THE SYSTEM FE2O3-SO3-H2O AT AMBIENT TEMPERATURE

The system Fe₂O₃-SO₃-H₂O is the simplest one that can approximate acid mine drainage, a common environmental problem. We have investigated this system by syntheses of Fe sulfates from the aqueous phase, thermodynamic measurements, and calculation of the stability fields using the extended Pitzer model of Tosca et al. (2007).

For the syntheses, charges were mixed and allowed to stand in the laboratory for at least 3 years, although it seems that such long equilibration times are necessary (1 year is perhaps enough). Liquids and solids were then separated and analyzed for the chemical (liquids) and phase (solids) composition. These minerals and phases have been identified: hydronium jarosite, ferricopiapite, paracoquimbite, rhomboclase, and (H₃O)Fe(SO₄)₂. We were able to establish roughly the aqueous compositions that coexist with these phases; an exact determination is difficult because many charges contain more than one phase, although sometimes the crystallization sequence (early versus late phase) can be discerned. Neither butlerite nor coquimbite was observed. The latter requires Al for its formation, as we found is a separate series of syntheses.

Thermodynamic measurements consisted of formation enthalpy measurements with an acid-solution calorimeter. In addition to these, low-temperature heat capacity of these phases are in progress now and will be used to calculate the standard entropies. Combination of the formation enthalpies with entropy estimates leads to approximate Gibbs free energies of formation. They may be used, together with the extended Pitzer model for this system (Tosca et al. 2007), to calculate the phase diagram and confront it directly with the results of our syntheses.

This confrontation suggests that i) the accuracy of the available thermodynamic data for the solid and liquid phases is satisfactory and ii) even the complexity of the extended Pitzer model cannot fully capture the details of the phase relationships in the studied system. Overall, however, we can state that these data can be used for modeling of processes in acid mine drainage. Currently, we are also performing similar work on quaternary systems and their minerals which are relevant to acid mine drainage.

References

Tosca, N.J., Smirnov, A. & McLennan, S.M., 2007: Geochimica et Cosmochimica Acta 71, 2680-2698.