Solution precipitation nucleation-crystallization

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

Primary nucleation is the classical form of nucleation. It occurs mainly at high levels of supersaturation and is thus most prevalent during unseeded crystallization or precipitation. This mode of nucleation may be subdivided into either homogeneous viz. spontaneously from clear solution, or heterogeneous viz. in the presence of dust particles in suspension, or solid surfaces. 

The reaction is sustained by addition of iron metal which reacts with the sulfuric acid formed, regenerating Fe(n) in solution. To ensure that the desired crystal form precipitates, a seed of a-FeO(OH) is added. However, with appropriate choice of conditions, for example of pH and temperature and by ensuring the presence of appropriate nucleating particles, the precipitation process may be adapted to prepare either the orange-brown y-FeO(OH), the red a-Fe203 or the black Fe304. 

The ratio of the instantaneous solute concentration c to the solute s solubility s, where the latter is the solute concentration in equihbrium with its crystalline or precipitated phase. Hence, RS = c/s, and a supersaturated solution experiences a thermodynamic driving force (AG = RT ln[RS]). A supersaturated solution will remain as a metastable state, because crystallization or precipitation requires a mechanism for relieving the supersaturated condition (eg., nucleation or addition of crystallite/precipitate). See Biomineralization... 

The area of conditions called the metastable zone is situated between the solubility and supersolubility curves on the crystallization phase diagram (Fig. 3.1). The supersolubility curve is defined as the line that separates the conditions where spontaneous nucleation (or phase separation or precipitation) occurs, from those where the crystallization solution remains clear if left undisturbed (Ducruix and Giege, 1992 Ducruix and Giege, 1999). 

By a change of temperature or pressure, it is often possible to cross the phase limits of a homogeneous crystal. It supersaturates with respect to one or several of its components, and the supersaturated components eventually precipitate. This is an additive reaction. It occurs either externally at the surfaces, or in the crystal bulk by nucleation and growth. Reactions of this kind from initially homogeneous and supersaturated solid solutions will be discussed in Chapter 12 on phase transformations. Internal reactions in the sense of the present chapter occur after crystal A has been brought into contact with reactant B, and the product AB forms isothermally in the interior of A or B. Point defect fluxes are responsible for the matter transport during internal reactions, and local equilibrium is often established throughout. 

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

The process of crystallization or precipitation involves the nucleation and growth of crystals (particles) from a supersaturated solution and is often followed by the agglomeration of crystals. Supersaturation is the driving force for precipitation, which is defined by the difference between the real solute concentration c and equilibrium solute concentration c P,T) at given pressure and temperature. The strict definition of supersaturation is linked to the difference of chemical potential Ap ... 

HOMOGENOUS NUCLEATION. In crystallization from solution, homogenous nucleation almost never happens, except perhaps in some precipitation reactions. The fundamentals of the phenomenon, however, are important in understanding the more useful types of nucleation. 

Induction Period. By a common definition, the time elapsed between mixing two reacting solution and the appearance of first crystals of precipitate is termed an induetion period. In diluted or slowly reacting systems preeipitation starts slowly, then accelerates, and eventually slows down. This initial part is well characterized by an induction period. The induction period r/ jhas beeome an experimental basis to determine the nucleation rate J, using a simple relationship. 

Precipitation has generally been observed to occur in three steps (1) nucleation, (2) crystal growth, and (3) agglomeration and the ripening of the solids. It is possible for a solution that is only slightly supersaturated with respect to a solid phase to be stable indefinitely. When the degree of supersaturation is increased or when fine particles of a substance are mixed with this solution, precipitation occurs. 

Another interesting growth form exhibited by several substances is the development of regular isometrical shapes towards the end of the precipitation process. The phenomenon was described for the case of barium sulphate by Melikhov and Kelebeev (1979) who proposed the mechanism that, in a strongly supersaturated solution, homogeneous nucleation followed by growth yields isotropic crystals of about 2 nm in size. Their surfaces then develop zones with enhanced growth rates which lead to the formation of anisotropic needles. 

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

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