PROCESS AND LANCE FOR THE PRODUCTION OF A BATH OF MOLTEN METAL OR ALLOYS

Abstract: Liquid argon, nitrogen or carbon dioxide is poured onto the surface of a bath of molten metal in a furnace. According to the invention, liquefied gas is discharged onto the surface in an amount which ranges from about 0.025 to 0.100 lb/cu.in. of metal in the furnace, or 0.01 to 0.05 lb per minute per square inch of exposed metal surface area in the furnace, while a skirt is preferably set around the open end of said furnace. Oxygen concentration above the bath remains lower than about 3.0%, while hydrogen and nitrogen pick up are reduced.

Description: 1 ― FIELD OF THE INVENTION The invention relates to a process for the production of a bath of molten metal or alloys wherein liquid nitrogen, argon or carbon dioxide is discharged above the bath of molten metal or alloys throughout the process and to a related apparatus to discharge said liquid above said bath, more particularly to a lance for discharging the said liquid gas. 2 ― PRIOR ART It is known from British Patent 987,190 to cast continuously a molten metal from a ladle into an ingot mould and to shield the jet of molten metal with a solidified or liquefied inert gas such as liquid nitrogen (when the presence of this element in the metal is not harmful) or argon and to also shield the surface of molten metal in said ladle to avoid oxygen, hydrogen and nitrogen pick-up from the surrounding atmosphere. It is known from D.E.-A-3.109.066 to protect the upper surface of a magnesium or magnesium alloy bath by discharging liquefied argon or nitrogen at a rate such any combustion of magnesium is avoided due to the fact that the oxygen content of the surrounding atmosphere remains below 1%. For example, such a result is obtained for a magnesium bath by discharging liquefied argon at a rate of 0,033 kg/mn and it is clear that such a result with a very low consumption is obtained without disturbing the liquid metal by successively introducing metal pieces during the heating and melting. In electrical furnaces, molten metal comes from the heating up of pieces of metal or of scrap metal which are progressively melted in said furnace, while new pieces of metal or scrap metal are added throughout the melting phase. Almost any open face surface of molten metal can be protected against oxygen, hydrogen and/or nitrogen pick-up by injection of liquid argon, nitrogen (if nitrogen pick-up by injection of liquid argon, nitrogen (if nitrogen pick-up is not a problem) or carbon dioxide snow above the said surface. Said process makes it possible to prevent contamination from atmospheric oxygen and also from humidity generating hydrogen in the melt of from nitrogen in cases whre liquid nitrogen is not used. Furthermore, it is possible with said process to protect the pieces of scrap metal or new stocks of metal in the stage of pre-heating above the liquid bath of molten metal prior to melting. The atmosphere above the metal is selected according to the nature of metals, alloyed metals, alloys or pure metals and it must be maintained above and around the elements of the charge throughout the whole melting and holding operations, from the very moment the charge begins to heat up to the moment the metal is tapped. Contrary to the shielding of the surface of molten metal with argon, nitrogen or carbon dioxide in the gaseous state, where the injection velocity of said gases creates turbulence and hence an ingress of atmospheric air diluting the inert atmosphere, protection of the metal with liquefied gases makes it possible for said liquefied gases to reach the bottom of the furnace or the surface of the molten metal: they first vaporize as cold heavy gases (which are heavier than the atmosphere at room temperature) which in turn, heat-up, expand and flush out all the atmospheric air in the furnace. However, there are some limitations to this protection against hydrogen, nitrogen and/or oxygen pick-up. When the pieces of metal are partly covered by water, this water can come into contact with the molten bath and generate hydrogen bubbles in the bath along with some metal oxides. Hydrogen can also be generated by the flames of the burners, if any are used to heat the molten metal. Oxygen can be generated from deeply oxidized scraps of metal introduced in the bath and nitrogen can be generated namely in arc furnaces in the region of electrodes. As long a liquid argon, nitrogen or carbon dioxide snow is poured onto the surface of the molten bath, air above the surface of said bath is removed, thus removing oxygen and humidity (water). However, the very low level of residual oxygen in the vessel, usually below 1%, at the beginning of the process cannot be maintained as soon as the level of molten metal in the furnace reaches about two-thirds of the height of said furnace. Oxygen concentration rapidly increases to reach about 3% to 5% (volume concentration) at this height, which, though still being considered as a good protection, is not completely satisfactory. When, according to the process disclosed in the priority application Serial No. 077,168 filed on July 24, 1987, liquid nitrogen or liquid argon is poured into the furnace during the production of molten metal, it is necessary for the level of diphasic argon or nitrogen to be as low as possible: the inventors discovered during their experiments that the presence of nitrogen or argon gas in the lance used to deliver the liquid