Arc furnace
The electric arc furnace (electric arc furnace) is an industrial furnace that is used , among other things, to melt down steel scrap for reuse as a new steel product. Expert circles often speak of EAF ( English Electric Arc Furnace ).
On the one hand, steel can be produced from iron ore via the blast furnace and converter route . However, the production of steel in electric arc furnaces is more costly than in the blast furnace route. All types of steel can be produced with electric arc furnaces, but due to their high cost they are mainly used for the production of quality and stainless steels. It is energetically favorable to melt the steel scrap in an electric arc furnace. In 2017, 1688 million tons of crude steel were produced worldwide, 28% of which with electric arc furnaces. The share in Europe was 42%, that of Germany 29% (Oct. 2016).
Products
The electric arc furnace is used for the production of structural steels, quality steels and stainless steels.
The design as a smelting reduction furnace (engl .: Submerged Arc Furnace , SAF) is for the melting of alloy components , ferro-alloys or so-called pictures used. The products from smelting reduction furnaces are mainly used in metallurgy and thus in metallurgy . The manufacture of calcium carbide , metallurgical silicon and synthetic crystals is less common .
function
In the electric arc furnace process, chemical energy can also be used to melt the input material in addition to electrical energy. A large part of the total energy is converted into thermal energy (up to 3500 ° C), which leads to the melting of the input material; a further part leads to the heating of the furnace lining. The heat above the arc , which burns between the electrode and the charge, is mainly transferred to the charge by radiation. In the AC electric arc furnace, several arcs burn between the charge (or the melt) and the tip of the three electrodes. In the case of a direct current arc, the arc is transferred from four bottom electrodes (+) through the load to one electrode (-).
In addition to scrap steel, sponge iron or pig iron can also be processed in the electric steel process. In addition to the liquid crude steel, a layer of slag forms on the melt from the non-metallic input materials (burnt lime / magnesium oxide) and oxides of the alloy materials . This has the task of binding unwanted components and protecting the steel bath from further oxidation and heat loss as well as protecting the furnace from overheating. Shortly before tapping, the slag is drained from the furnace in a slag bucket and is then transported away by a special vehicle and emptied at the slag bed. The liquid steel is poured into a steel ladle, which stands on a remote-controlled ladle trolley and transports the steel to the ladle furnace for further processing .
In the past, it was common practice to drain the melt into a ladle after the desired amounts of alloy components had been introduced into the steel bath and then cast it in the casting plant. Today, in most cases, the electric furnace is used as a pure smelting unit to produce a base melt with low carbon , sulfur and phosphorus contents. The final analysis is only made after tapping in the ladle furnace. This results in a higher accuracy of the analysis and also a considerable saving of energy. Despite the high energy costs for electricity as well as for natural gas and oxygen (for auxiliary burners in the vessel), this process is very flexible with regard to the amount of steel types to be produced and the various steel qualities.
The meltdown time, which essentially depends on the electrical output of the furnace transformer and the type and nature of the material used, is decisive for production . Typical cycle times (the time between two taps , tap-to-tap time) are between 45 and 90 minutes. The pure melting time with the use of an electric arc (power-on time) is around 30 to 70 minutes. The difference between the two times contains the sum of the power-off times during which the arc is switched off. This includes: B. charging, sampling or maintenance work. In order to achieve these times while specifying the furnace capacity and the material used, the furnace transformer must be dimensioned so that a specific electrical output in the range of around 0.5 to 1.4 MVA / t is achieved.
The furnace transformer located immediately next to the furnace is a special power transformer , usually oil-cooled and housed in its own casing for protection. The systems, which are operated with three-phase alternating current, achieve outputs of a few 10 MVA to over 100 MVA and have step switches for power transformers to set the undervoltage that is fed to the furnace via the electrode connections. The supply usually takes place in two stages: A power transformer which transforms from the high-voltage network such as the 110 kV level to an intermediate voltage of around 30 kV and is usually located with the electrical high-voltage switchgear outside the production hall. The furnace transformer located directly next to the furnace transforms the intermediate voltage to voltages of a few 100 V up to a few kV, which is fed directly to the electrodes in AC furnaces. The currents on the electrode side are a few 10 kA during operation, and in large ovens also over 100 kA, which is why the connecting rails to the electrodes must be kept as short as possible and are designed as waveguides. Water circulates inside the waveguide for cooling. The connection between the transformer and the electrodes is often made using a Knapsack circuit .
