Temperature control on hot pots in the aluminium production

Summary

1 Introduction

2 Aluminium
2.1 A short history of aluminium
2.2 Application fields for aluminium
2.3 From the bauxite to the final product
2.3.1 Bauxite mining
2.3.2 The Bayer process
2.3.3 The primary aluminium electrolysis
2.3.4 Refining and casting
2.3.5 Process balance
2.3.6 Recycling

3 The primary aluminium electrolysis
3.1 The bath
3.2 The electrolysis cell and the potroom
3.3 State of the technology
3.3.1 Process improvements in the last 30 years
3.3.2 Nowadays challenges

4 Bath properties and pot operation
4.1 Ratio
4.1.1 Aluminium fluoride
4.1.2 Sodium fluoride
4.2 Temperature
4.3 Electrical conductivity, resistance and anode-cathode distance
4.4 Other properties
4.4.1 Alumina solubility
4.4.2 Density
4.4.3 Surface properties
4.4.4 Thermal conductivity
4.4.5 Metal solubility
4.4.6 Vapour pressure

5 Experimental work
5.1 General information about Alumar
5.2 State of the art at Alumar and objective of this work
5.3 Measurement procedure and tools
5.3.1 Temperature and ratio measurement procedure
5.3.2 Selection of the cells
5.3.3 Temperature measurement
5.3.4 Ratio measurement
5.3.5 Superheat measurement
5.4 Analysis of the measurement system
5.4.1 Gauge capability for temperature measurements
5.4.2 Error sources in the temperature measurement
5.4.3 Gauge capability for ratio determination by chemical analysis
5.4.4 Error sources in the ratio measurement
5.4.5 Error sources in the superheat measurement
5.5 Action of the modifier
5.5.1 Temperature distribution in the cells
5.5.2 Identification of outliers
5.5.3 Relation between temperature, time and modifier power
5.5.4 Relation between ratio and temperature
5.5.5 Superheat
5.5.6 Economic analysis of the modifier action
5.6 Restrictive conditions
5.6.1 Description of the program
5.6.2 Original system
5.6.3 New system
5.7 The modifier calculation program
5.7.1 Old versus new program logic
5.7.2 Current program logic
5.7.3 New program logic

6 Conclusions

7 Future works

8 Bibliography

9 Appendixes
9.1 Appendix 1: Definitions and abbreviations
9.2 Appendix 2: Influences of additives on the bath
9.3 Appendix 3: Influences of operating parameters on the bath
9.4 Appendix 4: Temperature values for the gauge analysis
9.5 Appendix 5: Ratio values for the gauge analysis
9.6 Appendix 6: Monitoring messages in the new program

1 Introduction

The process of aluminium production is, even nearly 150 years after its discovery, not totally understood. Such a large quantity of factors influences the production process that no standard can be applied on it. In an aluminium reduction plant there are no two identical electrolysis cells. Differences in the start-up and operational disturbances alter the thermal and electrical behaviour of each cell individually. So every cell has to be monitored individually through the continuous changing bath composition and temperature variation. Every cell needs an individual dynamic optimisation of the chemical and thermal input, e.g. the AlF3- and CaF2-addition and the regulation of the anode-cathode distance. Self-regulating mechanism are the melting and freezing of the sidewall ledge, external mechanisms the lifting and lowering of the anodes.

In this work a cell control system at Alumar (Brazil) was analysed and improved. The focus laid on controlling the cell temperature through a variation of the anode-cathode distance. The main objective was to analyse the effect and influence of a so-called temperature resistance modifier on hot pots (electrolysis cells).

When the temperature of an electrolysis cell exceeds a certain limit, the pot operates outside its optimal working conditions, thus it has to be cooled down. This can happen by reducing the anode-cathode distance and therefore the resistance of the pot. The reduction is controlled by the so-called temperature resistance modifier. The concept of the temperature modifier was introduced less than one year ago at Alumar and is also within the Alcoa group a quite unexplored field.

This work is structured in two parts, first an introduction to theory, then the practical part. Chapter 2 gives some general information, from the history of aluminium to a brief overview over the entire production process. Chapter 3 focuses on the general functioning of the aluminium electrolysis. Chapter 4 details the properties of the cryolite bath in the reduction cell and their impacts on the pot operating characteristics.

