Electric arc furnace. History of invention and production

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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
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Electric arc furnace. History of invention and production

The history of technology, technology, objects around us

Directory / The history of technology, technology, objects around us

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The entire history of metallurgy is a struggle for quality, for improving the physical and mechanical properties of metal. And the key to quality is chemical purity. Even tiny impurities of sulfur, phosphorus, arsenic, oxygen, and some other elements sharply impair the strength and ductility of the metal, make it brittle and weak. And all these impurities are found in ore and coke, and it is difficult to get rid of them.

During smelting in a blast furnace and in an open-hearth furnace, the main part of the impurities is converted into slag and removed from the metal along with it. But in the same blast furnaces and open-hearth furnaces, harmful elements from combustible gases enter the metal and worsen its properties. Electrometallurgy, a branch of metallurgy, where metals and their alloys are obtained using electric current, helped to get really high-quality steel. This applies not only to steel smelting, but also to the electrolysis of metals and, in particular, their molten salts - for example, the extraction of aluminum from molten alumina.

Electric arc furnace

The bulk of alloyed high-quality steel is smelted in electric arc furnaces.

In arc steel-smelting furnaces and plasma-arc furnaces (PAF), heat generation occurs due to energy transformations of an arc discharge occurring in air, vapors of melted materials, inert atmosphere or other plasma-forming medium.

According to the general theory of furnaces M.A. Glinkov arc steel-smelting and plasma-arc furnaces are heat-exchange furnaces with a radiation mode of operation, since the energy conditions at the boundary of the process zone, that is, at the liquid metal bath mirror, create electric arcs and a refractory lining of the working space. In addition, in arc steel-smelting furnaces, vertically arranged graphite electrodes create non-uniform arc radiation, which depends on the diameter of the electrodes and the parameters of the electrical regime.

According to the conditions of heat exchange between the arcs, the surfaces of the working space and the metal, the features of the electrophysical processes of the arc discharge, the energy and electric modes, the entire melting in arc furnaces from the beginning of the melting of the solid metal charge to the draining of the liquid metal is divided into stages.

Before the start of melting, the dome-shaped roof of the furnace is lifted, taken aside and charge materials are loaded into the furnace from above. Then the vault is put in place, through the holes in it the electrodes are lowered into the furnace and the electric current is turned on. Cast iron, scrap iron and other materials begin to melt quickly.

As the charge melts, "wells" are formed under the electrodes and around them, into which arcs and electrodes are lowered. There comes a stage of "closed" combustion of arcs, when the melting of the charge occurs in the "wells", from below by heat transfer by radiation to the nearby layers of the charge and by heat conduction through a layer of liquid metal accumulated on the hearth. The cold charge at the periphery of the working space is heated due to the heat accumulated by the lining: in this case, the temperature of the inner surface of the lining is intensively reduced from 1800-1900 to 900-1000 degrees Kelvin. At this stage, the lining of the working space is shielded from arc radiation, so it is advisable to provide the maximum thermal power, taking into account the electrical capabilities of the furnace transformer.

When the amount of deposited liquid metal is sufficient to fill the voids between the pieces of the solid charge, the electric arcs open and begin to burn above the mirror of the metal bath. The stage of "open" burning of arcs begins, during which there is intense direct radiation of arcs on the lining of walls and roof, the temperature rises at a rate of up to 30-100 degrees Kelvin per minute and it becomes necessary to reduce the electric power of the arcs in accordance with the heat-receiving ability of the lining.

Electric arc furnace
Scheme of an electric arc melting furnace. The furnace has a welded steel casing 3. The furnace casing is lined from the inside with heat-insulating and refractory bricks 1, which can be basic (magnesite, magnesite-chromite) or acidic (dinas). Hearth 12 of the furnace is stuffed with refractory mass. The melting space is limited by walls 5, a hearth 12 and a vault, which is also made of refractory bricks and has openings for the passage of electrodes. In the walls of the furnace there is a working window 10 for controlling the progress of melting and a taphole for tapping the finished steel through the chute 2 into the ladle.

