Prebaked Anodes for Aluminium Electrolysis

Introduction

The worldwide aluminium production has reached a level of 50 Mio tons for which 30 Mio tons of carbon anodes are used, representing a commercial value of 10 billion $. Undoubtedly, this gives the anode product the highest market value of all manufactured carbons. In this paper the manufacturing process, the corresponding plant equipment and the quality control system applied by modern large smelters using state of the art pot technology (400 - 500 kA) are explained. A description of the entire chain of the anode manufacturing steps for a mega-smelter unit producing 800’000 tpy Al as shown in Figure 1, from raw material storage to the processing of the carbon, including the recycled material after anode usage in electrolysis, will be given. In order to understand the relevance of the production equipment features and of the anode quality figures, the role of prebaked anodes in the electrolysis process and their performance are first briefly described.

Figure 1: Aluminium Smelter

The electrolysis cell

The production of Aluminium1 is achieved through direct current electrolysis of alumina dissolved in a cryolithe / AlF3 bath at a temperature of 950 °C. The prebaked carbon anode conducts the current into the melt and participates in the reduction process. CO2 and CO gas evolve as the process off-gas. Figure 2 shows that the carbon anode block is progressively consumed and that the produced aluminium droplets are gathered at the bottom side on the cathode. The anodes are protected from air oxidation to a given extent by a cover made from recycled bath mixed with alumina. An assembly of rod / yoke / stubs is rodded through iron casting to the anode stubholes and is clamped to the transversal beam. The current density is relatively low (less than 10 kA/m2) but the electrical energy consumption reaches a high level of 13 MWh/t of Al. The disturbing effect of the magnetic fields on the stability and flatness of the metal pad, and the release of the process heat for maintaining an appropriate thermal balance, limit the energy density of the electrolytic process.

Figure 2: Aluminium Electrolysis Cell

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Figure 3: Aluminium Potroom

Therefore the pot active anode surface in a 400 kA pot producing 3 tons of Al per day extends more than 40 m2. Each new anode exhibits a surface area of about 1 m2 and is consumed at a rate of about 10 cm per week so that after a one month cycle the remaining butt should be replaced with a new rodded anode. As shown in Figure 3 the pots are hooded to capture the off-gas containing some fluorine and these are arranged side by side. The alumina is conveyed pneumatically and dosed to the bath by point feeders maintaining the optimum concentration in the bath close to 2-3 %. A multipurpose crane is used for metal tapping and beam rising, as well as for all operations related to the changing of the anodes (Figure 4).

The role of anodes in the pots

Current conductor aspects

In the total cell voltage of ~ 4 V, about 1/4 is related to anode ohmic and polarization surface overvoltage2. As shown in Table 1, the direct difference in between a bench mark and poor anode voltage related performance can reach 300 mV. Through the presence of carbon dust in the bath, due to excess and

Figure 4: Anode Change

selective oxidation, an indirect negative impact of 150 mV is observed3. For a given cell amperage, the appropriate selection of the anode size, but also of the number and geometry of stubholes are important to minimize energy consumption, which is a critical element of the Aluminium production cost. Achieving low anode resistivity, good oxidation resistance and avoiding dust contamination of the bath are of paramount importance for low and stable pot voltage. In order to decrease bubble voltage drop of 250 mV by one third, vertical slots have successfully been introduced in the last decade4. Usually two slots (10 mm wide and 300 mm height) collect the electrolysis gas, thus minimizing the surface area covered by the bubbles hindering the current path. These also aid the dissolution of alumina in the bath so that there are fewer risks of alumina sludge deposits on the cathode block.

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Table 1: Voltage drop and anode quality

Thermal aspects

About half (800 kW for a 400 kA pot) of the total electrical power input is dissipated by thermal conduction / convection or radiation. As shown in Table 2, the anodic part plays a decisive role for maintaining an appropriate heat balance that is mandatory for a high current efficiency in the cell. About 40 % of this heat is dissipated by the anode stubs /yoke /rod and the anode cover. The thermal conductivity of the anode and its cover layer are therefore an important element in the cell stability that is also related to an appropriate ledge profile for avoiding horizontal current through the metal pad. In a configuration where insufficient heat is lost through suboptimum stub size and inferior anode thermal conductivity, a reduction of the anode cover would be the wrong response to the thermal balance issue as severe air burn on the unprotected anode top occurs. The anodic part of the cell is therefore one limiting parameter of the productivity of the cell, one of the limits that has been somewhat rescinded, thanks to advanced carbon manufacturing technologies, despite the deterioration of the quality of raw materials.

