United States Patent (19) 11 4,083,941 Jayawant et al. 45 Apr. 11, 1978

54 PURFCATION OF ANHYDROUS HYDROGEN FLUORIDE FOREIGN PATENT DOCUMENTS 75) Inventors: Madhusudan Dattatraya Jayawant, 1,174,382 12/1969 United Kingdom ...... 423/484 Hockessin, Del.; Geoffrey Walsh 342,831 7/1972 U.S.S.R...... 423/484 Meadows, Kennett Square, Pa. Primary Examiner-Edward Stern (73) Assignee: E. I. Du Pont de Nemours and 57 ABSTRACT Company, Wilmington, Del. A process for preparing very high purity anhydrous hydrogen fluoride by contacting anhydrous hydrogen (21) Appl. No.: 788,670 fluoride with at least 2.3% by weight of persulfuric acid 22 Filed: Apr. 18, 1977 based on the anhydrous hydrogen fluoride or at least 0.7% by weight of hydrogen peroxide based on the 51 Int. C.’...... CO1B 7/22 anhydrous hydrogen fluoride at ambient temperature (52) U.S. C...... 423/488; 423/483; and either at least 0.6% by weight of methanol based on 423/484 the anhydrous hydrogen fluoride or at least one mole of (58) Field of Search ...... 423/484, 488, 483 sulfuric acid per mole of hydrogen peroxide at a tem perature of from 0 to 75° C and distilling the resulting 56) References Cited mixture to recover very high purity anhydrous hydro U.S. PATENT DOCUMENTS gen fluoride. 3,166,379 1/1965 Bradley et al. . 423/484 3,689,370 9/1972 Osaka et al...... 423/484 X 16 Claims, No Drawings 4,083,941 1. 2 1977 solves the iron contamination problem by a pro PURIFICATION OF ANHYDROUS HYDROGEN cess comprising contacting the anhydrous hydrogen FLUORDE fluoride with a permanganate or dichromate and then reducing the excess permanganate or dichromate with a BACKGROUND OF THE INVENTION heavy metal free reducing agent and then distilling the 1. Field of the Invention resulting mixture. This invention relates to a process for the preparation R. Osicki in Przemysl Chem, 42,32-3 (1963) discloses of pure anhydrous hydrogen fluoride from anhydrous a 30% HO and Ba(OH), addition to aqueous hydrogen hydrogen fluoride. More specifically, this invention fluoride and isothermal distillation to remove Mn, Cu, relates to a process which comprises treating anhydrous 10 Al, As, Ca and Na. However, sulfite is not removed, the hydrogen fluoride sequentially with hydrogen peroxide starting hydrogen fluoride is not anhydrous and the and methanol or sulfuric acid prior to distillation and main accomplishment is the removal of iron. W. Roland recovering anhydrous hydrogen fluoride with reduced et al. in East German Patent 62,309 (June 20, 1968) impurities after distillation. discloses 80% or 100% HF stirred with either 30% or 2. Prior Art 15 5% HO at 25 to 30° C followed by HF distillation. Anhydrous hydrogen fluoride is formed by heating a This procedure, however, does not sufficiently reduce mixture of fluorspar and sulfuric acid. The main impuri sulfite ions. ties in the thus prepared hydrogen fluoride are fluosul fonic acid, silicon tetrafluoride, sulfur dioxide, sulfuric SUMMARY OF THE INVENTION acid and water. These impurities are usually removed Now in accordance with the invention a process for by fractional distillation and the resulting hydrogen the preparation of pure anhydrous hydrogen fluoride fluoride has a purity of about 99.8% or better. How with decreased amounts of arsenic and sulfite impurities ever, hydrogen fluoride thus produced usually also has been discovered which does not introduce new contains minor amounts of certain other undesirable impurities via the reagents used, said process compris impurities which include arsenic and sulfite ions. In the 25 ing contacting anhydrous hydrogen fluoride having electronics industry, aqueous solutions of hydrogen arsenic and sulfite impurities with at least 2.3% by fluoride are used as cleaning agents and etchants in the weight of persulfuric acid based on the anhydrous hy production of semiconductors, diodes and transistors. A drogen fluoride or at least 0.7% by weight of hydrogen high degree of purity and extremely low levels of the peroxide based on the anhydrous hydrogen fluoride and above impurities are required to prevent minute quanti 30 either at least 0.6% by weight of methanol based on the ties of said impurities from remaining on the