Modern arc steel-smelting furnaces operate on three-phase current of industrial frequency. In direct arc furnaces, electric arcs occur between each of the three vertical graphite electrodes and the metal. The lined casing in arc steel-smelting furnaces has a spherical shape. The working space is covered from above by a domed vault. The casing is mounted on a supporting structure with a hydraulic (rarely electromechanical) mechanism for tilting the furnace. To drain the metal, the furnace is tilted by 40-45 degrees, to download the slag - by 10-15 degrees (in the other direction). The furnaces are equipped with mechanisms for lifting and turning the roof - for loading the charge through the top of the furnace, moving the electrodes - for changing the length of the arc and controlling the power introduced into the furnace. Large furnaces are equipped with devices for electromagnetic mixing of liquid metal in the bath, systems for removing and cleaning furnace gases.

Electric arc furnace
Plasma arc furnace

Domestic plasma-arc furnaces have a capacity of 0,5 to 200 tons, power - from 0,63 to 125 MW. The current strength on powerful and super-powerful plasma-arc furnaces reaches 50-100 kA.

Depending on the technological process and the composition of slags, the lining of plasma-arc furnaces can be acidic (when smelting steel for shaped castings) or basic (when smelting steel for ingots).

Electric arc furnace
Diagram of a plasma-arc furnace: 1- plasma torch; 2 - electrode; 3 - hole with cover

A feature of the design of plasma-arc furnaces with refractory lining as a variety of arc-heated smelting bath furnaces is the presence of one or more DC plasma torches and a bottom electrode - anode. To preserve the atmosphere of the plasma-forming gas, the working space of plasma-arc furnaces is sealed using special seals. The presence of a water-cooled electrode in the hearth creates the danger of an explosion, therefore, plasma-arc furnaces are equipped with a system for monitoring the condition of the hearth lining and an alarm warning about the melting of the hearth electrode with liquid metal.

Currently, plasma-arc furnaces with a refractory lining are in operation with a capacity of 0,25 to 30 tons and a power of 0,2 to 25 MW. The maximum current strength is up to 10 kA.

The most energy-intensive melting period in furnaces of both types is the melting period. It is then that up to 80 percent of the total energy consumption is consumed, and mostly electrical. The duration of the entire melting, depending on the adopted technology of electric steel smelting, can be 1,5-5 hours. The electrical efficiency of arc steel-smelting furnaces is 0,9-0,95, and the thermal efficiency is 0,65-0,7. The specific consumption of electrical energy is 450-700 kWh per ton, decreasing due to a decrease in the specific heat-releasing surface for larger arc steelmaking furnaces.

Plasma-arc furnaces have lower rates. Their electrical efficiency is 0,75-0,85. This is due to additional losses in the plasma torch during the formation of the plasma arc. Thermal is about 0,6, since there are additional losses in the water-cooled structural elements. A feature of the operation of plasma-arc furnaces is the use of expensive plasma-forming gases, which necessitates the creation of exhaust gas regeneration systems and the use of technologically acceptable cheap gas mixtures.

New opportunities in steelmaking appeared in connection with the successful development in the late 1980s of bottom (through the hearth) tapping of metal from electric arc furnaces. Such an exhaust system has been successfully implemented, for example, in the steel-smelting shop of the Thyssenstahl plant in Oberhausen (Germany), in the 100-ton furnaces of the plant in Friedriksferk (Denmark), etc. They can operate continuously for quite a long time, for example, Danish 100-ton units - within a week. When the melt is released, which lasts no more than 2 minutes, the furnace tilts only 10-15 degrees instead of 40-45 degrees (for conventional units). This makes it possible to almost completely replace the refractory wall lining with water-cooled panels, to drastically reduce the consumption of various materials and electricity, and to completely cut off furnace slag.

Surprising as it may seem at first glance, the modern ultra-high power arc steel furnace has a much lower specific energy consumption than the open hearth furnace. In addition, the work of a steelmaker of an open-hearth furnace is much harder and more tiring than the work of a converter or electric steel smelter.

Author: Musskiy S.A.

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