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COMBUSTION. Fluorine can be burned in a flame employing hydrocarbon fuel under conditions such that fluorine provides some of the oxidation normally furnished by excess air. The products of reaction include hydrofluoric acid and carbon fluorides, the latter being inert gases with no odor. However, the hydrofluoric acid formed is objectionable and should be removed. Therefore, it is desirable, if any appreciable quantities of fluorine are to be vented, that the burner gases he scrubbed with water or alkaline solutions t o remove the hydrofluoric acid. This presents a n appreciable corrosion problem and equipment has to be constructed of carbon, lead, etc., to withstand dilute hydrofluoric acid solutions. This system has the disadvantages of requiring the combustion of some fuel a t all times and is particularly disadvantageous for widely varying loads. REACTION WITH l $ r . 4 ~ ~ T ~ h. e disposal of fluorine by water scrubbing is not satisfactory, because fluorine does not altvays rea c t with water, for reasons unknown. Explosions have been encountered in such a system under some circumstances but not in others. Since there is insufficient information on how to control the reaction, i t is not satisfactory for design a t the present stage of development. REACTION WITH CARBON.The reaction of fluorine with carbon evolves so much heat that it is difficult t o design safe equipment t o handle such a process. Furthermore, the existence of an explosive compound F C (6) rules this method out. ABSORPTION BY LIME. The use of a lime slurry to remove fluorine in a liquid system is successful if sufficient contact time IS provided. Because lime is a much more dilute alkali than caustic soda, the destruction of the intermediate compound, OFz, is more difficult than in the case of caustic soda. Insufficient experience has been accumulated to establish a satisfactory quantitative basis for design. Furthermore, difficulties attending the handling of slurries are involved. ABSORPTION BY FLUORIDES OF LOWERVALE~TE. When inorganic fluorides, such as silver, antimony, cobalt, etc., are treated in their lower valence state with fluorine, they are oxidized t o the higher valence state and the fluorine is absorbed. The compound of higher valence can then be reduced to its lower state by hydrogen, and the hydrofluoric acid formed can be condensed out in a refrigerated trap. This method is decidedly unsatisfactory for small installations and, for large installations, has the usual hazards of handling hydrogen as well as involving a series of operations.
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From the foregoing examples, it is clear that disposal of fluorine varies with the conditions involved, but the use of caustic, or of the salt-soda lime combination, represents the most convrnient method for many installations: ACKNOWLEDGMENT
The authors acknowledge with appreciation the contributions made by the Hooker Electrochemical Company, E. I. du Pont de Semours K. Company, Inc., The Harshaw Chemical Company, The Johns Hopkins Cniversity, Columbia University, the Army (’orps of Engineers, Nanhattan District, The Kellex Corporation, and the inany individuals in these organizations, to the develop nients described in this paper. The work reported in this paper \vas done at the Kellex Corporation under contract from the \ranhattan District. LITERATURE CITED
(I! Bichowsky. F. R.,and Rossini F. D., “Thermochemistry of Chemical Substances”, New York, lteinhold Pub. Corp., 1937. ( 2 ) Bockemueller, 0.. “Organische Fluorverbindungen”. Stuttgart,
F. Enke, 1936. (3) Landau, R., Kellex Corp., Research Paper E7, Manhattan District Volumes (1947). (41 Latimer, W. M., “Oxidation Potentials”, New York, PrenticeHall, 1938. (5) Moissan, H., Compt. rend., 102, 1543 (1886). (6) Ruff, 0..and Bretschneider, O., Z . anorg. allgem. Chem., 217, 1-18 (1934); Simons, J. H., and Block, L. P., J . Am. Chem. Soc., 61, 2962-6 (1939). PREBEXTED before the Symposium on Fluorine Chemistry as paper 37, Division of Industrial and Engineering Chemistry, 110th Neeting of the AMERICANCHEMICAL SOCIETY, Chicago, 111. The work described in this paper is covered also in a comprehensive report of work with fluorine and fluorinated compounds undertaken in connection with the Manhattan Project. This report i3 soon t o be published as Volume I of Di\-ision VI1 of the Manhattan Project Technical Series.
ANALYSIS AND DISPOSAL OF FLUORINE S. G. Turnbull, ,4.F. Benning, G. W. Feldmann, -4.L. Linch, R . C. McHarness, and YI. IC. Richards E. I . DU PONT DE NEMOURS & COMPANY, INC., WILMINGTON, DEL.
