Speciation of the H2SO4–Fe2(SO4)3–FeSO4–H2O system and development of an expression to predict the redox potential of the Fe3 +/Fe2 + couple up to 150 °C

• Speciation of the quaternary acidic iron sulfate system is simulated up to 150 °C.
• The model is extensively validated by experimental data.
• A novel expression is developed to easily calculate the redox potential.
• The potential can be predicted with only temperature and nominal Fe3 +/Fe2 + ratio.
• Predicted redox potentials increase with nominal Fe3 +/Fe2 + ratio and temperature.

A thermodynamic model is developed and shown to reliably simulate the speciation of the quaternary H2SO4–Fe2(SO4)3–FeSO4–H2O system through a wide range of solution compositions and temperatures (25 °C–150 °C). The main species involved (including H+, Fe2 +, Fe3 +, SO42 −, HSO4−, FeHSO4+, FeSO4°, FeHSO42 +, Fe(SO4)2−, FeSO4+, Fe2O3 and H2O) were investigated, and their thermodynamic data were collected and critically assessed to evaluate their activity coefficients and equilibrium constants. Results show that the species distribution depends highly on the initial total amount of iron, acidity, nominal Fe3 +/Fe2 + ratio and temperature. Modeling results also reveal that most of the Fe(III) is distributed as complexes or precipitates and the free Fe3 + accounts for only a minor percentage, whereas a large amount of Fe(II) exists in the form of free Fe2 +, with the rest dissolved as complexes. Moreover, based on the model results, the temperature dependence of the real Fe3 +/Fe2 + ratios and their activity coefficient ratios was subsequently obtained and employed to systematically study the redox potential of the Fe3 +/Fe2 + couple. The redox potentials predicted by the model increase with the nominal Fe3 +/Fe2 + ratio and temperature when the nominal Fe3 +/Fe2 + ratios are 10:1, 100:1 and 1000:1, with its values in the range of 736–956 mV (all potentials in the present study are quoted with respect to the Standard Hydrogen Electrode at 25 °C) from 25 °C to 150 °C in the studied acidic iron sulfate solutions. At a nominal Fe3 +/Fe2 + ratio of 1:1, the predicted potentials are relatively stable with respect to temperature, typically between 671 mV and 679 mV, because the real Fe3 +/Fe2 + ratio in solution is much lower for such a condition. The speciation model explains the change of redox potential with temperature for all nominal Fe3 +/Fe2 + ratios. An expression is also developed to calculate the redox potential and is supported by experimental results from the literature. It appears that the redox potential can be easily and accurately determined only based on the variables of temperature and nominal Fe3 +/Fe2 + ratio, independent of the nominal concentrations of H2SO4 and Fe3 +. Moreover, this expression is probably applicable to more general cases at or around room temperature. The proposed model was validated by reliable prediction of measured redox potential from 25 °C to 150 °C. Comparison of previously published results of ferric solubility, together with an analysis of the calculated pH and ionic strength, provides additional justification for the model developed herein. These findings contribute to the research on aqueous speciation of acid iron sulfate solutions, the reduction kinetics of the Fe3 +/Fe2 + couple, as well as the mechanistic analysis in industrial leaching processes of chalcopyrite and other sulfide minerals.