CA3012163C - Method of monitoring individual anode currents in an electrolytic cell suitable for the hall-heroult electrolysis process - Google Patents

Method of monitoring individual anode currents in an electrolytic cell suitable for the hall-heroult electrolysis process Download PDF

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CA3012163C
CA3012163C CA3012163A CA3012163A CA3012163C CA 3012163 C CA3012163 C CA 3012163C CA 3012163 A CA3012163 A CA 3012163A CA 3012163 A CA3012163 A CA 3012163A CA 3012163 C CA3012163 C CA 3012163C
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anode
current
sensing
assembly
cell
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CA3012163A1 (en
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Jie BAO
Barry Welch
Sergey AKHMETOV
Yuchen YAO
Cheuk-Yi CHEUNG
Ali Jasim BANJAB
Maria Skyllas-Kazacos
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Dubai Aluminium PJSC
NewSouth Innovations Pty Ltd
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Dubai Aluminium PJSC
NewSouth Innovations Pty Ltd
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25CPROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/20Automatic control or regulation of cells

Abstract

A method of anode current monitoring in an electrolytic cell suitable for the Hall-Héroult electrolysis process, said method comprising : - providing a plurality of sensing assemblies (10A, 10B, 10C) at a plurality of locations along the anode busbar, each sensing assembly comprising at least one sensing element (101, 102, 121, 122, 141, 142) and converting means, for converting a measured analog signal into a digital output, - measuring with at least one of said sensing element(s) at least one set of values of a representative parameter of current at at least one sensing time, - digitalizing said analog signals of values into digital outputs, using said converting means, said digital outputs representing the current flow in the anode beam in the vicinity of the sensing assembly having generated said digital output. This method allows to detect and monitor abnormal conditions of cell operation such as anode effects.

Description

Method of monitoring individual anode currents in an electrolytic cell suitable for the Hall-Fleroult electrolysis process Technical field of the invention The invention relates to the field of fused salt electrolysis, and more precisely to the monitoring of the Hall-Heroult process for making aluminium by fused salt electrolysis. In particular, the invention relates to a particular approach determination and monitoring of individual anode currents of the Hall-Heroult type using simple equipment and digital communication for easy wiring, installation and maintenance.
Prior art The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (A1203) is dissolved in molten cryolite (Na3AIF6), and the resulting mixture (typically at a temperature comprised between 940 C
and 970 C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called "pot") used for the Hall-Heroult process typically comprises a steel shell, a lining usually made from refractory bricks, a cathode usually covering the whole bottom of the pot (and which is usually made from graphite, anthracite or a mixture of both), and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte.
Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.7 V to 5 V) which splits the aluminium oxide in aluminium ions and oxygen ions. The oxide ions are reduced to oxygen at the anode, said oxygen reacting with the carbon of the anode. The aluminium ions move to the cathode where they accept electrons supplied by the cathode; the resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface from where it needs to be removed from time to time, usually by suction.
Typical Hall-Heroult cells comprise tens of individual anode assemblies, each anode assembly comprising one (or two) anodes connected to an anode rod, said anode rod being mounted on the anode busbar (so-called "anode beam"). Anodes made from carbon are consumed during cell operation and need to be replaced. As their thickness is decreasing due to burn-off, their height needs to be adjusted regularly. In order to improve the monitoring and control of cell operation, it would be desirable to be able to individual anode currents and their distribution, as an indication of spatial variations of cell operation conditions. However, the currently available cell monitoring systems are largely dependent on the cell resistance. The cell resistance represents a combination of the average anode-cathode distance (ACD), global bath composition and bath properties in the cell, which is
2 not able to reflect the spatial variations in the cell. As such they do not help detect localized abnormalities, such as so-called anode effects, until they become severe and apparent on the cell resistance.
To obtain more information of the process, prior art has proposed several methods in view of measuring the current through each anode. According to a first known solution, anode current is calculated from Hall Effect sensors. Such a method is described for example by J.W. Evans and N. Urata ("Wireless and Non-Contacting Measurement of Individual Anode Currents in Hall-Heroult Pots; Experience and Benefits", Light Metals 2012, TMS, p. 939-942). This first proposition requires however significant modelling effort for each smelter cell design and may incur significant costs in deployment and maintenance. It is limited to low sampling rates (typically 1 Hz) for acquisition of anode current data.
Other prior solutions have proposed to calculate anode current from voltage drop measurement across a length on the anode rod. To this end, individual sensors, which are located on respective anode rods, transmit individually current levels to a remote computer. This solution is for example described in US 4,786,379 and WO
94/02859. This second method has however specific drawbacks. Thus, it can often lead to poor signal quality due to the poor contact of signal pickup points, as the electrical contact to the rod need to be disconnected each time the anode is replaced. Moreover, this may cause damage to wiring.
To eliminate the above drawbacks, it has been suggested to measure voltage points on the anode busbar, the number of voltage measurement points being one more than the number of anodes. The current specific to each rod is then determined from difference calculation (e.g., J. Keniry and E. Shaidulin, "Anode signal analysis: the next generation in reduction cells", Proceedings of TMS Light Metals, New Orleans (2008) p.329-331). This alternative method may however be complex, as it needs to solve a system of simultaneous equations of Kirchhoff circuit laws based on the model of the entire superstructure.
Another approach is based on the calculation of the currents that pass the anode bus bar, at the left and the right of each rod. To this end, the electric potential drops in a specific length are first acquired at the same time. The values of currents in the beam are then obtained by the Ohm's law and the rod current is obtained according to Kirchhoff's law.
(e.g. Li et al., "Experiments on measurement of online anode currents at anode beam in aluminium reduction cells", Light Metals 2015, TMS, p. 741-745).
3 The methods based on the measure of voltage points on the busbar have several advantages, by comparison with methods based on the measure of voltage points on the anode rod. In particular, they lead to more reliable systems, which require less maintenance. However, they imply specific drawbacks, especially for what concerns global structure of the apparatus. Furthermore, the signals transmitted can be susceptible to noises.
The problem that the present invention endeavours to resolve is therefore to propose a method of anode current monitoring in an electrolytic cell, which can be carried out with a simpler apparatus than in prior art. Moreover, the invention wishes to provide such a method, which makes it possible to obtain reliable results, in particular for what concerns the noise issue in signal transmission.
