RT Journal Article
T1 Evolución del sistema Al203-Na3AlF6 durante su proceso decristalización por descenso de temperatura
A1 López-Acevedo Cornejo, María Victoria
A1 López De Andrés, María Sol
AB A partir del diagrama de estabilidad determinado por FOSTER(1960) para el sistema Al2O3 - Na3AlF6, y aplicandoalgunas leyes y normas que regulan la cristalización en soluciones acuosas, se ha desarrollado un procedimientoteórico que permite modelizar el comportamiento de dicho sistema, durante su proceso de cristalización, enforma de corindón y criolita, mediante la técnica conocida comúnmente como "flux growth" (disolución no acuosade alta temperatura). Los datos empleados para llevar a cabo dicha modelización, se corresponden con las variablesde entrada aplicadas en una serie de experimentos, realizados con esta técnica y con el mismo sistema. La concordanciaentre los resultados experimentales obtenidos y las previsiones estimadas teóricamente, pone en evidencia lavalidez del procedimiento propuesto y constituye una nueva aportación para el conocimiento formal de los procesosimplicados en dicha técnica de cristalización.[ABSTRACT]A procedure which allows to foresee theoretically the behaviour of the Al203-Na3AlF6  system during its crystallisationprocess 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 beencarried out a series of experiments with the same technique and system. The entrance variables of these experimentshave 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 alsobeen obtained. The position which occupies these crystallme phases in the crucible shows the way in which its nueleationand their later growth have taken place: the phases located in the higher part of the crucible nueleated and grewin 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 phasein their stability fields (before reaching the eutectic point). The initial concentration of alumina and cryolite, in relationto the initial temperature, detennines the temperature at which, the first nueleus, in each case, are created. Thecryolite concentration in its interval of stability originates in all of the experiments already carried out a solid residuein which two levels of different composition are distinguished: the highest enriched with cryolite and the lowestenriched with corundum. The characteristics of the crystalline aggregates situated in this low level allow to differentiatetwo fonnation process: one characterized by continuous episodes of primary nueleation in which thosenueleus do not increased its size and originate a cryptocrystalline material, and another one characterized by an onlyepisode of heterogeneous nueleation which originates visible crystals in plates and micrometric density fonn. In theeutectic, all the phases which can be crystallised are developed from the components of the residualliquid as theirrespective equilibrium concentrations are being obtained...- The position of the heating plates in the furnace determine the temperature gradient originated in the workingchamber: if the temperature decreases towards the crucible basis, the nueleation of the vapour phase it isnot favoured in the highest part of the crucible. Whereas, if the gradient is inverted, the nueleation of the vapourphase takes place in the highest part of the crucibles. This fact varies the proportion between the different componentsof 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 obtainedand the forecasts theoretically estimated from the procedure proposed. The agreement between experimentaland theoretical result supposes an advance for the formal knowledge of the crystallisation process throughthe "flux" technique.
PB Real Sociedad Española de Historia Natural
SN 0583-7510
YR 2004
FD 2004
LK https://hdl.handle.net/20.500.14352/49836
UL https://hdl.handle.net/20.500.14352/49836
LA spa
DS Docta Complutense
RD 4 abr 2025