Molar Ratio of Ca2+ to Fe2+ in the Supersaturated Solution of Iron Carbonate and Calcium Carbonate and in the Precipitate: Relation and Interpretation

Abstract

Abstract Calcium and iron ions are commonly observed in treated waters. Their carbonate salts are widely found in production lines of separation plants and gas and oil wells. Extensive studies have been made on calcium carbonate salts. On the other hand, little attention has been paid to ferrous iron carbonates and iron sulfide [Tomson et al., 1991; Jonson et al., 1991; Braun, 1991; Greenberg et al., 1992, Yean et al., 2008]. This is probably attributed to the reactivity of ferrous iron and the difficulty in the laboratory to completely prevent the reactions of ferrous iron from oxygen. Several experimental designs and sampling procedures are proposed in the literature. Nonetheless, none of them guarantee complete avoidance of oxygen. In addition, little information is available about ferrous iron carbonate, no reports have been found in the literature focusing on the interaction of iron/calcium carbonate common to nearly all oil field systems. Most of the reports found in the literature focus on the topic in terms of impurities and their influence in crystal structures and/or in their geochemical interaction at low temperatures (20-30°C) [Langerak, et al., 1999; Meyer, H.J., 1984; Herzong et al., 1989]. This work has the following objectives; 1) to study the interaction between ferrous iron and calcium ions simultaneously toward carbonate ions in supersaturated solution in a continuous stirred tank reactor (CSTR), 2) to establish an acceptable experimental design as a preparation step to study the behavior of iron sulfide salts. The experiments were conducted in 0.5 M NaCl solution at 55 °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 significantly affect the solubility of calcium carbonate. This might be due to 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 solid. It was found that the relation was greater than unity; Rsolid/Rsolution = 2.39. Although calcium carbonate has higher solubility and also the saturation index of calcium carbonate was lower than that of ferrous iron carbonate during the experiment, however, 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.