Abstract:
To degas a molten metal, a receptacle containing the molten metal and a layer of slag over the molten metal is positioned in a chamber, and the chamber is evacuated. As the pressure in the chamber reduces, gas is generated at the interface between the molten metal and the slag, which causes the slag to foam. To inhibit overflowing of slag from the receptacle, a gauge outputs a signal indicative of the level of the surface of the slag, and the rate of evacuation of the chamber is reduced to reduce the rate of gas generation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of application Ser. No. 11/793,749 filed Jun. 20, 2007, now U.S. Pat. No. 7,815,845. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus for and a method of degassing molten metal, in particular molten steel. 
     Purification of molten metal, especially molten steel, by subjecting the molten metal to a vacuum has been known for some time. In such a process, the molten metal is poured into an open receptacle, or “ladle”, and covered with a layer of fused (liquid) mineral slag, which both insulates and isolates the molten metal, and is chemically formulated to aid the purification process. The ladle is positioned within a degassing chamber connected to a vacuum pumping arrangement for evacuating the chamber. The pumping arrangement typically comprises one or more primary pumps for exhausting gas drawn from the chamber to atmosphere, and one or more secondary mechanical vacuum booster pumps connected between the primary vacuum pumps and the degassing chamber. The pumping arrangement is operated to subject the chamber to a steadily decreasing pressure (increasing vacuum), which causes gaseous and metallic impurities to leave the liquid phase and be evacuated from the atmosphere above the melt. 
     However, as the pressure reduces a point may be reached at which vigorous chemical reactions occur at the interface between the molten metal and the molten slag, causing a rapid generation of gas that quickly inflates the slag layer by foaming. If uncontrolled, the foaming slag can rise up and overflow from the lip of the ladle, resulting in major loss of slag and potential disruption to the purification process. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides apparatus for degassing a molten metal, the apparatus comprising a chamber for receiving a receptacle containing molten metal and a layer of slag over the molten metal, a vacuum pumping arrangement for evacuating the chamber, a gauge for outputting a signal indicative of the level of a surface of the slag, and control means for using the signal to control the rate of evacuation of the chamber to inhibit overflowing of slag from the receptacle. 
     The apparatus can thus enable any sudden increase in the level of the slag surface to be detected and combated by a corresponding automatic prompt reduction in the rate of evacuation of the chamber, reducing the rate at which gas is generated at the interface between the molten metal and the slag and hence the degree of foaming. Once the level of the slag surface has receded, the evacuation rate of the chamber can be increased again. 
     Any one of a number of different techniques may be used to provide an indication of the level of the slag surface within the receptacle. Examples include lowering a probe into the receptacle, and using a variation in an electrical property of the probe, such as inductance or resistance, to determine the level of the slag surface. A gas sensor may be used instead of a probe. Another alternative is to use a video camera to produce an image of the inside of the receptacle, and to use variations in the image as an indication of the level of the slag surface within the receptacle. In the preferred embodiment, the gauge comprises a radar transceiver for outputting a radar beam towards the slag and receiving an echo of the radar beam from the slag surface. The gauge is preferably positioned a fixed distance above the receptacle such that the period between output of the radar beam and the reception of the echo is indicative of the distance between the gauge and the slag surface, and thus the distance of the slag surface from the top of the receptacle. The signal output from the gauge may be indicative of the length of that period, with the control means being configured to control the rate of evacuation of the chamber in response thereto. 
     Whilst the evacuation rate of the chamber may be controlled in response to the current level of the slag surface, both the current level of the slag surface and the current rate of change of the level of the slag surface may be used to control the evacuation rate. The control means may be configured to determine the rate of change of the level of the slag surface from data contained within a plurality of signals received from the gauge over a predetermined period of time. 
     The control means is preferably configured to adjust the speed of rotation of at least one pump of the vacuum pumping arrangement to control the rate of evacuation of the chamber. The control means preferably comprises a pump controller for controlling the power supplied to a variable speed motor of the pump, and thus the speed of rotation of the pump. The pump controller is preferably configured to change the frequency of the power supply to the motor to adjust pump speed, for example by transmitting an instruction to an inverter to change the frequency of the power supplied thereby to the motor. However, the controller may be configured to adjust another parameter of the power supply, such as the size (or amplitude) of the voltage or current of the power supply to the motor. 
     In the event that a reduction in the frequency of the power supplied to the motor, or a reduction in another parameter of the power supply, does not cause the level of the slag surface to recede, the frequency of the power supplied to the motor, or said another parameter, may be reduced to zero so that the pump is effectively switched off, thereby significantly reducing the rate of evacuation of the chamber. Therefore, the control means may be configured to turn off at least one pump of the vacuum pumping arrangement in dependence on said signal. Therefore, in a second aspect the present invention provides apparatus for degassing a molten metal, the apparatus comprising a chamber for receiving a receptacle containing molten metal and a layer of slag over the molten metal, a vacuum pumping arrangement for evacuating the chamber, a gauge for outputting a signal indicative of the level of a surface of the slag, and control means for switching off at least one pump of the vacuum pumping arrangement in dependence on the signal to inhibit overflowing of slag from the receptacle. 
