Electrochemical behaviour of sulphate anion in molten salts in connection with aluminium industry

1
Electrochemical behaviour of sulphate anion in
molten salts in connection with aluminium
industry

• Jana Hajasova under supervision Prof. Geir Martin
  Haarberg
• Department of Materials Technology
• NTNU Trondheim

2
Introduction

• Aluminium
• - produced by the electrolytic reduction of
  Al2O3 dissolved in cryolite-based bath in the
  Hall Héroult process
• Electrolyte
• - liquid cryolite and additives AlF3, CaF2,
  LiF, MgF2
• Impurities in the process of Al electrowininng
• - originate from
• the raw materials which are alumina (Fe, Ti),
  anode carbon (S) and bath components mostly AlF3
  (Si, S, Fe, P)
• tools, current leads (Fe) and lining (Si)

3
Introduction

• Impurities may affect the electrolysis
• reaction with the components of the electrolyte
• interaction with carbon in the anode, bath or the
  carbon lining
• electrolytic decomposition of the impurities with
  lower decomposition potential
• Impact
• current efficiency
• metal quality
• environment

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Introduction

• Origin of sulphur
• - petroleum coke used for anode production (1
  4 wt ) thiophene and thiols
• - cryolite, alumina and aluminium fluoride (up
  to 1 wt ) sulphates
• Forms of sulphur present in the bath
• S6, S4 (SO2), S0 or S2-
• Sulphur leaves the aluminium cell
• - off gases in the form of COS, SO2, H2S, CS2

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Introduction

• Behaviour of sulphur species in molten salts
• stable compounds
• sulphate can be reduced by carbon and by several
  metals (Ni, Fe, Cd, Pb, Zn), reduction products
  are sulphite, sulphur and sulphide
• some workers - sulphate ion can not be reduced
  cathodically
• some authors - sulphate ions can decompose -
  anodically to form SO2, SO3, O2, O2- and
  sulphites or cathodically to form S2- and O22-
• in molten Na2SO4 (900 C, Pt) at anode SO42-
  SO3 1/2O2 2e-
• limiting cathodic process SO3 2e- SO2
  O2-
• variety of electrochemical reactions (many
  oxidation states of S)
• - depends on the gas atmosphere, the
  electrode potential, the electrode material,
  composition of electrolyte, salt thickness

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Experimental

• Electrochemical study of behaviour of sulphate
  anion in the system NaCl-Na2SO4
• Different concentration of Na2SO4 (from 0.05 to
  4wt )
• Various sweep rates (from 50 mV/s up to 1V/s)
• Experimental conditions
• closed, vertical, electrically heated laboratory
  furnace under Ar atmosphere
• Telectrolyte 850 C and 935 C measured by a
  Pt-Pt10Rh thermocouple
• CVs measured by ZAHNER digital electrochemical
  analyser in anodic and cathodic direction with
  three electrode system (WE, CE, RE)

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Experimental
Apparatus scheme
RE Ag/Ag CE glassy carbon WE thin wire
(W, Mo, Cu, glassy carbon)
crucible - glassy carbon, alumina (d 4,5 cm)
placed in glassy tube
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Results cyclic voltammetry in NaCl

• NaCl 0,3 wt Na2SO4
• WE Mo RE Mo
• T 850 C
• recorded in anodic direction

NaCl 0,05 wt Na2SO4 WE Cu RE Ag/Ag T
850 C sweep rate 300mV/s recorded in cathodic
direction
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Results cyclic voltammetry in NaCl
NaCl 0,5 wt Na2SO4 WE W, RE Ag/Ag T
935 C, 300mV/s, recorded in cathodic direction
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Results cyclic voltammetry in NaCl
NaCl 0,5 wt Na2SO4 WE W RE Ag/Ag T 935
C recorded in cathodic direction
peak current is linearly dependant on sqrt sweep
rate
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Results cyclic voltammetry in NaCl
NaCl Na2SO4 sweep rate 300 mV/s WE W, RE
Ag/Ag recorded in cathodic direction at T
935 C
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Conclusion

• behaviour of sulphate anion is possible to study
  electrochemically in molten NaCl
• Mo, GC and Cu are not suitable materials for WE
• - they are not very stable in the melt and are
  consumed during electrolysis
• - probably due to the reaction with one or more
  of the reduction products
• W as working electrode
• - at low concentrations of Na2SO4 stable
• - at higher concentrations of Na2SO4 (0,5 wt
  and up) unstable
• temperature has no effect on speed of Na2SO4
  dissolution
• reduction peaks could be due to electrolytic
  reduction of sodium sulphate
• it might continue

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• Future plans
• electrolyte chlorides and fluorides or mixtures
  of F- and Cl- , cryolite
• working electrode W, Co, Ni, Cr, (Pt)
• CV - to study mechanism of electrochemical
  reactions and their kinetic parameters
• chronopotentiometry and chronoamperometry -
  determination of diffusion coefficients
• off gases analysis
• - titrimetric determination of S2- and SO2
  with iodine
• - gas chromatography
• electrolyte analysis
• - XRD powder diffraction analysis for
  qualitative and quantitative determination of
  crystallic structures of studied matters

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Acknowledgment

• The Research Council of Norway

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Thanks for attention