Steel Manufacturing Process from Scrap
STEEL MAKING FROM SCRAP BY ELECTRIC ARC FURNANCE (EAF):
The basic principle of electric steelmaking today is that the furnace is a “melting machine” that produces liquid steel with required chemistry, temperature, and mass in time to feed steel to successive ladle treatments and continuous caster, which finally determines the production rhythm.
Typical tap-to-tap times are in the range of 40–60 min, which is also the total furnace time per heat.
Melting Practice and Foaming Slag:
Although Ultra High Power (UHP) furnaces are used, fast melting by using only electric power is difficult and not the most economic practice either. Importing extra energy and assisting melting technique can greatly accelerate scrap melting and bring economic benefits.
Accordingly, the current state of the art in EAF steelmaking is to use as much as possible chemical energy, besides electric energy, to accommodate tap-to-tap times to the pace of the downstream continuous caster.
Regarding the application of electric energy, at the start of melting, after basket charge, not all the available power can be applied, as the electrodes may be still in a high position, too close to the roof. Then, when the melting operation has advanced further, changing to the tap maximum power may be applied. This is not the case for 100% flat bath operations like in Consteel EAFs or 100% DRI/HBI charging through the fifth hole.
Chemical energy is introduced by oxygen, carbonaceous materials, and natural gas, more and more through injectors rather than lances. The energy-generating reactions are:
The refining step usually does not require full power, which with already flat bath could be dangerous for the lining. At that time, the foaming of the slag is a must. For the slag to foam, the production of CO gas is necessary, by means of the injection of carbon and oxygen through lances or burners. For foaming purposes, several carbonaceous materials are useful, depending on local cost and availability: anthracite, petroleum coke, coke breeze.
Another requirement for good slag foaming has to do with slag chemistry. Basicity should be more than 1.5 and FeO content should be neither very low nor very high.
The main slag formers in the EAF are lime, dolomitic lime, oxides in scrap, gangue in DRI, oxidation products from metallic charge, ash of carbonaceous additions, MgO picked-up from refractories as well as the slag remaining from the previous heat.