Effect of Calcium and Iron (II) Ions on the Precipitation of Calcium Carbonate and Ferrous Carbonate

This work was intended to study the interaction between ferrous-iron and calcium ions simultaneously toward carbonate ions in supersaturated solution in a continuous stirred tank reactor (CSTR) and to establish an acceptable experimental design as a preparation step to study the kinetics of other types of ferrous iron salts. The experiments were conducted in 0.5 M NaCl solution at 55 degrees C. The pH values were between 6.2 and 7.8. It was found that calcium ions have a strong influence in increasing the solubility of iron carbonate. On the other hand, ferrous iron did not affect the solubility of calcium carbonate significantly. This might be caused by increasing the stability of ferrous iron carbonate complexes in the presence of calcium. Also, the molar ratios of calcium to iron in the solution were compared with their molar ratios in the precipitated salts. It was found that the relation was greater than unity; R(solid)/R(solution) = 2.39. Thus, calcium carbonate has a preference to precipitate. This result seems to be in agreement with water dissolution rates of divalent ions and also with reaction-rate constants of iron carbonate and calcium carbonate reported in the literature.