Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS...
Transcript of Towards Elimination of the Anode Effect and Perfluorocarbon ...web.mit.edu/dsadoway/www/ECS...
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.
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
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
premise of the current work:
understanding the anode effect
+ understanding PFC generation
+ plan for reducing emissions
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
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
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
results of studies in lab cells
+ AE linked to formation of highly resistive surface film
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
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
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
various types of electrodes
disk rodtubular
BN sheath
electrode surface
BN sheath
electrode surface
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
in an industrial cell
accurate voltage sensing
robust current shunting
control circuit
enabling technologies:
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
vacuum current breaker
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!!!
Sadoway, MIT ECS Meeting, Philadelphia, May 2002
acknowledgments
The Aluminum Association
U.S. Environmental Protection Agency,Climate Protection Division