LOW RESISTANCE ANODE ASSEMBLIES
BRIDGING THE IRON TO CARBON INTERFACE
A summary of :
“LOW RESISTANCE ANODE ASSEMBLY USING STEEL STUBHOLE
CONDUCTORS ACROSS THE CAST IRON TO CARBON INTERFACE”
Will Berends
HATCH
Lawyers Statement
• IP Protection for Described Technology:
– Patents/pending:
• CA 2,838,113
• US 62119508
• CN201420801618.3
• RU2014149274
• GCC2014/28532
• PCT CA2014/051178
• +International patents pending
Presentation
• Description of Anode Nails
(i.e. Stubhole Conductors)
• Full Scale Lab Testing
• Field Testing at Alouette
• Future Development
The Stub to Carbon (STC) Connection
• Cast iron connection
(i.e. the thimble)
• High electrical resistance
when cold
• Stub suffers corrosion
• Non-uniform current
distribution through anode
Main Causes of STC Electrical Resistance
• Varying contact pressure
and area due to:
– Iron solidification -shrinkage
gap
Stub
– Surface condition of stub and
stubhole
– Stub size and stubhole shape
– Differential thermal expansion
of steel stub, iron and carbon
Cast iron
Anode Top
What are Anode Nails?
• Hardened steel nails
inserted into stubhole
Iron
Thimble
Stub
• Head of nails are
embedded into the iron to
bridge the shrinkage gap
• Electrical conductivity of
the nails is in addition to
the regular STC
conductivity
• Carbon steel nails are
recycled into the cast iron
Nails
Anode Carbon
Stub
Iron Thimble
Anode Carbon
Nails
Anode Nails Overview
Anode Nails
Stem
Stubs
Anodes
Bath
Where are Anode Nails Applied?
Bottom Surface
Bottom corner or
groove
Side walls or flutes
How are Anode Nails Installed Manually?
•
Lengthened pneumatic nailer with hardened steel nails
•
Manual nailing station beside anode conveyor
How are Anode Nails Installed Automatically?
• Robotic Production Equipment
– Rodshop cycle times of 30-60 seconds/anode
– Automatic reloading of nails
Development of Anode Nails
• First concepts were tested with scale models using tin
solder connections with similar solidification
shrinkage to iron
Computer Modeling of Anode Nails:
ANSYS & PROCAST
• Iron solidification, thermal & electrical, and stress
• Predicted a reduced resistance due to contact
pressure of nails and contact area
Lab & Field Testing
• Objectives
– STC Resistance versus Temperature
– Current Distribution from each Stub
– Impact on Pot Noise & Current Efficiency
– Impact on Critical Current Density, Current pickup rate and
pot stability
Full Scale Lab Test: Equipment
• Electric kiln, 20 - 850°C
• 10 Amp Micro-ohmmeter
• Measurement averaging
circuit to eliminate
Seebeck effect
• 16 samples
– 8 Reference (without nails)
– 8 with nails
Full Scale Lab Test: Resistance Test Results
Contact Resistance Savings for Clean Stubhole with 45 Nails
Full Scale Lab Test: Distribution Test Result
A
Single Stub Current Distribution Improvement using Stubhole Conductors
B
Full Scale Lab Test: Results
• Impact of Anode Nails:
Reduction in Contact Resistance
Improved Current Distribution
Field Test: Alouette In-pot Test Arrangement
• Two series of tests, clean and
dirty stubholes
• Each test included 3 pairs of
test and reference anodes in 3
pots, set 1 day apart
Field Test: Nail Condition
• Condition of Anode Nails after 24 day pot cycle:
Field Test: Stem vs Stub Voltage
•
V-Stem correlates to total current
through anode assembly, ʄ(temperature
and Thermal Coeff. Of Resistance
TCOR)
•
Instantaneous V-STC = function of
Instantaneous Current per Stub and
STC Resistance
•
STC Resistance = ʄ(temperature,
TCOR), STC slowly decreases with
increasing temperature
•
Instantaneous Current per Stub =
ʄ(instantaneous resistance through
STC, anode, frozen bath, bubble layer
and bath layer close to each stub)
•
All of current through Stem is shared
unequally through the stubs due to
varying total resistance from each Stub
to metal pad (~zero potential).
