Ferrous carbonate precipitation kinetics: A temperature ramped approach

Recent evidence indicates that ferrous carbonate (FeCO$\sb3$, siderite) plays a critical role in forming corrosion resistant scale. Although FeCO$\sb3$ also limits the solubility of iron in some anaerobic systems, its precipitation kinetics have received little attention. The temperature dependent precipitation kinetics of ferrous carbonate were studied using a powerful new "temperature ramped" approach. Activation energies calculated using three second order and two first order precipitation models (53.8-123 kJ/mol) all qualitatively indicate that precipitation is controlled by surface reaction rate. Precipitation is extremely slow at ambient temperatures, but becomes limited by diffusion above approximately 110 C. Surface reaction rate equations must, therefore, be used to model low temperature precipitation, and mass transfer equations should be used to estimate high temperature FeCO$\sb3$ growth. Seed crystals were prepared and Ostwald ripened in a reactor under anoxic conditions. Precipitation was induced by linearly increasing (ramping) the temperature and recording the change in iron concentration with respect to time and temperature. Rather than performing a series of isothermal experiments, temperature dependent kinetics were determined by fitting nonlinear models to temperature ramped experiments. The techniques developed in this research could easily be adapted for use with a number of other reduced metal systems