“The future of aluminium smelting

“The future of aluminium smelting : - the environmentally friendly industry"

Barry Welch

[email protected]

A large number of valuable metals Rare earths, silicon, manganese, sodium, calcium, lithium, are dependent on an

electrical energy supply!

"Clean electricity" is a "commodity" in short supply,

- it should be used efficiently and responsibly.

- It should contribute globally to a better environment.

Aluminium: The flagship for metal electrowinning can lead the way!

What are the options?

• Inert anode's? – Eliminate the carbon dioxide that is

released?

• Eliminate anode effects? – According to the definition that is almost

being achieved! • But is the definition correct? • The first step is to get a better

understanding of the initiation and evolution process!

• Eliminate background PFC co-evolution. – Requires spatial control of conditions in

the cell • requires better sensing!

What is the potential for Inert anodes for Al? • The primary driver

– - eliminate the CO2 footprint arising from the present anode!

– eliminate the CO2 equivalent footprints from anode effect PFC's?

• The secondary benefits

– reduce the capital cost of the plant by eliminating anode production

– reduce the opex by eliminating anode change and carbon materials costs

• The materials requirements – an electronic conducting

material – resistant to high oxidation

potentials – insoluble in, and unreactive

to, cryolite-alumina systems • The operating requirements.

– A feasible design to satisfy the operating process requirements

– suitable materials to operate higher oxidation potentials

– safe methodology for removal of the liquid metal.

– Ability to shut down and restart the cell in event of power interruption.

The big question: is it to be developed for totally new technology or should it be

Retrofitable to existing technology? Where to install?

Two Approaches 1. Ceramic “pot” 2. Oxidized “designer alloy” Quotes - BJW TMS 2009

– The materials science for in electrode surfaces is probably solved…

– but through a combination of ignoring the key issues, and focusing on retrofit approach, there is still much to be done before we see the first commercial cells…

– The industry cannot be proud of the poor planning and analysis of what the issues are !!

… too many assumptions have been made!

The common Electrode surface material for all in recent years: - Nickel Ferrite (NiO.Fe2O3) as the basis sintered with other metals

especially Cu

The progress to date!

• More than 60 years R&D by five major smelting companies

• More than $US600,000,000 expended on trying to get a suitable material

• Probably more than 100 press announcements during the last 15

years implying "we have the material and we will have an operating smelter cell within a year or so!"

• No inert anode material will produce metal of the quality produced by the present process

• One smelting company has a developed prototype cell ready for full scale up and testing - but it will only BE ALLOWED to progress if future cost will be taken over by another company!

The successful developments and trials. 1. The electrodes have a very high ohmic resistance because of the

materials used - High ohmic voltage at current density 2. Limited corrosion and metal contamination cannot be avoided –

Reduces Al metal value 3. Require the electrolyte to be that maintained near saturation with

alumina – minimise corrosion

– achieve a modest life. Feeder design challenges, OPEX 4. Have a restricted current density range for operation. How do you

control it?

5. Consequently require: – a very high electrolyte volume – a more dispersive alumina feeding system – good agitation of the is the electrolyte for alumina dissolution

Features arising from various scale trials

Inert anode

• Material pre-heat energy

• Process conversion energy

• Cell heat loss + ~20%

– Total energy consumption

• Cell productivity (current density limit)

• Cell life (as little or no ledge!)

Savings ?

• anode gross carbon cost

• Loss of premium for quality

• Other penalties.

• Higher electricity , & reconstruction cost,

Conventional process

• 700 kWh/t

• 9000 kWh/t

• 8500 kWh/t

• 18200

• 1.5 t / day

• - $250 /t Al

• + $ 100 / t Al

The Inert anode retrofit option - no carbon plant @ 200/220 kA

• 900 kWh/t

• 5850 kWh/t

• 6200 kWh/t

• 12950

• 1.65 t / day

Globally environmentally worse - because of the high proportion of energy generated

by coal fired power stations!

Retrofit not a likey option!

