Abstract
The solubility of zirconia gels versus pH is studied in order to state first the influence of F- ions alone, then the influence of the simultaneous presence of F- and amorphous silica. Two concentrations of F- ions are chosen, 0.094 and 0.314 mol L-1, corresponding to initial molar ratios F/Zr = 0.6 and 2.0. Metal fluorocomplexes formed at pH 0.5 and 2.5 are characterized by 19F NMR. The influence of F- on the solubility of zircon is also studied. It is shown that the formation of silicon complexes rich in fluorine shifts the equilibriums between the zirconium fluorocomplexes toward the complexes poor in fluorine and that, in the presence of F-, the crystallization of zircon by hydrothermal synthesis can be forecasted, for pH values in the range 0-10.Key words: zirconia gels, silica, zircon, fluoride ions, solubility, 19F NMR.
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