Object of the invention The inventors have identified that the drawbacks of prior art are mainly linked to the way signals are transmitted from the individual sensors to the central unit. Thus, in known methods, these signals are forwarded in an analog form. Therefore, this implies the need for numerous and long signal wires, to carry the raw voltage signals of a fraction of millivolts to the central unit, for signal amplification and data acquisition.
Hence it leads to high wiring and significant maintenance costs. Furthermore the analogue signals transmitted, according to prior art, are highly susceptible to noises, particularly when signal/noise ratio is low.
According to the invention, the above problem is solved by converting the signal into a digital form, in the sensing assembly itself. Therefore, the output of each sensing assembly, also called smart sensing assembly, delivers a digital signal, which is transmitted to the central unit. As only digital signals are transmitted, the data communication is virtually immune to electronic/magnetic interference (EMI) and other sources of noises. The adoption of the digital communication bus minimizes the number of signal wires. For example, only two wires are required for a local controller communication bus if the signals are transmitted electronically.
Typically, representative parameter of current is voltage drop and/or temperature at said location. The individual anode current is determined by measuring the voltage drops and temperatures at each location on the anode busbar. Any appropriate routine can be carried out, such as those described by Yao et al, as well as by Li et al. The routine
4 according to Keniry and Shaidulin ("Anode signal analysis ¨ The next generation in reduction cell control", Light Metals, TMS 2008, p. 838-843) may also be used.
Advantageously, the number of sensing assemblies is equal to the number of anode rods plus one (additional assemblies are needed near risers). Typically, each intermediate sensing assembly is located on the anode busbar, between two adjacent anode rods, whereas each of the two end sensing assemblies is located in the vicinity of the facing anode rod, outside the latter.
Advantageously all sensing assemblies are connected with the central unit through a local controller network bus in a daisy chain configuration. The central unit receives signals in the digital form from the sensing assemblies and computes individual anode currents.
Advantageously, this central unit delivers a cell operating information. To this end, it performs preliminary detection and diagnosis of abnormal conditions. This cell operating information is digitally transmitted to a signal receiver, for example a cell controller or a potroom computer server. Data of this cell operating information can be transmitted electronically (using appropriate signal wires) or optically (using a fiber-optic cable).
A first object of the invention is a method of anode current monitoring in an electrolytic cell suitable for the Hall-Heroult electrolysis process, said cell comprising - a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block carrying at least one current collector bar and two electrical connections points, - a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being contained in an outer metallic shell, - a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and a metallic anode rod connected to an anode busbar (so-called "anode beam"), - a plurality of alumina feeders by which alumina powder is fed into the liquid bath, said method comprising - providing a plurality of sensing assemblies at a plurality of locations along said anode busbar, each sensing assembly comprising at least one sensing element and converting means, for converting a measured analog signal into a digital output, - measuring with at least one of said sensing element(s) at least one set of values of a representative parameter of current at at least one sensing time,
5 PCT/IB2017/050666 - digitalizing said analog signals of values into digital outputs, using said converting means, said digital outputs representing the current flow in the anode beam in the vicinity of the sensing assembly having generated said digital output.
5 In an advantageous embodiment, said method further comprises:
transmitting said digital outputs to a common central unit, and calculating from said digital outputs the current flow of each individual anode assembly.
In specific embodiments, said representative parameter of current can be the voltage drop at the vicinity of said location; each sensing assembly can be a voltage sensing assembly;
.. said sensing assemblies can be all identical, except for their individual address. Their number should be sufficient to enable determination of anode current for each anode assembly. All sensing assemblies are connected with the central unit through a local controller network bus in a daisy chain configuration. A plurality of sets of values of a representative parameter of current, at a plurality of sensing times can be measured by said plurality of sensing assemblies. In an advantageous embodiment each sensing assembly samples at a programmed rate or predetermined multi-sampling rates (including periodically intermittent or on-demand fast sampling). The sampling rate is preferably between 0.01 Hz and 200 Hz.
Advantageously the temperature of the anode beam is measured simultaneously to enable more precise determination of the anode current.
Said central unit delivers a cell operating information; this information can be digitally transmitted to a signal receiver. The process according to the invention can comprise a .. step of detecting abnormal conditions on the basis of said determined individual anode currents. In an advantageous embodiment this step comprises comparing the oscillation of at least one individual anode current with a predetermined threshold, and/or if the FFT
(Fast Fourier Transform) spectrum shows a spike above a given intensity. The method according to this embodiment can further comprise taking samples of individual anode current to determine at least one parameter of bubble dynamics, if the oscillation of at least one individual anode current is superior to said predetermined threshold and/or if the FFT spectrum shows a spike above a given intensity. If said parameter of bubble dynamics decreases, low alumina concentration can be detected. This process of monitoring individual anode current can be integrated into a process control system, and the method can further comprise moving upwards said anode if said parameter of bubble dynamics increases or does not substantially change.
6 Another object of the invention is a sensing assembly configured to carry out essential processing steps of the process according to the invention. Each sensing assembly measures and samples a voltage drop on the anode busbar (left or right to an anode rod). It includes the functionalities of signal amplification, signal filtering, data acquisition, analogue to digital signal conversion, digital signal processing and digital signal (data) communication. These functionalities can be realized by using integrated circuits, typically including an amplifier, an analogue filter, a microcontroller that performs sampling, analogue to digital signal conversion, digital signal processing and a network controller for data communication.
The sensing assembly for use in the method according to the invention comprises:
- a microcontroller configured to carry out essential process steps of the method according to the invention, said microcontroller comprising a CPU, a RAM and a processor, - at least one analog to digital converter capable of digitizing input measured analog signals into digital signals, - an input channel for analog data.
All sensing assemblies are connected with a central unit through a local controller network bus in a daisy chain configuration. The central unit receives anode busbar current signals in the digital form the sensing assemblies, computes individual anode currents, performs data analysis and transmits the cell operating information digitally to an appropriate signal receiver (e.g., a cell controller or a potroom computer server). Data can be transmitted electronically (using appropriate signal wires) or optically (using a fiber-optic cable). As only digital signals are transmitted, the data communication is virtually immune to electronic/magnetic interference (EMI) and other sources of noises. The adoption of the digital communication bus minimizes the number of signal wires (e.g., only two wires are required the local controller communication bus if the signals are transmitted electronically).
All sensing assemblies are typically identical. They may be powered by a common DC
supply which can be integrated into the central unit or alternatively be powered from the cell operating voltage (after DC-DC voltage boost and voltage regulation).