gas generates turbulences in said lance and thus some splashes occurred in the molten metal which could be very dangerous for people present in the vicinity of the furnace. It also destroys the inert atmosphere due to the pulsating flow, which provides non-maintenance of liquid in the furnace or on the meal surface and an ingress of air due to gas velocity. SUMMARY OF THE INVENTION Many attempts have been made to try to solve this problem. A first proposed solution has been to stop filling the furnace with metal as soon as the same reaches about two-thirds of the height of the furnace and to maintain the liquefied gas injection above the molten bath up to the tapping of said molten metal. One can readily appreciate that this solution is not satisfactory because of its poor efficiency. Another proposed solution the inventors had in mind consists of increasing the flow of liquefied gas which is poured onto the surface of molten metal, in order to flush out and at least dilute the oxygen present above the surface of molten metal. However, this proposal gives only a partial solution to said problem. A certain amount of liquefied gas is required to remain on the surface of molten metal throughout melt down and superheat to maintain the inert atmosphere. As soon as the critical liquified gas mass is exceeded (this amount varying with the size, power and, hence, liquid metal meniscus of the particular furnace) the situation can become dangerous. This critical mass of liquefied gas is thus determined experimentally: it must be smaller than the mass where explosions begin to take place. Convection movements are present in the molten metal, particularly in electrical furnaces, where the surface of molten metal forms a converging meniscus: as soon as the liquefied gas reaches the wall of said furnace, it tends to penetrate the molten metal, then creating a lot of minor explosions at the surface of the metal, projecting said molten metal on the walls of the furnace and running a risk for the operator working in the vicinity of said furnace. Of course, a cover is generally provided with the furnace, but it is not used, in practice, by the operators, because it is cumbersome and they further prefer to look at the melt throughout the entire process. After analyzing the situation, the inventors came to the conclusion that the furnace, without a cover, must be considered as an "open-end vaporizer" and not only as a "hot plate". The liquefied gas thus vaporizes not only because of the heat generated by the surface of the molten metal (the "hot plate"), but also due to the heat radiated by the furnace wall or walls and the pieces of metal still above the molten bath. Then they further reached the conclusion that, as the molten metal level rises, the total vaporizing capacity of the furnace decreases, in terms of the heat radiated from the furnace walls, but this is more than compensated for by the increased liquid metal bath temperature. Hence, more vaporization is occurring. This increase in vaporization rate coupled with the reduced furnace height above the bath creates a situation similar to the use of inert gases in their gaseous form, and an ingress of atmospheric air occurs due to the velocity of the rising hot gas "hitting" the colder atmosphere. A slight increase in liquefied gas flow to the critical mass flow rate can be made but experience has shown that this still does not prevent a slight rise in oxygen concentration above the bath. In a process for the production of a bath of molten metal or alloy of metals in a furnace by discharging a liquefied gas selected from the group consisting of nitrogen, argon and carbon dioxyde, the process according to the invention comprises the steps of introducing pieces comprising at least one of said metals in said furnace, said pieces forming a charge, heating said charge, and discharging said liquefied gas above the charge, said discharging step starting at the beginning of the heating up of said charge, up to the tapping of said molten metal or alloy, the amount of said liquefied gas discharged in the furnace being about between 0,69 kg/liter and 2,77 kg/liter of metal in the furnace and preferably said process further comprises setting a sheath of an appropriate material above the upper open end of said furnace in order to surround said open end, the lower end of said sheath being in an about sealing relationship with the top rim of said open end of said furnace, said sheath being set around said open end no later than the time when the level of molten metal in the furnace reaches two-thirds of the depth of the furnace, the height of said sheath being at least equal to one third of said depth. As stated above, the height of that sheath will be substantially about one-third of the depth of the furnace of higher. This is generally the height required to get about 3% by volume, or sometimes less, of oxygen in the atmosphere above the molten metal throughout the process, inasmuch as the flow rate of liquefied gas is maintained about within the limits set forth above. However, the minimal height of this sheath, preferably cylindrical, can be determined as follows: pieces of metal are introduced in the furnace and melted while liquefied gas, as defined above, is continuously poured into the metal and even sometime before introducing the pieces of metal according to a flow rate as set forth below. Oxygen concentration is measured with oxygen probe placed