Presentation on Electric Arc Furnace (A)

The electric arc furnace is an electric furnace that uses high temperature produced by electrode arc to smelt ore and metal. When the arc is formed by gas discharge, the energy is concentrated and the temperature of the arc zone is above 3000 ℃. For smelting metals, EAF is more flexible than other steelmaking furnaces, which can effectively remove sulfur, phosphorus and other impurities. The furnace temperature is easy to control, and the equipment covers a small area. It is suitable for smelting high-quality alloy steel.

Introduction

Through the metal electrode or nonmetal electrode to produce electric arc heating industrial furnace is called electric arc furnace. According to the form of arc, EAF can be divided into three-phase EAF, consumable EAF, single-phase EAF and resistance EAF. The furnace body of EAF is composed of furnace cover, furnace door, tapping tank and furnace body. The furnace bottom and wall are built with basic refractory or acid refractory.

According to the transformer capacity per ton of furnace capacity, EAF can be divided into ordinary power EAF, high power EAF and ultra-high power EAF. EAF steelmaking is to input electric energy into EAF through graphite electrode, and take the arc between electrode end and charge as heat source for steelmaking. Electric energy is used as the heat source in EAF, and the atmosphere in EAF can be adjusted, which is very beneficial to the smelting of steel grades with more oxidizable elements.

Shortly after the invention of EAF steelmaking, it was used to smelt alloy steel and developed greatly. With the improvement of EAF equipment and smelting technology, the development of electric power industry, the cost of EAF steelmaking is decreasing. Now EAF steelmaking is not only used for the production of alloy steel, but also for the production of ordinary carbon steel. The proportion of its output in the total steel output of major industrial countries is rising.

Development History

In 1888-1892, p.l.t.h é roult developed coal instead of energy by using a high temperature of electrode arc, and invented an arc furnace for direct smelting. At first, the EAF was only used for the production of calcium carbide and ferroalloy, and it was developed to make steel in 1906, and thus the scrap steel could be recycled economically and in scale. The electric energy is converted into heat energy by the arc between the end of the graphite electrode and the charge, and then the furnace charge is melted and the subsequent high-temperature metallurgical reaction is completed. Because it uses electric energy and is easy to adjust the atmosphere in the furnace, it can smelt various kinds of alloy steel including the oxidizable elements.

With the development of power industry, the continuous improvement of process equipment and the improvement of smelting technology, arc furnace is widely used and its production capacity and scale are increasing. The maximum capacity of EAF was 100t in 1930s, 200t in 1950s and 400t in the early 1970s. Especially in the past 50 years, the technical performance of EAF has been gradually improved, and the production cost has decreased significantly. The proportion of EAF steel in developed countries in Europe and America has exceeded 50%. The development of modern EAF smelting technology advances with the times. 

In the 1960s-1970s, the development of ultra-high power supply and related technologies is mainly carried out. HP and UHP are compared with the general power arc furnace (RP). They are mainly divided according to the capacity of transformer per ton of furnace capacity, and there is a trend of higher and higher in recent years. This means that the heat energy of the electric arc furnace is greatly increased in unit time, which makes the melting time significantly shortened, thus improving the production capacity, reducing electrode consumption, reducing heat loss and reducing the power consumption. The result is that the production capacity is increased again, and the cost also decreases greatly. High voltage long arc operation, water-cooled furnace wall, water-cooled furnace cover, foam slag technology, and external heat source melting have been widely used in conjunction with ultra-high power electric arc furnace. Ladle refining and intensified oxygen utilization have also been adopted.

In the 1980s, the development of LF and EBT technology made the modern EAF steelmaking process of EAF smelting and refining outside the furnace mature. It is worth noting that since then, people have paid more attention to the use of secondary combustion and flue gas sensible heat, namely, the preheating of scrap steel. Different preheating methods of scrap produce different types of modern EAFs, including ordinary arc furnaces preheating with basket scrap, flue shaft furnace with supporting claws, double shell arc furnace and Consteel arc furnace. At present, the equipment and production technology of EAF are still in the process of continuous development.

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Electric furnace is the abbreviation of an electric arc furnace. It is a kind of process equipment with scrap as the main raw material, some molten iron or pig iron as the additional part, electricity as the main energy and chemical energy as the auxiliary energy for smelting. Because there is no coking, sintering and iron making process in the process flow, it is also called a short process, which is different from the long process of converter smelting.

Due to the characteristics of low greenhouse gas emission and low energy consumption, the short process has been widely concerned by metallurgical workers and has experienced rapid development in the historical development process. At present, the annual output of converter steel accounts for about 2 / 3 of the world's steel output, and that of electric furnace steel accounts for about 1 / 3 of the world's steel output.

Classification of electric furnaces
According to the power supply mode, the electric furnace can be divided into DC electric furnace and AC electric furnace.

