Evolución del sistema Al2O3-Na3AlF6 durante su proceso de cristalización por descenso de temperatura

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.

A procedure which allows to foresee theoretically the behaviour of the Al203-Na3AlF6 system during its crystallisation process in corundum (Al2O3) and cryolite (Na3AlF6) forms by the technique commonly known as flux growth (high temperature solution) has been developed from the stability diagram determined 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 (Al203) and cryolite (Na3AlF6) crystals have been obtained. Rarely, diaoyudaoita (NaAl11O17), villiaumite (NaF) and chiolite (Na5Al3F14) have also been obtained. The position which occupies these crystalline phases in the crucible shows the way in which its nucleation and their later growth have taken place: the phases located in the higher part of the crucible nucleated and grew in vapour phase, out of the flux, and the phases located in the lower part were done in solution. The initial alumina and cryolite concentration determines the exclusive 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, determines the temperature at which, the first nucleus, 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 formation process: one characterized by continuous episodes of primary nucleation in which those nucleus do not increased its size and originate a cryptocrystalline material, and another one characterized by an only episode of heterogeneous nucleation which originates visible crystals in plates and micrometric density form. In the eutectic, all the phases which can be crystallised are developed from the components of the residual liquid 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 nucleation of the vapour phase it is not favoured in the highest part of the crucible. Whereas, if the gradient is inverted, the nucleation 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 calculations 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.