Anode failure and consumption mechanisms

The first issue is the thermal shock cracking related to poor resistance to stresses induced by the temperature gradient when the cold new anode is set into the hot bath5. When the anode is set in the pots, a layer of frozen bath is formed which takes about one day to melt as the bath temperature only slightly exceeds the liquidus temperature. The heat transfer dynamic from the bath to the carbon anode is driven by the temperature difference between the bath and its liquidus temperature (the so-called superheat). Usually a crack-free carbon anode can easily withstand the thermal stresses, but obviously the presence of lamination flaws in the bulk of the anodes ends up with horizontal slabbing of the block. In case of unprecedented high levels of inelasticity and of coefficient of thermal expansion, corner cracking may be experienced despite normal bath conditions. The loss of a corner piece, as illustrated in the Figure 5, creates major disturbances of the cell, as a short circuit between anode and metal can occur.

Voltage Drop Good Anode Poor anode VariationComponents (mV) (mV) (mV)

Clamp busbar to rod 15 25 10Rod 25 30 5Welding rod to yoke 10 20 10Yoke with stubs 45 55 10Stub to anode 105 150 45Anode body 150 180 30Surface overvoltage 530 600 70Bubble layer 170 250 80Total Anode 1'050 1'350 300Equilibrium voltageBath voltage 1'440 1'590 150Cathode voltage External VoltageTotal Voltage 4'150 4’600 450

1'210

300150

Cell Voltage 4.15 VExternal Voltage 0.15 VCurrent Efficiency 94 %Specific Energy Consumption 13.2 MWh/tAlHeat Losses Anode cover/Rod/Yoke/Stubs 313 kW 40 %Peripheral Crust 63 kW 8 %Side + End Walls 258 kW 33 %Cathode Bars 70 kW 9 %Bottom 78 kW 10 %Total Heat Loss 782 kW 100 %

400 kA cell figures

Table 2: Heat loss breakdown (400kA cell)

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Figure 5: Anode corner cracking

As there is a partial blockage of the electrolysis process during the first day the increase in current is progressive6. Setting the new anode bottom to the same level as the others in normal operation would lead to a massive over- shoot of its current as the daily consumption rate of the other anodes is about 1.7 cm, while the interpolar distance (anode to metal) is just 2 to 3 times this value. The excess current passing through an anode that is set too low massively deteriorates the cell current distribution with strong magnetic effects and a loss of current efficiency. The precise setting of the new anode, to a bottom height of about 1 to 2 cm above the neighboring anodes, is a pre-requisite for smooth cell operation and performance. During the first week, the anode top may be exposed to air attack as its temperature rapidly exceeds its point of ignition7 (~500 °C). The lack of a good thick cover, but also its collapsing due to inappropriate sizing and composition, can lead to a dramatic area reduction of the anode top section as shown in Figure 6. When the air burn reaches the stubhole area, the anode may

Figure 6: Severe air burn

even be lost as it falls into the bath (drop-off). The extent of air burn depends on three factors, these are: the anode top temperature, the anode cover thickness and the intrinsic air reactivity of the anode. The top temperature depends on the combined effect of the anode thermal conductivity and of its covering. At the bath interface the CO2 primary gas can react with the carbon and produces some CO. This reaction occurs at the carbon surface, but also in its bulk through gas permeation that is related to the hydrostatic pressure of the bath and to the anode permeability. The extent of CO2 burn depends on the intrinsic anode reactivity and on its permeability8,9. The internal attack can be quite deleterious as the weakening of the binder matrix through selective burning will produce carbon dust that will in turn contaminate the bath as shown in Figure 7.

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Figure 7: Carbon dust floating on the bath

At a constant pot voltage target value the increased bath resistivity (up to 20 % in severe cases) means a dangerous decrease of the interpolar distance, with a dramatic drop of current efficiency. In case of poor air burn behavior, with reduction of the anode sides, followed by a poor CO2 burn with severe dusting, as shown in Figure 8, the reduction of the working surface area will deteriorate the current distribution of the cells. With the reduced interpolar distance, the contact of anode-metal results in carbon spikes, as illustrated in Figure 9. Anode burn-off is the ultimate burden of such a dramatic scenario. Further to that, the recycling of internally attacked (soft) butts will massively deteriorate the quality of the next generation anodes. Such a vicious circle can eventually lead to a smelter metal output reduction of up to 5%, as well as increased operation costs.

Figure 8: Poor anode butts

Figure 9: Anode with spikes

Carbon consumption figures

The specific net carbon consumption describes the amount of carbon that has been consumed for the production of one ton of metal. The theoretical consumption of the reduction of alumina by carbon is 334 kg/t Al and the electrolytic reduction in the cells depends on the current efficiency as shown in Figure 10. The demand of anodes for a given metal production represents the gross anode consumption, while the excess carbon consumption integrates any carbon loss by burning and dusting or by discarded butt pieces (burn-off, drop-off and swimmer). The lowest net carbon consumption (gross consumption minus recycled butts) reaches 400 kg/t Al with a current efficiency of 95%, while the typical gross

Figure 10: Anode consumption breakdown

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consumption is close to 560 kg/t Al, unless collar paste is used around the stubs to prolong the anode life cycle by several days thus minimizing the amount of recycled butts. In this instance, the anode gross consumption can practically reach half a ton per ton of metal. A too high cycle time shows a critical decrease of the section area, which leads to poor anode current distribution in the pot.

