Characterization of carbon dust

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Contents

1. Introduction ... 9

References ... 13

2. Carbon dust in aluminium electrolysis ... 15

2.1. Carbon dust – a definition ... 15

2.1. The formation of carbon dust ... 18

References ... 21

3. Characterization of carbon dust ... 24

3.1. Grain size and carbon content ... 24

3.2. Characterization by optical microscopy ... 25

3.2.1. Overview images of raw carbon dust samples ... 27

3.2.2. Overview images of washed carbon dust samples ... 28

3.3. Discussion ... 30

3.4. Conclusions ... 30

4. Influence of carbon dust contents on current efficiency and anode carbon consumption ... 32

4.1. Experimental ... 32

4.1.1. Experimental cell ... 32

4.1.2. Apparatus ... 34

4.1.3. Experimental conditions ... 35

4.1.4. Anodes ... 36

4.1.5. Carbon dust ... 37

4.1.6. Cell performance ... 38

4.1.7. Treatment of the experimental data ... 38

4.2. Results ... 41

References ... 50

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5. The influence of carbon dust

on the electrical properties of the electrolyte ... 52

5.1. Introduction ... 52

5.2. Preliminary measurements of changes in electrical conductivity/resistivity by ac method ... 59

5.2.1. Experimental method ... 60

5.2.2. Experimental cell and apparatus ... 60

5.2.3. Experimental conditions ... 62

5.2.4. Results ... 62

5.2.5. Summary ... 64

5.3. Measurements of electrical conductivity/resistivity changes by dc method ... 64

5.3.1. Experimental apparatus and cell ... 65

5.3.2. Electrolyte composition ... 66

5.3.3. Results ... 67

5.3.4. Discussion ... 75

5.4. The Bruggeman equation ... 75

5.5. Excess Joule heating due to carbon dust ... 77

References ... 79

6. Characterisation of carbon dust in the electrolyte in aluminium cells ... 81

6.1. Introduction ... 81

6.2. Study of the behavior of carbon dust in the electrolyte used for aluminium electrolysis ... 82

6.2.1. The first set of the experiments ... 83

6.2.1.1. Experimental part ... 83

6.2.1.2. Results of electrolysis ... 84

6.2.1.3. Characterization of the anode surface ... 86

6.2.1.4. Conclusions ... 106

6.2.2. The second set of the experiments ... 106

6.2.2.1. Experimental part ... 106

6.2.2.2. Results of electrolysis ... 108

6.2.2.3. Characterization of the anode surface ... 111

6.2.2.4. Conclusions ... 131

7. Summary conclusions ... 133

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7 APPENDICES ... 135 A1. The results of gas analysis and calculated data

for graphite anode and 0 wt. carbon addition ... 137 A2. The results of gas analysis and calculated data

for prebaked anode and 0.67 wt. carbon addition ... 137 A3. The results of gas analysis and calculated data

for prebaked anode and 4 wt. carbon addition ... 139 A6. The results of gas analysis and calculated data

for prebaked anode and 8 wt. carbon addition ... 140 A8. A plot of the confidence band curves at 95 confidence level

for CE (Oxy) vs. the carbon addition to the melt ... 140 A9. A plot of the confidence band curves at 95 confidence level

for CE (P&W) vs. the carbon addition to the melt ... 141 A10. A plot of the confidence band curves at 95 confidence level

for CC (gas analysis) vs. the carbon addition to the melt ... 141 A11. A plot of the confidence band curves at 95 confidence level

for CC (anode weight) vs. the carbon addition to the melt ... 142 A12. A plot of the confidence band curves at 95 confidence level

for DBR vs. the carbon addition to the melt ... 142 A13. A plot of the confidence band curves at 95 confidence level

for Uel vs. the carbon addition to the melt ... 143 A14. A plot of the confidence band curves at 95 confidence level

for IRel vs. the carbon addition to the melt ... 143 A15. A plot of the confidence band curves at 95 confidence level

for ratio IRel/Uel vs. the carbon addition to the melt ... 144 A16. Anode effect phenomenon ... 145 References ... 146