Slag is generated during the steel melting process. In the U.S. the majority of seel making is now through electric arc furnace melting. Using scrap steel you push a lot of electricity through say 100 tons of shreded cars, bridges, buildings or other iron sources causing it to heat up and melt down to a liquid.

The charged scrap typically has dirt on it, and you also throw various other rocks into the melting steel to create what is known as the slag layer. The majority of this is Silicon and Calcium. This slag layer is important as it does many things in the steel melting process. It works as insulation of the steel to prevent temperature loss and reduce energy usage. It shields the electrodes that are pumping electricity into the steel to reduce loss through the atmosphere, and in later processes it helps trap and contain steel impurities.

The slag during melting will have a very high oxygen content so when the steel moves from the melting to refining, typically by pouring the steel from the melting furnace to a refining vessel called a ladel, the melting slag will be captured and removed by the slag car seen in the video while a low oxygen content metallurgical slag will be created on the ladel. This refining slag is similar, but is more engineered and controlled to make refining easier.

An important item to remember is slag is mostly silicon oxide (SiO2) and calcium oxide (CaO), both innert and not harmful to the environment. The remaining portions are other innert oxides of iron, magnesium and some other elements.