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

Transcription

1 The future of aluminium smelting : - the environmentally friendly industry" 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! Barry Welch Barry@Barry.co.nz

2 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!

3 What is the potential for Inert anodes for Al? The primary driver - eliminate the CO 2 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?

4 The common Electrode surface material for all in recent years: - Nickel Ferrite (NiO.Fe 2 O 3 ) as the basis sintered with other metals especially Cu 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!

5 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!

6 Features arising from various scale trials 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

7 The Inert anode retrofit option - no carbon 200/220 ka Material pre-heat energy Inert anode 700 kwh/t Conventional process 900 kwh/t Process conversion energy Cell heat loss + ~20% Total energy consumption 9000 kwh/t 8500 kwh/t kwh/t 6200 kwh/t 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, 1.5 t / day - $250 /t Al + $ 100 / t Al 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!

8 Environmental & Energy - Cermet Component Units BAT Carbon Anode Retrofit Cermet Anode Process Energy kwh/tonne Al 6, Thermal Energy kwh/tonne Al 6, ** Electrical Energy Requirement kwh/tonne Al 13,000 18,200 CO 2 e Footy prints Total CO 2 e for Hydro or Nuclear kg CO 2 e/tonne Al Total CO 2 e for Natural Gas Power kg CO 2 /Tonne Al Total CO 2 e for Coal Power kg CO 2 /Tonne Al 15,000 18,000

9 Experienced operating scale up cells with IA's

10 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..

11 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 COF 2 formed at high cd s or lower Al 2 O 3 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!

12 Co-evolution of AlOF 2 - A constraint on current density Evolution at E o ~1.665V according to the reaction Al 2 O 3 + 2AlF 3 = 3AlOF 2 (g) + Al. Becomes more favoured in acidic electrolytes Tarcy LM 1986 The data also indicates that as the material cools it undergoes a disproportion reaction: 6AlOF 2 (g) = Al 2 O3 + 4AlF O 2 (g) this becomes quantitative at temperatures below 700 C would Inflexion at expected potential

13 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

14 Some misleading diversions! Used a low temperature electrolyte (K 3 AlF 6 ) to reduce corrosion rates Resistivity of the electrodes surface counters benefits Ni Fe 2 O 4-17% Cu Cermet Testing performance of electrode assembly in conventional carbon anode cell Electrode potential too low Carbon dust depolarizes X 2.5 Tunnel (retrofit) vision Channing 2005 Erroneous assumptions on reactions.. Mann (Light Metals 2008)

15 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

16 The difference between actual and completion electrode potentials can be provided by heat transfer to the electrode interface In my earlier presentation I discounted direct formation of CF4 at anode effects! Al Producing Reactions in the Smelting Cell (ΔG o reaction) kj at 960 o C (ΔH o reaction) Enabling Potential Completion Potential kj at 960 o C Volts Volts 0.75Al2O3*H2O + 3C + 1/2AlF3 = 2Al + 3CO(g) + 3/2HF(g) Al2O3 + 3C + 3S = 3COS(g) + 2Al This reaction almost certainly never occurs in 1/2Al 2 O 3 + 3/2C + Na 3 AlF 6 (l) = 2Al + 3/2COF 2 (g) + 3NaF(l) present 1078 installed 1438 (1.85V*) cells! Al 2 O 3 + 3C = 2Al + 3CO(g) Al 2 O 3 + 3/2C = 2Al + 3CO 2 (g) Note : the chemical reaction 2COF 2 (g) +C = 2CO(g) + CF 4 (g) Is enabled, as soon as surface intermediates are formed. Under-potential evolution ΔG o = -46kJ 123 (~1.74*V) 2Na3AlF6 + 3/3C = 3/2CF4(g) + 2Al + 6NaF * Imposes limit on Electrode Potentials

17 Generally the literature discusses the anode process via the three reactions! Overall independent cell reactions Std State Cell voltage Al2O3 + 3C = 2Al + 3CO(g) Al2O C = 2Al + 1.5CO2(g) Na3AlF6 + 3C = 3CF4(g) + 4Al + 12NaF % Of REQUIRED ENERGY 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) In the cell environment COF2 will decompose 2COF2(g) + C = 2CO(g) + CF4(g) kj If this happens it lowers the enabling voltage If that happens we would expect an increase in CO content of cell gases during an anode effect!

