DC electric arc furnace's formation and structure

Electric arc furnace is an electric furnace that uses the electric heating of electric arc to smelt metal. In the electric arc furnace, there are one or more electric arcs. By the action of arc discharge, the electric energy is transformed into heat energy to supply the heat required to heat the smelting materials. Due to the high arc temperature, large electrothermal transformation capacity, high electrothermal efficiency and easy control of furnace atmosphere and furnace operation, electric arc furnace is widely used in industry, especially suitable for smelting refractory and advanced materials.

Classification of excentric bottom tapping electric arc furnace

Electric arc furnaces used in industry are divided into three categories. The first type is direct electric arc furnace. In this kind of electric arc furnace, the arc occurs between the electrode and the melted charge, and the charge is heated (directly) by the contact of the arc. This kind of electric arc furnace mainly includes three-phase steelmaking electric arc furnace, DC electric arc furnace and vacuum consumable furnace. The second type is indirect electric arc furnace. In this kind of electric furnace, the arc occurs between two special electrode rods, and the charge is only heated by the heat transfer (indirect) of the arc. This kind of electric arc furnace is mainly used for smelting copper and copper alloy, but it has been replaced by other smelting furnaces because of its disadvantages such as high noise and poor smelting quality. The third type is submerged arc furnace.

Formation and structure of DC electric heat furnace 

Arc is one of the phenomena of gas conduction, which is usually called arc discharge (gas conduction phenomenon is called gas discharge in Physics). There are other phenomena in gas discharge, such as glow discharge and spark discharge. Arc discharge is a self-excited discharge, which is characterized by low voltage (only tens of volts), high current density (up to hundreds of AMPS per square centimeter), continuous release of dazzling arc light, more heat and high temperature.

The electric arc in electric arc furnace is mainly the conduction phenomenon of gas (including metal vapor) excited by electrons emitted by hot cathode. Gas conduction must ionize to produce charged particles, positively charged positive ions, negatively charged electrons or negative ions. The positive electric quantity carried by the gas of electric ions is equal to the negative electric quantity, so it is called plasma, and the arc is also called arc plasma.

In electric arc furnace, gas ionization is generally caused by the following reasons:

1. Collision ionization caused by electrons emitted from the cathode. The hot electrons emitted from the cathode due to high temperature collide with gas molecules on the way to the anode, so that the molecules obtain ionization work and dissociate into positive ions and electrons, that is, neutral molecules become charged particles.

2. Thermal ionization. At high temperature, gas molecules have great kinetic energy. When colliding with each other, the molecules obtain energy and ionize, which is called thermal ionization.

3. Photoionization. The energy irradiated by atomic absorption light may also be excited to the ionized state. But in general, photoionization is not the main way of gas ionization.

4. Gas ionization caused by electric field. Under the action of electric field, charged particles such as positive ions, electrons or negative ions ionized from the gas move to the positive and negative poles respectively, and are accelerated by the electric field force, and the kinetic energy increases. When they collide with gas molecules, the molecules ionize t to generate new charged particles. These new charged particles are also accelerated by the electric field force and move towards the poles respectively. In the process of movement, they collide with gas molecules and cause new ionization. This is called a chain ionization process.

The degree of ionization caused by electric field mainly depends on the intensity of electric field and the free path of particle. The greater the electric field strength, the more work the electric field force does to the charged particle. The greater the kinetic energy of the particle, the higher the ionization degree caused by collision. The free path is also related to the gas density and the volume of the charged particle itself. If the density is thin, the free path is large. Therefore, the ionization degree of gas caused by electric field is high under vacuum. The volume of electrons is much smaller than that of ions, the free path is longer, and it is easier to ionize molecules or atoms. In other words, electrons play a major role in gas ionization caused by electric field.

It should be pointed out that in the arc, on the one hand, there is the ionization process of gas neutral molecules dissociating into positive ions and electrons, on the other hand. There is the recombination process of positive ions capturing electrons and compounding into neutral molecules. Under certain conditions, the two processes will reach dynamic equilibrium and maintain a certain degree of ionization.