Effect of Zircon on Sintering, Composition and Microstructure of Magnesia Powders

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.


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 SiO 2 , Al 2 O 3 , Cr 2 O 3 , TiO 2 , ZrO 2 and WO 3 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 ZrO 2 was soluble into MgO crystals.Han et al [1] studied sintering of MgO-based refractories with added WO 3 , and demonstrated that the low melting phases MgWO 4 and CaWO 4 formed in MgO boundaries, resulting in an increase of the liquid volume.The formation of CaWO 4 lead to change of the CaO/SiO 2 (C/S) ratio and decreased the amount of 3CaO•SiO 2 (C 3 S) and 2CaO•SiO 2 (C 2 S), and increased amount of liquid, so that the densification was improved by phase liquid sintering.Guo et al [5] discussed the effect of ZrSiO 4 on sintering of sintered-magnesia and the properties of magnesia brick made of presynthesized magnesia-zirconia.They indicated that ZrSiO 4 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-based refractories to form the matrixes which are very important for properties of refractories.Reaction sintering occurred during firing, ZrO 2 , 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.

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 <0.88 mm and that of zircon is <0.044 mm.The compositions of the batches are given in tab.II.The powder mixtures were mixed in a ball mill for 3h, and then were pressed with a pressure of 150 MPa to form cuboid samples with a width of 25 mm, height of 25 mm and length of 125 mm.

Samples
Z0 Z3 Z6 Z9 Magnesia 100 97 94 91 Zircon 0 3 6 9 The pressed samples were dried at 110 o C for 24h and sintered at 1400, 1500, 1600, and 1650 o C. The heating rate was 3 o C/min from ambient temperature to 1000 o C and 1.5 o /min from 1000 o C to the designed temperature, then soaked for 3h.The phase composition was analyzed by X-ray diffraction using Cu Kα radiation (model Philips X'pert pro).The bulk density (BD) and apparent porosity (AP) of sintered samples were obtained by Archimedes' Principle with kerosene oil as medium.The permanent linear shrinkage (PLS) and the modulus of rupture (MOR) were measured.Microstructures of the samples were studied by scanning electron microscopy (SEM) (model Philips XL30TMP) with attached energy dispersive analysis (EDAX Phoenix) for semi quantitative elemental analysis.

Sintering
Figs. 1 and 2 show the effects of zircon content on the BD and AP of samples sintered at different temperatures, respectively.The PLS of the samples sintered at different temperature as a function of zircon content is given in Fig. 3.It was found that the effects of zircon content on BD, AP and PLS of samples depended on the sintering temperature.When the samples were sintered at 1400, 1500 and 1600 o C BD and PLS of samples with zircon content of 3 wt% were higher than for samples without zircon, and AP of the former was smaller than that of the latter.When the zircon content increased from 3 wt% to 9 wt% BD, PLS and AP changed a little.However, when the samples were sintered at 1650 o C BD, PLS increased and AP decreased with increase zircon content up to 6 wt%.When the zircon 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 1500 o C 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 1650 o C .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 1650 o C increased.However, when the zircon content increased from 6 wt% to 9 wt% MOR of the samples sintered at 1600 and 1650 o C decreased.

Phase analysis and microstructure
Fig. 5 shows an XRD pattern of samples sintered with various zircon contents at 1650 o C for 3h.It revealed that only periclase was detected in samples Z0 and Z3.However, t-ZrO 2 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 1650 o C for 3h respectively.Sample Z0 (Fig. 6a) has a porous microstructure with more angular and welldefined edged periclase grains.Sample Z6 (Fig. 6b) exhibits a dense microstructure, with round and ellipsoidal periclase grains.White ZrO 2 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.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 ZrO 2 , forsterite and more glass phase.
Tab. III Composition (wt %) of phases in Fig. 6 (c, d The existence of ZrO 2 and forsterite may improve thermal shock resistance of bricks.

Discussion
Three reactions may occur in the powder mixtures of zircon and MgO, namely decomposition of zircon (Equation 1), solid solution of ZrO 2 into MgO (Equation 2), and a reaction between SiO 2 and MgO (Equation 3).The phase compositions of the samples depend on the zircon content and solubility of ZrO 2 in periclase.According to the MgO -ZrO 2 phase diagram [6], the solubility of ZrO 2 in periclase is about 2.6 wt% at 1650 o C. Therefore, the phase composition of sample Z3 (2.1 wt% ZrO 2 ) is only periclase because ZrO 2 has dissolved into MgO completely.However, the phase compositions of Z6 and sample Z9 are periclase, t-ZrO 2 and forsterite since the ZrO 2 content of sample Z6 (4.03 wt% ZrO 2 ) and sample Z9 (6.04 wt% ZrO 2 ) are more than 2.6 wt%.MgO SiO Mg SiO + = (3) On the other hand, SiO 2 derived from the decomposition of zircon changes the ratio of CaO/SiO 2 , 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-ZrO 2 -CaO-SiO 2 phase diagram (as shown in Fig. 7).Fig. 7 MgO-ZrO 2 -CaO-SiO 2 phase diagram [7] It is obvious that the eutectoid point of the sample with a zircon content of 3 wt% is much lower than that of a sample without zircon and also lower than for samples Z6 and Z9.The liquid phase improves sintering of samples, resulting in an increase of BD, PLS and strength of sintered samples, but decrease of AP.However, when the zircon content increases from 6 wt% to 9 wt% AP, BD and PLS of samples change a little.There may be two reasons.One is that two much ZrO 2 remained in the grain boundaries inhibiting MgO boundary migration.The second one is increase of viscosity of the liquid phase because of the increase of the SiO 2 content in the liquid.When the zircon content increases from 6 wt% to 9 wt% the strength of the samples decreases.The reason may be that the microcracks formed by phase transition of ZrO 2 during cooling increase in the samples.Based on the results mentioned above, the 6 wt% zircon content may be appropriate.In this case the sample has a dense microstructure, a reasonable ZrO 2 content and lower content of glass phase.

Fig. 3 4
Fig. 3 Variation of PLS with zircon content Fig. 4 Variation of MOR with zircon content