Chapter 5 contains the experimental part of the work. In a first time the quality of the measurement system was tested. Then the relations between temperature resistance modifier and time as well as two bath parameters, the temperature and the ratio were inquired. Thereafter substantial modifications on the program that calculates the temperature modifier were analysed and tested:

- The program logic includes several restrictive conditions whose task it is to prevent that the temperature modifier action could harm the pot and destabilise the reduction process. All these conditions were analysed and tested one by one to check their sense and their effect. The objective was to augment the percentage of pots on which the temperature modifier can be applied, i.e. to cool more hot pots down
- The formula that calculates the temperature modifier value was analysed and a new formula was tested. The objective was to adapt the temperature modifier better to the pots needs and turn its action more flexible.

In chapter 6 the final conclusions are drawn including the benefits and disadvantages of all the realised tests. Chapter 7 gives suggestions for future researches and improvements.

Definitions of the most important key words can be found in Appendix 1: Definitions and abbreviations.

2 Aluminium

2.1 A short history of aluminium

Officially two spellings exist, aluminium and aluminum. An anecdote says that when Alcoa was officially registered as a company, the employee on duty omitted the second i.

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Figure 2.1: Héroults electrolytic cell [41].

Aluminium is a quite new material. It was first isolated at the beginning of the 19th century. Only the development of new techniques, especially the availability of cheap electric current, enabled the start of a large-scale production little more than 100 years ago. The Hall-Héroult process (Figure 2.1), which is still used today, was invented in 1886 by Hall in the United States and Héroult in France, independently from each other. It has remained basically unchanged until today. The principle is the dissolution of alumina in cryolite and its reduction through electric current [13].

With an expected annual production of 26 million tonnes in 2003, it is the most important metal behind steel [43].

Figure 2.2 shows the development of the worldwide primary aluminium production. The basic understanding of bath properties, chemical processes and improvements in the production technology ensured this steady growth.

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Figure 2.2: Primary aluminium production since 1854 [13] [43].

2.2 Application fields for aluminium

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Figure 2.3: Main uses of aluminium in 2000 [44].

Its low density, resistance against corrosion, durability, electrical and thermal conductivity and recyclability open a wide field of applications to the aluminium.

The newest, largest and fastest growing sector is the transports sector (Figure 2.3) since car manufacturers discovered new methods to apply aluminium in their products. The other main applications areas are the construction and the packaging sector [40].

2.3 From the bauxite to the final product

Bauxite ore is mined, converted to aluminium oxide in the Bayer process and then reduced to aluminium in an electrolysis (Figure 2.4).

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Figure 2.4: The aluminium production process [41].

2.3.1 Bauxite mining

With 8 wt %, aluminium is the third most frequent element in the earth crust after oxygen and silicon. The main mining areas are in the tropical and subtropical zones. Aluminium never occurs pure, only in very stable compounds with other elements. The most common minerals are silicates and hydroxides. Bauxite belongs to the hydroxide minerals.

The ore is broken in opencast mining and crushed in the mine site. The ground ore is then transported to the alumina factory.

2.3.2 The Bayer process

Bauxite is leached under elevated pressure and temperature with a concentrated caustic soda solution. Aluminium dissolves forming Al(OH)3. Impurities like iron and titanium oxides don’t dissolve, silica dissolves partially. This solution is then filtered. The solid impurities form the red mud containing up to 25 % aluminium, it is dumped in landfill sites. From the clean solution, the Bayer liquor, aluminiumhydroxyde-crystals are precipitated, washed and then dried. The dried Al(OH)3 is calcined in flash calciners at 1200 – 1300°C to drive the chemically bound water off and so to produce alumina, Al2O3 [23] [41].

Its main application field is the production of aluminium in the Hall-Héroult electrolysis; other fields are the ceramics and chemical industry [63].

2.3.3 The primary aluminium electrolysis

Alumina is dissolved in an electrolytic bath based on cryolite. Electric current is applied to reduce the alumina. Aluminium is deposited in a metal pool at the bottom of the electrolytic cell and siphoned off periodically. The oxygen reacts with the anode carbon to carbon dioxide and is released in the off gas [9].

The sum reaction is [15]:

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Equation 2.1

2.3.4 Refining and casting

The aluminium from the Hall-Héroult electrolysis reaches a purity of 99.9 %. This is sufficient for most applications. Higher purities can be obtained by electrolytic refining [23]. The aluminium is casted to ingots and then extruded or rolled.

2.3.5 Process balance

For the production of one tonne aluminium are necessary [23] [63]:

- 4 000 – 5 000 kg bauxite (which give 2 000 kg alumina)
- 20 – 50 GJ thermal energy for the Bayer process
- 60 – 200 kg sodium hydroxide
- 50 kg cryolite
- Additives
- 400 – 600 kg electrodes
- 13 – 15 MWh electrical energy.

