carbon dust-paper by sadler&welch

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1 REDUCING CARBON DUST? – NEEDS AND POSSIBLE DIRECTIONS BARRY SADLER AND BARRY WELCH Net Carbon Consulting Pty Ltd ([email protected]) and Welbank Consulting Ltd ([email protected]) INTRODUCTION In the last quarter century there has been a significant change in emphasis on the production and monitoring of anode quality. This has followed considerable efforts from organisations such as R&D Carbon Ltd. Contrasting with this, many people say the industry has changed very little. In support of that argument I quote Robert J Mills, a young engineer who joined Reynolds Metal Company in February 1991 for the startup of the Lister Hill Reduction Plant. In his first day’s diary he wrote: “The first impression I have is the importance that officials lay on the electrode. It seems in their estimation the electrode takes importance even ahead of charge (alumina addition) cryolyte, and even the pot itself.” He also wrote: “For a few weeks I was allowed to study blueprints and books. A few operation pamphlets were given to us but they were incomplete and not very plain (pamphlets being written by a foreign Norwegian)”. Perhaps these comments also support the viewpoint that very little has changed. Despite the many quality control checks now carried out on anodes they are often subject to strong vilification and blame for operating problems. Another example from Mills supporting the premise that little has changed is his quote on operations: “After each “light” electrodes should be examined by a rake to ascertain that the electrode is clean – that no coal dust or pieces of crust remain under the electrode, partly preventing the flow of current.” Even then carbon dust was a problem but there was a methodology for control. There was also a second methodology. I understand that under ordinary operations we plan to try to have “lights” five to six hours apart.” and “Overcharging the furnace is not good for it as it is usually spoken as of “putting it to sleep”. That is “the lights” won’t come on.” It seems, therefore that removing carbon dust was important. It also explains the reluctance many smelters have had of reducing the anode effect frequency towards the zero target. In the intervening years, anode technology has changed a lot with the major emphasis being on prebake anodes, and more recently, having anodes of a large mass and high geometric density in order to enable maximum line current with minimum interference by anode change. Producing Anodes Today Today almost all smelters are running at line currents higher than they were originally designed – typically 20% to 25% higher. Matching this, the anodes have usually been made longer and taller with an increase in mass of a similar proportion. This usually has occurred

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Page 1: Carbon Dust-Paper by Sadler&Welch

1

REDUCING CARBON DUST? – NEEDS AND POSSIBLE DIRECTIONS

BARRY SADLER AND BARRY WELCH

Net Carbon Consulting Pty Ltd ([email protected])

and Welbank Consulting Ltd

([email protected])

INTRODUCTION In the last quarter century there has been a significant change in emphasis on the production and monitoring of anode quality. This has followed considerable efforts from organisations such as R&D Carbon Ltd. Contrasting with this, many people say the industry has changed very little. In support of that argument I quote Robert J Mills, a young engineer who joined Reynolds Metal Company in February 1991 for the startup of the Lister Hill Reduction Plant. In his first day’s diary he wrote: “The first impression I have is the importance that officials lay on the electrode. It seems in their estimation the electrode takes importance even ahead of charge (alumina addition) cryolyte, and even the pot itself.” He also wrote: “For a few weeks I was allowed to study blueprints and books. A few operation pamphlets were given to us but they were incomplete and not very plain (pamphlets being written by a foreign Norwegian)”. Perhaps these comments also support the viewpoint that very little has changed. Despite the many quality control checks now carried out on anodes they are often subject to strong vilification and blame for operating problems. Another example from Mills supporting the premise that little has changed is his quote on operations: “After each “light” electrodes should be examined by a rake to ascertain that the electrode is clean – that no coal dust or pieces of crust remain under the electrode, partly preventing the flow of current.” Even then carbon dust was a problem but there was a methodology for control. There was also a second methodology. “I understand that under ordinary operations we plan to try to have “lights” five to six hours apart.” and “Overcharging the furnace is not good for it as it is usually spoken as of “putting it to sleep”. That is “the lights” won’t come on.” It seems, therefore that removing carbon dust was important. It also explains the reluctance many smelters have had of reducing the anode effect frequency towards the zero target. In the intervening years, anode technology has changed a lot with the major emphasis being on prebake anodes, and more recently, having anodes of a large mass and high geometric density in order to enable maximum line current with minimum interference by anode change. Producing Anodes Today Today almost all smelters are running at line currents higher than they were originally designed – typically 20% to 25% higher. Matching this, the anodes have usually been made longer and taller with an increase in mass of a similar proportion. This usually has occurred

