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Light Metals 2009
Edited by xx
TMS (The Minerals, Metals & Materials Society), 2009
Daniel Richard1, Patrice Goulet2, Olivier Trempe2, Marc Dupuis3 and Mario Fafard2
Hatch, 5 Place Ville Marie, Bureau 200, Montréal (Québec), Canada, H3B 2G2
Aluminium Research Centre ­ REGAL, Laval University, Science and Engineering Faculty,
Adrien-Pouliot Building, Sainte-Foy (Québec), Canada, G1K 7P4
GéniSim Inc., 3111 rue Alger, Jonquière (Québec), Canada, G7S 2M9
Keywords: Numerical Analysis, Contact Resistance, Anode, Stub Hole
Reduction of cell voltage through redesign of the stub holes of
cast iron rodded anodes is an attractive idea. In practice, stub hole
optimisation is not an easy task and in situ trials may yield what
seem to be counter-intuitive results.

A closer examination reveals a complex behaviour of the steel
stub - cast iron - carbon joint. It was shown in previous work [1]
to be a non-linear thermal-electrical-mechanical coupled system.
Minimisation of the stub-to-carbon voltage drop is a balancing act
between contact surface area and electrical contact resistance.

To gain insights into the merits of different designs, a finite
element demonstration model was built using the in-house code
FESh++. Alternative configurations were studied. Potential
industrial applications are discussed.
Cast iron is typically used to connect the steel stubs of the anode
hangers to the carbon anodes used in Hall-Héroult cells. The steel
stubs are positioned into specially designed holes in the carbon
anode - the stub holes - where molten iron can be cast. The
solidified cast iron in the stub hole plays the role of a mechanical,
thermal and electrical connection.

In his 1976 paper, Peterson [2] instrumented an anode with 30
voltage probes and 50 thermocouples. Temperature and potential
readings were taken during 24 hours while the anode was in
operation. Up to 25% of the anode voltage drop was attributed to
the steel-cast iron-carbon connection. Peterson tested a stub hole
of a different design in a furnace, which seemed to indicate
negligible contact resistance at high temperature [3]. The results
of these two studies seem to be quite contradictory. A clear
conclusion is however that an interface resistance (contact
resistance) at the cast iron to carbon transition increases the
anodic voltage drop.

In practice, it was found that the stub hole voltage drop varies
from design to design. This opens the question of the optimal
connector design, for which both voltage drop and anode
fabrication aspects must be taken into consideration.
Cast Iron to Carbon Contact Resistance
The basis for the analysis of electrical contact between rough
surfaces has been laid out by Holm [9] and Greenwood &
Williamson [10,11]. In principle, rough surfaces in contact have
an effective area of contact smaller than the nominal area. A
constriction of the flux lines and a reduced effective area
contribute to a localised voltage drop rationalised as an interface

Based on the experimental work of Sørlie & Gran [12], Richard et
investigated the intrinsic behaviour of the cast iron to carbon
contact [1], from which a pressure and temperature-dependent
electrical contact resistance (phenomenological) law was derived
[8]. Richard et al [1] has also shown the stub-cast iron-carbon
assembly to be an indirectly coupled thermal-electrical-
mechanical system.
Anode Stub Holes Geometry
While stub holes machined in baked anodes allow for more design
freedom, the shape of moulded stub holes in green anodes is
somewhat limited by fabrication constraints.

Given that a green anode is relatively fragile, the flute geometry
must prevent dimensional warping or material collapse. If the
flutes are angled, this angle must allow for the stub hole inserts to
rotate properly during demoulding. The flutes must have a
'conical' shape, i.e. their top must be wider and longer than their
bottom. The cylindrically shaped part of the connector must also
be 'conical', i.e. its radius must be greater on the top. There must
remain enough carbon in between two consecutive flutes while a
minimum flute width must also be respected. Retractable
mechanisms are also used in some instances to create more
complex shapes.

Additional constraints due to casting further bound the possible
stub hole shapes. To prevent molten iron to solidify until it has
totally filled up the stub hole, the ratio of flute width to length has
to be high enough. The mass of cast iron must be small enough to
allow a good rodding speed to be maintained while a minimum
hole width and volume must be provided to prevent spill over.
Classical stub holes are shown in Figure 1.

Figure 1 ­ Classical Stub Holes, reproduced from Hou et al [6]