Experimental determination of the solubility product of magnesite at 50 to 200°C

Accurate knowledge of magnesite thermodynamic properties over a wide range of temperatures is crucial for characterizing mineral stabilities in the system MgO-CO2-H2O and for modeling carbon dioxide fate in important natural and industrial processes. However, available databases, especially for its solubility product, are sparse and contradictory, leading to considerable uncertainties in the calculation of chemical equilibria and phase transformations among carbonates. In this study, the solubility of synthetic magnesite was investigated from 50 to 200 degrees C in 0.1 mol kg(-1) NaCl solutions and in some cases under constant CO2 partial pressure (4-30 bars) both by means of a hydrogen electrode concentration cell (HECC) and a traditional batch Ti-reactor. The obtained apparent solubility products (:2) were extrapolated to infinite dilution to generate the solubility products (Ksp degrees-mgs), allowing calculation of the thermodynamic properties of magnesite. Of all the temperature functions tested, the equation giving a reliable fit of our data in the investigated temperature range (50-200 degrees C) has the following form: log(10)K(sp degrees-mgs) = a + b/T (K)+ cT (K) with: a = 7.267, b = -1476.604 and c = -0.033918. Based on this equation and its first and second derivatives with respect to T, we were able to derive values at 25 C, 1 bar for magnesite thermodynamic functions: Delta(f)G(298.15)(o) = (-1026.48 +/- 2) kJ mol(-1) (log(10)K(sp degrees-mgs) = -7.80 +/- 0.3), Delta H-f(298.15)o = (-1111.75 +/- 2) kJ mol(-1), S-298.15(o) = (60.00 +/- 2) J mol(-1)K(-1), and C-p(o) (298.15) = (75.91 +/- 2) J mol(-1)K(-1) (uncertainties are 3 sigma). (c) 2011 Elsevier B.V. All rights reserved.