A SAFE analytical method for industrial use has been developed for the analysis of high quality fluorine. This method is based upon the reaction of hydrogen fluoride with sodium fluoride, the conversion of fluorine to chlorine by reaction with sodium chloride, and analysis of the chlorine for oxygen and inert gases by standard methods. The over-all error in fluorine purity by this method is indicated to be less than 0.4%. A safe method for the disposal of waste fluorine on a large scale has been developed and satisfactorily operated. The waste fluorine is burned with hydrocarbon gases, and acid products formed are removed from nontoxic carbon fluorides by scrubbing with water and sodium hydroxide.
A
SAFE, routine method for analysis of fluorine present in low or high concentrations a t atmospheric or superatmospheric pressures was required for proper control of the quality of fluorine, ( a ) as produced directly by electrolytic cells and ( b ) when packaged in cylinders after purification and compression. An analytical method reported in the literature ( 1 ) consisted of absorbing fluorine by mercury in glass equipment. This was considered unsatisfactory for our purposes because it would be extremely hazardous for unskilled operators, did not include the
determination of the hydrogen fluoride present as an impurity, and could not be used when the latter was present. An attempt \vas first made to develop a completely gasometric method in which the highly toxic and reactive fluorine could be converted to the less toxic and more easily handled chlorine, and the hydrogen fluoride transformed to the easily analyzed hydrogen chloride by reaction with chlorides. The reaction of fluorine with chlorides of the folloFving bivalent elements was studied: calcium, barium, magnesium, cadmium, and mercury. Although these chlorides did react to yield chlorine and metallic fluorides, it ivas found that the hydrogen fluoride impurity \vas only partially converted to hydrogen chloride, and was for the most part adsorbed upon the metallic fluorides formed in the reaction. hlloreover, fluorine reacted with moisture and with oxides that wcre difficult to remove from these metallic chlorides. This led t o the formation of oxygen, fluorine oxide, and hydrogen fluoride, which gave inaccurate results. Howcver, three satisfactory analytical methods xwre developed and are more fully described here. SODIUAI FLUORIDE-SODIUM CHLORIDE JlETHOD
The sodium fluoride-chloride method was developed for fluorine at concentrations above 50%. The hydrogen fluoride is first removed quantitatively in a copper or nickel tube by sodium flue
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ride pellets as the acid fluoride, S a F . H F . The fluorine is then converted to chlorine by passage through anhydrous C.P. sodium chloride in another tube. After the fluorine gas sample has been passed into the system (Figure 1) for several minutes to s w e p out all traces of inert gases, a sample is taken of the “converted gas”, which consists of chlorine, oxygen, and inert gases such as nitrogen. The surplus gas formed in the conversion is passed into cold 2 S sodium hydroxide, which reacts with the chlorine to form sodium hypochlorite, and allows the inert gases to pass to thc air.
v i
INERTS
AIR
INERTS Figure 1.
Fluorine Sampling System for NaF-NaCl Method
The quantity of fluorine used in the analysis is determined by measurement of the chlorine absorbed as hypochlorite in the caustic plus that taken in the gas samples. The former is determined by titration with sodium thiosulfate of the iodine liberated by the action of potassium iodide and acetic acid on the hypochlorite solution. The chlorine in the gas sample is determined by absorption in caustic. The gases unabsorbed by the caustic are passed through alkaline pyrogallol for the determination of oxygen. The residual unreacted gases are called “inert” and comprise mostly nitrogen. The quantity of hydrogen fluoride present in the fluorine is determined by maceration of the sodium fluoride pellets in a nickel or platinum dish in the presence of cold, neutralized potassium nitrate, which is used to eliminate any error that would result from sodium fluosilicate that may be present as a n impurity (5’). The hydrogen fluoride is then titrated with silicate-free sodium hydroxide by standard methods. From the quantities of fluorine, hydrogen fluoride, oxygen, and inert gases determined by these methods, the exact composition and purity of the fluorine gas sample can be calculated. VAPOR PRESSURE METHOD
I n the second method the quantity of hydrogen fluoride present in the fluorine is determined by a direct measurement of the hydrogen fluoride vapor pressure. This is obtained by condensation of a known quantity of fluorine a t liquid nitrogen temperature, followed by pumping off the fluorine and rvarming the container which holds the residual hydrogen fluoride to room temperature. The quantity of hydrogen fluoride is calculated by spplication of Boyle’s law for gases. The sodium fluoride-chloride method is preferred over the vapor pressure method because it is more easily handled by routine analysts, is not handicapped by the possible development of very small leaks in the system, and allows for the simultaneous determination of fluorine, hydrogen fluoride, oxygen, and incrt gases.
