developments in electrochemical science, engineering ... · anthracite + pitch alumina powder or...
TRANSCRIPT
"Developments in electrochemical science, engineering design, control, and materials
enhancing aluminium smelting technology"
Barry Welch
Aluminium smelting has advanced considerably
2
From cells producing less < 1 tonne Al / year in the 1890’s
... to cells producing
>1250 tonnes / year today!
In China they have Run cells at 700,000A – 5.1 tonnes/day 1800 tonnes / year
Al Electrowinning is not the simple process as per texts! i.e. Al2O3 + 3/2C ≠ 3/2CO2 + 2Al with Edecomp = 1.18V
& CO not being formed electrochemically as it is “polarized”
• There are between 5 & 8 products released from the anode !
– CO2(g), CO(g), COS(g), SO2 (g), HF(g) , Cdust , COF2(g) ,CF4 (g), C2F6(g)
– & anode gases react with materials of construction
• The Electrochemical Cathode product (Na-Al alloy) undergoes numerous reactions with materials of construction. Products include:
– NaCx , NaCN, NaOH, Na2CO3, Al4C3, Si, NaxAlpSiyOz
• The cell is dependent on work practices to Remove Al, replace & protect Anodes.
• Al2O3 has to be replenished
3
Liquid Aluminium
Consumable Carbon Anode
Al2O3 dissolved in molten Na3AlF6
+
-
Importance of historically following the changes
• Understanding why we introduced – Work practices
– Control actions
• Assessing how technology & scale-up have impacted them
• Raising questions as to the relevance of retaining the old ways – Manual versus Robotics
– Need or relevance of Data processing simplifications & manipulations
• E.g.“Resistance smoothing”
4
Design & limited Theory make mistakes
5
CAPEX
• Cell cost
– Low productivity
• < 18 kg Al /m2 Potroom /day
– Short cell life
• Services
– Anode production
– Cranes and transport vehicles
– Emissions capture and recovery
OPEX
• Electrical energy
– priced on competitive demand
– greater human usage
– Limited generation capacity with low generation emissions
• Alumina
– high degree of refinement for metal applications
– hi transport cost
• Anode carbon
– Purity and structural constraints limited supply.
• Labour
– Limited automation of work practices
• repairs and maintenance
– Dusty abrasive atmosphere
The Challenges For Al Smelting
6
• Emission – Prevent HF
release – Minimize SOx
– Minimize COx
– Avoid CFx FOCx
• Waste Minimization – Prevent /Destroy
toxic materials in spent cathodes
Responsible Citizen
Energy Lowering - Faradaic Efficiency (CE%) Improvements
7
Cell Designs have Changed
8
9
Sὄderberg Technology dominates at end of WWII
…and energy consumption has been reduced
10
kWh/lb.
kWh/lb.
kWh /lb.
kWh /lb.
kWh /lb.
kWh /lb.
kWh /lb.
54 kWh/kg
Al
37 kWh/kg
Al
17.6 kWh/kg
Al
20.4 kWh/kg Al
16,28 kWh/kg Al
27 kWh/kg Al
Problems with cell Life
11
0
500
1000
1500
2000
2500
3000
3500
1890 1910 1930 1950 1970 1990 2010
Typ
ical
ce
ll lif
e (
day
s)
Year
Trends in Cell life
All Materials of construction of Cell lining are porous!
• They are wetted by liquid electrolyte! – And this enables reactions down a
thermal gradient!
• Sodium which is Co-deposited
with aluminium is the most damaging reactant - Especially with disordered carbon structures, With the product undergoing a molar volume expansion.. – Na(Al) + x C ↔Na Cx – X Increases with the graphitisation
of the carbon and reduces the expansion ,
– but being weakly bound the sodium can undergo other chemical reactions.
12
Graphitised
ordered carbon
structure
Anthracitic carbon
quality zone
Binder carbon quality
zone
Pitch Carbon
Graphitized Carbon
Cathode Technology change 1950 to 1975 Applying Materials Science
Paste Rammed cells Life <700 days
Preformed & Heat-treated
Blocks (& higher) AlF3 bath
(life> 2000 days)
Alumina Powder
Anthracite + Pitch
Alumina Powder or Refractories
Gas or electrically calcined Anthracite + pitch baked to 1100 C
Blending Materials science with modern economic management!
