Face-Specific Growth and Dissolution Kinetics of Potassium Dihydrogen Phosphate Crystals from Batch Crystallization Experiments

The final shape distribution of crystalline materials is an important product quality that is controlled by the growth and possibly also by the dissolution rates of individual crystal facets. Knowledge of the kinetics under batch process conditions allows optimal process design with regard to desired shape distributions. In this work, potassium dihydrogen phosphate (KDP) was chosen as a model substance, for which face-specific growth and dissolution rates were determined in a batch crystallizer. The temporal evolution of the crystal population was tracked with a flow-through microscope using a shape estimation procedure presented by C. Borchert et al. (Image-Based in Situ Identification of Face Specific Crystal Growth Rates from Crystal Populations. Cryst. Growth Des. 2014, 14, 952-971). Effects of concentration and temperature on the kinetics were separately investigated. It was found that crystal growth is strongly affected by impurities at low supersaturation, whereas impurity effects are diminishing at higher supersaturation. The dissolution rates were found to be linearly dependent on the applied undersaturation, and no impurity effects were visible. The effects of temperature on both growth and dissolution kinetics were found to obey the Arrhenius law, and corresponding activation energies for growth and dissolution of KDP were determined.