Sizing of Precipitates

Measurement methods for particle suspensions can be categorized into two groups  

Ensemble techniques that process information produced simultaneously by many particles and require the use of complex mathematical algorithms to invert the data 

Dielectric Sedimentation Method. The measurement relies upon the sensing of changes in the dielectric constant of medium between a pair of capacitance sensing plates placed around the sedimentation tube. 

Sedimentation Field Flow Fractionator. The chromatography-related principle of this particle size and size distribution analyzer is based upon the interaction of the particle suspension under centrifugal field motion in a thin channel. The elution time of the particles is a function of particle size, particle density, flow rate of mobile phase, density of mobile phase, and the centrifugal force applied. After the size separation has occurred, the particles are detected in the mobile phase using a turbidity detection system. The dynamic range of the instrument is dependent on particle density and operating conditions and is typically within 0.03 /rm— 1 /rm range. 

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Fig. 20.3(a) A piece of a nickel-based super-alloy cut open to show the structure there are two sizes of precipitates in the alloy - the large white precipitates, and the much smaller black precipitates in between. 

Houcine, L, Plasari, E., David, R. and Villermaux, J., 1997. Influence of mixing characteristics on the quality and size of precipitated calcium oxalate in a pilot scale reactor. Transactions of the Institution of Chemical Engineers, 75, 252-256. 

The particle size of precipitated potassium heptafluorotantalate is one of the more important parameters. In order to achieve a certain particle size, potassium salts are added to the hot tantalum strip solution as a hot solution. The mixture is cooled down at a specific rate in order to enable the precipitation and ciystallization of K-salt in the form of small, individual crystals. 

Therefore, the scope of this chapter is to describe the results obtained using SAS with particular emphasis on the production of particles with controlled PS and PSD in the micrometric as well as in the nanometric range. We will try to understand the role of high pressure vapor-liquid equilibria (VLEs) in determining the morphology and particle size of precipitates. 

Factors That Determine the Particle Size of Precipitates The particle size of solids formed by precipitation varies enormously. At one extreme are colloidal suspensions, whose tiny particles are invisible to the naked eye (10 to 10 cm in diameter). Colloidal particles show no tendency to settle from solution and are not easily filtered. At the other extreme are particles with dimensions on the order of tenths of a millimeter or greater. The temporary dispersion of such particles in the liquid phase is called a crystalline suspension. The particles of a crystalline suspension tend to settle spontaneously and are easily filtered. 

Von Weimam discovered that the particle size of precipitates is inversely proportional to the relative supersaturation of the solution during the precipitation process ... 

Similarly, higher stirrer speeds decrease the initial local environment concentration causing the improvement of micromixing, slower nucleation rate, and larger mean crystal size of precipitate. 

Experimental techniques used in the assessment of kinetics of precipitation and sizing of precipitate must address the specificity of this process. In particular, they need to be concerned with rapid chemical reactions, high initial supersaturation, high-order nucleation kinetics, very short time-scale of concurrently occurring component-phenomena, and small size of the crystals. 

Precipitation or reactive crystallization is very common in industrial applications and laboratory practice. A large number of high-value added product and intermediate materials are produced via precipitation. The precipitation process is very complex and the properties of precipitate strongly depend on the kinetics of the component subprocesses and their conditions. All these factors, and also the fact that the typical size of precipitate is in the submicron to 100 fim range, make the precipitation process very unique. Frequently, different theoretical and experimental approaches than those used for typical crystallization are required. 

When precipitates grow according to the Ostwald ripening, the average size of precipitates used in the Eq. (22) is obtained ... 

Incorporation of carbonate and other impurities occurs during irreversible hydrolysis reactions of ceramic precursors towards precipitation of HA. These ions decrease the transformation rate and crystal size of precipitated HA (LeGeros 2008). 

DS Halverson. Precipitation from supercritical fluids effects of process conditions on the morphology and particle size of precipitation products. MS thesis, Princeton University, Princeton, NJ, 1989. 

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