THERMOGRAPHIC AND THERMOGRAVIMETRIC STUDY OF THE INTERACTION OF MAGNESIUM FLUORIDE WITH SULFURIC ACID

N. P. Kurin, N. S. Turaev, V. P. Pishchulin, F. I. Tishchenko

(Presented by the scientific seminar of the departments of PMAChP and OChT)

The interaction of magnesium fluoride with sulfuric acid is an endothermic process. Thermodynamic calculations show that, for the sulfuric-acid decomposition of magnesium fluoride under standard conditions, 98.27 kJ/mol must be supplied, i.e., 1.53 times more than for the decomposition of calcium fluoride.

In studying the process of sulfuric-acid decomposition of magnesium fluoride, we observed heating of the reaction mass when magnesium fluoride was mixed with sulfuric acid, as well as a decrease in the rate of temperature rise of the mass during its heating in the interval 150–180°C; at the same time, intensive evolution of hydrogen fluoride was observed. These two observed thermal effects indicate the staged nature of the process of decomposition of magnesium fluoride by sulfuric acid.

To establish the phase transformations in the system \( \mathrm{MgF_2—H_2SO_4} \), differential thermal analysis was carried out simultaneously with thermogravimetry. Such a combination of differential thermal analysis with thermogravimetry makes it possible simultaneously to observe the loss of weight of the sample, indicating a change in the composition of intermediate compounds, and the thermal effects accompanying heating [1].

Fig. 1. Thermogram (1) and thermogravigram (2) of the process of sulfuric-acid decomposition of magnesium fluoride

Differential thermal analysis in combination with thermogravimetry was carried out on a UTA-1 thermobalance apparatus according to the standard procedure [2]. For the study, “chemically pure” grade magnesium fluoride and “chemically pure” grade sulfuric acid at a concentration of 97.5% with a 10% excess were used. The heating rate of the sample was 8° per minute. The weighed portion of the reaction mass for thermobalance analysis was 0.2043 g; the weighed portion for differential thermal analysis was 0.4492 g. The thermogram averaged from six values (curve 1)

and the thermogravimetric curve (curve 2) are shown in Fig. 1. Studies of the \( \mathrm{MgF_2—H_2SO_4} \) system by thermography and thermogravimetry were carried out in the temperature range from \(-70^\circ\mathrm{C}\) to \(+400^\circ\mathrm{C}\). The thermogram of the system shows that at temperatures of \(-30\) to \(-23^\circ\mathrm{C}\) and at \(0^\circ\mathrm{C}\) two endothermic effects are observed in the system, not accompanied by a change in weight and apparently associated with melting of sulfuric-acid cryohydrates. Further, at \(+18\)–\(20^\circ\mathrm{C}\), the system heats up (an exothermic peak without weight change) as a result of the possible formation of intermediate compounds of the type \( \mathrm{MgF_2 \cdot H_2SO_4} \),

\[ \mathrm{Mg} \begin{matrix} /\,\mathrm{F\cdot HF}\\ \backslash\,\mathrm{SO_4H} \end{matrix}. \]

Upon further heating, three endothermic effects are observed on the thermogram (curve 1) at 170, \(250^\circ\mathrm{C}\), and \(320\)–\(330^\circ\mathrm{C}\), with the corresponding three stepwise changes in the weight of the sample on the thermogravimetric curve (curve 2). The first two weight changes are equal to each other and correspond to the successive evolution of two molecules of hydrogen fluoride. The last endothermic effect at \(320\)–\(330^\circ\mathrm{C}\) and the loss of weight are associated with boiling (evaporation) of excess sulfuric acid from the system.

Thus, on the basis of the thermograms and thermogravimetric curves obtained, and by analogy with the mechanism of the reaction of calcium fluoride with sulfuric acid [3, 4], the probable mechanism of the reaction of magnesium fluoride with sulfuric acid may be represented by the following scheme:

\[ \mathrm{MgF_2 + H_2SO_4 \to MgF_2 \cdot H_2SO_4 \to Mg} \begin{matrix} /\,\mathrm{F\cdot HF}\\ \backslash\,\mathrm{SO_4H} \end{matrix} \to \]

\[ \underset{170\text{–}180^\circ\mathrm{C}}{\longrightarrow} \mathrm{Mg} \begin{matrix} /\,\mathrm{F}\\ \backslash\,\mathrm{SO_4H} \end{matrix} + \mathrm{HF}\uparrow \underset{250\text{–}270^\circ}{\longrightarrow} \mathrm{MgSO_4 + 2HF}\uparrow . \]

It should be noted that, for the process of sulfuric-acid decomposition of magnesium fluoride, the decomposition temperatures of the intermediate compounds are much higher than for the process with calcium fluoride, for which the decomposition temperature of the first intermediate compound is \(100\)–\(110^\circ\mathrm{C}\), and for the second, \(225^\circ\mathrm{C}\), which indicates the greater strength of the magnesium intermediate compounds.

Conclusions

In studying the \( \mathrm{MgF_2—H_2SO_4} \) system by differential thermal analysis and thermogravimetry in the range \(-70\) to \(400^\circ\mathrm{C}\), an exothermic effect without weight loss at \(20^\circ\mathrm{C}\) and endothermic effects at 170 and \(250^\circ\mathrm{C}\), with simultaneous stepwise change in the weight of the system, were detected.

On the basis of the data obtained, a probable mechanism is proposed for the reaction of magnesium fluoride with sulfuric acid.

LITERATURE

1. G. V. Young. Instrumental Methods of Chemical Analysis, pp. 274–280, Moscow, Gosatomizdat, 1963.
2. Installation for thermoweight analysis UTA-1. State Geological Committee of the USSR, Leningrad, 1965.
3. W. Rogers, K. Muller, Chem. Engng. Progr., 59, No. 5, 85, 1965.
4. N. Chopey, Chem. Ing., 69, No. 9, 94–96, 1962.