Arc steel furnace: device, principle of operation, power, control system

An arc steel furnace (ASF) is a device that heats material by electrical bending.

Industrial appliances range in size from small units of approximately one ton capacity (used in foundries for making cast iron products) to 400 units per ton used for secondary steel processing. Arc steel furnaces and EAFs used in research laboratories may have a capacity of only a few tens of grams. Temperatures of industrial devices can reach 1800 °C (3272 °F), while laboratory installations exceed 3000 °C (5432 °F).

Arc steel furnaces (AFS) differ from induction furnaces in that the charged material is directly exposed to electrical bending, and the current at the terminals passes through the charged material.

Construction

An arc steel furnace is used to produce steel and consists of a refractory vessel. Mainly divided into three sections:

• The shell, which consists of the side walls and the bottom steel of the “bowl”.
• A pallet that consists of fire-resistant material.
• Roof. It can be heat-resistant lined or water-cooled. And also made in the form of a ball or a truncated cone (conical section). The roof also supports a fireproof delta at its center through which one or more graphite electrodes enter.

Individual elements

The hearth can be hemispherical in shape and is necessary in an eccentric kiln for tapping the bottom. In modern workshops, the arc steel furnace - EAF 5 - is often raised above the ground floor, so that ladles and slag pots can be easily maneuvered to either end. Separate from the structure is the electrode support and electrical system, as well as an inclined platform on which the tool stands.

Unique tool

A typical EAF 3 arc steel furnace is powered by a three-phase source and therefore has three electrodes. They have a circular cross-section and, as a rule, segments with threaded connections, so that new elements can be added as they wear out.

An arc is formed between a charged material and an electrode. The charge is heated both by the current passing through it and by the radiated energy released by the wave. The temperature reaches about 3000 °C (5000 °F), causing the lower sections of the electrodes to glow with incandescent lamps when an arc steelmaking furnace is operating.

The elements are automatically raised and lowered by a positioning system that can use any electric winch, lifts or hydraulic cylinders. The regulation maintains approximately constant current. How much power is consumed by an arc steel-smelting furnace? It is kept constant while the charge is melting, even though the scrap may move under the electrodes as it melts. The mast arms holding the element can either carry heavy bars (which can be hollow water-cooled copper tubes carrying current to the clamps) or "hot arms" where the entire top carries the charge, increasing efficiency.

The latter type can be made of copper-plated steel or aluminum. Large water-cooled cables connect the bars or brackets to the transformer located next to the furnace. Such a tool is installed in storage and cooled with water.

Arc furnaces: operating principle

First of all, the charge material is loaded into the arc steel-smelting furnace, which is placed inside the working chamber using a special ladle with a sliding bottom. The bucket is moved by a crane beam with a drive to open the lower part.

After the working chamber is fully loaded, it is covered with a roof. Then, through the holes in the cover, the electrodes are lowered until they come into contact with the charge to provoke a short circuit, after which the heating of the electric arc is turned on. Due to the heat emitted from the arc, the material being processed is melted. At the moment when the metal is completely melted, it will consist of its liquid form and the resulting slag. Alloying additives and deoxidizing agents are added through a viewing window to achieve the appropriate final composition of the finished steel. The slag itself, at the time of steel oxidation, is drained through the appropriate holes and when the cleaned metal is completely ready, it is also poured into a transport ladle. The average temperature of the molten metal at the time it is unloaded from the furnace is about 1600 degrees.

There are four steps that are performed in the smelting process:

