Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D1/00—Treatment of fused masses in the ladle or the supply runners before casting
  • B22D1/002—Treatment with gases
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
  • B22D21/002—Castings of light metals
  • B22D21/007—Castings of light metals with low melting point, e.g. Al 659 degrees C, Mg 650 degrees C
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B21/00—Obtaining aluminium
  • C22B21/0084—Obtaining aluminium melting and handling molten aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B21/00—Obtaining aluminium
  • C22B21/06—Obtaining aluminium refining
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B21/00—Obtaining aluminium
  • C22B21/06—Obtaining aluminium refining
  • C22B21/064—Obtaining aluminium refining using inert or reactive gases
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B26/00—Obtaining alkali, alkaline earth metals or magnesium
  • C22B26/20—Obtaining alkaline earth metals or magnesium
  • C22B26/22—Obtaining magnesium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
  • C22B9/006—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals with use of an inert protective material including the use of an inert gas
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
  • C22B9/05—Refining by treating with gases, e.g. gas flushing also refining by means of a material generating gas in situ
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/10—Reduction of greenhouse gas [GHG] emissions
  • Y02P10/122—Reduction of greenhouse gas [GHG] emissions by capturing or storing CO2
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/10—Reduction of greenhouse gas [GHG] emissions
  • Y02P10/146—Perfluorocarbons [PFC]; Hydrofluorocarbons [HFC]; Sulfur hexafluoride [SF6]

Definitions

• This invention relates to a method for preventing the ignition of molten reactive metals and alloys, e.g, aluminum, and lithium and alloys of one or more 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.
• a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof.
• the invention also relates to extinguishing a fire on the surface of molten metals and alloys by contacting the surface with the gaseous mixture described above.
• Molded parts made of magnesium are finding increasing use as components in the automotive and aerospace industries. These parts are typically manufactured in a foundry, where the magnesium is heated to a molten state in a crucible to a temperature as high as 1400 °F (800 °C), and the resulting molten magnesium is poured into molds or dies to form ingots or castings. During this casting process, protection of the magnesium from atmospheric air is essential to prevent a spontaneous exothermic reaction from occurring between the reactive metal and the oxygen in the air. Protection from air is also necessary to minimize the propensity of reactive magnesium vapors to sublime from the molten metal bath to cooler portions of a casting apparatus.
• the flux film itself can oxidize in the atmosphere to harden into a thick oxychloride deposit, which is easily cracked to expose molten magnesium to the atmosphere.
• the salt fluxes are typically hygroscopic and, as such, can form salt inclusions in the metal surface which can lead to corrosion.
• fumes and dust particles from fluxes can cause serious corrosion problems to ferrous metals in the foundry.
• salt sludge can form in the bottom of the crucible. Fifth, and not least, removal of such fluxes from the surface of cast magnesium parts can be difficult.
• Cover gases can be described as one of two types: inert cover gases and reactive cover gases.
• Inert cover gases can be non-reactive (e.g., argon or helium) or slowly reactive (e.g., nitrogen, which reacts slowly with molten magnesium to form Mg 3 N 2 ).
• nitrogen e.g., which reacts slowly with molten magnesium to form Mg 3 N 2 .
• air must be essentially excluded to minimize the possibility of metal ignition, i.e., the system must be essentially closed.
• workers either have to be equipped with a cumbersome self- contained breathing apparatus or they have to be located outside of the dimensions of the processing area (e.g., by using remote control).
• inert cover gases are incapable of preventing molten metal from subliming.
• Reactive cover gases are gases used at low concentration in a carrier gas, normally ambient air, that react with the molten magnesium at its surface to produce a nearly invisible, thermodynamically stable film. By forming such a tight film, the aerial oxygen is effectively separated from the surface of the molten magnesium, thus preventing metal ignition and minimizing metal sublimation.
• S0 2 has been investigated in the past as a reactive cover gas, as S0 2 reacts with molten magnesium to form a thin, nearly invisible film of magnesium oxysulfides.
• S0 2 is low in cost and is effective at levels of less than 1% in air in protecting molten magnesium from ignition.
• S0 2 is very toxic and consequently requires significant measures to protect workers from exposure (permissible exposure levels are only 2 ppm by volume or 5 mg/m 3 by volume).
• Another problem with S0 2 is its reactivity with water in humid air to produce very corrosive acids (H 2 S0 and H 2 S0 3 ). These acids can attack unprotected workers and casting equipment, and they also contribute significantly to acid rain pollution when vented out of the foundry.
