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Optimization of Electrode Consumption in EAF for Different Operating Conditions

[vc_row][vc_column][vc_column_text]Graphite electrodes in electric arc furnace production account for about 10-20% of processing costs. Past studies rarely considered factors related to electrode consumption and electric furnace operating experience, such as arc stability, foam slag efficiency, electrode water cooling and other factors. In this study, we thoroughly studied the changes in electrode consumption between a single electric furnace and different electric furnaces under different operating conditions, and obtained the best practices for optimizing electrode consumption. This will include the discovery of the “strong factor” of the oxidizing atmosphere in the electric furnace, indicating its important influence on electrode consumption.

Graphite electrode is an indispensable main production consumable material for electric arc furnace or ladle refining furnace. The phenomenon of graphite electrode consumption is very complicated. There are many factors that affect the consumption of graphite electrodes. The complexity is not only the influence factors, but also some factors are interdependent.

The production experience factors between electric arc furnace conditions and electrode consumption have been studied. Most of the derived relations seem to be based on a specific furnace type, which is why most empirical expressions are not necessarily applicable to most electric furnaces.

Various factors affecting electrode consumption in electric arc furnaces are not predictable, because many parameters are actually not measurable, such as the conductivity of the added scrap steel, the cooling efficiency of the electrode spray water, the efficiency of the manually operated oxygen lance, and so on. The conductivity and type of scrap steel are two completely different parameters, but they are more or less of the same importance for electrode consumption. This discussion will clearly show various types of electric furnaces or scrap steel types and operation-related factors, based on the measurement and evaluation of more than 15 electric furnaces using φ550mm and φ600mm electrodes to obtain an empirical relationship. In addition, these results will also create conditions for further intensive and detailed research on this topic and draw more acceptable empirical formulas.

  • Electrode consumption mode

Electrode consumption can be roughly divided into two categories, one is the consumption at the end of the arced electrode rod, and the other is the oxidation consumption at the circumferential surface of the electrode rod.

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  • Electrode tip consumption

1.Arc stability:

A lot of research has been done on this, and there are various methods for calculation. The focus here is on the consumption of the electrode tip. The continuous consumption of the graphite electrode at the tip is achieved by sublimation. The graphite temperature at the tip is very high, and the graphite temperature at the cathode point is about 4000 K. At this temperature, the electrode is directly converted to gas evaporation through evaporation. The model shows that when the current density is 2.9 kA/cm2 and the temperature is 4000 K, the evaporation rate of the graphite electrode cathode point is 0.6 Kg/m2s.

The arc can drift freely on the electrode end face, the current is in the range of 10 kA, and the drift speed is very high-10 to 100 m/s. The arc is deflected at a certain angle under the influence of the magnetic field. The change of the arc direction and the jump of the arc point obviously lead to the change of the arc length, which leads to the change of the voltage. See Figure 2. (Figure II)

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Due to the evaporation of the high temperature spots of the electrode, a crater-like depression will be formed at the end of the electrode. It is quite possible that the electron emission mechanism will help the arc deflect to the edge of the depression instead of staying inside the depression. The arc movement makes the electrode end surface smooth, which is consistent with the observation result.

The scrap terminal effect is another factor that affects the arc length and arc stability. Due to the effect of the magnetic field, the arc is aligned along the axis of the electrode. Any deviation of the arc will result in the appearance of magnetic force from the electrodes, which tends to bring the arc back to collinear, which is suitable for any type of scrap.

Since the distribution of scrap in the furnace is randomly arranged, the graphite electrode axis and the scrap steel axis are rarely collinear. Therefore, most of the time, the arc is subject to conflicting calibration forces from the two end points of the electrode and the scrap steel, resulting in Unstable arc. Plasma jets are formed at both ends and point in different directions. As a result, the jets collide with each other and drag the conduction path to a long twisted path. When the arc length increases with the increase of the curved path, a new shortened arc can be formed on the shortest path between the electrodes. This results in the alternating lengthening and shortening of the arc shown by the sawtooth voltage waveform.

This is why the angle and shape of the electrode tip are very good descriptive indicators, expressing the relationship between the arc offset angle of the electrode axis and the consumption pattern at the electrode end.

2.Tip crack/decomposition:

In addition to the sublimation related to the arc temperature, the crack development at the arc tip is also related to the temperature distribution of the electrode tip, the stability of the arc, and the duration of the arc at this point.

