Electric arc furnace

The arc furnace ( EAF ) is an aggregate ( oven ) for melting of steel scrap for reuse as steel new product. In professional circles from EAF (Electric Arc Furnace) is often spoken.

Steel can be produced on the one hand of iron ore on the blast furnace and converter. It is, however, to melt it in an arc furnace steel scrap energetically more favorable. The production of steel in electric arc furnaces, however, is more costly than in the blast furnace route. With arc furnaces all types of steel can be produced, but they are used mainly due to the high costs for the production of carbon and stainless steels. 2012 1542 million tonnes of crude steel were produced worldwide, of which 29 % with electric arc furnaces. The share in Europe rose during the same period 41%, that of Germany 32%.

Products

The arc furnace is used for the production of structural steels, carbon steels and stainless steels.

The design as a smelting reduction furnace (English: Submerge Arc Furnace, SAF ), is used for the production of calcium carbide and synthetic silicon crystals.

Function

In electric arc furnace process, electrical and chemical energy is used for melting the feedstock. While a large portion of the total energy is converted to thermal energy (up to 3500 ° C), which leads to the melting of the feedstock; another portion leads to the heating of the furnace lining. The heat to the arc which burns between the electrode and the feedstock is primarily transmitted by radiation to the feedstock. When AC electric arc furnace to burn multiple arcs between the feedstock (or melt) and the electrode tip of the three electrodes. (-) In direct current electric arc, the arc of the four ground electrodes ( ) of the feedstock to one electrode is transferred.

In the electric steel process can be processed with addition of steel scrap and sponge iron or pig iron. In addition to the liquid crude steel formed from the non-metallic charge materials ( burnt lime / magnesia ) and oxides of the alloying materials a slag layer on the melt. Its task is to prevent unwanted components and to protect the steel bath from further oxidation and thermal losses and to protect the furnace from overheating. Shortly before tapping the slag is discharged from the furnace in a slag pot, and is then removed by a special-purpose vehicle and discharged at slag bed. The liquid steel is poured into a steel pan that is on a remote controlled pan cars and transported for further processing the steel into the ladle.

Previously, it was common after introducing the desired amounts of alloying constituents in the steel bath, drain the melt in a pan and then shed in the casting. Today, in most cases, the electric oven is used as a pure smelting to produce a base with low melt carbon, sulfur and phosphorous contents. The final analysis is not created until after tapping in the ladle furnace. This results in a higher accuracy in analysis and also a significant energy savings. Despite high energy costs for electricity and for natural gas and oxygen ( for auxiliary burners in the vessel ), this method is very flexible in terms of the quantity of goods to steel grades and the various steel grades.

Critical to the production is the melting time, which depends mainly on the electrical performance of the furnace transformer as well as the type and nature of the feedstock. Typical cycle times ( the time between two tappings, tap-to -tap time ) are between 45 and 90 minutes. The pure melting time with arc use ( power-on time ) is about 30 to 70 minutes. The difference of the two times includes the sum of time-outs (power -off time ) where the arc is shut off. This includes, for example, Charging, sampling, or maintenance. To achieve these periods in the specification of the furnace capacity and the starting material, the furnace transformer must be dimensioned such that a specific electrical power in the range of about 0.5 to 1.4 MVA / T is obtained.

The furnace transformer located immediately next to the stove is a special power transformer, usually oil-cooled and housed in a protective own envelope. The operated with three-phase ac systems achieve benefits by some 10 MVA to 100 MVA and above are equipped with tap changers for power transformers to adjust the lower voltage which is supplied to the furnace through the electrode terminals. Electricity supply is usually done in two stages: a power transformer that transforms from the high- voltage network as the 110 kV level to an intermediate voltage of about 30 kV and is usually located with the high voltage electrical switchgear outside the production hall. The furnace transformer located immediately next to the oven transforms the intermediate voltage at voltages of several 100 V to several kV, which is supplied to the electrodes leads directly Wechselspannungsöfen. The currents on the electrode side be in operation some 10 kA, with large furnaces around 100 kA, which is why the connecting rails must be kept as short as possible to the electrodes and are designed as a waveguide. Inside the waveguide circulated for cooling water.