The outputs are:

- 1 t of aluminium
- 1 – 2 tonnes red mud
- 1.4 t CO2
- CO, CF4, C2F6.

One tonne of primary aluminium costs about 1300 to 1400 US$ [42].

2.3.6 Recycling

Recycling of aluminium is economically and environmentally interesting [23]:

- The quality of secondary aluminium is close to the quality of primary aluminium and sufficient for the most applications
- Often products are used in a closed material circle, i.e. cans are recycled to cans, cast pieces to cast pieces
- Recycling needs only about 5 – 10 % of the energy necessary to produce primary aluminium.

3 The primary aluminium electrolysis

3.1 The bath

The main constituent of the bath is cryolite. Molten cryolite is the only electrically conducting material able to dissolve alumina. A typical bath composition is 2 – 3 % Al2O3, 10 – 13 % AlF3 and 4 – 6 % CaF2. The operating temperature ranges from 940 to 985°C [28] [54].

The melting point of cryolite is 1010 ± 1°C [20], the addition of alumina lowers this temperature. Additives are added to improve physical and chemical properties of the bath [15] [20]. They have to be cathodically more stable than alumina to avoid their reduction at the cathode and the contamination of the aluminium. The most frequent additives are AlF3 and CaF2. More rarely LiF and MgF2 are used. They mainly influence melting point of the mixture, cell efficiency, bath properties and metal and alumina solubility [3]. A detailed table of the effects of the additives on the different bath properties is shown in Appendix 2.

3.2 The electrolysis cell and the potroom

The reduction takes place in electrolysis cells, so-called pots (Figure 3.1).

The pot consists of a steel shell lined with thermal insulation. The thermal insulation has to be dimensioned to guarantee

- A constant heat loss from the pot to maintain a certain layer of frozen cryolite on the side wall to protect the lining from the aggressive bath

- A sufficient heat stay in the pot to maintain the operating temperature.

The carbon layer on the bottom is the cathode. The real cathode, where the reduction occurs is the aluminium metal pool surface. From the top, carbon anodes are introduced into the pot. The current passing the cell heats the bath. The anodes are consumed during the electrolysis and have to be replaced regularly. They are covered with frozen bath and alumina to prevent their oxidation at the atmosphere. The anode-cathode distance (ACD) is approximately 5 cm. Alumina is added into the pot in short intervals (1 – 5 minutes) in small quantities (3 – 5 feeders à 1.5 – 2 kg each) to ensure a more less constant concentration in the bath. The liquid metal is siphoned off every second day.

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Figure 3.1: Hall-Héroult electrolytic cell for aluminium electrolysis [9].

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Figure 3.2: Top-view of Hall-Héroult cell with prebake anodes [20].

Figure 3.2 shows a top view of a Hall-Héroult cell with prebake anodes. The pots are electrically connected in line as shown in Figure 3.3. The voltage drop in a single cell is 4 to 5 V. The amperage reaches 300 000 A. The potline is connected to a high voltage rectifier (Figure 3.4).

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Figure 3.3: Connection in line of the cells with current flow from the left to the right [20

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Figure 3.4: Many pots connected in series to a potline with voltage rectifier [20].

3.3 State of the technology

3.3.1 Process improvements in the last 30 years

The Hall-Héroult process has remained basically unchanged since its invention and it will stay the predominant process in the future. Research in alternative methods like inert electrode systems has not yet led to an applicable industrial production process.

The improvements of the Hall-Héroult process were made possible by the following developments: increased cell size, magnetic modelling of the flow patterns in the cells, magnetic design of the electrical components, thermal modelling of the cell’s heat balance, reduction of the heat losses and energy saving by improving the cell insulation, computerised process and operation control, use of the pointfeeder-technology allowing more efficient alumina feeding, application of improved construction materials, use of additives, increase of the alumina, cathode and anode quality, introduction of environmental control, e.g. the dry scrubber system which absorbs the fluorides almost completely etc. The current efficiency could be improved by the following means: reduction of the ratio, lower operating temperature and improvement of the raw materials quality [7] [15] [23] [61].

3.3.2 Nowadays challenges

The main focus in the past laid on increasing the current efficiency and reducing the energy consumption. Nowadays it moved to achieving higher amperages and current densities, i.e. a higher productivity.

