Alumina Structural Hydroxyl as a Continuous Source of HF

  • Margaret Hyland
  • Edwin Patterson
  • Barry Welch

Abstract

The link between moisture on alumina and HF generation in aluminium reduction cells has been long established. The assumption has usually been that the ‘culprit’ is the loosely bound adsorbed water, generating HF via bath hydrolysis as this surface water is flashed off during alumina feeding. Structural water, or more correctly, structural hydroxyl, also makes a significant contribution to HF generation. Laboratory experiments show that hydroxyl can dissolve in molten cryolitic electrolytes and gives rise to electrochemically generated HF. The electrochemically generated HF could be readily distinguished from HF generated via thermal hydrolysis.

Experiments with aluminas of varying combinations of high and low surface adsorbed moisture and structural hydroxyl (as measured by their LOI (20–300) and LOI (300–1000), respectively) confirmed the importance of electrochemically generated HF from structural hydroxyl. While some of the structural hydroxide reacts rapidly at the time of feeding, it also contributes to the steady state HF emission. From plant studies it was estimated that up to 8 kg F/torme Al was generated from structural hydroxyl lor aluminas containing 0.4 wt % LOI (300–1000) and assuming 3wt% alumina in the bath. Structural hydroxyl is found in transition alumina phases in smelter grade aluminas. Their presence ensures that even conservative smelter specifications of surface areas of 60–80 m2/g can be met. Paradoxically, this surface area is specified to ensure that the HF adsorption capacity of the alumina is sufficient for scrubber requirements, but for reasons of both surface and structural water incorporation, having a high surface area also means that the alumina will generate more HF. This reopens the debate on the merits of high surface area aluminas.

Keywords

HF generation fluoride emissions smelter grade alumina structural hydroxy 

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References

  1. 1.
    E. Patterson, M. Hyland, B. Moxnes and B. Welch, ‘Reducing hydrogen fluoride emissions from aluminium electrolysis cells.’ Proceedings of the Seventh Australasian Smelting Technology Conference and Workshop, Nov. 11–16, Melbourne Australia, 92–105, 2001.Google Scholar
  2. 2.
    E. Patterson, M. Hyland, V. Kielland and B.J. Welch, ‘Understanding the effects of the hydrogen content of anodes on hydrogen fluoride emissions from aluminum cells.’ Light Metals 2001, 365–370.Google Scholar
  3. 3.
    E. Patterson, ‘Hydrogen Fluoride Emissions from Aluminium Electrolysis Cells.’ (PhD thesis, The University of Auckland, 2001).Google Scholar
  4. 4.
    M. M. Hyland, A.R. Gillespie and J.B. Metson, “Predicting moisture content on smelter grade alumina from measurement of the water adsorption isotherm.” Light Metals 1997, 113–117.Google Scholar
  5. 5.
    J.L. Henry, ‘A study of the factors affecting fluoride emission from 10,000 ampere experimental aluminum reduction cell.’ In Extractive Metallurgy of Aluminum, vol. 2, G. Gerard, Ed. Interscience, New York, 67–81, 1963.Google Scholar
  6. 6.
    W.E. Wahnsiedler, R.S. Danchik, W.E. Haupin, D.L. Brackenstose and J.W. Colpitts, ‘Factors affecting fluoride evolution from Hall-Heroult smelting cells.’ Light Metals 1978, 407–424.Google Scholar
  7. 7.
    M.L. Slaugenhaupt, J.N. Bruggeman, G.P. Tarcy and N.R. Dando, ‘Effect of open holes in the crust on gaseous fluoride evolution from pots.’ Light Metals 2003, 199–204.Google Scholar
  8. 8.
    E. Sum, C. Beeby, S. Campbell and K. Gadsby, ‘The effects of cell condition and operations on hydrogen fluoride evolution in an industrial cell.’ Proceedings of the Seventh Australasian Smelting Technology Conference and Workshop, Nov. 11–16, Melbourne Australia, 67–78, 2001.Google Scholar
  9. 9.
    N. Aljabri, K. Venkatasubramaniam and Y.A.M. AlFarsi, ‘HF Emission from Dubai’s electrolysis cell.’ Light Metals 2003, 487–489.Google Scholar
  10. 10.
    K. Grjotheim, H. Kvande, K. Motzfeldt and B.J. Welch, ‘The formation and composition of the fluoride emissions from aluminium cells.’ Canadian Metallurgical Quarterly, 11(4) (1972) 585–599.CrossRefGoogle Scholar

Copyright information

© The Minerals, Metals & Materials Society 2016

Authors and Affiliations

  • Margaret Hyland
    • 1
    • 2
  • Edwin Patterson
    • 1
  • Barry Welch
    • 2
  1. 1.Department of Chemical and Materials EngineeringNew Zealand
  2. 2.Light Metals Research CentreUniversity of AucklandAucklandNew Zealand

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