Intensified Preparation of Spherical Calcium Carbonate Particles in a CMC-Na-Thickened Precipitation System Using an Annular Swirling Flow Reactor

Sodium carboxymethyl cellulose (CMC-Na) can regulate CaCO3 crystal growth and promote the formation of spherical particles. However, the accompanying increase in solution viscosity can impair micromixing and mass transfer during precipitation. To address this limitation, an annular swirling flow reactor was employed to intensify the reactive crystallization of Na2CO3 and CaCl2 in a CMC-Na-thickened system, with conventional stirring serving as a reference. Particle image velocimetry (PIV) and computational fluid dynamics (CFD) simulations were conducted to characterize the reactor hydrodynamics. The results demonstrated that the mean residence times were less than 2.5 s under all investigated conditions, while the residence time distributions exhibited near plug flow characteristics, confirming its suitability for rapid precipitation processes. Increasing solution viscosity had little influence on the overall flow field structure but reduced the local velocity gradient, vorticity, and secondary flow intensity. In contrast, increasing the inlet flow rate enhanced mass transfer and alleviated swirl attenuation caused by viscous dissipation. At a relatively high flow rate (Q = 6 m3·h−1), the reactor maintained stable hydrodynamic performance despite viscosity variations. Compared with conventional stirring, the annular swirling flow reactor produced CaCO3 particles with better roundness, smoother surfaces, and narrower particle size distributions under thickened conditions. Calcite remained the predominant crystalline phase under all investigated conditions. Overall, the intensified swirling motion and secondary flows effectively compensated for viscosity-induced mixing deterioration, providing an efficient strategy for the continuous and controlled synthesis of spherical CaCO3 particles in CMC-Na thickened liquid–liquid precipitation systems.