Method for generating pollution credits while processing reactive metals6780220Abstract This invention relates to a method for generating pollution credits while processing molten magnesium, aluminum, lithium, and alloys of such metals by contacting the molten metal or alloy with a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof. Claims What is claimed is: Description FIELD OF THE INVENTION
TABLE 1
Global Ozone
Atmo- Warming Depletion
spheric Potential- Potential-
Com- Exposure Lifetime.sup.(3) GWP.sup.(3) ODP.sup.(3)
pound Guideline.sup.(1) (yrs) (100 yr ITH) (CFC-11 = 1)
SO.sub.2 2 ppmv*
BF.sub.3 1 ppmv*
NF.sub.3 10 ppmv* 740 10800*
SiF.sub.4 2.5 mg/m.sup.3 as F*
PF.sub.5 2.5 mg/m.sup.3 as F*
SF.sub.6 1000 ppmv 3200 22200*
SO.sub.2 F.sub.2 5 ppmv*
(CClF.sub.2).sub.2 1000 ppmv 300 9800* 0.85*
HF 3 ppmv ceiling*
NH.sub.4 F 2.5 mg/m.sup.3 as F*
NH.sub.4 PF.sub.6 corrosive, causes
burns.sup.(2) *
CF.sub.4 Moderately toxic 50000* 5700*
by inhalation
CHClF.sub.2 1000 ppmv 11.8 1900* 0.055*
.sup.(1) The Condensed Chemical Dictionary, edited by Gessner G. Hawley.
New York, Van Nostrand Reinhold Co. (1981). Note: ppmv = parts per million
by volume.
.sup.(2) Material Safety Data Sheet for ammonium hexafluorophosphate,
Sigma-Aldrich Corporation, Milwaukee, WI.
.sup.(3) World Meterological Organization Global Research and Monitoring
Project-Report No.44, "Scientific Assessment of Ozone Depletion: 1998,"
WMO (1999).
As each of these compounds presents either a significant safety or an environmental concern, the search continues to identify new reactive cover gases for protecting molten magnesium, aluminum, lithium, and alloys of such metals which are simultaneously effective, safe, environmentally acceptable, and cost-effective. SUMMARY OF THE INVENTION This invention relates in one aspect to a method for generating pollution credits while processing molten reactive metals and alloys of such metals, e.g., magnesium, aluminum, lithium, and alloys of one or more of such metals. Reactive metals are metals (and alloys) which are sensitive to destructive, vigorous oxidation in air. In brief summary, the invention provides a method for generating pollution credits comprising: (a) treating molten reactive metal or alloy of such metal to protect said metal or alloy from reacting with oxygen in air by (1) providing molten metal or alloy and (2) exposing said metal or alloy to a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof to yield protected metal or alloy having a protective film thereon; and (b) taking allocation of pollution credits. In one embodiment, this invention employs a method for treating molten reactive metal or alloy to protect it from reacting with oxygen in air. The method comprises providing molten reactive metal or alloy and exposing it to a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof. The gaseous mixture may further comprise a carrier gas. The carrier gas may be selected from the group consisting of air, carbon dioxide, argon, nitrogen and mixtures thereof. One advantage of the present invention over the known art is that the Global Warming Potentials of perfluoroketones and hydrofluoroketones are quite low. Therefore, the present inventive process is more environmentally friendly. By employing the method for treating or protecting molten reactive metals or alloys which is described herein, processors who handle molten reactive metals or alloys will be able to produce unit quantities of such metals and alloys and parts containing such metals and alloys as before while generating much smaller quantities of materials exhibiting significant GWP contribution or other environmentally desirable effect. