Blanketing metals and alloys at elevated temperatures with...

Details Blanketing metals and alloys at elevated temperatures with... Blanketing metals and alloys at elevated temperatures with...

2001-02-09

2003-02-18

King, Roy (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or treating free metal

C420S590000

Reexamination Certificate

active

06521018

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention pertains to the blanketing of metals and alloys with gaseous mixtures, and in particular to a method of blanketing metals and alloys at elevated temperatures using gases having reduced global warming potentials relative to the prior art.
Open top vessels such as crucible and induction furnaces used to melt nonferrous metals are operated so that the surface of metal during melting and the surface of the molten bath are exposed to ambient atmosphere. Air in the atmosphere tends to oxidize the melt, thereby: causing loss of metal, loss of alloying additions and formation of slag that causes difficulty in metal processing; shortening refractory life; and promoting nonmetallic inclusions in final castings, pickup of unwanted gases in the metals, porosity, and poor metal recovery. One solution is to enclose the melt furnace in a vacuum or atmosphere chamber for melting and/or processing of the metals. However, completely enclosed systems are very expensive and limit physical and visual access to the metals being melted.
As alternatives, liquid fluxing salts, synthetic slag, charcoal covers, and similar methods and compounds have been used in the high-volume, cost-sensitive field of metal reprocessing for minimizing metal oxidation, gas pickup, and loss of alloying additions. For example, the prior art teaches that rapid oxidation or fire can be avoided by the use of fluxes that melt or react to form a protective layer on the surface of the molten metal. However, this protective layer of thick slag traps good metal, resulting in a loss of up to 2% of the melt. It also can break up and be incorporated into the melt, creating damaging inclusions. In addition, metal in the slag is leachable and creates a hazardous waste product.
These prior art techniques also necessitate additional handling and processing, and cause disposal problems. These techniques often reduce furnace life or ladle refractory life, increase frequency of shutdowns for relining or patching of refractories, and produce non-metallic inclusions that have to be separated from the metal bath prior to pouring of the metal into a cast shape.
In searching for solutions to the above-described problems, metallurgical industries turned to inert gas atmosphere blanketing. One type of gas blanketing system is based on gravitational dispersion of cryogenically-liquified inert gas over the surface of a hot metal to be blanketed. For example, such cryogenic blanketing systems are disclosed and claimed in U.S. Pat. No. 4,990,183.
U.S. Pat. No. 5,518,221 discloses a method and apparatus for inerting the interior space of a vessel containing hot liquids or solids in induction furnaces, crucible furnaces or ladles during charging, melting, alloying, treating, superheating, and pouring or tapping of metals and metal alloys. The method and apparatus employ a swirl of inert gas to blanket or cover the surface of the metal from the time of charging of the furnace until the furnace is poured or tapped or inerting of the molten metal contained in a furnace or ladle or other vessel. The gas swirl is confined by a unique apparatus mounted on top of the furnace or vessel containing the material to be protected. Any inert gas that is heavier than air can be used to practice the invention. In addition to argon and nitrogen, depending upon the material being blanketed, gases such as carbon dioxide and hydrocarbons may be used.
While some cryogenic blanketing systems are quite effective, use of such systems is limited to metallurgical facilities and vessels that can be supplied by well-insulated cryogenic pipelines or equipped with cryogenic storage tanks in close proximity to the point of use of the liquid cryogen. This is not always practical, and some cryogenic blanketing systems have been plagued by poor efficiency due to premature boil-off of the cryogenic liquid and oversimplified design of dispersing nozzles that wasted the boiled-off gas.
Moreover, cryogenic dispensers often fail to uniformly disperse the cryogenic liquid over the blanketed surface, leading to a transient accumulation or entrapment of the liquid in pockets under the slag or dross, which may result in explosions in a subsequent rapid boil-off.
Other approaches have been taken for different molten metals and alloys in further attempts to solve the above-described problems. For example, U.S. Pat. No. 4,770,697 discloses a process for protecting an aluminum-lithium alloy during melting, casting and fabrication of wrought shapes by enveloping the exposed surfaces with an atmosphere containing an effective amount of a halogen compound (e.g., dichlorodifluoromethane) having at least one fluorine atom and one other halogen atom; the other halogen atom is selected from the group consisting of chlorine, bromine, and iodine, and the ratio of fluorine to the other halogen atom in the halogen compound is less than or equal to one. A passivating and self-healing viscous liquid layer is formed which protects the alloy from lithium loss due to vaporization, oxidation of the alloy, and hydrogen pick-up by the alloy.
Another approach for some molten metals, such as magnesium, is to use inhibitors in the air. The early practice was to burn coke or sulfur to produce a gaseous agent, CO
2
or SO
2
. An atmosphere of CO
2
was found to be superior to the commonly used commercial atmospheres of N
2
, Ar, or He because of the absence of vaporization of the magnesium, the absence of excessive reaction products, and the reduced necessity for the enclosure above the molten metal to be extremely air tight.
However, the use of these inhibitors has several drawbacks. For example, both CO
2
and SO
2
pose environmental and health problems, such as breathing discomfort for personnel, residual sludge disposal, and a corrosive atmosphere detrimental to both plant and equipment. Furthermore, SO
2
is toxic, corrosive, and can cause explosions.
While BF
3
has been mentioned as being a very effective inhibitor, it is not suitable for commercial processes because it is extremely toxic and corrosive. Sulfur hexafluoride (SF
6
) also has been mentioned as one of many fluorine-containing compounds that can be used in air as an oxidation inhibitor for molten metals, such as magnesium. A summary of industry practices for using SF
6
as a protective atmosphere, ideas for reducing consumption and emissions, and comments on safety issues related to reactivity and health are provided in “
Recommended Practices for the Conservation of Sulfur Hexafluoride in Magnesium Melting Operations
,” published by the International Magnesium Association (1998) as a “Technical Committee Report” (hereinafter “IMA Technical Committee Report”).
The use of pure SF
6
was generally discarded because of its severe corrosive attack on ferrous equipment. In addition, the use of pure SF
6
for protecting molten metals such as magnesium has been reported to have caused explosions. Although sulfur hexafluoride (SF
6
) is considered physiologically inert, it is a simple asphyxiant which acts by displacing oxygen from the breathing atmosphere.
Later, it was found that at low concentrations of SF
6
in air (<1%), a protective thin film comprising MgO and MgF
2
is formed on the magnesium melt surface. Advantageously, even at high temperatures in air, SF
6
showed negligible or no reactions.
However, the use of SF
6
and air has some drawbacks. The primary drawback is the release to the atmosphere of material having a high global warming potential (GWP).
It also was found that CO
2
could be used together with SF
6
and/or air. A gas atmosphere of air, SF
6
, and CO
2
has several advantages. First, this atmosphere is non-toxic and non-corrosive. Second, it eliminates the need to use salt fluxes and the need to dispose of the resulting sludge. Third, using such an atmosphere results in lower metal loss, elimination of corrosion effects, and clean castings. Fourth, a casting process using such an atmosphere provides a clean operation and improved working conditions. Fifth, the addition of CO
2
t

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