Institute of Mineral Engineering Department of Ceramics and Refractory Materials Univ.-Prof. Dr. rer. nat. R. Telle

Thermal Stability of Zircon (ZrSiO4) and its

Dependence on Impurities in Natural Zircon Raw-Materials

The interest in zircon (ZrSiO4) as a ceramic and refractory material is due to its excellent thermo physical properties such as low thermal expansion, low thermal conductivity, as well as a good corro-sion resistance, for example, against glass melts, slags and liquid metal alloys. Accordingly, zircon has a wide range of applications as a construction material in glass tanks, in iron and steel production, in energy technology, as moulds and cores in preci-sion investment casting or as protective coatings of steel-moulding tools. In contrast to all this high tem-perature applications extremely differing informa-tion concerning the thermal stability of ZrSiO4 and the temperature of the solid state dissociation ZrSiO4 ZrO2 + SiO2 exist, varying between 1258C und 1700C and leading to doubtful statements concern-ing service life and long-time behaviour. In this project the mechanism of the decomposition-reaction of ZrSiO4 and its decomposition tempera-ture, as well as its dependence on type and concen-tration of impurities in natural zircon raw materials have been investigated. Based on the heat-treatment of numerous natural and synthetic zircon raw-materials with known grain size and impurity-concentration and also zircon single crystals the dissociation temperature of ZrSiO4 has been specified to 167310C. The solid state disso-ciation of ZrSiO4 has been found to be a multiple-stage process, showing a demixing and gradual en-richment of SiO2 in form of discrete metastable, intermediate compounds like ZrSi2O6 around the simultaneously precipitated ZrO2-grains (figure 1). The eutectic temperature in the ZrO2-SiO2 system has been determined to 168710C.

Fig. 1: Initial state of decomposition of the ZrSiO4-

single crystal, showing a solid state reaction un-der formation of discrete metastable, intermedi-ate phases (1660C)

Taking these results into consideration the thermo-dynamic dataset for the ZrO2-SiO2 system in FACT-Sage has been optimized (decomposition tempera-ture in the database so far: 1538C), especially with a dissociation temperature of 1673C and a eutectic temperature of 1687C (figure 2). The combination of this newly created dataset with the databases available in the FactSage software package allows thermochemical calculations in the system ZrO2-SiO2-FeO-Fe2O3-TiO2-Ti2O3-MgO (+ small concentrations of CaO), which contains the main impurities in natural zircon raw materials like rutile, illmenite, magnetite, quartz, corundum etc.. The thermochemical calculations validate the ex-perimental results that even minor impurities lead to a degradation of the eutectic temperature in the bi-nary ZrO2-SiO2 system, which is just 14C higher than the dissociation temperature of the zircon.

Thermodynamic Materials Modelling

. Fig. 2: Calculated optimized phase diagram for the

quasi-binary ZrO2-SiO2 system Through this the solid state dissociation of the zircon will be superimposed by the formation of silicate melt at temperatures noticeable below the dissocia-tion temperature, in which a rapid solution of the ZrSiO4, i.e. substantially its SiO2-portion and a of-tentimes dendritic-like precipitation of ZrO2 is ob-served (figure 3). In addition, above 1650C the dissociation of zircon as a solid state reaction starts, resulting in an accel-erated decomposition-kinetics and leading to a rapid decomposition of the ceramic material. Thermochemical calculations have also been applied to verify the influence of specific impurity phases on the decomposition of ZrSiO4, which have also veri-fied by the experimental results. One example is the appearance of ZrO2 in samples heat-treated below 1600C, which is often associated with the existence of impurities like corundum (-Al2O3). Corundum reacts with the surrounding zircon grains according the reaction 2 ZrSiO4 + 3 Al2O3 = Al6Si2O13 + 2 ZrO2 under formation of zirconia and mullite. Ther-mochemical calculations also explain the absence of characteristic iron- and titanium-containing impuri-ties of the zircon raw materials (rutile, ilmenite, magnetite etc.) in samples heat treated above 1500C. It has been shown, that these impurities contribute to a very high degree to the formation of a low melting liquid phase, which can be found in the microstructures as amorphous solidified glass phase, showing an enrichment of all impurities of the zir-con raw-materials like Al2O3, Fe2O3, TiO2, CaO etc..

Fig. 3: Thermal decomposition of a natural zircon raw material at 1700C ([1]: amorphous solidified melt phase - 13.5 wt.% ZrO2, 81.1 wt.-% SiO2, 5.4 wt.-% Al2O3); white: ZrO2; grey: ZrSiO4)

Beside its dependence on the grain size shown in this work, the thermal stability of ZrSiO4 depends to a great extent on the type and concentration of impu-rities in the zircon raw-materials. In a subsequent project the influence of alkaline- and alkaline earth-oxides on the decomposition of zircon has been studied by preparing mixtures of MgO, CaO and Na2CO3 and different zircon raw-materials according the reactions 2 ZrSiO4 + 1 Na2O = 1 Na2ZrSi2O7 + 1 ZrO2 1 ZrSiO4 + 2 CaO = Ca2ZrSi4O12 + 3 ZrO2 1 ZrSiO4 + 2 MgO = 1 Mg2SiO4 + 1 ZrO2 and heat-treating them at temperatures between 900 und 1500C. The occurring phase-reactions and the resulting microstructures have been examined by x-ray diffraction and electron microscopy. Literature: R. Telle, A. Kaiser, M. Kemper, M. Lobert, T. Kocyan Investigation of the Phase Reaction ZrSiO4 ZrO2 + SiO2 and its Influence on the Corrosion of Refractories Stahl und Eisen Special, 46th International Colloquium on Refractories, Aachen 12 and 13 November 2003 35-43 A. Kaiser, M. Lobert, R. Telle Studies on the Thermal Stability of ZrSiO4 and its Corro-sion Resistance to Alkaline and Alkaline-Earth Oxides Stahl und Eisen Special, 48th International Colloquium on Refractories, Aachen 28 and 29 September 2005 124-129

Department of Ceramics and Refractory Materials Please contact:

Mauerstrasse 5, D-52064 Aachen phone: +49-(0)241-8094968 fax: +49-(0)241-8092226 www.ghi.rwth-aachen.de

Dr. rer. nat. Arno Kaiser phone.: +49-(0)241-8094979 fax: +49-(0)241-8092226 e-mail: kaiser@ghi.rwth-aachen.de

Institute of Mineral Engineering

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