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION Fluorocarbons used in the present invention include perfluoroketones (PFKs), and hydrofluoroketones (HFKs) which incorporate limited amounts of hydrogen in their structures. These fluorocarbons can be effective as reactive cover gases to protect reactive molten reactive metals such as molten magnesium from ignition. As is the case with known fluorine-containing reactive cover gases, these fluorocarbons can react with the molten metal surface to produce a protective surface film, thus preventing ignition of the molten metal. For convenience, the following description refers to molten magnesium, but it should be understood that the invention is also applicable to other reactive molten metals and alloys, including aluminum, lithium and alloys of one or more of magnesium, aluminum or lithium. For the protection of molten magnesium from ignition, fluorocarbons of the present invention are desirable alternatives to the most commonly used cover gas currently, SF.sub.6. The fluorocarbons of the present invention are low GWP fluorocarbon alternatives to SF.sub.6, i.e., the fluorocarbons of the present invention have measurably lower global warming potential relative to SF.sub.6 (i.e., significantly less than 22,200) and are not significantly worse in atmospheric lifetime, ozone depletion potential, or toxicity properties. Perfluorinated ketones (PFKs) useful in the present invention include ketones which are fully fluorinated, i.e., all of the hydrogen atoms in the carbon backbone have been replaced with fluorine atoms. The carbon backbone can be linear, branched, or cyclic, or combinations thereof, and will preferably have about 5 to about 9 carbon atoms. Representative examples of perfluorinated ketone compounds suitable for use in the processes and compositions of the invention include CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2, (CF.sub.3).sub.2 CFC(O)CF(CF.sub.3).sub.2, CF.sub.3 (CF.sub.2).sub.2 C(O)CF(CF.sub.3).sub.2, CF.sub.3 (CF.sub.2).sub.3 C(O)CF(CF.sub.3).sub.2, CF.sub.3 (CF.sub.2).sub.5 C(O)CF.sub.3, CF.sub.3 CF.sub.2 C(O)CF.sub.2 CF.sub.2 CF.sub.3, CF.sub.3 C(O)CF(CF.sub.3).sub.2, perflurocyclohexanone, and mixtures thereof. In addition to demonstrating reactive cover gas performance, perfluorinated ketones can offer additional important benefits in safety of use and in environmental properties. For example, CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 has low acute toxicity, based on short-term inhalation tests with mice exposed for four hours at a concentration of 100,000 ppm in air. Also based on photolysis studies at 300 nm CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 has an estimated atmospheric lifetime of 5 days. Other perfluorinated ketones show similar absorbances and thus are expected to have similar atmospheric lifetimes. As a result of their rapid degradation in the lower atmosphere, the perfluorinated ketones have short atmospheric lifetimes and would not be expected to contribute significantly to global warming (i.e., low global warming potentials). Perfluorinated ketones which are straight chain or cyclic can be prepared as described in U.S. Pat. No. 5,466,877 (Moore et al.) which in turn can be derived from the fluorinated esters described in U.S. Pat. No. 5,399,718 (Costello et al.). Perfluorinated ketones that are branched can be prepared as described in U.S. Pat. No. 3,185,734 (Fawcett et al.). All of these patents are incorporated by reference in their entirety. Hydrofluoroketones (HFKs) that are useful in the present invention include those ketones having only fluorine and hydrogen atoms attached to the carbon backbone. The carbon backbone can be linear, branched, or cyclic, or combinations thereof, and preferably will have about 4 to about 7 carbon atoms. Representative examples of hydrofluoroketone compounds suitable for use in the processes and compositions of this invention include: HCF.sub.2 CF.sub.2 C(O)CF(CF.sub.3).sub.2, CF.sub.3 C(O)CH.sub.2 C(O)CF.sub.3, C.sub.2 H.sub.5 C(O)CF(CF.sub.3).sub.2, CF.sub.2 CF.sub.2 