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Individual Anode Signals

There have been several studies that have used digital signal processing to analyse the signals from the current through an anode rod (Barber 1992; Walker 1995), the magnetic Chapter 5 – Industrial Electrolysis field produced by the current through an anode rod (Barclay, Hung, and Rieg 2001) and the vibration of an anode rod (Xue and Oye 1999).
One study measured the voltage fluctuations in each anode in two industrial cells at 50 Hz for 20 second bursts every three hours (Barber 1992). One cell was a conventional Hall- Héroult cell and the other was a drained cathode cell, which is an experimental, modified Hall-Héroult cell that contains a cathode material that is wetted by molten aluminium (Thonstad et al. 2001: 333). Measurements were performed on anodes with 4o slope and on flat anodes (Figure 5.5).
The DC component, AC component, AC/DC ratio and dominant frequency were determined as described in Chapter 4.4. Results after two and 12 days are summarised in Table 5.2 (the anode rota was 13 days). The differences discussed below have been confirmed to be statistically significant. In the conventional cell, it was found that the AC/DC ratio decreased from 2.2% to 1.3%
over 10 days of operation. It was also found that the dominant frequency increased from 0.9 to 1.1 Hz. These results were attributed to the progressive rounding of the anode bottom profile as it was consumed, which would presumably promote the more frequent release of smaller gas bubbles.
Anodes in the drained cathode cell also exhibited a fluctuating current, although the patterns were significantly different. After two days, a significantly lower AC/DC ratio was observed on the sloped anode, compared to the anodes in the conventional cell. The dominant frequency was also higher than for anodes in the conventional cell, although Barber states that the signal was more evenly spread across all frequencies, and there was no single dominant frequency, for the sloped anode. It is plausible that the lower AC/DC ratio was due to the bubbles evacuating the ACG more quickly under the sloped anode, reducing the AC component.
Another study performed similar measurements on two anodes in a conventional Hall- Héroult cell and obtained similar results to Barber (Walker 1995). Interestingly, Walker Cathode Cathode Sloped Anode Flat Anode concluded that the voltage fluctuation was primarily due to waves in the electrolyte/metal interface, rather than bubble release. However, his logic is not convincing. In particular, he assumed that similar signals on two different anodes could not be due to bubbles, since slight variations in anode geometry should significantly affect bubble behaviour. This is a subjective assessment and cannot be quantified. In comparing his results to Barber’s, Walker did not recognise the significant of the absence of the metal pad in the drained cathode cell.
Hall Effect sensors have been used to measure the magnetic field generated by the current flowing through anode rods at the Intalco smelter (Barclay, Hung, and Rieg 2001). It was preferred to have sensors on the front and back of the anode rod, to cancel any effect of the ambient magnetic field. However, a single sensor was also shown to perform adequately. It was found that different signals were produced in different anodes, which could sometimes be linked to cell condition. The acoustic signal from the vibration of an anode rod in a 160 kA cell has been measured (Xue and Oye 1999). It was demonstrated that an anode with a spike1 produced a different signal to a ‘normal’ anode.
The last two methods discussed could be considered to be alternatives to measuring the fluctuation in the current flowing through an anode rod. The signal produced via any of the methods could be analysed using digital signal processing.

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Pressure Fluctuations in Electrolyte

In parallel with the anode voltage measurements described above, the pressure fluctuations in the electrolyte have also been measured (Walker 1995), using the method described in Chapter 4.2.3. Measurements were made in the side channel adjacent to the two anodes where the anode voltage signals were measured. The pressure fluctuations were observed to be relatively well correlated to the voltage fluctuations, to the extent that they exhibited a similar dominant frequency.

Electrolyte/Metal Interface

There are two reported studies of the gas-induced wave height in the electrolyte/metal interface in 150 kA and 230 kA cells (Rolseth, Solheim, and Thonstad 1988; Olsen, Rolseth, and Moxnes 1999), using the method described in Chapter 4.3.1. In the former, waves up to 22 mm in height were measured in the 50 mm inter-anode gap. The maximum height of waves in the centre or side channel was only 13 mm. The frequency of waves was 0.2-0.6 Hz in the centre and side channels and 0.4-0.8 Hz in the inter-anode gap. The second study was more focused on the effect of a longitudinal slot and results were only reported for the side channel. The measured wave heights were comparable to the earlier study. The conclusion from this work was that higher waves and frequencies occurred in the inter-anode gap because the inter-anode gap was narrower than the centre or side channel. Thus the evacuating gas had more effect on the electrolyte/metal interface in this region. This logic appears sound.

Section A : Theory and Background .
Chapter 1 – Justification
1.1 Hall-Héroult Process
1.2 Bubble Removal.
1.3 Objective
Chapter 2 – Bubble Formation .
2.1 Introduction .
2.2 Bubble Nucleation
2.3 Bubble Growth (via Diffusion)
2.4 Bubble Growth (via Coalescence)
2.5 Bubble Detachment .
2.6 Bubble Release.
Chapter 3 – Effect of Bubbles on Resistance
3.1 Introduction
3.2 Conductivity of Electrolyte
3.3 Activation Polarisation
3.4 Concentration Polarisation
3.5 Current Path Length
3.6 Conclusions
Chapter 4 – Measurement Techniques .
4.1 Introduction
4.2 Bubble Behaviour
4.3 Resistance ..
4.4 Digital Signal Processing
Section B : Review of Relevant Studies
Chapter 5 – Industrial Electrolysis
5.1 Introduction
5.2 Whole Cell Signals
5.3 Gas Volume .
5.4 ACG Voltage
5.5 Anodic Overvoltage
5.6 Individual Anode Signals
5.7 Pressure Fluctuations in Electrolyte.
5.8 Electrolyte/Metal Interface
5.9 Conclusions
Chapter 6 – Laboratory-Scale Electrolysis
6.1 Introduction
6.2 Direct Observation
6.3 Gas Volume
6.4 Cell Voltage
6.5 Anodic Overvoltage .
6.6 Conclusions .
Chapter 7 – Physical Modelling of Electrolysis .
Chapter 8 – Conclusions From Relevant Studies
Section C : Development of Measurement Technique
Chapter 9 – Preliminary Experiments
Chapter 10 – Current Injection Using a Capacitor – Development
Chapter 11 – Current Injection Using a Capacitor – Validation on Copper
Electrowinning Cell .
Section D : Conclusions and Recommendations
Chapter 12 – Conclusions
Chapter 13 – Recommendations for Measurements on Aluminium Reduction Cells