Table 3 addresses the costs of metal production for good and poor anode behavior in competitive western world smelters. Raw material costs in line with a LME Al price slightly above 2000 USD/t, which was representative of the years around 2010, were taken. The total anode cost represents 15 % of the metal production costs in case of good anode performance, but it reaches 20 % in case of poor anode performance.

Table 3: Al production cost and anode performance

The cost of Al production related to the anodes

Indirect costs can increase through higher energy consumption, intensified maintenance and increased labor requirements. These represent an increment of 90 USD/t Al. This is much more harmful than the higher costs related to the increased anode consumption (35 USD/t Al). Therefore, the anodes represent an important variable portion of costs that is linked to the quality of the manufactured carbon blocks.

The anode manufacture for large modern smelters

The chapter below describes the 2013 state of the art carbon plant responding to the challenge of production encountered in large new smelters. In recent years mega smelters producing 800’000 tons per year have been constructed in a first phase and a doubling of capacity are already scheduled or have been completed in many cases. Therefore, for a modern 400 kA pot technology, 2 potlines with a

$/tAl $/tAl600 600500 15.5 MWh @ 36 $/MWh 53590 105

100 11550 55

460 4801'800 1'890

Coke 161 181Pitch 50 57Energy 30 $ 17 30 $ 19Maintenance 28 $ 16 18Labour 25 $ 14 16Capital Cost 110 $ 62 110$ 64

320 3552'120 2'215

0.60 t @ 480 $/t0.15 t @ 600$/t

28 $25 $

Bad Anodes630 Kg/t AL

1.92 t @ 310 $/t

per ton Anodeper ton Anode

560 kg/t AlGood Anodes

Component

14 MWh @ 36 $/MWh

0.60 t @ 480 $/t0.15 t @ 600 $/t

(320/2'120) = 0.15 (355+1'890-1'800)/2'215=0.20

AluminaPower

Overall TotalTotal Anodes

Sub-Total ex. AnodesCapitalMiscellaneous

Anodes related cost

LabourMaintenance

Anodes

1.92 t @ 310 $/t

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total of more than 700 pots are needed for such

AP DubalAP3X DX Y400 Y500

Current [kA] 390 370 400 500

# Potlines 2 2 2 2

# Pots/Line 360 378 336 312

Sp. Energy Cons. MWh/t 13.1 13.2 12.8 12.5

Current Efficiency [%] 93 95 92 91

Al Production [1'000tpy] 750 750 700 800

# Anodes/pot 40 36 48 48

Anode Length [mm] 1'600 1'645 1'570 1'750

Anode Width [mm] 650 690 665 740

Anode Height [mm] 660 655 625 620

Baked Anode Weight [kg] 1'035 1'115 960 1'135

Current Density [A/cm2] 0.86 0.90 0.80 0.80

Gross Anode Cons. [kg/tAl] 560 580 540 560

Good Baked Anodes [ktpy] 420 435 378 448

Rodded Anodes [k#/y] 405 390 394 394

SAMITechnology

Table 4: Smelter technology and anodes

a large capacity. Table 4 gives the figures for the first phase of mega-smelters installed, or in the process to be completed in 2014 for the different major technology providers. Typically, the anode block tends towards a baked size of 1750 mm x 700 mm x 650 mm, with 4 stubholes and 2 longitudinal slots. Typically, the green density level is close to 1.65 kg/dm3. The green and baked weights before slotting are close to 1’250 kg and 1’200 kg respectively. The features of the carbon plant delivering rodded anodes to the potrooms are shown in Table 5.

Table 5: Design data for 800 ktpy Al

Smelter Capacity tpy 800'000

Anode requirement tpy 448'000

Recycled Processing

Capacity tpy 150'000

Throughput tph 50

Shifts/Week #/week 8

Availability % 75

Harbor Facility

Coke Silos # x t 2 X 30'000

Pitch Tanks # x t 1 x 12'000

Green Anode Plant # 1

Paste Production tpy 500'000

Throughput 1 Line t/h 75

Anode Weight kg 1'250

Forming Capacity blocks/h 60

Production Shifts/Week # 19

Weeks/y # 50

Availability % 87

Baking Furnace # 2

Gross Production Baked tpy 470'000

Green Blocks Baked # 395'000

Baked Anode Weight kg 1'190

Fires # 8

Capacity per Fire tpy 60'000

Pits/Flues # 10 / 11

Sections # 2 x 72

Anodes per Section # 180

Fire Cycle Target h 32

Waste Gas Cleaning Nm3/h 220'000

Blocks for Slotting # 387'000

Slotted Block Weight kg 1'170

Rodded Blocks to Pot # 383'000

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During the anode manufacturing process there are 6 different process areas, namely:

1. Calcined petroleum coke and pitch storage, usually in the harbor area

2. Butts and scrap crushing and sieving

3. Proportioning of the baked and green recycled materials

4. The continuous green mill where the dry aggregate, paste and block forming take place

5. The anode baking and slotting

6. The anode rodding with butts cleaning and stripping.

The Raw Materials

In the flow sheet in Figure 11, the source of the raw materials is shown. The green petroleum coke is a byproduct from oil refineries. For anode manufacture, the relevant properties to be measured are given in Table 6.