above the surface of the molten metal at intervals throughout the melting step and is generally maintained under about 3% by volume. As soon as 3% is reacted (or 2.9% or 3.1%, depending on the above limit accepted) the remaining height H from the surface of molten metal to the top of said furnace is measured. This height is the minimal height of the sheath to maintain throughout the process the required level of oxygen concentration above the molten metal, under the desired limit, such as 3% by volume. The material of the sheath is generally a metal such as steel. However, in the case of high frequency induction furnaces, it is worthwhile to choose said material among non-inductive materials, such as ceramics, asbestos, or the like. The man skilled in the art will choose this material, its thickness, heat-conductivity, etc., in order to maintain the said sheath as cool as possible. As furnaces or ladles have generally a circular cross section, the sheath will be preferably cylindrical, of the appropriate height disclosed above, with a diameter slightly greater than that of the open end of said furnace or ladle. The weight of the sheath will be generally sufficient to give the desired seal, to avoid air-inlet at the interface between the top rim of the furnace and the sheath. In some cases, it could be worthwhile to improve said seal by the addition of a sealing cushion all around the base edge of the sheath, said cushion being made of an adequate material, such as asbestos, ceramic or the like, well known by the man skilled in the art. As to the amount of liquefied gas discribed above the molten metal, it has been found that this amount depends on the type of metals melted in the furnace. In the case of heavy metals, having a density from about 7,47 to 8, the liquid gas consumption, to maintain the appropriate level of oxygen above the melt, may be within about 0,69 to 1,38 kg/liter of metal in the furnace In the case of light metals, having a density about 2,7, the liquid gas consumption, to maintain the appropriate level of oxygen above the melt, may be within about 0,83 to 1,66 kg/liter of metal in the furnace. According to one embodiment of the invention, the flow rate of liquid inert gas is maintained at about the same value throughout the process, so that the amount of discharged gas is within the range of (0,069 to 2,77) kg × V, V being the total inner volume of the furnace (liter). Advantageously, the amount is maintained within the range of (0,069 to 1,66 kg) × V (liter). Alternatively, the flow rate can be measured with respect to the exposed metal surface area in the furnace. In this case, the amount of discharged liquefied gas is advantageously maintained within the range of 0,7 to 3,5 kg per minute per square centimeter of exposed metal surface area in the furnace. It is also an object of the present invention to provide a lance for preventing splashes in a bath of molten metal, and/or maintaining a continuous flow to ensure an inert atmosphere is retained when liquid nitrogen or argon is poured into a furnace during the production of said molten metal. Another object of the invention is to provide a lance which is self degassing, i.e., where about no gas reaches the tip of the lance where liquid gas is poured. A further object of the invention is to provide a lance for discharging liquid nitrogen or argon above a bath of molten metal or alloy, said lance being provided with self-degassing means to discharge only liquefied gas from the lance onto the surface of the molten metal or alloy. This lance is designed to prevent fluctuation phenomena due to the diphasic state of the fluid within the lance submitted to heat radiated by the furnace or metal containing vessels or the hot molten metal contained therein during the different steps of the process. The lance according to the invention is able to deliver a calm flow of liquid which makes it possible to control the volume of liquid flowing out of the liquefied gas container with a simple pressure gauge. At this point in the feed line, at the very outlet of the tank, the state of the liquefied gas is monophasic (liquid) and can be measured as such. A given installation can be calibrated once for a given liquid gas: the flow rate is function of the pressure of said liquid. According to the invention there is provided a self degassing lance for discharging liquid nitrogen or argon above a bath of molten metal or alloy throughout the production of molten metal or alloy, said lance comprising a first cylindrical body having first and second ends, connector means connected to said first end of said first cylindrical body, and adapted to be connected to a storage vessel containing said liquid argon or nitrogen, diffusor moans connected at said second end of said first cylindrical body adapted to discharge said liquid argon or nitrogen, a second cylindrical body comprising first and second ends, said second cylindrical body coaxially surrounding at least a part of said first cylindrical body, first and second end flanges respectively positionned on each end of said second cylindrical body and defining between said first and second cylindrical bodies a hollow chamber, said first cylindrical body comprising a first hole and said second cylindrical body comprising a second hole close to said first end flange (27), said holes (24-23)(114-115) being- adapted to vent nitrogen or argon gas without substantially disturbing the flow of liquid