Crystal contact nucleation

Secondary nucleation is the formation of nuclei attributable to the influence of the existing microscopic crystals in the magma. The first type of known secondary crystallization is attributable to fluid shear, the other due to collisions between already existing crystals with either a solid surface of the crystallizer or with other crystals themselves. Fluid shear nucleation occurs when liquid travels across a Crystal at a high speed, sweeping away nuclei that would otherwise be incorporated into a Crystal, causing the swept-away nuclei to become new crystals. Contact nucleation has been found to be the most effective and common method for nucleation. The benefits include the following... 

Correlations of nucleation rates with crystallizer variables have been developed for a variety of systems. Although the correlations are empirical, a mechanistic hypothesis regarding nucleation can be helpful in selecting operating variables for inclusion in the model. Two examples are (/) the effect of slurry circulation rate on nucleation has been used to develop a correlation for nucleation rate based on the tip speed of the impeller (16) and (2) the scaleup of nucleation kinetics for sodium chloride crystalliza tion provided an analysis of the role of mixing and mixer characteristics in contact nucleation (17). Pubhshed kinetic correlations have been reviewed through about 1979 (18). In a later section on population balances, simple power-law expressions are used to correlate nucleation rate data and describe the effect of nucleation on crystal size distribution. 

Supersaturation has been observed to affect contact nucleation, but the mechanism by which this occurs is not clear. There are data (19) that infer a direct relationship between contact nucleation and crystal growth. This relationship has been explained by showing that the effect of supersaturation on contact nucleation must consider the reduction in interfacial supersaturation due to the resistance to diffusion or convective mass transfer (20). 

Nucleation due to crystal-to-ciystal contact is greater for equal striking energies than ciystal-to-metal contact. However, the viscous drag of the liquid on particle sizes normaUy encountered hmits the velocity of impact to extremely low values. The assumption that only the largest crystal sizes contribute significantly to the nucleation rate by ciystal-to-crystal contact permits a simple computation of the rate ... 

Contact nucleation by collision of ciystals with the walls of the ciystallizer, agitator or neighboring crystals. 

Another important development which altered our view of crystallization processes was the realization of the importance of secondary nucleation due to contact between crystals and the impeller and vessel. Secondary nucleation of this type has been shown (2-6) to often have a dominant role in determining crystallizer performance. Our understanding of crystal growth, nucleation, fluid mechanics and mixing have all greatly improved. A number of review (2r 101 have appeared in recent years which describe the advances in these and... 

Contact nucleation in industrial processes results from collisions of crystals with the impeller used for circulation of the magma or with other crystallizer internals such as baffles, pipe and crystallizer walls, and even other crystals. Careful experimental studies have shown that the number of crystals produced by collisions between crystals and these objects depends upon the collision energy, supersaturation at impact, supersaturation at which crystals mature, material of the impacting object, area and angle of impact, and system temperature. The collision energy for contact nucleation is small and does not necessarily result in the macroscopic degradation or attrition of the contacted crystal. 

Another theory that could account for the effect of supersaturation on contact nucleation is based on the view that nuclei formed cover a range of sizes that includes the critical nucleus. Since only the nuclei larger than the critical nucleus are stable, the relationship of the size of the critical nucleus to supersaturation reflects the dependence of contact nucleation on supersaturation. This concept, which has been referred to as a survival theory, seems to have been refuted by measurements of the sizes of crystals formed by collisions. These sizes are much larger than the critical nucleus, and the survival theory would have little influence on the number of nuclei that survive. 

If primary nucleation dominates the process, i tends to larger values (say greater than 3), j and k approach zero, and Eq. (18) approaches Eq. (17). Should crystal-impeller and/or crystal-crystallizer impacts dominate, j approaches 1 on the other hand, if crystal-crystal contacts dominate, j approaches 2. 

The ease with which nuclei can be produced by contact nucleation is a clear indication that this mechanism is dominant in many industrial crystallization operations. Research on this nucleation mechanism is continuing with the objective of building an understanding of the phenomenon that will allow its successful inclusion in models describing commercial systems. 

Secondary nucleation results from the presence of solute particles in solution. Recent reviews [16,17] have classified secondary nucleation into three categories apparent, true, euid contact. Apparent secondary nucleation refers to the small fragments washed from the surface of seeds when they are introduced into the crystallizer. True secondary nucleation occurs due simply to the presence of solute particles in solution. Contact secondary nucleation occurs when a growing particle contacts the walls of the container, the stirrer, the pump impeller, or other particles, producing new nuclei. A review of contact nucleation, frequently the most significant nucleation mechanism, is presented by Garside and Davey [18], who give empirical evidence that the rate of contact nucleation depends on stirrer rotation rate (RPM), particle mass density, Mj>, and saturation ratio. 

Equation (18-39) displays the competing factors that stabilize secondary nucleation in an operating crystallizer when nucleation is due mostly to impeller-crystal contact. Any increase in particle size produces a fifth-power increase in nucleation rate, tending to counteract the direction of the change and thereby stabilizing the crystal-size distribution. From dimensional argument alone the size produced in a mixed crystallizer for a (fixed) nucleation rate varies as (B ). Thus, this fifth-order response of contact nucleation does not wildly upset the crystal size distribution but instead acts as a stabilizing feedback effect. 

Contact nucleation is the most common mechanism of secondary nucleation. Crystal-crystal-, crystal-impeller, and crystal-wall collisions are involved. Secondary nuclei arise from microabrasion (crystal surface damage) or ordered cluster removal by fluid shear forces, as noted above. Figure 4-12 shows that for a given substance, impeller speed and material of construction can both play a role. 

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