Table 6: Properties of green coke

Calcination in a rotary or shaft kiln allows the volatiles to be driven out of the green coke. The calcined coke is then tested as per Table 7 to ascertain its future behavior in the anode manufacture and subsequent performance in the pots. The anode is not consumed completely in the aluminium production cell, as the yoke stubs would contaminate the metal and be irrecoverable. Hence, the anode butts are recuperated and used as a

Unit Typical Range

S % 1 - 3V ppm 100 - 400Fe ppm 100 - 300Si ppm 100 - 300

GREEN COKE PROPERTIES

> 4 mm

Water Content

Volatile Matter

Hardgrove Grindability Index

Ash Content

% 5 - 10

Sieving Analysis % 40 - 70

- 60 - 100

% 8 - 12

Elements XRF

% 0.1 - 0.3

Figure 11: Aluminium production flow sheet

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Table 7: Properties of calcined coke

Table 8: Properties of anode butts

valuable coarse material for anode manufacture. To ensure these are not contaminated by the smelting process and used without

Table 9: Properties of coal tar pitch

affecting the anode quality, the analysis shown on Table 8 is performed on a routine basis. The coal tar pitch used for anode manufacture is derived from coal tar emanating from coke ovens used to produce metallurgical grade coke. After distillation, coal tar pitch is obtained and used as a binder for the petroleum coke dry aggregate. The analyses listed in Table 9 gives an overview of the typical binder quality.

Unit Typical Range

S % 1 - 3V ppm 100 - 400Fe ppm 100 - 300Si ppm 100 - 300

CALCINED COKE PROPERTIES

% 0.1 - 0.3

Air Reactivity 525°C (0.5°C/min)

CO2 Reactivity

Ash Content

Water Content

Oil Content

Grain Stability

Pulverizing Factor

%/min

kg/dm3Pressed Density

-

% 0.0 - 0.2

% 0.1 - 0.3

% 70 - 90

0.9 - 1.2

Sieving Analysis > 4 mm % 25 - 45

% 3 - 15

Elements XRF

Tapped Bulk Density 2 - 1 mm kg/dm3 0.80 - 0.86

0.85 - 0.92

kg/dm3 2.05 - 2.10

Å 25 - 32

µΩm 420 - 520

Real Density

Crystallite Size

Specific Electrical Resistance

0.1 - 0.4

Unit Typical Range

S % 1 - 3V ppm 100 - 400Fe ppm 150 - 500Si ppm 100 - 300Na ppm 300 - 1'000

BUTTS PROPERTIES

Ash Content

Grain Stability

%

% 0.0 - 0.2

Tapped Bulk Density kg/dm3 0.92 - 0.98

Sieving Analysis % 50 - 70

kg/dm3 2.07 - 2.10

°C 580 - 630

Hardness

Ignition Temperature 10°C/min

> 4 mm

Real Density

2 - 1 mm

Water Content

Elements XRF

0.2 - 0.6

mm 1 - 5

% 85 - 95

Unit Typical Range

S % 0.4 - 0.7Na ppm 100 - 250Pb ppm 100 - 300Zn ppm 100 - 500

COAL TAR PITCH PROPERTIES

Water Content

Softening Point Mettler

Real Density Water

Coking Value

Quinoline Insoluble

Toluene Insoluble

Ash Content Pitch

Elements XRF Pitch

0 - 360 °C

160°C

Fractionated Distillation % 3 - 6

% 56 - 62

% 26 - 36

kg/dm3 1.30 - 1.33

% 0.1 -0.3

°C 110 - 120

% 0.0 - 0.2

Viscosity mPa*s 1'200 - 4'000

% 5 - 15

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The green mill

Dry aggregate preparation

The green and the baked recycled materials are prepared in campaigns and stored separately in proportioning silos as the different sourced materials need to be accurately blended for a consistent paste preparation10. The stripped butts from the rodding area and the baked scrap blocks are reduced to a size of – 250 mm in a primary crusher (jaw type, hydraulic or low speed roll crusher) and eventually to – 50 mm in a secondary jaw or cone crusher. A rigorous quality control of the entering raw materials is imperative to ensure a stable anode quality. The pre-crushed materials are lifted up to a compact vibration sizer splitting two fractions of baked recycled materials 20 - 4mm and 4 - 0 mm. These are then stored in separate bins. The oversize + 20 mm material is fed to a cone crusher and in a closed loop back to the screen. For the green scrap (paste or block) a single fraction 20 – 0 mm is stored in a bin. The three fractions are proportioned according to their average amount (typically 45/45/10) and a combined fraction 16 – 0 mm of recycled material