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after the paste plant and anode baking facilities have been constructed and it has placed considerable stress on the carbon plant facilities. The result has been a reduction in the fire cycle length and hence the overall anode heat treatment (the combination of temperature and time) while endeavouring to ensure the electrode adheres to a quality standard defined using anode carbon tests developed more than 25 years ago. Typical trend graphs for monitoring anode quality from two different suppliers are shown in Figure 1. It should be noted that these anodes are made from different raw material sources, and therefore the data is not necessarily directly comparable, although some anode production facilities can produce electrodes from different raw materials to a constant quality. The industry looks closely at several carbon dioxide and air burn anode reactivity trends including total reactivity, residue, and dust level.

Figure 1: Some trends in properties of anodes from different production facilities. Impact of Anode Changes on Cell Performance and Conditions The changes in cell operating conditions accompanying increases in anode size and higher line current has usually stressed the cell’s heat balance limit. From the earlier period when the cells needed to control heat dissipation, it has changed to one of maximising heat dissipation. Simultaneously, with better control, the anode cathode distance (ACD) has been reduced and cells operate towards their practical minimum ACD. The net effect of these changes includes:

• The anode to cathode distance for operations has been reduced typically by 4 to 6mm.

• The cells are required to dissipate 15% to 25% more heat from the same geometric dimensions.

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• The electrolyte volume per kA of operation has been reduced by approximately 30%.

• The anode effect frequency has been lowered reducing the times the cells are in a turbulent state that assists removal of carbon dust from under the electrode.

• The cells retain a better overall anode cover because of the less frequent crust breaking, reduced breaking areas (by point feeders) and the emphasis on prevention of airburn in all parts of the anode to achieve low net carbons.

Several of these changes result in a greater tendency for the cell to accumulate carbon dust. The reduction in anode to cathode distance has increased the tendency for carbon dust to hinder the slow heat pickup of modern anodes (which have less energy available than previously, a greater thermal demand, and reduced heat transfer conditions). This results in a greater tendency for anodes to form spikes and other short circuiting descriptors such as bellies. As will be discussed below, operational problems and changes that result from the presence of a high amount of carbon dust in the cells include:

• Slower heat pickup of a newly set anode resulting in a more prolonged period of imbalance in the cell’s current distribution.

• Squeezing of the cell through the increased resistance from the unknown amount of carbon dust (which effectively acts as an insulator in the electrolyte).

• A greater tendency to have alumina solubility problems since the coarse floating carbon dust hinders the free dispersion and mixing of alumina that is discharged on to it. Hence alumina solubility problems and feeding irregularities.

• A greater tendency to form shorting paths such as anode spikes and protrusions (primarily driven by the slow heat pickup) and this leads to the:

o Temperature cycling with periods of high temperatures and low fluoride concentrations.

o Significant decrease in current efficiency through the current bypassing the electrolysis process.

Carbon Dust Sources The preceding comments may have implied that all the carbon dust arises from the anodes. While anodes are a major contributor, they are not the sole source, for example:

• The cathode wear in modern cells typically contributes between 0.5 and 1 kilogram of carbon per tonne of metal produced.

• Those companies that use carbon collars to protect the anode stubs can have an input from residual powdered carbonaceous stub protection material being mixed in the recycle anode cover material. This can add between 0.5 and 6 kilograms of carbon per tonne Al when they are used.

• The recycle anode cover material often has a carbon content between 2 and 5 % and typically between 150 to 300 kilograms of cover material per ton of metal is spilt. This is another significant source of carbon dust in the electrolyte.

• The carbon in the secondary alumina in smelters is not monitored universally although it is an extremely important quality control indicator as well as its dust contribution. As seen in the following two graphs it can vary between 0.15% and 0.5%. All this carbon dust (which is generally very fine particle size) goes into the electrolyte since the alumina is consumed there.

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• For those smelters who use slots in their anodes. These become another potential source of carbon dust as the slots entrap a residual volume of reactive carbon dioxide.