Direct current furnace

DC electric furnace uses DC power as the main energy to provide heat source for the furnace. Generally, there is only one anode on the top of the furnace, forming a circuit with the cathode at the bottom of the furnace. It is characterized by very low electrode consumption, generally less than 1kg / ton steel, and stable molten pool. Because only one electrode and molten pool form arc column, the radiation damage to furnace lining is low, the consumption of refractory materials is relatively low, and the damage of water-cooled plate on the furnace wall is relatively small. The disadvantage is that the bottom electrode is relatively complex, there are several forms of water-cooled, conductive bottom type and needle-like distribution in the bottom refractory, but the service life is relatively short, the maintenance cost is high, resulting in high smelting cost. In addition, the rectifier equipment investment is relatively high, resulting in higher one-time investment.

The direct current electric furnace is not the main equipment in the electric furnace plant. In recent years, new progress has been made in the research of the bottom electrode, which basically solves the problem of short bottom life of the bottom electrode furnace. IThe low consumption of the graphite electrode shows a great advantage in smelting cost. It is expected that the DC electric furnace will be put into production in the new round of electric furnace construction.

Alternating current electric arc furnace

The three-phase alternating current furnace has been used in industrial production for more than 100 years. Its equipment is characterized by the use of three-phase alternating current power supply. When the three-phase graphite electrode contacts with scrap steel, an arc is generated under the action of huge current. The high temperature of the arc is used to melt scrap steel and molten steel. Compared with the direct current furnace, because there are three-phase electrodes, the electrode consumption is higher than that of the direct current furnace. The three-phase electrode has a polar circle, which is relatively close to the furnace wall, and the refractory consumption is relatively high, but the furnace bottom is relatively safer and has a long service life. 

At the beginning of this century, the EAF steelmaking technology has made great progress. On the basis of electric energy as the main energy, auxiliary energy and chemical energy have been introduced. With the development of automatic control technology, power input has increased from ordinary power to high power and ultra-high power, consumption has decreased and productivity has increased. In the case of all scrap steel as raw material, the smelting cycle of this furnace is less than 60 minutes, the power consumption is less than 280kwh / ton steel, the electrode consumption is less than 1.2kg/ton steel, and the annual steel output per nominal capacity has exceeded 10000 tons (i.e. the furnace with a nominal capacity of 100 tons, with an annual steel output of more than 1 million tons), the productivity is greatly improved, and at the same time, the process changes have also derived a variety of furnace types. 

  

 
Common type 3 AC electric furnace

Traditional electric furnace
The traditional electric furnace is the early mainstream electric arc furnace. Scrap steel is added from the top of the furnace, that is, the crown block basket is used to open the furnace cover and scrap steel is added. Each furnace needs to be charged twice to three times or more. The power consumption is relatively high, the electrode consumption is high, the noise is large when melting, the amount of flue gas at the moment of charging is large, the overflow is very obvious, the heat loss is large, the capacity of transformer is large, and the impact on the power grid is relatively large. Generally, mining In addition to the three-electrode holes, the fourth hole of the furnace cover is used for dust removal. The dust removal effect is relatively poor. In areas with strict dust and noise requirements, the way of increasing smoke hood for dust removal or building isolation hood, commonly known as doghouse, is used for secondary dust collection and noise collection. The early electric furnaces in China belong to the traditional type. However, the furnace technology is relatively mature, low failure rate, wide application, strong adaptability of raw materials, but the main technical and economic indicators are general.

Continuous charge furnace

In recent years, the continuous charging type electric furnace is one of the more furnace types newly put into operation. The biggest feature is that the scrap steel is continuously added in the smelting process, the furnace cover can be kept open, the energy input can be kept uninterrupted, the huge energy loss can be avoided, and the smoke and dust can not overflow. The high-temperature flue gas extracted from the furnace passes through the continuous charging tunnel, which also plays the role of preheating the scrap steel The consumption can be reduced by 30kwh-100kwh, and the molten pool is relatively stable, which reduces the huge noise and impact on the power grid produced by the traditional electric furnace during the electrode well penetration period, and the electrode consumption per ton of steel can be reduced by 0.1kg-0.3kg. In addition, in the smelting process, the molten pool is stable, the preheating temperature of scrap is generally not more than 400 ℃, the smelting cycle is shortened, the productivity is improved, the main technical and economic indicators are superior to the traditional electric furnace, but the one-time investment is higher than the traditional electric furnace.

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How Does Electric Arc Furnace Work?

1. Feeding
The operation of adding raw materials such as molten iron or scrap steel to the electric furnace is the first step of the steelmaking operation of the electric arc furnace.

2. Slagging
Slag making is an operation to adjust the composition, basicity, viscosity and reaction ability of slag in steel and iron production. For example, the purpose of oxygen blowing operation is to generate slag with enough fluidity and basicity, which can transfer enough oxygen to the metal surface, so as to reduce the sulfur and phosphorus below the upper limit of planned steel grade, and minimize the amount of splashing and slag overflowing during oxygen blowing.

