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Investigating the Effect of the Mg<sup>2+</sup>/Ca<sup>2+</sup> Molar Ratio on the Carbonate Speciation during the Mild Mineral Carbonation Process at Atmospheric Pressure
Aqueous
mineral carbonation of industrial wastes, such as fly ash,
is a promising sequestration technology to reduce CO<sub>2</sub> emissions
in small-/medium-sized plants. In this paper, the carbonation capacity
of a leachate rich in Mg<sup>2+</sup> and Ca<sup>2+</sup> contents
was examined to clarify the competition between the carbonation of
these two cations and the speciation of the resulting carbonate precipitate,
under the mild carbonation conditions using 20–80 °C and
atmospheric pressure. As confirmed, the carbonation precipitation
of the two cations was completed in 30–40 min. At room temperature,
increasing the Mg<sup>2+</sup>/Ca<sup>2+</sup> molar ratio was in
favor of the carbonation rate of Mg<sup>2+</sup>, which is maximized
at the Mg<sup>2+</sup>/Ca<sup>2+</sup> molar ratio of 2. In contrast,
the carbonation rate of Ca<sup>2+</sup> was decreased monotonically
as a result of the competition from Mg<sup>2+</sup>. For both cations,
their carbonation rate was maximized at 60 °C. In comparison
to the formation of predominant calcite and vaterite in the presence
of sole Ca<sup>2+</sup> in the leachate, the coexistence of two cations
resulted in the preferential formation of amorphous species, aragonite
and magnesian calcite. The quantity of the amorphous phase was increased
remarkably upon increasing the Mg<sup>2+</sup>/Ca<sup>2+</sup> molar
ratio at room temperature. An increase in the carbonation temperature
further deteriorated the crystallization of the carbonation precipitate,
resulting in the increase of the amount of amorphous species and the
phase change of calcium carbonate from calcite to aragonite