Where does the steel making slag come from?

The main source of steel making slag is: 

oxides of various impurity elements (such as Si, Mn, P, etc.) contained in steel materials (hot metal, scrap steel); 

added to remove sulfur and phosphorus in molten iron. 

Slag-forming material (lime, etc.) and flux (fluorite, etc.); 

ore, sinter, iron oxide, etc. added as oxidant or coolant to bring in impurities; 

lining refractory material that is eroded or washed down; 

Sediment impurities brought in by raw materials.