The purpose of this study was to investigate contact nucleation for various types of crystalline materials. Selected were aqueous solutions of two hydrated inorganic salts (MgS04 -7Hz0 and KAl(S04)z -12Hz0), one nonhydrated inorganic salt (&SO4) and one organic substance (citric acid). Nucleation data were determined as number of nuclei generated per contact as a function of supersaturation and impact energy. Both of these variables strongly influenced the number of nuclei generated per contact. These data show that crystal growth and contact nucleation are related, with more rapidly growing substances giving higher yields of nuclei. 1. Unless a seed crystal was cured before it was immersed in supersaturated solution, new crvstalp wniild form from microscopic particles (crystal dust) washed from the crystal surface. C2. No nucleation occurred on immersion of cured seed crystals into n slightly supersaturated solution unless there was contact between the crystal and another solid material. Nucleation under these conditions is called contact nucleation or collision breeding.3. When supersaturations were high enough to induce dendritic growth on the seed crystals, new crystals were formed by fluid movement shearing these dendrites from the parent crystal.Clearly ilie conditions described in these observations indicate that when secondary nucleation occurs in crystallization processes it must occur according to Observations 1 or 2. Moreover, in systems where external seeding is not used, contact nucleation, as described in Observation 2, must be the predominant nucleation mechanism.Clontz and McCabe (1971) and Johnson, Rousseau, and McCabe (1972) presented data quantifying the factors influencing contact nucleation for MgS04 * 7Hz0. The research renorted in this paper includes refinements to the previously reported work on MgS04 * 7H20 and additional studies on KAl(S04)2 -12H20, &SO4, and citric acid. This study will lead to a better understanding of secondary nucleation and provide a base for modeling and controlling nucleation in crystallization processes. CONCLUSIONS AND SIGNIFICANCEThe necessity of an impact between seed crystals and another object for nucleation to occur in slightly supersaturated aqueous solutions of the solutes investigated has been confirmed. In no case did nucleat: 'on occur without such an impact. Both impact energy and solution supersaturation were extremely important variables in determining the number of nuclei generated by a single contact with a seed crystal. Within the range of supersaturations considered, a linear relationship was found to exist between the number of nuclei and supersaturation for the three inorganic salts; this was not true for citric damage to the seed crystal. In all cases the number of nuclei generated was an exponential function of impact energy. Potassium sulfate was the only substance exhibiting a well-defined threshold energy; that is, impact energies below this level produced no nucleation. The relationship between a threshold energy and crystallizer stabili...
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