Precipitation phenomena in supersaturated solid solutions

Reactions in metallic systems in which a new phase precipitates from a supersaturated solution have very broad technological applications. For instance, such processes cause the precipitation hardening of alloys. The yield point and the strength of the alloy are increased as a result of the interaction of the dislocations with the precipitated particles. Accordingly, the quantitative treatment of a hardening problem requires that the number, size, distribution, and shape of the precipitated particles be known. In the following sections, certain kinetic problems which play an important role in this regard will be discussed [8]. 

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Precipitation and dissolution phenomena are extremely important in both natural waters and water treatment processes. Dissolution of minerals is a prime factor in determining the chemical composition of natural waters. Natural water chemical composition can be altered by precipitation of minerals and the subsequent sedimentation of these solids from supersaturated solutions. Water and wastewater treatment processes such as lime-soda softening, iron removal, coagulation with hydrolyzing metal salts, and phosphate precipitation are based on precipitation phenomena. 

In order for precipitation phenomena to take place, a supersaturation must occur. This will, in general, arise as a result of the undercooling of a homogeneous solid solution into a two-phase region (i.e. as a result of the crossing of a solubility line). Further discussion will be based upon the situation shown in Fig. 7-8. 

While this book treats several topics similar to those found in an analytical chemistry text, it endeavors to consider the spatial and temporal scales of the reactions in nature as distinctly different from those of the laboratoiy. For example, in chemical analysis, precipitates (frequently of metastable and active compounds) are formed from strongly oversaturated solutions, whereas in natural water systems, the solid phase is often formed under conditions of slight supersaturation often crystal growth and aging may continue over geological time spans. Interfacial phenomena are particularly important because chemical processes of significance often occur only at phase discontinuities. 

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