US007074378B2

(12) United States Patent (10) Patent No.: US 7,074,378 B2 Hart et al. (45) Date of Patent: Jul. 11, 2006

(54) PROCESS FOR THE PURIFICATION OF NF, 4,156,598 A 5/1979 Woytek et al. 4,543,242 A 9, 1985 Aramaki et al. (75) Inventors: James Joseph Hart, Fogelsville, PA 4,740,982 A 4, 1988 Hakuta et al. (US); Philip Bruce Henderson, 4,933,158 A 6, 1990 Aritsuka et al. Allentown, PA (US); Howard Paul 3.E. A 1942. It is al. Withers, Jr., Breinigsville, PA (US); 5. A "R. W., et al. Madhukar Bhaskara Rao, Fogelsville, 5,637,285 A 6/1997 Coronell et al. PA (US), Hoshang Subawala, 5,832,746 A 1 1/1998 Nagamura Macungie, PA (US) 6,352,676 B1 3/2002 Hsiung et al. (73) Assignee: Air Products and Chemicals, Inc., FOREIGN PATENT DOCUMENTS Allentown, PA (US) EP 1 201 291 A1 5, 2002 (*) Notice: Subject to any disclaimer, the term of this OTHER PUBLICATIONS past its, gjusted under 35 John T. Holmes, et al. Fluidized Bed Disposal of Fluorine, M YW- (b) by yS. I&EC Process Design and Dev., 1967, vol. 6, No. 4, p. (21) Appl. No.: 10/763,365 408-413, no month. Primary Examiner Edward M. Johnson (22) Filed: Jan. 23, 2004 (74) Attorney, Agent, or Firm—Geoffrey L. Chase (65) Prior Publication Data (57) ABSTRACT US 2005/O163695 A1 Jul. 28, 2005 (51) Int. Cl. This invention describes al improvement in a process for COIB 2/06 (2006.01) purifying a nitrogen trifluoride (NF) Stream containing (52) U.S. Cl...... 423.406. 423/241:423,239.1 unreacted F2 HF, and nitrogen oxides from an NFs reactor (58) Field of Classification Search 423/240 R wherein the F., and HF are removed and then the nitrogen ------s oxides removed by adsorption. The improvement in the See a E. i R.R. iii. 351 process resides in selectively removing the F from said NF pp p ry. stream without generating oxygen difluoride, removing HF (56) References Cited and then removing said nitrogen oxides by adsorption. Further purification can be effected as desired. U.S. PATENT DOCUMENTS 4,091,081 A 5/1978 Woytek et al. 20 Claims, 1 Drawing Sheet U.S. Patent Jul. 11, 2006 US 7,074,378 B2

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US 7,074,378 B2 1. 2 PROCESS FOR THE PURIFICATION OF NF, of 700 ppm. Typically, the NF, is generated by a process referred to as molten salt electrolysis wherein a salt com BACKGROUND OF THE INVENTION prised of ammonium fluoride and HF are electrolyzed. In the NF purification process, HF is removed first by contact with There is currently a large and growing requirement for 5 water or a caustic scrubber. Then, the OF present in the NF, in semiconductor manufacturing. One of the early pro stream is removed to a level below about 10 ppm by cesses for producing NF involves the direct fluorination of contacting the gas with an aqueous solution of Sodium ammonium ions by F whereby gaseous F is contacted with thiosulfate, hydrogen iodide, and sodium sulfide. The NF liquid (molten) ammonium acid fluoride (AAF) while gas stream is then passed through a Zeolite molecular sieve to eous NH is separately contacted with the liquid AAF to 10 remove Water. generate ammonium ions. The early processes are operated U.S. Pat. No. 5,637,285 discloses the production or NF to maintain a molar ratio of by-product HF to ammonia of by the direct fluorination of ammonium bifluoride at a melt 2.0 to 2.5 (melt ratio) in the reaction liquid and at tempera ratio of HF/NH of at least 2.55. tures above the melting point of ammonium bifluoride, NHHF, which is 127° C. Later processes for producing 15 BRIEF SUMMARY OF THE INVENTION NF, effect the direct fluorination of ammonium bifluoride using increased HF/NH melt ratios. These later processes This invention describes an improvement in a process for resulted in superior NF yields than when the lower melt purifying a nitrogen trifluoride (NF) stream containing ratio processes were used. unreacted F, HF and nitrogen oxides but substantially free Purification of the NF reaction product formed by the 20 of OF, wherein the F and HF are removed first and then the direct fluorination of ammonium bifluoride involves the nitrogen oxides are removed by adsorption. The improve removal of unreacted or unconverted F, as well as the ment in the process resides in removing the F from said NF byproducts HF, N, and trace levels of NF, and N.O. stream without generating oxygen difluoride, removing HF Conventional practice in prior purification processes effects and then removing said nitrogen oxides by adsorption. the removal of excess F and co-product HF by aqueous 25 Further purification can be effected as desired. KOH scrubbing followed by removal of trace levels of Significant advantages can be achieved by the process and nitrogen oxides and water present in the stream using a solid these include: molecular sieve adsorbent. Finally, the NF is further puri an ability to increase the life of absorber beds employed fied using distillation. in