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U N I T 5

Anode Structure And Property

Although there are two fundamental designs of anodes used in industrial aluminium electro winning, they are both formulated from similar materials and undergo the same range of reactions within the operating cell although their relative importance changes. For environmental reasons, there has been an increasing tendency to move to the prebaked anodes. However, the technical arguments presented are generally equally applicable to the Soderberg anode. The ingredients used to formulate the anodes in both instances are petroleum or pitch coke and a binding pitch (usually coal tar, but in some instances selected grades of petroleum pitch are used). On pyrolysis the pitch rigidly bonds the particulate coke. The specifications and physical properties of the pitch preferred for the two types of anodes differ with regard to such variables as softening point, coking value and viscosity characteristics.

Soderberg anodes.

Soderberg cell designs utilize heat generated within the cell to soften, and subsequently carbonise the pitch binder used to form the coke composite of the active anode. Electrical contact to the anode is achieved by steel pins (about 8 cm diameter), introduced through the pitch-carbon mixture, which is slowly converted to a carbonised mass when moving downwards to hotter zones as a consequence of the anode consumption. Cells with vertical pins are referred to as “Vertical Stud Soderberg” (VSS), while cells with horizontal pins introduced through the side are referred to as “Horizontal Stud Soderberg” (HSS). Of the two types, VSS cells are the more common as they are less labour-intensive and cause fewer operating disturbances. The pins are simply readjusted to compensate for the electrolytic consumption of the carbon.

Preparation of prebaked anodes.

The petroleum coke used as “filler” material for preparation of the anodes has varying structure, depending on the mechanism of its formation. Most of it is formed as a byproduct of petroleum refining processes, where processing conditions are optimised towards the petrochemical industry’s needs rather than the desired structural characteristics of the coke byproduct. Coke formed from coal tar pitch is also used for anodes. While this is invariably more expensive, due to the additional processing steps associated with its formation, its use is increasing. The major advantage of pitch coke is a higher purity, which improves the metal quality. However, the total amount of pitch coke available is limited compared to petroleum coke.

The Anode Effect

Under the constant current conditions used in operating smelters, an anode effect is normally manifested by a sudden increase in the cell voltage. The magnitude of this increase can vary from a few volts up to as much as 50 volts. Anode effects are generally characterized by unstable conditions with an oscillating voltage and arcing at the anode surface. Crackling can be heard, and carbon tetrafluoride (along with minor amounts of other fluorides) is evolved in addition to the oxides of carbon. Invariably in operating cells the onset of the anode effect is associated with depletion in the alumina concentration and the onset of concentration polarization for the normal electrode reaction. For a given alumina concentration and temperature, the current density at which the anode effect occurs, is fairly reproducible and is referred to as the “critical current density”, (CCD). The critical current density is reduced by lowering either the alumina concentration or the temperature of the electrolyte. It also depends on the manner in which the anode effect is induced. Orientation of the electrode has also been

shown to be important, as this affects the ease with which the gas can be removed from the electrode surface.

Theoretically it is reasonable to expect co-evolution of fluorides to occur with an increase in normal anode potential and therefore the cell voltage to increase by approximately 1 volt.

The unstable conditions that result, such as sparking and extreme turbulence, are not predictable, however. A number of explanations of the cause of this instability have been proposed. These include:

an insulating layer of solid electrolyte being formed on the anode

electrostatic attraction of the gas bubbles on the anode

an insulating gas film on the anode, perhaps due to non-wetting effects

kinetic phenomena.

The visual observations of anode effects tend to support the gas film explanation, in that sparking can be clearly observed in a small laboratory cell through the gas bubbles, while there is a strong tendency for very large gas bubbles to remain between the electrolyte and the anode surface. There have been several experimental investigations showing a close reciprocal relationship between the contact angle for anode wetting and the critical current density. This also supports the gas film concept and undoubtedly arises from co-evolution of the fluorides detected in the evolved gas.