is transferred into a small fraction bin located above the green mill scales area. This recycled material represents about one third of the total aggregate. The other two thirds of aggregate are made out of the fractions of coke that are prepared from the metered stream from the shift bin located in the paste plant. A compact multi-deck sizer (Figure 13) splits the stream in two fractions 8 – 2 and 2 - 0 mm and the oversize is directed towards a ball race mill circuit11 (Figure 14). There, it will merge with the overflows of the other two coke fractions. The 25 t/h vertical ball race mill, having steel balls close to 1 m diameter, is not sensitive to the size variability of the feed and has an integrated compact dynamic classifier. A filter above the mill collects the product that is transferred to the fines fraction bin. The sound level is very low (80 dB) compared to horizontal ball mill units (105 dB) and the iron contamination of the resulting fines (50 ppm) is four times lower. The target fineness of 4’000 Blaine is met with a consistency better than 200 Blaine. The nuisance dust is collected in bag filters (Figure 15). The collected material is fed back to the fines silo in a controlled manner.

Figure 12 : Modern Anode Production Steps

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Figure 13: Multi-deck sizer

Figure 14: Ball race mill

The four fractions are stored in bins equipped with load cells. With the installed loss in weight system (i.e. Figure 16), the metering is made continuously with the desired proportions that are typically 1/3 each of recycled, fines and 1/6 each of coarse and intermediate coke. Paste and green block production

The 75 t/h continuous plant consists of a thermal oil boiler, a preheating screw, a paste kneader and cooler,

Figure 15: Bag house filter

Figure 16: Fines scale

and eventually of a forming machine and an anode block cooling section, as illustrated in Figures 17 to 21. The first important element providing heat by means of oil circulation is a thermal oil boiler which is usually gas fired. Its power reaches 5 MW and it provides 300 °C hot oil (HTM) mainly to the preheater, but also to the pitch storage and dosing systems. After the proportioning of the four fractions and transfer thereof through a bucket elevator the ca. 65 t/h aggregate is heated to a temperature close to 200 °C into a four 30” diameter screws preheater (more than 200 m2 of surface exchange consuming up to 4 MW power). The hot aggregate falls into the intake section of the kneader that is powered by an 800 kW drive.

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The modern kneader12, with 750 mm barrel diameter (D) and 9.5 L/D process length (L), has a series of kneading elements with four flights mounted on a rotating shaft (max. of 75 rpm). The shaft also has backwards and forwards longitudinal movements so that efficient mixing is achieved by shearing of the paste in between the flights and the teeth. The liquid pitch (120 °C Mettler softening point) is dosed, from the green mill internal tank kept at a constant temperature close to 220 °C, either by a Coriolis flow meter or by a volumetric pump that is calibrated by stopping pitch feed for a while and entering into a tank equipped with load cells. It enters into the kneader through the first kneading teeth to avoid clogging in the initial aggregate intake section. The innumerable flow separations and reorientations with steady shear rates ensure maximum paste homogeneity despite the fact that the residence time is less than 3 minutes. The flap gate mounted at the end of the kneading zone, along with the rpm selection allows a consistent and high mixing energy reaching about 10 kWh/t. The temperature of the paste (~ 200 °C) is estimated from the signal of a thermocouple mounted into the last kneading bolt. The mixer shell is heated electrically prior to start-up or during short production stops.

Figure 17: Dry aggregate preheater

Figure 18: Paste kneader

In the transfer chute of the paste towards the cooler, water is dosed for cooling the paste to the right temperature needed for anode forming. About 0.5 % of water rate cools the paste by 10 °C.

Figure 19: Paste cooler

Figure 20: Anode block vibrator

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Figure 21: Green anode cooling

The intensive impeller cooler13 is a high speed 450 kW rotor mixer that produces a free flowing homogeneous paste to the forming machine. The residence time of 4 minutes is indirectly controlled by maintaining a constant load of 5 tons in the cooler. This is achieved by means of a flap gate integrated in the bottom of the rotating vessel. For vacuum pressed and vibrated anodes, temperatures of 130 °C and of 165 °C respectively are targeted14 (~2.5 m3/h and 1.2 m3/h of water are needed respectively). The forming of anodes is done by one hydraulic press having 60 blocks/h capacity or by two vibrators having both a production rate of 30 blocks/h15. In case of a tandem vibrating compactor, a travelling hopper scale feeding both units is required. After dumping the weighed paste (1’250kg) into the mold, the upper cover is lowered to the paste level and a vacuum is applied (30 mbar residual pressure). For the 2’000 ton press the hydraulic pressure is raised to about 150 bar and the holding time is about 1/10 of the one minute cycle. For the vibrator, the vibrations provided by the two fly weights driven by the motors rotating at 1’500 rpm generate a maximum unbalanced force of 500 kN on the block.