C(O)CH.sub.3, (CF.sub.3).sub.2 CFC(O)CH.sub.3, CF.sub.3 CF.sub.2 C(O)CHF.sub.2, CF.sub.3 CF.sub.2 C(O)CH.sub.2 F, CF.sub.3 CF.sub.2 C(O)CH.sub.2 CF.sub.3, CF.sub.3 CF.sub.2 C(O)CH.sub.2 CH.sub.3, CF.sub.3 CF.sub.2 C(O)CH.sub.2 CHF.sub.2, CF.sub.3 CF.sub.2 C(O)CH.sub.2 CHF.sub.2, CF.sub.3 CF.sub.2 C(O)CH.sub.2 CH.sub.2 F, CF.sub.3 CF.sub.2 C(O)CHFCH.sub.3, CF.sub.3 CF.sub.2 C(O)CHFCHF.sub.2, CF.sub.3 CF.sub.2 C(O)CHFCH.sub.2 F, CF.sub.3 CF.sub.2 C(O)CF.sub.2 CH.sub.3, CF.sub.3 CF.sub.2 C(O)CF.sub.2 CHF.sub.2, CF.sub.3 CF.sub.2 C(O)CF.sub.2 CH.sub.2 F, (CF.sub.3).sub.2 CFC(O)CHF.sub.2, (CF.sub.3).sub.2 CFC(O)CH.sub.2 F, CF.sub.3 CF(CH.sub.2 F)C(O)CHF.sub.2, CF.sub.3 CF(CH.sub.2 F)C(O)CH.sub.2 F, and CF.sub.3 CF(CH.sub.2 F)C(O)CF.sub.3. Some hydrofluoroketones can be prepared by reacting a fluorinated acid with a Grignard reagent such as an alkylmagnesium bromide in an aprotic solvent, as described in Japanese Patent No. 2,869,432. For example CF.sub.2 CF.sub.2 C(O)CH.sub.3 can be prepared by reacting pentafluoropropionic acid with magnesium methyl bromide in dibutyl ether. Other hydrofluoroketones can be prepared by reacting a partially fluorinated acyl fluoride with hexafluoropropylene in an anhydrous environment in the presence of fluoride ion at elevated temperature, as described in U.S. patent application Ser. No. 09/619306 (herein incorporated by reference). For example, HCF.sub.2 CF.sub.2 C(O)CF(CF.sub.3).sub.2 can be prepared by oxidizing tetrafluoropropanol with acidic dichromate, then reacting the resulting HC.sub.2 H.sub.4 COOH with benzotrichloride to form HC.sub.2 H.sub.4 C(O)Cl, converting the acyl chloride to the acyl fluoride by reaction with anhydrous sodium fluoride, and then reacting the HC.sub.2 H.sub.4 C(O)F with hexafluoropropylene under pressure. The gaseous mixture that comprises a fluorocarbon selected from the group consisting of perfluoroketones and hydrofluoroketones further comprises a carrier gas or carrier gases. Some possible carrier gases include air, CO.sub.2, argon, nitrogen and mixtures thereof. Preferably, the carrier gas that is used with the perfluroketones is dry air. The gaseous mixture comprises a minor amount of the fluorocarbon and a major amount of the carrier gas. Preferably, the gaseous mixture consists of less than about 1% of the fluorocarbon and the balance carrier gas. More preferably, the gaseous mixture contains less than 0.5% by volume (most preferably less that 0.1% by volume) fluorocarbon, selected from the group consisting of perfluoroketones, hydrofluoroketones and mixtures thereof. In order to keep the protective layer on the magnesium, the gaseous mixture is continuously, or nearly continuously, fed to the surface of the magnesium. Small breaks in the thin protective layer can then be healed without the possibility of such small breaks exposing molten magnesium to the air and initiating a fire. A cover gas composition is of low toxicity both as it is applied to the molten magnesium and as it is emitted from the process in which it is used. Cover gases comprising low toxicity hydrofluoroketones and perfluoroketones, and mixtures thereof, will be safe mixtures as applied to magnesium. However, all fluorine containing cover gas composition produce measurable amounts of hydrogen fluoride upon contact with the molten magnesium due to some level of thermal degradation and reaction with magnesium at temperatures of 650 to 800.degree. C. Hydrogen fluoride is corrosive and toxic and its concentration in the emitted gas should be minimized. A preferred cover gas composition will, therefore, produce minimal hydrogen fluoride. See Examples, below. Atmospheric lifetimes and global warming potentials for several fluorocarbons used in accordance with this invention, along with compounds currently known to be useful in the protection of molten magnesium as comparative examples, are presented in TABLE 2.