     In one arrangement, the pump controller receives directly the signals output from the gauge, and uses the signals to control the power supplied to the motor. In another arrangement, a system controller receives the signals output from the gauge, uses the signals to determine a target speed for the pump, and advises the pump controller of the target speed, for example, by advising the pump controller of the frequency of the power to be supplied to the motor. The functionality for determining the target speed can thus be provided by software stored on a single system controller, with the pump controller being responsive to the target speed received from the system controller to set its pump&#39;s speed. 
     In a third aspect, the present invention provides a method of degassing a molten metal, the method comprising the steps of positioning a receptacle containing the molten metal and a layer of slag over the molten metal within a chamber, evacuating the chamber, receiving from a gauge a signal indicative of the level of a surface of the slag, and using the signal to control the rate of evacuation of the chamber to inhibit overflowing of slag from the receptacle. 
     In a fourth aspect, the present invention provides a method of degassing a molten metal, the method comprising the steps of positioning a receptacle containing the molten metal and a layer of slag over the molten metal within a chamber, evacuating the chamber, receiving from a gauge a signal indicative of the level of a surface of the slag, and switching off at least one pump used to evacuate the chamber in dependence on the signal to inhibit overflowing of slag from the receptacle. 
     Features described above in relation to first aspect of the invention are equally applicable to the second to fourth aspects, and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the present invention will now be described with reference to the accompanying drawing, in which 
         FIG. 1  illustrates a first embodiment of a steel degassing apparatus; 
         FIG. 2  illustrates an example of a vacuum pumping arrangement for evacuating the degassing chamber of the degassing apparatus of  FIG. 1 ; 
         FIG. 3  illustrates a pump controller for driving a motor of a booster pump of the pumping arrangement of  FIG. 2 ; 
         FIG. 4  illustrates the connection of the pump controllers of the booster pumps of  FIG. 2  to the system controller; and 
         FIG. 5  illustrates a second embodiment of a steel degassing apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , an apparatus for degassing a molten metal, for example, molten steel, comprises a degassing chamber  10  for receiving a receptacle, or “ladle”  12 , containing molten metal  14  and a layer of slag  16  overlying the molten metal  14 . The chamber  10  is closed by a lid  18 , on which is mounted a gauge  20  for monitoring the level of the upper surface  22  of the slag  16  within the ladle  12 . In the illustrated example, the gauge  20  is in the form of a radar transceiver. The gauge  20  is connected to a controller  24  for controlling a vacuum pumping arrangement  26  connected to an outlet  28  of the chamber  10 . 
     With reference now to  FIG. 2 , an example of the vacuum pumping arrangement  26  comprises a plurality of similar booster pumps  30  connected in parallel, and a backing pump  32 . Each booster pump  30  has an inlet connected to a respective outlet  34  from an inlet manifold  36 , and an outlet connected to a respective inlet  38  of an exhaust manifold  40 . The inlet  42  of the inlet manifold  36  is connected to the outlet  28  from the chamber  10 , and the outlet  44  of the exhaust manifold  40  is connected to an inlet of the backing pump  32 . Whilst in the illustrated pumping system there are three booster pumps connected in parallel, any number of booster pumps may be provided depending on the pumping requirements of the enclosure. Similarly, where a relatively high number of booster pumps are provided, two or more backing pumps may be provided in parallel. An additional row or rows of booster pumps similarly connected in parallel may be provided as required between the first row of booster pumps and the backing pumps. 
     With reference to  FIG. 3 , each booster pump  30  comprises a pumping mechanism  46  driven by a variable speed motor  48 . Booster pumps typically include an essentially dry (or oil free) pumping mechanism  46 , but generally also include some components, such as bearings and transmission gears, for driving the pumping mechanism  46  that require lubrication in order to be effective. Examples of dry pumps include Roots, Northey (or “claw”) and screw pumps. Dry pumps incorporating Roots and/or Northey mechanisms are commonly multi-stage positive displacement pumps employing intermeshing rotors in each pumping chamber. The rotors are located on contra-rotating shafts, and may have the same type of profile in each chamber or the profile may change from chamber to chamber. The backing pump  32  may have either a similar pumping mechanism to the booster pumps  30 , or a different pumping mechanism. For example, the backing pump  32  may be a rotary vane pump, a rotary piston pump, a Northey, or “claw”, pump, or a screw pump. 
