Thermodynamics and kinetics of KDP crystal growth from binary and ternary solutions

The influence of solution thermodynamics and surface kinetics on the growth of single potassium dihydrogen phosphate (KDP) crystals from aqueous solutions in the presence of relatively high concentrations of organic compounds have been studied. Urea, 1-propanol and ethanol were used as the organic compounds. It was shown that the mean activity coefficient of the KDP species in the studied ternary system was successively derived based on the Pitzer activity equations and that the growth processes at different organic compound systems were described using the resulting activity-based driving force. The growth rates of single KDP crystals were measured in pure aqueous and water-organics solutions as functions of supersaturation, the velocity of the solution over a single crystal and the concentration of organics. The results reveal that, in pure solutions, the growth kinetics follows the Burton-Cabrera-Frank (BCF) surface diffusion mechanism and, at a relatively low solution velocity, the growth is controlled by both. diffusion and the surface integration mechanism according to the two-step model. In the studied range of organics concentrations, the influence of urea on the growth rate is less important, while that of alcohols is significant. This evidence was explained by the chemical nature of urea and alcohol molecules for adsorption on the growth surface.