Magnesia-Zircon Brick : Evolution of Microstructure , Properties and Performance with Increasing Sintering Temperature

Depending on phase components and densification, Magnesia-Zircon brick varies in appearance from white to veined and then brown with increasing sintering temperature. Properties including bulk density, apparent porosity and hot modulus of rupture as well as performance embodied with creep resistance and refractoriness continue to improve with sustaining enhancement of sintering temperature. Exceptionally, cold crushing strength first increases then decreases with rising sintering temperature and a peak exists at 1550C. Microstructural evolution suffers zircon decomposition companying by silica escape, forsterite formation, matrix solidification and zirconia coagulation, until a zirconia/forsterite composites belt tightly coating on magnesia aggregates. Excessive coagulation of zirconia caused by oversintering probably results in microcracks formation and defects enlargement thereby degrades cold crushing strength.


Introduction
The glass melting process involves large amounts of energy due to relatively high processing temperatures.In order to use the energy as efficient as possible, glass tank regenerators are developed to achieve heat recovery through preheating the combustion air and thereby provide better heat transfer via a high flame temperature [1].With modern regenerators, 50-75% of the enthalpy contained in the hot exhaust gases is returned to the system and thus energy is saved [2].
However, refractories installed in regenerator checker are chronically exposed to erosive and corrosive working conditions.Specially, the top courses are attacked by high temperature load and deposition of external oxides whilst the middle courses by condensation of alkali oxides and alkali sulfates mainly [3][4].Therefore, common refractories are very hard to meet the rigorous requirement of regenerator, such as Magnesia brick with poor corrosion resistance, Forsterite brick with insufficient thermal resistance, and Magnesia-Chrome brick with hazardous Cr 6+ contamination to environment [5][6].Until 1986, these existing corrosion and negative effect problems were solved by the development of Magnesia-Zircon brick [5].Nowadays, Magnesia-Zircon checker brick has been widely used as regenerator lining.As known, during firing a series of transformation occurs, which determines the final quality of Magnesia-Zircon brick [7].The objective of this research is to study the effect of the

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change of firing temperature on microstructure, physical/mechanical properties and performance of Magnesia-Zircon brick.

Experimental procedure
In this investigation, Magnesia-Zircon brick was prepared based on 77.5% sintered magnesia (0-5mm), 21% zircon (0-0.1mm) and 1.5% fumed silica.The starting materials were compounded together with binder in an intensive mixer for 10 minutes.Then, the mixture was pressed into cuboid specimens (240×115×53mm) under a pressure of 150MPa.After dried at 130 o C for 24 hours, the specimens were sintered in an elevator furnace in separate batches.Sintering  Chemical composition of magnesia and zircon was analyzed by XRF (Bruker S4, Germany) and the results were listed in Tab.I. Bulk density (BD) and apparent porosity (AP) of the sintered specimens was measured by means of Archimedes method with deionized water as immersion medium.Cold crushing strength (CCS) at room temperature was tested using Universal Testing Machine (Instron-5566, UK).Measurement of hot modulus of rupture (HMOR) was performed by using HMOR Tester (03AP, Precondar, China) at 1500 o C with residence time of 30 minutes in air.High temperature performance of the sintered specimens was assessed in terms of creep in compression (CIC) at 1400 o C and refractoriness under load (RUL) on 50 mm high by 50 mm diameter cylindrical samples under a 0.2 MPa load.The phase components of sintered specimens were identified by using XRD with CuK radiation (D/max 2500V, Riguka, JP).Microstructural observation on the mechanically polished samples was performed with optical microscope (STM6, Olympus, JP).The elemental composition of the matrix of sintered specimens was determined by electron probe microanalyser (EPMA-8705QH2, Shimadzu, Japan).

Results and discussion
Magnesia-Zircon brick varies in appearance with the variety of sintering temperature.As shown in Fig. 1., the appearance of the brick evolves from white to veined and then brown when the corresponding sintering temperature is 1400 o C, 1500 o C and 1600 o C.This is probably related to a difference in densification degree and phase components of the specimens.Fig. 2. presents the dependence of bulk density (BD) and apparent porosity (AP) on sintering temperature.Disparity of BD and AP among the specimens indicates different densification degree.The XRD patterns of the specimens are given in Fig. 3. and the phase components are tabulated in Tab.II.The following three reactions may occur in the mixture of magnesia and zircon during sintering, namely decomposition reaction of zircon (Equation 1), crystalline transformation reaction of zirconia from monolithic to tetragonal structure (Equation 2) and forsterite formation reaction between MgO and SiO 2 (Equation 3).

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T E D The major phase components of the specimens sintered at 1500 o C and 1600 o C are the final Mg 2 SiO 4 , MgO and t-ZrO 2 , indicating a full completion of the reactions.In contrary, the reactions are not completely finished in the specimen sintered at 1400 o C because residual undecomposed zircon and unstabilized zirconia (m-ZrO 2 ) can be still identified.Optical microscopic observation displayed in Fig. 4.(a) confirms that undecomposed zircon is surrounded by a ring of zirconia, as marked by arrow.Moreover, the matrix of the specimen sintered at 1400 o C looks still like compaction morphology rather than sintering texture.It is shown in Fig. 7. that the hot modulus of rupture (HMOR) continues to increase when raising the sintering temperature from 1400 o C to 1600 o C.This is attributed to the improved densification of matrix and enhanced bonding between matrix and aggregates with increasing sintering temperature.The inflection point of decelerated increase of HMOR

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emerges roughly at 1550 o C, implying microstructural evolution has generally completed under this temperature.This deduction is in good agreement with the performance of the specimens, which is embodied in terms of creep in compression (CIC) and refractoriness under load (RUL).

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As shown in Fig. 8., CIC expressed with the value of (Z 5 -Z 25 ) tends to be generally small (≤0.05%) and somewhat constant when the sintering temperature is above 1550 o C.This signals that the internal reactions and microstructural densification in the specimens has mostly completed.Temperature dependence of RUL plotted in Fig. 9. indicates similar refractoriness feature of specimens sintered above 1500 o C. Higher sintering temperature of 1600 o C has little improvement in performance of deformation resistance.Therefore, the sintering temperature of 1550 o C is thought to be suitable for Magnesia-Zircon brick because of its optimized combination of properties and performance obtained.

Conclusions
Based on the experimental results, the conclusions are drawn as follows.
was carried out at temperatures of 1400 o C, 1450 o C, 1500 o C, 1550 o C and 1600 o C respectively and with soaking time of 6 hours.