZnO? Assume anything you like for the cathode, i.e. air via some porous material:

ZnO + 2e + H2O → Zn(OH)2 → 2OH- + Zn

This is basically a Zinc Air battery being charged. The Zn need never enter solution, and no high temperatures required. It should be inexpensive?
 
Oh, I see that's a non trivial amount of charge per kg of Zn:

Zn @ 65g/mole
1 mole e = 96485 C, 3600 C / amp-hour
1 mole e = 26 amp-hours, or
52 amp-hours per mole of Zn,
800 amp-hours per kg of Zn @ 2V = 1.6 kWh per kg of Zn.

That's about $0.12 per kg Zn, with see Zinc @ $2.1/kg, about 6% of the cost. Seems reasonable.
 

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