Process and apparatus for minimizing the potential for explosions in the direct chill casting of aluminum lithium alloys

Abstract

Steam exhaust ports are located around a perimeter of a direct chill casting pit, at various locations from below the top of the pit to the pit bottom to rapidly remove steam from the casting pit with addition of dry excess air. Gas introduction ports are also located around a perimeter of the casting pit and configured to introduce an inert gas into the casting pit interior.

Claims

1. A process in direct chill casting wherein molten metal is introduced into a casting mold and cooled by impingement of a liquid coolant on solidifying metal in a casting pit having top, intermediate and bottom portions and including a movable platen comprising: detecting an occurrence of a bleed-out or a run-out; after detecting the occurrence of a bleed-out or a run-out: exhausting generated gas from the casting pit; and introducing an inert gas into the casting pit to impinge on the solidifying metal, the inert gas having a density less than a density of air, wherein the exhausting of generated gas is done continuously and in parallel with the introducing of the inert gas.

2. The process of claim 1, wherein the inert gas is helium.

3. The process of claim 1, wherein exhausting generated gas from the casting pit comprises exhausting by an array of exhaust ports about at least a periphery of a top portion of the casting pit.

4. The process of claim 3, wherein exhausting generated gas further comprises exhausting by arrays of exhaust ports about the intermediate and bottom portions of the casting pit.

5. The process of claim 1, wherein introducing an inert gas comprises introducing an inert gas through an array of gas introduction ports about a periphery of at least a top portion of the casting pit.

6. The process of claim 1, wherein introducing an inert gas comprises introducing an inert gas through arrays of gas introduction ports about a periphery of a top portion, an intermediate portion and a bottom portion of the casting pit.

7. The process of claim 1, wherein exhausting of generated gas comprises exhausting at a volume flow rate that is enhanced relative to a volume flow rate prior to detecting an occurrence of a bleed-out or a run-out.

8. The process of claim 1, wherein introducing an inert gas in to the pit commences within a maximum of about 15 seconds after detection of a bleed-out.

9. The process of claim 1, wherein exhausting of generated gas comprises exhausting to a location at least 20 meters from the casting mold.

10. The process of claim 1, wherein introducing an inert gas comprises impinging the inert gas upon a metal being cast at a flow rate substantially equal to a volumetric flow rate selected for a liquid coolant prior to detecting a bleed-out or run-out.

11. The process of claim 1, further comprising purifying inert gas via a gas purification system.

12. The process of claim 1, wherein after detecting the bleed-out or run-out, the process further comprising: stopping introduction of a metal into the casting mold; and stopping any flow of the liquid coolant.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified cross sectional side view of a direct chill casting pit in accordance with the present invention.

(2) FIG. 2 is a process flow diagram of a preferred embodiment of process of the present invention.

DETAILED DESCRIPTION

(3) An apparatus and method for casting Al—Li alloys is described. A concern with prior art teachings is that water and the Al—Li molten metal “bleed-out” or “run-out” materials come together and release hydrogen during an exothermic reaction. Even with sloped pit bottoms, minimum water levels, etc., the water and “bleed-out” or “run-out” molten metal may still come into intimate contact, enabling the reaction to occur. Casting without water, using another liquid such as those described in prior art patents affects castability, quality of the cast product, is costly to implement and maintain, as well as poses environmental concerns and fire hazards.

(4) The instantly described apparatus and method improve the safety of DC casting of Al—Li alloys by minimizing or eliminating ingredients that must be present for an explosion to occur. It is understood that water (or water vapor or steam) in the presence of the molten Al—Li alloy will produce hydrogen gas. A representative chemical reaction equation is believed to be:
2LiAl+8H.sub.2O.fwdarw.2LiOH+2Al(OH).sub.3+4H.sub.2(g).

(5) Hydrogen gas has a density significantly less than a density of air. Hydrogen gas that evolves during the chemical reaction, being lighter than air, tends to gravitate upward, toward the top of a cast pit, just below the casting mold and mold support structures at the top of the casting pit. This typically enclosed area allows the hydrogen gas to collect and become concentrated enough to create an explosive atmosphere. Heat, a spark, or other ignition source can trigger the explosion of the hydrogen ‘plume’ of the as-concentrated gas.