This is applied for about 40 seconds in its total cycle time of 2 minutes. As rapid compaction is critical, inflatable rubber bellows were introduced and mounted on the top of the cover weight creating an adjustable pneumatic force. During forming, the anode height is monitored through an in situ sensor. This allows an adaptation of the forming parameters (pressing load or vibrating time) for providing anodes at constant height and apparent density. This is possible provided that good control and consistency of the recipe and paste process parameters are achieved. The four stubholes, having typically 250 mm external diameter, 130 mm depth and 8 inclined flutes are formed by freely rotating metal nipples integrated in the cover weight. Pitch fumes collected in the different locations are transferred to the carbon plant regenerative thermal oxydizer, which is also connected to the bake furnace. This destroys the harmful pitch PAH (polyaromatic hydrocarbons).

The anode cooling for pressed anodes is simply achieved through natural air convection while for vibrated anodes,

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Figure 22: Anode baking furnace

one hour soaking in a water bath16 is required, as illustrated in Figure 21. The green anodes are moved on a steel plate conveyor. When the integral anode temperature is at least 10 °C below the softening point of the pitch, the anode can be moved with roller conveyors and stacked with a crane in the green anode storage area beneath the baking furnace. The paste plant has a central control room where all the measured process parameters are displayed and monitored. Samples of the fines fraction have to be taken to measure the Blaine fineness regularly throughout the shifts. The binder content can be checked in the laboratory to confirm the correct metering of dry aggregate and pitch in the paste (Table 10).

The baking furnace

From the green anode storage, the anodes are conveyed to a ring type open top baking furnace as shown in Figure 22.

Table 10 : In plant testing

Figure 23: Furnace inner structure

The building width is close to 40 m wide and more than 200 m in length. In ring type furnaces17, the anodes stacked in pits remain in place for about two weeks and the firing zone travels around the furnace. The anode takes half of this time to heat-up to a finishing temperature in the range of 1’100 °C to 1’200 °C. The remaining time is required for cooling and unloading of the blocks, as well as for refractory maintenance and eventually loading of new green anodes. The open top furnace has four fires with 18 sections each, which are separated by transversal headwalls. Each section is made up of 9 parallel pits separated by 10 flues. A multi-purpose crane takes a row of anodes at a time and stacks them vertically in 3 layers in the pits (Figure 23). The pit dimension exceeds 5 m in length and depth and accommodates the anode height with about 80 mm clearance. The anodes are packed in petroleum coke that burns at a rate of 10 kg by ton of baked anodes and hence protects the anodes from oxidation. As shown in Figure 24, the mobile firing equipment consists of the exhaust manifold connecting the off-gas to a ring main, the under-pressure bridge, three burner bridges operated with fuel and 2 coolers blowing in ambient air into the flues. The fire equipment is moved by

Unit Typical RangeIN PLANT TESTING

Blaine Value

Binder Content

Blaine 3'000 - 6'000

% 13 - 15

14

Figure 24: Fire equipment

one section every 28 to 32 hours. This fire cycle and the section anode load determine the fire capacity. A computerized process control system18 as shown in Figure 25 regulates the increasing temperature to a predetermined target curve by controlling the draft and fuel input to the heating sections. Processors on the fire bridges and on the under-pressure bridge collect the temperatures and under-pressure signals and transmit all the process data to a computer in the control room where the overall fire situation is visualized at a glance. First, the flues convey the heat introduced by gas burners placed on peepholes. Then, at the end of the baking cycle, cold air is introduced into the flues by fans placed on the large headwall openings cooling the anode blocks. These same headwall openings serve as off-gas outlets for the exhaust manifold.

Tie bricks linking the two flue walls guarantee the flue stability over the entire cavity depth. Baffles in the flues facilitate the temperature distribution through the height of the pit. In the heating sections, devolatilization of the pitch (1/3 of the binder content) occurs in the temperature range from 300 to 600 °C. For proper furnace operation including complete volatiles combustion (no soot and no tar deposits), sufficient under-pressure is maintained by a fan down-stream. The anode volatiles escape through the gaps in between the flue wall bricks and burn rapidly by the increasing temperature and sufficient supply of oxygen. Figure 26 shows the development of the flue variables in the first sections of a fire19. 50% of the energy required to bake the anodes is provided by the coal tar pitch volatiles evolving from the anode being baked. The extra fuel energy amounts to 1.8 GJ/t of baked anodes (500kWh/ta).

Figure 25 : Firing system

Figure 26:Temperature, draft and oxygen

development in the first fire sections

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Figure 27: Regenerative thermal oxidizer

The waste gases containing harmful condensed and volatile pitch PAH, volatile organic compounds VOC, carbon monoxide CO, SO2, NOx and particulate matter are treated with a regenerative thermal oxidizer (Figure 27). Due to the recycling of anode butts in the dry aggregate, some HF is present as well. The fluorine and SO2 are removed in a limestone absorber. After cooling to a temperature level below 400 °C, the anodes are unpacked and if necessary cleaned in dedicated equipment. These are eventually transported to the baked anode store. Anode slotting

Slots on the bottom side of the anodes are an effective way to reduce the anode bubble voltage drop in the pots20. This operation is made by machining the baked block rather than molding them during the forming stage of the green block as this results in density inhomogeneity and increased scrap rate Two slots of 1 cm width and about 350 mm average height, slightly inclined towards the inner pot side of the block, are cut simultaneously by two rotating discs (Figure 28) able to cut in excess of 40’000 anodes in their life time,

Figure 28: Slotting machine

Figure 29: Butts before cleaning

this represents more than one month operation for a large anode plant. The machine is mounted into a sound and dust proof cabin; the carbon material (up to 20 kg/block) that is recovered by a bottom screw conveyor and by aspiration/ bag filtration is recycled back to the green mill dry aggregate.

