Process for preparing nitrogen trifluoride

Details Process for preparing nitrogen trifluoride Process for preparing nitrogen trifluoride

2002-02-12

2004-11-23

Langel, Wayne A. (Department: 1754)

Chemistry of inorganic compounds

Nitrogen or compound thereof

Binary compound

Reexamination Certificate

active

06821496

ABSTRACT:

FIELD OF THE ART
The present invention relates to inorganic chemistry, and more particularly to a process for preparing nitrogen trifluoride.
PRIOR ART
Nitrogen trifluoride finds extensive application in the technology of semiconductors, high energy lasers, and chemical vapor deposition. In contrast to elemental fluorine, nitrogen fluoride is easy to transport in condensed state at a pressure of up to 75 kgf/cm
2
and packing density of up to 600 g/l (J. Fluor. Chem., 1991, 54, No. 1-3, p. 37).
Industrial technologies of producing nitrogen trifluoride are based on the electrolysis of molten ammonium hydrogen fluorides and on the direct fluorination of ammonium with elemental fluorine.
The most elaborated industrial process for nitrogen trifluoride synthesis is the electrochemical synthesis of NF
3
from ammonium hydrogen fluorides (Gmelin Handbook, 1986, v. 4, pp. 172-173; Handbuch der präparativen anorganishen Chemie, G. Brauer (Ed.) Moscow, “Mir” Publishers 1985, vol. 1, pp. 220-221 (Russian translation)).
The optimum process parameters for the electrochemical synthesis of NF
3
are: the temperature of 100-130° C., the melt composition corresponding to NH
4
F/HF: 1.1 to 1.8, current density of from 0.05 to 0.15 A/cm
2
. The current yield for NF
3
is 70%.
The process of electrochemical synthesis of NF
3
suffers from such disadvantages as explosion hazards involved by the formation of a mixture of nitrogen trifluoride with hydrogen, the use of gaseous ammonia in the step of obtaining ammonium hydrogen fluorides. Furthermore, the implementation of this process requires large capital inputs because of using costly nickel-containing structural materials, most stable in the corrosion-active medium of molten hydrogen ammonium fluorides.
For reducing the explosion hazard presented by mixtures of nitrogen trifluoride with hydrogen, a method of electrochemical synthesis was proposed, envisaging dilution of the anode gas with nitrogen to the NF
3
concentration less than 9.5% (U.S. Pat. No. 3,235,474, 204-63, Feb. 15, 1966). However, diluting the production gas with nitrogen adds much to the expenses for nitrogen trifluoride purification and condensation equipment.
It is known from the literature, that as starting material for producing nitrogen trifluoride by the method of electrochemical fluorination it is possible to use solutions of urea and other nitrogen-containing compounds: pyridine, hydrazine, guanidine, semicarbazide in anhydrous hydrogen fluoride (Z. anorg. allgem. Chem., 1969, v. 367, pp. 62-79). In accordance with this method, nitrogen trifluoride is obtained with a yield of from 16 to 38%.
The chemism of the process of electrochemical fluorination of urea is described by the following equations:
NH
2
CONH
2
+12F→2NF
3
+COF
2
+4HF  (1)
2NH
2
CONH
2
+24F→4NF
3
+CO
2
+CF
4
+8HF  (2)
The resulting electrolysis gas after alkaline purification from acid admixtures of CO
2
and COF
2
contained up to 10% CF
4
. In the fluorination of pyridine the content of CF
4
reached 34% of the volume of the obtained nitrogen trifluoride. This circumstance substantially complicates NF
3
purification, since it is difficult to separate NF
3
from carbon tetrafluoride. As it follows from the chemism of the process, represented by equations (1) and (2), in the process of electrochemical fluorination there is formed in total at least (with the 100% yield of NF
3
) 0.5 mole of such admixtures as CO
2
, COF
2
and CF
4
per mole of NF
3
. A large amount of admixtures in crude nitrogen trifluoride and the presence of hydrogen in the electrolysis gases adds to the cost of the process of gas purification from the admixtures and calls for taking measures to make the process explosion-safe.
Another known trend in producing nitrogen trifluoride is the direct fluorination of ammonia with elemental fluorine by the reaction:
4NH
3
+3F
2
→NF
3
+3NH
4
F.
Synthesis of nitrogen trifluoride by the direct fluorination of ammonia with elemental fluorine in the vapor phase in NF
3
:F
2