     The motor  48  of the booster pump  30  may be any suitable motor for driving the pumping mechanism  46 . In the preferred embodiment, the motor  48  comprises a three phase AC motor, although another technology could be used (for example, a single phase AC motor, a DC motor, permanent magnet brushless motor, or a switched reluctance motor). 
     A pump controller  50  drives the motor  48 . In this embodiment, the pump controller  50  comprises an inverter  52  for varying the frequency of the power supplied to the AC motor  48 . The frequency is varied by the inverter  52  in response to commands received from an inverter controller  54 . By varying the frequency of the power supplied to the motor, the rotational speed of the pumping mechanism  46 , hereafter referred to as the speed of the pump, or pump speed, can be varied. A power supply unit  56  supplies power to the inverter  52  and inverter controller  54 . An interface  58  is also provided to enable the pump controller  50  to receive signals from an external source for use in controlling the pump  30 , and to output signals relating to the current state of the pump  30 , for example, the current pump speed, the power consumption of the pump, and the temperature of the pump. 
     In the embodiment shown in  FIG. 4 , the pump controllers  50  of each of the booster pumps  30  are connected to the controller  24 . As illustrated, cables  60  may be provided for connecting the interfaces  58  of the pump controllers  50  to an interface of the controller  24 . Alternatively, the pump controllers  50  may be connected to the controller  24  over a local area network. 
     In use, the vacuum pumping arrangement  26  is operated to evacuate the degassing chamber  10  to degas the molten metal  14  contained within the ladle  12 . Gas is drawn from the chamber  10  into the inlet manifold  36 , from which the gas passes through the booster pumps  30  into the exhaust conduit  40 . The gas is drawn from the exhaust conduit  40  by the backing pump  32 , which exhausts the gas drawn from the chamber  10  at or around atmospheric pressure. During evacuation of the chamber  10 , the level of the surface  22  of the slag  16  is monitored using the gauge  20 . The gauge outputs a radar beam towards the slag  16 . The beam is first reflected from the surface  22  of the slag  16 , and then from the interface  62  between the molten metal  14  and the slag  16 . As a result, the gauge  20  receives a first, relatively weak echo of the radar signal after a first time period, due to the reflection of the radar beam by the surface  22  of the slag  16 , and a second, relatively strong echo after a second time period, due to the reflection of the radar beam from the interface  62  between the molten metal  14  and the slag  16 . The distance d 1  between the gauge  20  and the surface  22  of the slag  16  is proportional to the duration of the first time period. As the distance d 2  between the gauge  20  and the top of the ladle  12  is a constant, the distance d 3  between the top of the ladle  12  and the surface  22  of the slag  16  is thus also proportional to the duration of the first time period. 
     The gauge  20  outputs to the controller  24  a signal including, inter alia, the length, or an indication of the length, of the first time period. The controller  24  uses the data contained within the signals to monitor both the current level of the surface  22  of the slag  16  and the rate of change of the level of the surface  22 , for example, due to foaming of the slag  16  during degassing. These parameters are used by the controller  24  to control the rate of evacuation of the chamber  10 , which in turn controls the rate of degassing of the molten metal  14 , and thus the degree of foaming of the slag  16 . In this embodiment, the controller  24  varies the speeds of the booster pumps  30  to control the evacuation rate of the chamber  10  by issuing a command to the pump controllers  50  to vary the speeds of the booster pumps  30 . For example, a target speed for the booster pumps  30  can be provided to the pump controllers  50  in the form of a target frequency for the inverters  52 . In response to the command received from the controller  24 , each pump controller  50  controls the frequency of the power supplied to its motor  32  according to the target frequency provided by the controller  24 . This target frequency may be zero, so that the booster pumps  30  are effectively switched off. Alternatively, the target frequency may be progressively decreased towards zero depending on the data contained within the signals received from the gauge  20 . 
     As a result, a rapid increase in the level of the surface  22  of the slag  16  due to foaming can be rapidly detected and combated by a corresponding automatic prompt reduction in the rate of evacuation of the chamber  10 , thereby reducing the rate at which gas is generated at the interface  62  between the molten metal  14  and the slag  16  and hence preventing the slag  16  from overflowing from the ladle  12 . Once the level of the slag surface  22  has receded, the evacuation rate of the chamber  10  can be increased again by issuing an appropriate command to the pump controllers  50  to increase the speeds of the booster pumps  30 . 
     In the embodiment shown in  FIGS. 1 to 4 , a system controller  24  determines a target speed for the booster pumps  30 , and advises the booster pumps  30  of the target speed. In the embodiment shown in  FIG. 5 , the gauge  20  is connected directly to the pumping arrangement  26 . In this embodiment, the signals output from the gauge  20  are received directly by the pump controllers  50 , each of which has stored therein the functionality of the controller  24  of the first embodiment for controlling the speed of its respective pumping mechanism.