TABLE 2
Atmospheric Global Warming GWP
Lifetime Potential (GWP) relative
Compound (years).sup.(1) (100 year ITH).sup.(1) to SF.sub.6
Hydrofluoroketone
HCF.sub.2 CF.sub.2 C(O)CF(CF.sub.3).sub.2 .ltoreq.0.1.sup.(4)
.ltoreq.10.sup.(4) 0.0005
Perfluoroketone
C.sub.2 F.sub.5 C(O)CF(CF.sub.3).sub.2 0.02.sup.(2) 1.sup.(2)
0.00005
Comparative Compounds:
Hydrofluorocarbons
FCH.sub.2 CF.sub.3 13.6 1600 0.07
CF.sub.3 CHFCHFCF.sub.2 CF.sub.3 17.1 1700 0.08
CF.sub.3 CHFCF.sub.3 36.5 3800 0.17
HCF.sub.2 CF.sub.3 32.6 3800 0.17
Segregated
Hydrofluoroethers
C.sub.4 F.sub.9 OCH.sub.3 5.0 390 0.02
C.sub.4 F.sub.9 OC.sub.2 H.sub.5 0.8 55 0.002
C.sub.3 F.sub.7 CF(OC.sub.2 H.sub.5)CF(CF.sub.3).sub.2 2.5.sup.(2)
210.sup.(2) 0.01
Non-Segregated
Hydrofluoroethers
HCF.sub.2 OCF.sub.2 CF.sub.2 OCF.sub.2 H 7.sup.(3) 1725.sup.(3)
0.08
HCF.sub.2 OCF.sub.2 OC.sub.2 F.sub.4 OCF.sub.2 H 7.1.sup.(3) 1840.sup.(3)
0.08
Other Fluorochemicals
SF.sub.6 3200 22200 1.00
NF.sub.3 740 10800 0.49
CClF.sub.2 CClF.sub.2 300 9800 0.44
CF.sub.4 50000 5700 0.26
C.sub.2 F.sub.6 10,000 11,400 0.51
.sup.(1) World Meterological Organization Global Research and Monitoring
Project-Report No. 44, "Scientific Assessment of Ozone Depletion: 1998,
Vol. 2," Chapter 10, Table 10-8, pp. 10.27 to 10.28.
.sup.(2) Unpublished data, 3M Company, St. Paul, MN.
.sup.(3) Marchionni, G., et al., Journal of Fluorine Chemistry, 95, (1999),
41-50.
.sup.(4) Estimated, as described below.
