RU2274601C1 - Nitrogen trifluoride production process - Google Patents

Abstract

FIELD: inorganic compounds technology.

SUBSTANCE: process comprises electrolysis of ammonium fluoride melts in electrolyzer with carbon or nickel anodes and steel cathodes provided with anode bell, said process being characterized by that electrolyzer contains louver cathodes and anode bell is arranged above electrodes. In this case, electrolyte mirror area under the bell constitutes not more than 0.2 total area electrolyte mirror, bell dipping depth in electrolyte is at least 0.25 electrode height, cathode-bell distance is at least 0.025 electrode height, anode-bell distance is at least 0.025 electrode height in case of carbon anode and 0.05 electrode height in case of nickel anode, predetermined electrolyte level in electrolyzer is maintained by feeding hydrofluoric acid gas while ammonia gas is supplied continuously, ammonia supply velocity being varied depending on content NF₃ in anode gas and weight portion of ammonia in electrolyte is maintained equal to 24-28 wt %.

EFFECT: enabled production of NH₃ with less important fluctuation of yield during prolonged electrolysis (70-80% on carbon electrode and 70-75% on nickel electrode) and ensured explosion-proof production.

2 cl, 1 dwg, 2 tbl, 7 ex

Description

The invention relates to inorganic chemistry, to methods for producing nitrogen trifluoride.

Nitrogen trifluoride (NF ₃ ) is used in the chemical industry as a fluorinating agent and as a fluorine-containing raw material. It can be used as an oxidizing agent for high-calorie fuels, in the electronic industry as a cleaning gas for cleaning crystals of semiconductors and silicon wafers, as well as in chemical vapor deposition apparatus (CVD).

One of the main industrial methods for producing nitrogen trifluoride is the electrolysis of ammonium fluoride melts in hydrogen fluoride in electrolytic cells with metal or carbon electrodes.

It is known that during electrolysis, along with the target compound NF ₃ , a number of gaseous impurities are formed, such as ammonia cleavage and fluorination products: N ₂ , N ₂ O, N ₂ F ₂ ; products of fluorination and anodic oxidation of water: OF ₂ , O ₂ ; destruction products of the carbon base of the anode under the action of oxidizing agents: volatile fluorocarbons, CO ₂ .

When conducting the electrochemical method, the main factors affecting the ratio of the reactions of formation of the target compound NF ₃ and impurities are:

- the magnitude of the anodic polarization of the carbon or metal anode (or the magnitude of the voltage across the cell, which is usually oriented in practice);

- the concentration of water in the electrolyte;

- the concentration of ammonia in the electrolyte.

Before the main stage of electrolysis (production electrolysis), a preliminary stage of electrolyte dehydration is carried out. This is done to reduce the content of oxygen impurities and oxygen-containing compounds in the anode gas, which are formed due to the presence of water in the electrolyte. For the same purpose, the replenishment of the flow rate of the starting materials (NH ₃ and HF) during electrolysis is carried out in small portions - weighing from 1 to 10% of the initial load.

The main stage is the production of NF ₃ (operating electrolysis) on both carbon and metal anodes, carried out at a voltage of more than 5.5 V, which corresponds to the rectilinear portion of the current-voltage curve and ensures the flow of the electrochemical reaction of the discharge of fluorine ions and the formation of fluorine atoms.

In an electrolyte of the composition NH ₄ F + (1.1-1.8) HF according to the state diagram of the NH ₃ -HF system [Ruff O., Stoub LZ Anorgan. Chem., 212, 399, 1933] ammonia is in the form of a hydrogen fluoride complex corresponding to ammonium bifluoride: NH ₃ · 2HF. In the presence of free hydrogen fluoride, this complex partially dissociates according to the equation:

The main electrolysis products of pre-dehydrated electrolyte (NF ₃ and N ₂ ) are formed as a result of chemical fluorination of ammonia, which is in the adsorption layer of the anode in the form of ammonium bifluoride or ammonium ions, by highly active fluorine atoms (F) according to the reactions:

