Continuous precipitation of nanosized calcium phosphates, including HAp: the role of operating conditions and highly hydrolyzed PVA additive on particles’ characteristics, phase composition, and precipitation kinetics

The research describes the continuous precipitation of hydroxyapatite nanoparticles from aqueous solutions of (NH₄)₂HPO₄ and Ca(NO₃)₂, carried out in the Koflo static mixer. The role of a specific turbulence distribution and its intensity, solutions’ concentrations, and the addition of polyvinyl alcohol (PVA) on both the chemical and phase compositions, and final product features have been investigated. The driving force of the process, nucleation sources, and mechanisms controlling particles’ growth have also been determined. It was shown that the shape of HAp particles is primarily influenced by the unit power input ε_mix and may be changed within petals, whiskers/needles, or spheres. The mean size of single particles varied from 390 to 94 nm, however, agglomeration was observed. Its share was reduced to some extent by i) an increase of ε_mix and/or ii) the use of a small amount of PVA (up to 2 % w/v). The conditions limiting the formation of other than hydroxyapatite CaP phases (i.e. monetite, brushite), allowing to obtain high purity HAp (≥ 99 %) were defined. As the sources of nuclei, primary heterogeneous nucleation and secondary one were indicated. It was also recognized that the particle growth in the system was controlled by bulk diffusion.