nitrogen or argon. According to particular embodiements: the diameter of the first hole is smaller than that of the second hole and the area ration between the first and second holes is smaller than 0.5 and preferably about 0,25; said second hole is located near said first end of said first cylindrical body; advantageously, said first cylindrical body comprises a rectilinear portion connected to said connector means and a curved portion (30) (103) connected to said diffusor means (34) (105). said second cylindrical by extends over about all the length of the rectilinear portion of said first cylindrical body; said said second cylindrical body extends over about all the length of the first cylindrical body; said second cylindrical body extends almost to the second end of said first cylindrical body; said curved portion is oriented downward while said holes are located in the upward area of the walls of said first and second cylindrical bodies. BRIEF DESCRIPTION OF THE DRAWINGS Other and further features of the invention will be clearly understood by reference to the following description of various embodiments of the invention chosen for purpose of illustration only, along with the claims and the accompanying drawings, wherein: Fig. 1 is a schematic view, partially in cross-section, of an installation using an induction furnace according to the invention. Fig. 2 is a cross-section view of a lance according to the invention. Fig. 3 is a cross-section view of a preferred embodiment of a lance according to the invention. Fig. 4 is a schematic view of a test installation using the lance. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a schematic view of an induction furnace 1 of cylindrical shape (having an internal diameter D1). In the vertical wall 2 of the furnace 1 (having a bottom wall 13) are embedded helicoidally wound electrical conductors 3, to heat the bath of metal 4 by induction currents wherein some scraps of metal 12 (or new stocks) are not yet molten. The top rim 6 of the lateral wall 2 of the furnace bears a cylindrical sheath 7 made of an appropriate metal or the like. The internal diameter D2 of said sheath is slightly greater than the internal diameter D1 of the furnace 1. An L-shaped lance 8 is provided with a vertical portion 31 approximately arranged along the longitudinal axis of the cylindrical sheath 7 and a horizontal portion 33 connected through the valve 9 and the flexible hose duct 35 to the liquid argon or nitrogen storage vessel 10, said portions being connected together by an elbow portion 30. The lance 8 is used to dispense inert liquid 11 like argon or nitrogen onto the surface 14 of the molten bath. The cylindrical sheath 7 has a height H which is about one third of the depth of the furnace, from the rim 6 to the bottom wall 13. The inventors recognized that when the surface 14 of the molten metal 4 reaches beyond about two-thirds of the total depth of the furnace, oxygen concentration in the atmosphere 5 above the molten bath dramatically increases, whatever the flow rate of inert liquid 11 onto the surface 14. They also recognized that this concentration can be maintained about within the same range than before said molten metal reaches about two-thirds of the depth of the furnace by setting a cylindrical sheath 7 on the rim 6 of the furnace, said sheath surrounding the tip of the lance 8. This sheath must be set no later than when two-thirds of the furnace are filled and preferably as soon as liquid injection begins. When the flow rate of the inert liquid increases along with the introduction of metal in the furnace (this flow rate varies between about 0,7 and 3,5 g per minute per square centimeter of exposed metal surface area in the furnace or an approximate total liquid consumption of between about 0,69 and 2,77 kg/liter, preferably between about 0,69 and 1,66 g/cm³ of metal in the furnace), valve 9 can be equipped, if necessary, with a well known regulation device 15 of the type increasing said flow rate when the level of molten metal in the furnace increases. But it is also easy to have a manual valve with a pressure gauge (not represented on the figure) to control the flow rate of the inert liquid, increasing said flow rate within the above defined range or maintaining it within said range at a value corresponding to a furnace full of metal. The total consumption of liquefied gas from the beginning of the heating up of the metal charge until the tapping of the molten metal or alloy depends on such factors as melt down time and the amount of surface area of molten metal exposed to the atmosphere. Advantageously, the flow rate of said liquefied gas discharged in the furnace is about between 0.69 and 2,77 kg/liter of metal in the furnace, preferably about between 0,69 and 1,66 kg liter of metal in the furnace. Alternatively, the flow rate can be measured with respect to the surface area of molten metal exposed to the atmosphere in the furnace. Advantageously, the flow rate of the liquefied gas discharged in the furnace is about between 0,7 and 3,5 g per minute per square centimeter of molten metal exposed to the atmosphere in the furnace. Figure 2 shows an example of a first embodiment of a lance used to discharge intert liquid onto the surface of molten metal during molten metal production. The lance 8 comprises a first cylindrical body 22 and a second cylindrical body 20, coaxial with the first one and surrounding partially the same on