• S0 2 also has a tendency to form reactive deposits with magnesium which produce metal eruptions from the furnace (especially when SO 2 concentrations in the air are allowed to drift too high). Though S0 2 has been used commercially on a large scale for the casting of magnesium alloys, these drawbacks have led some manufacturers to ban its use.
• Fluorine-containing reactive cover gases provide an inert atmosphere which is normally very stable to chemical and thermal breakdown. However, such normally stable gases will decompose upon contact with a molten magnesium surface to form a thin, thermodynamically stable magnesium oxide/fluoride protective film.
• U.S. Patent No. 1,972,317 (Reimers et. al.) describes the use of fluorine-containing compounds which boil, sublime or decompose at temperatures below about 750 °C to produce a fluorine- containing atmosphere which inhibits the oxidation of molten magnesium.
• Suitable compounds listed include gases, liquids or solids such as BF 3 , NF 3 , SiF 4 , PF 5 , SF 6 , S0 2 F 2 , (CC1F 2 ) 2 , HF, NH 4 F andNH 4 PF 6 .
• gases, liquids or solids such as BF 3 , NF 3 , SiF 4 , PF 5 , SF 6 , S0 2 F 2 , (CC1F 2 ) 2 , HF, NH 4 F andNH 4 PF 6 .
• BF 3 , SF 6 , CF 4 and (CC1F 2 ) 2 as fluorine- containing reactive cover gases is disclosed in J. W. Fruehling et al., described supra.
• Each of these fluorine-containing compounds has one or more deficiencies. Though used commercially and effectively at lower levels than S0 2 , BF 3 is toxic and corrosive and can be potentially explosive with molten magnesium.
• NF 3 , SiF 4 , PF 5 , S0 2 F 2 and HF are also toxic and corrosive.
• NH 4 F and NH PF 6 are solids which sublime upon heating to form toxic and corrosive vapors.
• CF has a very long atmospheric lifetime.
• (CC1F 2 ) 2 a chlorofluorocarbon, has a very high ozone depletion potential (ODP).
• ODP of a compound is usually defined as the total steady-state ozone destruction, vertically integrated over the stratosphere, resulting from the unit mass emission of that compound relative to that for a unit mass emission of CFC-11 (CC1 3 F). See Seinfeld, J. H. and S. N.
• SF 6 was considered the optimum reactive cover gas for magnesium.
• SF 6 is effective yet safe (essentially inert, odorless, low in toxicity, nonflammable and not corrosive to equipment). It can be used effectively at low concentrations either in air ( ⁇ 1%) or in C0 2 to form a very thin film of magnesium oxyfluorides and oxysulfides on the surface of molten magnesium.
• This magnesium oxide/fluoride/sulfide/sulfur oxide film is far superior at protecting the magnesium from a vigorous exothermic oxidation reaction than is the magnesium oxide film inherently present on the metal surface.
• the magnesium oxide/fluoride/sulfide/sulfur oxide film is sufficiently thin (i.e., nearly invisible to the naked eye) that the metal surface appears to be metallic. This superior protection is believed to result from the greater thermodynamic stability of a nonporous magnesium sulfide/sulfur oxide and/or magnesium oxide/fluoride film as compared to the stability of a thick porous film of either magnesium oxide, sulfide or fluoride alone.
• a reactive cover gas only a small portion of the gas passed over the molten magnesium is actually consumed to form that film, with the remaining gas being exhausted to the atmosphere.
• This invention relates to a method for treating molten reactive metals and alloys of such metals, e.g., aluminum, lithium, and alloys of one or more of such metals, to protect them from reacting with oxygen in air with the proviso the reactive metal contains less than 50 percent by weight of, or in some embodiments is substantially free of, magnesium.
• the method comprises providing molten reactive metal 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.
• This invention also relates to a method for protecting molten reactive metal from reacting with oxygen in air.
• the exposed surface of molten reactive metal is exposed to or contacted with a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof.
• the gaseous mixture can react with the molten reactive metal at its exposed surface to produce a nearly invisible, thermodynamically stable film.
• the oxygen in air can be effectively separated from the surface of the molten reactive metal and thus can prevent metal ignition and can minimize metal sublimation.
• this invention relates to molten reactive metal wherein a protective film is formed on a surface of the reactive metal.
• This film is formed by a reaction of the molten reactive metal with a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones and mixtures thereof.