(6) It is understood that the molten “bleed-out” or “run-out” material when combined with the ingot cooling water that is used in a DC process (as practiced by those skilled in the art of aluminum ingot casting) will create steam and water vapor. The water vapor and steam are accelerants for the reaction that produces the hydrogen gas. Removal of this steam and water vapor by a steam removal system will remove the ability of the water to combine with Al—LI creating Li—OH, and the expulsion of H.sub.2. The instantly described apparatus and method minimizes the potential for the presence of water and steam vapor in the casting pit by, in one embodiment, placing steam exhaust ports about the inner periphery of the casting pit, and rapidly activating the vents upon the detection of an occurrence of a “bleed-out”.

(7) According to one embodiment, the exhaust ports are located in several areas within the casting pit, e.g., from about 0.3 meters to about 0.5 meters below the casting mold, in an intermediate area from about 1.5 meters to about 2.0 meters from the casting mold, and at the bottom of the cast pit. For reference, and as shown in the accompanying drawings described in greater detail below, a casting mold is typically placed at a top of a casting pit, from floor level to as much as one meter above floor level. The horizontal and vertical areas around the casting mold below the mold table are generally closed-in with a pit skirt and a Lexan glass encasement except for the provision to bring in and ventilate outside air for dilution purpose, such that the gasses contained within the pit are introduced and exhausted according to a prescribed manner.

(8) In another embodiment, an inert gas is introduced into the casting pit interior space to minimize or eliminate the coalition of hydrogen gas into a critical mass. In this case, the inert gas is a gas that has a density less than a density of air and that will tend to occupy the same space just below the top of the casting pit that hydrogen gas would typically inhabit. Helium gas is one such example of suitable inert gas with a density less than a density of air.

(9) The use of argon has been described in numerous technical reports as a cover gas for protecting Al—Li alloys from ambient atmosphere to prevent their reaction with air. Even though argon is completely inert, it has a density greater than a density of air and will not provide the inerting of the casting pit upper interior unless a strong upward draft is maintained. Compared to air as a reference (1.3 grams/liter), argon has density on the order of 1.8 grams/liter and would tend to settle to the bottom of a cast pit, providing no desirable hydrogen displacement protection within the critical top area of the casting pit. Helium, on the other hand, is nonflammable and has a low density of 0.2 grams per liter and will not support combustion. By exchanging air for a lower density of inert gas inside a casting pit, the dangerous atmosphere in the casting pit may be diluted to a level where an explosion cannot be supported. Also, while this exchange is occurring, water vapor and steam are also removed from the casting pit. In one embodiment, during steady state casting and when non-emergency condition pertaining to a ‘bleed-out’ is not being experienced, the water vapor and steam are removed from the inert gas in an external process, while the ‘clean’ inert gas can be re-circulated back through the casting pit.

(10) Referring now to the accompanying drawings, FIG. 1 shows a cross-section of an embodiment of a DC casting system. DC system 5 includes casting pit 16 that is typically formed into the ground. Disposed within casting pit 16 is casting cylinder 15 that may be raised and lowered, for example, with a hydraulic power unit (not shown). Attached to a superior or top portion of casting cylinder 15 is platen 18 that is raised and lowered with casting cylinder 15. Above or superior to platen 18 in this view is stationary casting mold 12. Molten metal (e.g., Al—Li alloy) is introduced into mold 12. Casting mold 12, in one embodiment, includes, coolant inlets to allow coolant (e.g., water) to flow onto a surface of an emerging ingot providing a direct chill and solidification of the metal. Surrounding casting mold 12 is casting table 31. As shown in FIG. 1, in one embodiment, a gasket or seal 29 fabricated from, for example, a high temperature resistant silica material is located between the structure of mold 12 and table 31. Gasket 29 inhibits steam or any other atmosphere from below mold table 31 to reach above the mold table and thereby inhibits the pollution of the air in which casting crewmen operate and breathe.

(11) In the embodiment shown in FIG. 1, system 5 includes molten metal detector 10 positioned just below mold 12 to detect a bleed-out or run-out. Molten metal detector 10 may be, for example, an infrared detector of the type described in U.S. Pat. No. 6,279,645, a “break out detector” as described in U.S. Pat. No. 7,296,613 or any other suitable device that can detect the presence of a “bleed-out”.

