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

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

(Submitted by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

1. Production of Magnesium Fluoride

The production of hydrogen fluoride—the main intermediate product for obtaining fluorine and various fluorine compounds—is acquiring ever greater practical importance.

The principal industrial source for obtaining hydrogen fluoride is fluorspar. Recently, much attention has been devoted to the question of extracting fluorine from the off-gases of superphosphate production, since the reserves of fluorine in apatites and phosphorites are large.

Thus, in Italy, at the superphosphate plants of Montecatini, hydrofluoric acid has for more than 10 years already been obtained from off-gases as a by-product [1].

In processing apatites, the fluorine contained in them is released in the form of silicon tetrafluoride. To obtain hydrogen fluoride, silicon tetrafluoride is subjected either to direct processing by pyrohydrolysis [2], treatment with methanol [3], or hydrolysis in the presence of sulfuric acid, or else it is processed through intermediate fluorides; in the latter case, in particular, a solution of ammonium fluoride and silicon dioxide (“white carbon black”) is obtained. After evaporation and crystallization, ammonium fluoride is either treated with sulfuric acid [1, 4] to obtain hydrogen fluoride, or subjected to thermal decomposition, or converted into other fluorides with their subsequent decomposition to hydrogen fluoride.

Methods have been worked out in detail for producing hydrogen fluoride by thermal decomposition of sodium bifluoride obtained in the conversion of technical ammonium fluoride-bifluoride with sodium fluoride, as well as by sulfuric-acid decomposition of calcium fluoride, which is formed when an ammonium fluoride solution is treated with phosphogypsum or with waste from furnaces for producing hydrofluoric acid [5]. Despite the large number of research works, to this day not one of the methods for processing fluorine-containing gases into hydrogen fluoride has found final industrial implementation. The reason for this is, above all, the low economic efficiency of the methods—the majority of methods yield an economic effect not through the production of hydrogen fluoride, but as a result of obtaining by-products such as, for example, “white carbon black,” ammonium sulfate, and others. In many cases, in particu-

during pyrohydrolysis, the use of high temperatures leads to increased corrosion of the equipment; in methods using sodium bifluoride, the degree of conversion reaches only small values, and the recycle solutions are large in volume; moreover, in most cases the reaction gas obtained has a low concentration of hydrogen fluoride.

We have developed a method for processing fluorine-containing gases into anhydrous hydrogen fluoride via magnesium fluoride. The first part of the scheme—obtaining a solution of ammonium fluoride from fluorine-containing gases—we leave without any changes. Subsequently, the ammonium fluoride solution is treated with magnesium sulfate:

\[ 2\mathrm{NH}_4\mathrm{F}+\mathrm{MgSO}_4=(\mathrm{NH}_4)_2\mathrm{SO}_4+\mathrm{MgF}_2. \]

The magnesium fluoride thus obtained is filtered off and, after drying, is sent for sulfuric-acid decomposition, while the ammonium sulfate can be used in agriculture. The hydrogen fluoride released during sulfuric-acid decomposition is condensed and passes through a purification stage, and the magnesium sulfate is again directed to the precipitation of \(\mathrm{MgF}_2\).

Table 1

Effect of ion concentration in solution on the degree of precipitation, precipitation rate, and size of the smallest particles of magnesium fluoride

When fluorine-containing gases from superphosphate plants are captured and after the resulting solution is treated with ammonia and “white soot” is separated, an ammonium fluoride solution with a concentration of about 10% is obtained. We used such a solution in the precipitation of magnesium fluoride.

The precipitation of magnesium fluoride was carried out in a fluoroplastic-4 reactor with a stirrer, placed in a thermostat. A solution of the precipitant, \(\mathrm{MgSO}_4\), with a 5% excess over the stoichiometric amount, was fed continuously over 30 min into the ammonium fluoride solution with stirring. The total required reaction time was 60 min, the pH of the medium was 6, and the temperature was 75–80°C. This temperature was chosen on the basis of the best filterability of the precipitates obtained; the use of higher temperatures leads to decomposition of ammonium fluoride into ammonium bifluoride and ammonia.

Next, in order to find conditions for obtaining better-filtering precipitates of magnesium fluoride, the degree of precipitation, precipitation rate, and diameter of the smallest particles were determined as a function of solution concentration (Table 1) and of the acidity of the medium (Table 2).

A high precipitation rate and, correspondingly, larger crystals of magnesium fluoride are characteristic of lower concentrations and higher acidity of the medium—the average particle size was 20–40 microns; in individual cases the particles reached 400 microns in diameter. However, at the same time the degree of precipitation decreased from 99.85% to 95–97%.

Thus, the optimal conditions for obtaining and precipitating magnesium fluoride crystals are

Table 2

Effect of medium acidity on the degree of precipitation, precipitation rate, and size of the smallest magnesium fluoride particles

Note: The composition of the solution corresponds to experiment 4 in Table 1.

temperature — 75–80° C,
reaction time — 1 hour,
pH of medium — 4–6,
excess precipitant — 5%,
precipitant concentration — 10%.

Under these conditions the degree of precipitation is more than 99%.

An important characteristic of the precipitate is the specific volume resistance of the precipitate to filtration. It was determined on the basis of equation 1 [6]

\[ r_0=\frac{1}{h}\left(\frac{\Delta P\cdot \tau}{\mu\cdot g}-R_{\mathrm{f.p.}}\right), \tag{1} \]

where \(r_0\) is the specific volume resistance of the precipitate to filtration, \(M^{-2}\);
\(R_{\mathrm{f.p.}}\) is the resistance of the filter partition, \(M^{-1}\);
\(h\) is the height of the precipitate, \(m\);
\(\Delta P\) is the excess pressure of the filtrate, \(kg/m^2\);
\(\tau\) is time, sec;

\(\mu\) is the dynamic viscosity coefficient of the filtrate, \(\dfrac{kg\cdot sec}{m^2}\);

\(g\) is the acceleration of free fall, \(m/sec^2\).

For comparison, values of the specific volume resistance of the precipitate to filtration were determined both for magnesium fluoride—\(5.3\cdot 10^{13}\ M^{-2}\)—and for calcium fluoride—\(1.98\cdot 10^{14}\ M^{-2}\)—under optimal conditions.

Conclusions

1. The investigations carried out showed the possibility of processing ammonium fluoride solutions into magnesium fluoride and ammonium sulfate.
2. The optimal conditions for precipitating magnesium fluoride were found.
3. The values of the specific volume resistance of the precipitate to filtration were determined for magnesium fluoride and calcium fluoride.

Literature

1. G. Einecke, Die Flusspatlagerstätten der Welt, Dusseldorf, 1956.
2. N. P. Galkin, V. A. Shubin, N. S. Krylov. ZhKhP, No. 9, 635, 1962.
3. FRG Patent No. 1036827, 1959.
4. I. M. Boguslavskii et al. ZhKhP, No. 7, 450–452, 1961.
5. I. M. Boguslavskii et al. Investigations in the Chemistry and Technology of Fertilizers, Pesticides, and Salts. Nauka Publishers, Moscow, 1966.
6. V. A. Zhuzhikov. Filtration. Goskhimizdat, Moscow, 1961.