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  • Science of Sintering, 41 (2009) 11-17 ________________________________________________________________________

    _____________________________

    *) Corresponding author: pengconghua@yahoo.com.cn

    doi: 10.2298/SOS0901011P UDK 661.846:661.883.1:662.785 Effect of Zircon on Sintering, Composition and Microstructure of Magnesia Powders C. Peng*), N. Li, B. Han The Hubei Province Key Laboratory of Refractory and Ceramics, 181, Wuhan University of Science and Technology, 947 Heping Dadao, Wuhan Hubei, 430081, PR China Abstract:

    The effects of zircon on sintering, composition and microstructure of fused magnesia powders were studied by XRD, SEM and EDAX. With the increase of zircon content up to 6 wt%, the strength of sintered samples increased but the apparent porosity decreased. 6 wt% is an appropriate content of zircon to possess better properties of samples, and in this case the samples have a dense microstructure and lower content of glass phase. The presence of a liquid phase resulting from zircon addition is the main reason to improve sintering of magnesia powders. Keywords: Fused magnesia; Zircon; Sintering; Densification

    1. Introduction

    Magnesia-based refractories have been widely used in cement rotary kilns and steel ladles for their high melting point, and good resistance to basic slags and clinkers. However, they have some shortcomings, such as high thermal conductivity, poor thermal shock resistance and penetration resistance [1]. In order to avoid the shortcomings mentioned above, oxides such as SiO2, Al2O3, Cr2O3, TiO2, ZrO2 and WO3 were added into refractories [2, 3]. These oxides may react with MgO to form a second phase to improve sintering of MgO. Chaudhuri et. al [4] studied the effects of titania, ilmenite and zirconia on properties of magnesia refractories, and found that calcium titanate formed in samples with added titania, but ZrO2 was soluble into MgO crystals. Han et al [1] studied sintering of MgO-based refractories with added WO3, and demonstrated that the low melting phases MgWO4 and CaWO4 formed in MgO boundaries, resulting in an increase of the liquid volume. The formation of CaWO4 lead to change of the CaO/SiO2 (C/S) ratio and decreased the amount of 3CaOSiO2 (C3S) and 2CaOSiO2 (C2S), and increased amount of liquid, so that the densification was improved by phase liquid sintering. Guo et al [5] discussed the effect of ZrSiO4 on sintering of sintered- magnesia and the properties of magnesia brick made of pre-synthesized magnesia-zirconia. They indicated that ZrSiO4 could accelerate the sintering of magnesia remarkably, and properties of this magnesia brick were superior to those of common magnesia products.

    Fine powder mixtures of fused magnesia and zircon were used to prepare the MgO-

    http://www.doiserbia.nbs.bg.ac.yu/Article.aspx?id=0350-820X0701003N##

  • C.Peng et al. /Science of Sintering, 41 (2009) 11-17 ___________________________________________________________________________

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    based refractories to form the matrixes which are very important for properties of refractories. Reaction sintering occurred during firing, ZrO2, forsterite and glass phase were in-situ formed in the matrix which gave an important effect on the composition and properties of the samples. This paper describes some of the results.

    2. Experimental procedures

    Fused magnesia and zircon were used in this investigation. The chemical compositions of the raw materials are given in tab. I. The particle size of magnesia powder is

  • C.Peng et al./Science of Sintering, 41 (2009) 11-17 ___________________________________________________________________________

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    content increased from 6 wt% to 9 wt% BD, PLS and AP changed a little. On the other hand, the sintering temperature had an effect on the sintering of samples. The difference of AP, PLS and BD between samples sintered at 1400 and 1500oC were only small. However, BD and PLS of the samples increased, and AP of the samples decreased when the sintering temperature rose from 1500 to 1650oC .

    0 3 6 92.60

    2.65

    2.70

    2.75

    2.80

    2.85

    2.90

    2.95

    3.00

    3.05

    3.10

    3.15

    3.20

    Bulk

    den

    sity

    /g.c

    m-3

    Zircon content /%

    1400 1500 1600 1650

    0 3 6 912

    14

    16

    18

    20

    22

    24

    26

    28

    App

    aren

    t por

    osity

    /%

    Zircon content /%

    1400 1500 1600 1650

    Fig. 1 Variation of BD with zircon content

    Fig. 2 Variation of AP with zircon content

    Fig.4 gives the effect of zircon content on MOR of the samples sintered at different

    temperatures. It was found that with the increase of zircon content up to 6 wt% MOR of samples sintered at 1400, 1500, 1600 and 1650oC increased. However, when the zircon content increased from 6 wt% to 9 wt% MOR of the samples sintered at 1600 and 1650oC decreased.

