STUDY OF THE PROCESS OF PRODUCING HYDROGEN FLUORIDE FROM AMMONIUM FLUORIDE SOLUTIONS

N. P. KURIN, N. S. TURAEV, V. P. PISHCHULIN, F. M. TISHCHENKO

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

2. Study of the kinetics of the interaction of magnesium fluoride with sulfuric acid

Magnesium fluoride obtained by the conversion of ammonium fluoride with sulfuric-acid magnesium, after drying, is then subjected to sulfuric-acid decomposition according to the reaction

\[ \mathrm{MgF_2 + H_2SO_4 = MgSO_4 + 2HF}. \]

A 10 g weighed portion of magnesium fluoride was mixed with sulfuric acid at a concentration of 97.5% and an excess of 5% and was subjected to decomposition at a specified temperature; the hydrogen fluoride evolved was absorbed by a solution of caustic potash. After the reaction had been carried out for a specified time at a specified temperature, the reacted mass and the solution from the absorption system were analyzed for fluorine-ion content; for this purpose fluorine was distilled from the sample in the form of fluorosilicic acid, followed by titration of the distillate with a solution of thorium nitrate in the presence of sodium alizarinsulfonate indicator [1]. From the analytical data, the degree of decomposition of magnesium fluoride was calculated. The results of the study in the temperature range 100–250°C are presented in Fig. 1. As can be seen from Fig. 1, the reaction of decomposition of magnesium fluoride by sulfuric acid proceeds rather intensively during the first 5–10 min, after which the process slows sharply, and the degree of decomposition increases very slowly.

The rate of reaction increases significantly with increasing temperature; thus, after 5 min of reaction at 100°C the degree of decomposition of magnesium fluoride reached 39.5%, whereas at 150° it was 56.2%, at 200°—67.2%, and at 250°—already 77.0%.

A further increase in temperature to 300°C is inexpedient, since the data obtained for the degree of decomposition at 300°C coincide completely with those at 250°C; consequently, the optimum process temperature is 250°C—after 30 min of reaction with a 5% excess of sulfuric acid, the degree of decomposition of magnesium fluoride is 91.3%.

For processing the kinetic data we used the topochemical equation of Kazeev—Erofeev—Kolmogorov [2]:

\[ \alpha = 1 - e^{-k\tau^{n}}, \tag{1} \]

where \(\alpha\) is the fraction of reacted substance;

\(K\)—a constant proportional to the rate constant, \(\mathrm{min}^{-1}\);

\(\tau\)—reaction time, min;

\(n\)—a constant determined by the mechanism of formation and growth of reaction nuclei.

A graphical kinetic analysis of the experimental data is presented in Fig. 2.

The results of the kinetic analysis (Fig. 2) showed that the process of decomposition of magnesium fluoride by sulfuric acid proceeds in two diffusion stages; in the first stage, up to a degree of decomposition of 70–73%, the constant \(n = 0.718\), and in the second, \(0.284\). The values of the apparent activation energies by stages, found on the basis of the data obtained and from the Arrhenius equation, are respectively \(16\,300\ \mathrm{J/mol}\) and \(11\,000\ \mathrm{J/mol}\). The expressions for the rate constant in the investigated temperature interval of 100–300°C have the following form, respectively:

Fig. 1. Dependence of the degree of decomposition of magnesium fluoride by sulfuric acid of 97.5% concentration with a 5% excess on the reaction time at various temperatures: 1—100°C, 2—150°C, 3—200°C, 4—230°C, 5—250°C.

Fig. 2. Dependence of \(\lg[-\lg(1-\alpha)]\) on \(\lg \tau\) for the process of sulfuric-acid decomposition of magnesium fluoride at various temperatures: 1—100°C, 2—150°C, 3—200°C, 4—230°C, 5—250°C.

\[ K_{\mathrm{I}} = 24.66 \cdot e^{-\frac{1960}{T}}, \tag{2} \]

\[ K_{\mathrm{II}} = 13.12 \cdot e^{-\frac{1320}{T}}, \tag{3} \]

and the expressions for the degree of decomposition of magnesium fluoride by sulfuric acid are written as follows:

\[ \ln(1-\alpha_{\mathrm{I}}) = -24.66 \cdot \tau^{0.718} \cdot e^{-\frac{1960}{T}}, \tag{4} \]

\[ \ln(1-\alpha_{\mathrm{II}}) = -13.12 \cdot \tau^{0.234} \cdot e^{-\frac{1320}{T}}. \tag{5} \]

Thus, both from the values of the coefficient \(n\) and from the values of the apparent activation energy, it may be said that the first and second stages of the process of sulfuric-acid decomposition of magnesium fluoride lie in the diffusion region of reaction.

Increasing the excess of sulfuric acid raises the degree of decomposition of magnesium fluoride from 91.3% at a 5% excess, at a temperature of 250°C and a reaction time of 30 min, to 94.0% at a 10% excess and to 97.1%.

at an excess of 20%. Stirring of the reacting mass during the process increases the degree of decomposition of magnesium fluoride by sulfuric acid of 97.5% concentration and with a 20% excess to \(\sim 97.0\%\) at a temperature of \(250^\circ\mathrm{C}\) in 20 min, as against 95.0% without stirring.

On the basis of the foregoing, the following should be regarded as the optimum conditions for sulfuric-acid decomposition of magnesium fluoride:

temperature — \(250^\circ\mathrm{C}\),
excess sulfuric acid — 20%,
reaction time — 30 min.

Under these conditions, the degree of decomposition of magnesium fluoride reaches 97.1–97.5%; the reaction gas contains HF—96–97%, \(\mathrm{SO_2}\)—less than 0.1%, \(\mathrm{H_2SO_4}\)—0.2–0.5%, the remainder being water vapor and air.

Conclusions

1. The kinetics of the sulfuric-acid decomposition of magnesium fluoride by sulfuric acid in the temperature range \(100\text{–}300^\circ\mathrm{C}\) has been studied.

2. It has been established that the process of decomposition of magnesium fluoride by sulfuric acid is satisfactorily described by the Kazeev–Erofeev–Kolmogorov equation and proceeds in two stages.

3. The apparent activation energy of the decomposition process in the first stage, up to a degree of decomposition of magnesium fluoride of 70–73%, is \(16.3\ \mathrm{kJ/mol}\); in the second stage it is \(11.0\ \mathrm{kJ/mol}\).

4. The optimum conditions for carrying out the sulfuric-acid decomposition of magnesium fluoride have been determined.

5. The overall yield of fluorine from ammonium fluoride solution into anhydrous hydrogen fluoride is 96–97.3%.

References

1. E. K. Kiseleva. Analysis of Fluorine-Containing Compounds. Khimiya Publishing House, Moscow–Leningrad, 1966.

2. V. V. Boldyrev. Methods for Studying the Kinetics of Thermal Decomposition of Solids. Tomsk State University Publishing House, Tomsk, 1958.