SCAVENGING OF GAS THROUGH A MELT IN A LADLE

Abstract: The disclosure relates to a method for scavenging of gas through a melt in a ladle by a nozzle disposed in the bottom of the ladle. By varying the gas pressure, the nozzle is caused to assume one open and one substantially closed position. At a low gas pressure, the nozzle allows the passage of a gas flow which is sufficient to keep the nozzle free of the melt but is insufficient to influence the melt proper. At a high gas pressure, the nozzle allows the passage of a gas flow of such magnitude that agitation and refining of the melt take place. The nozzle is, in this instance, of a cross-sectional area of the order of magnitude of between 0.2 and 15 mm<2>. The differential pressure at substantially closed nozzle, i.e. the difference between the gas pressure in the nozzle and the ferrostatic pressure from the melt, is greater than 0.05 bar, and the differential pressure at open nozzle lies in the range of between 5 and 250 bar.

Description: SCAVENGING OF GAS THROUGH A MELT IN A LADLE The present invention relates to a method for scavenging gas through a melt in a ladle during the processing stage, the gas being supplied through a nozzle device disposed beneath the surface of the ladle bath, and for keeping the nozzle device free of melt during the charging, transport and casting phases. In the steel-making, metallurgical and ferro-alloy industries, use is made of ladles for the transport and processing of melts. In order to achieve the desired agitation * and refining effects in the melt, an inert gas such as, for example, argon gas, is supplied through the bottom of the ladle. So-called scavenging stones have hitherto been employed in the art for supplying the gas. A scaven¬ ging stone consists of a porous body of ceramic material. The poro¬ sity of the material may be of such a nature as to permit the pas¬ sage of gas while the melt cannot permeate through the pores. However, the employment of scavenging stones is fraught with a number of drawbacks. It may be difficult to cause the scavenging stone to open for the gas flow and the scavenging stone may, in ad¬ dition, become blocked by the adhesion of metal or slag. The steel may also break out through the stone. Furthermore, the permitted gas pressure is limited, since excessively high gas pressure may destroy the scavenging stone. This drawback limits the gas flow which may be possible. Hence, the scavenging stone suffers from a poor degree of flexibility in respect of the employment of different gas flows. With reference to the employment of nozzles in converters, where they have displayed highly favourable properties, a desire has been voiced in this art for employing nozzles for the supply of gas in ladles. Experiments in this area have also been carried out. How¬ ever, as soon as the gas scavenging operation is discontinued, melt has run into the nozzle and blocked it. It has then not proved pos¬ sible to re-open the nozzle. As a result of this problem in opening and closing the nozzle, nozzles have hitherto not been put to ex¬ tensive use in conjunction with ladles. In this context, it might be mentioned that, in converters, there is no problem inherent in the opening and closing phase, since the converter may be tilted side¬ ways and the nozzle opening be brought to lie above the metal bath surface when gas scavenging is not in progress. The object of the present invention is to obviate the above- outlined drawbacks inherent in the employment of scavenging stones and to solve the problem of opening and closing of nozzles in ladles. The present invention is substantially characterised in that the nozzle device is, by the intermediary of a switchable valve ar¬ rangement, connected to a gas supply source of variable gas pressure which causes the nozzle device to assume one open and one substan¬ tially closed position, such that the nozzle, at low gas pressure, allows the passage of a gas flow which is sufficient to keep the nozzle free of the melt but is insufficient to influence the melt proper, and, at high gas pressure, allows the passage of a gas flow of such a magnitude that agitation and refining of the melt take place. The present invention is further characterised in that the noz¬ zle device is of a cross-sectional area in the range of between 0.2 and 15 mm 2 ; that the differential pressure in a substantially closed nozzle, i.e. the difference between the gas pressure in the nozzle and the ferros atic pressure from the melt, is greater than 0.05 bar; and that the differential pressure in an open nozzle lies in the range of between 5 and 250 bar. Preferably, the cross-sectional area lies in the range of between 0.1 and 5.0 mm 2 and the differential pressure for an open nozzle is in the range of between 10 and 50 bar. The present invention will now be described in greater detail below with reference to the accompanying Drawing which shows a schematic diagram of the scavenging system. Referring to the Drawing, a ladle 1 is shown with a melt 2. In the bottom of the ladle 1, there is disposed a nozzle device 3 for scavenging the melt. A three-way valve 4 is disposed at the ladle 1 and is connected to the nozzle device 3 and to quick-coupling devices 5 to which conduits from gas supply equipment 6 and 7, respectively, may be connected. Thus, the nozzle is opened and closed by variable gas pressures. Closure is obtained at low pressure when the nozzle merely allows the passage of a very slight gas flow. However, this flow is of sufficient magnitude to keep the nozzle free of melt but is insufficient to influence the melt proper. When the pressure is raised, the nozzle allows the passage of a desired gas volume for the contemplated process. When the gas scavenging is terminated, the pressure is once again reduced and the nozzle closes. This opening and closing effect in the nozzle is attained by combining a nozzle of slight inner diameter and a gas supply system which allows for both low and high gas pressures. Two examples are given below of different nozzle cross-sectional areas. The differential pressure is, here, taken to mean the difference between the pressure at the nozzle and the ferrostatic pressure from the melt. Nozzle cross-sectional Differential Flow area (mm 2 ) pressure (bar) nl/min 2.0 40.0 600.0 2.0 0.1 1.5 4.5 40.0 1300.0 4.5 0.1 3.0 At the high differential pressure, the nozzle is open. If a lower gas flow is required for the processing, a lower operational pressure is selected. Hence, an optimum gas flow may be set for each ladle size and process involved, by selecting nozzle cross-sectional area and gas pressure. At 0.1 bar, the nozzle gives such a slight flow that it is, to all intents and purposes, closed. As mentioned above, the term differential pressure is taken to mean the difference between the pressure at the nozzle and the fer¬ rostatic pressure from the melt. One basic pre-condition for the scavenging system according to the present invention is that a gas flow be always maintained through the nozzle, when the melt is charged into the ladle, which entails that gas must be supplied during the charging, during the transport of the ladle and during the casting process itself. This feature is made possible by the employment of the above-mentioned three-way valve 4 which is disposed on the ladle 1. Through the valve, gas supply to the nozzle device 3 may be effected from dif¬ ferent gas supply equipment units. For example, during charging, a stationary gas supply network 7 is connected, by the intermediary of the quick coupling devices 5, to the valve 4. On transport of the ladle 1, the conduit from the network 7 is disconnected and, in¬ stead, the nozzle 3 is supplied via the valve 4 from a mobile gas supply unit which accompanies the ladle during its travel. (Depen¬ ding upon the distance the ladle is to travel, it is possible, as an alternative, to employ a moving gas hose which supplies the nozzle with gas during transport.) During the casting phase, the nozzle is then connected, by the intermediary of the three-way valve and an available quick coupling, back to a stationary gas supply network. Thus, reduction of the method as described in the fore- going into practice has realised a method which, in a satisfatory manner, attains the desired agitation and refining effect in the melt in the ladle.

