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The crystal growth of pyrite in aqueous supersaturated solutions at pH 6.50, 25 °C was investigated using the seeded growth, pH-stat technique. Crystal growth started immediately after inoculation of iron(II) sulfide supersaturated solutions with pyrite seed crystals. The crystal growth rates measured from the desupersaturation curves were found to depend strongly on the relative solution supersaturation with respect to pyrite. Kinetics analysis yielded an apparent order of 3.5 ± 0.5 suggesting a surface diffusion controlled mechanism. The presence of organophosphorus compounds such as 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), N,N,N',N'-ethylenediaminetetrakis(methylenephosphonic acid) (ENTMP) in the supersaturated solutions at concentration levels between 0.1 and 1000 nM inhibited the crystal growth of pyrite by 70-100% with respect to the blank experiments. Moreover, in the presence of the compounds tested the crystal growth of pyrite was preceded by well-defined induction times which increased sharply with inhibitor concentration. At concentration levels as low as 1 mM, crystal growth was completely suppressed. The presence of nitrilotris(methylenephosphonic acid) (NTMP) had no inhibition effect on pyrite crystal growth. The influence ofthe inhibitors tested on the kinetics of crystal growth was attributed to the blocking of the active growth sites due to adsorption.

The kinetics of the spontaneous formation of iron sulfides were studied in low ionic strength deaerated aqueous solutions at 25°C at both nearly neutral and moderately acidic pH. The constant supersaturation approach was adopted for this investigation. In nearly neutral pH (7.1–7.4) mackinawite precipitated directly, while at pH 5.4–5.6 amorphous FeS formed, showing marked sensitivity to oxidation by atmospheric oxygen. The rates of precipitation were found to be proportional to the relative solution supersaturation and the value m = 2 was found for the apparent rate order of precipitation. The investigation was extended at 80°C at comparable supersaturations with the 25°C experiments using the free drift method. At pH > 7, only iron oxides were produced while at pH 5.5 amorphous FeS precipitated. The rates of precipitation measured, showed a first order dependence on the relative solution supersaturation. The apparent orders of 2 and 1 for the precipitation of amorphous iron sulfide at 25°C and 80°C, respectively, in combination with an estimated activation energy of 40 kJ mol −1 and with the insensitivity of the kinetics measured on the fluid dynamics suggested a surface controlled process.