The perfluoroketones and hydrofluoroketones used in accordance with the invention have much lower global warming potential (GWP) than the fluorocarbons known in the art such as SF.sub.6, hydrofluorocarbons, and hydrofluoroethers. As used herein, "GWP" is a relative measure of the warming potential of a compound based on the structure of the compound. The GWP of a compound, as defined by the Intergovernmental Panel on Climate Change (IPCC) in 1990 and updated in Scientific Assessment of Ozone Depletion: 1998 (World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project--Report No. 44, Geneva, 1999), is calculated as the warming due to the release of 1 kilogram of a compound relative to the warming due to the release of 1 kilogram of CO.sub.2 over a specified integration time horizon (ITH). ##EQU1## where F is the radiative forcing per unit mass of a compound (the change in the flux of radiation through the atmosphere due to the IR absorbance of that compound), C is the atmospheric concentration of a compound, .tau. is the atmospheric lifetime of a compound, t is time and x is the compound of interest. The commonly accepted ITH is 100 years representing a compromise between short-term effects (20 years) and longer-term effects (500 years or longer). The concentration of an organic compound, x, in the atmosphere is assumed to follow pseudo first order kinetics (i.e., exponential decay). The concentration of CO.sub.2 over that same time interval incorporates a more complex model for the exchange and removal of CO.sub.2 from the atmosphere (the Bern carbon cycle model). Carbonyl compounds such as aldehydes and ketones have been shown to have measurable photolysis rates in the lower atmosphere resulting in very short atmospheric lifetimes. Compounds such as formaldehyde, acetaldehyde, propionaldehyde, isobutyraldehyde, n-butyraldehyde, acetone, 2-butanone, 2-pentanone and 3-pentanone have atmospheric lifetimes by photolysis ranging from 4 hours to 38 days (Martinez, R. D., et al., 1992, Atmospheric Environment, 26, 785-792, and Seinfeld, J. H. and Pandis, S. N., Atmospheric Chemistry and Physics, John Wiley & Sons, New York, p. 288, 1998). CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 has an atmospheric lifetime of approximately 5 days based on photolysis studies at 300 nm. Other perfluoroketones and hydrofluoroketones show similar absorbances near 300 nm and are expected to have similar atmospheric lifetimes. The very short lifetimes of the perfluoroketones and hydrofluoroketones lead to very low GWPs. A measured IR cross-section was used to calculate the radiative forcing value for CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 using the method of Pinnock, et al. (J. Geophys. Res., 100, 23227, 1995). Using this radiative forcing value and the 5-day atmospheric lifetime the GWP (100 year ITH) for CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 is 1. Assuming a maximum atmospheric lifetime of 38 days and infrared absorbance similar to that of CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 the GWP for HCF.sub.2 CF.sub.2 C(O)CF(CF.sub.3).sub.2 is calculated to be 9. The perfluoroketones and hydrofluoroketones of the invention typically have a GWP less than about 10. As a result of their rapid degradation in the lower atmosphere, the perfluoroketones and hydrofluoroketones have short lifetimes and would not be expected to contribute significantly to global warming. The low GWP of the perfluoroketones make them well suited for use as an environmentally preferred cover gas. Also, the PFKs and HFKs of this invention can react more fully with molten magnesium than does SF.sub.6. As a result less unreacted cover gas can be emitted to the atmosphere; less cover gas can be required to produce a comparably performing protective film; or both. Consequently, useful concentrations of the cover gas can be lowered, thus reducing the global warming impact. The full substitution of fluorocarbons of the present invention for SF.sub.6 can be accomplished without increasing the risk to worker safety since these materials (PFKs, and HFKs) are of low toxicity, are non-flammable, and are generally very innocuous materials. Substitution for SF.sub.6 with a PFK, or HFK, alone or as a mixture thereof, can provide protection of molten magnesium in various processes, such as magnesium refining, alloying, formation of ingots or casting of parts. This substitution can be straightforward and can provide the same