Each sensing assembly unit is identical but has a unique identifier number stored in its firmware. The information of identifier number is used during configuration of the distributed measurement system and is included in the transmission of anode current measurement. In this way, the sensing assemblies can be mass produced and universally
7 installed to reduce their costs and the location of the measurement is also obtained for individual anode current monitoring. This scheme also enjoys easy maintenance, as a faulty unit can be easily identified and replaced.
The central unit processes anode current information received from all sensing assemblies and calculates individual anode currents and performs preliminary detection and diagnosis of abnormal conditions. It transmits processed cell information to a remote computer server. The central unit is programmed such that it can decide the level of details of cell information depending on the process anomalies it detects, to avoid communication congestion.
.. The central unit also performs automatic calibration using an optimization algorithm.
Figures Figures 1 to 15 all relate to embodiments of the invention.
Figure la and lb each show a schematic view of a different embodiment of the invention, showing an anode busbar provided with sensing assemblies and central unit, for carrying a monitoring process according to the invention.
Figure 2 is a flowchart, showing the different steps of an embodiment of the monitoring process according to the invention.
Figure 3 is a schematic view, showing with more details the information exchange .. between the electronic components of the figure la.
Figures 4, 5 and 6 refer to Example 1.
Figure 4 shows schematically the anodes (Al to A20) and the feeders (F1 to F4) in an experiment in which feeder Fl was blocked.
Figure 5 shows the anode current (black line) determined for each of the four anodes (Al, A2, A3, A18) near feeder Fl, as well as the PCF concentration (in ppm) measured in the exhaust gas of this cell (grey).
Figure 6 shows the anode current (black line) determined for each of the four anodes (A8, A9, A10, All) near feeder F4, as well as the PCF concentration (in ppm) measuredin the exhaust gas of this cell (grey).
Figures 7 to 9 refer to Example 2.
Figure 7 shows the current determined over nine days for anode A20 (curve Cl) and anode A19 (curve 02) before and after the replacement of both anodes A19 and A20. It can be seen that the current of anode A20 had increased steadily over more than one week up to an abnormal level.
Figure 8 shows a Fast Fourier Transform (FFT) analysis of the current recorded at anode A19 in stages 1 to 4.
8 Figure 9 shows the same analysis for anode A20.
Figures 10 to 12 refer to Example 3.
Figure 10 shows the current evolution (figure 10a) in Amperes and the current pick-up rate (in Amperes/hour) (figure 10b) of a new anode.
Figure 11 shows the FFT analysis for a new anode after 30 minutes of anode setting.
Figure 12 shows the same analysis after 20 hours of anode setting. Figure 13 shows the current of a new anode with abnormal anode current evolution due to low set position.
Figure 14 shows a functional diagram for the central unit used for the implantation of the process according to an advantageous embodiment of the invention.
Figure 15 shows a functional diagram for the sensing assembly according to an advantageous embodiment of the invention Description 1. General presentation An aluminium smelter comprises a plurality of electrolytic cells arranged the one behind the other (and side by side), typically along two parallel lines. These cells are electrically connected in series by means of conductors, so that electrolysis current passes from one cell to the next. The number of cells in a series is typically comprised between 50 and over 100, but this figure is not substantial for the present invention. The cells are arranged transversally in reference of main direction of the line they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of a respective line, i.e. the circulation direction of current.
Each electrolytic cell substantially comprises a not shown potshell, a superstructure and a plurality of anodes, two of which 2A and 2B are illustrated on figure 1. This superstructure comprises a fixed frame (not shown on the figures) and a mobile metallic anode beam 4, hereafter called "anode busbar", which extends at the outer periphery of the fixed frame.
Each anode 2A, 2B is provided with a respective metallic anode rod 6A, 6B for mechanical attachment and electrical connection to the anode busbar. For example, anode busbar 4 may be provided with any appropriate known means, such as a pair of hooks (not shown on the figures), adapted to cooperate in a usual way with anode rods, for this attachment.
The Hall-Heroult process as such, the way to operate the latter, as well as the cell arrangement are known to a person skilled in the art and will not be described here in more detail. It is sufficient to explain that the current is fed into the anode busbar, flows from the anode busbar to the plurality of anode rods and to the anodes in contact with the
9 liquid electrolyte where the electrolytic reaction takes place. Then the current crosses the liquid metal pad resulting from the process and eventually will be collected at the cathode block. In the present description, the terms "upper" and "lower" refer to mechanical elements in use, with respect to a horizontal ground surface. Moreover, unless otherwise specifically mentioned, "conductive" means "electrically conductive".
2. Sensing assemblies As shown in two different embodiments on figure la and figure 1 b, the anode busbar 4 supports the plurality of sensing assemblies 10. An electrolysis cell with n anodes requires n+1 sensing assemblies plus a number of extra units depending on the number and location of anodic risers 60 (shown on figure 1b). Seven of these sensing assemblies 10A, 10B , 10C, 100, 10E, 10M, 10N are illustrated on figure 1 b; they are connected to a central unit 20. Each assembly is mounted on the anode busbar 4. By way of example, the assembly is housed in a protection case (not shown on the figures) with gasket seals.
As shown in Figure la, each assembly has first sensing elements 101, 102, 121, 122, 141, 142 for voltage and temperature measurements, as analogue signal inputs.
In a preferred embodiment, one sensing assembly is provided between two adjacent anode rods. By way of example, the distance between sensing elements of one single same assembly (e.g. 101 and 102 of 10A) is between about 70 mm and about 120 mm.
Moreover, the distance between facing sensing elements of adjacent different assemblies (e.g. 102 and 121 of 10A and 10B) is of the order of the distance between anodes. Each sensing assembly is linked to the adjacent assembly (assemblies) by a DC power supply cable 50 (shown on figure 1b).
As will be explained in more detail below, the sensing assembly 10 also includes an analogue low pass filter, a signal amplifier, as well as a microcontroller or equivalent unit, configured to execute the process steps necessary to carry out the process according to the invention. The microcontroller typically comprises a microprocessor with analogue/digital converters, digital input/output channels, random access memory, EPROM, solid state storage and communication controller. The sensing assembly
10 is electrically linked to a local controller area bus (typically a CAN bus 30) transceiver for digital communications. The local controller area bus transceiver implements digital signal by-passing to ensure uninterrupted transmission from normal smart sensing assemblies to central unit, should one sensing assembly be faulty.