The axial force originates from the convergence phenomenon of the current flowing to the cathode point of the electrode tip, as well as the inward radial force. When the cathode spot, arc length, and arc current change rapidly, the force also changes rapidly. This force change causes the rupture of the graphite structure at the tip and causes cracks. Once a crack occurs, its propagation is much easier due to the rapid change in the magnitude and direction of the force. This may cause the disintegration of graphite, which is already close to the disintegration temperature of the graphite electrode.

However, these electrode losses are not continuous and highly depend on the scrap steel quality/charge quality and arc stability. In the scrap furnace, the crack length at the tip of the electrode is often seen to be about 1.5 times the tip diameter, or smaller cracks are often seen. Sometimes cracks cannot be seen from the surface of the electrode, but cracks can be seen after the electrode is broken and during inspection. For the same reason, cracks appear inside the electrode.

These internal cracks may cause a small part of graphite fragments to fall off the tip without being exposed. This loss mainly occurs when the arc is unstable and the electrode tip surface temperature increases. (Picture 3)

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  • Oxidation of electrode

During the melting of scrap, the temperature distribution of the electrodes in the electric furnace will change. For various electric arc furnace process conditions, the temperature of the electrode is close to the so-called “steady-state temperature” distribution, which represents the highest temperature that can be reached in the electrode during the steelmaking process.

Flue gas produced by electric arc furnace:

The surface of the graphite electrode reacts with the flue gas/ambient gas in the furnace, resulting in oxidation of the graphite electrode. The oxidation rate is highly dependent on:

  • The temperature of the graphite electrode surface.
  • The content of oxygen atoms or molecules in contact with the graphite surface.
  • The flow rate of the flue gas.

Various flue gas analysis shows that the generation of electric furnace flue gas is caused by different reactions in different stages of steel smelting. In the initial melting process, most of the flue gas generated by the arc jet “tearing” the melting charge. The flue dust is initially composed of liquid steel droplets, but depending on the flue gas temperature and atmosphere, it may quickly and rapidly oxidize to Fe2O3 or Fe3O4. According to visual observations and various studies, the densest flue gas is mainly in the melting phase. Scrap containing galvanized steel vaporizes at this stage because the boiling point of zinc is lower than 1000°C.

If a burner or oxygen lance is used, the generated gas reaction will increase the production of smoke. The oxygen in the smoke reacts with the electrode surface. Due to the negative pressure suction of the smoke, the smoke flows through the furnace at a certain rate. , Thereby increasing the oxidation rate of the graphite electrode surface. The real oxidation occurs on the surface of the electrode. The temperature of the electrode tip reaches 2000°C, and the electrode temperature is 800°C at a distance of 2 meters from the tip.

2.Arc temperature and furnace environment:

As far as the ambient temperature in the furnace is concerned, the heat generated in the arc increases the ambient temperature in the entire furnace through radiation or convective heat transfer. In the furnace of an electric furnace, the convective heat exchange between the inside of the electrode and the environment largely depends on the process operation of the electric furnace, and is mainly concentrated in the initial melting period of each basket of scrap. In the continuous production of the electric furnace, each tapping process takes a few minutes, and the ambient temperature in the furnace drops by 50%, so the convective heat exchange time between the electrode and the environment is the time for tapping and processing the tapping port. time. Once 70% of the scrap is melted, the convective heat transfer of the electrodes in the furnace is negligible. In the operation of an electric furnace using hot molten iron, the convective heat transfer ratio of the electrode is even lower. However, regardless of the conditions of the smelting period, the main component of heat transfer to the environment is radiation. To support this, many studies have shown that measuring radiation is a reasonable method to understand the plasma temperature. Since the convective heat transfer loss of the electrode is negligible, the electrical power generated in the arc column is balanced with the radiant power loss. therefore:

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Here:

σ = conductivity,

E = voltage gradient

Prad = radiated power loss.

Since σ and Prad can be expressed as a function of temperature, and the E value can be obtained by experimental observation or analysis by measuring reactance, arc voltage, arc length, etc., the axial temperature gradient of the arc column can be known. The Lowke’s method is used to calculate the radiation of the iron-containing vapor plasma, which is used to simplify the method of estimating the arc axial temperature. Table 1 shows the arc column temperature calculated by previous studies under different iron content and σ range.