Задание

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Выполнение на английском языке

U N I T 9

Cell Dynamics And Quality Control

One of the most common operating faults in the industry has been to make decisions based on the measurement of a single variable. A classic example of this has been control strategies based on a constant cell voltage (or resistance) since cell voltage is also dependent on temperature and alumina content. Although typical operating conditions for a cell can be specified, the value of these variables continuously changes. This is a consequence of such routine operations as alumina feeding, metal tapping or anode changing. Invariably the change in one parameter causes changes in others because of their inter-relationships.

Alumina-induced changes.

This is the most regular disturbance, occurring every feeding period. To maintain a material balance, over 100 kg of alumina must be added within a short space of time. However, in practice, usually slightly less than the theoretical amounts are added and this results in a slow reduction in the peak alumina concentration achieved after each feed, ultimately leading to an anode effect. The alumina added together with the frozen electrolyte contained in the broken crust causes an immediate cooling of the bath. It is another aspect of cell dynamics induced through the intermittent feeding process. Heat adsorption continues for some time as the alumina dissolves and the broken crust melts. Then the temperature slowly recovers although it no steady-state temperature exists. As the alumina concentration is depleted there is a steady increase in temperature.

Changes in temperature or alumina concentration can substantially alter the driving force for heat transfer from the bath through the ledge to the sidewalls. For the higher values of superheat, the thermal resistance of the sidewall can prevent conduction through the sidewalls of the equivalent amount of heat. Therefore the protective side-freeze tends to melt and lowers the thermal resistance. Similarly at low values of superheat that follow alumina addition extra freezing of the ledge occurs since the heat being conducted through the sidewall exceeds that transferred to the interface. Normally the thickness of the side freeze is at maximum shortly after the heavy feeds that follow an anode effect and reaches its minimum during an anode effect. Because of the melting and freezing that occurs on the sidewall there will be a cycle of the bath volume and thereby the height will also cycle. The variation in bath volume can be measured by following the changes in concentration of an additive (such as chloride ion) that may be used as an analytical tracer.

Changes associated with anode effects.

It is well known that the cell voltage rises in the period before the onset of an anode effect. All this increase is applied to the bath volume between the anode and cathode surfaces and is manifested by an increase in the rate of heat generation.

The heat generating voltage within the electrolyte can be measured by subtracting the ohmic voltage drop through the anode and cathode and the voltage equivalent of the enthalpy of reaction from the cell voltage. Working from the arbitrary reference of the minimum cell voltage, which occurs about 50 minutes before the anode effect, there can be calculated the subsequent increase in the rate of heat generation. Often an anode effect is suppressed within half a minute of its onset, but sometimes it persists for several minutes and requires manual suppression.

Quality Control.

The performance of the electrolytic cell, and the quality of the metal produced are directly related to the quality standards maintained for the raw materials, and the composition of the electrolyte used in the smelting process. It is well known that impurities such as vanadium and phosphorous cause a lowering in current efficiency. These and other impurities such as silicon, iron, nickel, also find their way into the metal product through co-reduction. With the more sophisticated applications of aluminium impurities should normally be avoided and it is becoming more important to monitor the purity of the raw materials used. Alumina and petroleum coke receive the greatest attention for purity because they are used in the greatest amount. However, it is also important to monitor the quality of the electrolyte constituent as well.

The alumina feeding technology is based on volumetric additions of the powder at fixed time intervals to maintain the desired concentration range. Thus flow and physical and chemical characteristics that influence the mass of alumina added to the cell in a given volume are important also. Hence angle of repose, bulk and real densities, and moisture content are all important quality standards for smooth cell operation.

ISO methods for feed materials.

Over the years international technical committees have refereed analytical methods and test procedures and these have resulted in a set of ISO Standard Methods. In some instances it is recognized that the methods do not give absolute accuracies, but use of the ISO methods enables direct comparison between smelters and suppliers.

Not only is it necessary to maintain uniformity in analytical procedures, but also it is also important that correct sampling and specimens handling is maintained – thus the ISO standards also encompass these as well as the important physical properties.

