Dr LJ Erasmus June 2013
Self baking Electrode Mr Söderberg, Sem& Westley Det norske Aktieselskabfor Electrokemisk Industri, Oslo 1919 6/14/2013 2
SöderbergElectrodes A Triumph of Materials Engineering Anthracite Pitch Requirements Mechanical integrity Ability to sustain enormous temperatures Oxidation resistance
SöderbergPaste Unbaked paste Anthracite 78% Tar 22% Bulk density 1.63 kg/m 3 Plastisity 39% (20 50%) Baked paste Bulk density 1.45 kg/m 3 Compressive strength 14.6 N/mm 2 Bending strength 3.34 N/mm 2 Modulus of elasticity 2.81 kn/mm 2 Electrical resistivity 76.7 µωm (room temp) Volatiles 12.6% 6/14/2013 4
Söderberg Electrode Paste Production Process Anthracite CRUSHER + 16 mm - 16 +4 mm SCREEN +2 mm SCREEN - 4 +1 mm CALCINING FURNACE BALL MILL -63 µm Ball Mill Prod. -4 mm fraction Medium size Coarse size SCALE BINDER 120 C 180 C PREHEATER PREHEATER 180 C Sold MIXER CASTING WATER COOLIN G PACKAGING / STACKING 6/14/2013 5
Types of electrode pastes
Electrode Column designs
Electrode slipping Electrode slip mechanism Maintenance ring Casing Upper ring Pneumatic cylinder Pneumatic slip cylinders Lower ring Pneumatic cylinder Slip ring
Electrode Management Casing Design Solid and liquid paste levels Electrode baking 450 C Isotherm Electrode slipping Furnace operation Shut down practice
Casing design Steel Current capacity Sufficient steel to provide sufficient strength to carry the electrode Tensile strength Sufficient windows in fins for baked paste bridge formation Shearing strength
Casing
SöderbergElectrode Column WINDOWS CASING FINS PASTE CYLINDERS ELECTRODE CASING CONTACT CLAMPS LIQUI D PASTE BAKED ELECTRODE 6/14/2013 12
Solid and Liquid Paste Levels SOLID PASTE LEVEL LIQUID PASTE LEVEL LIQUID PASTE BAKED ELECTRODE 6/14/2013 13
Solid level control Segregation as the solid pushes into the liquid Electrode break Liquid level = 2.2 x Electrode diameter Maximum liquid paste temperature = 80 C Prevent boiling and fuming, with subsequent segregation
Liquid level control High liquid level Segregation Course anthracite particles settles to the bottom Electrode break Low liquid level Not sufficient hydrostatic head to force gases down Not sufficient depth to carry un-molten paste cylinders Electrode Break
Baking Mechanism Baking when the paste devolatilise Between 450 C 500 C The carbon residue then binds the anthracite Volatiles forced down electrode Due to higher temperatures volatiles are cracked to carbon and hydrogen This carbon strengthens the bond between anthracite particles
Baking Mechanism The baking region is critical and must be managed A constant baking rate must be maintained to prevent shifts of baking zones Stoppages can cause paste shrinkage Weak points in the electrode
Temperature Zones 25 C -Solid paste 70 C Completely softened 300 C Solidification sets in 700 C Solidified
The 450 C Isotherm PLC SCADA THERMOCOUPLE LIQUID PASTE ±450 C h h BAKED ELECTRODE 6/14/2013 19
Electrode Baking W = I 2 R (Maximum Current) Baking Temperature Temperature from Fins Temperature from Contact Clamps = + + Temperature from Environment 6/14/2013 20
Electrode Baking Zones CASING PASTE CHARGE FINS 80 ºC SOFT PASTE POWER SUPPLY 200 ºC CONTACT SHOE HIGH ELECTRODE CURRENT NOLMAL CONDITIONS HIGH SLIPPING RATE 500 ºC 1200 ºC BAKING ZONE BAKED ELECTRODE 2000 ºC 2500 ºC FURNACE CHARGE
Temperature profile 6/14/2013 22
Baking zones under different operating conditions
Soft Break Casing failure Too high current in casing
Common Reasons for Electrode Breakages
Baked Electrode Properties High thermal and electrical conductivity Compressive and tensile strength Low Coefficient of Thermal Expansion Thermal Shock resistance Uniform anthracite distribution Low consumption 6/14/2013 26
In Action 6/14/2013 27
In Action 6/14/2013 28
Production of extruded graphite electrodes Process uses calcined petroleum coke as filler and coal tar pitch as binder May involve multiple re-impregnation and re-baking steps. 6/14/2013 29
Petroleum coke quality Sulphur Cause puffing during baking Ni & V Oxidation catalysts Accelerate pin-holing 6/14/2013 30
Graphite electrodes 6/14/2013 31
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Energy efficiency The greatest source of energy in the future will be using it more efficiently, Bill Colton, Exxon s vice-president 6/14/2013 33