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Stub Repair at the typical smelter
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ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 1 of 13
Reducing Anode Stub Volt Drop by Improved Stub Repair Barry Woodrow Stimir hf, Hafnarfjrur, Iceland.
Introduction
The configuration of anode rods in pre-bake aluminium smelters varies according to the
reduction technology used, but in all cases the rod is carefully designed to achieve the
highest possible electrical efficiency.
The aluminium reduction process inevitably results in damage to the rod fabric. High
operating temperatures and aggressive chemical conditions both contribute to erosion of the
anode stub, significantly reducing electrical efficiency and potentially leading to mechanical
failure of the anode-to-stub joint.
fig. 1 A typical rod assembly
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 2 of 13
Other physical changes also occur, including stub toe-in and stub elongation, both
phenomena resulting in mechanical fitting-in difficulties and causing increased electrical
resistance.
Anode stubs must therefore be repaired or replaced to maintain the electrical and mechanical
integrity of the anode rod.
Volt Drop The Hidden Cost
It is not unusual for departmental costing systems to obscure the return on investment benefit
to Rodding Plant machinery. Efficiencies are not always obvious, with capital costs accruing
in the Rodding Plant whilst operating efficiencies are achieved in the Pot Rooms.
Control of anode volt drop is a major factor in achieving good reduction efficiencies. This is
often seen as totally a Pot Room function, yet significant contributions can be made in the
Rodding Plant.
fig. 2 Volt drop across a rodded anode
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 3 of 13
For a typical total reduction cell voltage of 4,60 volts it is generally accepted that 0,30 volt is
attributable to anode drop which is defined as the volt drop measured from the anode bus to
the bottom of the anode block. The following table details typical contributions to the total
anode drop.
fig. 3 Anode volt drop
From these figures is may be seen that 6,5% of the total energy used in the reduction process
is lost to volt drop. Further, the stub-carbon drop is no less than 40% (0,12 V) of the total
anode drop and is attributable to processes directly under the control of the Rodding Plant.
Hence, it can readily be seen that improvements in stub repair techniques will lead to anode
drop and increased electrical efficiencies.
In order to more clearly understand how these improvements may be achieved, it is
necessary to first examine the types of stub damage that may occur.
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 4 of 13
Stub Damage
Poor welds and weld failures in a repaired stub are quality problems which need to be
addressed at source. However, the geometry of the stub cut prior to welding the new stub is a
major factor is determining weld quality. A cut which is anything but flat and absolutely
horizontal will never provide the basis for an optimal weld.
Stub erosion is caused by molten bath, either as a result of overflow across the anode, or
through the underside of the butt. In both cases, steel is dissolved from the stub,
contaminating both the bath and molten aluminium, and eroding the stub profile. Eroded stubs
are mechanically weaker, increasing the risk of failure of the anode seal, and have a higher
electrical resistance, causing an increase in volt drop over the anode assembly.
Stub toe-in occurs in all pre-bake smelters, causing alignment difficulties between the stubs
and anode holes. Every time a rod goes through the reduction cycle, the much greater
expansion of the steel yoke compared to the carbon anode results in bending of the stubs, or
"toe-in". After repeated cycles the stubs will no longer fit into the holes in the carbon anode
block.
Stub elongation is a phenomenon of all pre-bake technologies. As stubs age through
repeated cycles in the reduction cells, progressive elongation of the stub occurs, and stub
diameter reduces. In the cell, stub steel becomes malleable and the weight of the anode, in
combination with forces resulting from bath and beam movement, cause the rods to stretch,
by as much as 1 mm in each reduction cycle. Dissimilar stub lengths across the yoke lead to
alignment difficulties when sealing the anode block with cast iron; where rod clamping is not
used during sealing, the rod will rest on the longest stub and will not be perpendicular to the
anode. Thicker pancakes of iron under shorter stubs change the electrical properties of the
joints, affecting current distribution across the anode and increasing volt drop. Thicker
pancakes also cause thimble-stripping problems. Some cell configurations can tolerate only a
very small increase in overall length of the assembly.
