THERMOGRAPHIC ANALYSIS OF THE FORMATION OF \(MgAl_2O_4\) OF COMPOSITION Sh-64

P. G. Usov, V. I. Vereshchagin, V. A. Amelina

(Submitted by the scientific seminar of the inorganic departments of the Chemical-Technological Faculty)

The reaction of spinel formation proceeds with a small release of heat. Thermodynamically, the reaction is considered possible already from \(298^\circ K\) [1]. But the thermal effect of the spinel-formation reaction is very insignificant \((\Delta H_{298} = -4.52\ \text{kcal/mol})\), and therefore the occurrence of the reaction at this temperature is by no means obligatory.

Many authors have recorded the beginning of the spinel reaction from oxides in the range \(700–800^\circ C\) [2–3]. A clearly expressed exothermic effect cannot be observed, apparently also because the process of spinel formation occurs over wide temperature limits.

The aim of our work is to study the physicochemical processes occurring during the formation of spinel from composition Sh-64 \((63.9\%\, Al_2O_3 + 36.1\%\, MgO)\) when the mixture is heated to temperatures of \(1040–1060^\circ C\) in pure form and with the introduction of certain additives. The method of derivatography was used, including the recording of differential-thermal curves (DTA), weight-loss curves (TG), and weight-loss-rate curves (DTG). The curves were recorded on a Hungarian derivatograph manufactured by “MOM” at a heating rate of \(10\ \text{deg/min}\). In Fig. 1 the derivatogram of the Sh-64 batch without additives is presented.

On the DTA curve, a number of endothermic effects are recorded at temperatures of \(180^\circ C\), \(300^\circ C\), \(420^\circ C\), \(770^\circ C\), and a small effect at \(920^\circ C\). From the DTG and TG curves it may be concluded that all endothermic effects are associated with decomposition processes. Comparison of the thermal curves of pure magnesium oxide (Fig. 2) shows that all processes occurring on the derivatogram of the Sh-64 batch correspond—

Fig. 1. Thermogram of composition Sh-64.

Fig. 2. Thermogram of precipitated magnesium oxide.

will be processes occurring in only one component of the charge—magnesium oxide. At \(180^\circ\mathrm{C}\) adsorbed water is removed, and at a higher temperature—chemically bound water.

Of particular interest is the small endothermic effect at \(740\text{–}760^\circ\mathrm{C}\), which corresponds to the decomposition temperature of dolomite,

\[ \mathrm{CaMg(CO_3)_2}. \]

Probably, in air, carbonation of magnesium oxide takes place together with calcium impurities.

A very small recurring exo-effect at \(920^\circ\mathrm{C}\) is connected, in all probability, with structural changes in the lattice of \(\mathrm{MgO}\), freed from water during the transition to periclase.

The addition of \(0.1\%\ \mathrm{Y_2O_3}\), introduced in the form of an aqueous solution of the salt \(\mathrm{Y(NO_3)_3}\) (Fig. 3), does not change the general character of the derivatogram of charge Sh-64. The endothermic effect and weight losses at \(420^\circ\mathrm{C}\) increase, which is associated with the formation of brucite \(\mathrm{Mg(OH)_2}\) during treatment of the charge with an aqueous solution [4]. Dehydration of brucite takes place in the region of \(420^\circ\mathrm{C}\) [5]. Attention is drawn to the insignificant exothermic effect at \(800^\circ\mathrm{C}\), which increases noticeably if the given charge is preliminarily pressed at a specific pressure

\[ P = 1500\ \mathrm{kg/cm^2} \]

and prepared

Fig. 3. Thermogram of composition Sh-64 with an addition of 0.1%

powder with particle sizes of 0.2–0.5 mm (Fig. 4). The appearance of an unstable exoeffect at 530–540°C is apparently associated with the burning-out of organic impurities introduced during pressing (oil).

A distinct exothermic effect on the DTA curve, with a maximum at 840°C and not associated with weight losses, appears on the derivatogram of the Sh-64 batch with an addition of 1% \(BeO\) (Fig. 5). This can be attributed to the reaction of spinel formation,

\[ MgAl_2O_4, \]

and the magnitude does not contradict the calculated value at this temperature \((\Delta H_{1073^\circ} = -6.22\ \text{kcal/mol})\). The appearance and increase of this effect on the DTA curves

Fig. 4. Thermogram of composition Sh-64 with an addition of 0.1%, pre-ground before pressing.

Fig. 5. Thermogram of composition Sh-64 with an addition of 1%.

X-ray data for the products of thermal analysis (DTA, TG, derivative thermography) up to 1040°C of batch Ш-64 with various microadditives.

Table I

charges Sh-64 is associated with acceleration of the synthesis process and reaction from the onset of the reaction (800°C) upon introduction of the corresponding additives. In accordance with this explanation of the effect at 820–840°C are the X-ray analysis data for the samples described (Table I). This is also explained by the fact that during heat treatment up to 1040–1060°C the specimens sinter somewhat, while the formation of spinel from oxides proceeds with an increase in volume by 8%. Consequently, the lower the temperature at which spinel synthesis is completed, the more sintered the specimen is by the end of the analysis (by 1040°C). It is evident from Table I that the increase in the spinel phase \(MgAl_2O_4\) and the decrease in the content of \(\alpha-Al_2O_3\) in the Sh-64 batch with various additives occur in the same sequence in which the corresponding derivatograms were considered.

References

1. P. P. Budnikov, F. G. Kerbe, G. M. Matveev. Glass-like Materials. Riga, “Zinatne,” 1966.

2. G. F. Hütting, D. Zinker, H. Kittel. Z. Electrochem., 1934, 47, 36.

3. W. Jander, H. Pfister. Z. Anorg. Chem., 1938, 239, 95.

4. G. V. Samsonov et al. Physicochemical Properties of Oxides. Handbook. Moscow, “Metallurgiya,” 1969.

5. L. G. Berg, I. P. Burmistrova et al. A Practical Guide to Thermography. Publishing House of Kazan University, 1967.

6. P. P. Budnikov. Chemistry and Technology of Oxide and Silicate Materials. Kiev, “Naukova dumka,” 1970.