THE ELECTRIC ARC FURNACE
The electric arc furnace as the name suggests is a furnace in which heat is generated with the aid of electric arc produced by graphite electrodes. The main components of the electric arc furnace are the furnace shell with tapping device and work opening, the removable roof with the electrodes and a tilting device. The furnace shell is circular and with a refractory lining. The work opening and the tapping device are arranged opposite each other for tapping purposes, the complete furnace is tilted to an angle of about 42 degrees. Normally, the furnace is charged with its roof removed. When scrap is added, a charging bucket travels over the furnace, the bottom opens and the scrap is charged into the furnace within a few minutes. During the process, a control system advances the slow burning electrodes. High voltage is transformed into low voltage and high amperage. The most important parameter for the efficiency of an electric arc furnace is the "specific apparent power of the transformer" - in terms of 1 t of charge. Values range from 300 to 750 kVA/t (kilo-volt-ampere per tonne). In some cases, as much as 1,000 kVA/t has been installed.
THE MELTING PROCESS
The electric arc furnace process generally follows the following pattern.
- Charging
- Melting
- Oxidising
- Deoxidising or refining
Besides scrap or sponge iron, the charge also includes the ores, fluxes (lime, flourspar), reducing agents (carbon) and alloying elements in the form of ferroalloys. These can be added through the work opening before or during oxidizing.
Process begins with the ignition of the electric arc. After melting, further scrap can be added. An additional injection of oxygen or some other fuel-gas mixture can accelerate the melting phase. The maximum transferable electric power and the heat stability of the refractory lining determine the time needed for melting. The most up-to-date furnaces with a hi specific apparent power (UHP furnaces) achieve melting periods of about 40 to 60 minutes and tap-to-tap times of about 1.5 hours.
During the refining stage, iron oxides included in the slag react with the carbon of the bath. This gives rise to the gaseous carbon monoxide, which causes the heat to boil, and rinses impurities such as phosphorus, hydrogen, nitrogen and non-metallic compounds from the heat. These impurities escape as gases or are included in the slag. Sulphur cannot be completely eliminated.
The advantages of steelmaking in the electric arc furnace are :
- All possible grades of steel can be melted
- Low capital outlay
- The melting process can be programmed and automated
- Good efficiency
But it has some shortcomings as well. Because it uses scrap , the EAF route can only be used to produce steel grades with low purity requirements. Major new developments in steel making have taken place in EAF based steel making. Innovations such as DC arc technology, scrap preheating, post combustion, oxygen and carbon injection etc have led to a tremendous increase in productivity and a decrease in electric consumption.