Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-02-09
2003-03-25
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C252S003000, C169S046000, C169S043000, C169S044000, C169S047000, C420S402000
Reexamination Certificate
active
06537346
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for preventing the ignition of molten magnesium by contacting the molten magnesium with a gaseous mixture comprising a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof. This invention also relates to molten magnesium and solid magnesium comprising a film formed on a surface of the magnesium. This film is comprised of a reaction product of molten magnesium and a fluorocarbon selected from the group consisting of perfluoroketones, hydrofluoroketones, and mixtures thereof. The solid magnesium may be in the form of ignots or castings. The invention also relates to extinguishing a fire on the surface of magnesium by contacting the surface with the gaseous mixture described above.
BACKGROUND OF THE INVENTION
Molded parts made of magnesium (or its alloys) 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. (760° 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. In either situation, an extremely hot magnesium fire can result within a few seconds of air exposure, causing extensive property damage and, most tragically, serious injury to and devastating loss of human life.
Various methods have been investigated to minimize the exposure of molten magnesium to air. See J. W. Fruehling et al.,
Transactions of the American Foundry Society, Proceeding of the
73
rd
Annual Meeting, May
5-9, 1969, 77 (1969). The two most viable methods for effectively separating molten magnesium from air are the use of salt fluxes and the use of cover gases (sometimes referred to as “protective atmospheres”). A salt flux is fluid at the magnesium melt temperature and it effectively forms a thin impervious film on the surface of the magnesium, thus preventing the magnesium from reacting with oxygen in the air. However, the use of salt fluxes presents several disadvantages. First, 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. Second, the salt fluxes are typically hygroscopic and, as such, can form salt inclusions in the metal surface which can lead to corrosion. Third, fumes and dust particles from fluxes can cause serious corrosion problems to ferrous metals in the foundry. Fourth, 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.
As a result, there has been a shift from using salt fluxes to using cover gases to inert molten magnesium. 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
). For inert cover gases to be effective, air must be essentially excluded to minimize the possibility of metal ignition, i.e., the system must be essentially closed. To utilize such a closed system, 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). Another limitation of inert cover gases is that they 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.
The use of various reactive cover gases to protect molten magnesium from ignition has been investigated as early as the late 1920s. An atmosphere containing CO
2
is innocuous and economical yet forms a protective film on a magnesium surface which can prevent ignition for over 1 hour at 650° C. However, the CO
2
-based films formed are dull in appearance and unstable, especially in the presence of high levels of air, and consequently offer little protection for the magnesium surface from ambient oxygen. In effect, the CO
2
behaves more like an inert cover gas than a reactive cover gas.
SO
2
has been investigated in the past as a reactive cover gas, as SO
2
reacts with molten magnesium to form a thin, nearly invisible film of magnesium oxysulfides. SO
2
is low in cost and is effective at levels of less than 1% in air in protecting molten magnesium from ignition. However, SO
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 SO
2
is its reactivity with water in humid air to produce very corrosive acids (H
2
SO
4
and H
2
SO
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. SO
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 SO
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 oxyfluoride protective film. U.S. Pat. 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
, SO
2
F
2
, (CClF
2
)
2
, HF, NH
4
F and NH
4
PF
6
. The use of BF
3
, SF
6
, CF
4
and (CClF
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 SO
2
, BF
3
is toxic and corrosive and can be potentially explosive with molten magnesium. NF
3
, SiF
4
, PF
5
, SO
2
F
2
and HF are also toxic and corrosive. NH
4
F and NH
4
PF
6
are solids which sublime upon heating to form toxic and corrosive vapors. CF
4
has a very long atmospheric lifetime. (CClF
2
)
2
, a chlorofluorocarbon, has a very high ozone depletion potential (ODP). The 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 (CCl
3
F). See Seinfeld, J. H. and S. N. Pandis,
Atmospheric Chemistry and Physics: From Air Pollution to Climate Change
, John Wiley & Sons, Inc., New York, (1998). Currently, there are efforts underway to phase out the production of substances that have high ODPs, including chlorofluorocarbons and HCFCs, in accordance with the Montreal Protocol.
UNEP
(
United Nations Environment Programme
), Montreal Protocol on Substances that Deplete the
Milbrath Dean S.
Owens John G.
3M Innovative Properties Company
Jordan Robert H.
King Roy
McGuthry-Banks Tima
LandOfFree
Molten magnesium cover gas using fluorocarbons does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Molten magnesium cover gas using fluorocarbons, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Molten magnesium cover gas using fluorocarbons will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3048587