NF purification; The following patents and articles describe processes for 30 an ability to selectively remove fluorine in a single step; production and purification of NF an ability to produce a valuable fluorine-containing prod U.S. Pat. No. 4,091,081 discloses a process for the uct while effecting removal of unreacted fluorine; and, production of NF, by the direct fluorination of ammonium an ability to reduce corrosion problems associated with bifluoride at temperatures above 260° F. and below 400° F. the caustic scrubber, which often is responsible for a An HF/NH ratio of 2 to 2.5 is maintained. Purification is 35 2% loss in production due to downtime. effected by passing the reaction product through a mist eliminator pad and then through an aqueous KOH scrubber. BRIEF DESCRIPTION OF SEVERAL VIEWS OF Residual HF and unreacted F are removed in the scrubber. THE DRAWINGS After scrubbing, the temperature of the stream is reduced to condense water and then the stream passed through molecu- 40 The FIGURE is a flow diagram for the production and lar sieve driers for further purification. purification of NF U.S. Pat. No. 4,156,598 describes a process for the production NF by the direct fluorination of ammonium DETAILED DESCRIPTION OF THE bifluoride. Purification of the gaseous reaction stream INVENTION involves passing the gaseous reaction stream through a mist 45 eliminator pad to remove entrained ammonium fluoride or This invention describes an improvement in a process for bifluoride and then through a bath capable of forming a salt purifying nitrogen trifluoride (NF). In newly adopted pro of HF and F, e.g., an aqueous KOH bath. Alternatively, cesses for producing NF in high yields, gaseous F., as the sodium fluoride is used in place of KOH, but F is not fluorine reactant, is contacted with a vigorously mixed removed by this method. To extend the lifetime of the 50 ammonium acid fluoride complex wherein the ratio of adsorber columns NF is removed to a level below 0.03 equivalents of HF to NH typically is such that a significant volume percent before adsorption of N2O and water. amount of fluorine, e.g., up to about 20%, typically 5 to 15% U.S. Pat. No. 4.543.242 describes a process for the by volume can remain in the NF product stream. The NF production and purification of NF, NF, is purified by stream also contains byproduct HF, and nitrogen oxides but removing HF by condensation. Further purification of NF 55 is substantially free of oxygen difluoride (OF), i.e., typi is accomplished by wet scrubbing of NF using aqueous cally, there is no OF in these streams. Any NF stream KOH followed by treatment using a molecular sieve. having residual fluorine and HF contained therein, but U.S. Pat. No. 4,933,158 discloses a process for the substantially free of OF, can be employed as a feedstream purification of NF produced by various methods, including for the process. the direct fluorination of ammonium bifluoride. Removal of 60 It has been discovered that when the removal of residual NO, CO, and NF from an NF stream is accomplished by HF and fluorine is effected using aqueous reactants such as passing the stream through a thermally treated natural Zeo aqueous potassium hydroxide, some of the fluorine reacts to lite. produce Small amounts, e.g., 50 to 100 ppm of oxygen U.S. Pat. No. 4,980,144 describes a process for purifica difluoride. Furthermore, it has been found that this oxygen tion of NF containing hydrogen fluoride, and oxygen dif- 65 difluoride byproduct then is capable of reacting with the luoride. In the described process, NF is generated under molecular sieve in the adsorber columns thereby shortening conditions such that OF is generated in amounts in excess the service life of the adsorbent. Thus, processing NF US 7,074,378 B2 3 4 feedstocks containing a significant level of fluorine by nesium oxide, strontium oxide, lanthanum oxide, and cal conventional methods can cause problems associated with cium oxide; specific examples of anhydrous hydroxides adsorber life. include anhydrous aluminum hydroxide, anhydrous calcium A key to the improvement in the NF purification process hydroxide, anhydrous strontium hydroxide, and anhydrous resides in removing unreacted F from the NF reaction magnesium hydroxide; specific examples of carbonates product stream without generating oxygen difluoride and include calcium carbonate and sodium carbonate, and, spe under Such conditions so as not to remove a substantial cific examples of carbides and Sulfides include silicon car amount of NF. This selective removal of F without remov bide, iron Sulfide, etc. For reasons of efficiency and economy ing NF is accomplished by contacting the NF reaction metals and non-metals such as tungsten, silicon, Sulfur and stream containing unreacted fluorine with a metal or non 10 carbon are selected to provide a valuable metal fluoride metal