(12) In the embodiment shown in FIG. 1, system 5 also includes exhaust system 19. In one embodiment, exhaust system 19 includes, in this embodiment, exhaust ports 20A, 20A′, 20B, 20W, 20C and 20C′ positioned in casting pit 16. The exhaust ports are positioned to maximize the removal of generated gases including ignition sources (e.g., H.sub.2(g)) and reactants (e.g., water vapor or steam) from the inner cavity of the casting pit. In one embodiment, exhaust ports 20A, 20A′ are positioned about 0.3 meters to about 0.5 meters below mold 12; exhaust ports 20B, 20B′ are positioned about 1.5 meters to about 2.0 meters below the mold 12; and exhaust ports 20C, 20C′ are positioned at a base of casting pit 16 where bleed-out metal is caught and contained. The exhaust ports are shown in pairs at each level. It is appreciated that, in an embodiment where there are arrays of exhaust ports at different levels such as in FIG. 1, there may be more than two exhaust ports at each level. For example, in another embodiment, there may be three or four exhaust ports at each level. In another embodiment, there may be less than two (e.g., one at each level). Exhaust system 19 also includes remote exhaust vent 22 that is remote from casting mold 12 (e.g., about 20 to 30 meters away from mold 12) to allow exit of exhausted gases from the system. Exhaust ports 20A, 20A′, 20B, 20W, 20C, 20C′ are connected to exhaust vent 22 through ducting (e.g., galvanized steel or stainless steel ducting). In one embodiment, exhaust system 19 further includes an array of exhaust fans to direct exhaust gases to exhaust vent 22.

(13) FIG. 1 further shows gas introduction system 24 including, in this embodiment, inert gas introduction ports (e.g., inert gas introduction ports 26A, 26A′, 26B, 26B′, 26C and 26C′) disposed around the casting pit and connected to an inert gas source or sources 27. In one embodiment, concurrent to positions of each of ports 26B and 26B′, and 26C and 26C′, there are positioned excess air introduction ports to assure additional in-transit dilution of the evolved hydrogen gas. The positioning of gas introduction ports is selected to provide a flood of inert gas to immediately replace the gases and steam within the pit, via a gas introduction system 24 that introduces inert gas as and when needed (especially upon the detection of the bleed-out) through inert gas introduction ports 26 into casting pit 16 within a predetermined time (e.g., about a maximum of 30 seconds) of the detection of a “bleed-out” condition. FIG. 1 shows gas introduction ports 26A and 26A′ positioned near a top portion of casting pit 16; gas introduction ports 26B and 26B′ positioned at an intermediate portion of casting pit 16; and gas introduction ports 26C and 26C′ positioned at a bottom portion of casting pit 16. Pressure regulators may be associated with each gas introduction port to control the introduction of an inert gas. The gas introduction ports are shown in pairs at each level. It is appreciated that, in an embodiment, where there are arrays of gas introduction ports at each level, there may be more than two gas introduction ports at each level. For example, in another embodiment, there may be three or four gas introduction ports at each level. In another embodiment, there may be less than two (e.g., one) at each level.

(14) As shown in FIG. 1, in one embodiment, the inert gas introduced through gas introduction ports 26A and 26A′ at top 14 of casting pit 16 should impinge on the solidified, semi-solid and liquid aluminum lithium alloy below mold 12, and inert gas flow rates in this area are, in one embodiment, at least substantially equal to a volumetric flow rate of a coolant prior to detecting the presence of a “bleed-out” or a “run-out”. In embodiments where there are gas introduction ports at different levels of a casting pit, flow rates through such gas introduction ports may be the same as a flow rate through the gas introduction ports at top 14 of casting pit 16 or may be different (e.g., less than a flow rate through the gas introduction ports at top 14 of casting pit 16).

(15) The replacement inert gas introduced through the gas introduction ports is removed from casting pit 16 by an upper exhaust system 28 which is kept activated at lower volume on continuous basis but the volume flow rate is enhanced immediately upon detection of a “bleed-out” and directs inert gas removed from the casting pit to the exhaust vent 22. In one embodiment, prior to the detection of bleed-out, the atmosphere in the upper portion of the pit may be continuously circulated through an atmosphere purification system consisting of moisture stripping columns and steam desiccants thus keeping the atmosphere in the upper region of the pit reasonably inert. The removed gas while being circulated is passed through the desiccant and any water vapor is removed to purify the upper pit atmosphere containing inert gas. The purified inert gas may then be re-circulated to inert gas injection system 24 via a suitable pump 32. When this embodiment is employed, inert gas curtains are maintained, between the ports 20A and 26A and similarly between the ports 20A′ and 26A′ to minimize the escape of the precious inert gas of the upper region of the casting pit through the pit ventilation and exhaust system.