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The value of the radiative heat transfer rate can be used to estimate the temperature gradient. At the beginning, it is transmitted from the arc axis along the radial direction. The unit of power transmission is kW/cm, that is, the current and voltage gradient within the radius are equal. Energy transfer Q determines the temperature gradient through emissivity:

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The thermal conductivity is:

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Kabs is the absorption coefficient in cm-1.

When a photon moves across a temperature gradient, energy is transferred in a manner similar to heat conduction. The thermal conductivity Keff is in W/cmK.

The probability P of radiation escape or photon transmission varies with the distance from the axis. Zolloweg’s research shows the following:

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Since K is a constant, it can be assumed that the transmission rate of radiation is proportional to r -0.5, and the relationship between the average probability of escape and the position of free radicals is calculated. Some results indicate that the probability of escape at the boundary is almost 50% higher, so that the total radiation escape can be calculated.

Figure 4 shows the relationship between KA’s relative radiation power and current.

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The calculations provided here are for the arc column far away from the electrode tip, assuming a cylindrical shape. Near the electrode tip where the arc column is too narrow and not cylindrical, the power loss is mainly caused by convection. The heat is transferred to the electrodes by convection. Some measurements performed by Montgomery et al. indicate that radiation accounts for approximately 40% of the arc power. Due to the error of radiation calculation and the lack of knowledge and understanding of iron content, these two factors make it impossible to measure accurate temperature. The voltage gradient can be known from the arc length measurement, and its range is usually between 7 and 9 V/cm, from which the temperature range can be calculated to be between 8,000 K and 10,000 K.

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Flame formed by burner

3.Burner and furnace door gun:

In the case of the same ambient temperature in the furnace, due to the local influence of the burner oxygen and combustion energy, the oxidation of the graphite electrode tip is strengthened.

The burner is designed to heat and cut scrap steel or charge, and is most effective in the melting stage. When the scrap steel is close to the flat bath condition after melting, its efficiency is reduced. The jet formed by the burner may be long enough to approach the electrode and heat the electrode, causing the electrode at a distance of 1 meter from the tip to overheat. Two special characteristics of the burner are crucial to the influence of the electrode: oxygen source and heat source.

In the flat molten pool state after the scrap is melted, when the burner flame or oxygen jet is more likely to approach the electrode, it is to reduce the amount of oxygen blown by the burner. At this time, the oxidation rate of the electrode is 1.5 times the normal oxidation rate.

A similar effect can also be seen on the No. 1 electrode near the furnace door. No. electrode.). In most AC arc furnaces, the average life of the No. 1 electrode is shorter than the other two electrodes. Under certain conditions, the life of the No. 1 electrode is 10% less than the other two electrodes. Such an observation made on a stainless steel electric arc furnace in Korea is shown in Figure 6. Through long-term measurement of the diameter and shape of the electrode tip, the electrode tip consumption and surface oxidation consumption are calculated.

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Compared with No. 2 electrode and No. 3 electrode, the life of No. 1 electrode is always lower. Table 2 calculates the increase in the oxidation loss of the No. 1 electrode of the AC arc furnace. On the No. 1 electrode, an oxygen lance is used in the smelting process, and the electrode consumption increases by about 30%.

electrode No.1 No.2 No.3 total
Average life (number of furnaces used by electrodes 11.33 13.67 14.83 Kg/t
Tip consumption (kg/t) 0.49 0.51 0.48 1.48
Oxidation (kg/t) does not use oxygen lance 0.35 0.32 0.32 0.99
Oxidation using oxygen lance (kg/t) 0.48 0.36 0.34 1.18

Effect on oxidation loss near the oxygen lance electrode

The surface of the electrode is directly oxidized due to the combustion of the flame or oxygen blowing from the oxygen lance, and the temperature of the electrode surface rises sharply due to the joint action of the Joule heat inside the electrode and the heat transfer heat of the arc heat. Therefore, the graphite electrode tip reacts with oxygen in the surrounding air more quickly than before to form carbon oxide.

This reaction is called combustion, and it is not only limited to the circumferential surface of the cylindrical electrode, but also in the entire electrode hole.

The internal combustion of the electrode will damage the structure of the electrode, which will loosen and damage the small fibrous structure.

  • In conclusion

Although electrode consumption is related to many parameters, not all parameters need to be calculated using empirical relationships. This is because some parameters are not independent, but interdependent, and selecting one parameter will also represent the other parameters. In this way, through extensive observation and understanding of the performance of the electric furnace electrode, it can be seen that compared with the actual consumption of the new ultra-high power furnace, the relationship between the current-related electrode tip consumption is not satisfactory, and the early model only considers the current As the reason for the tip consumption, this needs to be improved.