18 Increasing Interfacial Electrode Potential gradient ΔH = Energy required for complete reaction including phase changes Electrochemistry First Law of thermodynamics =ΔH Reaction /nf reaction =energy gradient required to be transfered Across interface if no heat transfer Heat transfer required for completion of reaction Electrode overpotential enables and assists forward reaction =ΔG Reaction /nf reaction at equlibrium and I reaction =0 Reaction is enabled V No current flow /reaction possible reaction not enabled 18 i

19 Answers needed for proof of COF 2 being cause of AE s! Does COF 2 form (as intermediate) in cells? Does COF 2 readily decompose at Electrode potentials below that required for direct CF 4 formation? Does the anode gas increase in CO during an AE? And is the increase approximately in the proportion of 2 x CO / CF 4?

20 1. Does COF 2 form (as intermediate) in cells? Calandra 1980 & 82 Cyclic 1000V/s(Na 2 O NaF & Al 2 O 3 -Na 3 AlF 6 ) with Reversal at different potentials Demonstrated F- co-discharge initiates at voltages well below necessary for CF 4 direct formation. And A surface intermediate C x F is formed that can passivate the surface!

21 1b. Does COF 2 form (as intermediate) in cells? COF 2 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.

22 1c. Does COF 2 form in the predicted potential band i.e. Near 1.8V in cells? Yes as demonstrated by Haverkamp (not published) in 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%

23 1c. Does COF 2 form in the predicted potential band ie. Near 1.8V in cells? Yes as demonstrated by Haverkamp (not published) in 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%

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25 And it passivate because the F requires co-oxidation of O 2- to be stripped from surface Resulting Current (A) Coulombs Moles F Increasing Volts 20V/s 1.55E-07 Gas Vol cm Atoms F Moles CF4 Atoms C 9.36E E+16 >5E+15 Potential / current range PFC Formation continues Area = Coulombs secs Area = Coulombs 0.00 Decreasing Volts 200V/s Sweep Voltage

26 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.

27 Does the anode gas increase in CO during an AE? And is the increase approximately in the proportion of 2CO / CF 4? 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

28 Anode Gas (%) Start of Anode Effect Anode Gas (%) Start of Anode Effect Composition of Pot Gas During AE (No Anode Effect Quenching) Carbon Dioxide (CO 2 ) Carbon Monoxide (CO) Tetrafluoromethane (CF 4 ) Time (min.) (No Anode Effect Quenching) Tetrafluoromethane (CF 4 ) Hexafluoroethane (C 2 F 4 ) Time (min.) Measurements were made without moving the anodes to cause electrical shorting or AE killing Continuous emission of CF4 during AE.

29 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

30 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 2.35V Maximum PFC s zone* Direct CF4 Enabled Background PFC s zone* Extra CO + CF4 (via COF2) enabled 1.85V Cell control band! T <~ 975 o C AlF 3 < ~7% i D < ~0,73 Al 2 O 3 > ~1.8% T < o C AlF 3 < ~7-12% i D < ~ Al2O3 > ~2% 1.75V T < o C AlF 3 < ~12-16% i D < ~ Al2O3 > ~2.2%

31 High Current & Low ACD Strategy changes shape of basic curve! 1. Control curve modified through: Low Anode Cathode distance High AlF3 concentration 2. Operating in a different zone of the anode potential curve Alumina concentration lower Hi AlF3 &CD Zone 3. Back EMF set at 1.65V brings in error V = E nernst + α + β*f(i) + I* R ohmic R = δv/δi = R ohmic + δ/ δ I {β*f(i)}

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

33 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

34 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, 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!

35 An early warning example for prevention

36 Cell millivoltage (mv) Cell voltage curve prior to AE onset for Constant Cell current modern cell (0.5 sec data) Next voltage 13.16V 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) :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 Time

37 The better understanding of AE Mechanism And Control Limits Has been Combined with Multi Operating Variable analysis 37

38 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!

39 .. 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

40 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!

41 Conclusions.. Co-operative research between academics and Industry can help the industry achieve its goals But I have said enough!

42 This is an anode effect! What 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!

43 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.

44 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

45 Consider an electrode pair segments Change the Bath depth The anode current density changes Polarisation consequently changes The total voltage doesn t change (much!) Can reach the anode electrode potential that enables PFC emissions. Lower the number of anodes The anode current density changes Polarisation consequently increases Can reach the anode electrode potential that enables PFC emissions. Refer Alzarouni s presentation