• This film can be nearly invisible and thermodynamically stable and can effectively separate oxygen in air from the surface of the reactive metal and thus can prevent metal ignition and can minimize metal sublimation.
• this invention relates to solid reactive metal comprising a film formed on a surface of the reactive metal.
• This film is comprised of a reaction product of molten reactive metal and a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof.
• This solid reactive metal may be in the form of ignots or castings.
• this invention relates to a method for extinguishing a fire on the surface of molten reactive metal comprising contacting a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof, with said surface of said molten metal.
• reactive metal means a metal or alloy which is sensitive to destructive, vigorous oxidation when exposed to air, such as aluminum, or lithium or any alloy comprising at least one of such metals, with the proviso the reactive metal contains less than 50 percent by weight of, or in some embodiments is substantially free of, magnesium.
• the following description of illustrative embodiments of the invention shall refer to aluminum. It should be understood that the present invention can also be used with lithium, or alloys of one or more of the metals.
• 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.
• Fluorocarbons of 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 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.
• PFKs perfluoroketones
• HFKs hydrofluoroketones
• fluorocarbons of the present invention are desirable alternatives to the most commonly used cover gas currently, SF 6 .
• the fluorocarbons of the present invention are low GWP fluorocarbon alternatives to SF 6) i.e., the fluorocarbons of the present invention have measurably lower global warmthing potential relative to SF 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. PFKs containing fewer carbon atoms in the carbon backbone may exhibit higher toxicity.
• Representative examples of perfluorinated ketone compounds suitable for use in the processes and compositions of the invention include CF 3 CF 2 C(0)CF(CF ) 2 , (CF 3 ) 2 CFC(0)CF(CF 3 )2, CF 3 (CF 2 )2C(0)CF(CF 3 ) 2 , CF 3 (CF2) 3 C(0)CF(CF 3 )2, CF 3 (CF 2 ) 5 C(0)CF 3 , CF 3 CF 2 C(0)CF 2 CF 2 CF 3s CF 3 C(0)CF(CF 3)2 , perfluorocyclohexanone, and mixtures thereof.
• perfluorinated ketones can offer additional important benefits in safety of use and in environmental properties.
• CF 3 CF 2 C(0)CF(CF 3 ) 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 3 CF 2 C(0)CF(CF 3 ) 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. Patent No. 5,466,877 (Moore et al.) which in turn can be derived from the fluorinated esters described in U.S. Patent No. 5,399,718 (Costello et al.).
• Perfluorinated ketones that are branched can be prepared as described in U.S. Patent No. 3,185,734 (Fawcett et al.).
• Hydrofluoroketones 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. HFKs having fewer carbon atoms in the carbon backbone may exhibit higher toxicity.
• hydrofluoroketone compounds suitable for use in the processes and compositions of this invention include: HCF 2 CF 2 C(0)CF(CF 3 )2 5 CF 3 C(0)CH 2 C(0)CF 3 , C 2 H 5 C(0)CF(CF3) 2 , CF 2 CF 2 C(0)CH 3 , (CF 3 ) 2 CFC(0)CH 3 , CF 3 CF 2 C(0)CHF 2 , CF 3 CF 2 C(0)CH 2 F, CF 3 CF 2 C(0)CH 2 CF 3 , CF 3 CF 2 C(0)CH 2 CH 3 , CF 3 CF 2 C(0)CH 2 CHF 2 , CF 3 CF 2 C(0)CH 2 CHF 2 , CF 3 CF 2 C(0)CH 2 CH 2 F, CF 3 CF 2 C(0)CHFCH 3 , CF 3 CF 2 C(0)CHFCHF 2 , CF 3 CF 2 C(0)CHFCH 2 F, CF 3 CF 2 C(0)CF 2 CH 3 , CF 3 CF 2 C(0)CF 2 C(0)
• 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.
• a Grignard reagent such as an alkylmagnesium bromide in an aprotic solvent
• CF 2 CF 2 C(0)CH 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.
• HCF 2 CF 2 C(0)CF(CF 3 ) 2 can be prepared by oxidizing tetrafluoropropanol with acidic dichromate, then reacting the resulting HC 2 H COOH with benzotrichloride to form HC 2 H C(0)C1, converting the acyl chloride to the acyl fluoride by reaction with anhydrous sodium fluoride, and then reacting the HC 2 H C(0)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, C0 2 , argon, nitrogen and mixtures thereof.
• 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.
• 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.
