Pyrrhotite dissolution in acidic media

• The pyrrhotite dissolution in acidic media was investigated.
• The regression models predict similar rates for dissolution reactions near pH 2.
• The reaction with oxygen is dominant at pH > ∼3.

Non-oxidative dissolution rates for hexagonal pyrrhotite (Fe1−xS) were measured for pH values ranging from 0 to 1.3 and temperatures ranging from 25 to 70 °C. These results showed that pyrrhotite dissolution rate increases with decreasing pH or increasing temperature.The 16 new rate data from this study were combined with 46 data taken from the literature (1 datum was discarded) to develop a rate equation for non-oxidative pyrrhotite dissolution rH+(mol/m2s)=1.58×107e-65,900R1TMH+1.46 This fit spans a range of pH values from 0 to 5 and temperatures from 20 to 90 °C. It is most reliable for low pH (⩽2.75), where most rate data were measured. This equation reasonably predicts rates for all pyrrhotite (Fe1−xS) compositions regardless of the value of x, including monoclinic pyrrhotite, hexagonal pyrrhotite and troilite (FeS).In addition, an equation that expresses pyrrhotite rate as a function of temperature and PO2PO2 was developed using 35 rate data taken from the literature rO2(mol/m2s)=1.10×10-2e-30,200R1TPO20.352 and an equation that expresses the rate of oxidation of pyrrhotite as a function of temperature and Fe(III) concentration was developed using 48 previously published data rFe(III)(mol/m2s)=0.516e-33,600R1TMFe(III)0.368 The correlation coefficients for these regression models were relatively low due the narrow range of experimental conditions, relatively few data, and possible uncontrolled variables. These equations predict similar rates for all three reactions near pH 2 where many of the experiments were carried out leading to concerns that some of the experiments may have measured combined rates. For natural conditions, the rates of the H+ and Fe(III) reactions are predicted to be relatively fast and similar at pH < 3. The O2 reaction is dominant at higher pH > ∼3.

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