In chemical fluorination, namely, the production of NF ₃ , kinetic, thermodynamic, and structural factors influence the process. The combination of these factors in the reaction zone (adsorption layer of the anode) leads to the fact that the electrochemical method does not make it possible to obtain the target compound with a theoretical (100%) yield. A characteristic feature of the electrochemical method of producing NF ₃ is that the balance of production and consumption of fluorine atoms, as well as the ratio between reactions (2-5) of obtaining the main electrolysis products (NF ₃ and N ₂ ) are determined by the concentration of ammonia in the electrolyte. According to our data, the maximum current outputs NF ₃ (about 75% on the nickel anode and about 80% on the carbon one) are achieved with a mass fraction of ammonia in the electrolyte of 25-26%.

According to reactions (2-5), the ammonia consumption for the formation of one mole of NF ₃ is two times less than for the formation of one mole of N ₂ . Therefore, in order to maintain a given concentration of ammonia in the electrolyte, replenishment with ammonia should be carried out taking into account the actual yield of these compounds.

In the process of producing NF ₃ explosive gas mixtures can form, in particular, these are mixtures of NF ₃ with hydrogen and ammonia.

There are various ways to prevent or minimize the occurrence of explosion hazards and, in particular, by producing NF ₃ in specially designed electrolyzers.

A known method of producing NF ₃ in an electrolyzer with separation of the anode and cathode chambers of an electrochemical cell by a fluoropolymer diaphragm [Val. Japanese application 88130790, cl. With 25 V 9/00, publ. 06/02/1988, Chem. Abstr. 1989, Vol. 110, 15084].

A known method of producing NF ₃ in an electrolytic cell with a two-level arrangement of electrode chambers, preventing the mixing of anode and cathode gases [US Patent 5779866, cl. With 25 V 9/00, op. February 4, 1998].

Conditions for safe operation during electrochemical production of NF ₃ are:

1. A sufficient degree of separation of the cathode and anode electrolysis gases, in which the concentration of hydrogen in the anode gas does not exceed 1 vol.% _.

2. Prevention of the release of elemental fluorine at the anode (as established by the authors, this requirement is practically fulfilled if the concentration of ammonia in the electrolyte does not fall below 22 wt.%, Which corresponds to the composition of NH ₃ · 3HF).

3. The absence of ammonia in the gas phase of the electrolyzer (this requirement is practically fulfilled, since gaseous ammonia supplied under the electrolyte layer greedily reacts with hydrogen fluoride and, at our low feed rates, according to our data, up to 1 m ³ per hour, does not enter the gas electrolyzer phase); the vapor pressure of ammonia over the molten electrolyte in the operating range of temperatures and electrolyte compositions is not more than 0.5 mm Hg

Thus, of the three listed conditions for safe operation, only the first is completely determined by the design features of the cell, the other two are carried out by technological measures.

The closest is the method (prototype) of producing NF ₃ in an electrolyzer with a gas separation baffle in the form of an anode bell immersed in the electrolyte and partially covering the working surface of the electrodes, while the distances between the electrodes and the baffle are set from 30 mm for small cells to 200 mm for industrial electrolyzers [US Patent 5085752, cl. With 25 V 1/24, publ. 02/04/1991].

The disadvantages of this method are the low specific volumetric productivity of the electrolyzer and the high voltage of the electrolysis, due to the large interelectrode distances (up to 400 mm in industrial electrolysis cells) and the partial overlapping of the working surface of the electrodes by a gas separation partition. At large interelectrode distances, gas evolution from the electrodes does not provide efficient mixing of the electrolyte in the entire volume of the electrolyzer, and thus, it is not possible to maintain a given ammonia concentration for all working electrodes. In addition, the method is characterized by a high corrosion rate of that part of the gas separation partition that is located between the electrodes (due to its bipolar operation). The voltage is up to 8 V, and the achieved current output of the target compound is about 65%.