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 5 of 13
Stub Repair Available Technologies
A number of technologies have been offered for stub repair including stub straightening to
reduce toe-in, stub milling to reduce elongation, and stub cutting to prepare for stub
replacement.
fig. 4 Anode rods hanging in the overhead conveyor
In many smelters, stub straightening has a demonstrable cost benefit; straight stubs reduce
operational difficulties and improve the stub-carbon volt drop. However, particularly where
larger diameter stubs demand investment in pre-heating, stub straightening may be a
marginal benefit when compared to replacement of bent stubs.
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 6 of 13
Stub milling to remove elongation caused by repeated reduction cycles is a relatively new
technology and the cost-effectiveness is largely unproven. Since elongation inevitably means
reduced stub diameter, this technology can be considered as merely a temporary delay in
stub replacement, and the use of a stub saw to remove elongation may be a less expensive
alternative both from the investment and operational perspectives.
Stub cutting is one function that must always be applied in any pre-bake smelter, since stubs
must inevitably be replaced from time to time. Consolidating all stub repair functions into a
single cut / weld operation may often prove to be the most economical solution in many
smelters.
Stub Cutting Burn or Saw ?
Until recently, smelters had little choice in how stubs are cut. Traditionally, gas burning
prevailed. Gas torches use a considerable quantity of energy, are noisy, release greenhouse
gases, produce uneven cut surfaces, and result in a hazardous working environment for
employees. Implimentation of the ISO 14000 standard means that smelters are taking a
critical look at gas cutting. As a general rule, gas cutting is difficult to automate, meaning
added cost for supervisory labour. Successful gas cutting requires that the rod be unhooked
and laid horizontally, adding time and cost to the repair operation.
In some Rodding Plants, electric or hydraulic oscillating saws (mechanical hacksaws) are
used. These generally require that the rod is laid horizontally, with a cost penalty for the
unhooking and subsequent re-hooking operations. Repeated handling of the rod also means
increased personnel hazards.
Electrically or hydraulically operated saws circular saws are also in use. These blades are
relatively thick and remove significant mass, resulting in increased noise levels and higher
energy use, and blade maintenance is both difficult and expensive. The use of even thicker
segmented blades has simplified maintenance but with the trade-off of increased capital cost.
Some systems are designed such that rods remain suspended in the overhead conveyor,
improving the efficiency of the operation.
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 7 of 13
Stimir hf. of Iceland has an innovative automated Stub Cutting machine which uses a band
saw, a proven technology well established for many industrial applications. Compared to
other cutting methods, a band saw uses less energy, is virtually free of vibration and is much
quieter. The cut surface is clean and planar, and blade replacement is both easy and
inexpensive.
The band saw cut stubs horizontally whilst the rods still hang in the overhead conveyor. With
no requirement to unhook the rod from the overhead conveyor, complete automation of the
stub cutting operation is now readily achieved, and in combination with an automatic welding
station the complete stub repair operation may be automated.
Operation and Control Philosophy
Stimirs stub saw was originally developed in 2001 when it was installed for Alcan Iceland and
used to cut 125 mm diameter stubs on a three stub yoke.
fig. 5 The Stimir Stub Saw installed at Alcan Iceland 1x3 125mm stubs
The stub saw is intended to be installed off-line in the rod repair loop. Anode rods are
delivered to the machine by the overhead conveyor, and removed in the same way. All cutting
operations take place whilst the rod is hanging from the overhead conveyor.
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 8 of 13
There are a number of options for control of the saw. At Alcan Iceland, at the start of each
shift the rod repair supervisor visually inspects the rods to be repaired and fixes a bar code
label to each yoke. The machine reads each bar code to determine the actions required,
sequencing automatically through all queued rods. Alcan Icelands original requirement was
simply to cut off any damaged stub at a single fixed height.
