Electrode consumption forms a large part of the operational costs of EAF steelmaking. In addition, electrode prices have been very volatile during recent years. Therefore, it is important to understand what causes the consumption of electrodes and how it could be reduced.
The consumption of graphite electrodes doesn’t only depend on the quality of the electrodes but also the handling of the electrodes and EAF process practices. This article discusses the EAF process phenomena that cause the consumption and the process practices that can be used to mitigate the excess consumption.
Main factors of graphite electrode consumption during the EAF process
The consumption of graphite electrodes in electric steelmaking is related to different parameters used by the steel production technologies and electrical parameters of the supply system. These parameters are, for example, the quality of the electrodes and the amount of electric current. The four most important components of electrode consumption mechanisms are tip consumption, sidewall oxidation, stub loss, and top joint breakage. (Migas & Karbowniczek 2013)
Tip consumption of graphite electrodes occurs when the temperature is 3000°C or above. The high temperature of the electric arc causes the graphite material to sublimate. The graphite converts directly from a solid to carbon monoxide in this phenomenon, gas. Various factors affect the sublimation rate of graphite. These factors include the magnitude of used current when the arc is present, the diameter of the electrode tip, the duration of time that current is passing through the electrode, the resistivity of the electrode, and arc stability. (LMM Group 2007)
Electrode sidewall oxidation is caused by the reaction between oxygen and graphite in the furnace atmosphere, which forms carbon monoxide. Electrode surface area and smelting time are the two major factors affecting the amount of electrode sidewall oxidation. (Rongxing Carbon 2007) Oxidation and the penetration of cracks can also cause a small section of the electrode or the joint to break. Poor scrap loading and other factors also affect the top joint breakage. (Migas & Karbowniczek 2013)
Of course, also the negligent handling and storing of electrodes may cause excessive consumption. For example, small fractures caused by transportation or handling may cause the electrode to break from that point during the melt. Or if the electrode is clamped at the electrode joint it may break due to excessive stress applied to the joint.
How to reduce electrode wear
The graphite electrode consumption is a large part of the cost of steel production in the EAF process route, but this cost can be decreased with optimized EAF process practices. With real-time melting information, voltage and current can be dynamically controlled.
Using higher voltages reduces the consumption of electrodes as the arc current decreases. However, this also increases the length of the arc which might be problematic if not kept under control. The long arc might cause damages to the furnace side walls if it is allowed to run too long.
In the early phases of the heat, the unmelted scrap protects the sidewalls from this long arc but after it has melted away the arc should be shortened, and voltages lowered. This can be done by measuring the amount of unmelted scrap protecting the sidewalls and adjusting the arc length based on these levels near the edges of the furnace.
This has been proven to lead to even a 10% reduction in electrode consumption. In addition, this optimal control of voltage levels and electric current also reduces the wear of the protecting refractory mass. These benefits combined have been proven to decrease the operating costs of EAF significantly.
Real-time information from the EAF can be measured reliably for example with our ArcSpec system. ArcSpec monitors and analyses the emitted light from the EAF with Optical Emission Spectrometry (OES). The system makes it possible to operate the EAF with higher voltage and longer arc without significantly increasing the refractory wear. The system has also a built-in feature to optimize the voltage and electric current levels to reduce electrode consumption, among other benefits.
References
LMM Group 2007. Study on the technology of reducing the consumption of graphite electrode [online]. Dalian: LMM Group.
Migas, P. and Karbowniczek, M., 2013. Selected Aspects of Graphite Applications in Ferrous Metallurgy [online]. Krakow: AGH University of Science and Technology.
Rongxing Carbon 2007. The consumption mechanism of graphite electrode in electric steelmaking [online]. Zhengzhou: Rongxing Carbon. Available from: https://www.graphite-product.com/news/PI/2020-03-24/187.html# [Accessed 6 November 2020]
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