utility as a reactive cover gas that only SF.sub.6 does currently. Surface films produced with the fluorocarbons of the present invention can be more stable to higher temperatures than those formed with SO.sub.2, enabling work with higher melt temperatures (e.g., additional alloys, more complex casting parts). Improvements realized through the use of fluorocarbons of the present invention as reactive cover gases can include a significant reduction in the emission of a potent greenhouse gas (i.e., SF.sub.6), a potential reduction in the amount of fluorine-containing reactive cover gas required to provide protection, and a reduction in total emissions. This substitution can be done without increasing risks for workers since the fluorocarbons of the present invention are all safe materials with which to work, have low toxicity, are nonflammable, and are not a detriment to production equipment. The use of perfluoroketones, or hydrofluoroketones, or mixtures thereof, in a gaseous mixture demonstrate the ability to also put out fires that are already occurring on the surface of molten magnesium. Therefore, the gases also may be used to extinguish fires on molten magnesium. As discussed above, the use of a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof as a cover gas for handling molten magnesium instead of cover gases such as SF.sub.6 provides an opportunity to reduce the emission of undesirable pollutants while producing similar, even increased amounts of magnesium. Accordingly, one can use the present invention to produce protected magnesium or other reactive metal or alloy and receive allocation of pollution credits. In some applications, a magnesium producer can convert a facility which utilizes cover gas comprising SF.sub.6 to instead utilize a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof as a cover gas. Pollution credits may be allocated according to a function of: (1) how much protected reactive metal or alloy is processed or produced; (2) how much of a reduction in emissions or use of higher GWP cover gas (e.g., SF.sub.6) is achieved; or (3) any other recognized system. As used herein, "allocation" of pollution credits is meant to include any system wherein credits are awarded, assigned, designated, or otherwise credited by any public or private agency for the processing of reactive metals or alloys. EXAMPLES The present invention is further illustrated, but is not meant to be limited by, the following examples. The standard test procedure for evaluating the efficiency of each test fluorocarbon cover gas is given below. An approximately 3 kg sample of pure magnesium was placed in a cylindrical steel crucible having an 11.4 cm internal diameter and was heated to 680.degree. C. Cover gas was continuously applied to the 410 cm.sup.2 surface of the molten magnesium through a 10 cm diameter ring formed of 95 mm diameter stainless steel that was placed about 3 cm over the molten magnesium. The tubing was perforated on the side of the ring facing the molten magnesium so that the cover gas flowed directly over the molten magnesium. A square 20 cm.times.20 cm, 30 cm high stainless steel chamber with an internal volume of about 10.8 liters was fitted over the crucible to contain the cover gas. The top of the chamber was fitted with two 8.9 cm diameter quartz viewing ports and ports for a skimming tool and thermocouple. A cover gas inlet, two gas sampling ports and a door for adding fresh magnesium and for removing dross from the chamber were placed on the sides of the chamber. A stream of the cover gas was pumped from the chamber into the flow cell of an FTIR spectrophotometer (Midac I2000 Gas Phase FTIR) with a mercury cadmium telluride (MCT) detector. Using Modified Extractive FTIR (EPA Method 320), the volumetric concentration of HF and the test cover gas (in ppmV) were measured continuously during experimentation. Once the mixtures had stabilized, concentrations were measured over a period of 5 to 10 minutes, average values of these concentrations were calculated, and those average values were used to make a relative comparison of the test cover gases. In all cases, initial magnesium melting was done using a standard cover gas of 0.5% SF.sub.6 in CO.sub.2 at a flow rate of 5.9 L/min. The experimental gas mixture was then substituted for the standard cover gas mixture by utilizing a train of rotameters and valves. Dry air (having a -40.degree. C. dew point) at a flow rate of 5.9 L/min was used