Figure 3 illustrates data transfer means along the superstructure, in a more detailed manner. This figure schematically shows assemblies 10A, 10B and 10C of figure la, as well as assembly 10N which is located at the end of the busbar opposite to assembly 10A.

Each assembly is provided with a respective microcontroller 11A to 11N. Each microcontroller has a respective input 12A to 12N, which receives data from sensing elements (shown on figure la but not on figure 3). Each microcontroller has moreover a respective output 13A to 13C, which delivers data in a digital form.
5 Each assembly is also provided with two connecting ports 14A to 14C, as well as 15A to 15C which are connected in a daisy chain. The terminal sensing assembly 10N
has only one connecting point 14N. First connecting port 14A of end assembly 10A is linked to central unit 20 via line 25 which is advantageously a CAN bus. Moreover, second connecting port 15A of end assembly 10A is linked to first connecting port 14B
of adjacent 10 assembly 11B via CAN bus 30A. Each intermediate assembly (like 10B), i.e. each assembly which is not provided at one end of the chain, is linked with a first adjacent assembly (like 10A) at its first port (like 14B) via a first CAN bus (like 30A), and is linked with a second adjacent assembly (like 10C) at its second port (like 15B) via a second CAN bus (like 30B).
Figure 15 shows a functional diagram for the sensing assemblies 200 according to an advantageous embodiment of the invention. The large arrows indicate flux of data.
Sensing assembly comprises a microcontroller 210. Said microcontroller 210 comprises a CPU 211, a RAM 212 and a chip 213 with embedded firmware containing a computer program that enables the microcontroller 210 to execute essential steps of the process according to the invention; said chip 213 may also include the identifier of the sensing assembly. Analog data enter the microcontroller 210 through analog input channels 214 and are digitized by an analog-to-digital converter 215.
Anode beam voltage is measured at the anode beam by a voltage measurement signal pickup 250; this signal is amplified by a beam voltage amplifier 251, and filtered by a low pass filter 210 before entering the microcontroller 210 through one of the analog input channels 214. Anode beam temperature is measure at the anode beam by a beam temperature sensor 253; this signal is amplified by a temperature signal amplifier 254, and filtered by a low pass filter 255 before entering the microcontroller through one of the analog input channels 214.
The microcontroller 210 receives electrical power 219 from the central unit 100 (figure 14) through a power regulator and isolator 221 and an overvoltage protector 222.
Part of the power 224 is derivated through a power converter 225 to supply appropriate electrical power to the beam voltage amplifier 251 and the temperature signal amplifier 254.
11 Said microcontroller 210 exchanges digital data with the neighbouring sensing assemblies (and possibly with the central unit 100 if adjacent to the sensing assembly under consideration) through a CAN bus receiver 232, a digital signal isolator 231 and a CAN
bus connector 233 for a Daisy chain configuration.
3. Central unit The central unit 20,100 typically comprises a microprocessor, a local controller area bus (CAN bus), transceivers for communications with all sensing assemblies and an Ethernet network adaptor or a fibre optic network adaptor for communication with at least one remote computer servers. The central unit is also equipped with a power regulator with overvoltage protection to produce regulated voltage from smelter cell voltage and provide power to all sensing assemblies through the same daisy chain connection as used for data communications.
Figure 14 shows a functional diagram for the central unit 120,100 according to an advantageous embodiment of the invention. The large arrows indicate flux of data. The central unit 100 comprises a microcontroller 110 and a power unit 120. The microcontroller 110 comprises a CPU 111, a RAM 112 and a chip with embedded firmware 113 containing a computer program that enables the microcontroller to execute essential steps of the process according to the invention. Since the process generates huge amount of data, these data may be stored on additional solid state memory 115, possibly externally, and/or additional RAM chips 114 can be provided, possibly externally.
The microcontroller 110 receives electrical power from a power unit 120. Said power unit 120 receives electrical current 119 from the cell voltage; it comprises a power regulator and isolator 121 and an overvoltage protector 122. Said power unit 120 also supplies electrical current 123 to the sensing assemblies 200.
Said microcontroller 110 exchanges digital data with the first sensing assembly through a CAN bus connector 130, a digital signal isolator 131 and a CAN bus transceiver 100. Said microcontroller 110 also exchanges digital data with the local area network through an Ethernet transceiver and network controller 140. If this data exchange is carried out through optical fibres, an Ethernet-to-optical fibre converter 141 is necessary.
4. Use In use, each sensing assembly samples, at a programmed rate or prescheduled multi-sampling rates (including periodically intermittent or on-demand fast (high-frequency) sampling). For example, the duration between two successive samplings may be between
12 200 Hz) and about 0.01 Hz, and preferably between about 10 Hz and about 0.5 Hz. Each pair of sensing elements permits acquisition of values of a representative parameter of anode current, such as voltage drop between the two locations of the elements of said pair.
Sampling rate may be constant or variable. In an embodiment the sampling rate is constant, for instance between 0.1 and 0.01 Hz. The inventors have found that this sampling rate represents a sensible compromise between the depth of investigation (which would render it desirable to monitor the process continuously at high sampling frequency) and the constraints posed by the transmission and handling of such an .. enormous amount of data.
In another embodiment the sampling rate is scheduled in advance, and there is a base sampling rate (as in the first embodiment) on which short periods of faster sampling rate are superimposed at intervals that are regular or irregular, and for durations that are constant or variable. Such periods of faster sampling rate may be of the order of 1 to 100 Hz. In an advantageous embodiment the interval between two periods of fast sampling rate are of the order of 20 minutes to 60 minutes. The sampling can also be modified as a result of the monitoring, in order to cope with specific conditions of the pot.
The need and sampling frequency of high frequency sampling depends on the events and specific conditions that are supposed to be monitored. FFT analysis can be applied on any kind of periodically sampled data, but the sampling frequency needs to be commensurate with the time scale of the specific event to be monitored. As an example, bubble dynamics typically shows up on a FFT spectrum between 0.5 and 1 Hz, which requires a sampling rate in the order of 10Hz. (Theoretically 2 Hz would be sufficient, but practically 10 Hz are preferred here due to imperfect low pass anti-aliasing filters).
