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Graphite electrodes are used in both DC and AC electric arc furnaces. Companies who use electric arc furnaces have to cover the cost of these consumable electrodes during production, so optimizing the usage of graphite electrodes is a money-saving opportunity. This paper aims to discuss the production process of graphite electrodes as well as industry practices that are used to enhance the life of electrodes.
To produce a graphite electrode, carbon is required. The carbon source for graphite electrodes comes from the petroleum industry as a by-product from the oil refining process. In the refining of crude oil, hydrocarbon chains are cracked in coking units and the resulting fuels are then separated as kerosene, gasoline, and diesel. As chains of hydrocarbons are being cracked in this process, pure carbon becomes deposited on the sidewalls of the coking units. The carbon on the sidewalls accumulates up to a certain point when it is the ground out of the unit. This carbon is called petroleum coke, or petcoke. This is the carbon source for graphite electrodes.
After the petcoke is obtained from the oil refinery, it is blended with pitch to make a plastic-like material. (Making a UCAR® Graphite Electrode) This blend of petcoke and pitch is then extruded through a circular die and cut off in sections. It is then baked at a temperature of above 800 degrees Celsius for a period of one to two weeks. After baking, the electrode is impregnated with more pitch to increase the density of the electrode which lowers electrical resistance and makes it stronger. The electrode is then re-baked at a slightly lower temperature to drive off volatiles found in the pitch. Finally, the electrode is heated to a temperature of about 3000 degrees Celsius to recrystallize the carbon into graphite. This final heating is called graphitization. Graphitization is very important in electrode manufacturing because it provides better mechanical strength and also improves electrical conductivity. The final step in electrode production is machining. The electrodes are machined to specific tolerances. This is especially important at the ends of the electrodes where the joints connect. (D. Klein) A good connection between joints is necessary for mechanical and electrical properties of the electrode.
Electrode consumption takes on two forms: Continuous consumption and discontinuous consumption. (Richard J. Fruehan) Continuous consumption contributes 90% of total electrode consumption while discontinuous consumption only accounts for 10%. Even though discontinuous consumption accounts for far less than continuous consumption, discontinuous consumption can also account for furnace downtime which can cost a lot of money. So even though it is a small percentage, it is worthwhile to try to prevent it for the sake of productivity.
In continuous consumption, the tip and sidewalls oxidize and the mass of the electrode is reduced. Tip consumption is a function of current and angle. Higher currents and steeper tip angles both yield faster oxidation rates of electrodes. (A. Lefort) Higher currents increase the temperature of the electrode which favors a faster oxidation rate. The steeper the tip angle, the closer the electrode needs to be to the steel bath to arc. Steel is more likely to splash onto the electrode if it is closer to the bath. One way to reduce the oxidation rate of electrodes is to cool them. Water cooled electrodes have been shown to reduce the oxidation rate of the sidewalls by 40% and the tip by 50%. One issue with water cooled electrodes is the flow rate of the water. If the flow rate is too high, water will pour into the furnace and increase the heat loss. The loss of heat can end up costing more than the money saved from reducing electrode consumption, so this is an important factor to consider in this process.
In discontinuous consumption, one of the most obvious techniques to prevent electrode breakage is simply to not ram the electrodes into the steel scrap. Graphite is a soft, brittle material and steel scrap is pretty hard and durable. If these two materials come head-to-head, the steel wins. Another method of discontinuous consumption is tip spalling. (A. Lefrank) This is more prevalent in DC furnaces than in AC furnaces because the DC electrodes see higher temperatures and therefore higher thermal stresses. The temperature gradient can be significant in the tip of a DC electrode and the thermal expansion of graphite can cause enough stress on the tip to break off small pieces. (J. E. Surma) Normally, the arc in a DC furnace will move about randomly, but occasionally the arc will sit in one spot, heating up that portion of the electrode tip creating thermal stresses that cause the tip to spall. Arc deflection control is a practice that has been implemented so when the arc becomes fixated on one spot, it will essentially “push” the arc out from that spot to get it to move about in a random fashion once more.
Another practice to prevent the discontinuous consumption of electrodes is to ensure proper torque is applied when installing new electrode segments. The vibrations from the furnace and electromagnetic forces due to the flow of electricity give electrodes a good shaking. This can jostle the segments loose from their joints. This is mainly a problem in DC furnaces because AC furnaces are designed to ensure that the electromagnetic forces work to their advantage. In AC furnaces, the phase sequence is counterclockwise, so the forces on the electrodes due to the electromagnetism of the system only ever tightens the electrode joints. Below is a chart of recommended torque according to electrode diameter from SGL Carbon's website and an AC furnace schematic of forces due to phase sequence.
One final practice that helps prevent the failure of electrode joints is to turn off the water spray for a few minutes after the addition of another electrode segment. (Richard J. Fruehan) The temperature gradient is much steeper when the water spray operation is active. This thermal gradient can cause a problem when it reaches the joint because of the thermal expansion of graphite. Even if the electrode was tightened with the proper amount of torque, the effects of temperature on the electrode joint is enough to loosen it. This coupled with the furnace vibrations could be enough to cause the electrode joint to fail.
In conclusion, graphite electrodes are an essential part of electric arc furnace steelmaking. Since they are consumable, any way to improve the life and efficiency of an electrode saves money in the steelmaking industry. Practices such as water cooling electrodes and optimizing current and tip angle are effective ways of reducing the oxidation of electrodes while taking care not to impact the scrap during melting, ensuring proper torque during installation, and turning off the water spray while adding another segment all help reduce breakage and joint failure.
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D. Klein, K. Wimmer. "DC Electrodes - A Key Factor for Progress in EAF Production." Metallurgical Plant and Technology International 18:4 (1995): 54-63.
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J. E. Surma, D. R. Cohn, D. L. Smatlak, P. Thomas, P. P. Woskov, C. H. Titus, J. K. Wittle, R. A. Hamilton. "Graphite Electrode DC Arc Technology Development for Treatment of Buried Wastes." Waste Management '93 Symposia. Tuscon, 1993.
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