Electric Arc Furnace Working Principle and Operation
Electric arc furnaces work on the principle of the electrode forming an arc between itself and the top layer of metal to be melted.
The operation begins with a delivery of steel scrap to the scrap bay, located at one end of the EAF building. The scrap comes in many forms, from shredded crushed cars to kitchen white goods, steel sections, and plates. These are loaded onto a scrap container known as a basket, which has bottom-opening doors, along with some pig iron (an iron ingot product of the blast furnace) as required for the finished grade of steel.
The basket is loaded to ensure that the topmost layer is of light scrap, then a layer of heavy scrap, alternating until the basket contains the correct mix and weight of scrap.
On its way to the furnace, the basket may pass through a large pre -heat system, which uses electric arc furnace industrial heat in the form of exhaust gasses to heat the scrap, thereby increasing efficiency using waste heat.
Next the basket of scrap is transported on its steel wheeled trolley to the furnace where the furnace roof is retracted and ready for the charge.
As I mentioned earlier I was engineer at an aluminum smelter that had an EAF for melting iron, used to secure the rods to the carbon anode blocks. (See my article on Aluminum Smelting from Bauxite). I found charging to be the worst and most dangerous part of the operation- as any molten metal lying in the furnace gets displaced, it spills out the top and sets fire to any oil or grease in the vicinity- very scary!
Anyway, once the charge is in the furnace, the lid is replaced on the furnace and the electrodes lowered onto the top layer of light scrap. An arc is struck and the electrodes start sinking into the scrap, their rate of advance and voltage are controlled automatically, the oxygen burners are energized, and melting commences.
The electric arc furnace temperature of the molten metal is around 2970F, but alarmingly the temperature of the arc is something in the region of 11000F!
A normal melt of 100T would take just over an hour to complete, using about 400kWh of power.
The formation of slag is an important component of EAF steel production, and is encouraged for several reasons as listed below;
1. Slag forms on the top of the molten steel and acts as a sponge to gather the impurities in the metal.
2. Slag acts as an insulation layer on top of the metal preventing excessive heat loss.
3. Slag helps reduce the wearing away of the refractory.
Slag formation is aided by the injection of magnesite and dolomite in the form of magnesium oxide and lime from calcium oxide. (These can also be added to the scrap charge.) Now the melt has reached its end product, another basket of scrap can be added and once all metal is completely molten, samples can be taken and the molten steel adjusted by addition of more slag producers and injection of oxygen with inclusion of chemicals to burn away the typical impurities of sulphur, aluminum manganese, and phosphorus (their oxides being held in the slag).
This can cause the slag to foam vigorously and spill out the slag overflow.
Once the samples have confirmed, the correct temperature and composition of the molten steel it is then tapped into the pre-heated cradle where alloys may be added along with more lime.
Reference Web: P2Pays - EAF working principles and operation.