(16) The number and exact location of exhaust ports 20A, 20A′, 20B, 20B′, 20C, 20C′ and inert gas introduction ports 26A, 26A′, 26B, 26B′, 26C, 26C′ will be a function of the size and configuration of the particular casting pit being operated and these are calculated by the skilled artisan practicing DC casting in association with those expert at recirculation of air and gases. It is most desirable to provide the three sets (e.g., three pairs) of exhaust ports and inert gas introduction ports as shown FIG. 1. Depending on the nature and the weight of the product being cast, a somewhat less complicated and less expensive but equally effective apparatus can be obtained using a single array of exhaust ports and inert gas introduction ports about the periphery of the top of casting pit 16.

(17) In one embodiment, each of a movement of platen 18/casting cylinder 15, a molten metal supply inlet to mold 12 and a water inlet to the mold are controlled by controller 35. Molten metal detector 10 is also connected to controller 35. Controller 35 contains machine-readable program instructions as a form of non-transitory tangible media. In one embodiment, the program introductions are illustrated in the method of FIG. 2. Referring to FIG. 2 and method 100, first an Al—Li molten metal “bleed-out” or “run-out” is detected by molten metal detector 10 (block 110). In response to a signal from molten metal detector 10 to controller 35 of an Al—Li molten metal “bleed-out” or “run-out”, the machine readable instructions cause movement of platen 18 and molten metal inlet supply (not shown) to stop (blocks 120, 130), coolant flow (not shown) into mold 12 to stop and/or be diverted (block 140), and higher volume exhaust system 19 to be activated simultaneously or within about 15 seconds and in another embodiment, within about 10 seconds, to divert the water vapor containing exhaust gases and/or water vapor away from the casting pit via exhaust ports 20A, 20A′, 20B, 20B′, 20C and 20C′ to exhaust vent 22 (block 150). At the same time or shortly thereafter (e.g., within about 10 seconds to within about 30 seconds), the machine readable instructions further activate gas introduction system and an inert gas having a density less than a density of air, such as helium, is introduced through gas introduction ports 26A, 26A′, 26B, 26B′, 26C and 26C′ (block 160). It is to be noted that those skilled in the art of melting and direct chill casting of aluminum alloys except the melting and casting of aluminum-lithium alloys may be tempted to use nitrogen gas in place of helium because of the general industrial knowledge that nitrogen is also an ‘inert’ gas. However, for the reason of maintaining process safety, it is mentioned herein that nitrogen is really not an inert gas when it comes to interacting with liquid aluminum-lithium alloys. Nitrogen does react with the alloy and produces ammonia which in turns reacts with water and brings in additional reactions of dangerous consequences, and hence its use should be completely avoided. The same holds true for another presumably inert gas carbon di oxide. Its use should be avoided in any application where there is a finite chance of molten aluminum lithium alloy to get in touch with carbon di oxide.

(18) A significant benefit obtained through the use of an inert gas that is lighter than air is that the residual gases will not settle into the casting pit, resulting in an unsafe environment in the pit itself. There have been numerous instances of heavier than air gases residing in confined spaces resulting in death from asphyxiation. It would be expected that the air within the casting pit will be monitored for confined space entry, but no process gas related issues are created.

(19) The process and apparatus described herein provide a unique method to adequately contain Al—Li “bleed-outs” or “run-outs” such that a commercial process can be operated successfully without utilization of extraneous process methods, such as casting using a halogenated liquid like ethylene glycol that render the process not optimal for cast metal quality, a process less stable for casting, and at the same time a process which is uneconomical and flammable. As anyone skilled in the art of ingot casting will understand, it must be stated that in any DC process, “bleed-outs” and “run-outs” will occur. The incidence will generally be very low, but during the normal operation of mechanical equipment, something will occur outside the proper operating range and the process will not perform as expected. The implementation of the described apparatus and process and use of this apparatus will minimize water-to-molten metal hydrogen explosions from “bleed-outs” or “run-outs” while casting Al—Li alloys that result in casualties and property damage.

(20) There has thus been described a commercially useful method and apparatus for minimizing the potential for explosions in the direct chill casting of Al—Li alloys.

(21) As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.