Characterization of synthetic slag

An important parameter to characterize synthetic slag for its suitability to desulphurize molten steel is sulphide capacity of slag. On the basis of ionic theory, a modified sulphide capacity of slag is

(1)

                                                                                                
Where  is % sulphur in slag and  and  are henrian activity of oxygen and sulphur in steel. In terms of property of slag sulphide capacity  is

(2)

Where  is weight percent of sulphur in slag with a gas having partial pressure of oxygen  and   . For a given slag, higher is the value of , better is for desulphurization.  and  are related with

(3)

At  and ,  and 0.133 respectively. Within the temperature range of desulphurization  the ratio  does not depend significantly on temperature. Alternatively can be calculated by

(4)

For a slag with   and  according to equation 4 is  at 1873.
Another important parameter is partition coefficient of S at equilibrium; it is defined as                   

(5)

                                                                                                
 is activity of oxygen in steel and is determined by the amount of deoxidizer. Extent of desulphurization depends on extent of deoxidation. An aluminum killed steel can desulphurize steel much effectively that that when either  or  is used to deoxidize steel

Large value of  ensures efficient desulphurization, also large value of  requires low value of  and high value of  as well. By equation 5