Figure 2: Trends in carbon dust levels in secondary alumina at 2 different smelters More than a quarter of a century ago, prior to point feeders, the airburn and carboxy reactivity of the anodes was higher, but carbon dust was generally less of a problem. This is because it was removed from the cell automatically by airburn and the dust being carried out with the strong cell gas flame.

Figure 3: Another source of dust in Modern cells –slots in some qualities of anodes

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Today there are four primary mechanisms for removal of the carbon dust, these being: 1. A very limited amount with tapped bath. 2. Self cleaning by being transpired with the gas flame and burning. If it doesn’t burn it

can be carried on to the alumina fluoride reactors and is then recycled. The importance of this mechanism is illustrated in Figure 4. Here it is seen where a fine carbon dust is deposited on the cover material when the strong flame is not maintained and it is not burnt.

3. The third method and one more commonly used is regular skimming of the bath. Because it is both labour intensive and an unpleasant task this is usually done in one location such as in the metal tap hole.

4. By cleaning the bath surface during anode change usually with the mechanical grab.

Figure 4: Carbon dust precipitated by incomplete combustion of the emergent cell gases. (This recycles with the cover material and enters the electrolyte through spillage) The extent of carbon dust accumulation and the magnitude of problems is essentially a balance between the various generation reaction rates and the removal processes. Unfortunately, quantifying the amount of carbon removed by the different mechanisms is difficult, because of the variations in the mechanisms (as exemplified by the combustion above), sampling difficulties, and the absence of good analytical methods. Various reports on the percent carbon suspended in the bath range from 1 to 5%. This variation may be real or due to either sampling or measurement errors! Impact of dust on performance From the physical properties of the carbon it is evident that there will be some segregation occurring within the electrolyte with the coarser material floating on the surface. The finer material will be more generally dispersed within the electrolyte. The surface material often extends down about 2cm once it accumulates in the open cavity when an anode is being changed. As it flows within the cavity it is obvious that it adds to the viscosity of the electrolyte and its flow is sluggish.

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Impact on Alumina Feeding and Solubility Carbon dust that accumulates under a point feeder hole can have an adverse affect on the solubility. Rather than allowing the alumina powder to disperse it is observed to form a clump and has difficulty in penetrating to or mixing freely with the electrolyte – conditions necessary for good solubility. With poor alumina solubility it is common for sludge accumulation which leads to irregular feeding and control or an increase in anode effect frequency, or both. The increase in anode effect frequency usually results in a higher temperature and, unfortunately, the wrong actions being taken to solve the consequential problem rather than the primary problem. Changes in Bath Resistance Although carbon is an electronic conductor, for current to flow through suspended particles there is a need for an electrochemical reaction to occur on either side of the particle. The possible electrochemical reactions that can occur in a smelting cell require an added voltage gradient. This increases the equivalent resistance and therefore bypassing the particle, rather than current flowing through it, becomes the preferred path. That is to say, the electronic conductor acts as a resistor. This situation is equivalent to the bubble resistance in cells and the best models for bubble resistance show that the increase in resistance is proportional to the volume fraction of the material. Since carbon dust has a similar or lower density than bath, the increase in resistance of the bath will be proportional to the weight percent. A 5% increase in fine dust in the bath would result in a reduction of the anode to cathode distance of approximately 2mm. However the coarser particles that have a denser volume fraction in the layer adjacent to the anode surface will increase the resistance much more. They present the same problem for current flow when not in direct contact with either the electrode or each other. Based on limited measurements it has been estimated that a heavily dusted cell typically operates at an anode to cathode distance that is 4mm to 6mm less than normal. Operating at lower anode to cathode distances usually results in a lower current efficiency and consequently extra heat generation when a cell operates at a constant voltage (or control resistance which is usually imposed). Consequently the cell heats up. Many control systems tend to reduce the cell voltage when the cell is in the heating trend. This will reduce the ACD further. Consequently the resistance imposed by the carbon dust in the electrolyte results in a warming of the cells and a reduction in current efficiency as well as a reduction in anode to cathode distance. Shorting Between Anodes and Cathodes During anode change the coarse material aggregates on the bath surface in the open cavity. Because of its black body radiation the contacting particles tend to partially freeze cryolite and make it a solid material. Setting the anodes on top of this it completes the freezing process and adheres to the anode under surface – typically extending in some zones by 2cm to 3cm. This effectively reduces the anode to cathode distance. However it also reduces the speed of re-melting of the freeze formed on the anode because of:

• The higher melting point of the bath around the carbon particles. • The reduced cross sectional area that comes in contact with the dissolving electrolyte. • The low superheat of the electrolyte in that zone that further retards the dissolving

process.