component capable of converting the fluorine to a product. fluoride, preferably in Solid form, or in a liquid or gaseous The fluorine reactor 8 can be in the form of a packed bed, form having Substantially lower vapor pressure than NF, a fluidized bed or trickle bed reactor or any combination of for efficient removal. A metal or non-metal element, metal the above. Ambient temperatures and pressures may be used oxide, metal Sulfide, metal nitride, metal phosphide, metal 15 in the reactor due to the high reactivity of the metal arsenide, metal carbide, metal carbonate, anhydrous metal component with the unreacted fluorine. The fluorine-con hydroxide, metal silicide, metal germanide, metal boride and taining product compound typically cannot be converted metal aluminide may be used for contacting the NF reaction back to the metal or nonmetal element or metal oxide by product under conditions for forming a metal and/or non simple means (e.g., pressure or temperature Swing cycles), metal fluoride. and is periodically removed in the product-fluoride form Hydrogen fluoride in the reaction product normally does from the fluorine reactor 8 via line 10 and replaced with not react with the metal or non-metal element component fresh starting material. The frequency of replacement is used for converting the fluorine to a fluoride, e.g., a metal based on the reaction capacity for a particular starting Such as tungsten or non-metal element Such as carbon, to any material and concentration of F (and possibly HF) in the substantial degree. HF, therefore, can be removed subse 25 feed. Volatile metal and non-metal fluorides can be removed quent to F removal. However, small amounts of HF will by pressure reduction when the metals or non-metals are react and can be removed by the metal or non-metal element reacted to desired completion or removed as vapor along component. HF will react to a substantial degree with some with the effluent NF stream. of the other components such as metal oxides. If these An effluent stream in line 12 from the fluorine reactor 8 components are used, then HF can be removed prior to 30 generally consists of a mixture of NF and N, and the removal of F. If removal of hydrogen fluoride is effected fluoride-product if volatile, along with some nitrogen prior to removal of fluorine, such removal must be done oxides. It generally will contain less than 15 ppm HF, less under conditions such that oxygen difluoride is not formed. than 200 ppm F, approximately 100 ppm NO and no Removal of hydrogen fluoride, in that case, is accomplished detectable OF. In prior processes, the fluorine reacted with in the absence of a water containing medium, e.g., by 35 the aqueous hydroxide ion in the KOH caustic scrubber cryogenic condensation. generating oxygen difluoride. But, by using a non-aqueous To facilitate an understanding of the invention, reference and anhydrous compound Such as a metal or nonmetal to is made to the drawing. An NF reaction product stream 2 react with the fluorine, no or only a trace amount of oxygen containing typically 40 vol% NF, 35 vol% HF, 15 vol% difluoride is formed. In contrast to other NF purification F, 9 vol % N, 1 vol % NF, and trace NO, and 40 processes, an oxygen difluoride removal step is not required. substantially free of OF, obtained by the direct fluorination An alternate embodiment to the above process is to effect of ammonium bifluoride, is employed for illustration. The HF separation after selective removal of fluorine in fluorine fluorine content typically ranges from 5 to 15%. In this reactor 8. The HF condenser/separator when placed subse embodiment of the process, the stream 2 is first passed to a quent to fluorine reactor 8 can use multiple methods for condenser/separator 4 to remove HF. The HF is condensed 45 effecting removal which includes condensation, Scrubbing in in condenser/separator 4 and removed as a liquid. Conden an aqueous alkaline solution, e.g., aqueous potassium or sation of HF can be effected at subatmospheric to super Sodium hydroxide, or adsorption. Aqueous or alkaline sys atmospheric pressure and temperatures below the boiling tems for removing HF prior to removal of fluorine will result point. Typical condensation temperatures range from -78 to in the generation of oxygen difluoride and thereby defeat the -196° C. at atmospheric pressure. The condenser/separator 50 purpose of selective removal of fluorine without generating is preferably operated under conditions sufficient to remove oxygen difluoride. the maximum amount of HF with negligible NF loss. The effluent gas from the fluorine reactor 8 is then sent via Consequently, the vapor feed stream from condenser/sepa line 12 to an adsorber 14 to remove nitrogen oxides such as rator 4 typically will contain less than 10% HF and prefer NO, as well as possibly water, and the like from the effluent ably less than 