    0 3 6 9-7.0-6.5-6.0-5.5-5.0-4.5-4.0-3.5-3.0-2.5-2.0-1.5-1.0-0.50.0

    1400 1500 1600 1650

    Line

    ar s

    hrin

    kage

    /%

    Zircon content /% 0 3 6 9

    20

    22

    24

    26

    28

    30

    32

    34

    36

    38

    40

    42

    Col

    d m

    odul

    us o

    f rup

    ture

    /MP

    a

    Zircon content /%

    1400 1500 1600 1650

    Fig. 3 Variation of PLS with zircon content

    Fig. 4 Variation of MOR with zircon content

    At the same time, the MOR of the samples sintered increased with rising of the

    sintering temperature.

    3.2 Phase analysis and microstructure

    Fig.5 shows an XRD pattern of samples sintered with various zircon contents at 1650oC for 3h. It revealed that only periclase was detected in samples Z0 and Z3. However, t-ZrO2 and forsterite were detected in sample Z6 to which 6 wt% zircon was added. No crystal phase containing CaO was detected, showing that CaO and other impurities are in glass phase.

    Fig. 6(a, b) shows the microstructure of samples Z0 and Z6 soaked at 1650oC for 3h

  • C.Peng et al. /Science of Sintering, 41 (2009) 11-17 ___________________________________________________________________________

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    respectively. Sample Z0 (Fig. 6a) has a porous microstructure with more angular and well-defined edged periclase grains. Sample Z6 (Fig. 6b) exhibits a dense microstructure, with round and ellipsoidal periclase grains. White ZrO2 is embedded in the intergranular gaps of periclase grains. The formation of forsterite and the dense microstructure are the reasons for strength increase of the sintered samples.

    10 20 30 40 50 60 70 80 90

    ZZ

    Z

    P

    P

    P

    PFF F

    Inte

    nsity

    (a.u

    .)

    2 (degree)

    Z0

    Z3

    Z6

    Z9

    P: PericlaseZ: t-ZrO2F: Forsterite

    FZ

    P

    Fig. 5 XRD of samples with different zircon content fired at 1650oC

    (a) (b)

    (c) (d) Fig. 6 SEM micrographs of samples sintered at 1650oC: (a) sample Z0; (b) sample Z6; (c) magnified SEM micrograph of sample Z0; (d) magnified SEM micrograph of sample Z6;

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    Tab. III gives the composition of phases in Fig. 6 (c, d) by EDAX. It is possible that there is calcium silicate and a little glass phase in samples without zircon addition. In the samples with zircon additive there are ZrO2, forsterite and more glass phase. Tab. III Composition (wt %) of phases in Fig. 6 (c, d)

    MgO SiO2 CaO ZrO2 Possible phase 1 4.3 25.6 70.1 - C3S or C2S +glass phase2 - 19.3 80.7 - C3S or C2S 3 - - 6.1 93.9 t-ZrO24 24.8 36.1 39.2 - glass phase + M2S 5 53.2 61.3 4.01 - M2S + glass phase

    The existence of ZrO2 and forsterite may improve thermal shock resistance of bricks.

    3.3 Discussion

    Three reactions may occur in the powder mixtures of zircon and MgO, namely decomposition of zircon (Equation 1), solid solution of ZrO2 into MgO (Equation 2), and a reaction between SiO2 and MgO (Equation 3). The phase compositions of the samples depend on the zircon content and solubility of ZrO2 in periclase. According to the MgO - ZrO2 phase diagram [6], the solubility of ZrO2 in periclase is about 2.6 wt% at 1650oC. Therefore, the phase composition of sample Z3 (2.1 wt% ZrO2) is only periclase because ZrO2 has dissolved into MgO completely. However, the phase compositions of Z6 and sample Z9 are periclase, t-ZrO2 and forsterite since the ZrO2 content of sample Z6 (4.03 wt% ZrO2) and sample Z9 (6.04 wt% ZrO2) are more than 2.6 wt%.

    4 2 2ZrSiO ZrO SiO + (1)

    .. ''2 2

    MgOMg Mg oZrO Zr V O + + (2)

    2 22 4MgO SiO Mg SiO+ = (3) On the other hand, SiO2 derived from the decomposition of zircon changes the ratio

    of CaO/SiO2, resulting in formation of a phase with a lower melt point. The eutectoid points of samples are given in Tab. IV based on the MgO-ZrO2-CaO-SiO2 phase diagram (as shown in Fig. 7).

    Fig. 7 MgO-ZrO2-CaO-SiO2 phase diagram [7]

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    It is obvious that the eutectoid point of the sample with a zircon