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Accordingly there is a high risk of the adhering conducting material not having re-melted before it can come into contact with the cathode (see under-surface of spike in Figure 3). Hence some shorting occurs. This is reflected by more rapid increase in anode currents than is normal for that stall location. More importantly, however the increased current is all direct short circuiting lowering the current efficiency. If, as is often the case, it persists long enough for the anode material to also come into contact with the metal pad, spike formation initiates and the short circuiting becomes more severe. Typically in a shorting electrode more than 60% of the current flowing through that electrode bypasses the electrochemical processes resulting in a loss in current efficiency of between 1% and 3% depending on the number of anodes in the cell and their duration of shorting. In cells with 20 anodes or less this can have a severe impact and if the shorting frequency reaches 10% of the anodes and the anode shorting duration averages 6 days, there is a permanent loss in current efficiency of about 1.5%. The problem of reactive slots The early introduction of slots was typically accompanied by comments that it doesn’t matter how deep or how many you put in, it’s all beneficial. However, subsequently, some smelters have had bad experiences. In particular, these bad experiences have been associated with incorrect slot orientation (transverse versus longitudinal depending on what is optimum for the cells) and an increase in the reactivity of the carbon surrounding the slots. When formed in the green anode, it is extremely difficult to achieve uniform density and compaction of all the material added to the vibroformer or press and usually there is an area of poor compaction/low density around the slot. This gets worse as the slots are made taller. Insufficient support of slots during baking can exacerbate this problem by allowing further disruption to the anode structure by slumping. An example of poor anode structural integrity near slots is shown in figure 5. For newly installed anodes the slot cavity above the electrolyte is filled with high CO2 concentration cell gases that are continuously replenished. Accordingly if the anode has a high carboxy reactivity tendency, the anode slots will rapidly widen as illustrated in Figure 3 and may even form a keyhole shape. Furthermore if there is strong differential reactivity and a high dusting tendency from carboxy attack, the slots will introduce a considerable amount of dust. These anode structural integrity problems and dusting issues associated with slots are covered in more detail in the following sections.

Figure 5: Damage to baked anode structure adjacent to a formed slot. Core was drilled through slot

Slot surface

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Options for minimisation and avoidance of dust problems While it is easy to blame the anode quality, it is obvious from the above that there are also operational issues that impact on the tendency for the dust to either form initially or accumulate in the cells. Like many other aspects of smelting operations, the issue is seldom one-sided. Producing anodes with low dusting tendency Traditional approaches to reducing the dusting tendency of anodes have focussed on:

• Baking the anodes at an appropriate heat treatment (i.e. peak temperature and soaking time).

• Keeping the level of sodium, a catalyst for the carbon + CO2/air reactions, in the anode as low as possible.

• Keeping the air permeability of the anode low to reduce the access of CO2 and air to the internal structure of the anode.

• Achieving a high degree of structural integrity in the anode carbon, both at the micro (figure 6a) and macro scale (figure 6b).

Figures 6a (left) and 6b (right): Poor anode structural integrity, showing compaction faults in an anode core (left) and internal cracking in an anode cut in half (right). These traditional steps to reduce anode dusting tendency remain valid and there have been large quantities of literature published that cover these in considerable detail. Only a summary of the most relevant points on each topic will be covered here. More attention will be given to other dusting issues and opportunities that are more current. For example, some of the “new” sources of dust stem from:

• The widespread adoption of formed anode slots (as noted earlier), and • Changes to coke quality with the greater use of relatively high sulphur cokes in

merchant calcined filler coke blends. There are, however at least two innovations that offer promise to help reducing dust:

• The use of “undercalcined” cokes, and • Selective addition of low reactivity cokes into the anode binder fines matrix.

These anode dusting issues and opportunities will now be expanded on.

