What Causes Electric Arc Furnace Electrode Consumption?

What Causes Electric Arc Furnace Electrode Consumption?

Graphite electrodes are primarily used in smelting electric arc furnaces as conductive materials. Compared to other conductive materials, the greatest advantage of graphite electrode materials lies in their excellent electrical and thermal conductivity, as well as their toughness. They can withstand the impact of high currents and do not soften or melt at high temperatures.  Therefore, they are widely used in the field of high-temperature electric steelmaking. In steelmaking electric arc furnaces, graphite electrodes are used as conductive materials to transfer heat energy to the furnace charge through arc discharge, melting scrap steel.

Therefore, the research on reducing electrode consumption and improving the quality of graphite electrodes is a prominent issue that electric arc furnace metallurgy experts and enterprises urgently need to address.

Therefore, exploring the mechanism of electrode consumption, implementing effective measures to reduce electrode consumption, and striving to lower production costs have become crucial aspects of reducing steelmaking costs and conserving energy in electric arc furnaces.  Especially in today's rapidly developing steel industry, with the rapid growth of electric arc steelmaking, the demand for electrodes is also increasing.

However, due to limitations in capital, energy, production technology, and other conditions, the development of graphite carbon products currently struggles to meet the special needs of electric arc furnace steelmaking production. There is still a considerable gap between the electrode consumption indicators and those of foreign countries, with low quality and a prominent supply-demand contradiction.

For example, most of the electrodes used in ultra-high power electric arc furnaces (EAFs) in large domestic steel plants are imported from Japan and other countries. Therefore, reducing electrode consumption has become one of the important issues that electric arc furnace steel-making enterprises need to consider. It is an important way to save energy, fully implement the requirements for energy conservation and consumption reduction for steel enterprises put forward in China's "Eleventh Five-Year Plan" for national development in the new era, and it is also an important way to reduce the production cost of electric arc furnace steel, improve economic efficiency, and enhance the core competitiveness of enterprises.

The electrode is the final component of the short net, which generates a powerful electric arc through the ends of two interconnected graphitized electrodes to melt the furnace material and heat the molten steel.  Essentially, the electrode serves as the central hub for converting electrical energy into thermal energy. During operation, the electrode is subjected to high temperatures, oxidation from furnace gases, and impacts from collapsing materials. Notably, the junction where the two electrodes are connected exhibits higher resistance and lower conductivity compared to other areas, making it prone to tripping, oxidation, detachment, and fracture. Consequently, this leads to significant consumption of the electrode, prolongs smelting time, and reduces productivity.

During the steelmaking process, electrodes are exposed to high temperatures, leading to the consumption of their surfaces through the carbon-oxygen reaction with oxygen. Graphite electrodes are stable at low temperatures but prone to oxidation at high temperatures. Generally, carbon products begin to oxidize at around 450°C in air, while graphite products with a higher degree of graphitization start to oxidize at around 600°C.  Oxidation increases rapidly after exceeding 750°C and intensifies with rising temperatures.  When heated to 900°C in steam, they are oxidized.  The main factors affecting the side oxidation of graphite electrodes are high temperatures and oxidizing atmospheres. This results in electrode consumption, especially with the successive application of new technologies such as furnace door oxygen lances, oil-oxygen fluxing, EBT cluster oxygen lances, and furnace wall oxygen lances, which increase the intensity of oxygen supply in the furnace and enhance the oxidizing atmosphere, further increasing electrode consumption.

Due to direct contact between the electrode tip and the arc, the tip electrode sublimates and is consumed;  the electrode part comes into contact with the molten pool, and its carbon element is absorbed by the molten pool, resulting in erosion consumption;  the electrode is subjected to electromagnetic force, mechanical force, and impact force from solid raw materials during operation, leading to fracture and spalling consumption.

The consumption of electrodes in electric arc furnaces can be divided into chemical consumption and physical wear and tear:

1.1 Physical loss

The physical wear of electrodes primarily refers to the consumption at the tip and side of the electrode, which is mainly caused by mechanical external forces and electromagnetic forces.  This includes issues such as loosening and breakage at the electrode joint, electrode cracks, and detachment of the joint thread.  The reasons for this are poor quality of the electrode itself, such as low strength;  equipment issues, such as improper selection of electrode diameter, poor electrode clamps, lifting, and control devices;  and operational issues, such as improper charging, large pieces of scrap steel collapsing and impacting the electrode during the melting period, and loose connection between two electrodes.

1.2 Chemical loss

It mainly refers to the consumption of the electrode surface, including the consumption at the electrode tip and the consumption at the periphery.  Local heating at the electrode tip causes graphite sublimation and contact between the electrode tip and the molten steel, leading to the absorption of graphite. The consumption at the electrode tip is mainly caused by graphite sublimation at high temperatures and melting in steel slag.  Under normal operating conditions, tip consumption can account for up to 50% of the total electrode consumption.  Side consumption is the main reason for the oxidation of the electrode, accounting for about 40% of the total consumption. 

The oxidation reaction rate is closely related to temperature:

1) When the temperature ranges from 550℃ to 750℃, the oxidation reaction rate is controlled by the electrode itself, and the influence of graphite quality and temperature on electrode consumption is stronger than that of air.

2) When the temperature exceeds 800℃, the flow rate of air begins to control the reaction. The influence of air flow rate and air pressure on electrode consumption is stronger than that of temperature and electrode mass.

The larger the contact area between the electrode and the air, the greater the intensity of the oxidation reaction involved, resulting in increased consumption.

The contact between the electrode perimeter and steel slag, as well as the contact with furnace gas, leads to oxidation loss.