0
500
1000
1500
2000
2500
3000
3500
1890 1910 1930 1950 1970 1990 2010
Typ
ical
ce
ll lif
e (
day
s)
Year
Trends in Cell life
14
Summary of Cathode Materials
• Carbon materials optimization has conflicting requirements
• Complicated by jointing difficulties – Differential properties
• Galvanic & electrochemical corrosion
• Some possible design solutions with materials selection?
– Permeability modification?
15
2Al + (x+3)SiO2 (refractory Slag) + Bath ⇨ Al2O3*x SiO2 +3Si +Heat
+ molar Volume expansion
Thermite HEAVE CAUSED BY METAL FLOOD TO SUBCATHODE
Parallel Current paths Note: 4Al + 3C = Al4C3 generates 0.09 to 0.15V
A
l
A
l
Aluminium
Electrolytic Path
I2 = [(V+~0.1) / Rbath]
Electronic
Path
I1 = [ V/Rc ]
R bath
-
+
16
Growth of Al4C3 deposit in sub cathodic pores once solubility product exceeded
Note: 4Al + 3C = Al4C3 generates 0.09 to 0.15V
Na in Al
A
l
A
l
Al3+
Na +
Al 3+
Al 3+ C 4-
Ping!!
17
-
+ Anode reactions: Na ⇒ Na+ + e Al ⇒ Al3+ + 3e NaCx ⇒ Na+ +xC e
Cathode reaction: 3C +12e + 4Al3+ ⇒ Al4C3
Bath reaction: Na+ + Na3AlF6 ⇒ 6NaF + Al3+
NaCx
Na on Cathode block also causes a Growth of Al4C3 deposit once solubility product exceeded
4[Al3+] + 3[C4-] = Al4C3
Na Co-deposited with Al - or intercalated has same effect!
Na (al)
A
l
A
l
Al 3+
Al 3+
C 4-
Ping!!
Na+
Na+
Na+
Na+
Overall Cell Reaction: 12Na(Al) + 4Na3AlF6 + 3C ⇒ 24 NaF + Al4C3
18
“The cells are dependent on work practices to Remove Al -Metal Tapping”
Ladle was used for low
kA Cells
Tap-hole also used, but slowly phased out…
19
Vacuum assisted tapping
Crucible removed by manpower or….
Alumina “control” Technology up to ~1950 ( & beyond)! Volumetric side addition of the mass needed for 6 to 8 hours
20
***Ledge was pseudo-eutectic of alumina cryolite Temperature cycling lead to formation of a hard bottom alumina crust on the cathode surface!
• Three steps
– Spread Al2O3 along side of cell
– Crust break
– Wait for “anode effect”
From 1950 to 1975 – automated mechanically assisted (timed) Alumina feeders
21
• Minimum feed about 20 kg
• Typical feed 60 to 100 kg – Time interval 30 to 90
minutes typically – Mass of feed usually exceeds
the feed zone saturation solubility limit
• Normally forms sludge/muck
• More alert and than it is possible
to dissolve added during anode effect termination
1959 to 1980 – Explosive Changes
• All companies had fundamental & applied R&D activities – Explosion in fundamental knowledge , especially bath chemistry and
electrode processes
• Materials science for the space age, structure-property relationships – Better carbons & composite carbide refractories for cell lining
• Analogue control instrumentation – Solid state transistors replacing Valve
• ⇒Digital control & computing – Modelling developments
22
The industry emphasis was also to retrofit improvements. And this has continued. This led to simplifications in both Data processing and theoretical explanation. These simplifications need to be rectified for fine tune advancements
More recent advances through applying fundamental and applied knowledge (1955 to 2000)
23
Demand feed Logic –from late 1960’s Enabled smaller Al2O3 additions reducing sludge formation
• Alumina feed philosophy- Overfeed the alumina, then wait for the voltage to rise a predetermined amount
• alternative wait for the Rate of increase in voltage to exceed a certain value
24
• Early Limitations - quality of signal because of high potline AEF (including pre programmed ones) – Need to get accurate voltage rise
signal of <100 mV in order to ensure do not risk AE’s • Enter the “Resistance
Smoothing” approach
• Aim: – To compensate for impact on cell
voltage of short term fluctuations in line current for Ohmic contributions
• “Normalized Voltage”
Vn = (Vactual – Bemf)*Iavge/Iactual + B emf
With a wise word of warning
25
Mid I970’s – Signal smoothing (Tabereaux et al LM 1983 p479)
Resistance control. A common value of 1.65 V used for B EMF not converted back to voltage. R cell = [Vcell -1.65]/I actual
But the control changes are Voltages not resistances!