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Bake anodes properly Baking anodes to minimise the differential reactivity between the binder/fines matrix and the calcined filler coke has been a long held objective of baking furnace operators. This has typically meant aiming for an anode bake-out temperature of around 1080 - 1150oC at a typical (flue gas) soaking time of about 30 hours. Anode heat treatments much below this target temperature/time combination represent underbaking, and higher heat treatments (overbaking) increase anode thermal conductivity (increasing the risk of airburn) and/or increase the amount of desulphurisation during baking (increasing anode reactivity [1]). Improvements to anode cover reduce the risks to anode performance associated with higher thermal conductivity, but the problem of desulphurisation with overbaking is getting worse (see later). Some companies have expressed baking targets in terms of the difference in structural ordering (as measured by Real Density) between baked anode and the original filler coke (figure 7). Note that the higher baking specification for Company A is aimed at minimising anode dusting tendency in cells which are very sensitive to dust and thermal conductivity is not an issue as the anodes are afforded very good protection from airburn. The lower Company B baking specification is more typical of plants that need to balance anode dusting tendency and thermal conductivity issues.

2030

2040

2050

2060

2070

2080

2090

800 900 1000 1100 1200 1300 1400Baking Temperature (Equivalent Degrees)

Rea

l Den

sity

(kg/

m3 )

Company B

Company A

Coke RD

Figure 7: Target baking temperature ranges based on anode structural ordering (Source: Rio Tinto Aluminium) One of the more recent changes that needs to be watched with the high baking temperature approach is the increasing quantity of relatively high sulphur (e.g. > 4%) cokes now being used in blends. As sulphur in blend coke components increases, the amount of desulphurisation during baking also increases [1]. This is the result of the high sulphur coke blend components behaving according to their actual sulphur level, and not according to the average sulphur level of the blend. Desulphurisation during baking can increase anode reactivity and dusting tendency as shown in figure 8. The dust on high heat treatment/overbaked anodes is a very fine powder around the top edges of the anode butt and appears to be the detritus from oxidation rather than dislodged particles from differential attack.

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The single most important action a carbon plant can take to reduce anode dusting tendency is to bake anodes well. Avoiding underbaking is essential, and although dusting from overbaking is much less severe, excessive heat treatment should also be minimised; careful control of baking heat treatment in a target range is required.

A B C

0

1

2

3

4

5

6

LAYER

CD

R

%

Figure 8: Anode baking data from a closed baking furnace, showing the influence of baking layer on equivalent baking temperature (left), with the top layer anodes “overbaked” which results in desulphurisation, and carboxy reactivity dust (right) showing the high baked top layer anodes tend to have a slightly higher dusting tendency than anodes from layers with lower baking conditions. These results were confirmed in plant observations with butts from the top layer anodes showing significant amounts of fine powder dust on the butt top surfaces. The optimum anode baking temperature/soak time varies with the raw materials used, carbon plant processing conditions, and cell design/operation. Given this, it is preferable for each plant to determine the baking heat treatment that is optimal for their circumstances. This can be done by monitoring the heat treatment (Best done using the green coke Lc method [2]) of a significant number of anodes (e.g. a full furnace section). These anodes are then cored and tested for heat treatment sensitive properties and ideally will then be tracked through Potrooms to establish anode consumption. Charts such as figure 9 can then be constructed to show the relationship between heat treatment and anode properties/ performance. Depending on the anode parameters that a plant is most sensitive to, the target baking heat treatment range can then be defined from these diagrams.

0

1

2

3

4

5

6

800 900 1000 1100 1200 1300 1400

Baking Temperature (Equivalent Degrees)

CO

2 Rea

ctiv

ity D

ust (

%)

Figure 9: Influence of heat treatment during baking on anode dusting tendency (left) and anode performance (right). Note that these data are from different plants, and compared to figure 8, these plants use a coke that is not prone to desulphurisation (Source: Rio Tinto Aluminium)

A B C

1100

1200

1300

Layer

BT _

degE

Top Middle Bottom

Top Middle Bottom

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Keep anode Sodium levels as low as possible The relationship between sodium and anode reactivity/dusting is well accepted and has resulted in the drive to push sodium in anodes as low as possible. Removing butts from cells immediately after disconnection from the buswork (Reducing the opportunity for butts to absorb bath), adopting a “no white” standard for butt appearance after shotblasting, and no recycling of anode carbon that has become detached from the rod assembly in the cell, have meant that some plants are consistently achieving baked anode sodium concentrations lower than 200ppm. There is no “acceptable” level for sodium and efforts to consistently maintain anode sodium concentrations as low as possible and avoid any “spikes” due to, e.g. butt cleaning problems, will be rewarded with better anode performance. Keep anode air permeability low Dusting is almost exclusively the result of sub-surface attack of the anode structure - reducing the access of oxidant gases to the internal anode structure will reduce dusting; lower anode permeability levels will achieve this. Opportunities to improve anode permeability include:

