improved stub repair

ELEVENTH INTERNATIONAL CONFERENCE ON

NON-FERROUS MINERALS & METALS 2007

Barry C. Woodrow Stmir hf, Iceland

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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




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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




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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.




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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.




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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.




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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.




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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.




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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




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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




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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.




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fig. 9 A Stimir MIG Welder for Balco, India 1x4 140mm stubs




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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




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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.

improved stub repair

Documents

anode stub volt

total anode

control of anode volt

stubcarbon drop

rodded anode

anode bus

anode stubs

anode block