Plant and method for vacuum degassing liquid steel

Abstract

The invention relates to a plant for vacuum degassing liquid steel, comprising: at least one vacuum chamber 2, suitable to temporarily receive liquid steel inside it; a vacuum generation system 10, connected to said at least one vacuum chamber 2 via an intake duct 20. The vacuum generation system 10 comprises at least two compression stages connected together in series, of which: a first compression stage 11 works closer to the aforesaid at least one vacuum chamber and is composed of one or more screw pumps 110; and a second compression stage 12 works farther with respect to said at least one vacuum chamber 2 to bring the gases at least to atmospheric pressure and is composed of one or more liquid ring pumps 120.

1 - 20 . (canceled) 
     
     
         21 . Plant for vacuum degassing liquid steel, comprising:
 at least one vacuum chamber ( 2 ), suitable to temporarily receive liquid steel inside it;   a vacuum generation system ( 10 ), connected to said at least one vacuum chamber ( 2 ) via an intake duct ( 20 ),   characterised in that the vacuum generation system ( 10 ) comprises at least two compression stages connected together in series, of which a first compression stage ( 11 ) works closer to said at least one vacuum chamber ( 2 ) and is formed by one or more screw pumps ( 110 ), and a second compression stage ( 12 ) works farther with respect to said at least one vacuum chamber ( 2 ) to bring the gases at least to atmospheric pressure and is formed by one or more liquid ring pumps ( 120 ) and in that said one or more screw pumps ( 110 ) are dimensioned to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1:200, if the discharge pressure is comprised between 50 and 120 mbar absolute.   
     
     
         22 . Plant according to  claim 21 , wherein said one or more screw pumps ( 110 ) are dimensioned to be able to work with compression ratios comprised between 1:3 to and 1:10, if the discharge pressure is atmospheric, and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios comprised between 1:25 and 1:200, and preferably between 1:70 and 1:90. 
     
     
         23 . Plant according to  claim 21 , wherein the vacuum generation system ( 10 ) is dimensioned to bring the vacuum chamber ( 2 ) to a degree of vacuum between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar. 
     
     
         24 . Plant according to  claim 21 , wherein the vacuum generation system ( 10 ) comprises at least one intermediate compression stage, which is positioned between the first stage ( 11 ) and the second stage ( 12 ) and is connected to them in series, said intermediate compression stage being formed by one or more screw pumps ( 110 ) having similar characteristics to those of the first stage ( 11 ). 
     
     
         25 . Plant according to  claim 21 , wherein one or more of said compression stages are each formed by two or more pumps connected in parallel. 
     
     
         26 . Plant according to  claim 21 , wherein the intake duct ( 20 ) comprises a by-pass duct ( 21 ) able to exclude from the gas flow the compressor stages formed by screw pumps ( 110 ). 
     
     
         27 . Plant according to  claim 21 , wherein each screw pump ( 110 ) comprises two screw rotors, kinematically synchronised with each other via electric axis. 
     
     
         28 . Plant according to  claim 21 , comprising at least one filtration device of the water used by said one or more liquid ring pumps ( 120 ) suitable to remove dust accumulated in the water itself during operation of the pump or a replacement device of the water itself. 
     
     
         29 . Plant according to  claim 21 , wherein, in the section comprised between the vacuum chamber ( 2 ) and the vacuum generation system ( 10 ) the intake duct ( 20 ) comprises a connection branch ( 28 ) to the atmosphere equipped with a control valve ( 23 ). 
     
     
         30 . Plant according to  claim 21 , comprising at least one gas filtration device ( 25 ) positioned between the vacuum chamber ( 2 ) and the vacuum generation system ( 10 ). 
     
     
         31 . Plant according to  claim 21 , wherein, in the section comprised between the vacuum chamber ( 2 ) and the vacuum generation system ( 10 ) the intake duct ( 20 ) comprises a connection branch ( 28 ) to the atmosphere equipped with a control valve ( 23 ) and wherein said plant comprises at least one gas filtration device ( 25 ) positioned between the vacuum chamber ( 2 ) and the vacuum generation system ( 10 ), said plant comprising at least one shut-off valve ( 22 ) that is installed in said intake duct ( 20 ) between the vacuum chamber ( 2 ) and the filtration device ( 25 ), downstream of the branching point of the connection branch ( 28 ) to the atmosphere. 
     
