How an electric arc furnace work?

Electric Arc Furnace

     Arc furnace provides a simple and effective way of melting various grades of scrap and then going ahead to refine the metal to desired specification. It is also useful in making all kinds of steels including tool steels and alloy steels. This provides a method of utilising low cost scrap, which is available in abundance.The key advantage in EAF melting is that refining is possible and you can also produce low carbon steels. An electric arc furnace is mainly used for making steel and consist of devices like refractory-lined vessel and electrodes. Electrodes are normally round in section and comes in segments with threaded couplings, so that as the electrodes wear, new segments can be added. The arc forms between the charged material and the electrode. The charge so formed is heated both by current passing through the charge and by the radiant energy evolved by the arc.Through automatic positioning system electrodes are raised and lowered. For positioning electric winch hoists or hydraulic cylinders are used. The regulating system maintains an approximately constant current and power input during the melting of the charge, even though scrap may move under the electrodes while it melts. The mast arms holding the electrodes are used to convey the current to the electrode holders. The transformer is installed in a vault to protect it from the heat of the furnace.

   The refractory lined vessel having a removable roof is separated from the electrical system. The bottom of the furnace, is lined with refractory bricks and granular refractory material. There is a tilting platform on which the furnace is built so that the liquid steel can be poured into another vessel for transport in the steel making process. To prevent the liquid steel from the contaminants like nitrogen and slag modern furnaces have a bottom tap-hole on the spout. In some of the latest plant, scrap pre-heating is applied with different method to decrease the electric consumption and to increase the productivity.

Operation

   Scrap metal is delivered to a scrap bay, located next to the melt shop. The furnace is filled with the scrap. After putting the scrap inside furnace the roof again cover the top of the furnace where the melt down goes on. The electrodes are lowered onto the scrap, an arc is struck and the electrodes are then pushed into the layer of shred at the top of the furnace. Voltage selected for this level of operation is usually small. Low voltages protect the roof and walls from excessive heat and damage from the arcs.

  After reaching the base of the furnace and the electrodes can be raised slightly, thereby increasing the length of the arcs and increasing power to the melt. This helps in the formation of molten pool even more rapidly. Modern furnaces are designed with some additional features. In this oxygen is pushed into the scrap. sometimes chemical heat is provided by wall-mounted oxy-fuel burners. Both processes accelerate scrap meltdown. The formation of slag is an important part of steel making which floats on the surface of the molten steel. Slag not only acts as thermal blanket but also reduce erosion of the refractory lining. For a furnace with basic refractories causes the slag to foam, allowing greater thermal efficiency, and better arc stability and electrical efficiency. Once srap has been completely melted down, often another bucket of scrap is charged into the furnace and melted down. After the second charge is completely melted, refining operations take place to check and correct the steel chemistry and superheat the melt above its freezing temperature in preparation for tapping. Once the temperature and chemistry are

correct, the steel is tapped out into a preheated ladle through tilting the furnace.

     Alternating current furnaces have three moving graphite electrodes. Heating and melting of metal is enabled by radiant energy of the arc burning between the electrodes and metal, and the temperature in the arc zone reaches 4000 °С. The uniform burning of arc is regulated by means of moving of current-carrying electrodes transversely to the surface of the melt. It is possible to regulate the radiant energy by stretching and contracting the arc with two

moving electrodes.

Principles of control

    Control of electrode movement is, in its turn, based on maintaining of constant level of the burning electric arc external impedance. During the previous decade they have undergone a drastic change due to the uprise of the new computing techniques generation in the industry such as controllers and digital controlled electric drives. The voltage of furnace power transformer may vary in steps via the tap changing device. But, as a rule, the transformer taps change is not applied in small furnaces during the technological process. The problem of electrode failure at arc ignition in up-to-date electric drives is easily solved by adjusting current limiting set points in the electrode movement controlled by hydraulic cylinders, and the overcharge alarm is transmitted to the master controller to ensure the appropriate system response. With the help of modern automation equipment it becomes possible to integrate the control system into a top-level network for organization of recording systems of different types,

control or document management systems.The use of EAFs allows steel to be made from a 100% scrap metal feedstock, commonly known as 'cold ferrous feed' to emphasise the fact that for an EAF, scrap is a regulated feed

material. The primary benefit of this is the large reduction in specific energy (energy per unit weight) required to produce the steel.

    EAFs can be rapidly started and stopped, allowing the steel mill to vary production according to demand. Although steelmaking arc furnaces generally use scrap steel as their primary feedstock, if hot metal from a blast furnace or direct-reduced iron is available economically, these can also be used as furnace feed.

Applications

Electric Arc Furnace has following applications:

· Electric arc furnace produces many grades of steel.

· Concrete reinforcing bars common merchant-quality standard channels, bars, and

flats.

· Special bar quality grades used for the automotive and oil industry.

· A typical steel making arc furnace is the source of steel for a mini-mill, which may

make bars or strip product.