1. First of all, preparatory work is carried out on the working area of ​​the furnace, in which the hearth can be repaired, the bottom filled with magnesite powder and the charge can be directly loaded;
2. Next, the melting process itself begins, which lasts on average from 80 to 180 minutes. At the peak moment of melting, the maximum power level of the electric arc is applied. This provokes accelerated melting of the charge and forms slag separation (due to silicon oxidation). Some models of arc furnaces additionally use oxygen gas burners, which are installed either in the walls or in the surface of the vault to slightly speed up the process of melting the material being processed. If the melting procedure, due to the technological features of the enterprise, cannot completely melt the charge, oxygen is introduced inside the working space, increasing the effect of the electric arc. At the end of the second period, almost the entire mass of phosphorus is removed from the metal structure, due to the presence of basic ferrous slag in it;
3. At the third stage, the oxidation process begins, the duration of which is approximately 30 - 90 minutes. The resulting slag is also drained, along with which phosphorus and the remains of slag-forming additives in the form of lime are removed. After draining the slag waste, a special additive is added to the ore to initiate the process of carbon oxidation, which subsequently begins a boiling effect, accompanied by dephosphorization of the metal and the removal of hydrogen and nitrogen bubbles. At the third stage, periodic removal of foaming slag also occurs. By the end of this stage, all existing oxidized slag is removed so that the phosphorus particles in its composition do not dissolve in the liquid metal when the process of material reduction begins;
4. The final procedure is to carry out the recovery process, which takes from 50 to 120 minutes. To create the required grade of steel, additives are added to saturate it with manganese and chromium. After this, aluminum and ferrosilicon are added to the liquid material to provoke the removal of oxygen. When it is necessary to remove sulfur molecules from molten metal, highly basic slag is added by adding lime, special spar, and fireclay. In addition, deoxidation is carried out with ground ferrosilicon and coke. The final step is to drain the liquid metal along with the final stage of slag so that it can pass from the liquid phase into the sulfur-like slag and non-metallic inclusions.

There are a number of parameters that limit the process of metal melting: temperature of the lining, power of the electric arc. Thus, there is no danger of overheating of the lining, at maximum heating power and low temperature of the inner surface of the furnace. The lining has its own heating limitations, the temperature of which should not exceed 1800 degrees. To ensure maximum temperature resistance, the floor is made from magnesite brick, which in some cases eliminates the use of magnesite powder. As for the walls and vault, they are lined with magnesite-chromite bricks, the integrity of which is maintained for up to 250 melting procedures. Pod is able to withstand much more heats and retains its qualities up to 5000 cycles (if magnesite powder is poured during each heat).

Due to the fact that during the processing and melting of metal, during the oxidation period, a large amount of dusty gases are released, the average temperature of which can reach 1400 degrees, they must be processed by wet cleaning and only then released into the atmosphere. The gas released during the oxidation period can reach a volume of 200 cubic meters.

To reduce electricity consumption, the following actions are recommended in arc furnaces:

• A second arc furnace of smaller power and volume can be used to perform the oxidation and reduction steps. Thus, with the help of the ladle-furnace mechanism, liquid metal is moved and the costs of ensuring the idling operation of the main furnace are reduced;
• If you warm up the loaded charge and enter the furnace, this will also save energy when starting the process. Preheating is carried out using fuel combustion;
• The efficiency of the unit can be increased by installing oxygen gas burners, which will reduce electricity consumption for heating and melting metal by about 10-15%. A portioned addition of an oxygen jet is also used, which speeds up the time for complete heating of the metal;
• It is possible to provide additional heating of the charge using a mechanism that ensures recirculation of hot gases that heat the lining;
• Using the temperature of the drained slag, you can heat water and warm up other elements necessary for the work process of the enterprise;
• In some models of arc furnaces, electrodes are installed at an angle of 45 degrees relative to the vertical. This arrangement makes it possible to release gases vertically upward through the shaft, thereby heating the charge. In addition, the gases give off thermal energy to the material being processed and, at the outlet, having a low temperature, cool the ends of the electrodes, which greatly increases their working life.

Tapping and other operations

The arc steel-smelting furnace EAF 50 is built on an inclined platform, so that liquid steel can be poured into another container for transportation. The tilting operation to transfer molten steel is called tapping. Initially, all steel-making vaults of an arc furnace had an outlet chute covered with refractory, which was washed out when it was tilted.

But often modern equipment has an eccentric bottom outlet valve (EBT) to reduce the inclusion of nitrogen and slag in the liquid steel. These ovens have a hole that runs vertically through the hearth and shell and is offset from the shape of an egg. It is filled with fireproof sand.

