PURPOSE: To ensure compactness and small castability, by blowing preheated air into continuous current melted blast furncae slag as necessary, and by changing the slag into granulated slag after dispersing N2 gas in the melted slag with air bubbles.

CONSTITUTION: Oxidizing gas, such as, air preheated not less than approx. 200°C, etc. as necessary is blown into continuous current melted blast furnace slag -especially, continuous current melted blast furnace slag as in a furnace front system. N2 gas contained in melted slag is discharged with the oxidizing gas and changed into granulated slag. Flowing slag quantity greatly varies and the constant maintenance of the quantity rate of cooling water to melted slag quantity is difficult especially in the preparation of granulated slag in the furnace front system, but this pre-treatment can produce conpact granulated slag without exactly managing water quantity. This granulated slag ensures the little elution of Ca component and has a property difficult to concrete