The objectives of the invention are:

development of a more efficient design of the electrolyzer, which allows to increase the yield of the target compound - NF ₃ ;

reducing the energy intensity of the process by reducing the voltage on the cell and energy losses due to the generation of Joule heat;

development of scaling criteria for an electrochemical cell.

The essence of the invention lies in the fact that a method has been developed for producing nitrogen trifluoride by electrolysis of hydrofluoric melts of ammonium fluoride in an electrolyzer with carbon or nickel anodes and steel cathodes, with an anode bell, characterized in that louvre cathodes are used in the electrolyzer, and the anode bell is placed above the electrodes, this:

- the area of the electrolyte mirror under the anode bell is not more than 0.2 of the total area of the electrolyte mirror (A),

- the immersion depth of the bell in the electrolyte is at least 0.25 of the height of the electrodes (B),

- the distance of the cathode-bell is not less than 0,025 from the height of the electrodes (V),

- the distance of the anode-bell is not less than 0.025 from the height of the electrodes if the anode is carbon, and not less than 0.05 from the height of the electrodes if the anode is nickel (G);

- a given level of electrolyte in the electrolyzer is supported by the supply of gaseous hydrofluoric acid, and gaseous ammonia is fed continuously, whereby the operating electrolysis is started at a mass fraction of ammonia in the electrolyte of 25-26% and the feed rate of ammonia is changed depending on the content of NF ₃ in the anode gas (D);

- upon receipt of NF ₃ (in production electrolysis), it is not allowed that the mass fraction of ammonia in the electrolyte be lower than 24% and higher than 28% (E).

The method is characterized in that upon receipt of NF ₃ in an electrolytic cell with carbon or nickel anodes, the top (above the electrodes) location of the anode bell and the louvered cathodes, the fulfillment of conditions (A-D) provides a sufficient degree of separation of the anode and cathode electrolysis gases, in which the hydrogen content in the anode gas does not exceed 1 vol.%, and NF ₃ in the cathode gas is practically absent at a nominal anode current density of 0.1 A / cm ² , and this is verified as in laboratory electrolyzers at 30-50 A with a height of elec childbirth 150 mm, and in the experimental electrolyzer at 4000 A with a height of the electrodes 600 mm.

The claimed connection between the design parameters of the electrolyzer, such as the immersion depth of the bell in the electrolyte, the distance between the electrodes and the bell, with the height of the electrodes allows the design to be scaled for electrolyzers of different capacities (unit power). In this case, the fulfillment of explosion safety conditions (A-G) even in electrolyzers of an industrial scale of 10-20 kA with electrodes of the order of 600 mm high is ensured at interelectrode distances of not more than 30-40 mm, that is, significantly shorter than those stated in the prototype.

The fulfillment of condition (D) ensures the production of NF ₃ with current outputs close to the maximum possible, and the fulfillment of condition (E) ensures the production of NF ₃ with current output of at least 60%. Exceeding the range of the mass fractions of ammonia in the electrolyte specified in condition (E) leads, under the conditions of the method, to a significant decrease in the current output NF ₃ .

The proposed method is carried out in an electrochemical cell, shown in the drawing. The cell includes a steel casing (pos. 1), a cover (pos. 2), an anode half cap (pos. 3), one flat carbon or nickel anode (pos. 8), two steel louvered cathodes (pos. 9), an anode bell ( item 7). The cover and the anode half cover are electrically insulated from each other and from the cell body by dielectric gaskets (pos. 11, 12). The current leads of the cathodes and anode are brought out of the cell through the cover and the anode half-cover, respectively, while they are electrically isolated from the covers by means of dielectric inserts (item 4). The anode bell is attached to the anode half cap in one way or another, for example, by welding or flange connection; in the latter case, the bell is removable (preferred). The louvre cathodes are made in the form of a series of steel plates welded to a continuous vertical frame at an angle of 10-20 ° to the vertical. Reflective plates (pos. 10) are attached to the upper part of the frame, where the blinds end, the purpose of which is to direct the flow of gas-filled catholyte away from the bell. The width of the blinds in the cathodes is 10-20 mm for small cells and about 50 mm for cells of industrial sizes with an electrode height of about 600 mm. A section of the anode bell is preferably 10-15 mm higher than the electrodes, but it can also be placed directly at the upper ends of the electrodes. In both cases, the surface of the anode is fully utilized, and the anodic dissolution of the bell due to bipolar operation is minimized. The anodic and cathodic electrolysis gases are separately removed from the cell: the anodic gas through the hole in the anode half cover (key 5), the cathode gas through the hole in the cell cover (key 6). The holes in the covers (pos. 5, 6) can be used for the initial loading of reagents (NH ₄ F salts and hydrofluoric acid) into the cell, and during electrolysis, one of the holes (pos. 6) is used to feed the electrolyte with gaseous NH ₃ and HF. Reagents are siphoned under the electrolyte layer into the cathode compartment of the cell.