Cell Amperage – more recent years
Firs
t Po
int
fed
~1
96
5
Ind
ivid
ual
An
od
e A
dju
st~1
98
0
~ 1
80
kA M
ag C
om
p P
FPB
19
76
26
AP
27
-30
, Len
gth
ened
cel
ls f
or
cap
acit
y cr
eep
19
86
to
30
0kA
AP
3X
cap
acit
y cr
eep
th
ru R
etro
fits
of
larg
er
ano
des
& h
igh
er C
urr
ent
den
sity
19
90
20
05
Ch
ines
e lo
nge
r ce
lls 2
00
3 t
o 2
01
5, t
o 6
00
kA
CE% Improvements Design + Chemistry + Control
1950 Centre break, side-by-side
Cells 3% AlF3 985 oC
7% AlF3 T = 975 o C
Demand feeding needed for operating
at 7%
Point Fed Cells
10 to 12 % AlF3 955 to 965 oC
Some of CE% loss due to current
bypass / shorting! Some by Na in
Cathode!
27
Raising CE% through applying fundamental knowledge
• 1955 Grjotheim
– Demonstrated that Al dissolved in pure cryolite forming a sodium fog
• & Na flame
• Bredig et al. ORNL 1959
– demonstrated that the NaF was a good solvent for Na
• Petit Foster & Frank 1976
– presented curves showing how the Na in Al & Na3AlF6 was reduced by AlF3 additions.
– Hence lowering metal solubility and the back reaction.
AlF3 additions limited by the adverse impact on alumina solubility, and electrolyte resistance or ACD
28
~2005 High Current – Strategy basis changes!
2. Operating in a different zone of the anode potential curve
30
1. Control curve modified through: Low Anode Cathode distance High AlF3 concentration
3. Back EMF set at 1.65V brings in error
• V = Enernst + α + β*f(I) + I* Rohmic
• R = δV/δI = Rohmic + δ/ δ I {β*f(I)}
Hi AlF3 &CD Zone
Many cell conditions have changed over the years! Do our design, work ,control ,& operating and practices match the
new requirements?
31
0
0.000002
0.000004
0.000006
0.000008
0.00001
0.000012
0.000014
0
1
2
3
4
5
6
0 0.2 0.4 0.6 0.8 1
Re
sist
ance
Ce
ll V
olt
s
Fraction Target Current
Errors in Resistance calculations
R Traditional
Using 1.65V as Bemf is no longer appropriate – errors too great!
Bath volume halved in the last 25 years! Alumina removal rate in cell up 25%
And we have much more information from the cells in the computer - but do have we automatic analysis, and do we use it for problem diagnostics??
Al Electrowinning is not the simple process as per texts! i.e. Al2O3 + 3/2C ≠ 3/2CO2 + 2Al with Edecomp = 1.18V
& CO not being formed electrochemically as it is “polarized”
• There are between 5 & 8 products released from the anode !
– CO2(g), CO(g), COS(g), SO2 (g), HF(g) , Cdust , COF2(g) ,CF4 (g), C2F6(g)
– & anode gases react with materials of construction
• The Electrochemical Cathode product (Na-Al alloy) undergoes numerous reactions with materials of construction. Products include:
– NaCx , NaCN, NaOH, Na2CO3, Al4C3, Si, NaxAlpSiyOz
• The cell is dependent on work practices to Remove Al, replace & protect Anodes.
• Al2O3 has to be replenished
32
Liquid Aluminium
Consumable Carbon Anode
Al2O3 dissolved in molten Na3AlF6
+
-
Some Reactions occurring in an Operating Cell – depending on anode Potential Gradient?