Designs
The arc furnace can be designed as a direct current furnace (consisting of a melting electrode and a bottom electrode) or as an alternating current furnace (consisting of three melting electrodes). The arc length is regulated by means of an electrode regulator. High demands are placed on the power supply of the furnaces, which result from the uneven burning of the arc; there is a risk of undesired network feedback .
The furnace vessel itself consists of three parts (bottom vessel, upper vessel, lid) and can be tilted hydraulically. The steel structures are usually water-cooled on the outside and lined with refractory material on the inside . The capacity (furnace capacity) is given in tons and usually refers to the amount of liquid steel, i.e. H. the tap weight. The range of sizes extends from around 1 t (smaller foundries ) to 300 t (large steelworks ).
The bottom vessel , which has to hold the entire amount of liquid steel, is lined with brick-shaped refractory material. It also contains the tap opening through which the liquid steel is poured into the pan. This opening is designed either as an extended 'snout' with a discharge channel or as an eccentric bottom opening. The latter has the advantage that the furnace does not have to be tilted so much during tapping. Eccentric tapping also prevents the slag remaining in the furnace from running into the ladle; this is particularly desirable for the further treatment of the melt in the ladle. In modern processes, a remainder of liquid steel remains in the furnace (hot heel) after tapping, so that better arc ignition conditions prevail in the subsequent process and the bottom vessel is better protected from the arc radiation. In more recent designs, nozzles are also installed that blow in oxygen as a reaction gas or flushing gases such as argon or nitrogen under high pressure via a porous floor stone ( Tuyeres ).
In addition to the bottom container , the upper vessel must accommodate the solid charge . It is also lined on the inside or provided with water-cooled copper heat sinks, the surface of which is sealed by refractory gunning mix and also by spraying process slag. Auxiliary burners (natural gas / oxygen) are usually installed in the upper vessel . Opposite the tap hole is the slag door in the upper vessel. In older processes, the slag was poured into a separate slag bucket via the tapping nozzle. In the meantime, the resulting process slag is drained through this slag door by tilting the furnace in the opposite direction into a separate level or trough, from which it is removed and transported away. The lockable door also serves other purposes, e.g. B. the liquid steel sampling, the temperature measurement, the manual addition of additives, the visual inspection and also the additional process treatment by means of external, swing-in oxygen lances , which are often combined with carbon lances .
The hinged lid is also lined on the inside with refractory material. When the lid is swung open, scrap, sponge iron, liquid pig iron and additives (e.g. alloying agents such as chrome, etc.) are charged into the furnace. Baskets with a bottom flap are used for scrap and other solid additives. Liquid pig iron is charged using tiltable refractory pans. When the lid is closed, the graphite electrodes are moved into the furnace vessel through openings. In some designs, finer piece goods, such as. B. sponge iron, can be continuously tracked via an additional cover opening and a conveyor belt system.
In modern processes, the electrical energy is supplemented by chemical energy (oxygen, also in connection with carbon or natural gas) depending on the energy availability and costs. A special design is the CONARC furnace (CON = Converter, ARC = Arcing) from SMS Siemag AG , in which both energies are used efficiently. The furnace consists of two vessels and combines the advantages of the electric arc furnace and the classic converter blowing process. While the charge in one vessel is treated electrically using swiveling graphite electrodes, the charge in the other vessel can be decarburized using a top lance that can also be swiveled by means of oxygen injection.
Emissions
The electric arc furnace process emits gas and dust-containing substances. Effective extraction systems and filters are therefore required. In addition, there are sound emissions and electromagnetic radiation ( arc radiation and radio waves from the arc ignition processes). Due to the high electrical electrode currents, strong alternating magnetic fields also arise . Compared to crude steel production , the electric arc route saves around 55% energy.
See also
literature
- Manfred Jellinghausen: Steel production in an electric arc furnace . Verlag Stahleisen, Düsseldorf, ISBN 3-514-00502-8 .
Individual evidence
- ↑ World Steel Association: World Steel in Figures 2017
- ^ The European Steel Association (EUROFER): European Steel in Figures. 2016 edition, covering 2011-2015.
- ↑ Stahl-online.de: Raw steel production in November 2016. Media information from December 13, 2016.
- ↑ Tamini AC or DC Furnace Transformers , accessed March 6, 2019
- ↑ Siemens brochure on industrial transformers , accessed on May 10, 2019
- ↑ Tamini Group: Electric Arc Furnace Transformers (PDF, English) ( Memento from March 20, 2013 in the Internet Archive ).
