Method for reducing to metallic chromium the chromium oxide in slag from stainless steel processing

Abstract

The present invention relates to a method for reducing to metallic chromium the chromium oxide material found in slag formed in an electric arc furnace during the process of making stainless steel in that furnace. The chromium oxide content of the slag can be effectively reduced to a relatively low concentration by maintaining the slag in a liquid phase while at the same time blowing into the furnace via a carrier gas certain amounts of powdered aluminum dross. In particular, powdered aluminum dross can be blown into the furnace in amounts ranging from about 10 to 20 kg of dross per ton of molten steel in the furnace. Alternatively, powdered aluminum dross can be blown into the furnace in a thrown-in amount which satisfies the equation: 0.5≦[Thrown-In Al dross (ton)×100]/[Slag in Furnace (ton)×Cr 2 O 3 (wt %)]≦1.0. Via such a method used during the stainless steel making process, the recovery of valuable chromium and the rate of chromium oxide reduction can be increased. Further, the cost of the stainless steel making process can be reduced by using relatively inexpensive powdered aluminum dross as the chromium oxide reducing agent instead of more expensive conventional reducing agents.

1 . A method for reducing to metallic chromium the chromium oxides found in slag formed in an electric arc furnace during a stainless steel making process, which method comprises maintaining said slag in a liquid phase while at the same time blowing into said furnace, via a carrier gas, powdered aluminum dross in a thrown in amount which satisfies the equation:
   0.5≦[Thrown-In Al dross (ton)×100]/[Slag in Furnace (ton)×Cr 2 O 3  (wt %)]<1.0.   
     
     
         2 . A method according to  claim 1  wherein the particle size of the powdered aluminum dross ranges from about 1 mm to 5 mm. 
     
     
         3 . A method according to  claim 2  wherein the powdered aluminum dross is blown into said electric arc furnace via a steel tube along with at least one inert carrier gas selected from the group consisting of nitrogen (N) and argon (Ar). 
     
     
         4 . A method according to  claim 3  wherein the amount of powdered aluminum dross blown into said electric arc furnace exceeds the chemical equivalent amount needed to reduce all of the chromium in the slag. 
     
     
         5 . A method according to  claim 1  wherein the blowing in of powdered aluminum dross to said electric arc furnace occurs at a point in time after the addition of any oxygen introduced into said furnace during the stainless steel making operation has been completed. 
     
     
         6 . A method according to  claim 1  wherein the blowing in of the powdered aluminum dross to said electric arc furnace occurs at a point in time after consumption of power during the operation of electric arc furnace is between about 300 to 400 kW/ton of metal in said furnace. 
     
     
         7 . A method according to  claim 1  wherein the basicity of slag in the electric arc furnace is maintained between about 1.1 and 1.7. 
     
     
         8 . A method according to  claim 7  wherein the alumina content of the slag within the electric arc furnace is maintained at 10 wt % or greater. 
     
     
         9 . A method according to  claim 1  wherein the aluminum content of the blown in powdered aluminum dross is 30 wt % or greater. 
     
     
         10 . A method for reducing to metallic chromium the chromium oxides found in slag formed in an electric arc furnace during a stainless steel making process, which method comprises maintaining said slag in a liquid phase while at the same time blowing into said furnace, via a carrier gas, powdered aluminum dross in a thrown-in amount of from about 10 to 20 kg of dross per ton of molten steel in said furnace. 
     
     
         11 . A method according to  claim 10  wherein the particle size of the powdered aluminum dross ranges from about 1 mm to 5 mm. 
     
     
         12 . A method according to  claim 11  wherein the powdered aluminum dross is blown into said electric arc furnace via a steel tube along with at least one inert carrier gas selected from the group consisting of nitrogen (N) and argon (Ar). 
     
     
         13 . A method according to  claim 12  wherein the amount of powdered aluminum dross blown into said electric arc furnace exceeds the chemical equivalent amount needed to reduce all of the chromium in the slag. 
     
     
         14 . A method according to  claim 10  wherein the blowing in of powdered aluminum dross to said electric arc furnace occurs at a point in time after the addition of any oxygen introduced into said furnace during the stainless steel making operation has been completed. 
     
     
         15 . A method according to  claim 10  wherein the blowing in of the powdered aluminum dross to said electric arc furnace occurs at a point in time after consumption of power during the operation of electric arc furnace is between about 300 to 400 kW/ton of metal in said furnace. 
     
     
         16 . A method according to  claim 10  wherein the basicity of slag in the electric arc furnace is maintained between about 1.1 and 1.7. 
     
     
         17 . A method according to  claim 16  wherein the alumina content of the slag within the electric arc furnace is maintained at 10 wt % or greater. 
     
     
         18 . A method according to  claim 10  wherein the aluminum content of the blown-in powdered aluminum dross is 30 wt % or greater.