Chemistry:Prebaked consumable carbon anode

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Short description: Type of anode for aluminium smelting

Prebaked consumable carbon anodes are a specific type of anode designed for aluminium smelting using the Hall-Héroult process.

Use and end-of-life disposal

During the smelting process, these anodes are suspended within the electrolysis cell(s) containing the aluminium oxide or aluminium fluoride. The process consumes the anode at a rate of roughly 450 kg of anode per tonne of aluminium produced.[1]

"Spent" anodes have little industrial use and are generally discarded; however, anodes that have been used to process aluminium fluoride may contain some amount of hydrogen fluoride and require hazardous waste disposal procedures.[2] Efforts to find industrial use for spent anodes have led to proposals to use the anodes as a cost-effective alternative for coke in small-scale foundries that lack a ready supply of coke, and cannot afford modern electric furnaces.

Industrial standards

The properties of the anode are largely set during the baking process and must be carefully controlled to ensure an acceptable output efficiency and reduce the amount of undesirable byproduct produced.[3] To that end, the aluminium smelting industry has settled on a range of acceptable values for commercial mass-produced anodes for the purpose of consistent, optimal performance.

Industrial Standards for Prebaked Carbon Anodes[4][5][6]
Property Standard Range
Baked apparent density ISO 12985-1 1.53-1.64 gcm-3
Electrical resistance ISO 11713 55-62 μΩ for pressed anodes
Compressive strength ISO 18515 40-48 MPa
Young's modulus RDC-144 3.5-5.5 GPa
Tensile strength ISO 12986-1 8-10 MPa for pressed anodes
Thermal conductivity ISO 12987 3.5-4.5W mK-1
Coefficient of thermal expansion RDC-158 3.5-4.5 x 10-6 K-1
Air permeability ISO 15906 0.5-1.5 nPm
Carboxy reactivity residue ISO 12988-1 84-96%
Air reactivity residue ISO 12989-1 0.05-0.3% per minute
Grain stability N/A 70-90%

Significance of the industrial standards

Density

Higher baking temperatures result in higher density anodes, which exhibit reduced permeability and therefore extend the operational life of the anode.[7] However, excessive density will result in thermal shock and fracturing of the anode upon first use in an electrolysis cell.[8]

Electrical resistance

Efficient aluminium smelting requires low resistance on the part of the anode. Low resistance results in greater control over the electrolysis cell's voltage and reduces the energy loss associated with resistive heating.[9] However, anodes with low electrical resistance also exhibit increased thermal conductivity. Anodes that conduct too much heat will oxidize rapidly, reducing or eliminating their smelting efficiency, called "air burn" in industry parlance.[10]

Mechanical strength (compressive strength, Young's modulus, tensile strength)

Anodes are subject to a variety of mechanical stresses during creation, transportation and use. Anodes must be resistant to compressive force, resistant to elastic stress,[11] and resistant to impact without becoming brittle.[12][13] The relationship between compressive strength and Young's modulus in prebaked anodes usually results in a compromise in the anode's resistance to compressive force and elastic stresses.[14]

Thermal conductivity and thermal expansion

Low anode thermal conductivity results in "air burn", as noted in Electrical Resistance, above.[15][16]

Low thermal expansion coefficients are desirable to avoid thermal shock.[17][18]

Carbon reactivity and air permeability

Anodes should be relatively impermeable to both carbon dioxide and air generally to reduce the opportunity for "carbon dioxide burn" and "air burn", both of which will reduce the anode's smelting efficiency.[19]

Grain stability

High grain stability indicates high anode structural integrity, increasing the smelting efficiency of the anode. High Grain stability also minimizes particle degradation during anode fabrication.[20]

References

  1. ↑ "Aluminium for Future Generations – Anode Production". http://primary.world-aluminium.org/processes/anode-production.html. 
  2. ↑ Hocking, M.B. (1985). Modern Chemical Technology and Emission Control. Berlin: Springer-Verlag. pp. 244. ISBN 9783642697753. 
  3. ↑ Fisher, Keller and Manweiller (January 2009). "Anode plants for tomorrow’s smelters: Key elements for the production of high quality anodes". Aluminium International Today. http://www.rd-carbon.com/data/documents/publications/raw-materials-carbon-products/anode_plants_for_tomorrow_smelters.pdf. Retrieved 28 October 2015. 
  4. ↑ Marsh, H. and K. Fiorino. Carbon Anodes. in Fifth Australasian Aluminium Smelter Technology Workshop. 1995. University of New South Wales Kensington Campus, Sydney, Australia: L. J. Cullen Bookbinders
  5. ↑ Sadler, B.A. and B.J. Welch. Anode Consumption Mechanisms- A Practical Review of the Theory & Anode Property Considerations. in Seventh Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne, Australia
  6. ↑ Barclay, R. Anode Fabrication, Properties & Performance. in 7th Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne
  7. ↑ Sadler, B. Anode consumption and the ideal anode properties. in Fourth Australasian Aluminium Smelting Technology Workshop. 1992. Sydney, Australia
  8. ↑ Sadler, B.A. and B.J. Welch. Anode Consumption Mechanisms- A Practical Review of the Theory & Anode Property Considerations. in Seventh Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne, Australia
  9. ↑ Sadler, B. Anode consumption and the ideal anode properties. in Fourth Australasian Aluminium Smelting Technology Workshop. 1992. Sydney, Australia
  10. ↑ Thyer, R., Anode coating is reducing air burn, in CSIRO research in materials processing and metal production. 2007, Commonwealth Scientific and Industrial Research Organisation: Melbourne. p. 1-2
  11. ↑ Sadler, B.A. and B.J. Welch. Anode Consumption Mechanisms- A Practical Review of the Theory & Anode Property Considerations. in Seventh Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne, Australia
  12. ↑ Tomsett, A. Anode Baking Furnace Operation. in 7th Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne, Australia
  13. ↑ Barclay, R. Anode Fabrication, Properties & Performance. in 7th Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne
  14. ↑ Barclay, R. Anode Fabrication, Properties & Performance. in 7th Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne
  15. ↑ Sadler, B.A. and B.J. Welch. Anode Consumption Mechanisms- A Practical Review of the Theory & Anode Property Considerations. in Seventh Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne, Australia
  16. ↑ Kuang, Z., J. Thonstad, and M. Sørlie, Effects of Additives on the Electrolytic Consumption of Carbon Anodes in Aluminium Electrolysis. Carbon, 1995. 33(10): p. 1479-1484
  17. ↑ Sadler, B.A. and B.J. Welch. Anode Consumption Mechanisms- A Practical Review of the Theory & Anode Property Considerations. in Seventh Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne, Australia
  18. ↑ Barclay, R. Anode Fabrication, Properties & Performance. in 7th Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne
  19. ↑ Marsh, H. and K. Fiorino. Carbon Anodes. in Fifth Australasian Aluminium Smelter Technology Workshop. 1995. University of New South Wales Kensington Campus, Sydney, Australia: L. J. Cullen Bookbinders
  20. ↑ Barclay, R. Anode Fabrication, Properties & Performance. in 7th Australasian Aluminium Smelting Technology Conference & Workshops. 2001. Melbourne



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