surface of anhydrous hydrogen fluoride or at least one mole of said electronic industry products after they have been sulfuric acid per mole of hydrogen peroxide, at a tem etched or cleaned with the hydrogen fluoride, thereby perature of from 0 to 75 C, distilling the resultant adversely affecting their function. mixture and recovering a very high purity anhydrous Several processes have been proposed for the prepa 35 hydrogen fluoride with decreased arsenic and sulfite ration of high purity hydrogen fluoride which substan impurities in the distillate. tially reduce the presence of the undesirable impurities Thus, the process of the present invention involves referred to above. Among such are processes that in first contacting anhydrous hydrogen fluoride contain volve repeated fractional distillation in fluoroplastic ing arsenic and sulfite impurities with at least 0.7% by equipment. However, such methods are suitable only 40 weight of hydrogen peroxide and then contacting the for the preparation of small quantities of high purity hydrogen peroxide and anhydrous hydrogen fluoride hydrogen fluoride and not considered to be economical with either at least 0.6% by weight of methanol based for producing commercial quantities. U.S. Pat. on the anhydrous hydrogen fluoride or at least one mole 3,166,379 discloses a more attractive process from the of H2SO4 per mole of hydrogen peroxide at a tempera standpoint of commercial utility which is based on the 45 ture of from 0 to 75° C followed by distillation and conversion of the impurities to nonvolatile products by recovery of the hydrogen fluoride with substantially oxidizing said impurities with permanganate or chro less of the aforesaid impurities. mate salts and recovering pure hydrofluoric acid with The process of the present invention also involves decreased impurities by simple distillation. Such an contacting anhydrous hydrogen fluoride containing oxidation process is suitable for the purification of aque 50 arsenic and sulfite impurities with at least 2.3% prefera ous hydrofluoric acid but it suffers from the drawback bly 2.3 to 5% by weight of persulfuric acid based on the that when anhydrous hydrogen fluoride is distilled in anhydrous hydrogen fluoride at a temperature of from the presence of permanganate or chromate salts, vola 0' to 75 C, distilling the resultant mixture and recover tile manganese or chromium compounds contaminate ing anhydrous hydrogen fluoride with decreased ar the distillate. Thus, for example, although the arsenic 55 senic and sulfite impurities. and sulfite levels are greatly decreased, the presence of In the present application reference to anhydrous large amounts of manganese or chromium, 100 ppm or hydrogen fluoride means hydrogen fluoride that is at higher, renders the hydrogen fluoride unsuitable for least 97% by weight hydrogen fluoride. Hydrofluoric electronic and other applications which require a high acid means aqueous hydrofluoric acid containing more degree of HF purity. A solution to this problem is 60 than 30% water. claimed in U.S. Pat. No. 3,689,370 which describes a Hydrogen fluoride is made from fluorspar, a naturally process involving the addition of an inorganic ferrous occurring calcium fluoride by reaction with sulfuric salt to the anhydrous hydrogen fluoride after the per acid. Anhydrous hydrogen fluoride is used as a cleaning manganate or chromate treatment to reduce excess agent and etchant in the electronics industry. However, oxidizing agents. However, this process results in a high 65 in such application, the concentrations of impurities level of iron contamination. Allowed U.S. application such as arsenic and sulfite must be very low. Generally Ser. No. 634,438, filed Nov. 24, 1975, by Geoffrey W. based on 49% hydrogen fluoride the arsenic level must Meadows, now U.S. Pat. No. 4,032,621, issued June 28, be no more than 0.03 ppm and the combined sulfite and 4,083,941. 