CRYSTALLIZATION FROM DILUTE METAL SOLUTIONS
BS>The nucleation and crystal growth from solutions of zinc in mercury were studied; the test amalgam formed one pole of a concentration cell whose emf was used as a continuous measure of the concentration. The metastable region in which spontaneous nucleation could not occur extended to 2 deg C below the saturation temperature. Nucleation at temperatures below the metastable region normally produced a crop of fine crystals mainly suspended in the liquid phase, which sometimes subsequently aggregated into a solid cap at the surface or walls. The rate of nucleation varied approximately as exp (--const/ DELTA T/sup 2/). Stirring increased the nucleation rate by only a factor of about 5. The rates observed were immensely greater than those reported by Hollomon and Turnbull for pure metals subdivided into small drops; this would suggest heterogeneous nucleation on foreign particles. If a first crop of crystals were allowed to form and then to be removed from the system, the subsequent rate of nucleation was not signiflcantly reduced by this treatment; this would suggest that the rate cannot depend on the presence of foreign particles. One possible explanation is that nucleation may occur only at the glass wall, and the growing crystallites are subsequently floated into the solution. Another depends on the further observation that if a small number of crystals is added to a solution cooled at such a rate as to keep it within the metastable region (where spontaneous nucleation cannot otherwise occur), an immense crop of nuclei is produced equal in number to that obtained with spontaneous nucleation at temperatures below the metastable region. In other words it would appear that nuclei breed nuclei with a high multiplication factor. The high rates of nucleation of a bulk liquid phase may be due to this breeding of nuclei and not to heterogeneous nucleation. If massive zinc is added to an amalgam that is in equilibrium with a solid phase that has been obtained by cooling a solution from higher temperatures, the massive zinc dissolves and is reprecipitated as very fine crvstals; this is attributed to the metastability of the alpha phase at the temperatures in question. From emf data on 2-phase amalgams, heats of solution were calculated. The solid phase in these experiments could be largely freed from mercury by centrifugation and was found to consist of hexagonal plates and prisms rounded by stirring. (auth)
- Research Organization:
- Imperial Coll. of Science and Tech., London
- NSA Number:
- NSA-14-024596
- OSTI ID:
- 4141044
- Journal Information:
- Acta Met., Vol. Vol: 8; Other Information: Orig. Receipt Date: 31-DEC-60
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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