Energy density of electric arc furnace

Producing one ton of steel in an electric arc furnace requires about 400 kWh per short ton (1.44 gigajoules) or about 440 kWh per ton (1.6 GJ); the theoretical minimum energy required to melt one ton of scrap is 300 kWh (1.09 GJ) (melting point 1,520 °C (2,768 °F).Thus, a 300-ton, 300-MVA EAF would require about 132 MWh of energy to melt the molten steel, and about 37 minutes of "energization time" (the time the steel is melted by the arc).Electric arc steelmaking is only economical where electricity is plentiful and reliable, with a well-developed grid.In many places, plants operate during off-peak hours when utilities have excess generating capacity and electricity prices are lower.This compares favorably with the estimated global energy consumption of steel production of approximately 20 GJ per ton[9] (1 gigajoule equals approximately 270 kWh) by all methods.

Operation 

Scrap metal is transported to a scrap bin located next to the smelting plant.Steel scrap is generally divided into two main grades: shreds (white goods, cars and other objects made of similar light gauge steel) and heavy melt (large slabs and steel girders), and some direct reduced iron for chemical balance ( DRI) or cast iron. Some furnaces melt almost 100% DRI.Scrap is loaded into large drums called baskets with a "flip" door at the bottom.Take care to layer the scrap in the basket to ensure good furnace operation; the heavy melt is placed on top of a thin layer of protective chips on top of which more chips are placed.These layers should be present in the furnace after charging.Once loaded, the baskets may enter a scrap preheater that uses hot furnace exhaust gases to heat the scrap and recover energy, increasing plant efficiency.

The scrap basket is then brought to the smelter shop, the roof is lifted off the furnace, and the scrap in the basket is loaded into the furnace.Charging is one of the more dangerous operations for electric arc furnace operators.The mass of falling metal releases a great deal of potential energy; any liquid metal in the furnace is often moved up and out by the solid scrap, and if the furnace is hot, the grease and dust on the scrap can be ignited, causing a fireball to erupt.In some double-shell furnaces, while the first shell is melting, the scrap is loaded into the second shell, which is preheated with exhaust gases from the active shell.Other operations are continuous charging preheating the waste on a conveyor belt before discharging it into a furnace, or loading it from a shaft set above the furnace through which the exhaust air passes directly.Other furnaces can be filled with hot (molten) metal from other operations.

After charging, the furnace roof swings back and the melting begins.The electrode is lowered onto the scrap, the arc is struck, and the electrode is set to drill into the debris layer on top of the furnace.Choose a lower voltage for the first part of the operation to protect the roof and walls from overheating and arc damage.Once the electrode reaches the heavy melt at the bottom of the furnace and the arc is shielded by the scrap, the voltage can be increased and the electrode raised slightly, prolonging the arc and increasing power to the melt.This allows the molten pool to form faster, reducing tapping time.Oxygen is blown into the waste, burning or cutting the steel, and additional chemical heat is provided by wall-mounted oxy-fuel burners. Both processes accelerate scrap melting. Supersonic nozzles enable oxygen jets to penetrate the foaming slag and reach the liquid pool.

An important part of steelmaking is the formation of slag, which floats on the surface of molten steel. Slag is usually composed of metal oxides and serves as a destination for oxidized impurities, acts as a thermal blanket (stops excessive heat loss) and helps reduce erosion of the refractory lining.For furnaces using basic refractory materials, including most carbon steel production furnaces, the usual slagging agents are calcium oxide (CaO, in the form of quicklime) and magnesium oxide (MgO, in the form of dolomite and magnesite). These slagging agents are either loaded into the scrap or blown into the furnace during the melting process. Another major component of EAF slag is iron oxide produced by the combustion of steel with injected oxygen.Later during heating, carbon (in the form of coke or coal) is injected into this slag layer, which reacts with iron oxides to form metallic iron and carbon monoxide gas, which then causes the slag to bubble, resulting in improved thermal efficiency, and better arc stability and electrical efficiency.The slag layer also covers the arc, preventing radiant heat from damaging the furnace roof and side walls.Once the initial scrap charge is melted, another barrel of scrap can be loaded into the furnace, although EAF development is moving towards a single-charge design. The scrap charging and melting process can be repeated as many times as necessary to achieve the desired thermogravity the number of charges depends on the density of the scrap; lower density scrap means more expense.After all scrap is completely melted, a refining operation is performed to check and correct the chemical composition of the steel and to superheat the melt above its freezing point in preparation for tapping.Introducing more slagging agent and blowing more oxygen into the molten pool burns off impurities such as silicon, sulfur, phosphorus, aluminum, manganese and calcium and removes their oxides into the molten slag. After these elements burn out first, the carbon is removed because of their greater affinity for oxygen. Metals such as nickel and copper, which have a lower affinity for oxygen than iron, cannot be removed by oxidation and must be controlled solely by waste chemistry, such as the introduction of the aforementioned direct reduced iron and pig iron.There is always foamy slag, and it often overflows the furnace and pours into the slag pit from the slag door.Temperature sampling and chemical sampling by automatic spray gun. Oxygen and carbon can be measured automatically with special probes dipped into the steel, but for all other elements, a "cold" sample  a small sample of solidified steel  is analyzed on an arc emission spectrometer.Once the temperature and chemical composition are correct, the molten steel is removed by tilting the furnace into a preheated ladle.For ordinary carbon steel furnaces, once slag is detected during tapping, the furnace will quickly tilt back to the slag removal side, thereby minimizing slag being brought into the ladle. For some special steel grades, including stainless steel, slag is also poured into ladles, where it is processed in ladle furnaces to recover valuable alloying elements.During tapping, some alloying additions are introduced into the metal stream, and more flux, such as lime, is added to the top of the ladle to start building a new slag layer.Typically, tons of molten steel and slag are left in the furnace to create a "hot heel", which helps preheat the next batch of scrap and speed up its melting.During and after tapping, the furnace "turns around": clear the solidified slag on the slag door, check visible refractories, check for leaks in water-cooled components, check for electrodes that are damaged or lengthened by adding new segments; after tapping is complete, tap Mouth filled with sand. For a 90-ton medium-power furnace, the entire process from one heat to the next usually takes about 60-70 minutes (tapping time).

Periodically, the furnace is completely emptied of molten steel and slag to allow refractory inspections and, if necessary, more extensive repairs.Since refractory materials are typically made from calcined carbonates, which are highly susceptible to hydration with water, any suspected leaks from water-cooled components are taken extremely seriously, rather than outright concerns about potential steam explosions. Excessive refractory wear can lead to breakouts, liquid metal and slag can seep into the refractory and furnace shell and escape into the surrounding area.