3 4. sulfate no more than 3 ppm. The degree to which these Since the period of time is dependent on the tempera impurities are present in commercial or normally avail ture of the liquids being mixed or contacted, it will be able anhydrous hydrogen fluoride depends largely on understood that the contact time can vary very widely. the source of fluorspar. In commercial manufacture, At higher temperatures the contact time will be very technical grade anhydrous hydrogen fluoride is finally low, e.g., 1 to 2 hours. Similarly at lower temperatures purified by one or two distillation steps which do not the time of contact can be very long, e.g., several days. sufficiently reduce the concentration of the aforesaid Under ambient temperatures associated with winter, the impurities to levels acceptable in electronics uses. The contact time may be in the range of 20 to 80 hours. process of the present invention removes impurities Thus, depending on the temperature, the contact time such as arsenic and sulfite that are not effectively re 10 can range from 1 to 80 hours on a batch basis. Thus, the moved by said conventional distillation. period of time is whatever is required at a given temper The arsenic and sulfite impurities in commercial an ature to convert essentially all of the arsenic. hydrous hydrogen fluoride rarely exceed 0.05% and Although hydrogen peroxide does not oxidize the usually are less than 0.01%. Accordingly, the stoichio volatile arsenic impurity in anhydrous hydrogen fluo metric amount of oxidizing agent required is not great. 15 ride as rapidly as potassium permanganate, unlike potas However, a large excess of HO, has been found to be sium permanganate it does not form a contaminating necessary in order to achieve the desired low levels of byproduct. Hydrogen peroxide is readily available and arsenic and sulfite in the purified HF. forms only water as a byproduct. Relatively high con The amount of persulfuric acid required may vary centrations of hydrogen peroxide are, however, re widely. The concentration of the persulfuric acid used 20 quired. Based on the equation for the oxidation of As should be such to minimize the presence of water in the to As distillation step. What is meant by persulfuric acid is HSOs. Persulfu As + HO, + 2H - As + 2HO ric acid may be prepared by combining the sulfuric acid and hydrogen peroxide of this invention described 25 about 400 times the stoichiometric amount required to herein. oxidize the arsenic originally present in the anhydrous The amount of hydrogen peroxide required depends hydrogen fluoride is required. In order to reduce the on the amount of impurities present in the hydrogen arsenic to levels within the range set for electronic fluoride. The hydrogen peroxide amount referred to applications (less than 0.03 ppm in 49% HF) a large herein of at least 0.7% by weight based on the anhy 30 excess of hydrogen peroxide is required. drous hydrogen fluoride is based on 100% hydrogen The main impurity in commercially available anhy fluoride. Less than 0.7% by weight will not reduce the drous hydrogen fluoride, besides arsenic, is sulfite and sulfite and arsenic impurities sufficiently. The upper sulfate. The use of hydrogen peroxide alone does not level of hydrogen peroxide is limited only by conve reduce the sulfite and sulfate content of the anhydrous nience and economy. The preferred amount of hydro 35 hydrogen fluoride sufficiently to make it acceptable for gen peroxide is from 0.7% to 1.5% by weight. electronic uses. However, the subsequent treatment What is meant by sulfite content as used herein is the with methanol or sulfuric acid followed by distillation combined amount of sulfite and sulfate present in the results in an anhydrous hydrogen fluoride within the anhydrous hydrogen fluoride although in some in limits of sulfite and sulfate set for electronic applica stances both sulfite and sulfate are specifically men tions. Thus, methanol or sulfuric acid is added to the tioned. treated anhydrous hydrogen fluoride to reduce the Hydrogen peroxide of up to 70% by weight is com sulfite level. mercially available. Generally in attaining the levels of An aqueous solution of formaldehyde may also be hydrogen peroxide required in the process of this inven used, but because of undesirable properties, which in tion, hydrogen peroxide of from 50% to 70% by weight 45 clude the availability of formaldehyde in only up to in water may be used. The concentration