Environmental & Energy - Cermet

Component

Units

BAT

Carbon Anode

Retrofit

Cermet Anode

Process Energy kWh/Tonne Al 6,600 9250

Thermal Energy kWh/Tonne Al 6,200 6800**

Electrical Energy

Requirement kWh/Tonne Al 13,000 18,200

CO2 e Footy prints

Total CO2e for Hydro or

Nuclear kg CO2e/Tonne Al 1380 0

Total CO2e for Natural

Gas Power kg CO2/Tonne Al 8500 8451

Total CO2e for Coal

Power kg CO2/Tonne Al 15,000 18,000

•

Experienced operating scale up cells with IA's

Retrofit problems – concentration gradient’s

• Common zone of high corrosion using dispersed conventional feeding

• Anode gas bubbles from in the anodes are totally different and don’t impact the mixing process the same!

– Like champagne bubbles !!

• If you don’t get mixing right…..

Earlier Inert Anode dreams and obstacles

The Dreams • Potential voltage saving by

operating at very small ACD, and through low anode polarization

– Based on a misunderstanding of process v’s reaction enabling energy

• Measured low polarization – which is thru corrosion

• A drained high current density cell! – but COF2 formed at high cd’s or

lower Al2O3

The Obstacles • What electronically conducting

material are you going to use to get the current to anode surface?

• Methodology for maintaining alumina concentration uniform and saturated.

• Ability of a cermet bonded to metal to withstand thermal cycling

• Materials of construction for superstructure resistant to hot pure oxygen!

Co-evolution of AlOF2 - A constraint on current density

Evolution at Eo ~1.665V according to the reaction

Al2O3 + 2AlF3 = 3AlOF2(g) + Al.

– Becomes more favoured in acidic electrolytes

The data also indicates that as the material cools

it undergoes a disproportion reaction:

6AlOF2(g) = Al2O3 + 4AlF3 + 1.5O2(g)

this becomes quantitative at temperatures below 700°C would

Inflexion at expected potential

Tarcy LM 1986

Retrofit problems – radiant heat loss !

• Need a clear liquid surface at a high superheat for alumina dispersion and dissolution – You don’t have conventional anodes

to support a crust cover

– But fume transport and radiant heat will result in the crust collapse anyhow!

• So we have another materials problem!

• you need to halve the top heat loss to approach energy equivalents in retrofit cells – There is not enough flexibility in

changing anode current density

Mann (Light Metals 2008)

Some misleading diversions !

• Testing performance of electrode assembly in conventional carbon anode cell – Electrode potential too low

– Carbon dust depolarizes

• Used a low temperature electrolyte (K3AlF6) to reduce corrosion rates

– Resistivity of the electrodes surface counters benefits

• Tunnel (retrofit) vision

• Erroneous assumptions on reactions …..

Ni Fe2 O4 - 17% Cu Cermet

Channing 2005

X 2.5

Part 2:Eliminating all PFC evolution from conventional cells First step: Developing a better understanding of PFC emissions from smelter cells. Present concepts wrong / questionable!

Then solve two problems

1. better definition of cell operating control limits to avoid onset

2. Introduce new diagnostics for detection of deviations causing background PFCs

In my earlier presentation I discounted direct formation of CF4 at anode effects!

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!

Generally the literature discusses the anode process via the three reactions!

Overall independent cell reactions Std State Cell voltage

% Of REQUIRED

ENERGY

Al2O3 + 3C = 2Al + 3CO(g) 1.074 32

Al2O3 + 1.5C = 2Al + 1.5CO2(g) 1.191 62

4Na3AlF6 + 3C = 3CF4(g) + 4Al + 12NaF 2.527 75

These ignore another chemical compound that is known to Exist – COF2(g) !

They also tend to ignore the energy required for complete reaction!

Al2O3 + 2Na3AlF6(l) + 3C = 4Al + 3COF2(g) + 6NaF(l)

1.863 75

In the cell environment COF2 will decompose

2COF2(g) + C = 2CO(g) + CF4(g) -45.8 kJ

If this happens it lowers the “enabling” voltage 1.806

If that happens we would expect an increase in CO content of cell gases during an anode effect!

