Arc steelmaking furnace (DSP) is a device that heats the material using electric bending.
Industrial appliances range in size from small units, about one tonne of capacity (used in foundries for the manufacture of cast iron products) to 400 units per tonne used for steel recycling. Arc steel furnaces, chipboards used in research laboratories, can have a capacity of only a few tens of grams. Industrial devices can reach temperatures of up to 1800 ° C (3272 ° F), while laboratory settings exceed 3000 ° C (5432 ° F).
Arc steelmaking furnaces (DSP) differ from induction furnaces in that the feed material is directly exposed to electric bending, and the current at the terminals passes through the charged material.
Construction
An arc steelmaking furnace is used to produce steel and consists of a refractory vessel. It is mainly divided into three sections:
- The shell, which consists of the side walls and the lower steel of the "bowl".
- A pallet that consists of refractory material.
- Roof. It can be heat-resistant lining or water-cooled. And also made in the form of a ball or a truncated cone (conical section). The roof also supports a refractory delta in its center, through which one or more graphite electrodes enter.
Individual items
The hearth may have a hemispherical shape and is necessary in an eccentric furnace to tap the bottom. In modern workshops, the steel-arc furnace - chipboard 5 - often rises above the first floor, so that buckets and slag pots can be easily maneuvered at any end. Apart from the structure, there is an electrode support and an electrical system, as well as an inclined platform on which the tool stands.
Unique tool
A typical DSP 3 steelmaking arc 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.
An arc is formed between the charged material and the electrode. The charge is heated by both the current passing through it and the radiated energy released by the wave. The temperature reaches about 3000 ° C (5000 ° F), as a result of which the lower sections of the electrodes glow with incandescent lamps when the arc steel furnace is operating.
Elements are automatically raised and lowered by a positioning system that can use any electric hoist, hoists or hydraulic cylinders. Regulation maintains approximately constant current. And what power is consumed by the steelmaking arc furnace? It is maintained constant during charge melting, even though the scrap can move under the electrodes during melting. The mast sleeves holding the element can either carry heavy tires (which can be water-cooled hollow copper pipes leading current to the clamps) or “hot sleeves” where the entire upper part carries a charge, increasing efficiency.
The latter type can be made of copper-coated steel or aluminum. Large water-cooled cables connect busbars or brackets to a transformer located next to the furnace. A similar tool is installed in the storage and cooled by water.
Tapping and other operations
DSP 50 steelmaking arc furnace is built on an inclined platform, so that liquid steel can be poured into another container for transportation. The tilt operation to transfer molten steel is called tapping. Initially, all the steel smelting arches of the arc furnace had a discharge chute, covered by a refractory, which was washed out when it was tilted.
But often modern equipment has an eccentric lower discharge valve (EBT) to reduce the incorporation of nitrogen and slag into molten steel. In these furnaces there is a hole that extends vertically through the hearth and shell and is offset from the center in a narrow "nose" in the shape of an egg. It is filled with refractory sand.
Modern plants can 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-based furnaces have a similar arrangement, but have electrodes for each shell and one set of electronics.
Oxygen elements
AC furnaces usually have a pattern of hot and cold spots along the perimeter of the focus located between the electrodes. In modern, oxygen-fuel burners are installed in the side wall. They are used to supply chemical energy to the negative zones, which makes the heating of steel more uniform. Additional power is provided by supplying oxygen and carbon to the furnace. Historically, this was done using copies (hollow mild steel pipes) in the slag door, now it is mainly done using wall-mounted injection units that combine oxygen-fuel burners and air supply systems into one vessel.
A medium-sized modern steel furnace has a transformer with a rated capacity of about 60,000,000 volt-amperes (60 MVA), with a secondary voltage of 400 to 900 and a current exceeding 44,000. It is expected that in a modern workshop this furnace will produce 80 metric tons of molten steel approximately 50 minutes from loading cold scrap to release.
For comparison, the main oxygen furnaces can have a capacity of 150–300 tons per batch or “heat” and generate heat for 30–40 minutes. There are huge differences in the details of the design of the furnace and its operation, depending on the final product and local conditions, as well as on ongoing research to increase the efficiency of the installation.
The largest, intended only for scrap (in terms of branch weight and rated power of the transformer), is a DC device exported from Japan with a tap weight of 420 metric tons and powered by eight 32 MVA transformers for a total capacity of 256 MVA.