2% HF. 55 stream. Conventional adsorbents such as silicate-based The effluent from the condenser/separator 4 is sent via molecular sieves or Zeolites, e.g., 3A, K-Zeolite, chabazite, line 6 to a fluorine reactor 8 to effect removal of unreacted a mordenite or carbon-based molecular sieves are used in the F. The fluorine reactor is filled with a metal or non-metal adsorber to effect the removal of residual water, nitrogen component capable of converting the unreacted fluorine to a oxides, and other byproducts. Finally, the effluent from the fluoride for effective removal. A metal, metal oxide, metal 60 adsorber rich in NF is sent via line 16 to a distillation compound, nonmetal element, or anhydrous metal hydrox column 18 for final purification. A purified NF product is ide that can react with gaseous F to form a fluorine removed via line 20 as a middle cut, with N removed via containing product is Suitable. Specific examples of metals line 22 as an overhead stream. Residual metal fluoride which and non-metal elements for forming metal and non-metal may have been carried through the process is removed via fluorides include silicon, tungsten, iron, Zinc, Zirconium, 65 line 24. Sulfur, and carbon; specific examples of metal oxides In Summary, operating the process under conditions for include aluminum oxide, Zirconium oxide, iron oxide, mag effecting the removal of fluorine from an NF reaction US 7,074,378 B2 5 6 product stream without forming byproduct oxygen difluo removing the nitrogen oxides by adsorption in an ride extends the life of the adsorber beds and enhances the adsorber; followed by purification process. recovering a purified NF product. The following example is illustrative of another embodi 2. The process of claim 1 wherein the fluorine is selec ment. 5 tively removed by contacting the stream with a metal An NF reaction product stream 2 is generated in con component under conditions for forming a metal fluoride. ventional manner by the fluorination of ammonium bifluo 3. The process of claim 1 wherein the fluorine is selec ride at an NH/HF melt ratio of approximately 2.6. The tively removed by reaction with a non-metal component reaction product stream typically contains 45% NF, 32% under conditions for forming a non-metal fluoride. HF, 13% F, 9% N, 1% NF, and 500 ppm N.O. HF is 10 4. The process of claim 2 in which the metal component removed from this stream by passing it through a shell and for selectively removing fluorine is selected from the group tube condenser/separator 4 made of Monel(R) pipe with consisting of a metal element, metal oxide, anhydrous metal cooling liquid at -78° C. from a dry ice/limonene bath hydroxide, metal Sulfide, metal nitride, metal phosphide, circulating through the shell side at a pressure of 3 psig and metal arsenide, metal carbide, metal carbonate, metal sili flowing at 20 standard liters per minute (sLm). HF is 15 cide, metal germanide, metal boride and metal aluminide. liquefied and with even lower cooling liquid temperatures 5. The process of claim 4 wherein the metal element in can be removed from the stream to a concentration <1.0%. said metal component is selected from the group consisting Fluorine is removed from the stream without forming OF of tungsten, iron, Zinc, silicon, aluminum, magnesium, cal in a reactor constructed of Monel(R) pipe of 6-inch diameter cium, and Zirconium or mixtures thereof. equipped with a cooling jacket for circulating a cooling 6. The process of claim 4 wherein the fluorine is removed liquid. It is filled with 17 kg of tungsten metal powder (Type by contact with a metal component selected from the group M63 from Osram Sylvania Co.) to give a bed height of consisting of aluminum oxide, anhydrous aluminum hydrox 8-inches. This reactor is heated with a resistance heater to an ide, calcium oxide, anhydrous calcium hydroxide, strontium initial temperature of 80° C. To effect removal, the stream, oxide, anhydrous strontium hydroxide, magnesium oxide, now containing 65% NFs, 1% HF, 19% F, 13% N, 1.5% 25 anhydrous magnesium hydroxide and lanthanum oxide. NF and 700 ppm NO after removal of HF, is passed 7. In a orocess for purifying an NF stream containing through the tungsten-filled reactor whereupon the F2 reacts fluorine, hydrogen fluoride, nitrogen oxides, and Substan completely with the tungsten metal powder to generate tially free of oxygen difluoride, the improvement which tungsten hexafluoride (WF) gas at 16.7 g/min. The tem comprises: perature of fluorine reactor 8 is maintained between 100 and 30 first removing hydrogen fluoride and, 200° C. by means of the cooling fluid circulating through the removing fluorine without removing a substantial amount jacket. (The resistance heater is turned off because the of NF and without generating oxygen difluoride; and reaction generates Sufficient heat to Sustain these tempera then, tures.) The