of the hydro 37% concentration in water, formaldehyde is not pre gen peroxide used should be such to minimize the pres ferred. ence of water in the distillation step. The methanol is added to the anhydrous hydrogen The hydrogen peroxide treatment or oxidation reac fluoride and hydrogen peroxide mixture and agitated to tion of the present invention may take place at ambient 50 insure adequate contacting of the methanol and the temperatures or from 0 to 75 C, preferably at from 0 mixture. The methanol may alternatively be injected to 20 C. Said treatment is conducted with hydrogen into the line used to unload the tank car of treated anhy fluoride under relux or nonreflux conditions with some drous hydrogen fluoride. Premixing of the methanol agitation by a stream of inert gas bubbles or a suitable and hydrogen peroxide prior to the addition to the stirring device. Oxidation is usually complete more 55 anhydrous hydrogen fluoride is hazardous and should rapidly as the temperature is increased. The boiling be avoided. point of hydrogen fluoride is about 20 C and pressur Sulfuric acid is also effective in the present invention ized equipment is required if higher temperatures are in reducing the sulfite impurity of anhydrous hydrogen used. However, when convenient, the hydrogen perox fluoride after the herein described treatment with hy ide aqueous solution may also be added to an empty 60 drogen peroxide. The sulfuric acid is reagent grade of a vessel or tank car of transport and the anhydrous hydro purity that would not introduce enough water to gen fluoride added thereto or the hydrogen peroxide greatly affect the hydrogen fluoride liquid-vapor equi may be added together with the anhydrous hydrogen librium. Generally 95% purity or greater is desirable. fluoride to said vessel or tank car. In the period of time The amount of sulfuric acid required is at least one mole of at least 20 hours, often required for transportation of 65 of sulfuric acid (100% basis) per mole of hydrogen the car to its destination, with only the agitation due to peroxide. Generally an amount of from 1 to 5 moles per movement during transportation at ambient tempera mole of hydrogen peroxide is used. Preferably this tures the reaction with hydrogen peroxide is complete. amount is 1.5 to 5 moles. The most preferred amount of 4,083,941 5 6 sulfuric acid is from 1.2 moles to 3 moles per mole of The anhydrous hydrogen fluoride purified in the hydrogen peroxide. process of the present invention is anhydrous hydrogen The purification of anhydrous hydrogen fluoride by fluoridehaving a concentration of at least about 97% by treatment with a mixture of potassium persulfate, potas weight hydrogen fluoride. sium sulfate and potassium bisulfate and then distillation 5 In the following examples which further illustrate the gave only a minor decrease in arsenic content (25 ppm invention all references to parts and percentages are by decreased to 9.4 ppm on a 49% hydrogen fluoride ba weight unless otherwise indicated. The commerical sis). Treatment of anhydrous hydrogen fluoride with anhydrous hydrogen fluoride used in all the following 6% potassium peroxydisulfate decreased the arsenic examples was tested for arsenic and sulfite and sulfate content to 0.22 ppm which was still considerably higher 10 based on 49% hydrogen fluoride and found to contain than the desired arsenic level of 0.03 ppm or less. It 25 ppm of arsenic and 25 ppm of sulfite and sulfate. seemed, therefore, that the salts of persulfuric acids were not effective enough to oxidize the arsenic impu EXAMPLE 1 rity in anhydrous hydrogen fluoride. It was found, how This example illustrates the use of hydrogen peroxide ever, that when sulfuric acid was added to the oxidized 15 to decrease the arsenic level of commercial anhydrous reaction mixture of anhydrous hydrogen fluoride and hydrogen fluoride to meet the electronic grade specifi hydrogen peroxide, that the arsenic and sulfite was cation. effectively reduced to the level desired. The equipment used for the purification of anhydrous Methanol has been found to be particularly effective hydrogen fluoride consisted of a cold-rolled mild steel in the process of the