• the optimum ratio of a fluorocarbon to carrier gas will depend, in part, upon the carrier gas, operating conditions, molten metal or alloy, and equipment used. In order to keep the protective layer on the aluminum, the gaseous mixture is continuously, or nearly continuously, fed to the surface of the aluminum.
• a cover gas composition is of low toxicity both as it is applied to the molten aluminum 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 aluminum. However, all fluorine containing cover gas composition produce measurable amounts of hydrogen fluoride upon contact with the molten aluminum due to some level of thermal degradation and reaction with aluminum at temperatures of 650 to 800°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.
• GWP global warming potential
• fluorocarbons known in the art such as SF 6 , hydrofluorocarbons, and hydrofluoroethers.
• GWP global warming potential
• ITH integration time horizon
• F is the radiative forcing per unit mass of a compound (the change in the flux of radiation through the atmosphere due to the JJR absorbance of that compound)
• C is the atmospheric concentration of a compound
• ⁇ is the atmospheric lifetime of a compound
• t is time
• 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).
• concentration of an organic compound, x, in the atmosphere is assumed to follow pseudo first order kinetics (i.e., exponential decay).
• concentration of C0 2 over that same time interval incorporates a more complex model for the exchange and removal of C0 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.
• CF 3 CF 2 C(0)CF(CF 3 ) 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 3 CF 2 C(0)CF(CF 3 ) 2 using the method of Pinnock, et al. (J. Geophys. Res., 100, 23227, 1995).
• the GWP (100 year ITH) for CF 3 CF 2 C(0)CF(CF 3 ) 2 is 1. Assuming a maximum atmospheric lifetime of 38 days and infrared absorbance similar to that of CF 3 CF 2 C(0)CF(CF 3 ) 2 the GWP for HCF 2 CF 2 C(0)CF(CF 3 ) 2 is calculated to be 9.
• the perfluoroketones and hydrofluoroketones of the invention typically have a GWP less than about 10.
• 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.
• the PFKs and HFKs of this invention can react more fully with molten aluminum than does SF 6 .
• 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 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 6 with a PFK, or HFK, alone or as a mixture thereof, can provide protection of molten magnesium in various processes, such as aluminum 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 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 2 , enabling work with higher melt temperatures (e.g., additional alloys, more complex casting parts).
• 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 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.
• SF 6 potent greenhouse gas
• 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 aluminum. Therefore, the gases also may be used to extinguish fires on molten aluminum.
• the reactive metal will be aluminum, lithium, or an alloy containing one or both of these metals which contains less than 50 percent by weight of, or in some cases is substantially free of, magnesium.
• 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°C. Cover gas was continuously applied to the 410 cm 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 x 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 12000 Gas Phase FTIR) with a mercury cadmium telluride (MCT) detector.
• FTIR spectrophotometer Modac 12000 Gas Phase FTIR
• MCT mercury cadmium telluride
• 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 l A the previous concentration to a minimum fluorocarbon concentration of 0.03 to 0.06%.
• Comparative Example CI C F 9 0CH 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).
• C 4 F 9 OCH 3 available as NOVECTM HFE-7100 Engineering Fluid from 3M Company, St. Paul, MN 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.
• C F OCH 3 produced a thin flexible surface film on molten magnesium immediately after skimming so that no evidence of metal burning was observed.
• concentration of C 4 F 9 OCH 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 4 F OCH 3 tested.
• CF 3 CF 2 C(0)CF(CF 3 ) 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 CI using C 4 F OCH 3 .
• the CF 3 CF 2 C(0)CF(CF 3 ) 2 was prepared and purified using the following procedures.
• the reactor contents were allowed to cool and were one-plate distilled to obtain 307.1 g containing 90.6% CF 3 CF 2 C(0)CF(CF 3 ) 2 and 0.37% C 6 F ⁇ 2 (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 3 CF 2 C(0)CF(CF 3 ) 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 l,l,l,2,4,4,5,5,5-nonafiuoro-2-trifluoromethyl-pentan- 3-one.
• the reactor was sealed and, heated to 60°C, while stirring, reaching a pressure of 12 psig (1400 torr).
• 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.
• glc gas-liquid chromatography
• 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.
• CF 3 CF 2 C(0)CF(CF 3 ) 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 6 as a cover gas and in Comparative Example CI using C 4 F OCH 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 test series using F 9 OCH 3 , where evidence of metal burning was noted when the cover gas concentration was reduced to about 625 ppm.
• 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,
• 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

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