=ΔHReaction/nF reaction =energy gradient required to be transfered

Across interface if no heat transfer

Incr

easi

ng

Inte

rfac

ial E

lect

rod

e Po

ten

tial

gra

die

nt

Heat transfer required for completion of reaction

Electrode “overpotential” enables and assists forward reaction

No current flow /reaction possible – reaction not enabled

Electrochemistry 101 + First Law of thermodynamics

18

=ΔGReaction/nF reaction at equlibrium and I reaction =0 Reaction is “enabled”

ΔH

= E

ner

gy r

equ

ired

fo

r co

mp

lete

rea

ctio

n

incl

ud

ing

ph

ase

chan

ges

i

V

Answers needed for proof of COF2 being cause of AE’s!

Does COF2 form (as intermediate) in cells?

Does COF2 readily decompose at Electrode potentials below that required for direct CF4 formation ?

Does the anode gas increase in CO during an AE?

And is the increase approximately in the proportion of 2 x CO / CF4 ?

1. Does COF2 form (as intermediate) in cells?

Calandra 1980 & 82– Cyclic Voltammetry @ 1000V/s(Na2O–NaF & Al2O3-Na3AlF6) with Reversal at different potentials

• Demonstrated F- co-discharge initiates at voltages well below necessary for CF4 direct formation.

• And …

– A surface intermediate CxF is formed that can passivate the surface!

1b . Does COF2 form (as intermediate) in cells?

COF2 identified in 1998 (Doreen in LM ) by careful experimental design that minimized gas contact time with C as Al2O3 Was electrochemically depleted from the cell

COF2 Also detected With continuous gas analysis when electrowinning rare earths from oxide fluoride melts! Especially at higher temperatures and superheat's.

1c . Does COF2 form in the predicted potential band – i.e. Near 1.8V in cells?

1. Completely vertically oriented microelectrode of area 0.7 cm².

2. Cell voltage Increased 5 V at 20 V per second, and reversed at 200 V per second.

3. Data recorded at ~2,000 Hz

4. Alumina concentrations from 0.75 to 4wt%

Yes as demonstrated by Haverkamp (not published) in 1992

1c . Does COF2 form in the predicted potential band – ie. Near 1.8V in cells?

1. Completely vertically oriented microelectrode of area 0.7 cm².

2. Cell voltage Increased 5 V at 20 V per second, and reversed at 200 V per second.

3. Data recorded at ~2,000 Hz

4. Alumina concentrations from 0.75 to 4%

Yes as demonstrated by Haverkamp (not published) in 1992

And it passivate because the F requires co-oxidation of O2- to be stripped from surface

0.00

0.50

1.00

1.50

2.00

2.50

0 0.5 1 1.5 2 2.5 3

Re

sult

ing

Cu

rre

nt

(A)

Sweep Voltage

Potential / current range PFC Formation continues

Decreasing Volts 200V/s

Increasing Volts 20V/s

Area = 0.015 Coulombs

Area = 0.042 Coulombs

0.032 secs

Coulombs 0.015

Moles F 1.55E-07

Gas Vol cm3 0.004

Atoms F 9.36E+16

Moles CF4 2.34E+16

Atoms C >5E+15

Does the anode gas increase in CO during an AE? This data clearly shows it does!

Holliday & Henry (JoM October 1957) Collected gas samples and analyse them by mass spectrometry and chemical techniques.

Does the anode gas increase in CO during an AE? And is the increase approximately in the proportion of 2CO / CF4 ?

Tabereaux Richards et al LM1996 confirms this and also confirms the proportion increase! CO Increase approximately 40% CF4 increase 16% CO2 decrease > 50% This also indicates that heat transfer is playing an important role in the release of CO

Composition of Pot Gas During AE

(No Anode Effect Quenching)

Time (min.)

An

od

e G

as (

%)

Star

t o

f A

no

de

Effe

ct

Carbon Dioxide (CO2)

Carbon Monoxide (CO)

Tetrafluoromethane (CF4)

100

90

80

70

60

50

40

30

20

10

00 1 2 3 4 5 6 7

(No Anode Effect Quenching)

Time (min.)

An

od

e G

as (

%)

Star

t o

f A

no

de

Effe

ct

1 2 3 4 5 60

20

18

16

14

12

10

8

6

0

2

4

Tetrafluoromethane (CF4)

Hexafluoroethane (C2F4)

Measurements

were made without

moving the anodes

to cause electrical

shorting or AE

killing

Continuous emission of CF4 during AE.

Implications of correcting the understanding of AE reaction mechanism.