present invention in reducing the 20 distillation vessel 3 inch I.D., 8 inch deep and inch sulfite contaminant of the anhydrous hydrogen fluoride. wall thickness immersed in a heating bath. A mild steel Although formaldehyde will also remove the sulfite head was attached to the distillation vessel by a inch contaminant, methanol is advantageous due to its lower clamped flange sealed by a Teflon (E) gasket. A mechan water content. The amount of water introduced with ical stirrer with the stirrer shaft passing through a Te methanol is not sufficient to greatly affect the hydrogen 25 flon (E) bearing in the head was used to mix reagents fluoride liquid-vapor equilibrium. Additionally, metha added to HF in the distillation vessel. The head carried nol contains few undesirable impurities. Sufficient four threaded inlets to which were connected a distilla methanol should be added to reduce the sulfite content tion column, a thermocouple well, a sample inlet-port to that required for electronic usage. Generally, an fitted with a Teflon (R) septum and an HF cylinder when excess of methanol over the amount of unreacted hy 30 the vessel was being charged. All surfaces contacting drogen peroxide is not required. Thus, from 0.6 to 2% HF were either mild steel or Teflon (R) and threaded by weight of methanol based on the anhydrous hydro connections were sealed with Teflon (E) pipe tape. gen fluoride is a sufficient amount. Preferably from 0.6 The distillation column consisted of a 12 inches long, to 1% by weight of methanol is used. The process of the inch diameter mild steel pipe packed with mild steel present invention is accomplished without introduction 35 turnings. The column was connected by a tee and elbow of undesirable contaminants which are volatile in an to a 18 inches long inch diameter mild steel pipe essentially anhydrous system. Following the addition of which served as a condenser and the distillate was col the methanol the hydrogen fluoride may be stirred or lected in a Teflon (R) dropping funnel. The column and maintained under reflux for a few minutes prior to distill condenser were water jacketed and the receiver was lation. Since the impurities are converted to residues 40 cooled in an ice water bath. The temperature in the with low volatility compared to hydrogen fluoride, column jacket was controlld by circulating water from elaborate fractionation is not necessary to separate said a reservoir maintained at the desired temperature and impurities in the present process. Thus, distillation at refrigerated water was supplied to the condenserjacket. either atmospheric or higher pressure is effectively The temperature in the distillation vessel was measured conducted in a column with an efficiency of one theo 45 by a iron/constantan thermocouple enclosed in a steel retical plate or less. well with the tip positioned inch from the bottom of The process of the invention must be conducted in the vessel. The column head temperature was similarly equipment which is not attacked by anhydrous hydro measured at the center of the tee side arm at the top of gen fluoride to avoid contamination by extraneous ma the condenser. terial. Thus all surfaces of the distillation vessel, col 50 600 parts of hydrogen fluoride (commercial anhy umn, column packing, condenser and receiver which drous) was charged to the distillation vessel, cooled to come into contact with hydrogen fluoride must be inert 0 C in an ice water bath. 7.7 parts of 70% hydrogen towards it. Suitable materials of construction are metals peroxide (sp. gr. 1.284 g/ml) were injected from a 10 ml such as low carbon steel, nickel and nickel alloys such syringe using a hypodermic needle to penetrate the as Inconel, Hastalloy Alloys B, C and D, Carpenter 20 55 Teflon (E) septum on the vessel head. The hydrogen , Durinet 20 and platinum. Of these, low carbon steel is fluoride was stirred for two hours during which time preferred from the standpoint of economy. Stainless the temperature increased to 10 C. The temperature steels are generally not suitable due to the possibility of was then maintained at 10' to 13' C without stirring for trace contamination from alloy constituents. Polymeric 