     
         32 . Method for vacuum degassing liquid steel, comprising the following operating steps:
 a) providing at least one vacuum chamber ( 2 ) suitable to temporarily receive liquid steel inside it;   b) placing liquid steel in said vacuum chamber ( 2 );   c) evacuating the vacuum chamber ( 2 ) through a vacuum generation system ( 10 ) creating in said chamber a predefined degree of vacuum and maintaining it for a predetermined period of time so as to complete the operation of degassing the liquid steel;   d) bringing again the vacuum chamber ( 2 ) to atmospheric pressure and withdrawing the degassed liquid steel;   characterised in that the vacuum evacuation step c) is conducted by means of a vacuum generation system ( 10 ) comprising at least two compression stages connected together in series, of which a first compression stage ( 11 ) works closer to said at least one vacuum chamber ( 2 ) and is formed by one or more screw pumps ( 110 ), and a second compression stage ( 12 ) works farther with respect to said at least one vacuum chamber ( 2 ) to bring the gases at least to atmospheric pressure and is formed by one or more liquid ring pumps ( 120 ) and in that said one or more screw pumps ( 110 ) are dimensioned to be able to operate with compression ratios not exceeding 1:12, if the discharge pressure is atmospheric, and with compression ratios not exceeding 1:200, if the discharge pressure is comprised between 50 and 120 mbar absolute.   
     
     
         33 . Method according to  claim 32 , wherein said one or more screw pumps ( 110 ) are dimensioned to be able to operate with compression ratios comprised between 1:3 and 1:10, if the discharge pressure is atmospheric, and, if the discharge pressure is between 50 and 120 mbar absolute, with compression ratios of between 1:25 and 1:200, and preferably between 1:70 and 1:90. 
     
     
         34 . Method according to  claim 33 , wherein, in evacuation step c), the vacuum chamber ( 2 ) is brought to working at a degree of vacuum between 0.2 and 5 mbar, and preferably between 0.5 and 1.5 mbar. 
     
     
         35 . Method according to  claim 32 , wherein said evacuation step c) provides for the direct aspiration of the gases from said vacuum chamber ( 2 ) through the said vacuum generation system without a preventive filtration step of the gases, independently of the level of dust concentration in the gases themselves. 
     
     
         36 . Method according to  claim 32 , wherein said evacuation step c) provides for the aspiration of the gases from said vacuum chamber ( 2 ) through said vacuum generation system with a preventive filtration step of the gases, to reduce the dust concentration in the gases themselves before their passage through the vacuum generation system ( 10 ). 
     
     
         37 . Method according to  claim 32 , wherein evacuation step c) comprises:
 an initial evacuation step c 1 ) wherein the vacuum chamber ( 2 ) is brought from atmospheric pressure up to about 300 mbar using only the liquid ring pumps ( 120 ) of the vacuum generation system ( 10 ); and   a final evacuation step c 2 ) wherein the vacuum chamber ( 2 ) is brought from the pressure of about 300 mbar to the predefined degree of vacuum also using the screw pumps ( 110 ).   
     
     
         38 . Method according to  claim 32 , wherein during evacuation step c), the aspiration capacity of the vacuum generation system ( 10 ) is varied to reduce foaming phenomena of the slag in the liquid steel, the aspiration capacity being varied by slowing or excluding one or more of the pumps ( 110 ,  120 ) of the vacuum generation system ( 10 ), preferably the liquid ring pumps ( 120 ), said variation of aspiration capacity being preferably carried out when the internal pressure of the vacuum chamber ( 2 ) is between 300 mbar and 1 mbar. 
     
     
         39 . Method according to  claim 32 , comprising a treatment step f) of the water used by said one or more liquid ring pumps ( 120 ), said step being preferably carried out during evacuation step c), said treatment consisting in a filtration of the water from the dust or a replacement of the water itself. 
     
     
         40 . Method according to  claim 32 , comprising a mixing step e) of the molten steel at least during the evacuation step c).