The productivity is directly proportional to the amperage but with increasing cell amperage the heat balance control becomes more difficult. Other focuses are: improvement of the bath chemistry, development of new sensors and measurement tools which permit a better and quicker analysis of the bath composition, improvement of materials for side-walls, cathodes and anodes, sophistication of the mathematic models, increasing of the cell size and reduction of the energy consumption [15] [23] [27] [36].

4 Bath properties and pot operation

The following chapter gives a general overview over the main bath properties and the effects of additives, mainly aluminium fluoride. The three most important properties for this work, the ratio, the temperature and the resistance are treated in detail, including their interactions, effects on other bath properties, effects of pot operating factors on them and their control. The other properties are mentioned briefly at the end of the chapter. Appendix 2 and 3 give a qualitative overview over the effects of every factor and parameter. Definitions of the most important terms can be found in Appendix 1: Definitions and abbreviations.

4.1 Ratio

Cryolite is the main component of the bath. Additives are used to improve the physicochemical properties of the bath. The most important additive is aluminium fluoride. Three definitions exist to characterise its content in the bath^[1] ; in Europe it is expressed as excess of AlF3 in wt %, in the United States as bath ratio (wt % NaF/wt % AlF3), in the literature sometimes as cryolite ratio (mol% NaF/mol% AlF3) [53] [64]. A low ratio bath is also called acidic. In the presence of additives like LiF or MgF2 these ratio definitions are not valid, other definitions have to be applied. Al2O3 and CaF2 do not influence the definition [12] [16] [29].

To maintain the bath ratio constant is crucial for a stable pot operation. Ratio variations influence numerous operating conditions like the liquidus temperature, the alumina solubility, the electrical and thermal conductivity, the density, etc.

4.1.1 Aluminium fluoride

The concept of ratio was of steadily increasing importance in the last two decades when the smelters started to use more acid baths, i.e. containing more aluminium fluoride. The main advantage is the drop of the liquidus temperature, consequently the operating temperature can be lowered which saves energy and increases the current efficiency. On the other hand the ratio decrease causes an increase in the bath resistivity and voltage. A compromise has to be found between best efficiency and smallest voltage.

Aluminium fluoride is consumed during the process. Even using dry scrubber offgas cleaning systems – which are able to recover > 99 % of the emitted fluorides – AlF3 has to be added regularly, about 15 – 25 kg/ t of produced aluminium [15]. The AlF3-consumption rises with increasing [5]:

- Temperature: the volatility of AlF3 increases
- AlF3 concentration: the partial pressure of AlF3 increases
- Alumina impurities: they react with AlF3 and consume it:

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Equation 4.1

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Equation 4.2

- Water content of the alumina. AlF3 is consumed after the following reaction.

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Equation 4.3

- Cell age.

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Figure 4.1: The system NaF-AlF3 with operating window [9].

Figure 4.1 shows the NaF-AlF3 phase diagram with the operating window of the aluminium reduction. The corresponding liquidus line is very steep, this means that a small change in the bath composition, i.e. a slight ratio variation leads to a large increase or decrease of the liquidus temperature and vice-versa.

In an electrolysis cell exist liquid and solid bath (Figure 3.1). The frozen side-ledge has a different chemical composition from the liquid bath, it consists of nearly pure cryolite. If the pot temperature varies, e.g. decreases during an anode change, bath freezes out. This releases solidification energy, so the operating temperature is kept more less constant, the side ledge acts as a thermostat. But due to this freezing out of bath, the ratio of the bulk bath drops [58].

The tendency to steadily decrease the bath ratio difficults the pot control because the more acid the bath is, the more the ratio varies with the operating temperature. The pots become very sensitive to fluctuations in the heat and energy input, i.e. the amperage [53].

Another difficulty is the time lag that sometimes exists between the addition of aluminium fluoride and the reaction of the system. This “inactive state” of the pots still could not be explained. It is supposed that it is related to dynamic variations in the heat balance and side ledge and top crust melting and solidification processes. Also the aluminium fluoride could react with sodium or sodium fluoride [7] [15].

Other problems that arise when operating above a certain AlF3 content or below a certain operating temperature are: rise of the anode effect-frequency, increase of the sludge formation and an elevated corrosiveness of the bath [28] [54] [58]. In practice most companies work with ratios between 1.08 and 1.12.