The electrolyzers intended for carrying out the method may differ in the number of electrochemical cells, the size of the electrodes, technical means of maintaining a given temperature regime and monitoring electrolysis parameters, but the mutual arrangement of structural elements corresponding to the drawing and the fulfillment of conditions (A-D) are common to them.

To perform the method in an electrolyzer with carbon or nickel anodes, louvre cathodes and an anode bell located above the electrodes that satisfy the conditions (A-D), an electrolyte consisting of an NH ₄ F salt and hydrofluoric acid is prepared or loaded into it, moreover, the molar ratio of NH ₄ F / HF is 0.65-0.77, which corresponds to the composition of NH ₄ F + (1.54-1.3) HF, the concentration of free HF from 45.4 to 41.3 wt.% And the concentration of NH ₃ in the electrolyte from 25 to 27 wt.%.

Before receiving TFA, dehydrating electrolysis is carried out using one of the known methods. When dehydrating electrolysis, the concentration of water in the electrolyte is reduced to 0.05 wt.%, While the volume fraction of N ₂ O in the anode gas is reduced to 1%.

Before starting production electrolysis, the concentration of ammonia in the electrolyte is checked by sampling for chemical analysis. If necessary, adjust the composition of the electrolyte by feeding HF or NH ₃ so that the mass fraction of ammonia in it is in the range from 25 to 27%, preferably from 25 to 26%.

After that, they immediately switch to working electrolysis, setting the nominal anode current density equal to 0.1 A / cm ² . When working electrolysis periodically, for example, 1 time per hour, control the content of NF ₃ in the anode gas (chromatographic analysis). The electrolyte level in the electrolyzer is maintained within specified limits by periodically feeding the cell with gaseous hydrogen fluoride (if necessary, according to the readings of the level gauge).

Ammonia gas is fed into the cell, preferably continuously, but also periodically. A distinctive feature of the method is that the feed rate of ammonia per unit of passed amount of electricity is associated with the content of NF ₃ in the anode gas and change it in accordance with the equation:

where M _NH3 is the mass of ammonia supplied to the electrolyzer per 1 kAh of the passed amount of electricity, g / kAh;

With _NF3 - the content of NF ₃ in the anode gas, vol.%.

At the beginning of the working electrolysis, the minimum feed rate with ammonia is set according to formula (6), which corresponds to the maximum achievable NF ₃ content in the anode gas under the conditions of the method (about 80% for the carbon anode and about 75% for nickel), namely 127 g / kAh for coal and 132 g / kAh for nickel.

In practice, the content of NF ₃ in the anode gas is obtained equal to or close to the indicated values. When continuously fed with ammonia at the indicated rate, this indicator (NF ₃ content in the anode gas) either remains constant or decreases due to a decrease in the ammonia concentration in the electrolyte (as shown above, the ammonia consumption for N ₂ formation is two times higher than for NF ₃ formation ) When reducing the volume fraction of NF ₃ in the anode gas by a certain amount (minimum, reliably determined), for example by 5%, the feed rate of ammonia is increased in accordance with equation (6). The concentration of ammonia in the electrolyte gradually increases to the optimum, due to an increase in the yield of NF ₃ and a decrease in the consumption of ammonia. Thus, the concentration of ammonia in the electrolyte is regulated, maintaining it near the optimal value, and not at the local point of the cell (the place of sampling for analysis), as is done in existing methods, but in the entire volume of the cell, because the criterion is used (composition of the anode gas) associated with the operation of all anodes.