about the whole longitudinal portion 33 of the lance 1. The first cylindrical body 22 es extended by an elbow 30, on its downstream end, which, in turn, is prolonged by an about vertical portion 31 of said lance extending about along the vertical axis of said furnace 1 (figure 1). A first end 28 of said first cylindrical body 22 is adapted ot be connected to the vessel 10 by means of a valve 9 and a flexible hose 35. The second cylindrical body comprises two end flanges, a first one 27 located upstream near the valve 9 and a second one 29 located downstream near the elbow 30. The two cylindrical bodies 20 and 22 along with the two end flanges 27 and 29 define a hollow chamber 21, having a first hole 24 close to the end flange 29, on the top of the said first body 22, and a second hole 23 close to the end flange 27, on the top of said second body 20. Tabs 36 are connected to both cylindrical bodies to maintain their coaxial alignment. A diffuser 34 is connected at the lower end of the vertical portion 31 of said lance. When the inert liquid flows (horizontally in Fig. 2) inside the first cylindrical body 22, inert gas vaporized from said inert liquid 26 can escape through the hole 24, and the escaped gas flows counter-flow to the liquid in the hollow annular space 21 defined between said first and second cylindrical bodies. Said inert gas, which is cold, escapes through the port 23 after flowing around the said second cylindrical body, thus maintaining the cold temperature of the first cylindrical body. Furthermore this cold gas cools the sheath 20 of the lance 8 (second cylindrical body) allowing said lance to withstand the heat generated by the bath of molten metal when it is used according to figure 1. This lance thus prevents any water condensation falling on the molten bath with the risk of generating hydrogen by heat decomposition of the water. The distance between the lower end of the diffuser and the surface of molten metal will be maintained as small as possible, namely beyond two-thirds of metal in the furnace. This distance, smaller than the distance between the top end of the skirt and the level of molten metal, will be preferably maintained between about 25,4 and 101,6 mm. Fig. 3 is a view of the preferred embodiment of the lance according to the invention. It comprises a first cylindrical body 101 having a first, about horizontal, portion 102, a curved portion 103 and then a second, about vertical, portion 104 at the end of which is screwed a diffuser 105, having, for example, holes of 40 micrometers diameter. This first cylindrical body is surrounded by a second cylindrical body 112 having a first about horizontal portion 106, a curved portion 107 and an about vertical portion 108, all portions respectively coaxially surrounding the corresponding portions of said first cylindrical body. In both ends, said second cylindrical body comprises end flanges 109, 110 defining a hollow cylindrical chamber 113 between the inner wall of said second cylindrical body and the outer wall of said first cylindrical body. Spacer means 116 are provided between said first and second cylindrical bodies to maintain them in coaxial alignment, end flanges 109 and 110 also maintaining said coaxial alignment. The first cylindrical body comprises an inner vent hole 114 at the end of said first portion 102, located near the connection between said first portion 102 and said curved portion 103. The second cylindrical body comprises an outer vent hole 115 located near the end flange 109. The area ratio between said inner and said outer vent holes is about 0.5. The end flange 110 is as close as possible to the stainless steel diffuser 105 connected to the first cylindrical body 104 by a female connector 118 and a compression nut 117. A drip washer 1101 having a diameter about 5 to 10 times the diameter of said first cylindrical body 104 is set between the diffuser 105 and the female connector 118 to vaporize water generated by condensation on the lance, when radiating heat from the metal bath is not sufficient to keep the lance above freezing temperature. This circular drip washer 1101 may comprise, if necessary, a rim 1102 along the circumference if the conditions are such that a lot of water is generated and there is a risk that such water falls in the bath of molten metal. The way of using the lance to inert a bath of molten metal will now be explained with reference to Fig. 4. The lance is preferably set about horizontally, the diffuser 132 being a few inches above the molten metal fill level. A pressure relief valve 128 is connected to the output of the liquid argon cylinder 126 just after the flow rate command valve 123 and then to one end of a cryo-hose 129. The opposite end of the hose 129 is connected to the lance 131 having a diffuser 132 at the tip thereof. An oxygen probe 134 controls the oxygen level by means of an oxygen analyzer 133. A gauge 127 is provided in the cryo-hose 129 to indicate the pressure of argon or nitrogen in said hose. The pressure flow control of the liquid argon and thus the flow rate of liquid argon is very reliable. This system does not measure the liquid flow rate at the tip of the lance, but at the liquid outlet of the cylinder just before the flexible hose going to the lance. The lance can be calibrated either for nitrogen or for argon. Flows slightly differ between nitrogen and argon. The flow rate of liquid is a function of the pressure of the liquid in the