.. In one embodiment there is a constant sampling rate at a frequency f1 (typically comprised between 0.01 and 0.1 Hz on which are superposed bursts of high frequency sampling at a frequency f2 comprised between 0.1 and 10 Hz; the duration d2 of such a high frequency sampling burst is typically between 0.2 and 3 minutes (preferably between 0.5 and 2 minutes), and the spacing between two of such high frequency sampling bursts is typically between 0.2 h and 2 h (preferably between 0.5 h and 1 h).
Each anode current, i.e. the current value in each anode rod, is then calculated upon the basis of the above values of the representative parameter. This calculation is carried out according to any appropriate known method. For example, considering anode rod 6A, the sensing members of 101/102 and 121/122 of the respective assemblies 10A and
13 calculate the currents that pass the anode busbar, at the left and the right of said rod. To this end, the electric potential drops in a specific length are first acquired at the same time.
The values of currents in the beam are then obtained by the Ohm's law and the rod current is obtained according to Kirchhoff's law, as described by Li et al.
("Experiments on measurement on online anode currents at anode beam in aluminium reduction cells", Light Metals 2015, TMS, p. 741-745).
The input analogue signal of each sensing assembly is of the order of a few millivolts, depending on the distance between the voltage measurement points and the busbar materials. This input signal is filtered and amplified and converted to digital form by the microcontroller. The latter also calculates current on the busbar, using the Ohm's law and calibration algorithm. The sensing assembly digitally transmits the busbar current values to the central unit via a controller area network bus (e.g., CAN bus) electronically, or via an optic fiber (e.g., using a CAN bus-to-fiber converter).
The central unit collects all busbar current values and calculates (by Kirchhoff's current law) and analyses all anode current measured simultaneously. Current values on the anode beam can be calculated by sensing assemblies. The anode current values can be calculated by the central unit. In an embodiment, the central unit receives anode busbar current signals in the digital form from the smart sensing assemblies, computes individual anode currents, performs data analysis and transmits the cell operating information digitally to an appropriate signal receiver (e.g., a cell controller or a potroom computer server). Data can be transmitted electronically (typically by using appropriate signal wires or wireless transmission means) or optically (using a fiber-optic cable).
In a more detailed manner, referring to figure 3, input 12A to 12N of each microcontroller 11A to 11C receives data from sensing elements, in analog form. The output 13A
to 13N
of each microcontroller delivers data in a digital form.
We describe here the process stages carried out with in a typical embodiment of the invention.
All sensing assemblies are installed on the anode busbar. An electrolysis cell with n anodes requires n+1 sensing assemblies plus extra units depending on the number and location of anodic risers. One sensing assembly unit is placed on each side (left and right) of every anode rod.
(i) Analogue voltage signals across a distance on the anode busbar at the locations left and right of anode rods and the temperature at the above locations are acquired using analog input channels 12A to 12N of the microcontroller. For example, the voltage drop
14 between 101 and 102 and the local temperature are measured using analogue input channels 13A and 12A of sensing assembly 10A.
(ii) The voltage and temperature signals are then converted into a digital form using the Analogue to Digital Converter (typically built in the microcontroller), 11A in this example.
Using the resistivity of the anode busbar corrected with temperature, the current on the anode busbar i is calculated from the above voltage drop. The beam current at the location left to anode 2A is then calculated using the following formula:
V
/A -(a + bT)-1 A
where V is the voltage drop measured, / is distance on the anode busbar between the voltage measurement points, A is the cross sectional area of the anode busbar, and T is the temperature measurement. This temperature correction is necessary because the impact of small variations of the busbar temperature on its electrical conductivity is sufficient to perturbate the busbar resistance and thus the determination of the beam current.
Similarly, the beam current at the location right to anode 2A can be determined using sensing assembly 10B, denoted as /B.
(iii) All the data representative of current at different locations on the busbar is transmitted to the central unit through all sensing assemblies (connected in a daisy chain-tyoe arrangement) using a communication hardware and software protocol (typically CAN bus) in every sampling period. The central unit processes these data to determine anode currents values. Each anode current is determined from the difference between the beam current left and right to the anode rod. For example, the current of anode 2A, denoted as /24 can then be calculated from the following equation:
/2A = - /B.
The central unit also communicates with a cell controller or a computer server via a local area network (e.g., Ethernet) using, e.g. the deterministic TCP/IP protocol.
At the locations where there are risers, additional sensing assemblies are needed. For example, as shown in Figure 1, due to the riser, an additional sensing assembly 100 is required to calculate the anode current for 20, which is the difference of the beam current from 100 and that from 10E.

(iv) The above analog signals can be sampled at different sampling rates, as scheduled or on demand, which can be programmed in the software (in the firmware) of the micro controller.
For example, normal sampling rate can be 2 Hz to observe the trend of individual anode 5 current. Periods of fast sampling can be scheduled, e.g., to have a 1 minute period for every 30 minutes, in which a 10Hz sampling rate is implemented. This is useful to obtain fast current signal to reflect CO2 bubble dynamics. The Central unit sends commands to all sensing assemblies to apply the scheduled fast sampling rate (to all sensing assemblies) and return to normal sampling rate after 1 minute.
10 The central unit can also start a fast sampling period triggered by abnormalities detected from certain anode current signals (e.g., 2 Hz signals). In this case, the fast sampling rate (e.g., 10Hz) is applied to sensing assemblies located to the left and right of the anode in question. For example, if anode 2B is in question, a fast sampling rate (e.g., 10 Hz) will be applied to sensing assemblies 10B and 10C for a period of time (e.g., 1 to 5 minutes) to
15 collect more information on bubble dynamics. The beam current signal from 10B will be down-sampled, in this example, to 2Hz so that it can be used, together with the beam current signal from 10A (2Hz data) to calculate the anode current of 2A.
The beam current 10N and 10M are used to calculate the current of anode 2M.
All anode beam current information is transmitted from the originating assemblies to the central unit through all intermediate assemblies. As an example, anode beam current information is transmitted from originating end assembly 10N to then central unit 20 via the data communication bus through all intermediate assemblies 10(N-1) to 10A (without the intervention of these assemblies). This allows the central unit to calculate all anode currents.
The central unit also decides the sampling rates depending on the anomalies it detects to obtain richer process data for diagnosis of abnormal operating conditions. For example, the central unit transmits processed anode current information to above mentioned remote computer server via a local area computer network (LAN). During normal operations, less detailed current distribution information is transmitted to avoid communication channel congestion. When certain abnormal condition is detected, significant details of the process data are communicated with the remote computer server to allow further diagnosis. This approach allows implementation of very high sampling rate to detect fast process dynamics (e.g., bubble dynamics) without causing the burden of large LAN
throughput in large scale deployment. The central units also coordinate with each other to give a high priority to the faulty cell to transmit detailed process data.