V-Stem ~ Current
V-STC = ʄ(Current,
Stub Resistance)
Bath
layer
Metal layer ~zero potential
Field Test: STC Resistance Variation
STC Resistance varies due to:
•
Yoke expansion and anode bending force
•
Unequal stub temperatures
•
Stub & stubhole condition
Thermal Expansion
Other resistance through anode to metal pad include:
•
Anode Thickness
•
Anode and STC Temperature
•
Frozen Bath
•
Bubble coverage under anode
•
Bath resistance through ACD
High Contact Pressure
Thermal Anode Bending
V1
V2
V3
V-Stubs provides more detailed information than just V-Stem
Stem
V6
V5
V4
Field Test: Anode Resistance Variation
•
Anode assembly resistance may vary >3:1 due to temperature and
carbon thickness, plus additional resistance from frozen bath coverage,
causing an high imbalance in current distribution among anodes.
Resistance
variance
Steel °C
STC °C
Anode °C, mm
‘Cold’ New
Anode
•
‘Hot’ Old
Anode
Old anodes (after the slots are gone), have lower than average
resistance, higher than average current density, and higher noise that
exceeds CCD more often.
Field Test: Alouette Pot B137 Voltage Signal
STUBS – NO NAILS
STEM
STUBS – WITH NAILS
STEM
Field Test: Alouette Pot B137 Voltage Signal
> Critical Current Density
Loss of anode slots
High Std Dev Stem & Stubs
40 hours
> Critical Current Density
Loss of anode slots
Low Std Dev Stem & Stubs
18 hours
Field Test: Test #1 V, I, R- Clean Stubholes
Measurement of STC Contact Resistance from Iron to Top Surface of Anode
Field Test: Test #2 V, I, R - Dirty Stubholes
Measurement of STC Contact Resistance from Iron to Top Surface of Anode
Field Test: Power Savings During Heat-up
Test 2 Savings ~ 160-380 kWh per each anode setting event +
ongoing savings from reduced resistance over anode cycle
Field Test: Dirty Stubholes Resistance
Each 1 µΩ savings in STC contact resistance ~ 10 kWh/mt
Alouette Test #2
Field Test: Close up of Bubble Induced
Low Voltage Anode Effect LVAE
• Portions of anode bottom
regularly exceed Critical
Current Density for Low
Voltage Anode Effect.
STUBS
No Nails
• LVAE occurs in the bath film
layer and cycles with bubble
formation and release
• Not detected by stem current
• Non-propagating if adequate
alumina is available in
surrounding bath
STEM
Field Test: Current Stability at Steady State
CCD
range
CCD
range
NO NAILS
WITH NAILS
Observations:
–
–
–
–
Peak current density under portions of the anode may exceed CCD due to bubble growth
and self correct with bubble release and evacuation
The current distribution within the anode, shows a consistency that may be a function of
location of the stub relative to the centre channel. This behaviour may enable optimization
The anode nails demonstrate a lower current deviation in each stub and between stubs
The reduced pot noise, and reduced CCD indicate greater pot stability and more uniform
electrolysis occurring across the anode bottom surface.
Measures of Current Stability
Std Dev of avg stub current (red)
Nails reduce this by 34%
Std Dev across stubs (colours)
Nails reduce this by 14%
STUBS
Avg stub
current
Std Dev of each stub current (colours)
Nails reduce avg of these SD’s by 29%
Std Dev of Stem current
Nails reduce this by 19%
STEM
Note: All SD based on 12 hour moving avg to mitigate
effect of aging anode and pot events (setting, tapping)
Why such improvements?
1. Lower STC contact resistance reduces power
consumption across the pot potential (top to bottom)
2. More even STC resistance for all stubs enables a
more even horizontal shunting of current between
stubs in response to varying local resistance between
the each stub and the metal pad. This is not indicated
in voltage savings but in improved current stability.
Summary: Impact of Anode Nails
– Significant decrease of STC resistance at low temperatures
– Improvement in radial current distribution from stub to anode
– Potential decrease in smelting energy consumption > 1/2%
– Avg decrease in time for new anodes to reach full current ~ 40%
– Improved current stability = reduced pot noise, improved CE% (TBD)
– Decreased time at CCD = fewer LVAE’s & PFC’s (TBD)
– Potential more uniform preheat of cathodes in startup (TBD)
Future Development
• Optimize location of nails to tune anode resistance to
further reduce current Std Dev and to reduce CCD &
LVAE’s
• Long term in-pot tests to measure:
– Resistance Savings & Energy Efficiency
– Pot Noise & Current Efficiency
– LVAE & PFC emissions
Thank you
WBerends@Hatch.ca
Questions?