Effect of polyacrylic acid on direct aqueous mineral carbonation of flue gas desulfurization gypsum

• Effect of PAA on carbonation of FGD gypsum was studied.
• PAA extended induction period to over 1.5 h.
• PAA increased Ca2+ in filtered solution to 60% of gypsum.
• In-situ liquid XRD revealed the crystallization sequence of calcite with time.
• Overall carbonation yield was reduced due to CO2 emission.

We have previously precipitated highly pure calcium carbonate (CaCO3) crystals from a filtered solution extracted in the course of the direct mineral carbonation of flue gas desulfurization gypsum. However, the maximum amount of pure CaCO3 obtained by the process was less than 5% of the gypsum. We thus attempted to manipulate the induction period by adding a polyacrylic acid (PAA), with the expectation of dissolving more Ca2+ in the solution. This was because it is known to inhibit the formation of CaCO3. The PAA was actually found to prolong the induction period to over 1.5 h, with the addition of 2.7 g/L PAA specifically increasing the amount of dissolved Ca2+ to 60% of the gypsum. It was also found that the addition of PAA lowered the overall carbonation yield via CO2(g) emission, thus reducing the CO2 dissolution. In addition, ammonium carbonate ((NH4)2CO3) was discovered to be usable for producing pure CaCO3 in the presence of PAA. An in-situ liquid-phase X-ray diffraction of the filtered solution revealed the sequential formation of calcite crystals from the amorphous phase on exposure to air. The morphology of the obtained calcite comprised twinned pseudo-spherical or dumbbell-shaped crystals, unlike the typical rhombohedral calcite phase obtained without PAA. It is clearly demonstrated that the precipitation of clean CaCO3 in direct mineral carbonation of FGD gypsum can be enhanced by additives as PAA for a better economic viability.

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