4.1.2 Sodium fluoride

Sodium is a constitutional element of cryolite. Furthermore it enters the bath as an alumina impurity. In the molten electrolyte, it is reduced to Na at the cathode [57]:

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Equation 4.4

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Equation 4.5

The sodium reduction leads to changes in the bath ratio (NaF/AlF3) and to bath generation – it is introduced as an impurity and stays in the bath. It is neither volatilised like AlF3, nor reduced to metal like Al [30].

An important effect of NaF is the reduction of the interfacial tension between the bath and the metal, i.e. it destabilises the metal – electrolyte surface [22].

4.2 Temperature

Different temperatures are important for the correct operation of a pot:

- The liquidus temperature (temperature where the bath begins to freeze)

- The operating temperature (bath temperature of the operating pot)

- The superheat (difference between operating and liquidus temperature).

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Figure 4.2: Part of the Na3AlF6 – 5 wt % CaF2 – AlF3 – Al2O3 phase diagram [51].

A constant bath tempera-ture close to the liquidus temperature is important to guarantee a stable pot operation. A low liquidus temperature is one of the main objectives in decreasing the ratio. As shown in Figure 4.2 the liquidus temperature drops with increasing AlF3-con-tent, hence the operating temperature can be lowered [14] [51] [58].

With decreasing bath temperature [30] [48] [58] [64]:

- The side ledge thickness increases; the heat losses are lowered but also the quantity of liquid bath is reduced
- The current efficiency rises, a drop of 10ºC can increase the efficiency up to 2 %
- The evaporation diminishes
- The aluminium solubility in the bath lowers.

The pot has to operate with a certain superheat, usually 8 to 10°C. If the superheat rises too much problems occur [30]:

- The current efficiency drops
- The amount of fumes increases
- The heat losses increase
- The side-ledge melts, impacting negatively on the potlife.

A too low superheat causes [30]:

- The alumina solubility and solution rate to decrease
- Metal rolling; the density difference between metal and bath decreases (Figure 4.6).

The bath temperature is influenced by [14] [30] [48] [51] [58] [64]:

- Current density; the higher it is, the more heat is generated
- ACD; the bigger the distance is, the more heat is generated
- Alumina feeding; the alumina dissolution is an endothermic process, requiring 150 to 200 kJ/mol. After feeding the bath temperature drops up to 5 ºC within seconds. Until the restoration of the original bath temperature it lasts seconds or minutes
- Alumina feeding pauses; in a pause the temperature rises 0.2 ºC/min
- Electrical resistance of the bath
- Metal height
- Length and frequency of anode effects
- Cell operations like anode changing, siphoning, etc
- Side ledge thickness and cell insulation.

To control the operating temperature in daily operation the bath composition, pot voltage and metal height are used. A continuous temperature measurement is not possible yet, in most smelters an operator measures the temperature manually every second day.

As shown in these last two chapters it is important to measure both, bath ratio and temperature to get a complete image of the pot conditions.

4.3 Electrical conductivity, resistance and anode-cathode distance

The main current carriers in the electrolyte are sodium ions [18] [20]:

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Equation 4.6

Losses in current efficiency have two main causes. The most important one is the reoxidation of aluminium in the bath [27]:

Equation 4.7

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Figure 4.3: Electrolyte conductivity and energy liberated in the interpolar gap as a function of the AlF3 content [53] [58].

The other reason is the electronic conductivity caused by the solubility of the metallic aluminium in its own salt, the cryolite. This partly short-circuits the bath [17] [62].

The best current efficiencies are obtained with low temperature and low ratio values.

The electrical resistance determines the stability of the pot. When it varies little, the pot is stable. It is influenced by the [2] [13]:

- Alumina concentration; if the alumina does not totally dissolve, it sinks to the ground of the cell where it forms mud. This mud isolates the cathode and turns its surface irregular. This causes irregularities in the metal pool surface and the current flow like resistance variations and horizontal current flows. As a result the electrical resistance increases

- Bath composition
- Metal height
- Anode-cathode distance.

The anode-cathode distance (ACD) is one of the most important control variables. It directly influences the cell stability through parameters like cell voltage, heat input and noise. If the ACD increases, the cell voltage and heat input increase and the noise decreases. The bath has a low electrical conductivity so the voltage drop is high. This voltage drop generates the heat, which keeps the electrolyte molten. So the ACD is intrinsically related to the heat generation and all its effects like superheat, side-ledge thickness, etc. The variation of the ACD in order to maintain the pot stable is the main theme of this work and was treated in detail in chapter 5.