Modern plants may have two shells with one set of electrodes that are transferred between them. The first part heats the scrap, and the other is used for melting. Other DC furnaces have a similar arrangement, but have electrodes for each shell and one set of electronics.

Vacuum melting furnaces

The scope of application of such units is quite wide. They are used in mechanical engineering, construction and many other industries. Used for melting high-quality steels and heat-resistant alloys based on iron, nickel and cobalt, as well as non-ferrous metals and alloys. In addition, these furnaces can be used for zone cleaning, heat treatment of metal parts, glass melting, and production of single crystals. An ideal furnace for melting steel, it can produce low-carbon steel that is not contaminated by nitrogen, hydrogen and oxygen.

Oxygen elements

AC furnaces typically have a pattern of hot and cold spots around the perimeter of the fireplace, located between the electrodes. Modern ones install oxygen-fuel burners in the side wall. They are used to supply chemical energy to negative zones, which makes heating of the steel more uniform. Additional power is provided by introducing oxygen and carbon into the furnace. Historically done with lances (hollow mild steel tubes) in the slag door, it is now primarily done with wall-mounted injection units that combine oxy-fuel burners and air systems into one vessel.

A modern, average-sized steel furnace has a transformer rated at about 60,000,000 volt-amperes (60 MVA), with a secondary voltage of 400 to 900 and a current in excess of 44,000. In a modern shop, such a furnace would be expected to produce 80 metric tons of liquid steel approximately 50 minutes from cold scrap loading to release.

By comparison, basic oxygen furnaces can have a capacity of 150–300 tons per batch, or "heat up" and produce heat for 30–40 minutes. There are huge differences in the details of furnace design and operation, depending on the end product and local conditions, as well as the research being done to improve the efficiency of the installation.

The largest, scrap-only (in terms of tap mass and transformer power rating), is a DC unit exported from Japan, with a tap mass of 420 metric tons and fed by eight 32 MVA transformers for a total power of 256 MVA.

To produce a ton of steel in an electric arc furnace it takes approximately 400 kilowatt-hours short, or about 440 kWh metric. The theoretical minimum amount of energy required to melt scrap steel is 300 kWh (melting point 1520 °C / 2768 °F). Therefore, a 300 tonne 300 MVA EAF will require approximately 132 MWh of energy and have an on-time of approximately 37 minutes.

Steel production using an electric arc is only economically viable if there is sufficient electricity available with a well-developed network. In many locations, mills operate during off-peak hours when utilities have excess production capacity and meter prices are lower.

Types of melting furnaces

Metal smelting devices are classified into the following types:

• Gas ovens. The coolant is highly heated air or flue gas.
• Induction furnaces. Melting of metal is carried out thanks to internal heat, which is generated from the work of eddy currents.
• Arc electric. Melting occurs from a high degree of electric arc.
• Muffle furnaces. The central element is the muffle.

Operation

An arc steel furnace pours steel into a small ladle machine. Scrap metal is delivered to a pit located next to the smelting shop. Scrap generally comes in two main grades: scrap (white goods, cars and other items made from similar lightweight steel) and heavy melt (large slabs and beams), as well as some direct reduced iron (DRI) or chemical grade cast iron. balance. Individual furnaces melt at almost 100% DRI.

Next stage

The scrap is loaded into large buckets called baskets with clamshell doors for the base. It is necessary to ensure that the scrap is in the basket to ensure good operation of the oven. A strong melt is laid on top with a light layer of protective shred, on top of which another part is laid. All of them must be present in the oven after loading. At this time, the basket can go into a scrap preheater, which uses the plant's hot, waste gases to melt and recover energy, increasing efficiency.

Which melting furnace to choose and where to buy it

When choosing such equipment, you need to focus on exactly what tasks you plan to perform with its help.

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Today you can purchase equipment for the metallurgical industry from major suppliers or directly from manufacturers. As a rule, both of them offer delivery, commissioning, maintenance, repairs and other services.