Claims: WHAT WE CLAIM AND DESIRE TO SECURE BY LETTERS PATENT IS: 1. An apparatus for scavenging gas through the melt in a ladle during the processing phase, the gas being supplied through a nozzle device disposed beneath the surface of the ladle bath, and for keeping the nozzle device free of melt during the charging, trans- port and casting phases, characterised in that the nozzle device is, by the intermediary of a switchable valve arrangement, connected to a gas supply source of variable gas pressure which causes the nozzle device to assume one open and one substantially closed position so that the nozzle, at a low gas pressure, allows the passage of a gas flow which is sufficient to keep the nozzle free of the melt but is insufficient to influence the melt, and, at a high gas pressure, al¬ lows the passage of a gas flow of such magnitude that agitation and refining of the melt take place. 2. The apparatus as claimed in Claim 1, characterised in that the nozzle device is of a cross-sectional area of the order of mag¬ nitude of between 0.2 and 15 mm 2 ; that the differential pressure at substantially closed nozzle, i.e. the difference between the gas pressure in the nozzle and the ferrostatic pressure from the melt, is greater than 0.05 bar; and that the differential pressure at open nozzle lies in the range of between 5 and 250 bar. 3. The apparatus as claimed in Claim 2, characterised in that the cross-sectional area of the nozzle device is preferably of the order of magnitude of between 1.0 and 5.0 mm 2 ; and that the dif¬ ferential pressure at open nozzle preferably lies in the range of between 10 and 50 bar. 4. The apparatus as claimed in Claim 1, characterised in that the nozzle device is, by the intermediary of the valve arrangement, connected to a mobile gas supply source during the transport phase.

Inventors: BRUNNER MIKAEL, GRIPENBERG HENRIK, BRUNNER, MIKAEL, GRIPENBERG, HENRIK
Applicants: AGA AB, AGA AKTIEBOLAG
Application year: 1988
Main classification: F27D3
IPC codes: B22D1