Most consistent with the method is a continuous change in the feed rate of ammonia according to the signal of the NF ₃ content sensor in the anode gas.

With periodic replenishment of the electrolyte with ammonia, formula (6) makes it possible to calculate the mass of ammonia, which must be fed into the electrolyzer for the ampere hours worked between recharge. In this case, the average value of the concentration of NF ₃ in the anode gas is substituted into the formula (6) from the analyzes performed in the interval between the recharge. The frequency of recharge is determined empirically based on data on the composition of the anode gas and chemical analysis of the electrolyte.

Example 1

The process of obtaining NF _{3 is} carried out in a steel electrolytic cell with external electric heating, a volume of 6.9 dm ³ . The cell has one nickel anode measuring 100 · 150 · 4 mm (width, height, thickness) and two steel louvre cathodes located on both sides of the anode. The anode bell is made of Steel-3 with a thickness of 2 mm and is located above the electrodes, with the lower section of the bell located almost at the upper ends of the electrodes.

3.5 kg of NH ₄ F salt and 2.6 kg of HF are charged in portions into the cell. An electrolyte is obtained with a concentration of NH ₃ , free HF and H ₂ O — 26.4 wt.%, 42.1 wt.% And 0.5 wt.%, Respectively.

When heated to operating temperature (120-130 ° C), the electrolyte electrolyzer is characterized by the following indicators:

- the immersion depth of the bell in the electrolyte is 40 mm (0.27 of the height of the electrodes);

- distance cathode - bell 10 mm (0.067 from the height of the electrodes);

- distance anode - bell 10 mm (0.067 from the height of the electrodes);

- the proportion of the electrolyte surface under the bell is 8.9% of the total surface.

After heating the electrolyte to operating temperature, dehydrating electrolysis begins at a current of 6 A, which corresponds to an anodic current density of 0.02 A / cm ² . Dehydrating electrolysis is carried out until the volume fraction of N ₂ O in the anode gas is reduced to 1%, which corresponds to a concentration of H ₂ O in the electrolyte of about 0.05 wt.% _. The mass fraction of ammonia in the electrolyte at the end of dehydrating electrolysis is 25.1%.

Then they immediately switch to working electrolysis, increasing the current to 30 A, which corresponds to an anode current density of 0.1 A / cm ² .

The electrolyte level during electrolysis is maintained by periodic feeding of gaseous HF, ammonia is fed 2 times a day in portions of about 40 g (52-53 dm ³ ), which is about 2.5% of the initial load. This corresponds to the known methods of replenishing the consumption of ammonia during electrolysis (1-10% of the initial load).

Working electrolysis is carried out for 5 days, the voltage is from 5.4 to 6.3 V, the content of NF ₃ in the anode gas is from 63 to 75 vol.% (Average 67 vol.%), The concentration of NH ₃ at the point of selection of the electrolyte for analysis varies for this period in the range from 25.9 wt.% to 24.3 wt.%. There are no pops during electrolysis.

Example 2

The experiment of Example 1 is repeated, but with working electrolysis, the ammonia feed is changed depending on the average TFA content in the anode gas in the period between the feeds, according to equation (6). An anode gas sample is taken for analysis every two hours, that is, six times between recharge, which, as in example 1, is done twice a day. The mass of ammonia fed to the feed lies in the range from 47 to 50 g. The volume fraction of NF ₃ in the anode gas during the entire experiment is in the range from 70 to 75% (average 73.6%), the electrolysis voltage is in the range of 5 , 5-6.1 V, the concentration of NH ₃ at the point of selection of the electrolyte for analysis in the range from 26.0 wt.% To 25.5 wt.%.