• Mix hot and cool the paste before forming, and/or • Form/press anodes under vacuum. • Optimising aggregate particle size distribution for packing effectiveness. • Operating at the optimum pitch level for the raw materials and operating conditions

used. While optimisation techniques based on feedback of green anode properties (e.g. Green Dry Density pitch scans [3] are effective with conventional mixing/forming technologies, they are less effective when more intensive paste processing equipment is used. Pitch optimisation then becomes a combination of experience (e.g. “eyeballing” green anodes), and longer term feedback from baking (i.e. amount of block sticking, packing material adhesion, and anode deformation), baked anode core test results, and ideally, anode performance (via an anode tracking system).

It should be noted that anode air permeability can get too low for some plants. Highly compacted green anodes that lead to very low permeability baked anodes are difficult to bake due to the restrictions on pitch volatile release during baking. This can cause severe internal cracking unless the heat up rate during baking is slow and well controlled. Achieve a high level of anode structural integrity Good structural integrity means that the anode structure is free from flaws such as “compaction faults” (figure 6a), cracks (figure 6b) and other flaws due to problems such as paste segregation. These defects increase dusting tendency due to weak bonding of coke and butt particles and/or increased access of oxidant gases to the anode structure. Achieving a high degree of micro and macro structural integrity in the anode carbon means doing all the basics right when making an anode. The introduction of formed slots in anodes has, however lead to additional anode structural integrity issues. Avoid poor anode structural integrity near slots This has been touched on several times already but it is important to recognise that formed slots are “not for free”. They affect anode quality significantly, and depending on factors including the orientation of anodes during baking, the impact of the lower anode quality on anode performance can outweigh the improvements in cell dynamics from the slots. This is driving more smelters to cut rather than form slots, and in several instances has lead to slots

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being removed from anodes altogether. It should also be recognised that deeper slots are not for free either as they tend to make slot structural integrity issues worse. These slot related structural integrity issues result from:

• Additional edges (6/slot) that are often loose and friable. • Additional vertical surfaces – slot formers interrupt the forces that push paste against

slot formers, reducing anode quality (i.e. lower density and increased internal flaws) adjacent to the slots, with the area near the top of the slot most affected. Increasing slot height increases these problems.

• Damage to the slot surface due to poor slot former condition: e.g. rough surfaces, deformed edges, bent/deformed/misaligned former blades, etc. Hot green anodes can be severely damaged as they are pushed off the former if slot blades are not in good condition.

• Cracking/collapse of the slot during handling. As slots become higher, damage increases from moving hot anodes over changes in height/direction in conveyors, and from the high loads applied by grabs on overhead cranes and stackers. In many plants the constraint on how high slots can go will be the ability of the anodes to withstand the loads imposed to pick up anode packets for stacking or loading in baking furnaces. Horizontal end – to – end anode loading/stacking is an advantage with longitudinal slots as the load from grabs is not applied across the slots.

• Collapse of unsupported slots during baking is common (although not always seen) with all anode baking orientations but it is much worse when the slots do not face a flue wall. Slot down or slot to headwall orientations do not allow packing coke into the slot and result in slower bake out of the slots. This gives the carbon around the slot more time in the “soft” temperature range (i.e. > pitch Softening Point, < onset of carbonisation) when the green anode can deform under its own weight. The result of this can be seen in figure 5, with paste slumping into the slot, and the carbon adjacent to the slot deforming as the slot partially collapses.

Options to reduce dust generation from slots include: • Don’t form slots – increasingly they are being cut in baked anodes with suitable

diamond saws as this avoids most of the slot dusting problems. Anode structural integrity only becomes an issue with cut slots if the baked anodes contain significant horizontal cracks (from poor forming or baking) that the slot cuts intersect. This can cause the “wings” from the bottom of the anode to drop off during anode cutting, handling, or setting in the cells.