Overflow

The vessel is then taken to the smelting shop, the furnace roof is opened and the material is loaded into it. Overfilling is one of the most dangerous operations for operators. A lot of potential energy is released by tons of falling metal. Any liquid substance in the furnace is often forced upward and outward by the solid scrap and grease. Dust on the metal will ignite if the furnace is hot, causing a fireball to ignite.

In some double-shell devices, scrap is loaded into the second while the first is melting, and is preheated by the exhaust gas from the active part. Other operations include continuous loading and temperature control on a conveyor belt, which then discharges the metal into the furnace itself. Other devices may be loaded with hot material from other operations.

Advantages and disadvantages

The use of electric arc furnaces for steel smelting is widely used in the metallurgical industry. The main advantages of using such equipment are the ability to carry out the following operations:

• melting the charge regardless of its composition,
• rapid heating of metal in a furnace,
• temperature control,
• deoxidation of the metal and resulting in a material with a low content of impurities.

When remelting steel in a furnace unit, all conditions are created to reduce the waste of alloying components. This ensures that metal losses due to oxidation at high temperatures are reduced.

Electric arc units are widely used for industrial purposes for melting various metals. With their help you can obtain high-quality strong steel alloys. The efficiency of an arc furnace largely depends on the quality of the heating device. Therefore, you should purchase reliable equipment from well-known and trusted manufacturers.

Voltage

After charging, the roof folds back over the furnace and melting begins. The electrodes are dropped onto the scrap metal, an arc is created, and they are then set so that they spread into a layer of crumbs at the top of the device. Low voltages are chosen for this operation to protect the roof and walls from excessive heat and arc damage.

Once the electrodes have reached the heavy melt at the base of the furnace and the waves are shielded with a crowbar, the voltage can be increased and the electrodes raised slightly, extending and increasing the power for the melt. This allows the molten pool to form more quickly, reducing tapping time.

Oxygen is blown into the scrap metal, burning or cutting the steel, and additional chemical heat is provided by wall burners. Both processes accelerate the melting of the substance. Supersonic nozzles allow oxygen jets to penetrate the foaming slag and reach the liquid bath.

Oxidation of impurities

An important part of steelmaking is the formation of slag, which floats on the surface of the molten steel. It is typically composed of metal oxides and also acts as a collection site for oxidized impurities, as a thermal blanket (stopping excessive heat loss), and also helps reduce erosion of the refractory lining.

For a basic refractory furnace producing carbon steel, common slag formers are calcium oxide (CaO in the form of burnt lime) and magnesium oxide (MgO in the form of dolomite and magnesite.). These substances are either loaded with scrap or blown into the furnace during melting.

Another important component is iron oxide, which is formed when steel is burned with introduced oxygen. Later, when heated, carbon (in the form of coal) is injected into this layer, reacting with iron oxide to form metal and carbon monoxide. This causes the slag to foam, resulting in greater thermal efficiency. The coating prevents damage to the roof and sides of the stove from radiant heat.

Electric arc furnace design

Regardless of design features, all arc furnaces are designed almost identically. Thermal steelmaking units consist of the following basic elements:

• mechanical device;
• electrical department;
• automated system control;
• device for feeding materials into the working part;
• the container in which the melting is carried out;
• waste disposal system;
• gas purification.

The cylindrical body of the furnace includes detachable parts - a casing and a bottom. The frame is highly resistant to significant temperature influences.

The design has holders in which graphite electrodes are installed. Electricity supply cables are connected to them. During operation of the furnace, a constant arc is formed between the electrodes. Thanks to it, a temperature arises in the device, which ensures the melting of metals.

What does an electric arc furnace look like?

Devices designed for automatic control of the entire system are built into the closed body of the furnace structure. The melting process is controlled using doors. There are several cavities in the frame to remove waste. Through them, various additives are also introduced to adjust the composition of the metal.

The charge can be loaded into the furnace through the working window or from above. Devices that feed material through a special opening are usually small in size. It is fashionable to load scrap metal into such units manually using a wide shovel.

Furnaces with top charge feed are more powerful and larger devices. They have a rather complex design. The mechanism of the device can be of three types:

• rotary vault;
• roll-out housing;
• rollable vault.

The most common are arc units with a rotating mechanism.