Examples 3-6

In examples 3-5, the electrolyzer corresponding to example 1 is used.

In example 6, in contrast to examples 3-5, the electrolyzer contains one carbon anode measuring 100 × 150 × 20 mm (width, height, thickness). In all these examples, the electrolyzer contains two steel louvre cathodes, the height of which corresponds to the height of the anode, and the anode bell, the lower slice of which is located at the upper ends of the electrodes.

In examples 3-6, the dimensions of the electrochemical cell are changed, affecting the quality of gas separation, in accordance with table 1.

In each of examples 3-6, the procedure (process) corresponding to example 2 is repeated. During working electrolysis, additionally, every 3-4 hours, hydrogen concentrations in the anode gas and TFA in the cathode gas are determined by chromatographic analysis. Working electrolysis in each of examples 3-6 is carried out for 5 days.

Examples 3-6 provide a rationale for the relationship between the parameters (structural dimensions) of the electrochemical cell included in the claims.

Table 1. Cell parameters and gas separation indicators Example 3 Example 4 Example 5 Example 6 Anode material Nickel Nickel Nickel Coal The height of the electrodes (N _e ), mm 150 150 150 150 Depth of immersion of the bell in the electrolyte, mm 45 (0.3 · N _e ) 40 (0.27 · N _e ) 30 (0.2 · N _e ) 40 (0.27 · N _e ) Distance cathode - bell, mm 3.0 (0.02 · N _e ) 10.0 (0.067N _e ) 10.0 (0.067N _e ) 10.0 (0.067N _e ) Distance anode - bell, mm 10.0 (0.067N _e ) 6.0 (0.04 · N _e ) 10.0 (0.067N _e ) 3.5 (0.023N _e ) The fraction of the electrolyte surface under the bell from the total surface of the electrolyte,% 10.3 8.8 15.1 18.8 Volume fraction of H ₂ in the anode gas during working electrolysis,% Up to 1 0.2-0.5 Up to 1 0.5-0.8 Volume fraction of NF ₃ in the cathode gas during working electrolysis,% Ots. Up to 2 Up to 1 Up to 1 A parameter that is outside the recommended range The cathode - bell distance is less than normal The distance of the anode - bell is less than normal The immersion depth of the bell in the electrolyte is less than normal The distance of the anode - bell is close to the minimum

Example 7/1, 2.

The process is carried out in a pilot plant in a steel electrolyzer with carbon anodes and steel louvre cathodes equipped with an anode bell for separating anode and cathode gases. The bell is located above the electrodes; the distance between the bell slice and the upper ends of the electrodes is 15 mm. The anodes are placed in sections in the cell - 4 sections with 3 anodes in each. The louvre cathodes are located on both sides of each anode section. Anodic and cathodic gases are separately removed from the cell through the corresponding gas collectors. The reaction mixture is heated using water vapor of the required pressure supplied to the tubular heat exchanger of the electrolyzer; removal of excess heat during electrolysis - using water supplied to the same heat exchanger.

The cell is characterized by the following indicators.

The volume of electrolyte, m ³ 1,5. The distance from the electrolyte mirror to the cell lid, cm 8-12. The fraction of the surface of the electrolyte under the anode bell,% 14.8. The number of anode plates, pcs. 12. The size of the anode plates (height · width · thickness), mm 60028570. Depth of immersion of the bell in the electrolyte, mm 150 (0.25 · N _e ). Distance anode - bell, mm 15 (0.025 · N _e ). Distance cathode - bell, mm 15 (0.025 · N _e ). Current load at anode current density of 0.1 A / cm ² , A 4000.

The cell corresponds to the claimed method of producing NF ₃ , and with minimal interelectrode distances.

In an electrolyzer, in portions of 200-300 kg, 1110 kg of NH ₄ F salt and 860 kg of gaseous HF are loaded. An electrolyte is obtained with a mass fraction of ammonia of 25.5%, free HF of 43.1% and H ₂ O of 1.5%.

