The secondary nucleation of magnesium sulfate due to the action of fluid forces on a seed crystal was studied by introducing the hypothesized nuclei so generated into a region of high supersaturation. The number of nuclei observed is strongly dependent on the supersaturation at the seed and the severity of shear forces. Classical nucleation theory is used to interpret the results. These results are consistent with other experimental observations described in the recent literature on contact nucleation. The growth and nucleation process inferred is similar to that illustrated by Clontz and McCabe (1972) SCOPEThe objective of this work was the experimental demregion of relatively large supersaturation had to exist onstration that fluid mechanical forces could effect the within the nucleating chamber. The magnesium sulfate generation of secondary nuclei. This objective was system was studied and the, experimental results were achieved only insofar as nuclei were generated but a analyzed using classical nucleation models. CONCLUSIONS AND SIGNIFICANCEThe interpretation Powers (1963) has reported experiments in which nucleation occurred when a sucrose crystal was held on a rotating glass rod in a supersaturated solution. However, both La1 et al. (1969) and Clontz and McCabe (1972) have suggested that fluid shear alone could not produce breeding with MgS04. 7Hz0 at a AT < 4"C, in the region of supersaturations where needle breeding did not occur and where the crystals grow in a nondendritic form. It may have been that the degree of supersaturation was, in fact, too low so that nuclei or embryos which were formed dissolved; also the magnitude of the shear was not considered. In this work we hypothesize that secondary embryonic species can be produced by fluid shear at a growing crystal surface. We suggest that these potentially may have been present in previous work (La1 et al., 1969; Clontz and McCabe, 1972) but may have dissolved because of insufficient level of supersaturation and thus not observed.According to the Gibbs-Thomson equation the critical size varies inversely as the supersaturation: at a given supersaturation, all particles smaller than the critical size will dissolve, and only those larger will survive. The higher the supersaturation, the smaller the critical size, and hence, the greater the number of particles finally surviving. This has been called the survival theory (Garabedian and Strickland-Constable, 1972; StricklandConstable, 1972). According to the hypothesis in this work, fluid forces at a growing crystal surface can cause the formation of potential secondary nuclei. If the supersaturation in which these develop is great enough, then the critical size required is very small. In the work to be described here, these potential nuclei are visualized as formed due to fluid forces at a growing crystal surface, they are introduced quickly into a region of great supersaturation and there they exist as particles for which the sizes are greater than the critical size associated with this region. ...
The classical theory of nucleation has been acceptably well developed, particularly as it pertains to vapor-liquid transformation in pure systems. When liquid solution-solid crystal transformations are considered the theory is still applied, typically, but with less confidence. Perhaps the best measure of the extent to which one can accept classical theory for these latter systems is provided by a study of the experimental results of Nielsen (1969Nielsen ( , 1971. He carried out homogeneous nucleation studies for a variety of ionic crystal-aqueous solution systems. H e was able to show reasonable agreement between classical theory and experiment for some salts (e.g., 2,2 electrolytes), while success was mixed for others. The major goal of this work is the characterization of the crystals which are produced through homogeneous nucleation by the Nielsen process. In order to do this, a preliminary experimental study of the process is required to identify the mechanisms by which the observed product is formed. This paper describes work serving to standardize the experimental procedure. ExperimentalPrecipitation experiments were carried out using a modified Nielsen apparatus, Figure 1. Nielsen's original apparatus was activated by two syringes operating in parallel, each of capacity 5 mL. These operated simultaneously to force the two reactant streams to intimately mix in a downstream tee. The device provided a sample of the precipitate for subsequent study-primarily the counting of the product crystalline entities; it also permitted a measure of the reaction or induction time t i , i.e., the period elapsed between the time of mixing and the visually determined onset of precipitation. The general idea behind this was to mix reactant solutions rapidly so that the precipitation mechanisms occurred from a well-mixed homogeneous solution. The mixing occurred in a specially designed mixing tee whose characteristic mixing time was estimated to be about one millisecond. Obtaining a particular relationship between the number of particles, Nvs. initial supersaturation, So, ensured that homogeneous nucleation had occurred.The apparatus modification involved in this work included a larger volume of reagents, say a maximum of 525 mL, and a mixing tee of simpler configuration. The motivating idea behind this latter change was to enable easy characterization of the tee in the event that future scale-up and modifications were to be considered. The tubing sizes given in Figure 1 are about the same as those used by Nielsen (1961). The larger quantities of reactants were desirable as the goals of the experiments included providing product crystals in sufficient quantities for more elaborate characterization and further processing. Plexiglass vessels D served to contain the reactants under air pressure as they were fed to the mixing tee. In Figure I , C is the pressure gauge, B, the pressure regulator and A, an in-line filter; vessels D were fed from vessels I whose contents were transported through filters G (0.22 wm) by the diaphragm ...
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