Concomitant Precipitation of Solid-State Miscible Product-Impurity Phases in Solution Crystallization. Part 1: Theoretical Basis and Workflow for Efficient Impurity Rejection

Crystallization is routinely employed in industrial chemical synthesis to remove undesired impurities and obtain the product as a crystalline solid. A common impurity retention mechanism in crystallization is the formation of solid solutions, where the impurity is partially miscible in the solid state with the product. If the impurity level is high enough to exceed the solid-state miscibility limit or solvus, a second solid-solution phase may form. This second crystallization event is often erratic, causing an unexpected rise in the impurity level, which can then lead to significant challenges and potentially render the product out-of-specification. This two-part contribution is focused on the formation of the aforementioned second solid-solution phase. Part 1 describes the thermodynamic origin of the formation of two crystalline solid solutions in solution crystallization. A thermodynamic model is introduced based on varying solid-state miscibilities between the product and impurity, solvent solubilities and crude quality. This model is verified with experimental data from the so-called SLIP test of two model systems exhibiting partial solid-state miscibilities. Next, a workflow is presented on how to approach and resolve impurity challenges undergoing this impurity retention mechanism. This mechanism-based workflow utilizes the solid form landscapes of product and impurity in conjunction with the thermodynamic model for rational selection of solvent system for effective purification via either crystallization, reslurrying, polish filtration, or combinations thereof. Its utility is demonstrated in Part 2 through three industrial case studies to arrive at robust and scalable solutions.