Preparation of pure calcium carbonate by mineral carbonation using industrial byproduct FGD gypsum

Abstract:

Mineral carbonation is one of the geological approaches for the sequestration of anthropogenic CO₂ gas. Its concept is based on the natural weathering processes in which silicate minerals containing divalent cations such as Ca or Mg are carbonated to CaCO₃ or MgCO₃ in the reaction with CO₂gas. Raw materials for the mineral carbonation have been extended to various industrial solid wastes such as steel slag, ashes, or FGD (flue gas desulfurization) gypsum which are rich in divalent cations.

These materials have economic advantages when they are produced in CO₂ emission sites. Flue gas desulfurization (FGD) gypsum is such a byproduct obtained in at coal-fired power plants. Recently, we carried out a research on the direct mineral carbonation of FGD gypsum for CO₂sequestration. It showed high carbonation reactivity under ambient conditions and the process can be described as follows:

CaSO₄·2H₂O + CO₂(g) + 2NH₄OH(aq) → CaCO₃(s) + (NH₄)₂SO₄(aq) (1)

At the early stage of the process, calcium carbonate (CaCO₃) exists as a dissolved ion pair during the induction period. High-purity CaCO₃ could be precipitated from dissolved calcium carbonate solution extracted during the induction period. The effect of experimental parameters on pure CaCO₃ was evaluated: CO₂ flow rate (1–3 L/min), ammonia content (4–12%), and solid-to-liquid (S/L) ratio (5–300 g/L). FE-SEM (field-emission scanning electron microscopy) and XRD (X-ray diffraction) study revealed that the precipitated CaCO₃ was round-shaped vaterite crystals. The induction time was inversely proportional to the CO₂ flow rate and the yield for pure CaCO₃ increased with the ammonia content. The formation efficiency for pure CaCO₃ decreased with S/L (solid/liquid) ratio. It was 90% (mol/mol) when the S/L ratio was 5 g/L. However, S/L ratio didn’t affect the maximum solubility limit of dissolved CaCO₃.