Anode rodding

Even though the rodding shop falls under the responsibility of the aluminium reduction plant, its design and operation have a huge impact on the anode quality and performance. Figures 29 to 31 show the important steps for anode butts cleaning and recycling (stripping). The first operation on the butts returning from the potrooms is the cleaning of the

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Figure 30:Anode cover cleaner

Figure 31: Butts after shot blasting

cover (up to 300 kg per block). The thorough removal of this cover material, rich in cryolite is essential to ensure that the next anode generation has a good resistance to oxidation3,7. Sodium is not only a strong catalyst of CO2 and air combustion but also gaseous Fluorine, that escapes during baking, attacks and destroys the baking furnace refractory. Cleaning is made in three stages. These are: pre-cleaning by pneumatic rams or push bars that loosen the anode cover, fine cleaning by rotating tools or chains and eventually by shot blasting. A black butt (the bench mark value of sodium content in recycled butts being as low as 200 ppm) leaving the cleaning area is a good start for the manufacture of high quality anodes. The anode cover is recycled in the bath treatment plant. After coarse

Figure 32: Iron Casting

crushing, it is mixed with about ¼ of alumina prior to it being used as anode cover in the potrooms. The butts are then stripped down and conveyed towards the primary crusher. The cast iron thimble is removed from the stubs and several steps of rod and stubs repair follow in order to provide perfect rod/yoke to the casting area (Figure 32). There, after coating the stubs with graphite emulsion which prevents bonding of the iron thimble, the rod is positioned so that the stubs can be centered. The cast iron prepared in an induction furnace is poured into each stubhole. Centering of the stubs is essential for optimum contact with the carbon as the air gap resulting from the cooling of the cast iron is dependent upon the thimble thickness21. The newly rodded assemblies are loaded into a pallet that is transported to the potrooms.

Anode quality control

Routine testing of the key anode properties and of the corresponding raw materials is mandatory for avoiding any anode performance deterioration. The standardization of analytical methods on carbon materials by the ISO committee and the availability of the commercial

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testing equipment are decisive in this area. The baked anodes are core drilled

Table 11 : Properties of baked anodes

(cylinders Ø 50mm and length 250mm). and besides these being measured for impurities by XRF and for the real density, as an indicator of the heat-treatment degree, the other key properties to be monitored in order to avoid any disturbance of the smelter performance are shown in Table 11. The thermal conductivity, the CO2 reactivity, the air permeability and the air reactivity of the anodes are the decisive factors for the net carbon consumption figure and also for the estimation of the amount of carbon dust generated by the anodes.

Outlook Aluminium smelting technology has massively increased the productivity of a single potline22 through the cell amperage and thankfully also to the number of installed pots. Besides the decrease of the pot voltage23 to less than 4 V, the rectifier voltage limit has recently been increased from 1’500 V to 2’000 V 24. Evidently the increase of the pot size will also continue; validations of 600kA pots are already under progress. This means that one potline with 500 pots can reach a production capacity of 800’000tpy, that is twice the amount reached in the period (2010 to 2013). The number of anodes in the pot will be increased from 48 to 52 and its size will reach 1’800 mm so that the current density will remain below 0.9 A/cm2

. With a green weight of 1’340kg and a rate of 60 blocks/h, a paste plant throughput of 80 tph is ultimately the target. This means that one green mill line will still cover the need of one potline. The bake furnaces (2 units), with deeper pits, will accommodate the longer anode column of 5.4 m. Thanks to the higher anode baked weight of 1’280 kg, nine pits with a conservative 30 hours fire cycle will provide the right amount of blocks (350’000) to the potrooms. It can be concluded that the future challenges of the carbon production before the end of the decade will be the design of: ‒ the assembly of the anode with stubs,

yoke and rod

‒ the forming machines to cope with longer green anode sizes

‒ baking furnaces with deeper pits.

