How low should be?:           

Consider dephosphorization in a slag of  at 1773K. Initial %P in metal is 0.1 and mole fraction of  in slag = 0.01. Let us calculate which will allow dephosphorization.

(6)

At 1773 K,    

 can be determined by equation 3 and 4. We substitute the values in equation 5. We get . Now the question before us is how to attain such a low value of in a slag of given composition? Such a low value of can be attained when we use basic oxides which have a very strong tendency to form a stable chemical compound. The different basic oxides have different ability to lower . The following expression describes the relative effects of basic oxides on .

(7)

Alkaline oxides and BaO are stronger than CaO but they are corrosive to the refractory lining and hence not used.

Consider a slag

We calculate at different temperatures

Decrease in temperature increases which favours dephosphorization reaction.