To produce a ton of steel in an electric arc furnace, approximately 400 kilowatt-hours for a short value or about 440 kWh per metric are required. The theoretical minimum amount of energy required to melt steel scrap is 300 kWh (melting point 1520 ° C / 2768 ° F). Therefore, a 300-ton EAF with a power of 300 MVA will require about 132 MWh of energy, and the turn-on time is approximately 37 minutes.
Steel production using an electric arc is economically viable only if there is a sufficient amount of electricity with a well-developed network. In many places, mills operate during off-peak hours, when utilities have excess production capacity and the price of a meter is lower.
Operation
An electric steel furnace pours steel into a small bucket machine. Scrap metal is delivered to a recess located next to the smelter. Scrap, as a rule, can be of two main types: scrap (white goods, automobiles and other items made of similar lightweight steel) and heavy melt (large slabs and beams), as well as some direct reduced iron (DRI) or cast iron for chemical balance. Individual furnaces melt almost 100% DRI.
Next stage
The scrap is loaded into large buckets, called baskets, with folding beds for the base. Care must be taken to ensure that the scrap is in the basket in order to ensure good operation of the furnace. A strong melt is laid on top with a light layer of protective shred, on top of which another part lies. All must be present in the oven after loading. At this time, the basket can go into the scrap pre-heater, which uses the hot, off-gases of the plant to melt and recover energy, increasing efficiency.
Overflow
Then the vessel is delivered to the smelting shop, the roof of the furnace is tilted and 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 replaced by solid scrap and grease up and out. Dust on the metal ignites if the stove is hot, which causes a fireball to burst.
In some double-shell devices, scrap is loaded into the second, while the first is melted, and preheated with flue gas from the active part. Other operations are: continuous loading and working with temperature on a conveyor belt, which then unloads the metal into the furnace itself. Other devices may be loaded with hot material from other operations.
Voltage
After charging, the roof leans back over the furnace and melting begins. The electrodes are lowered onto the scrap metal, an arc arises, and then they are mounted so that they spread in the crumb layer in the upper part of the device. Low voltages are selected for this operation to protect the roof and walls from excessive heat and damage from arcs.
Once the electrodes have reached a heavy melt at the base of the furnace and the waves are shielded with a crowbar, the voltage can be increased and the electrodes slightly raised, lengthening and increasing the power for the melt. This allows you to more quickly form a molten bath, reducing the time of taps to release.
Oxygen is blown into scrap metal, burns or cuts steel, and additional chemical heat is provided by wall burners. Both processes accelerate the melting of the substance. Ultrasonic nozzles allow oxygen streams to penetrate into foaming slag and reach a liquid bath.
Impurity oxidation
An important part of steelmaking is the formation of slag, which floats on the surface of molten steel. It usually consists of metal oxides, and also acts as a place to collect oxidized impurities, like a thermal blanket (stopping excessive heat loss), and also helps reduce erosion of the refractory lining.
For a furnace with basic refractories producing carbon steel, the usual slag formers are calcium oxide (CaO in the form of calcined lime) and magnesium (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 by burning steel with oxygen introduced. 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 leads to foaming of the slag, which provides greater thermal efficiency. The coating prevents damage to the roof and side walls of the furnace from radiant heat.
Burning impurities
Once the scrap metal has completely melted and a flat bath is reached, 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 overheat the melt above its freezing temperature in preparation for release. 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 in the slag.
Carbon removal occurs after these elements burn out first, as they have a greater resemblance to oxygen. Metals that have a lower affinity than iron, such as nickel and copper, cannot be removed by oxidation and should only be controlled by chemistry. This, for example, the introduction of direct reduction iron and cast iron, mentioned earlier.
Foamy slag remains everywhere and often overflows the furnace to pour out of the door into the intended pit. Temperature measurement and chemical screening are done with automatic copies. Oxygen and carbon can be mechanically measured by special probes that are immersed in steel.
Production Benefits
Using a control system for arc steel-smelting furnaces allows us to produce steel from 100% of raw materials - scrap metal. This significantly reduces the energy required for the production of the substance, compared with the primary manufacture of ores.
Another advantage is flexibility: while blast furnaces can not significantly vary their work and can work for several years, this instance can be quickly started and stopped. This allows the steel mill to vary production depending on demand.
A typical arc steelmaking furnace is the source of steel for a mini-mill that can produce a bar or strip product. Mini-factories can be located relatively close to metal products markets, and transport requirements are less than for an integrated plant, which is usually located near the shore for access to shipment.
Electric arc furnace: device
A schematic cross section is an electrode that rises and falls with a rack and pinion drive. The surface is lined with refractory bricks and the lower lining. A door allows access to the inside of the device. The furnace body rests on the rocker arms so that it can be tilted for tapping.