An electric arc furnace (EAF) uses electricity to generate intense heat. It melts steel scrap and iron to make molten steel.
The major charge materials in an EAF are recycled steel scrap, direct reduced iron, and sometimes supplemental pig iron from the blast furnace. They must meet certain quality requirements to ensure high-quality, low-carbon steel.
The Arc
An electric arc furnace (EAF) melts metals to turn them into ingots, which are then rolled or cast into shapes like bars and slabs. It is a key technology for modern steel manufacturing and has made the industry more efficient by reducing its energy usage and carbon footprint.
The EAF uses a combination of steel scrap, Direct Reduced Iron (DRI), and sometimes supplemental pig iron from the blast furnace as raw materials, and electricity to heat the furnace. High-voltage electric arcs between graphite electrodes are used to generate heat and melt the steel. The EAF can produce various types of steel, depending on the mix of its charge material.
While the EAF is more efficient than a blast furnace, it still requires substantial power to operate and maintain its high-voltage electric arcs between electrodes. This makes it less suitable for locations with limited access to fossil fuels. It also requires more maintenance due to the arc’s high current demands and erratic behavior.
The EAF can be powered by alternative energy sources such as wind or hydroelectricity to minimize these costs. However, securing these sources at reasonable prices remains a challenge.
Electric arc furnaces have advantages over other melting technologies, such as cupola or induction furnaces. These other types of furnaces use coal, coke, or limestone fuel as their primary heat source, making them primarily suitable for processing iron ore. Cupola furnaces, in particular, are best suited for producing cast iron and other low-carbon metals.
The EAF is the only melting technology that uses both electricity and fuel to smelt metals, allowing it to be used for both ferrous and nonferrous applications. It is especially useful for smelting nonferrous metals such as aluminum and phosphorous, which require a higher temperature range than those of steel. The EAF’s arc temperature is sufficiently high to melt nonferrous metals but not so hot as to burn the electrodes. It can also be used to smelt phosphate rock and other ores, which contain low levels of phosphorus. In these cases, the slag is tapped from the top of the furnace and cooled with air or water before being poured into a slag pit.
The Hearth
For steelmakers, the electric arc furnace (EAF) represents a new and revolutionary way to produce high-quality, high-grade clean steel. This is because the EAF process uses electricity to heat metal rather than fossil fuels, reducing the plant’s reliance on carbon-intensive energy sources and making it much more environmentally friendly.
The EAF uses extreme heat—up to 3500°C—to melt and refine scrap metal, primarily steel, placed inside the furnace. The molten metal is then transformed into the raw material for steel production. In addition, the electric arc can be used to process other materials, such as alumina, slag, and phosphate rock.
While blast furnaces have long been the main method for iron and steelmaking, they are now largely being replaced by the more advanced, cleaner-burning EAF. The EAF process allows plants to operate more efficiently, and it can use recycled metal as the raw material, further reducing their emissions. The EAF can also be operated using 100% renewable energy, compared to the traditional blast furnace, which must use a form of fossil fuel to reduce iron ore into pig iron.
An EAF is a squat, cylindrical vessel with a dish-shaped hearth lined with chrome-magnesite refractory bricks and a dome-shaped roof. It sits on a hydraulically operated platform that can tilt the furnace for tapping and slag removal.
When a charge of scrap or other raw material is loaded into the furnace, the electrodes are lowered into the shred, and an arc is struck between them. The arc generates direct and radiant heat, melting the scrap. The arc’s power can then be increased to a higher voltage, lengthening the arcs and transferring more power to the metal.
These arcs interact with the scrap, converting it into a molten pool of metal and forming metallic oxides in the bath that help the molten metal melt even faster. This is called “gaseous reactivity.” In addition to adding oxygen, the arcs produce carbon dioxide, either burned in-furnace or sent through an exhaust system to be captured and vented outside.
The EAF can be run in a number of different configurations, depending on the needs of the steelmaker. Some facilities have two EAFs, each with its own shell and set of electrodes; one preheats scrap while the other melts it down to make steel.
The Roof
A typical EAF is a squat steel cylinder with a dish-shaped refractory hearth and three electrodes (carbon rods) extending down through its roof. Powerful electric current arcs between these electrodes during the melting process. The intense heat melts the scrap metal and causes chemical reactions that produce steel. This molten steel is poured out through a tapping spout on the right side of the furnace (see figure).
The bottom of the hearth is lined with magnesite bricks. The walls and roof are made of replaceable water-cooled panels, covered inside by sprayed-on refractories and a blanket of slag. Large openings in the wall, usually with water-cooled doors, are used for off-gas removal and lance injection for sample taking, testing, and maintenance.
During the initial stages of melting, the EAF arc is erratic and unstable, with wide swings in current and rapid movement of the electrodes. These conditions are necessary to create a long arc between the electrodes and to penetrate the scrap into a liquid pool of metal in the hearth of the furnace. Once this condition is achieved, the arc becomes much more stable, and power input can be increased.
The temperature can be increased for refining in the later stages of the melt. This can involve injecting oxygen or carbon into the slag phase to react with it to form different products such as carbides and nitrides. The slag, which has a lower density than the steel, floats on top of the molten steel and protects it from contamination.
Scrap metal is delivered to the EAF by a crane and placed in large buckets. These buckets normally have clamshell-style doors, which open up by retracting two segments at the bottom. Care is taken to layer the scrap in the baskets, with heavy melted material on top of a light layer of protective shred. The buckets then pass to the scrap pre-heater, which uses hot furnace off-gases to heat the scrap, increasing plant efficiency.
Once the scrap is loaded in the furnace, the refractory brick movable roof and the electrodes are raised to lower buckets into the furnace. The electrodes progressively wear during the melting process, so their positions need to be carefully adjusted to minimize short electrodes that waste energy and reduce production output.
The Electrodes
The electric arc furnace is an efficient melting apparatus and a good alternative to more traditional steel-making processes. It can handle a wide range of charge materials and produce a full spectrum of steel grades. This flexibility is a large part of its appeal. Its operational details, however, require careful planning and execution. The electric arc furnace is also less costly and more efficient than basic oxygen or open-hearth furnaces.
The charged material (scrap steel and alloy material) is directly exposed to an electric arc in an electric furnace. Current from the electrode terminals passes through the charged material, causing ions to pass away from the cathode and toward the anode, which is positively charged. Electrodes are typically made of low-cost, high-conductivity graphite and bonded with other alloys to give the electrodes strength, conductivity, and arc resistance.
When the arc is turned on, a powerful electric current ionizes the arc plasma and creates intense heat. This heat melts and chemically reacts the scrap metal and other materials, forming molten steel. Once the steel reaches the correct chemical composition and physical properties, it is tapped into giant electrode molds for further processing.
A major challenge of EAF steelmaking is sourcing the right types of scrap metal. The quality of the scrap is crucial to ensure that the EAF process produces high-quality, cost-effective steel. Its major constituents must be low-cost and free of unwanted contamination, such as slag, scale, and dust. A good scrap metal supply chain is vital for the success of this process.
Some plants use twin shells in which the first shell preheats scrap, while the second is used for meltdown. This is a very effective way to maximize the production of high-quality steel and reduce power consumption. Modern plants may also employ continuous charging-in which the scrap is continuously pre-heated and discharged into the active furnace, utilizing off-gases from the first shell. This allows the EAF to operate continuously, making it a versatile and flexible tool for the steel industry.
