Numerical Determination of Nucleation Parameters in Polymorphic Crystallization from Solutions
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NUMERIRCAL DETERMINATION OF NUCLEATIlON PARAMETERS IN POLYMORIPHIC CRYSTALLIIZATI[ON FROM SOLUTIIONS M.MERLO, A. SANTOS and L.GAGO DUPORT Dept. Estructura y Propiedades de los Materiales. U. de Cddiz. Apdo. 40 Pto.Real. (Cddiz).Spain.
ABSTRACT A numerical model based in nucleation and growth algorithms has been designed to compute the temporal sequences of precipitation and phase transformation kinetics in metastable solutions of polymorphic or hydrated substances. A outline of the model is made for the particular case of crystallization of several hydrated phases from nickel nitrate solutions RINTRODUCTILON The development of numerical models for the description of nucleation processes tends to reflect the difficulty of experimentally accessing the involved process parameters. If not a quantitative approximation, referring to a specific system, the aim tends to be the investigation of the physical aspects connected with the nucleation mechanism. This is the case with the Ising models adapted to the study of the microscopic nature of the metastable state [1],. From another point of view, but also based on growth and nucleation algorithms, numerical models are also frequently used in the interpretation of experimental data on crystallization [2]. Their main advantage, compared to the normal methods (determination of Tmax, and continuous cooling or tind, at constant supersaturation) lies in the possibility of reproducing the continuous crystallization process making it possible to evaluate the temporal evolution of parameters such as supersaturation, growth rate or crystal size distribution. The information on the nucleation is in all cases inferred and, therefore restricted to the specified assumptions of each procedure. When numerical simulation models are used this is done in function of the type of algorithm used, which acts as an initial growth condition and on which depends the optimization of the model with the respect to the experimental data. The model that is described in this work has been designed for the treatment of kinetic data, both for nucleation and crystal growth, based on experiments of various microscopic techniques. It allows, furthermore, for the analysis of the influence of nucleation algorithms based on different thermodynamic considerations (referring mainly to the surface tension or the grain size distribution), of the parameters involved in the process. An example of the type of information obtained is shown in the application of various nickel nitrate structural phases monitored experimentally by calorimetry (in continuous cooling processes at different rates) or by X-ray powder diffraction at constant temperature. NUCLEATIION AND GROWTH ALGORIITHMS. As input values are used experimental data of various techniques sensitive to some macroscopic property variable to the crystallization (calorimetry, conductimetry, diffraction etc.). With the aim of making the model independent of the technique, the experimental values have been expressed in terms of crystallized mass fraction per time unit
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