Method for solidifying liquid steel slag

Abstract

The liquid steel slag ( 2 ) is poured in at least four successive layers ( 6 - 8 ), comprising a lowermost layer ( 6 ), an uppermost layer ( 7 ) and at least two intermediate layers ( 8 ), in a reservoir ( 3 ) and the layers of liquid steel slag are allowed to solidify. After having applied the uppermost layer ( 7 ), the solidified steel slag is cooled down more quickly by means of water. The average temperature of each of the intermediate layers ( 8 ) is kept at least until the start of the water cooling step, and this for at least one hour, above a minimum temperature which is equal to or higher than the temperature at which β-dicalcium silicate is formed. In this way, more crystalline phases different from dicalcium silicates are formed so that the formation of fines by the transition of β into γ-dicalcium silicates can be considerably reduced. A glassy material and/or a phosphorus containing compound is preferably added to the liquid slag to further reduce the formation of fines.

1 . A method for solidifying liquid steel slag to produce solid steel slag, which liquid steel slag comprises at least the elements calcium, silicon and oxygen and has a basicity, defined as the ratio between its calcium content, expressed as wt. % CaO, and its silicon content, expressed as wt. % SiO 2 , which is higher than 1.2 so that, when the liquid steel slag is cooled down to room temperature, β-dicalcium silicate is formed therein as from a first temperature, which method comprises the steps of:
 pouring at least four successive layers of said liquid steel slag in a reservoir, which successive layers comprise a lowermost layer, an uppermost layer and at least two intermediate layers; 
 allowing the liquid steel slag to solidify in the reservoir; and 
 cooling the solidified steel slag in the reservoir by applying water onto the solidified steel slag, 
 characterised in that 
 said reservoir has a bottom delimited by side walls, which bottom has a surface area of S m 2 ; 
 said successive layers of liquid steel slag are poured on top of one another in said reservoir, with at least said intermediate layers having a volume of at least S×0.03 m 3  and; 
 said water cooling step is started after having poured said uppermost layer of liquid steel slag in the reservoir; and 
 the temperature of each of said intermediate layers, determined as a volume weighted average over the respective intermediate layer, is kept at least until the start of said water cooling step, and this for at least one hour, preferably for at least two hours, between 1300° C. and a minimum temperature which is equal to or higher than said first temperature. 
 
     
     
         2 . A method according to  claim 1 , characterised in that said minimum temperature is equal to or higher than 700° C., preferably equal to or higher than 750° C., more preferably equal to or higher than 800° C., most preferably equal to or higher than 850° C. and even more preferably equal to or higher than 900° C. 
     
     
         3 . A method according to  claim 1 , characterised in that the temperature as from which β-dicalcium silicate is formed in the steel slag when cooling the liquid steel slag down to room temperature is lowered by adding a phosphorus containing compound to the liquid slag, in particular a phosphate and/or a pyrophosphate containing compound. 
     
     
         4 . A method according to  claim 1 , characterised in that the basicity of the liquid steel slag is lowered by adding a glassy material thereto after the liquid steel slag has been separated off from the liquid steel, which glassy material has a further basicity, defined as the ratio between its calcium content, expressed as wt. % CaO, and its silicon content, expressed as wt. % SiO 2 , which is smaller than 0.20, and preferably smaller than 0.15. 
     
     
         5 . A method according to  claim 4 , characterised in that said glassy material comprises at least 50 wt. %, preferably at least 60 wt. % silicon expressed as SiO 2 , said glassy material preferably comprising glass that has preferably been crushed, in particular soda-lime-silica glass. 
     
     
         6 . A method according to  claim 4 , characterised in that both said liquid steel slag and said glassy material are applied in a same slag pot, said glassy material being preferably applied into the slag pot before applying the liquid steel slag therein. 
     
     
         7 . A method according to  claim 4 , characterised in that per 100 parts by weight of the liquid steel slag, less than 10 parts by weight, preferably less than 9 parts by weight and more preferably less than 8 parts by weight of said glassy material are added to the liquid steel slag. 
     
     
         8 . A method according to  claim 1 , characterised in that the volume of said intermediate layers of liquid steel slag is at least S×0.04 m 3  and preferably at least S×0.05 m 3 . 
     
     
         9 . A method according to  claim 1 , characterised in that the volume of said intermediate layers of liquid steel slag is at most S×0.5 m 3 , preferably at most S×0.4 m 3 , more preferably at most S×0.3 m 3  and most preferably at most S×0.2 m 3 . 
     
     
         10 . A method according to any one of the  claim 1 , characterised in that said water cooling step is started at least one hour, preferably at least two hours and more preferably at least three hours after having poured said uppermost layer of liquid steel slag in the reservoir. 
     
     
         11 . A method according to  claim 1 , characterised in that said water cooling step is started less than ten hours, preferably less than seven hours and more preferably less than five hours after having poured said uppermost layer of liquid steel slag in the reservoir. 
     
     
         12 . A method according to  claim 1 , characterised in that said successive layers have a total height of less than 2.0 m, preferably less than 1.8 m, more preferably less than 1.6 m and most preferably less than 1.4 m. 
     
     
         13 . A method according to  claim 1 , characterised in that said predetermined time intervals are shorter than 90 minutes, preferably shorter than 75 minutes, more preferably shorter than 60 minutes and most preferably shorter than 50 minutes. 
     
     
         14 . A method according to  claim 1 , characterised in that said predetermined time intervals are longer than 5 minutes, preferably longer than 10 minutes and more preferably longer than 15 minutes. 
     
     
         15 . A method according to  claim 1 , characterised in that the temperatures of the liquid steel slag upon being poured in said successive layers are higher than 800° C., preferably higher than 900° C. and more preferably higher than 950° C. 
     
     
         16 . A method according to  claim 1 , characterised in that the temperature of each of said intermediate layers, determined as a volume weighted average over the respective intermediate layer, drops at an average rate which is not higher than 50° C./min, preferably not higher than 40° C./min, more preferably not higher than 30° C./min and most preferably not higher than 20° C./min as from being applied until being covered by a further intermediate layer or by said uppermost layer. 
     
     
         17 . A method according to  claim 1 , characterised in that the temperature of each of said intermediate layers, determined as a volume weighted average over the respective intermediate layer, is kept before the start of said water cooling step, for at least one hour, preferably for at least two hours, below 1200° C. and preferably below 1100° C. 
     
     
         18 . A method according to  claim 1 , characterised in that said successive layers of liquid steel slag are poured at predetermined temperatures and at predetermined time intervals on top of one another in said reservoir and the temperature of each of said intermediate layers, determined as a volume weighted average over the respective intermediate layer, is kept above said minimum temperature by keeping said predetermined temperatures sufficiently high and said predetermined time intervals sufficiently short. 
     
     
         19 . A method according to  claim 1 , characterised in that the solidified steel slag is cooled down by applying water onto the uppermost steel slag layer at least until said temperature of each of said intermediate layers is lower than 400° C., preferably lower than 300° C. and more preferably lower than 200° C. 
     
     
         20 . A method according to  claim 1 , characterised in that the basicity of the liquid steel slag is higher than 1.4 and in particular higher than 1.6.