Operating pH influences homogeneous calcium carbonate granulation in the frame of CO₂ capture

The era of industrialization has caused the drastic increase in atmospheric carbon dioxide concentrations that now require various remediation strategies such as CO₂ capture and storage. In this study, calcium carbonate granulation is proposed as a new conversion route for CO₂ trapped in water matrices into dense solids. Herein, we demonstrate the effectiveness of the fluidized-bed reactor to produce compact calcium carbonate pellets from captured CO₂ via a granulation reaction at different pH conditions and in the absence of seed materials. Constant values of calcium-to-carbonate ratio, influent carbonate concentration and influx flow rate were used while operating pH was varied from 8.5 to 11.0. Optimal operating condition with carbonate removal and granulation efficiencies of 92% and 90%, respectively was found at pH of 10.0 ± 0.2, where the lowest daily effluent concentration of carbonate ions was measured at 16.6 mg L⁻¹ via the alkalinity test. At optimum operating pH, large compact granules ranging from 1 – 2 mm in diameter (∼93.6 g) were obtained with overall particle size distribution leaning towards bigger sizes. Morphological analyses of the granules revealed their smooth surfaces and subrounded shapes, while crystalline and elemental analyses identified these as high purity calcium carbonate. Moreover, spontaneous homogeneous nucleation, particle aggregation, crystal growth and granulation are proposed as the main mechanisms of calcium carbonate granulation.