What is an electric arc furnace | Types of electric arc furnaces, Operation of an Electrical Arc Furnace

 

What is an electric arc furnace | Types of electric arc furnaces, Operation of an Electrical Arc Furnace.

 

An electric arc is a flow of electric current in the form of a continuous spark in a circuit. We know that the atmosphere is a poor conductor of electric current. When a high voltage is applied across an air gap, the air gets ionised under electrostatic forces, and it starts acting as a conductor for the flow of current. Thus, the current starts to flow in the air gap in the form of a continuous spark called an arc.

 

In order to strike an arc, high voltage is required, which can be obtained from a step-up transformer. But once the arc is initiated, a normal voltage is sufficient to maintain the arc. Alternatively, an arc can also be produced by short-circuiting two low voltage electrodes momentarily and then moving them back. On doing so, an arc will strike between two electrodes through which electric current flows from one electrode to another.

 

The arc initiated between two electrodes produces heat, and by using different types of electrodes, such as carbon or graphite, temperatures up to 3500 °C can be obtained. Thus, the heating of material can be done by using this principle in electric arc furnaces.

 

 

Electric arc furnace :

 

Typically, arc furnaces are made in a cylinder, but conical furnaces are used because they have a larger surface area per volume, use less power, lose less radiation, and have a lower melting point. The heater's curve is made of a reasonable acidic or fundamentally stubborn coating supported by a metal edge. A clamping mechanism holds the electrodes in place inside the furnace from the top or sides of the chamber. They are positioned so that they are easy to change or adjust. A charge entryway is provided on the heater shell, through which the charge to be warmed is kept and liquid metal is taken out.

 

 

Electrodes Used in Arc Furnace :

 

Conductivity, insolubility, mechanical strength, non-reactiveness, and other characteristics will determine the electrodes used in an electric arc furnace. Most of the time, carbon and graphite electrodes with a diameter of 18–27 cm are used. Carbon electrodes are used to make ferroalloys, aluminium phosphorous, and other materials in small furnaces. Self-baking electrodes are utilised for the electrochemical furnace and electrolytic production of aluminum.

 

 

Types of Electric Arc Furnaces :

 

There are 3 types of electric arc furnaces,

 

1-  Direct electric arc furnace,

2-  Indirect electric arc furnace,

3-  Submerged electric arc furnace.

 

 1-       Direct Electric Arc Furnace :

 

As can be seen in the image below, a direct electric arc furnace produces the arc between the charge to be heated and the electrodes. The charge serves as an additional electrode in this kind of furnace. The arc is created between the two carbon or graphite electrodes and the charge in two different places. Because the arc is created directly with the charge to be heated, this method of heating can achieve high temperatures. Additionally, the charge will generate additional heat as current flows through it.

 

Two electrodes are positioned vertically from the furnace's top into the charge in a single-phase arc furnace. In contrast, in a three-phase arc furnace, the charge itself forms a star point when three electrodes are positioned at the corners of an equivalent triangle to produce three arcs.

 

Additionally, a direct arc furnace's ability to automatically stir the charge as a result of the current's electromagnetic force is one of its most important features. When compared to the cupola method, this type of arc furnace, which is primarily used for steel production, offers advantages.

 

 2-    Indirect electric arc furnace,

 

In a direct arc furnace, we have seen that the arc formed will be in direct contact with the charge to be heated. But in an indirect furnace, the arc formed will be not in direct contact with the charge. In an indirect furnace, the arc is formed between two electrodes near the charge, and heat produced by the arc is transferred to the charge by radiation as shown below.

 

In this furnace, the electrodes are projected through the chamber at each end along the horizontal axis. The arc is formed by bringing the electrodes in contact for a very short time and then withdrawing them back. Since there is no direct contact of the arc with the charge, temperatures obtained in an indirect arc furnace will be lower than that of a direct arc furnace. Also, there is no inherent stirring action in charge due to the absence of current flow and thus furnace must be rocked mechanically.

 

The heat produced in the charge is not only by radiation but also by conduction from the refractory lining during rocking action. By varying arc length i.e., by varying the distance between electrodes, power input to the furnace can be regulated. This type of furnace is usually of single-phase and its size is limited. The applications of indirect electric arc furnaces are melting of non-ferrous metals, iron foundries where there is a need for molten metal, etc.

 

 3-  Submerged electric arc furnace :

In a submerged arc furnace, the electrodes are immersed in the charge as shown in the below figure. This type of furnace is used for the manufacture of ferroalloys such as Fe-Mn, Si-Mn, Fe-Si, Mg-Si and etc. The resistance offered by the charge to the flow of current will generate heat in it. The power to the furnace is controlled by varying the applied voltage or by varying arc length.

 

 

Power Supply and Control of the Arc Furnace :

An electric arc furnace typically receives a power supply with a high current and low voltage.The melting of ferrous and non-ferrous metals is the most common use for the electric arc furnace. This requires a lot of power, which means a lot of current.

The voltage in an electric arc furnace is in the volts range, and the current is in the hundreds of amperes range. Electric arc furnaces use low voltage and high current for the following reasons:

 

-      Since heat is known to be inversely proportional to the square of the current, a high current is utilized. Insulation and safety considerations limit the voltage that can be delivered to the furnace.

 

The refractory lining in the furnace will last longer if low voltage and high current electrodes are used close to the charge but away from the roof.

 

-      Altering the voltage applied or the length of the arc, which can be accomplished by moving the electrodes, can be used to control the power input to the furnace.

 

 

The Operation of an Electrical Arc Furnace

 

The Workings of an Electric Arc Furnace The electric furnace's workings include charging the electrode, melting the metal, and refining. The heavy and light scrap in the large basket is preheated with the help of exhaust gas. Sulfur and burned lime are added to accelerate the formation of slag.

The charging of the heater happens by swinging the top of the heater. If necessary, hot metal charging is also carried out.

The meltdown phase comes next. During this time, the electrodes are lowered onto the scrap. Arcis is then formed between the metal and the electrode. Low voltage is chosen for this because it provides protection. The voltage is increased to accelerate the melting process after the arc is shielded by electrodes. Manganese, silicon, and carbon all undergo oxidation during this process. Large arc production requires a lower current. This also has less heat loss. The melting process can be sped up by thoroughly bathing the electrodes.

During melting, the refining process begins. For the practise of single oxidising slag, the removal of sulphur is not necessary. This only requires the removal of phosphorous. Regardless, in two-fold slag practice, both (S and P) should be removed. Deoxidizing is followed by the removal of oxidizing slag in double slag practice. It is then deoxidized with the assistance of aluminum, ferromanganese, or ferrosilicon. The heat will be deoxidized when the required temperature and bathing chemistry are reached. The molten metal is now ready to be tapped.

Tube pressure panels or hollow annulus spraying can be used to cool the furnace.