to create the test cover gas by evaporating a flow of test fluid in it such that a volumetric concentration of 0.03 to 1 volume % fluorocarbon in air was produced. During testing, the molten magnesium was observed for a period of about 20 to 30 minutes (equivalent to 10 to 15 chamber volumes exchanges of cover gas) to monitor any visible changes to the surface that would indicate the start of magnesium burning. The existing surface film was then removed by skimming the surface for about 3-5 minutes. The new surface film that formed was then observed for a period of at 15-30 minutes The concentration of the fluorocarbon component of the cover gas mixture was started at about 1% by volume in air and reduced sequentially in steps of 1/2 the previous concentration to a minimum fluorocarbon concentration of 0.03 to 0.06%. Comparative Example C1 C.sub.4 F.sub.9 OCH.sub.3 (methoxy nonafluorobutane), a hydrofluoroether, has been described as an effective fluorocarbon cover gas for molten magnesium in World Published Application WO 00/64614 (Example 5). In this comparative example, C.sub.4 F.sub.9 OCH.sub.3 (available as NOVEC.TM. HFE-7100 Engineering Fluid from 3M Company, St. Paul, Minn.) was evaluated as a fluorocarbon cover gas at 1% and at decreasing volumetric concentrations in air. In all cases, the volumetric flow rate for the cover gas/air mixture was 5.9 L/min. At nominal concentrations of about 1, 0.5, 0.25 and 0.125% (corresponds to 10000, 5000, 2500 and 1250 ppmV, respectively), C.sub.4 F.sub.9 OCH.sub.3 produced a thin flexible surface film on molten magnesium immediately after skimming so that no evidence of metal burning was observed. When the concentration of C.sub.4 F.sub.9 OCH.sub.3 was reduced to 0.0625% (i.e., 625 ppmV), some evidence of burning was observed on the molten magnesium surface as white blooms, but no fire resulted. Exposure to fresh molten magnesium during skimming caused the HF concentration to remain essentially unchanged or to be increased at all volumetric concentrations of C.sub.4 F.sub.9 OCH.sub.3 tested. The HF concentrations measured at the various volumetric concentrations of C.sub.4 F.sub.9 OCH.sub.3 tested are presented in TABLE 3.
TABLE 3
Concentration of Concentration of Concentration of
C.sub.4 F.sub.9 OCH.sub.3 in Air Hydrogen Fluoride over Hydrogen Fluoride
over
Over Molten Stable Surface Molten Fresh Molten
Magnesium Magnesium Film Magnesium Film
(ppm by volume) (ppm by volume) (ppm by volume)
8300 4500 4100
4100 2000 2200
2000 980 1000
800 590 480
The data in TABLE 3 show that significant hydrogen fluoride is produced at 800 ppm volumetric concentration of C.sub.4 F.sub.9 OCH.sub.3 (i.e., 480-590 ppm HF), the minimum concentration required to protect molten magnesium from ignition. Example 1 CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 (1,1,1,2,4,4,5,5,5-nonafluoro-2-trifluoromethyl-pentan-3-one), a perfluoroketone, was evaluated as a cover gas to protect molten magnesium from ignition using essentially the same procedure as described in Comparative Example C1 using C.sub.4 F.sub.9 OCH.sub.3. The CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 was prepared and purified using the following procedures. Into a clean dry 600 mL Parr reactor equipped with stirrer, heater and thermocouple were added 5.6 g (0.10 mol) of anhydrous potassium fluoride and 250 g of anhydrous diglyme (anhydrous diethylene glycol dimethyl ether, available from Sigma Aldrich Chemical Co.). The anhydrous potassium fluoride was spray dried, stored at 125.degree. C. and ground shortly before use. The contents of the reactor were stirred while 21.0 g (0.13 mol) of C.sub.2 F.sub.5 COF (approximately 95.0 percent purity) was added to the sealed reactor. The reactor and its contents were then heated, and when a temperature of 70.degree. C. had been reached, a mixture of 147.3 g (0.98 mol) of CF.sub.2.dbd.CFCF.sub.3 (hexafluoropropylene) and 163.3 g (0.98 mol) of C.sub.2 F.sub.5 COF was added over a 3.0 hour time period. During the addition of the hexafluoropropylene and the C.sub.2 F.sub.5 COF mixture, the pressure was maintained at less than 95 psig (7500 torr). The pressure at the end of the hexafluoropropylene addition was 30 psig (2300 torr) and did not change over the 45-minute hold period. The reactor contents were allowed to cool and were one-plate distilled to obtain 307.1 g containing 90.6% CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 and 0.37% C.sub.6 F.sub.12 (hexafluoropropylene dimer) as determined by gas chromatography. The crude fluorinated ketone was water-washed, distilled, and dried by contacting with silica gel to provide a fractionated fluorinated ketone of 99% purity and containing 0.4% hexafluoropropylene dimers. A sample of fractionated CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 made according to the above-described procedure was purified of hexafluoropropylene dimers using the following procedure. Into a clean dry 600 mL Parr reactor equipped with stirrer, heater and thermocouple were added 61 g of acetic acid, 1.7 g of potassium permanganate, and 301 g of the above-described fractionated 1,1,1,2,4,4,5,5,5-nonafluoro-2-trifluoromethyl-pentan-3-one. The reactor was sealed and heated to 60.degree. C., while stirring, reaching a pressure of 12 psig (1400 torr). After 75 minutes of stirring at 60.degree. C., a liquid sample was taken using a dip tube, the sample was phase split and the lower phase was washed with water. The sample was analyzed using gas-liquid chromatography ("glc") and showed undetectable amounts of hexafluoropropylene dimers and small amounts of hexafluoropropylene trimers. A second sample was taken 60 minutes later and was treated similarly. The glc analysis of the second sample showed no detectable dimers or trimers. The reaction was stopped after 3.5 hours, and the purified ketone was phase split from the acetic acid and the lower phase was washed twice with water. 261 g of CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 was collected, having a purity greater than 99.6% by glc and containing no detectable hexafluoropropylene dimers or trimers. The perfluorinated ketone, CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2, was then evaluated as a fluorocarbon cover gas at 1% and at decreasing volumetric concentrations in air (i.e., at about 1.0, 0.5, 0.25, 0.12, 0.06 and 0.03% by volume; corresponds to 10000, 5000, 2500, 1250, 600 and 300 ppm, respectively). At all concentrations tested, CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 produced a thin flexible surface film on the molten magnesium during skimming and prevented metal ignition. The film visually appeared to be thinner and more elastic than the surface film produced in the initial molten magnesium protection using SF.sub.6 as a cover gas and in Comparative Example C1 using C.sub.4 F.sub.9 OCH.sub.3 as a cover gas. The silvery-gray film produced was stable and did not change appearance over at least 30 minutes. This is in contrast to the series using C.sub.4 F.sub.9 OCH.sub.3, where evidence of metal burning was noted when the cover gas concentration was reduced to about 625 ppm. The HF concentrations measured at the various volumetric concentrations of CF.sub.3 CF.sub.2 C(O)(CF.sub.3).sub.2 tested are, presented in TABLE 4.
TABLE 4
Concentration of Concentration of Concentration of
CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 Hydrogen Fluoride over Hydrogen
Fluoride
in Air over Molten Stable Surface Molten over Fresh Molten
Magnesium Magnesium Film Magnesium Film
(ppm by volume) (ppm by volume) (ppm by volume)
10400 420 670
4800 470 775
2400 360 640
1200 280 370
560 180 120
480 120 100
280 40 40
The data in TABLE 2 show that, at equal volumetric concentrations, significant less hydrogen flouride is produced using CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 compared to C.sub.4 F.sub.9 OCH.sub.3 as a cover gas. For example, at 2000 ppm C.sub.4 F.sub.9 OCH.sub.3, 980 ppm of HF was produced over the stable surface film and 1000 ppm of HF was produced over the fresh molten film. In contrast, at 2400 ppm CF.sub.3 CF.sub.2 C(O)CF(CF.sub.3).sub.2 (a slightly higher fluorocarbon concentration), only 360 ppm of HF was produced over the stable surface film and 640 ppm of HF was produced over the fresh molten film. In summary, the perfluorinated ketone outperformed the hydrofluoroether as a cover gas for molten magnesium (i.e. protected the molten magnesium at lower concentrations) and also generated less hydrogen fluoride as a degradation product upon exposure to the molten metal surface. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope of this invention. Accordingly, it is to be understood that this invention is not to be limited to the illustrative embodiments set forth herein, but is to be controlled by the limitations set forth in the following claims and any equivalents thereof.
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