Precipitation kinetics ripening

It is frequently observed that precursor and metastable phase form before the formation of stable phase. For example, Schoonen and Barnes (1991) experimentally clarified that precursor, FeS , is requisite for the formations of marcasite and pyrite (FeS2). The changes from precursor through metastable phase to stable phase are commonly recognized. This process is called Ostwald ripening. Therefore, precipitation kinetics of precursor and metastable phase has to be elucidated. 

In order to monitor the mechanical properties in relation to the microstructure, the knowledge of the precipitation state at the end of a thermo-mechanical treatment is of prime importance. In this purpose, Arcelor develops models that allow for the prediction of the influence of the process parameters on the state of precipitation. The model Multipreci, developed at IRSID is one of them. It (Hedicts the precipitation kinetics of mono- and di-atomic particles in ferrite and austenite as a function of the time-temperature history. It is based on the classical theories for diffusive phase transformation and treats simultaneously the nucleation, growth and ripening phenomena. The state of precipitation that is predicted includes the particle size distribution, their number and volume fraction. From these values, the effect of the precipitates on the mechanical properties can be calculated. 

Secondary phases predicted by thermochemical models may not form in weathered ash materials due to kinetic constraints or non-equilibrium conditions. It is therefore incorrect to assume that equilibrium concentrations of elements predicted by geochemical models always represent maximum leachate concentrations that will be generated from the wastes, as stated by Rai et al. (1987a, b 1988) and often repeated by other authors. In weathering systems, kinetic constraints commonly prevent the precipitation of the most stable solid phase for many elements, leading to increasing concentrations of these elements in natural solutions and precipitation of metastable amorphous phases. Over time, the metastable phases convert to thermodynamically stable phases by a process explained by the Guy-Lussac-Ostwald (GLO) step rule, also known as Ostwald ripening (Steefel Van Cappellen 1990). The importance of time (i.e., kinetics) is often overlooked due to a lack of kinetic data for mineral dissolution/... 

Dissolution-precipitation models. Dubinina and Lakshtanov (1997) developed a kinetic model that describes isotopic fractionation between a mineral and fluid involved in one of three types of dissolution-precipitation processes (Fig. 11). Type I (mineral synthesis) considers successive dissolution of an unstable phase, A, of uniform isotopic composition and precipitation (crystallization) of phase B. Type II (Ostwald ripening) involves the partial dissolution of phase B which has a non-uniform isotopic composition... 

The effect of ageing of Th02(am, hyd) precipitates may be interpreted in terms of increasing particle size via dissolution-precipitation equilibria, so called Ostwald ripening. This effect is kinetically favoured at higher aqueous Th concentrations, i.e., at low pH. 

Sugimoto, T. (1978) Kinetics of reaction-controlled Ostwald ripening of precipitates in the steady state. Journal of Colloid and Interface Science, 63, 369-377. 

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