cylinder, the diameter of the Tee junction between the cylinder 126 and the flexible hose 129 and the opening of the command valve 123. The lance line, having stabilized in temperature, allows monophasic liquid flow. Indications shown by the gauge 127 are remarkably steady, yet the gauge needle can be animated by very short span strokes that are due to the liquid out of measuring assembly tending toward the diphasic state. The lance and its hole system help separate the phases, as does the diffuser which is really a phase separator. If during operations the pressure on the gauge rises and fluctuates, no pressure setting needs to be done but instead the diffuser has to be moved higher up above the metal bath, variations in pressure (up) meaning that the diffuser is too close to the heat source and acts as a vaporizer which builds up a back pressure. During operation of the lance, the gas phase escapes through the hole 24 (Fig. 2) or 114 (Fig. 3) and the hollow chamber 21 or 113 is rapidly filled with cold gas which flushes out air at ambient temperature at the beginning of the operation of the lance, through the hole 23 or 115. The inner sleeve 22 (first cylindrical body) or 102 (horizontal portion) is thus rapidly cooled by the cold gas thus reducing the vaporization of the liquid phase flowing in said inner sleeve. This is why the lance according to the invention makes it possible that less or about no turbulences occur in the liquid flow which is a condition for inerting the bath of molten metal efficiently. EXAMPLES OF THE INVENTION Example 1 The furnace is charged at intervals as the metal melts. The charge for a ferrous alloy is usually made of returns (gates, risers), discarded castings, non-ferrous scrap, ferro-alloys, virgin metal, etc. If the metal melted is non-ferrous, the charge will also be made of returns (gates, risers), discarded castings, non-ferrous scrap, alloying elements, virgin ingots of a known analysis, etc. The "cold-charge" is of course bulky and cannot be introduced in the furnace at once, in its entirety. The furnace thus is loaded with whatever can be put in to fill it and recharged at variable interval as the charge "melts down". This operation goes on until the furnace is full of molten metal. Usually, alloying elements are added last. The metal is introduced by hand, electro-magnet devices, bucket, conveyors, and similar equipment. The liquefied gas is introduced in the furnace a few minutes after starting to charge the same when said charge begins to get hot and thus when enough heat is present to vaporize the liquid gas. There is no need to introduce liquid nitrogen or argon into a cold furnace whre it would accumulate onto the bottom for no practical purpose. Furthermore, an accumulation of cold liquefied gas on the bottom could be detrimental t the lining. On the top rim of an induction furnace having a circular open end of 0,045 m and a depth of 0,60 m was placed a skirt or cylindrical sheath of 0,20 m height and 0,60 m diameter. A flow rate of liquid argon of 1,13 kg at 3 bar was poured into the furnace as soon as the charge bacame hot until the furnace was full, the diffuser being at a distance of about 7,6 cm. Up to half of the furnace depth, the oxygen content above the molten metal was less than 1%, then 1.5% at two-thirds of the depth and 3.0% when the furnace was full. Comparative Example 2 The same measurements were made as in Example 1 under the same conditions and with the same metal bath but without said skirt. When the furnace was one-third full, the oxygen content was about 1.0%, then 1.5% at about half full and then about 3.0% at two-thirds of the depth, and it reached 6.0% when the furnace was full. Example 3 An 0,28 m diameter furnace is charged with 136 kg of Alloy 303 stainless steel to a depth of metal in the furnace of 0,28 m. Liquefied argon is discharged above the charge in the furnace starting at the beginning of the heating up of said charge up to the tapping of the molten charge. During the 72 minute heat, 42, 45 kg of liquid argon are consumed at a flow rate of 0,589 kg per minute. The amount of the liquefied gas discharged in the furnace in terms of the volume of metal in the furnace 2,49 kg/liter and the flow rate in terms of the exposed metal surface area in the furnace is 0,98 g per minute per square centimeter. At this liquefied gas flow rate, the oxygen content above the molten metal is 2%. Example 4 A 0,4 m diameter furnace is charged with 589,7 kg of an alloy containing 85% Cu, 5% Sn, 5% Pb and 5% Zn to a depth of metal in the furnace of 0,50 m. Liquefied nitrogen is discharged above the charge in the furnace starting at the beginning of the heating up of said charge up to the tapping of the molten charge. During the 110 minute heat, 90,7 kg of liquid nitrogen are consumed at a flow rate of 0,825 kg per minute. The amount of the liquefied gas discharged in the furnace in terms of the volume of metal in the furnace is 1,38 kg/liter and the flow rate in terms of the exposed metal surface area in the furnace is 0,63 g per minute per square centimeter. At this liquefied gas flow rate, the oxygen content above the molten metal is 3.5% to 6.0%. Example 5 A 0,127 m diameter furnace is charged with 31,75 kg of Alloy 8620 steel to a depth of metal in the furnace of 0,31 m. Liquefied argon is discharged above the charge in the furnace starting it the beginning of the heating up of said charge up to the tapping of the molten