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FLUORIDE CHEMISTRY IN THE BAYER PROCESS

Wilson, D.J., Aboagye, A.A., Heath, C.A., Rosenberg, S.P., Tichbon, W. and Whitaker, C.R.

Sodium fluoride is a common contaminant in many alumina refineries. At the Worsley refinery, fluoride originates from the bauxite, and enters the process during milling and digestion. While the concentration of fluoride in process liquors is generally quite low, it undergoes a range of reactions with other species in the liquor stream that can cause serious problems with refinery operations and product quality. For example, fluoride is able to form a range of sparingly soluble double salts with other common impurities in Bayer liquors, which manifest as scales in evaporators, and under extreme conditions may also co-precipitate with gibbsite in the precipitation circuit. It is therefore surprising that the open literature of the alumina industry contains few references to the impact of fluoride on the Bayer process. In this paper, the results of laboratory investigations that describe the solubility behaviour of kogarkoite (a sodium fluoride sulphate double salt) in Bayer liquors, and the interactions of fluoride with a number of calcium species are presented. We also discuss the extent of fluoride substitution in the aluminate ion in solution, and describe the impact of kogarkoite co-precipitation with gibbsite on the operation of the Worsley refinery. In addition, details of a cost-effective fluoride control process stemming from these studies, applied at Worsley, are given.