68 hours. The temperature was raised to 20.8 C. With materials such as polyethylene, unplasticized polyvinyl 60 the column jacket adjusted to 21.5C and the condenser chloride and fluorocarbon polymers such as Teflon (E) jacket to 5' C, distillation of hydrogen fluoride com can also be used, and of these Teflon (E) or similar fluo menced. The temperature in the distillation vessel was rocarbon polymers are preferred. gradually increased to 22.8 C and the column jacket Although the process of the invention is primarily temperature to 23.5' Cover a period of four and a half directed to the preparation of very pure anhydrous 65 hours and 370 parts hydrogen fluoride was collected. hydrogen fluoride for critical uses, such as those in the An aqueous solution of hydrogen fluoride was pre electronics field, it also results in the production of pared by slowly running the anhydrous HF collected in technical grade hydrogen fluoride of improved quality. the Teflon (E) dropping funnel, which served as a re 4,083,941 7 8 ceiver, onto pure ice made from distilled double deion ized water. The strength of the solution was adjusted to EXAMPLES 4 and 5 49% on a weight basis by adding the required amount of These examples illustrate that if sufficient hydrogen pure water. peroxide is not added to the hydrogen fluoride the ar The aqueous HF was analyzed for arsenic by reduc senic content of the purified HF exceeds the electronic ing arsenic impurities to arsine with hydrogen, absorb grade specification. ing the arsine from the hydrogen gas stream in a pyri : The equipment was similar to that used in the preced dine solution of silver diethyldithiocarbamate to form a ing examples except for the mild steel distillation vessel colored complex, and measuring the absorbance spec which was 43 inch deep instead of 8 inches. The proce trophotometrically. Sulfur impurities were oxidized 10 dure was similar to that described in Example 1. with hydrogen peroxide to sulfate which was then de In Example 4, 2.6 parts of 70% HO was added to termined turbidometrically after the addition of excess 300 parts of commercial anhydrous hydrogen fluoride barium chloride. The analytical results were expressed and held for 48 hours at 12 C prior to distillation. In as parts per million of arsenic and sulfite plus sulfate in Example 5, 1.3 parts of 70% H2O, was added to 306 49% hydrogen fluoride. 15 parts of commercial anhydrous hydrogen fluoride and The results obtained on the purified sample are sum held for 70 hours at 10 C prior to distillation. marized in Table 1. Comparison with the corresponding Aqueous solutions of the anhydrous hydrogen fluo analyses on the commercial anhydrous hydrogen fluo ride were prepared as described in Example 1 and ana ride shows that the arsenic and sulfite plus sulfate impu- 20 lyzed for arsenic and sulfite plus sulfate. The results are rity levels were substantially decreased by the purifica given in Table 1. The arsenic content of the purified HF tion procedure. The arsenic level was less than that increased as the HO added was decreased relative to required for electronic grade HF (0.03 ppm) whereas the HF. the sulfite plus sulfate level did not meet the specifica Table 1 tion (3 ppm). 25 % %HO, Hold CHOH Impurities ppm EXAMPLE 2 Ex. Based Temp. Time Based based on 49% HF No, on HF C Hr. on HF As SO, +SO This example illustrates treatment of commercial 1 0.90 0-13 70 None 0.02 6.9 anhydrous hydrogen fluoride with hydrogen peroxide 2 0.90 020 71 0.65 0.02

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UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. : 4,083,94l O DATED April ll, l978 INVENTOR(S) : MADHUSUDAN DATATRAYA JAYAWANT and GEOFFREY WALSH MEADOWS It is Certified that error appears in the above-identified patent and that said Letters Patent are hereby Corrected as shown below: O Column 8, line 42, "end the passed" should be -- end tube passed -- . Column 8 line 55, "sued" should be -- used -- . Column 8, line 57, after "12 inches" insert "long, 3/4 inch". O eignedA. and escaled thisP Third Day of October 1978 SEAL O Attest:

DONALD W. BANNER RUTH C. MASON Attesting Officer Commissioner of Patents and Trademarks O

O o