Formation Processes of Spinel (MgAl2O4) by a Thermal Decomposition of a Freeze-Dried Sulfate

HATTORI Takeo, MOHRI Jun-ichi

doi:10.2109/jcersj1950.89.1030_287

The freeze-dried salt was prepared from the mixed solution of magnesium and aluminum sulfates in composition of spinel. The thermal decomposition processes of the resultant salt were studied with DTA, TG and X-ray powder diffraction methods, and the formation processes of spinel were discussed. The results obtaind were as follows: (1) The salt seemed to be composed of the mixture of two compounds of MgSO₄⋅6H₂O and Al₂(SO₄)₃⋅17H₂O, in which magnesium sulfate was amorphous and aluminum sulfate crystalline. (2) On heating the freeze-dried salt, the crystalline Al₂(SO₄)₃⋅17H₂O decomposed to a crystalline anhydride which then decomposed to an amorphous alumina. In contrast, the amorphous MgSO₄⋅6H₂O decomposed to an amorphous anhydride. This anhydride was then crystallized, and finally decomposed to an amorphous magnesia. (3) Once the amorphous magnesia was formed, some part of this reacted to form spinel immediately with non-crystalline alumina which had been already formed. (4) The amorphous magnesia which did not react with this alumina then crystallized as the temperature raised. This crystalline magnesia did not completely react with alumina up to higher temperature. Thus the spinel has been formed at two stages. The temperature of the first stage to give spinel was as low as 840°C in this study which was very low when compared with the case of the solid-solid reaction, for example, as of alumina and magnesia pellets.

Formation Processes of Spinel (MgAl<sub>2</sub>O<sub>4</sub>) by a Thermal Decomposition of a Freeze-Dried Sulfate

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