Designs

The arc furnace can be used as direct current electric furnace (consisting of a melting electrode and a ground electrode ), or as an alternating current furnace run (consisting of three melting electrode ). The arc length is controlled by an electrode controller. To the power supply of the furnace, high demands are made ​​, resulting from the nonuniform burning of the arc; there is a risk of unwanted network perturbations.

The furnace vessel itself consists of three parts ( floor planter, upper vessel, lid ) and can be hydraulically tilted. The steel structures are lined on the outside usually water-cooled and on the inside with refractory material. The capacity ( the furnace capacity ) is given in tonnes and refers generally to the amount of liquid steel, ie the tap weight. The range of sizes ranging from about 1 t ( smaller foundries ) to 300 t (large steel mills ).

The base or which must absorb the entire amount of liquid steel is internally lined with refractory material ziegelförmigem. It also contains the taphole, the liquid steel is poured into the ladle. This opening is performed either as an extended ' snout ' with discharge chute or as eccentric bottom opening. The latter has the advantage that the furnace during tapping does not need to be so strongly tilted. A rotating with the shaft of the remaining in -furnace slag in the ladle having an eccentric racking prevented, this is particularly desirable in the further treatment of the melt in the ladle. In modern process processes remains after tapping a residue of liquid steel in the oven ( Hot Heel), so that in subsequent process prevail better Lichtbogenzündbedingungen and the bottom vessel is better protected by the arc radiation. In more recent designs the nozzles are also installed, the blowing through a porous bottom brick oxygen as the reactant gas or purge gases such as argon or nitrogen under high pressure ( Tuyeres ).

The upper vessel must, in addition to the bottom vessel absorb the solid feedstock. It's on the inside is also lined or equipped with water-cooled copper heat sinks whose surfaces are sealed with refractory gunning mix and also through on spraying process slag. In the upper vessel are mostly auxiliary burner (natural gas / oxygen) installed. Compared to the tap hole is located in the upper vessel, the slag door. In older processes, the slag was poured over the tapping nozzle into a separate slag pot. Meanwhile, the resulting process slag is discharged through the slag door this opposite tilting the furnace in a separate layer or bowl from which it is removed and transported away. The lockable door also serves other purposes, such the liquid steel sampling, temperature measurement, the manual addition of supplementary materials, visual inspection and also the additional process treatment by external, retractable oxygen lances, which are often combined with carbon lances.

The swiveling lid is also lined on the inside with refractory material. When swung open lid scrap, sponge iron, pig iron and liquid additives (eg alloying elements such as chromium, etc.) are charged into the furnace. For scrap and other solid additives therefor baskets with a bottom flap can be used. The charging of hot metal is via a tiltable refractory pans. With the lid, the graphite electrodes are moved through openings in the furnace vessel. In some embodiments, finer cargo, such as Sponge iron, to be introduced continuously via an additional lid opening and a conveyor belt system.

In modern process processes the electric power is replaced by chemical energy (oxygen, in combination with carbon or gas ), depending on the availability of energy and costs. A special design is the CONARC oven (CON = Converter, ARC = arcing ) of SMS Demag AG, where both energies are used efficiently. The furnace consists of two vessels, and combines the advantages of the arc furnace and the classic converter blowing process. While the charge is electrically treated in a vessel of pivoting graphite electrodes, the charge in the other vessel through a co- swiveling top lance by oxygen injection can be decarburised.

Emissions

The electric arc furnace process emits gaseous and dusty materials. Are therefore required effective exhaust systems and filters. In addition, acoustic emission and electromagnetic radiation (arc radiation and radio waves through the Lichtbogenzündvorgänge ). Due to the high electrical electrode currents also arise strong alternating magnetic fields. Compared with the crude steel production approximately 55% energy is saved in the electric arc route.