Figure 7 shows the regression relationship between the electrode tip consumption rate (kg/hour) and the ratio I 2/P (square of current/power).

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The measurement was done in 20 electric furnaces using φ550mm and φ600mm electrodes. In these electric furnaces, there is a good correspondence between I 2/P and electrode tip consumption per hour. However, regardless of the influence of the electrode diameter, in most cases, the current density of the average electrode tip diameter is between 30 and 35 A/cm2. Using this model to calculate the tip consumption can get an empirical relationship:

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Here

Rtip = electrode tip consumption rate,

I 2 = the square of the current in kA,

P = active power (MW),

ID = current density (ampere/cm2)

IProxy = the best reference current considering proximity effect and skin effect under the condition of 5% harmonic distortion THD. Furnace coefficient, Sf calculation formula is:

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Here:

The index x ranges from 0.2 to 0.6, depending on the electrode tip angle and the type of scrap.

fI = arc stability factor.

To calculate the electrode tip consumption based on the produced steel kg/t, you can use the standard formula to calculate:

Among them, POT = power-on time.

There is also a good correlation between electrode tip consumption rate and I 2/P. In some cases, stainless steel electric arc furnaces have high harmonics, and transformers are also a problem. Compared with similar electric arc furnaces, the electrode consumption rate is higher. This shows that the harmonic content will affect the skin effect and the actual current density, resulting in an abnormally high electrode tip consumption rate. In the presence of harmonics, near the polar center circle (PCD), the proximity effect reduces the current carrying capacity of the electrode tip. Therefore, considering the harmonic distortion THD of approximately 5% in any furnace, the proximity effect is taken into account in the formula, and the arc stability factor F1 is calculated from the standard deviation of the current and the power factor.

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According to the empirical formula proposed here, it is possible to calculate the optimal current of the required power to obtain the minimum electrode consumption.

The oxidation mode is prone to great changes, depending on the impact of oxygen from the burner. The closer the electrode is to the burner or oxygen lance, the more violent the surface oxidation consumption of the electrode rises. The distance between the burner flame and the electrode cannot be accurately measured. This is because the oxygen flow and flame of the burner are blocked, and the jet direction itself changes greatly. Assuming that the airflow direction and channel of the burner and the oxygen lance are not obstructed or blocked, consider the role of the burner and the oxygen lance.

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Calculation of side oxidation rate Rside:

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Here

V = the volume of the red hot truncated cone at the electrode end,

V’= truncated cone volume-the volume consumed per hour.

V’is based on the length loss and diameter loss ∆D caused by electrode tip consumption.

here ∆D = average electrode tip diameter reduction, the calculation formula is

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Here

ID = current density (ampere/cm2),

IProxy = the best reference current considering proximity effect and skin effect under the condition of 5% harmonic distortion THD,

CB = burner constant,

O2 = single furnace steel oxygen consumption (Nm3/t),

Sd = inner diameter of furnace,

Pcd = Diameter of the polar center circle.

In a 90-ton electric arc furnace producing stainless steel in Korea, under the following operating conditions, the electrode tip consumption and sidewall oxidation were calculated, and a distance of 600mm was verified. In the case of keeping other parameters unchanged, Table 3 shows the consumption changes of the diameter φ600mm electrode and the diameter φ700mm electrode as the current increases.

parameter name Value
Electric furnace diameter 6200mm
Operating current 55KA
Burner constant 0.6
Diameter of polar center circle 1250mm
Oxygen consumption 12Nm3/T
Time from tapping to tapping 55 min

 

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Normal state

The electrode sidewall oxidation consumption (kg/t) is calculated from Rside, TTT and the weight of molten steel tapping.

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Figure 11 The effect of current (kA) change on electrode tip consumption (kg/t)

This subject has conducted an in-depth study on the influence of factors such as wild wind suction speed, furnace ambient temperature, electrode water cooling and other factors. The next step of the research will refine the oxidation electrode consumption equation. The function includes the additional parameters discussed in this article, and the equation is applicable to all types of electric arc furnaces.

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The influence of current (kA) change on the sidewall oxidation (kg/t) of the electrode

 

Translated from the January 2021 issue of “Steel Technology” in the United States.

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