Ladle white steel valorization through a pyrometallurgical approach

This work deals with the thermodynamical modelling and the first feasibility laboratory experiments for the valorization of the white ladle steel slag. This slag is produced during the final deoxidation process of steel, performed in a ladle after the decarburization and desulphuration processes of the basic oxygen (BOF) or electric arc (EAF) furnaces. The white steel slag is a reactive and relatively high melting point (14501500°C) material, which thus cannot be used as an aggregate in the construction industry (like the black BOF or EAF slags) and cannot be quenched to exploit its reactivity (like the blast furnace slag). Today the white steel slag is entirely landfilled (about 2 Mt/year in Europe – Euroslag 2018). The valorization of the white steel slag occurs in a pyrometallurgical process, where the sensible heat of the molten slag is recovered. The process consists of an aluminothermic reduction of the silica present in the slag, which generates a Fe-Si ferroalloy along with the iron coming from the reduction of the iron oxides present in the slag and the iron retained in the slag during the spilling operation. The Fe-Si ferroalloy is totally recycled in the steelmaking to produce Si-killed steel. The slag associated with this ferroalloy consists of a calcium aluminate, containing all metals not reduced in the aluminothermic reduction. The process must be conducted so as to produce low melting point materials, both the Fe-Si ferroalloy and the calcium aluminate. Thus, the chemical composition of the reacting mass must be carefully controlled by adding the necessary constituents. These additions are constituted by metallic EoW aluminum, silica sand and scrap iron. In such a way it is possible to control the process and to adapt to any input slag composition. The aluminothermic reaction being highly exothermic, the entire process does not require energy input, being exothermic for about 50100 kWh/ton input slag. This process has been realized in a laboratory induction furnace, where two different slags have been processed. The overall quantity of reacted material has been of about 2 kg, enough to prove the feasibility of the process and to evidence the importance of a careful temperature control of the furnace.