effluent stream in line 12 from the fluorine removing the nitrogen oxides by adsorption in an reactor 8 now contains 76% NF, <2 ppm HF. <1 ppm F, 35 15% N, < 1 ppm NF. 100 ppm NO and 8% WF. From adsorber; followed by this it can be seen that nearly all the F reacted with the recovering a purified NF product; tungsten without forming OF and, also the HF and NF is wherein the fluorine is selectively removed by contact removed. NO is removed to a lower concentration. with a metal carbonate selected from the group con Further purification of the NF stream is done by passing 40 sisting of calcium carbonate and sodium carbonate the stream through an N2O/HO adsorber 14 made of 3-inch under conditions for forming a metal fluoride. diameter Monel(R)pipe packed with 3 A molecular sieve and 8. The process of claim 4 wherein the byproduct HF is mordenite molecular sieve having a combined bed length of removed by condensation prior to effecting removal of 24-inches. The effluent stream from adsorber 14 contains fluorine from the NF stream. 76% NFs, 16% N and 8% WF. NO is reduced to <15 ppm. 45 9. The process of claim 4 wherein HF is removed subse The effluent stream from the adsorber filled with a 3 A quent to the removal of fluorine and the HF is removed by molecular sieve and mordenite is sent to a distillation a method selected from the group consisting of condensa column 18 whereby the WF is removed from the NF as a tion, Scrubbing in an aqueous alkaline hydroxide solution higher boiling fraction from the bottom of the column via and adsorption. line 24 and the N is removed from the NF as a lower 50 10. The process of claim 4 in which a zeolite is used as an boiling fraction from the overhead via line 22. The purified adsorbent in the adsorber. NF product is removed as a middle boiling fraction via line 11. The process of claim 10 in which the Zeolite is selected 20. The purity of the NF processed in this manner is from the group consisting of mordenite and chabazite. 99.996%. The WF isolated from distillation column can be 12. The process of claim 9 wherein the residual impurities further purified to 99.9995% by distillation and can be 55 in the resulting NF stream from the adsorber are removed marketed as a valuable co-product for use in semiconductor by distillation. production. 13. The process of claim 3 wherein the non-metal com The invention claimed is: ponent is selected from the group consisting of carbon and 1. In a process for purifying an NF stream containing sulfur. fluorine, hydrogen fluoride, nitrogen oxides, and Substan 60 14. In a process for purifying NF present in a reaction tially free of oxygen difluoride, the improvement which product stream wherein said NF is produced by contacting comprises: gaseous F., as the fluorine reactant, with an ammonium acid first removing hydrogen fluoride and, fluoride complex under conditions for generating a reaction removing fluorine without removing a substantial amount product stream containing unreacted fluorine, byproduct of NF and without generating oxygen difluoride by 65 hydrogen fluoride, and nitrogen oxides but Substantially free contacting with a non-aqueous compound to react with of oxygen difluoride, the improvement which comprises: the fluorine; and then first removing byproduct hydrogen fluoride, and US 7,074,378 B2 7 8 removing fluorine without removing a substantial amount of tungsten, iron, Zinc, silicon, aluminum, magnesium, cal of NF and without generating oxygen difluoride by cium and Zirconium or mixtures thereof. reacting a non-aqueous compound with the fluorine; 18. The process of claim 14 wherein the fluorine is and then removed by contact with a metal component selected from removing the nitrogen oxides by adsorption; followed by the group consisting of aluminum oxide, anhydrous alumi recovering a purified NF product. num hydroxide, calcium oxide, anhydrous calcium hydrox 15. The process of claim 14 wherein the reaction product ide, strontium oxide, anhydrous strontium hydroxide, mag stream contains from 5 to 20% fluorine by volume. nesium oxide, anhydrous magnesium hydroxide and 16. The process of claim 15 in which a metal component lanthanum oxide. is used for selectively removing fluorine and it is selected 10 19. The process of claim 14 wherein fluorine is selectively from the group consisting of a metal element, metal oxide, removed by reaction with a non-metal component under anhydrous metal hydroxide, metal Sulfide, metal nitride, conditions for forming a non-metal fluoride. metal phosphide, metal arsenide, metal carbide, metal car 20. The process of claim 19 wherein the non-metal bonate, metal silicide, metal germanide, metal boride and component is selected from the group consisting of carbon metal aluminide. 15 and sulfur. 17. The process of claim 16 wherein the metal element in said metal component is selected from the group consisting