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Significant quantities of slag are generated and treated as a waste material or a by-product from steel and steel-related industries.Since slag itself usually contains valuable components,many trials to utilize them for various uses have been made in steel industries.In this paper,research and applied activities for slag treatment in steelmaking and slag utilizations of ferrous and non-ferrous slags are presented.Steelmaking slags are conventionally stored in slag yard.Molten slags are dumped into slag yard then stored for more than half years to be stabilized.As this treatment requires huge space and long time,POSCO has introduced new slag treatment facilities;slag atomization process and BSSF slag treatment device.Slag atomization process were developed by POSCO and BSSF adopted from Baoshan Iron & Steel Co.Both have similar concept of quenching slags but atomization process uses air and ocasionally water mist whereas BSSF does steel balls and water.Since products by these processes are fine aggregates,research to expand its use are ongoing.We are still weighing up merits and demerits of these facilites in order to decide POSCO s next generation slag treatment method.Flouspar are very efficient agent for steelmaking process but its hazardous effects on environment forces to strengthen regulations of its use.Slag itself is known as candidate material of flouspar substitute.Secondary steelmaking slag is well known example of flouspar substitute.Research to find other flouspar substitute materials has been made in steel-related industries.The physical and chemical characteristics of several kinds of slag from ferro-manganese production are investigated.Considering all aspects when being used as substitutes of fluorspar,FeMn refining slag which is generated from the production of low or medium carbon grade ferro-manganese was chosen as a fluorspar substituting material.It has a low melting point less than 1 250 ℃,high Mno content up t0 35% and low P and S content.It is applied to converter operation so that 3 ~ 7 kg FeMn refining slag per ton of liquid steel was added int0 100 ton capacity of converter several times during blowing.From the result of plant trial,more than 70% of fluorspar consumption was decreased.