Al Producing Reactions in the Smelting Cell (ΔGo
reaction) (ΔHo
reaction) Enabling Potential
Completion Potential
kJ at 960oC
kJ at 960oC Volts Volts
0.75Al2O3*H2O + 3C + 1/2AlF3 = 2Al + 3CO(g) + 3/2HF(g) 1537 0.86 2.65
Al2O3 + 3C + 3S = 3COS(g) + 2Al 1219 0.910 2.10
Al2O3 + 3C = 2Al + 3CO(g) 1351 1.072 2.333
Al2O3 + 3/2C = 2Al + 3CO2(g) 689 1099 1.189 1.899
1/2Al2O3 + 3/2C + Na3AlF6(l) = 2Al + 3/2COF2(g) + 3NaF(l) 1078 1438 (1.85V*) Note : the chemical reaction
2COF2(g) +C = 2CO(g) + CF4(g) Is enabled, as soon as surface intermediates are formed. “Under-potential evolution”
ΔGo= -46kJ 123 (~1.74*V)
2Na3AlF6 + 3/3C = 3/2CF4(g) + 2Al + 6NaF 1483 1940 2.563 3.356
* Imposes limit on Electrode Potentials
The difference between actual and completion electrode potentials can be provided
by heat transfer to the electrode interface
This reaction almost certainly never occurs in
present installed cells!
33
Enabled is one thing, completing reaction/product
formation is another!
For Al Smelting Anodes. • Sufficient energy must cross the
interface boundary to COMPLETE product formation!
– ΔH reaction needed -here
represented as an equivalent
Voltage even though energy in
excess of Δ G can be either as
V or heat!
• A mixture of products always
formed electrochemically at
anode! • An indication Heat transfer to the
carbon anode may be limiting!
CO+CF4 Enabled Via COF2 ~ 1.82 V
CO+CF4 Complete ~ 2.35 V
CO Complete
~2.33V
CO enabled ~ 1.1 V
CO2 enabled ~ 1.2 V
CO2 complete ~ 1.89 V
DDC
Independent
CF4 Enabled ~2.45 V
Independent CF4 complete ~ 2.95 V
34
Another Work-practice- Replacing and protecting Anodes. .. Time consuming, introduces disturbances and operating problems!
35
In a multi-electrode system - the current & Reactions between individual anode pairs is not controlled,
only the total current through the cell!
36
Fast-forward to today – Hi Productivity • Cells for higher productivity and
lower energy consumption
• Much reduced electrolyte volumes so rate of change of concentration is faster!
• Working potential band for avoiding anode effects is reduced
• Alumina solubility is reduced by higher aluminium fluoride concentrations
• Power supplies are more stable and anode effect frequenciess substantially reduced – Do we still need resistance
smoothing?
• The control band is tighter
37
Maximum PFC’s zone* Direct CF4 Enabled
Background PFC’s zone* Extra CO + CF4 (via COF2) enabled
T <~ 975oC AlF3 < ~7% iD < ~0,73 Al2O3 > ~1.8%
T < 975 -955oC AlF3 < ~7 -12% iD < ~0.73 – 0.88 Al2O3 > ~2%
T < 955 -935oC AlF3 < ~12 -16% iD < ~0.9 – 1.1 Al2O3 > ~2.2%
Cell control band! 1.75V
~1.80V
2.35V
Better signal Processing needed to avoid PFC’s
• Change due to electrode process – not resistance!
• Need to limit rise
• Monitor under/overfeed duration to stop going to too low alumina concentration
• Uniform feeding & mixing more important!
38
Individual Anode Current Monitoring – an added control tool!
• Helps determine changes in spatial conditions in cells. – Minimize background PFC’s
– Highlight equipment & work-practice errors
• Marginal gains in CE%
– But greater gains can be derived by lowering Voltage for Minimum energy/kg
• But a revision & update of control strategy also needed
39
Nothing is New – 1987 !!!!
Minimizing energy means less emissions & greater efficiency!
But there are controlling laws that are invariably ignored!
& CE% improvements are of less value than lowering voltage!