16 5. Detection of abnormal conditions Abnormal conditions of cell operation can arise in many circumstances, but one of the most intricate abnormal condition is the so-called anode effect. Well-known to persons skilled in the art, this effect is related to the built-up of an insulating gas layer under the anode, leading to an increase in anode potential. Indeed, during the course of the cell operation the anode is consumed and its carbon reacts with the oxygen released through electrolysis to form carbon dioxide. These carbon dioxide bubbles that form mainly underneath the anode need to be released continuously. The dynamics of such gas bubbles leads to a perturbation of anode currents that can be detected as electric noise .. (oscillation) on the current signal.
Other abnormal conditions that can be monitored by the method according to the invention are blocked alumina feeders or defective crust breakers, errors in manual setting of anodes, data input errors in the pot control system.
.. An example of a detection and diagnosis of abnormal conditions, carried out according to the invention, will now be described referring to the flowchart of figure 2.
All the steps of this flowchart are carried out for at least one anode of the cell, advantageously for the majority of these anodes and, preferably, for all the anodes of the cell.
While the detection/diagnosis method and the instrumentation scheme are independent to each .. other, the instrumentation scheme according to the invention does provide an effective way (requiring low maintenance and is less susceptible to electronic magnetic interference) to collect information required for the detection/diagnosis method.
a/ At step 100, the individual anode current (hereafter IAC) is calculated, and its .. oscillation is determined. Current oscillation can be determined by performing a Fourier transform on the current signal around a selected frequency. Then, the determined oscillation is compared with a predetermined threshold.
b1/ If the value of oscillation is superior to said predetermined threshold, anode current will be sampled at a higher rate (e.g. 10 Hz) over a period advantageously comprised between 10 sec and 100 sec (typically 1 minute) to capture bubble dynamics and its variation.
c1/ At downstream step 200, if a decrease in bubble dynamics has been determined from certain anode current signal, it implies reduced alumina concentration in the vicinity of the above anode, at stage 210.
17 c2/ On the other hand, if an increase or a lack of change in bubble dynamics has been determined, the magnitude M of the considered IAC is used to determine if there is anode setting issues.
Let us consider that this magnitude fulfils at least one of the following criteria:
- M is superior to x% of average IAC, where x is a predetermined value, and "average IAC" is calculated upon the basis of the IAC of all the anodes of the cell.
- M is superior to the sum of the current of multiple anodes:
If at least one criterion is met, this means that the considered anode is set too low.
Therefore, at step 221, this anode is required to be moved upwards by a certain distance.
If the magnitude M does not meet the criteria of step 221, no fault is detected from anode current signals.
b2/ If the value of oscillation is inferior to said predetermined threshold, the magnitude M
of the considered IAC is used further to at step 300 to determine the root cause.
c1/ Let us consider that this magnitude fulfils at least one of the following criteria:
- M is superior to x% of average IAC, where x is a predetermined value, and "average IAC" is calculated upon the basis of the IAC of all the anodes of the cell.
- M is superior to the sum of the current of multiple anodes:
If at least one criterion is met, and IAC noise level increases (step 310), this means that the considered anode is set too low. Therefore, at step 311, this anode is then moved upwards by a certain distance.
If the magnitude M does not meet the criteria of step 310 and/or if IAC level does not increase at step 310, no abnormalities are detected from individual anode current measurement.
c2/ Let us consider that this magnitude fulfils at least one of the following criteria:
- M is inferior to x% of average IAC, where x is a predetermined value and "average IAC" is calculated upon the basis of the IAC of all the anodes of the cell
18 - M is inferior to multiple anodes (see above explanation).
If at least one criterion is met, this means that the considered anode is set too high.
Therefore, at step 320, this anode needs to be lowered by a certain distance.
If at step 300 none of the possibilities 310 and 320 are fulfilled, no abnormalities are detected from individual anode current measurements.
6. Calibration According to an advantageous embodiment, the central unit 20 also performs automatic calibration using an optimization algorithm. It collects anode bus bar current measurements during each time when an anode is removed and uses them to progressively improve the calibration accuracy to capture the effects of the time-varying operating conditions and properties of bus bar materials, using a recursive optimization algorithm.
Due to the spatial variations in the properties of bus bar materials (e.g., resistivity) and actual distance between signal pickup points, proper calibration is advantageously required. When an anode is removed, the anode rod current, i.e. the difference between the currents on the anode bus bar left and right to the anode rod, will be zero. The data collected each time when an anode is removed for a cycle of anode replacement are used, together with the line current, to determine the calibration factor for each pickup point using an appropriate optimization algorithm.
During the j-th anode setting, the difference of anode bus bar current left and right to the anode rod, denotes as /i+ and If are expected to be zero, with respect to a reference current direction. Assume that there are N number of anodes and there will be N rounds of anode setting practice. The objective of the calibration is to minimise each individual anode current during anode settings.
All the cells are connected in series therefore the total line current is maintained. Hence, the sum of all individual anodes current at any given time has to equal to the total line current, denoted as IL.
Denote 07 and ar the calibration factors for the anode busbar current measurement left and right to the j¨th anode rod respectively. The calibration algorithm can be written as the following optimization problem with all the calibration factors as the decision variables:
19 min IVY ¨ )2 I ¨
\
where W is a weighting to penalize the error in individual anode current measurement during anode setting.
Constraints can be placed on the calibration factors to reflect the effect of the anode busbar structure on the resistivity at different locations.
The above optimization problem can be written in a recursive form so that the changes in the calibration factors, sa7 and AaT, are determined as the decision variables to improve the calibration results after N set of measurements during anode setting have been collected and used for initial calibration.
7. Additional advantages The invention can be carried out by using a distributed instrumentation scheme with smart sensing assemblies with digital communication for real time continuous individual anode current measurement. The smart sensing assembly has its own processor which takes the local voltage measurements, converts the signal to digital via an A/D
converter and carries out signal processing and communicates to other sensing assemblies and the central unit in digital form.
