Hard Gray Scale

  • Neal R. Dando
  • Stephen J. Lindsay


The dry scrubbing and secondary alumina handling systems for pre-bake cell technologies are hampered by accumulations of hard gray scale, or HGS. HGS is an amorphous reaction product formed from collision-induced reactions of bath fines, alumina and water. HGS formation is greatest in high attrition areas owing to the high surface energy of new alumina surfaces created by the fracture of alumina particles. If not systematically addressed with cleaning and maintenance procedures HGS can: greatly reduce the removal efficiency of gaseous fluoride in dry scrubbers, reduce the capability to convey alumina in sufficient quantities, reduce filter life and alter the concentration of fluoride on secondary alumina.

By moving to higher line load and/or pot hooding flow rates an acceleration of HGS formation in dry scrubbers and reacted alumina transport lines is likely, especially when existing equipment is utilized without upgrades. This accentuates the need for counter-measures to reduce or control HGS formation rates. This paper defines the chemistry of HGS, proposes a mechanism for HGS formation, discusses the conditions that are necessary to form HGS, identifies the key areas of concern in a smelter, and discusses practical alternatives to reduce rates of HGS formation.


Hard Gray Buildup Gray Scale Mineral Deposits Dry scrubber 


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  1. 1.
    Neal R. Dando, “Adsorption/Entrainment of Fluoride in Smelting Grade Alumina: Surface Chemical Speciation and Adsorption Mechanism,” Light Metals 2005, pp. 133–139.Google Scholar
  2. 2.
    Stephen J. Lindsay, “Effective Techniques to Control Fluoride Emissions,” Light Metals 2007, pp. 199–204.Google Scholar
  3. 3.
    Stephen J. Lindsay, “Bridging the Gaps Between Refineries and Smelters,” the proceedings of the 8th Australasian Smelting Technology Conference 2004, pp. 148–162Google Scholar
  4. 4.
    Slaugenhaupt, M. L., Bruggeman, J. N., Tarcy, G. P., Dando, N. R., “Effect of Open Holes on Vapor-Phase Fluoride Evolution from Pots”, Light Metals, 2003, pp. 199–204Google Scholar

Copyright information

© The Minerals, Metals & Materials Society 2016

Authors and Affiliations

  • Neal R. Dando
    • 1
  • Stephen J. Lindsay
    • 2
  1. 1.Alcoa Inc.Alcoa Technical CenterAlcoa CenterUSA
  2. 2.Alcoa Inc.Primary MetalsAlcoaUSA

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