• The cell voltage (resistance) Control band for avoiding PFC's must be narrowed.

• The Existing alumina concentration versus Voltage curve requires re-examining, – bubble resistance typically ignored

– impact of chemistry shift to higher aluminium fluoride

– impact of higher current density on the electrode base potential.

• Spatial Concentration gradients in the cell, and anode-set errors can enable PFC co-evolution at individual anode's Without showing signs of an anode effect

Impact of changing operations on control band • 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 frequency is substantially reduced – Do we still need resistance

smoothing?

• The control band is tighter

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

1.85V

2.35V

High Current & Low ACD – Strategy changes shape of basic curve!

2. Operating in a different zone of the anode potential curve

– Alumina concentration lower

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

Sign for PFC co-evolution at an individual anode -By current lowering

Consider an electrode pair segments

Change the anode cathode distance

– The current through the segment changes

• Polarisation consequently changes

– The total voltage doesn’t change (much!)

• Can reach the anode electrode potential that enables PFC emissions.

Lower the local alumina concentration

Polarisation consequently increases

The current through the segment decreases

The total voltage doesn’t change (much!)

Can reach the anode electrode potential that enables PFC emissions.

Refer Evans presentation

Typical imbalance in currents – cell emitting PFCs above the normal background levels

• W. Li, Q. Zhao, J. Yang, S. Qiu, X. Chen, On Continuous PFC Emissions Unrelated to Anode Effects, Light Metals 2011, 2011, 309-314

• So the methodology of defining the initiation of PFC emissions is flawed and needs changing.

• Needs to be linked to the average minimum voltage ** for normal operations, and the voltage rise!

An early warning example for prevention

Cell voltage curve prior to AE onset for Constant Cell current – modern cell (0.5 sec data)

4300

4350

4400

4450

4500

4550

4600

4650

4700

4750

4800

5:05:00 PM 5:12:12 PM 5:19:24 PM 5:26:36 PM 5:33:48 PM 5:41:00 PM 5:48:12 PM

Ce

ll m

illiv

olt

age

(m

V)

Time

Change in Cell Voltage during Underfeed

– Voltage rise through increased anode potential.

– - Rise less than 35 mV to PFC co-evolution leading to anode effect

– ( note the computer has an early detection system –minimises duration)

Next voltage 13.16V

37

The better understanding of AE Mechanism And Control Limits Has been Combined with Multi Operating Variable analysis

… and the work practices in Cells

• Anode effects tend initiate in zones of poor feeding within the cell – Through empty ore bins

– Through blocked feeder holes

– Through Bath flow inhibited by freeze under newly set anodes

– Through slow mixing as a consequence of too -few point feeders per kilo amp

• Consequently low voltage PFC emissions can be more prevalent and need to be accounted for!

.. Success there opens the door to attack background PFC’s

section of cell (~12) on PFC’s in the line duct

Welch – 1othMST Conference 11-13th June 2015 39

Consequence of implementing knowledge - still ongoing!

• Individual anode Currents should be monitored continuously!

• Need Changes in raw signal processing and control limits

• Important to Introduce Cross diagnosis for potential problems!

Conclusions

• …….. Co-operative research between academics and Industry can help the industry achieve its goals

• But

• I have said enough!

What is an anode effect?

• This is an anode effect!

• Some papers imply the cause is not know – Yet the literature between 1950 and

1980 makes the cause clear!

• Does non-wetting or a gas film preventing current flow? – No but there is arcing indicating a

resistive film on the electrode surface!

Anode effect's in modern smelting cells.

• Smelter cells have spatial and temporal changes of the electrolyte alumina concentration and interelectrode distances,

• The multiple electrodes can operate essentially as independent cells operating at the same voltage between anode stem and cathode metal pad.

• This is despite the cell being operated at a constant total current.

• These combinations enable localised PFC emissions, even though the cell voltage can be within the control band.


Change the anode cathode distance

– The current through the segment changes




Lower the local alumina concentration


The current through the segment decreases

The total voltage doesn’t change (much!)


Refer Evans presentation


Change the Bath depth

– The anode current density changes




Lower the number of anodes

The anode current density changes



Refer Alzarouni’s presentation

Documents

“The future of aluminium smelting