As explained above, the invention has many advantages for monitoring electrolytic cells in a Hall-Heroult electrolysis plant. By itself, such a structure of a distributed instrumentation scheme according to the invention provides additional advantages, such as;
(i) By using a digital communication daisy chainõ the number of wires needed for transmitting the measurement of local cell conditions is significantly reduced to theoretically two wires plus wires for power supply. This leads to significant benefits of reduction of workload of installation and maintenance.
(ii) By using digital communication, the signal transmission is immune to noises which can be a significant problem in instrumentation for Hall-Heroult cells due to the small voltage signals (several milli-volts) and significant level of noise caused by strong electrical and magnetic fields surrounding the cells.
(iii) Smart sensing assemblies process signals and provide functionalities including calibration, preliminary detection of anomalies of local cell conditions.
(iv) Each unit is identical but has a unique identifier number stored in its firmware. This scheme also enjoys easy maintenance, as a faulty unit can be easily identified and replaced.

Examples Operating modern cells at low energy input and electrolyte volume can be improved by monitoring individual anode current signals. Several publications have shown the impact 5 of increasing alumina concentration gradient while operating at a lower electrolyte volume.
Sequentially, an increase in anode potential beyond the enabling limits of the co-evolution of fluorocarbon species permits the formation of bubble resistive film under the anode and reduces anode wettability.
10 Example 1: PFC detection In this example, the impact of a blocked feeder on the spatial variations in alumina concentration in the electrolyte across a Hall-Heroult reduction cell was reflected and detected by individual anode current distribution. The effect of the resultant non-uniform 15 current distribution on the initiation of PFC emissions can be analyzed for corner and central anodes.
Individual anode electrodes are designed to operate under similar conditions of electrolyte composition and electrode potential. However, because of non-uniform alumina dispersion
20 in the electrolyte, the behavior of individual anodes will be distinctive with a risk of individual electrode potentials shifting to levels corresponding to the discharge of fluoride ions, with resultant PFC formation. Formation of PFCs is undesirable for environmental reasons and is indicative for the cell operating at lower current efficiency;
PFC
accumulation can lead eventually so-called anode effects.
In this example, an industrial Hall-Heroult cell comprising twenty anodes (A1-A20) and four feeders (F1-F4) was used. Figure 4 shows schematically the positioning of the feeders. One of the feeders (F1) was blocked, and this introduced an overall alumina concertation gradient in the cell. During this time, the total amount of alumina supply to the cell remained the same while the feeding rate proportionally was increased in the other three feeders. An increase in the co-evolution of fluorocarbon species was triggered shortly after blocking feeder F1 and this was associated with current redistribution predominately in the anodes around feeder F1. To facilitate the analysis, the cell was divided into four zones based on the feeder's location as illustrated by the different grey scheme in Figure 4.
21 The steady depletion of alumina in the vicinity of the anodes near feeder Fl (anodes Al, A2, A3 and A18) has resulted in a lowering of current at these anodes as shown in Figure 5.
On the other hand, due to constant cell current condition and better alumina concentration in other zones of the cell, an increase in current for anodes adjacent to feeder F4 (anodes A8, A9, A10, All) occurred as shown in Figure 6.
The example shows quick response on the co-evolution of fluorocarbon species when introducing a spatial change in alumina concentration in a modern industrial cell. This demonstrates the capability of the process according to the invention to detect at an early stage abnormal operating conditions that may lead to background PFC emission or even anode effects.
Example 2: Spike detection Irregular variation in individual anode current magnitude is suggested as an effective tool to detect local abnormalities in Hall-Heroult cells. An example supporting this was the formation of a spike in corner anode A20 of the same cell as illustrated in Figure 4 which was removed at 70% of its expected service life because its current has increased steadily to 50% in excess of the average. This is illustrated in Figure 7 where the curve Cl represents the typical current determined for anode A20 and curve 02 represents the typical current as determined at adjacent anode Al 9 that was changed at the same time.
Time domain analysis was divided into four stages as summarized in Figure 7 and Table 1. As summarized in Table 1 below, the diversion was linked with other anode changes that also impacted superheat, bath flow and mixing.
Table 1:
Analysis of anodes Al 9 and A20 during spike formation process.
Anode A19 Anode A20 Stage current current Comment magnitude magnitude Normal Normal Both anodes operate normally (prior of replacing 1 current current anodes 2 and 3).
(-1% from (+11% from target) target) Age of both anodes is 13 days.
22 Normal Drawing A reduction in superheat under corner anode A20 abnormal due to changing neighbour anodes.
current 2 higher (+12% from current by Presence of any undissolved material as crust lump target) 24% or carbon dust results in initiation of anode spike.
N Drawing Anode A20 continues to draw high abnormal ormal abnormal current which suggests that spike is already current 3 higher formed and current is flowing through the lower (-8% from current by ohmic resistance path due to short-circuit (localized target) 27% low ACD).
Drawing Normal abnormal As spike extends into metal pad, current draw of current hi her anode A20 further increases abnormally.

(-12% from g current by target) Age of both anodes is 21 days.
30%
The above analysis shows that the spike has formed as a consequence of the low superheat zone due to the recent anode set combined with the possible presence of undissolved material or carbon dust under the corner anode which strongly effects spike formation in anode A20. Furthermore, activities related to anode setting, such as cavity cleaning, could physically direct undissolved material to accumulate under the corner anode.
Fast Fourier Transformation FFT analysis of bubble frequency and amplitude components of anode A19 (at 10 Hz) did not show significant dynamics during stage 1 since the anode was still partially slotted. Both anodes require an extra day to complete the slot consumption process. As the slots start to disappear in stage 2, however, a bubble peak in the frequency range of 0.7 to 1.0 Hz starts to appear as illustrated in Figure 8. In stages 3 and 4, anode A19 has continued to show the normal bubble peak due to the release of cell gases.
Similarly, anode A20 shows the slight appearance of the same bubble peak at 1.0 Hz as anode A19 in stage 1 (see Figure 9). However, a change in anode A20 frequency response was observed for the first time in stage 2 where some low frequency component appears with a slight bubble peak shift to 1.5 Hz. When an anode is partially shorting and is therefore drawing a higher current the electrode potential is automatically lowered, and therefore the electrochemical current density will drop, thus lowering the rate of gas formation and changing its release frequency. The change could also be related to the
23 presence of a low superheat zone with some undissolved material which hinders bath mixing and initiates electrode potential changes as well as localized MHD
instability (Figure 9). It is noted that a new peak starts to appear in stages 3 and 4 where the bubble components split into two peaks, consistent with a change in gas release pattern due to the presence of the spike.