Задание

5. Обсудите с партнером проблемы контроля качества производства металла. Используйте материалы публикации. Составьте диалог.

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U N I T 6

Cathodes

Typical analysis of the metal produced in an aluminium smelter can show that it contains liquid aluminium of at least 99.65% purity, and this liquid metal acts as the cathode for the electrochemical reaction. The other components found in the metal generally originate from impurities in the raw materials such as refined alumina, fluorides and anode carbon. The refined alumina composition is dependent on the bauxitic raw material used, but invariably it contains small amounts of compounds containing sodium, silicon and iron.

From a materials balance point of view one would expect that all materials added to the cell would come out as an electrode product or, alternatively, be associated with the fumes evolution. Most of the likely impurities are in-volatile, as fundamental studies have shown that the presence of either alumina or cryolite have stabilizing effects on other oxides such as those of silicon, sodium and titanium. They form double salts with sodium or aluminium compounds present in the electrolyte. Consequently, only a few impurities can be lost as their volatile fluorides.

Cathode polarization.

For many years people have argued as to whether or not the cathode is polarized, but recent laboratory data tend to support that this does occur at high current densities. Because the measurements were made in laboratory cells with different mass transfer conditions, the data should not be interpreted as being representative of the situation in operating cells. However, it is indicative of trends, particularly for bath composition or cathode current density changes. The trends illustrate that lowering the cryolite ratio decreases the cathode polarization. However, the difference for the range of bath compositions used at the operating current densities would be less than 0.15 V. Increasing the current density increases the polarization for all baths. In industrial cells the cathode polarization would be reduced significantly by the high turbulence existing due to the electromagnetic stirring. This stirring accelerates the mass transfer of electrolyte to the cathode surface. However, it should be stressed that the stirring effect is strongly dependent on the design of the cell due to differing interaction between the electromagnetic forces, the depth of the metal pad and the cell condition. In all cases cathode polarization should be reflected by an increase in sodium content above the equilibrium value in the aluminium produced. However, this has not been observed and, in fact, the reverse appears to be the case. With the content of alumina and calcium fluoride in a normal industrial cell, the equilibrium sodium content may be between 130 and 140 ppm if the cell is operating at a cryolite ratio of 2.8. In practice, however, for the same conditions, the sodium content is only about half of the expected value. Cathode Failure.

Chemical reactions in the cathode start immediately after the initial material uptake, but chemical and physical processes continue throughout the life of the cathode lining. Evidence of this is the change in cathode resistance with time as well as the steadily increasing degree of distortion of the steel cathode lining support. Normally cells have to be closed down and have the cathode lining rebuilt after 1200 to 2800 days of operation. There are several different effects that signal the need to shut down a cell. These are:

the iron content in the aluminium produced becomes abnormally and unacceptably high

the voltage drop across the cathode lining, and hence the cell, becomes too high excessive deformation of the steel case is observed upward arching of the bottom of the carbon cathode lining.

If these factors are not closely monitored, a catastrophic failure of the cell may follow. The worst that can happen is to get a tap-out of metal directly through the bottom of the cell or through the bottom along the collector bars. This type of failure makes it necessary to rebuild the whole cathodic part of the cell, i.e. renewing both the bottom and the wall linings. A less severe failure is a tap-out through the side-wall ("side-" or "end-tap-out"). In this case the molten bath and metal penetrate through the lining materials of the sides and they may dissolve the steel shell. The cell has to be shut down, but sometimes only the side-walls need to be rebuilt.

Задание 6

Преобразуйте данные предложения из текста в активный залог.

Обратите внимание, что не во всех предложениях есть дополнение.

1. The measurements were made in laboratory cells with different mass transfer conditions.

2. The data should not be interpreted as being representative of the situation in operating cells.

3. In industrial cells the cathode polarization would be reduced significantly by the high turbulence existing due to the electromagnetic stirring.

4. Cathode polarization should be reflected by an increase in sodium content above the equilibrium value in the aluminium produced.

5. If these factors are not closely monitored, a catastrophic failure of the cell may follow.

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