Impact of Magnesia Grain Size on the In Situ Spinel Formation in Al2O3–MgO–C Refractories

Mehdi Naeemi, Ali Baghaei, Homeyra Heydari Boroujeni

Mehrgodaz Refractories Company, Sefid-Dasht, Chaharmahal & Bakhtiari 88751-16655/Iran

Revision 02.11.2017, 15.01.2018

Volume 10, Issue 2, Pages 80 - 85

Abstract

In order to evaluate the effect of magnesia grain size on spinel formation process, four compositions with different dead burned magnesia grain sizes along with fused alumina and graphite were prepared. After pressing, samples were tempered at 220 °C and coked under reducing atmosphere at 1600 °C. Permanent Linear Change (PLC) and physical properties including Bulk Density (BD) and Apparent Porosity (AP) were measured. Phase analyses were investigated by X-Ray Diffraction (XRD) technique and flexural strength of samples at 1400 °C in Ar atmosphere were determined by HMOR test. In addition, the samples were coked once again under coke bed at 1600 °C in order to assess the spinel formation process. The results of the PLC have shown that the use of finer magnesia grain sizes in the composition would increase the available surface area for spinel formation reaction. In fact, the PLC value of the sample M4 was +4 % while this value for other samples M3, M2 and M1 was +3,27, +2,44 and +2,36 %, respectively. Moreover, spinel formation decreased by the increase of magnesia grain size to a certain level. Regarding the results of second coking process, although some unreacted magnesia were characterized by XRD in sample M1, only negligible expansion were recorded for samples which could be due to the formation of spinel phase in the magnesia and alumina interface acting as an ion diffusion barrier. Flexural strength of samples at 1400 °C increased by using finer fractions of magnesia in samples because of the formation of spinel phase in the matrix as a ceramic bond which has been revealed by SEM/EDS investigations.

Keywords

alumina-magnesia-carbon, spinel, grain size, carbon containing refractories

References

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