The electrolyte is heated to a temperature of 120-130 ° C and dehydration electrolysis is started at a current of 1600 A and a voltage of 3.0 V. During dehydration electrolysis, the current is reduced so that the voltage across the cell does not exceed 4.5 V.

Dehydration is carried out within 360 hours. At the end of dehydration, the current is 300 A.

The progress of dehydration is judged by the change in the volume fraction of N ₂ O in the anode gas. Dehydration is completed when this indicator decreases to 1%. The mass fraction of ammonia in the electrolyte at the end of dehydrating electrolysis is 24.2%. 30 kg (40 m ³ ) of gaseous ammonia are fed into the electrolyzer, the mass fraction of ammonia in the electrolyte after feeding is 25.4%.

They immediately switch to working electrolysis, increasing the current to 4000 A.

Replenishment of the consumption of starting materials during electrolysis is carried out by supplying gaseous HF and NH ₃ . The supply of hydrogen fluoride maintain a given level in the electrolyzer, focusing on the level gauge, and the flow of NH _{3 is} carried out in two ways.

Example 7/1 uses periodic ammonia feed. The frequency of the recharge is determined according to the chemical analysis of the electrolyte (a sample of the electrolyte for analysis is taken each time from the same point in the cell). Mass fraction of NH ₃ in the electrolyte is maintained in the range from 24% to 28%. This method of feeding the electrolyte with ammonia corresponds to the known methods for producing TFA.

In example 7/2, ammonia replenishment is used, corresponding to the claimed method, namely, when the mass fraction of NH ₃ in the electrolyte is in the range from 25% to 26%, they start and carry out continuous ammonia replenishment based on the formula (6). Anode gas analysis is carried out every 3-4 hours (twice per shift) and, if necessary, the ammonia supply rate is changed by changing the pressure in the ammonia buffer tank in front of the constricting device (flow meter).

The following results are obtained (table 2).

Electrolyzers designed to produce NF ₃ and corresponding to the claimed method are characterized by small interelectrode distances. As a result, the electrolysis voltage in them is less than in existing methods. In laboratory electrolyzers that meet the conditions (A-G) and designed for a current load of 30-50 A, the voltage is from 5.3 to 6.5 V, in the experimental electrolyzer for 4000 A from 6 to 7.3 V.

The proposed method allows to obtain NF ₃ with smaller output fluctuations during prolonged electrolysis, for example, 70-80% on a carbon anode and 70-75% on nickel if the explosion-proof conditions of production are met in both cases.

Table 2. Performance electrolysis EXAMPLE 7/1 EXAMPLE 7/2 Electrolysis voltage, V 5.6-7.3 5.8-6.5 Volume fraction of NF ₃ in the anode gas,% 61-78 (average 66.4) 72-80 (average 75.2) Volume fraction of hydrogen in the anode gas,% Less than 1 Less than 1 Volume fraction of NF3 in the cathode gas,% Lack of Lack of Popping in the cell No No The duration of the experiment, days thirty thirty

Claims (2)

1. The method of producing nitrogen trifluoride by electrolysis of hydrofluoric melts of ammonium fluoride in an electrolyzer with carbon or nickel anodes and steel cathodes, with an anode bell, characterized in that the louvre cathodes are used in the electrolyzer, and the anode bell is placed above the electrodes, while the area of the electrolyte mirror under the anode bell is not more than 0.2 of the total area of the mirror of the electrolyte, the immersion depth of the bell in the electrolyte is at least 0.25 of the height of the electrodes, the cathode-bell distance is not less than 0.025 from the height of the electrodes, the distance of the anode-bell is not less than 0.025 from the height of the electrodes, if the anode is carbon, and not less than 0.05 from the height of the electrodes, if the anode is nickel, the electrolyte level is supported by the supply of gaseous hydrogen fluoride, and gaseous ammonia is fed continuously, and the rate of feeding with ammonia is changed depending on the content of nitrogen trifluoride in the anode gas. 2. The method according to claim 1, characterized in that the mass fraction of ammonia in the electrolyte is maintained in the range of 24-28 wt.%.