towards elimination of the anode effect and perfluorocarbon ...web.mit.edu/dsadoway/www/ecs...
TRANSCRIPT
![Page 1: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/1.jpg)
Towards Elimination of the Anode Effect and Perfluorocarbon Emissions
from Hall-Héroult Cells
Hongmin Zhu & Donald R. Sadoway
Department of Materials Science & EngineeringMassachusetts Institute of Technology
Cambridge, Massachusetts U.S.A.
![Page 2: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/2.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
background
p on anode effect Hall-Héroult cell produces CF4 and C2F6 (PFCs)
p PFCs have high GWP
p in US, Al smelting is #1 point source of PFCs
![Page 3: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/3.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
premise of the current work:
understanding the anode effect
+ understanding PFC generation
+ plan for reducing emissions
![Page 4: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/4.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
outline of today’s talk:
p electroanalytical studies p gas analysis during electrolysis
in laboratory cellsp method to avert AE and PFC
generation in lab cellsp extension to industrial cells
![Page 5: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/5.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
results of studies in lab cells
p anodic reaction inhibited when potential exceeds a critical value,EC ≈ 3.0 - 3.5 V
p anode becomes active again when potential is reduced below a so-called recovery value, ER ≈ 2.8 V
p PFCs generated only at extreme potentials, i.e., E > 4.0 V
![Page 6: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/6.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
results of studies in lab cells
+ AE linked to formation of highly resistive surface film
![Page 7: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/7.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
our experiments:
3-electrode cell featuringO rotating rod anode (WE)O reference electrodeO large counter electrode
electrolysis cell featuringO tubular anode for
improved gas sampling
![Page 8: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/8.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
cell for electrokinetic studies
cell cover
mullite reaction tubecounter electrodereference electrode
working electrode
furnace
crucible
electrode
BN sheath
thermocouplegas or vacuum
![Page 9: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/9.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
various types of electrodes
disk rodtubular
BN sheath
electrode surface
BN sheath
electrode surface
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Sadoway, MIT ECS Meeting, Philadelphia, May 2002
evidence of a passivating film84% Na3AlF6 - 11% AlF3 - 5% CaF2
E / V vs Al / AlF3
I / m
Acm
-2
-500
0
500
1000
1500
2000
2500
= 100 mV s-1v
0 RPM200 RPM400 RPM600 RPM
1% Al2O3
1.0 2.0 3.0 4.0 5.0 6.0
T=970ºC
CV on graphite RDE
![Page 11: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/11.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
evidence of a passivating filmI /
mA
cm-2
-500
0
500
1000
1500
2000
2500
1.0 2.0 3.0 4.0 5.0 6.0
0 RPM200 RPM400 RPM600 RPM
= 100 mV s-1v
84% Na3AlF6 - 11% AlF3 - 5% CaF2
E / V vs Al / AlF3
T=970ºC
CV on graphite RDE
1% Al2O3
![Page 12: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/12.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
evidence of a passivating film
0
400
800
1200
1600
0 2 4 6 8 10 12 14
average current duringa potential step,tubular electrodelinear sweep, rod electrode
I / m
Acm
-2
E / V vs Al / AlF3
T=970ºC
![Page 13: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/13.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
evidence of film formation
t / s
-100
0
100
200
300
400
500
600
0 5 10 15 20 25 30 35 40
I / m
Acm
-2
0.85 V
2.0 V
3.0 V
3.5 V 0.85 V
potentiostatic electrolysis
![Page 14: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/14.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
evidence of film formation
-100
0
100
200
300
400
500
600
0.0
2.0
4.0
6.0
0 20 40 60 80 100 120
I EI /
mA
cm-2
E / V vsAl / AlF
3
t / s
![Page 15: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/15.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
cell for analysis of anode gas
cell cover
mullite reaction tubecounter electrodereference electrode
working electrode
furnace
crucible
BN sheath
thermocouplegas or vacuum
electrode
![Page 16: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/16.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
controlled-potential electrolysis
04080
120160200240280
1.5 2.0 2.5 3.0 3.5 4.0
CC
F4/ p
pm
E / V vs Al / AlF3
![Page 17: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/17.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
controlled-potential electrolysis
0.00.51.01.52.02.53.03.54.0
2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
log
(CC
F4/ p
pm)
E / V vs Al / AlF3
![Page 18: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/18.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
controlled-potential electrolysis
in the high-voltage regimeln c = α + β E,
where β = 0.325 V–1
c.f. Nissen & Sadoway (1997)r CF4 = a exp (b E ),
where b = 0.331 V–1
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Sadoway, MIT ECS Meeting, Philadelphia, May 2002
data from an industrial cell
0
5
10
15
20
25
30
0 10000 20000 30000 40000 50000 60000
cell
volta
ge /
V
t / s
anode effect
![Page 20: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/20.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
a closer look at an anode effect
4.5
5.0
5.5
6.0
800 810 820 830 840 850
cell
volta
ge /
V
t / s
5.0 V, 10 s
4.9 V, 23 s
![Page 21: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/21.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
data from a laboratory cell
200
400
600
800
10000 500 1000 1500 2000 2500 3000 3500 4000
I / m
At / s
800 mA cm-2
galvanostatic electrolysis0.6 wt% Al2O3
![Page 22: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/22.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
data from a laboratory cell
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0 500 1000 1500 2000 2500 3000 3500 4000
E/ V
vs
Al /
AlF
3
t / s
galvanostatic electrolysis0.6 wt% Al2O3
![Page 23: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/23.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
data from a laboratory cell
2.0
2.5
3.0
3.5
0 500 1000 1500 2000 2500 3000 3500
9oc26jj, 9nv04ca, ha
700 mA600 mA466 mA
E / V
vs
Al /
AlF 3
t / s
s = 0.58 cm 2
![Page 24: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/24.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
AE averted by current stepping
2.4
2.6
2.8
3.0
3.2
3.4
600
700
800
900
0 500 1000 1500 2000
I / mA
t / s
current
potentialE / V
vs
Al /
AlF 3
![Page 25: Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS _PFC.pdfrecovery value, E R ≈2.8 V PFCs generated only at extreme potentials, i.e., E](https://reader034.vdocument.in/reader034/viewer/2022051923/6010d6d7c5a9016ef532b6e3/html5/thumbnails/25.jpg)
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
major findings:
onset of anode effect is potential dependent
AE can be averted by reducing current so as to prevent voltage from exceeding a critical value
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Sadoway, MIT ECS Meeting, Philadelphia, May 2002
in an industrial cell
accurate voltage sensing
robust current shunting
control circuit
enabling technologies:
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Sadoway, MIT ECS Meeting, Philadelphia, May 2002
vacuum current breaker
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Sadoway, MIT ECS Meeting, Philadelphia, May 2002
our proposal
shunting ≈1% cell current drops cell voltage for ≈1 min∴ 300 kA cell 3 kA/min
no AE no PFCs
150di Asdt
−=
+ no ANODE EFFECT!!!
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Sadoway, MIT ECS Meeting, Philadelphia, May 2002
acknowledgments
The Aluminum Association
U.S. Environmental Protection Agency,Climate Protection Division