The Alcan Iceland machine has subsequently been modified (2005) on site by Stimir to
enable the selection of a second cut 10-20mm above the bottom end of the stub, thus
removing elongation. The stub end is considerably harder than the main body of the stub,
having been carbonised during repeated cycles in the reduction cell. Trials using different
blades were undertaken and a higher performance band saw blade was selected for use after
the machine had been modified. This ability to fit a complete range of industry standard band
saw blades to the machine enables a very significant degree of optimisation, increasing
throughput whilst still maintaining an acceptable blade life.
fig. 6 A Stimir Stub Saw in operation at Alcan Steg in Switzerland 2x6 125mm stubs
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 9 of 13
In late 2003 a Stimir saw was installed at Alcan Aluminium Valais, Steg smelter in
Switzerland. This model was designed to cut 125 mm diameter stubs on a six stub yoke, and
was controlled by a PanelView touch-screen. At Steg, the machine was programmed to cut at
one of two preselected heights, enabling full or partial pin replacement. Several times during
each shift the supervisor inspected the queued rods and keyed in the relevant instructions at
the PanelView. The machine then processed all programmed instructions.
By programming three cut heights into the machine all stub repair functions may be
addressed by the Stimir saw. When operating in tandem with an automatic welding machine
installed downstream, complete stub replacement may be achieved automatically.
fig. 7 A Stimir Stub Saw ready for delivery to Sral in Norway 1x3 160mm stubs
Stimir has just shipped (June 2007) an automatic stub saw and MIG welding system to Balco
of Korba, India. The system, operating on Balcos 1x4 140mm stubs, is scheduled for
installation and commissioning in September 2007, and cuts stubs at one of three heights :
(i) about 10mm from stub end to remove elongation
(ii) about halfway up the stub to remove stub-end erosion
(iii) about 90% of the stub removed to remove badly eroded and/or toed-in stubs
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 10 of 13
fig. 8 A Stimir Stub Saw for Balco, India 1x4 140mm stubs
The actual positions of the cuts are supervisor programmable (password protected) and may
be changed to suit process conditions. Manual mode is also available for maintenance and
training purposes. Following the cutting operation, the rod is moved in the overhead conveyor
to an automatic MIG welding station, where a 50% or 90% length replacement stub is
automatically selected and welded in place.
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 11 of 13
fig. 9 A Stimir MIG Welder for Balco, India 1x4 140mm stubs
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 12 of 13
Performance
The machine cycle time for each stub is dependent upon the stub diameter. Experience
shows a cycle time of typically < 5 minutes for 180mm diameter stubs. The cycle time is
adjustable within certain limits; as a general rule, faster cycle times result in increased blade
wear. However, the use of high performance blades does permit a faster cycle time whilst
maintaining acceptable blade life.
Welding time varies according to a number of factors including head movement speed, wire
feed speed, and the number of passes. During the Factory Acceptance Test for the BALCO
system, each pass took about 40 seconds; total machine time per stub including a four-pass
weld is less than 3 minutes.
Benefits Stub Saw
Alcan Iceland carried out a detailed study of the benefits of installing the Stimir saw. The
highlights of the study were :
Very significant electrical energy saving due to reduced volt drop Very significant reduction in carbon dioxide emission with elimination of gas cutting Improved quality of stub welding due to smooth and horizontal stub surface Uniform stub lengths contributing to enhanced connectivity and improved current
distribution
Improved working environment and significant reduction in manpower requirements
Benefits Welding System
Thus far there have been no studies undertaken on the Stimir welding system. However, it is
clear that at the very least the benefits include :
Accurate and reproducible welding process Optimal use of consumables argon / CO2 mix, welding wire Improved working environment and significant reduction in manpower requirements
ELEVENTH INTERNATIONAL CONFERENCE ON
NON-FERROUS MINERALS & METALS 2007
Barry C. Woodrow Stmir hf, Iceland
page 13 of 13
Summary
Anode volt drop may be reduced by improved stub repair techniques.
High quality stub cutting is fundamental to successful stub repair. Increased energy efficiency
provides for a rapid pay-back of the investment, and reduced operating costs in the Rodding
Plant, whilst contributing to an improved environment.
06 July 2007
Biography
Barry C. Woodrow, B.Sc. is Commercial Manager for Stimir hf. of Hafnarfjrur, Iceland. A graduate
chemist, he has more than 35 years experience in non-ferrous metals and chemicals industries in
Zambia, Kenya, Nigeria, Italy and Iceland.