3. Slag out
According to different smelting conditions and purposes, the operation of slag discharging or raking is adopted in the process of EAF steelmaking. For example, when smelting with the single slag method, the oxidation slag must be removed at the end of oxidation; when reducing slag is made with the double slag method, the original oxidation slag must be completely discharged to prevent phosphorus return, etc.

4. Pool stirring
Energy is supplied to the molten metal pool to make the molten metal and slag move, so as to improve the dynamic conditions of the metallurgical reaction. Pool stirring can be realized by means of gas, mechanical, electromagnetic induction and other methods.

5. Dephosphorization
Chemical reaction to reduce phosphorus content in molten steel. Phosphorus is one of the harmful impurities in the steel. The steel with more phosphorus is easy to crack when it is used at room temperature or lower, which is called "cold brittleness". The higher the carbon content in steel, the more serious the brittleness caused by phosphorus. Generally, the phosphorus content of ordinary steel is not more than 0.045%, and the phosphorus content of high-quality steel is less.

6. Bottom blowing
N2, Ar, CO2, Co, CH4, O2 and other gases are blown into the molten pool of the furnace through the nozzle at the bottom of the furnace according to the process requirements to accelerate the melting and promote the metallurgical reaction process. The bottom blowing process can shorten the smelting time, reduce the power consumption, improve the dephosphorization and desulfurization operation, increase the amount of residual manganese in the steel, and improve the metal and alloy yield. It can make the composition and temperature of molten steel more uniform, so as to improve the quality of steel, reduce the cost and improve productivity.

7、 Melting period
The melting period of steelmaking is mainly for open-hearth and electric furnace steelmaking. The melting period of EAF steelmaking is from the beginning of power on to the end of the melting of all the trace materials in the furnace, and from the end of molten iron blending to the end of the melting of all the materials in the open hearth steelmaking. The task of the melting period is to melt and heat up the charge as soon as possible, and make the slag in the melting period.

8. Oxidation and decarbonization
The oxidation period of EAF steel-making usually refers to the process stage from the melting and cleaning of furnace burden, sampling and analysis to the completion of slag removal. It is also believed that it starts from blowing oxygen or adding ore to decarbonize. The main tasks of the oxidation period are to oxidize the carbon and phosphorus in the molten steel, to remove the gas and inclusions, and to heat the molten steel uniformly. Decarburization is an important process in the oxidation stage. In order to ensure the purity of steel, the amount of decarburization is required to be more than 0.2%. With the development of off furnace refining technology, the oxidation refining of EAF mostly moves to ladle or refining furnace.

9. Refining period
In the process of steelmaking, some elements and compounds harmful to the quality of steel are selected into the gas phase or discharged or floated into the slag through chemical reaction through slagging and other methods, so that they can be excluded from the molten steel.

10. Reduction period
In the operation of EAF, the period from the end of oxidation to the end of tapping is usually called reduction period. Its main task is to make reducing slag for diffusion, deoxidization, desulfurization, chemical composition control and temperature adjustment. The reduction period has been canceled in high power and superpower EAF steelmaking operations.

11. Off furnace refining
The steelmaking process in which the primary molten steel in a steelmaking furnace (converter, electric furnace, etc.) is transferred to another vessel for refining, also called secondary metallurgy. Therefore, the steelmaking process is divided into two steps: primary refining and refining. Primary smelting: the charge is melted, dephosphorized, decarburized and main alloyed in an oxidizing atmosphere. Refining: degassing, deoxidizing, desulfuration, removal of inclusions and fine-tuning of the composition of primary molten steel in vacuum, inert gas or reducing atmosphere. The advantages of steelmaking in two steps are: improving the quality of steel, shortening the smelting time in steelmaking workshop, simplifying the process and reducing the production cost. There are many kinds of off furnace refining, which can be roughly divided into two categories: off furnace refining under atmospheric pressure and off furnace refining under vacuum. According to the different treatment methods, it can be divided into ladle treatment and ladle refining.

12. Liquid steel mixing
The stirring of molten steel in the refining process outside the furnace. It can homogenize the composition and temperature of molten steel and promote the metallurgical reaction. Most metallurgical reactions are phase interface reactions, and the diffusion rate of reactants and products is the limiting link of these reactions. In the static state, the metallurgical reaction speed of molten steel is very slow, for example, it takes 30-60 minutes to desulfurize the static molten steel in the electric furnace, while it only takes 3-5 minutes to desulfurize the molten steel by stirring the molten steel in the furnace refining. When the molten steel is in static state, the removal rate of inclusions floats upward and is slow; when the molten steel is stirred, the removal rate of inclusions increases exponentially and is related to the stirring strength, type, characteristics and concentration of inclusions.

13. Ladle wire feeding
By feeding deoxidizing, desulfurizing and fine-tuning powder wrapped with iron sheet, such as Ca Si powder, or directly feeding aluminum wire, carbon wire, etc. into the ladle through wire feeder, the method of deep desulfurizing, calcium treatment and fine-tuning of carbon and aluminum in the steel can be carried out. It also has the functions of cleaning molten steel and improving the morphology of non-metallic inclusions.

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