Example 3: Monitoring the new anode current pick up curve In the presented study involving thirty repeated experiments where anode setting reference was fixed, a rapid initial increase in current pick-up rate during the first two hours changed to a slower uniform rate for the next twelve hours as illustrated in Figure 10. Occasionally small spikes occurred as highlighted after nine hours in Figure 10b. This type of spike is probably due to detachment of pieces of the frozen layer followed by re-freezing a new insulating coating. Removal of the coating may be aided by the induced bubble forces. The first two stages were confirmed by photographic pictures taken of new industrial anodes after anode setting for one, eight and twelve hours in three neighboring cells for the same stall number.
In the first one hour after anode setting, bath freeze has developed on the bottom and side vertical plane of the anode surface. The side freeze is noticeably thinner than at the bottom. On the bottom, its thickness was 35-40 mm which matches the predicted inter-electrode distance.
Fast Fourier Transform (FFT) analysis of the new anode current signal at 10 Hz data sampling frequency after 30 minutes of anode setting in Figure 11 shows a noisy signal which is attributed to MHD instability resulting from the anode bottom freeze.
The frequency response spectrum of anode current has shown a high bubble dynamic at low frequency range which perhaps due to energy input deficit at stable magnetohydrodynamic conditions following anode replacement. The early current pick is expected to be on the lower side of the anodes adjacent to those unchanged because of the mixing and heat transfer enhanced by their gas release.
The early current carried (presumably by the side) steadily increases to about 10% of the final current in the first two hours as seen in Figure 10 and then changes to a slower more uniform rate. During the second phase, the remainder of the freeze, which presumably is predominantly underneath the anode, dissolves slower and the rate of current pick up drops off. This is consistent with the second phase of Figure 10 and the photographical
24 observations reported above. The slow dissolution of freeze increases the available surface area.
After about twenty hours the process enters a third stage which exhibits low frequency dynamics for the same anode, imposing a clear path for bubbles to escape through the slotted anodes. In this stage the rate of current pickup drops off further, perhaps due to the continuing very low superheat of the adjacent anode and the balance in competition between the entropic energy" demand for the electrode reaction and for the energy transfer for heating the anode. This is confirmed by the absence of the bubble peak at 1 Hz frequency in Figure 12, and the appearance of a peak at 0.5 Hz.
While monitoring the current pick up, in some instances a more rapid rate of increase was observed perhaps due to freeze detachment or very low setting height of the anode as illustrated in Figure 13.

Claims (18)

25
1. A method of anode current monitoring in an electrolytic cell suitable for the Hall-Héroult electrolysis process, said cell comprising - a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block carrying at least one current collector bar and two electrical connections points, - a lateral lining defining together with the cathode a volume containing a liquid electrolyte and a liquid metal resulting from the Hall-Héroult electrolysis process, said cathode and lateral lining being contained in an outer metallic shell, - a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode (2A, 2B) and a metallic anode rod (6A, 6B) connected to an anode busbar (4) (so-called "anode beam"), and - a plurality of alumina feeders by which alumina powder is fed into a liquid bath, said method comprising - providing a plurality of sensing assemblies (10A, 10B, 10C) at a plurality of locations along said anode busbar, each sensing assembly comprising at least one sensing element (101, 102, 121, 122, 141, 142) and converting means, for converting a measured analog signal into a digital output, - measuring with at least one of said sensing element(s) at least one set of values of a representative parameter of current at at least one sensing time, and - digitalizing said analog signals of values into digital outputs, using said converting means, said digital outputs representing the current flow in the anode beam in the vicinity of the sensing assembly having generated said digital output.
2. The method according to claim 1, further comprising - transmitting said digital outputs to a common central unit (20), - calculating from said digital outputs the current flow of each individual anode assembly.
3. The method according to claim 1 or 2, characterized in that each sensing assembly is a voltage sensing assembly.
4. The method according to any of claims 1 to 3, characterized in that said representative parameter of current is the voltage drop at the vicinity of said location.
Date Recue/Date Received 2023-03-08
5. The method according to any of claims 1 to 4, wherein the temperature of the anode beam is measured simultaneously to enable more precise determination of the anode current.
6. The method according to any of claims 1 to 5, characterized in that the number of sensing assemblies is sufficient to enable determination of anode current for each anode assembly.
7. The method according to any of claims 1 to 6, characterized in that all sensing assemblies are connected with the central unit through a local controller network bus in a daisy chain configuration.
8. The method according to any of claims 1 to 7, characterized in that said central unit delivers a cell operating information.
9. The method according to any of claims 1 to 8, characterized in that said cell operating information is digitally transmitted to a signal receiver.
10. The method according to any of claims 1 to 9, comprising measuring with said plurality of sensing assemblies a plurality of set of values of a representative parameter of current, at a plurality of sensing times.
11. The method according to claim 10, characterized in that each sensing assembly samples at a programmed rate or predetermined multi-sampling rates.
12. The method according to claim 11, characterized in that the sampling rate is between 0.01 Hz and 200 Hz.
13. The method according to any of claims 1 to 12, comprising detecting abnormal conditions on the basis of said determined individual anode currents.
14. The method according to claim 13, characterized in that detecting abnormal conditions comprises comparing the oscillation of at least one individual anode current with a predetermined threshold, and/or if the FFT (Fast Fourier Transform) spectrum shows a spike above a given intensity.
15. The method according to claim 14, further comprising taking samples of individual anode current to determine at least one parameter of bubble dynamics, if the oscillation of Date Recue/Date Received 2023-03-08 at least one individual anode current is superior to said predetermined threshold and/or if the FFT spectrum shows a spike above a given intensity.
16. The method according to claim 15, detecting low alumina concentration if said parameter of bubble dynamics decreases.
17. The method according to claim 15 or 16, further comprising moving upwards said anode if said parameter of bubble dynamics increases or does not substantially change.
18. A sensing assembly for use in the method according to any of claims 1 to 17, comprising:
- a microcontroller (210) configured to carry out essential process steps of the method according to claims 1 to 16, said microcontroller comprising a CPU, a RAM and a processor, - at least one analog to digital converter (215) capable of digitizing input measured analog signals into digital signals, - an input channel for analog data.
Date Recue/Date Received 2023-03-08
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