charge. During the 17 minute heat, 6,4 kg of liquid argon are consumed at a flow rate of 0,376 kg/mn. The amount of the liquefied gas discharged in the furnace in terms of the volume of metal in the furnace is 1,6 kg/liter and the flow-rate in terms of the exposed metal surface area in the furnace is 2,94 g per minute per square centimeter. At this liquefied gas flow rate, the oxygen content above the molten metal is 0.8% to 1.8%. Example 6 An 0,20 m diameter furnace is charged with 113,4 kg of Alloy 8620 stainless steel to a depth of metal in the furnace of 0,44 m. Liquefied argon is discharged above the charge in the furnace starting at the beginning of the heating up of said charge up to the tapping of the molten charge. During the 44 minute heat, 19,95 kg of liquid argon are consumed at a flow rate of 0,453 kg per minute. The amount of the liquefied gas discharged in the furnace in terms of the volume of metal in the furnace is 1,38 kg/liter and the flow rate in terms of the exposed metal surface area in the furnace is 1,4 per minute per square centimeter. At this liquefied gas flow rate, the oxygen content above the molten metal is 1.8% or less. Example 7 A 0,40 m diameter furnace is charged with 340,19 kg of Alloy Stellite 6 to a depth of metal in the furnace of 0,762 m. Liquefied argon is discharged above the charge in the furnace starting at the beginning of the heating up of said charge up to the tapping of the molten charge. During the 200 minute heat, 226,8 kg of liquid argon are consumed at a flow rate of 1,134 kg per minute. The amount of the liquefied gas discharged in the furnace in terms of the volume of metal in the furnace is 2,30 kg/liter and in terms of the exposed metal surface area in the furnace is 0,84 kg per minute per square cm. At this liquefied gas flow rate, the oxygen content above the molten metal is 1.7% or less. By using the above disclosed lance and related method, not only oxygen and nitrogen pick-up were reduced (in this latter case, by using an inert gas which is not nitrogen), but also hydrogen pick-up from the atmosphere. According to the invention, continuously pouring or discharging a liquid inert gas onto the surface of the melt, namely at the time alloying elements are added to said melt, drastically reduces hydrogen pick-up, the sample taken showing the metal ready for casting without a degassing step. This was particularly true for aluminum, copper and their respective alloys. Furthermore for aluminum alloys, liquid argon or nitrogen advantageously replaced chloride and fluoride fluxes during melting while providing reduced non metallic inclusions (cleaner metal), increased tensile strength and elasticity, improved flowability, and increased metal temperature without metal losses (about 149°C), and allowed the melt to be held for a prolonged time at temperature with reduced metal losses. For copper and copper alloys, an increased flowability has been noticed, along with less slag and rejections and better surface quality. For a Copper-Beryllium alloy, the increase of beryllium recovery was from 40% to 91%. Zinc alloys protected according to the invention before casting show a more homogenous zinc dispersion while nickel and cobalt alloys show an increased fluidity, a reduced hydrogen pick-up with little or no slag formation and cleaner metal. Steels have shown reduced slag formation, increased fluidity, reduced hydrogen pick-up and increased elongation and yield strengths. In all cases increased fluidity permits either the lowering of the metal tap temperature if no pouring related problems are being experienced (by up to 66°C) or the reduction of mis-runs or other pouring temperature related problems.

Claims: 1. A process for the production of a bath of molten metal or alloy of metals in a furnace, by descharging a liquefied gas selected from the group consisting of nitrogen, argon and carbon dioxide, characterized in that said process comprises the steps of introducing pieces comprising at least one of said metals in said furnace, said pieces forming a charge, heating said charge, and discharging said liquefied gas above the charge, said discharging step starting at the beginning of the heating up of said charge, up to the tapping of said molten metal or alloy, the amount of said liquefied gas discharged in the furnace being about between 0,69 kg/liter and 2,77 kg/liter of metal in the furnace. 2. A process according to claim 1, characterized in that said process comprises setting a sheath of an appropriate material above the upper open end of said furnace in order to surround said open end, the lower end of said sheath being in an about sealing relationship with the top rim of said open end of said furnace, said sheath being set around said open end no later than the time when the level of molten metal in the furnace reaches two-thirds of the depth of the furnace, the height of said sheath being at least equal to one third of said depth. 3. A process according to claim 1 or 2, characterized in that the amount of said liquefied gas discharged in the furnace is about between 0,69 kg/liter and 1,66 kg/liter of metal in the furnace. 4. A process according to claim 1 or 2, characterized in that said molten metal has a density from about 7,47 to 8 and the amount of liquefied inert gas is within the range of 0,69 to 1,38 kg/liter of metal in the furnace. 