Polymorph Selection by Continuous Precipitation

When the dominant rate-mechanisms within a mixed-suspension mixed-product removal (MSMPR) crystallizer are secondary nucleation and size-independent linear crystal growth, the effluent crystal distribution is guaranteed to exhibit a single polymorphic solid form at steady state. However, multiple solid forms are often simultaneously observed during the continuous precipitation of CaCO₃. Accounting for agglomeration within the population balance reconciles model predictions with experiments. Here, we elucidate the steady state structure and linear stability features of an agglomeration-enabled continuous precipitator model. We demonstrate that one can make rational process design and operation decisions to select the effluent solid form, regardless of its thermodynamic stability. Specifically, we utilize these results to choose process conditions that yield pure, thermodynamically metastable vaterite during CaCO₃ precipitation, based on powder X-ray diffraction, solid-state ⁴³Ca NMR, and scanning electron microscopy. This new design framework enables predictive modeling of CaCO₃ precipitation, but more generally, it is expected to enable rational decision making during the design and operation of other agglomerative precipitation processes for which solid form selection is desired.