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Evolución del sistema Al203-Na3AlF6 durante su proceso de cristalización por descenso de temperatura

López-Acevedo Cornejo, Victoria and López Andrés, Sol (2004) Evolución del sistema Al203-Na3AlF6 durante su proceso de cristalización por descenso de temperatura. Boletín de la Real Sociedad Española de Historia Natural. Sección geológica , 99 (1-4). pp. 219-235. ISSN 0583-7510

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Abstract

A partir del diagrama de estabilidad determinado por FOSTER(1960) para el sistema Al2O3 - Na3AlF6, y aplicando
algunas leyes y normas que regulan la cristalización en soluciones acuosas, se ha desarrollado un procedimiento
teórico que permite modelizar el comportamiento de dicho sistema, durante su proceso de cristalización, en
forma de corindón y criolita, mediante la técnica conocida comúnmente como "flux growth" (disolución no acuosa
de alta temperatura). Los datos empleados para llevar a cabo dicha modelización, se corresponden con las variables
de entrada aplicadas en una serie de experimentos, realizados con esta técnica y con el mismo sistema. La concordancia
entre los resultados experimentales obtenidos y las previsiones estimadas teóricamente, pone en evidencia la
validez del procedimiento propuesto y constituye una nueva aportación para el conocimiento formal de los procesos
implicados en dicha técnica de cristalización.
[ABSTRACT]
A procedure which allows to foresee theoretically the behaviour of the Al203-Na3AlF6 system during its crystallisation
process in corundum (AL203) and cryolite (NA3ALF6) forms by the technique commonly known as flux growth
(high temperature solution) has been developed from ihe stability diagram detennined by FOSTER(1960). It has been
carried out a series of experiments with the same technique and system. The entrance variables of these experiments
have been used to develop the theoretical procedure. In all these experiments, corundum (A~O) and cryolite (Na3AlF6)
crystals have been obtained. Rarely, diaoyudaoita (NaAl11017)' villiaumite (NaF) and chiolite (Na5Al3F11) have also
been obtained. The position which occupies these crystallme phases in the crucible shows the way in which its nueleation
and their later growth have taken place: the phases located in the higher part of the crucible nueleated and grew
in vapour phase, out of the flux, and the phases located in the lower part were done in solution.
The initial alumina andcryolite concentration detennines the exelusive crystallisation of one or the other phase
in their stability fields (before reaching the eutectic point). The initial concentration of alumina and cryolite, in relation
to the initial temperature, detennines the temperature at which, the first nueleus, in each case, are created. The
cryolite concentration in its interval of stability originates in all of the experiments already carried out a solid residue
in which two levels of different composition are distinguished: the highest enriched with cryolite and the lowest
enriched with corundum. The characteristics of the crystalline aggregates situated in this low level allow to differentiate
two fonnation process: one characterized by continuous episodes of primary nueleation in which those
nueleus do not increased its size and originate a cryptocrystalline material, and another one characterized by an only
episode of heterogeneous nueleation which originates visible crystals in plates and micrometric density fonn. In the
eutectic, all the phases which can be crystallised are developed from the components of the residualliquid as their
respective equilibrium concentrations are being obtained.
..- The position of the heating plates in the furnace determine the temperature gradient originated in the working
chamber: if the temperature decreases towards the crucible basis, the nueleation of the vapour phase it is
not favoured in the highest part of the crucible. Whereas, if the gradient is inverted, the nueleation of the vapour
phase takes place in the highest part of the crucibles. This fact varies the proportion between the different components
of the system and, in consequence, the theoretic ca1culations obtained from them can be fiddled. However, in spite of this possible desviations, it is made clear the concordance between the experimental results obtained
and the forecasts theoretically estimated from the procedure proposed. The agreement between experimental
and theoretical result supposes an advance for the formal knowledge of the crystallisation process through
the "flux" technique.

Item Type:Article
Uncontrolled Keywords:Crecimiento de cristales, Cristalización en disoluciones de alta temperatura, Técnica del "flujo", Sistema AI203-Na3AIF6, Crecimiento de corindón, Crecimiento de criolita, Crystal growth, High temperature, Solution crystallisation, Flux growth technique, Al03-Na3AlF6 system, Corundum crystallisation, Cryolite crystallisation
Subjects:Sciences > Geology > Crystallography
ID Code:14857
Deposited On:17 Apr 2012 11:46
Last Modified:06 Feb 2014 10:10

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