5. A process according to claim 1 or 2, characterized in that said molten metal has a density of about 2,7 and the amount of inert gas is within the range of 0,83 to 1,66 kg/liter of metal in the furnace. 6. A process according to claim 4 or 5, characterized in that the flow rate is maintained constant throughout the process at a value so that the amount of discharged gas is within the range corresponding to a volume which is the total inner volume of the furnace. 7. A process for the production of a bath of molten metal or alloy of metals in a furnace by discharging a liquefied gas selected from the group consisting of nitrogen, argon and carbon dioxide, characterized in that said process comprises the steps of introducing pieces comprising at least one of said metals in said furnace, said pieces forming a charge, heating said charge, and discharging the liquefied gas above the charge, said discharging step starting at the beginning of the heating up of said charge, up to the tapping of said molten metal or alloy, the flow-rate of said liquefied gas discharged in the furnace being about between 0,7 and 3,5 kg per minute per square centimeter of exposed metal surface area in the furnace. 8. A process for the production of a bath of molten metal or alloy of mitals according to claim 7 characterized in that it comprises setting a sheath of an appropriate material above the upper open end of said furnace in order to surround said open end, the lower end of said sheath being in an about sealing relationship with the top rim of said open end of said furnace, said sheath being set around said open end no later than the time when the level of molten metal in the furnace reaches two-thirds of the depth of the furnace, the height of said sheath being at least equal to one third of sail depth. 9. A lance for discharging liquid nitrogen or argon above a furnace throughout the production of molten metal or alloy, characterized in that it comprises a first cylindrical body (22) (101) having first (28) and second ends, connector means (9-35) connected to said first end (28) of said first cylindrical body (22) (101), and adapted to be connected to a storage vessel (10) containing said liquid argon or nitrogen, diffusor means (34) (105) connected at said second end of said first cylindrical body (22) (101) adapted to discharge sail liquid argon or nitrogen, a second cylindrical body (20) (112) comprising first (27) (109) and second (29) (110) ends, said second cylindrical body (20) coaxially surrounding at least a part of said first cylindrical body (22), first (27) and second (29) end flanges respectively positionned on each end of said second cylindrical body (20) (112) and defining between said first (22) (101) and second (20) (112) cylindrical bodies a hollow chambre (21) (113), said first cylindrical body (22) (101) comprising a first hole (24) (114) and said second cylindrical body (20) (112) comprising a second hole (23) (115) close to said first end flange (27), said holes (24-23) (114-115) being adapted to vent nitrogen or argon gas without substantially disturbing the flow of liquid nitrogen or argon. 10. A lance according to claim 9 characterized in that the diameter of the first hole (114) is smaller than that of the second hole (115). 11. A lance according to claim 9 or 10 characterized in that the area ratio between the first (114) and second (115) holes is smaller than 0,5. 12. A lance according to one of claims 10 to 12 characterized in that the area ratio between the first (114) and second (115) holes is about 0.25. 13. A lance according to one of claim 9 to 12, characterized in that said second hole (23) (115) is located near said first end of said first cylindrical body (22) (101). 14. A lance according to one of claims 9 to 13, characterized in that said first cylindrical body (22) (101) comprises a rectilinear portion (102) connected to said connector means (34) (105) and a curved portion (30) (103) connected to said diffusor means (34) (105). 15. A lance according to one of claims 9 to 14, characterized in that said second cylindrical body (20) extends over about all the length of the rectilinear portion of said first cylindrical body (22). 16. A lance according to one of claims 9 or 14, characterized in that said second cylindrical body (112) extends over about all the length of the first cylindrical body (101). 17. A lance according to one of claims 14 to 16, characterized in that said second cylindrical body (112) extends almost to the second end (110) of said first cylindrical body (101). 18. A lance according to one of claims 14 to 17, characterized in that said curved portion (103) is oriented downward while said holes (114) (115) are located in the upward area of the walls of said first (101) and second (112) cylindrical bodies. 19. A lance according to one of claims 14 to 18, characterized in that it further comprises a washer (1101) between the diffuser (105) and the second end of said first cylindrical body (101). 20. A lance according to claim 19, characterized in that said washer (1101) has a diameter between about 5 to 10 times the diameter of said first cylindrical body (101) at said second end (117). 21. A lance according to one of claims 19 or 20, characterized in that said washer (1101) further comprises a rim (1102) around its circumference.

Inventors: HORNBY ANDERSON, SARA CALIFORNIA PLAZA, LUTGEN, NOEL CALIFORNIA PLAZA
Applicants: LIQUID AIR CORPORATION
Application year: 1988
Main classification: C21C7
IPC codes: F27B14, C22B9, F27D3, F27D23, C21C7