Unit Typical Range

S % 1 - 3V ppm 100 - 400Fe ppm 150 - 500Si ppm 100 - 300Na ppm 150 - 300

BAKED ANODES PROPERTIES

Apparent Density Baked

Specific Electrical Resistance Anode

Flexural Strength

Compressive Strength

Static Elasticity Modulus

Coefficient of Thermal Expansion

Thermal Conductivity

Air Permeability

Air Reactivity Anode

CO2 Reactivity Anode

Real Density

Crystallite Size

Ash Content

Elements XRF

% 0.2 - 0.5

Å 28 - 34

10-6K-1 3.8 - 4.5

W/mK 3.5 - 5.0

kg/dm3 1.56 - 1.62

kg/dm3 2.07 - 2.10

MPa 10 - 14

µΩm

GPa 4.0 - 6.0

50 - 60

MPa 40 - 60

% 88 - 95

% 70 - 90

nPm 0.5 - 2.0

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References

1. J Thonstad et al.,” Al Electrolysis, (2001), ISBN 3-87017-270-3 2. W. Haupin, “Interpreting the components of cell voltage”, Light Metals 1998, pp. 153 - 159 3. T. Tordai,”Anode dusting during the electrolytic Production of Aluminium Thesis #3808 , EPPFL, (2007). 4. M. Meier et al.,“ Production and performance of slotted anodes” Light Metals 2007, pp. 277 – 282. 5. M. Meier,” Cracking behavior of anodes”, (1996), ISBN 3-9521028-1-4. 6. R. Odegard et al. “Current pick-up and temperature distribution in newly set anodes”, Light Metals 1992, pp.555–561. 7. S.M. Hume, ”Anode Reactvity”, (1999), ISBN 3-9521028-2-2 8. R. Perruchoud et al.,”Dust generation / accumulation for changing anode quality and cell parameters”, Light Metals 1999, pp. 509 – 516. 9. W.K. Fischer and R. Perruchoud, “Factors influencing the reactivity behavior of anodes in Hall- Héroult cells”, Light Metals 1986, pp. 575 – 586 10. R. J. Akhtar et al.,“New green anode plant at Emal”, Light Metals 2013, pp. 1101 – 1104. 11. T. Moller,” Fine production for anode manufacturing”, Light Metals 2005, pp. 653 – 658. 12. H.U. Siegenthaler et al.,“New design of the Buss kneader zone for green paste preparation“, Light Metals 2010, pp. 969 – 973. 13. B. Hohl,”New technology for continuous preparation of anode paste”, Light Metals 1994, pp. 719 – 722. 14. K. L. Hulse, “Anode Manufacture”, (2000), ISBN 3-9521028-5-7 15. W.K. Fischer and M. Meier, “Advances in forming”, Light Metals 1999, pp. 541 – 546. 16. W.K. Fischer et al.,” Cooling of green anodes after forming”,

Light Metals 1999, pp. 547 – 554. 17. F. Keller and P.O. Sulger, “ Anode Baking”, ( 2008), ISBN 978-2-940348. 18. R. J. Akhtar et al.,“Anode quality and bake furnace performance of Emal”, Light Metals 2012, pp. 1175 – 1179. 19. M. Lustenberger, “Heat treatment of carbon anodes for the Al industry”, Thesis #3039 , EPPFL, (2004) 20. H. Gudbransen et al., “Field study of the anodic overvoltage in prebaked cells”, Light Metals 2003, pp. 166 – 171. 21. S. Beier et al., “FEM analysis of the anode connection in Aluminium cells”, Light Metals 2011, pp. 979 – 984. 22. Yan Feiya et al., “In depth analysis of energy saving and current efficiency improvement of Aluminium reduction cells”, Light Metals 2013, pp. 537 – 542. 23. Z.Dongfang et al., “Development and application of SAMI low voltage technology”, Light Metals 2012, pp. 607 – 612. 24. Fact sheet of ap-technology.com, “The world’s bench mark reduction technologies”, March 2013.

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Figures credits

1 EMAL, UAE – Abu Dhabi 2 Peter Entner, CH - Sierre 3 Rio Rinto Alcan, CDN - Alma 4 Al- Pechiney, F - Voreppe 5 R&D Carbon, CH - Sierre 6 R&D Carbon, CH - Sierre 7 R&D Carbon, CH - Sierre 8 R&D Carbon, CH - Sierre 9 R&D Carbon, CH - Sierre 10 R&D Carbon, CH - Sierre 11 R&D Carbon, CH - Sierre 12 R&D Carbon, CH - Sierre 13 Morgensen, D – Wedel 14 Claudius Peters, D- Buxtehude 15 Donaldson Inc., USA - Minneapolis 16 Schenk Ag, D - Darmstadt 17 Köllemann GmbH, D - Adenau 18 Buss Ag, CH – Pratteln 19 Eirich GmbH, D – Hardheim 20 KHD- Outotec, D - Köln 21 R&D Carbon, CH - Sierre 22 Innovatherm,D – Butzbach 23 R&D Carbon, CH - Sierre 24 Riedhammer GmbH, D - Nürnberg 25 R&D Carbon, CH - Sierre 26 R&D Carbon, CH - Sierre 27 CTP, A – Graz 28 T.T.Spa, I – Due Carrare 29 Hydro Al, N – Stabekk 30 Kempe Eng. , AUS - Geelong 31 Hydro Al, N – Stabekk 32 Westfalia Becorit, D – Lünen

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