Usually the ACD is 5 cm; it varies during the pot operation due to [54]:

- Vertical and horizontal current flows in the aluminium pad
- Gas bubbles that are formed under the anode surface and then released. Theu can cause pressure fluctuations and disturb the metal surface, at low ACD the metal surface can break up. A small density difference and a low interfacial tension augment this trend.

A low ACD, a high electrolyte immersion of the anodes and a high anodic current density cause the pot to be instable.

The conductivity and related parameters can be calculated with the following equation:

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Equation 4.8

The cell voltage of about 4.5 V is composed by:

- 1.2 V for the electrolytic reduction of alumina
- 1.5 V for the resistance of the bath
- 1.8 V for cathodic and anodic overvoltages, busbar losses, bubble formation, anode effects, etc.

The ACD cannot be measured continuously because of the corrosive electrolyte. It is estimated through the total cell potential drop and adjusted to keep the cell noise within specified limits.

The tendency of steadily increasing the amperage to improve the productivity, leads to an important increase in the thermal energy input – it is proportional to the square of the amperage. To ensure that the cell will not overheat the thermal insulation can be reduced to augment the heat losses or the conductivity can be augmented which results in a smaller voltage drop in order to avoid the heat generation. The voltage could also be decreased by diminishing the ACD but this reduces the stability of the pot due to an increase of the noise [58] [28].

4.4 Other properties

4.4.1 Alumina solubility

The alumina solubility is the ease and quantity with which alumina dissolves in the electrolytic bath. To ensure a good performance it has to dissolve rapidly, if it is too slow, mud and crust form and disturb the process [30] [64]. The alumina solubility is mainly influenced by bath temperature, ratio, additives, amount of alumina already contained in solution and the physical properties of the alumina like particle size and surface properties [30].

At an elevated alumina level in the bath, sludge forms and deposits under the metal pool, covering the cathode. If alumina depletes, the anode effect occurs: the resistance increases instantly, the temperature rises because of electric arcs, gas bubbles form, the bath composition alters and the side ledge melts [2].

Figure 4.4 shows the Na3AlF6 – AlF3 – Al2O3 phase diagram.

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Figure 4.4: Na3AlF6 – AlF3 – Al2O3 phase diagram [31].

The intersection between the three lines is called the double-solubility or pseudo-eutectic point. When the alumina concentration approaches this point, its dissolution slows down and sludge forms. It can also be observed that the lower the ratio, the lower the alumina solubility. With the trend to steadily increase the AlF3-content of the pots, the alumina control becomes increasingly difficult. [54]

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Figure 4.5: Bath resistance versus alumina concentration [31].

The alumina content cannot be controlled directly. The cell voltage or resistance varies with the alumina content (Figure 4.5). On this basis the alumina feeding is controlled [10].

4.4.2 Density

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Figure 4.6: Density difference evolution from liquid aluminium and cryolite bath with the temperature [5] [45].

The density difference between the metal pool and the electrolytic bath is a crucial element in the aluminium electrolysis. The densities are respectively 2300 and 2100 kg/m3 for the aluminium and the bath (Figure 4.6). The density difference is very small, so little disturbances like a swing in the temperature can cause the aluminium to roll over onto the top. This (partial) roll-over reduces the current efficiency because the metal has direct contact to the anode. The steady reduction of the operating temperature in order to optimise the current and energy efficiency and energy consumption affects negatively the density difference [13]. Other factors influencing the density are the AlF3 and Al2O3 – contents [5] [30].

4.4.3 Surface properties

The surface-active component is NaF. The surface tension mainly affects [13]:

- The electrolyte/metal interface: the surface tension influences the rate of aluminium dissolution, i.e. of metal reoxidation in the bath. The value should be as high as possible to obtain a good separation between the electrolyte and metal

- The electrolyte/carbon interface: it influences the selective absorption of electrolyte components in the carbon lining and the separation from carbon particles of the electrolyte. The smaller the surface tension, the more wettable the carbon

- The electrolyte/carbon/gas interface: it influences the wetting of the carbon material and the onset of the anode effect.

4.4.4 Thermal conductivity

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Figure 4.7: Thermal conductivity as a function of the AlF3 content at different alumina concentrations (numbers given in the picture [wt %]). The curves end where the melt is saturated with alumina [24].

The main influence factor for the thermal conductivity is the bath composition. When the thermal conductivity increases heat is lost, the operating temperature drops and more side ledge is formed.

[...]

^[1] In this work the concept of bath ratio was used, exceptions will be explicitly mentioned.