Cell-Cell Volts Volts (at Cell) Energy in = Vat cell*I
Vcell-cell = V at cell + V external I*V external = I2 * R external - Wasted Energy
Lowering energy through Voltage Reduction kWh/kg = 298* V cell-cell /CE%
• The Aim: – To reduce energy
• The First Question - how is the energy used at the cell?
41
Utilisation of energy at the cell (/t Al)
42
• Preheat all input Materials – impure alumina
– Gross carbon anode
– make up fluorides
– Anode cover material
• Thermal energy to maintain the reactor temperature
– The cell heat loss
• Enthalpies of all
conversion reactions
– making aluminium forming both CO and CO2 gases
– forming COS
– evolving HF
1.89 t Al2O3
~7.6 kg Na2O
~ 20 kg H2O(l)
~80 kg Recycle fluorides
All at ~25oC
500 to 600 kg C
10 to 20 kg S
All at 25oC
150 to 250 kg
Anode cover Material
~ 15 to 30 kg for bath
Make All at ~25oC
12 to 20 kg AlF3 at 25oC
Heat losses include impact at anode change
43
Process Outputs (Reaction Products) ( Typical Values / tonne Al Produced)
Energy In = V x I x t
~ 12,700 to 14,500 kWh
Cell Gases 1110 to 1350 kg
CO2
120 to 300 kg CO
~ 30 kg HF
~ 20 kg COS
960oC
~ 100 kg Cover @ ~600oC
~ 100 kg [Gross-Net] C @ ~900oC
Bath tap
~ 15 kg liquid bath
@ ~960oC
Metal tap
1 ton liquid Al
@ ~960oC
43
Materials Preheat Energy Demand
Material kWh/ t Al
Range Comment & Reduction Options Gains?
Secondary Alumina 690 ± 50 Partial pre-heat? Geothermal energy source above 400°C ?
- 300
Gross Carbon 215 ± 30
Better shape design for retaining covering material? Better anode setting pattern for covering? Reduce carbon reactivity towards CO formation? Heat source to achieve temperature above 450°C?
-25 -140
AlF3 & Anode cover Material
25
± 10 Develop a recyclable Alternative?
-10
Does Iceland have an opportunity to get geothermal energy at the required temperature? What would be the new design of the pre-heater and anode handling?
44
Reactions conversion Energy Demand
Reaction kWh/ kg Al
Range Comment & Reduction Options Gains?
Aluminium production xAl2O3(s) + 3C = 2x Al +3COx (g)
5810* ± 100 Depends on Net Carbon consumption & fraction of metal formed with CO(g) as net reaction product* Reduced by more calcined less reactive anode's of uniform reactivity. Better control of cell stability and conditions.
-90 -100
Formation of Sulphur products, HF, & Na that enters cathode
20 ± 10
Little flexibility as controlled by raw materials
0
• Based on lowest net carbon achieved (392 kg C/t Al) and CO /CO2 ratios measured in operating cells .
45
The concluding challenges for Energy
• How can you eliminate the extra energy required through materials impurities – Especially S & HF?
• How can cell heat-loss be reduced? – Can the need for ledge be
removed?
• Can the feed materials be preheated using “free” thermal energy?
46
In conclusion
• Times have changed – Simplifications are not necessary and now harmful
• Many academic interpretations have been made for experimental designs and conditions that ignore limitations of operations. – Academics must work closely with ,but with open eyes open
– And a multi discilinary approach is needed!
• Robotics, and automation are lagging – Including ones that lead to energy savings
But success stories exist!
47
Note: Some CO(g) is formed directly electrochemically despite the literature implying otherwise! (Rest by “Back-Reaction”)
48
CO limited by Surface coverage/Desorption
CO2
Cell Heat loss
• Reductions constrained by the need to maintain side ledge. – Benefits of increasing side
ledge diminish for large cells.
– Depends on whether operating for high productivity(& CE%) or minimum energy.
• Significant further reductions require totally new cell design concepts and operating practices – automation of work practices
– robotics for precision
49
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
0 100 200 300 400 500 600
kWh
loss
/ k
g A
l
Operating Current (kA)
Trends in Cell Heat loss (kWh/kg Al)