Incorporation of ionic impurities in crystals growing from solution. The case of lead ions in potassium chloride crystals

✍ Scribed by G. D. Botsaris; E. A. Mason; R. C. Reid

• Publisher: American Institute of Chemical Engineers
• Year: 1967
• Tongue: English
• Weight: 524 KB
• Volume: 13
• Category: Article
• ISSN: 0001-1541
• DOI: 10.1002/aic.690130432

✦ Synopsis

Single seed crystals of potassium chloride were grown from aqueous solutions under conditions of constont temperature, supersaturation, and impurity concentration. W i t h emission spectrography as the method of analysis, the distribution of various metal ion impurities between the grown potassium chloride crystal and the solution was studied. The distribution coefficients of Pb++, defined as the ratio of the impurity concentration in the crystal t o the concentration in solution, were found to be greater than unity a t very low concentrations.

The leod distribution coefficients varied between 2,700 (for solutions containing 10-8 moles of lead/mole of potassium chloride to 28 (for solutions of 10-5 moles of lead/mole of potassium chloride).

The results suggest a mechanism of nonequilibrium capture of the impurity. The magnitude of the distribution coefficient is shown to depend on the equilibrium distribution Coefficient between the solution and surface of the crystal, the rate o f growth of the crystal, and the rate of diffusion of the impurity through the crystal lattice. These results are believed to be of significance in the purification of materials by fractional crystallization. For instance, they indicate thot i n order to produce crystals of lead-free potassium chloride, the conventional techniques or recrystallization should be modified to include rejection of the first crystals grown.