Burning impurities

Once the scrap metal is completely melted and a flat pool has been achieved, another bucket can be loaded into the furnace. After the second charge is completely melted, refining operations are carried out to check and adjust the chemical composition of the steel and to superheat the melt above its freezing point in preparation for tapping. More slag formers are introduced and a lot of oxygen enters the bath, burning impurities such as silicon, sulfur, phosphorus, aluminum, manganese and calcium and removing their oxides into the slag.

Carbon harvesting occurs after these elements burn out first, as they are more similar to oxygen. Metals that have a lower affinity than iron, such as nickel and copper, cannot be removed by oxidation and must be controlled only by chemistry. This is, for example, the introduction of direct reduced iron and cast iron, mentioned earlier.

Foamy slag persists everywhere and often overflows the furnace to spill out of the door into the designated pit. Temperature measurements and chemical sampling are carried out using automatic copies. Oxygen and carbon can be measured mechanically with special probes that are immersed in the steel.

Induction oven

Induction casting furnaces, also called crucible furnaces, like arc furnaces, operate on electricity, but have a different operating principle. Induction furnaces involve a transformer with 2 windings. If electricity is passed through the first winding, it forms an electric field that provokes eddy currents in the 2nd winding. That is why, in place of the secondary winding, a crucible chamber filled with raw materials that absorb eddy currents is placed. Heat regulation depends on the power of the electrical current consumed by the device, that is, the more energy the structure consumes, the more powerful the eddy currents are formed.

Unlike arc furnaces, induction furnaces are not able to show the expected results without additional expensive electrical equipment. However, the advantages of an induction furnace are much greater than those of an arc furnace:

• It is easy to operate.
• Heats up quickly.
• Evenly distributes the temperature inside the chamber, since powerful heat-resistant fans are built into the oven.

It is also important that these stoves do not affect the cleanliness of the environment, since they do not emit gases and dust into the air. Thanks to this, it is allowed to use them, even in factories located inside the city. The only drawback of the device is that induction furnaces are less spacious than arc furnaces, so they melt 2-3 times less raw material per day.

Benefits for production

The use of a control system for arc steel-smelting furnaces makes it possible to produce steel from 100% raw materials - scrap metal. This significantly reduces the energy required to produce the substance compared to primary production from ores.

Another advantage is flexibility: while blast furnaces cannot vary their operation significantly and can operate for several years, this example can be started and stopped quickly. This allows the steel mill to vary production based on demand.

A typical arc steel furnace provides the steel source for a mini mill, which can produce either a bar or strip product. Mini-mills can be located relatively close to steel markets, and transportation requirements are less than for an integrated plant, which is typically located near shore for access to shipping.

Melting gold and silver

A gold smelting furnace is easy to make at home. It is also applicable for smelting silver.

The operating procedure is as follows:

• A fireclay brick is taken and cut into 2 parts. Using a Pobedit drill with a diameter of 48 mm, a through hole is made in one half, in the middle. And in the second hole is drilled to half the height.
• A spiral is drawn through the hole and both halves are tightened with bolts, holes for which are drilled from the sides.
• A graphite crucible is installed on top.
• A metal frame is made and both halves are inserted into it.
• All side gaps are covered with clay.
• Voltage is applied to the removed ends of the spiral.
• Pieces of gold or silver are thrown into the crucible.
• During the heating process, the non-ferrous metal melts.

Making furnaces for melting metal with your own hands is a complex process, but doable. To do this, you need to study the characteristics of the types of equipment. Decide which one is most preferable to the given conditions. Manufacturing costs will quickly pay for themselves.

At large enterprises of the Vtorchermet system, it is economically profitable to carry out not only the primary processing of scrap metal, but also its melting with subsequent rolling of the resulting semi-finished products - ingots. This increases the profitability of the operation of such enterprises, since the operating costs of purchasing equipment are recouped by the quick sale of products. Subject to compliance with technical production standards, it turns out to be of almost the same quality as steel produced at specialized metallurgical plants. The first link in this chain is furnaces for smelting metal from scrap metal.

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