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Table I rrcordi comparative result* obtained with these two methods at two of the cooperating laboratopies. The absolute error of the hydrogen fluoride determination is shown to be less than 0.15no for the sodium fluoridr-chloride method. The estimated over-all error in fluorinc purity is about 0.4‘“,. 4PPLIC4TIONS OF PREFERRED BIETHOD
The sodium fluoride-chloride method has brcn regularly used in the following indances:
1. To determine the purity of fluorine a t concentrntions of 50y0or greater in cylinders under pressures as high as 400 pounds per square inch. 2. To determine the hydrogen fluoride and other impurities Dresent in the fluorine produced bv electrolrtic cells in which the following variables were changed: temperature, concentration of hydrogen fluoride in electrolyte, time from start-up of cell, method of adding hydrogen fluoride to electrolyte. 3. To determine the effectiveness of sodium fluoride scrubbers and refrigeration coils of various design in the removal of hydrogen fluoride from fluorine. 4. To obtain analyses of fluorine in manifolds leading from Droduction t o consuming units, for better control of the latter, gnd for estimates of dailfplant production of fluorine. HYDRIODIC ACID METHOD
For the analysis of low concentrations of fluorine, the sodium fluoride-chloride method was not satisfactory because the fluorine reaction with salt was difficult to initiate and maintain. However, a method based on the reaction of fluorine with dilute hydriodic acid solutions was developed for the analysis of mixtures of inert gases and fluorine that contained less than 10% of the latter. The equations are:
+ 2HI +2 H F + 11 + 2Hz0 +4 H F + 02 2F2 + HzO +2 H F + FzO F2
2F2
(1) (2)
(3)
Reaction 1 is the main one Reaction 2 causes a relative error of only about 2y0 in the results calculated from 1. Reaction 3 is insignificant a t the 1 N acetic acid concentration used. DISPOSAL O F FLUORINE
Since it became necessary a t times t o dispose of substantial amounts of fluorine over short periods, a number of methods for the conversion of fluorine to less active and less toxic materials were investigated. The reactions involved in three of these are:
+ 2F2 +CFI + 3Fn +SFe Hz + --+ 2 H F
C
S
F2
(6)
Reactions 4 and 5 were found to occur smoothly enough, but with liberation of considerable quantities of heat a t a gas-solid interface. Since it was believed that the dissipation of this heat might present difficulty in large scale operations, these reactions were dropped from consideration. Another drawback t o reaction 4 is the formation of explosive carbon fluorides reported in the literature ( 2 ) and experienced by other investigators working on the disposal problem. Reaction 6 offered-a promising lead, for considerable quantities of hydrogen are formed simultaneously with fluorine in generation of the latter by electrolytic cells. However, the reaction with TABLE I. COMPARISON OF ANALYTICAL hfETHODS FOR DETER- hydrogen was found to be inhibited a t times, and also considerable YINISG VOLUMEPERCEXTHYDROGEN FLUORIDE IN FLUORINE heat was evolved. No reliable catalyst for initiating this reaction was found. h7aF-NaCI -Vapor Pressures o 1 h-0 2 No. 1 No. 2 It was observed, however, that fluorine could be reacted with 0.37 + 0.04 0.32 * 0.05 0.10 water. This reaction can occur in two ways: (a) a burning re0.85 * 0 . 1 2 0.55 i O . 0 9 0 . 7 0 == 0 . 0 5 ... action, in which water and fluorine gas actually combine at the 1.04 * 0.02 .... 0.95 * 0.02 ... gas-liquid interface with the formation of a purple flame, and 7-
-
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( b ) a slower reaction, in which no burning is noted. Reaction rate measurements on this slower reaction indicated that it is actually quite rapid, for a hundred-fold decrease in fluorine concentration can be achieved a t a contact time of 4.5 seconds with water; a thousand-fold decrease is effected in 8.9-seconds contact time. This “inhibited reaction” can be converted to the “burning reaction” by adding small amounts of volatile alcohols to the water. FLUORINE
I
BURNER
Figure 3.
FP
If
HF
$\ Figure 2.
Fluorine Ring Burner
Disadvantages of the reaction of fluorincj wit11 water as a means of disposal are mainly: violent explosions ivhich occur as the inhibited or nonburning reaction is suddcnly convcrtcd to the burning reaction, and introduction of water into systems intended t