=(1.1-2.0):1 molar ratios makes it possible to obtain the target product with the yield calculated on the basis of fluorine from 10 to 25% (J. Amer. Chem. Soc., 1960, 82, 5301). The low yield of nitrogen trifluoride is due to complexities in regulating the reaction temperature and to the interaction of NF
3
or of its intermediate synthesis products with ammonia, giving nitrogen and hydrogen fluoride. Moreover, difficulties arise with the gas separation from the forming sublimates of ammonium fluorides.
For raising the effectiveness of cooling the reaction mixture and increasing the yield of nitrogen fluoride, a method is proposed for the fluorination of ammonia in the presence of sulfur hexafluoride, hexafluoroethane or tetrafluoromethane (JP 2-255513, C01B 21/083, Oct. 16, 1990).
With the molar ratio of diluent gas to ammonia of (5-100):1 and of fluorine to ammonia of (3-20):1, the temperature in the reactor of 80 to 250° C., the yield of NF
3
was from 30 to 59.5%. The maximum yield of nitrogen trifluoride was obtained with the molar ratio NF
3
:SF
6
equal to 4:100. The results cited in said Japanese Patent indicate that the content of the target product in waste gases should not exceed 1-2 vol. %, consequently, its concentration to 99% along with regeneration of the diluent gas will require substantial capital inputs.
Furthermore, separating sublimates (melts) of ammonium hydrogen fluorides from diluted gases is a technically complicated operation. Therefore, the possibilities of industrial implementation of this method are not high.
For overcoming the above-indicated disadvantages, the authors of U.S. Pat. No. 4,091,081, C01B 21/52, May 23, 1978 and U.S. Pat. No. 5,637,285, C01B 21/06, Jun. 10, 1997 proposed methods for preparing nitrogen trifluoride by the fluorination with elemental fluorine of ammonia dissolved in a melt of ammonium hydrogen fluorides, at the process temperature of from 93 to 209° C. and an NF
3
:HF molar ratio equal to 1:(2.0-2.5) and 1:(2.55-2.85), respectively.
Carrying out the synthesis of nitrogen trifluoride in the liquid phase of a melt of ammonium hydrogen fluorides may provide an effective removal of the exothermal reaction heat and, in the main, solve the problem of separating ammonium hydrogen fluorides from the target product.
However, industrial implementation of these processes will require taking strictest explosion-safety measures in connection with using ammonium and elemental fluorine in the technology. Furthermore, with sufficiently high yields of nitrogen trifluoride (65%), the consumption of fluorine-containing starting materials is unjustifiably high, since more than 60% of fluorine used in the technology is converted into ammonium hydrogen fluorides, the regeneration of hydrogen fluoride from which is sufficiently complicated.
In the combination of essential features, the closest to the herein-proposed process is the method of producing nitrogen fluorides by the direct fluorination with elemental fluorine of ammonia derivatives containing nitrogen-hydrogen bonds, such as alkali metal amides, urea, biuret, sulfamide, formamide, hydrazine, ethylene diamine, melamine at a temperature of 0-300° C. in the presence of a catalyst: a metal fluoride forming an acid salt with HF (U.S. Pat. No. 3,961,024, C01B 21/52, Jun. 1, 1976). For instance, the fluorination of 50% mixtures of urea with sodium fluoride or sodium hydrogen fluoride by a gaseous mixture containing 50 vol. % of F
2
gave a gas having the composition: 10-17 vol. % of NF
3
and 3-13 vol. % of N
2
F
4
. Along with nitrogen fluorides, the production gas contained admixtures of COF
2
, CO
2
, CF
4
, and NO
3
F. The fluorination of mixtures of biuret with lithium fluoride or sodium hydrogen fluoride (1:1) by diluted elemental fluorine leads to the formation of a gas containing 6-47 vol. % of NF
3
and 2.6-26 vol. % of N
2
F
4
.
The disadvantages of the known method are: a low selectivity of the process of producing nitrogen trifluoride because of

Affiliated with

Also associated with

Rating

Process for preparing nitrogen trifluoride does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for preparing nitrogen trifluoride, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for preparing nitrogen trifluoride will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3296746