• Form anodes at higher temperatures and with vacuum to give higher paste formability around the slot formers.

• Maintain the slot former blades in excellent condition. • Reorient anodes in the baking furnace pits so that the slots face the flue walls -

without this orientation, it is difficult to avoid significant slot damage during baking. • If anode reorientation in the baking furnace pits is not feasible, pack wooden slats,

thick cardboard, etc into the slots to provide support during baking. The carbonised packing material must be completely removed from the slots before setting the anodes in the cells.

• Effectively clean all slots to remove packing coke and deformed carbon. Even after taking all these steps with formed slots, there is still a reasonable probability of having periods where slots will contribute to dusting. Slot cutting should be seriously

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considered by any plant introducing slots, and be evaluated as an alternative by plants that currently form slots.

Use undercalcined filler cokes In 2001, Samanos and Dreyer outlined [4] laboratory studies that suggested there may be potential benefits from using filler cokes calcined at lower than normal temperatures (i.e. “undercalcined” with a Real Density < 2.05g.cm-3). This work has since been implemented at a plant scale at several smelters. In a long awaited follow-up publication, a recent presentation [5] showed further impressive results with lower CO2 and Air reactivity, lower dusting tendency, greater “robustness” to variation in anode baking temperature, and lower plant anode consumption (despite a small reduction in anode density) by using undercalcined filler cokes. The improved plant performance was achieved with 50% - 100% undercalcined coke blended with normally calcined cokes. These results indicate that using undercalcined cokes has the potential to reduce differential reactivity in anodes, thereby reducing reactivity/dusting tendency and improving anode performance. The clear message from the presentation is that the use of undercalcined cokes will increase. It is likely that a limitation to this will be the ability of calciners to produce cokes to multiple Real Density specifications unless there is broader adoption of this approach by other smelters. Using undercalcined filler cokes is an approach that should be seriously evaluated by smelters and their coke suppliers.

Selective addition of low reactivity cokes to the aggregate An approach to reducing the preferential attack of the binder-fines matrix in the anode structure is to selectively add low reactivity coke to the fines fraction in the aggregate by adding it to the ball mill feed. While not new, (e.g. [6]) there is renewed interest in this approach (e.g. [7]) driven at least in part by a desire to maximise anode performance as coke quality declines. It is likely that the ability to selectively blend cokes to improve anode properties such as dusting will be increasingly retrofitted into existing plants and incorporated into new plant designs. Improved Work Practices in the Cell Rolofs and Wai-Poi [8] showed there were a number of changes that could be applied at the cell to reduce the problems inherent with carbon dust. Such actions include:

• Eliminating the use of stub collar protection. • Better energy management during anode change to increase the anode cathode

distance and increase the superheat so that less carbon is frozen under the electrolyte. • More systematic skimming of the carbon dust from the cell. • Elimination of slots in the anodes.

Other preventative measures that can be taken besides those implemented by Rolofs and Waipoi include:

• Better attention to cell cover. • Keeping the point feeder holes open so that a strong gas flame is maintained and more

of the carbon dust is burnt rather than recycled.

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References:

1. L. Edwards, K. Neyrey, L. Lossius; “A review of coke and anode desulfurisation”, Light Metals 2007, TMS, P. 895.

1. ISO standard 17499, “Carbonaceous materials used in the production of aluminium —Determination of the baking level expressed by the equivalent temperature, 2004

2. C. Vanvoren, “Adjusting paste plant parameters for optimum quality”, Second Australasian aluminium smelting technology workshop, Sydney, 1987, P. 3.

3. B. Samanos, C. Dreyer; “Impact of coke calcination level and anode baking temperature on anode properties”, Light Metals 2001, P. 681

4. J. Lhuissier; “About the use of under-calcined coke for the production of low reactivity anodes”, Presented at the 5th International pitch & calcined petroleum coke conference, Valencia, Spain, Sept., 2007

5. R.T. Tonti, R.D. Zabreznik, K. Ries; “Anode performance improvement with low reactivity coke additives to the binder matrix”, Light Metals 1992, TMS, P 635.

6. F. Figueiredo,et. al.; “Alumar Coke Blending Facility”, Light Metals 2007, TMS, P. 891.

7. B. Rolofs, N. Wai-Poi; “The effect of anode spike formation on operational performance”, Light Metals 2000, TMS, P. 189.