Electric Arc Furnaces (EAF) and Basic Oxygen Furnaces (BOF)

Electric arc furnaces (EAF) and Basic Oxygen Furnaces (BOF)

Electric arc furnaces (EAF) and basic oxygen furnaces (BOF) are two mainstream steelmaking technologies. EAFs use electrical energy, generating high-temperature arcs between graphite electrodes and scrap steel to melt metal. They are suitable for processing scrap steel, characterized by strong flexibility and low investment costs, making them ideal for small- to medium-scale production and short-process steelmaking.

With lower carbon emissions, they align with circular economy principles and are widely applied in regions with abundant electricity or stringent environmental requirements, accounting for approximately 30% of global steel production.

In contrast, BOFs primarily use hot metal (about 75%) and inject high-purity oxygen from the top, triggering oxidation reactions with carbon and silicon in the iron, releasing heat to efficiently produce steel in 15-20 minutes.

Known for their mature process and high production efficiency, BOFs are suited for large-scale, continuous production.

However, they rely on iron ore and coke, resulting in higher carbon emissions.

Despite this, BOFs remain the dominant steelmaking method globally, contributing over 70% of production.

Together, EAFs and BOFs represent scrap recycling and iron ore-based steelmaking routes, jointly supporting the modern steel industry.

Electric Arc Furnace (EAF)

Electric Arc Furnace (EAF) Definition and Working Principle

An electric arc furnace (EAF) is a type of electric furnace that uses the high temperatures generated by electric arc to melt ores and metals. The positive and negative electrodes in an EAF are positioned close to each other with only a small gap in between. When current is applied, it creates a discharge through the gas, forming an electric arc. The energy is highly concentrated during this process, with temperatures in the arc zone typically exceeding 3000°C and reaching up to 5000°C.

This intense heat releases a large amount of thermal energy near the electrodes, facilitating the conversion of electrical energy into thermal energy to heat and melt the raw materials.

Types and Applications of Electric Arc Furnaces

Electric arc furnaces include types such as basic oxygen electric arc furnaces and acidic electric arc furnaces, commonly used in the production of carbon structural steel and alloy steel.

Beyond steel production, EAFs are widely utilized in non-ferrous metal smelting, such as the melting of copper, aluminum, and nickel. Additionally, electric arc furnaces operated under vacuum conditions, known as vacuum electric arc furnaces, are employed for melting high-melting-point metals, reactive metals, and special steels.

Components and Characteristics of Electric Arc Furnaces

The primary components of an electric arc furnace include the furnace body, electrodes, and power supply. The furnace body, typically constructed from refractory materials, serves to contain the raw materials and molten metal. The electrodes, usually made of graphite or carbon, conduct the current and generate the electric arc. The power supply provides high-current, low-voltage electricity to produce sufficient heat.

Electric arc furnaces are characterized by their high process flexibility, effective removal of impurities such as sulfur and phosphorus, ease of temperature control, compact equipment footprint, and suitability for melting high-quality alloy steels. Furthermore, the electric arc steelmaking process, which uses electricity as its energy source, offers advantages such as lower greenhouse gas emissions and reduced energy consumption compared to other steelmaking furnaces.

Development Trends of Electric Arc Furnaces

With advancements in technology, electric arc furnace technology continues to evolve. For instance, direct current (DC) electric arc furnaces offer benefits such as stable and concentrated arcs, effective stirring of the melt pool, and uniform temperature distribution within the furnace. Additionally, as environmental and energy efficiency requirements increase, the energy efficiency and environmental performance of electric arc furnaces are being continuously improved.

Basic Oxygen Furnace (BOF)

The Basic Oxygen Furnace (BOF), also known as the Basic Oxygen Steelmaking (BOS) process, is a core equipment for efficient steelmaking that utilizes high-purity oxygen as an oxidizing agent under alkaline furnace lining conditions. Combining the 19th-century principles of alkaline converter steelmaking with modern oxygen blowing technology, it has become the predominant method for global crude steel production, accounting for over 70% of output. The following analysis explores this technology from the perspectives of its technical principles, operational processes, chemical characteristics, and historical development.

I. Technical Principles and Structural Features of the BOF

1. Alkaline Furnace Lining Design

The furnace lining is constructed using high-temperature-fired alkaline refractory bricks made from dolomite and mixed with pitch. This lining can react with acidic oxides (such as phosphorus and sulfur) at high temperatures to form stable slag, preventing harmful elements from re-entering the molten steel. This design effectively addresses the phosphorus removal challenges in traditional steelmaking, particularly suitable for the refining of high-phosphorus pig iron.

2. Oxygen Blowing Technology

High-purity industrial oxygen (over 99%) is blown into the melt pool through top, bottom, or side blowing methods. Oxygen reacts with carbon, silicon, and manganese in the molten iron to release substantial thermal energy (temperatures reaching 1650-1700℃), achieving rapid decarburization and temperature increase. The top-blowing method is notable for its efficiency, with the lance positioned approximately 1 meter above the melt surface and oxygen flow rates reaching Mach 3.

3. Reaction Process Classification

Metal Oxygen Absorption: Includes oxygen decomposition and the generation of iron oxides such as FeO and Fe₂O₃.

Impurity Oxidation: Carbon forms CO/CO₂ gases, while silicon and manganese form oxides that enter the slag. Phosphorus reacts with lime to form stable phosphates.

Slag Reaction: Lime (CaO) is added to adjust slag alkalinity, enhancing desulfurization and phosphorus removal efficiency.

II. Operation Process and Process Optimization of Basic Oxygen Furnace (BOF)

1. Charging Process

The converter is tilted at 45° to charge scrap steel and molten iron. Scrap steel is used as a coolant to balance the temperature, with a typical ratio of molten iron to scrap steel of 70:30. The charging process takes 2-3 minutes, during which a pollution control system collects dust and graphite fragments (kish).

2. Oxygen Blowing and Slag Making

After the converter is reset to a vertical position, the top-blowing lance descends to 2.5-3 meters above the furnace bottom and blows oxygen at a pressure of 0.8-1 MPa for approximately 20 minutes. During the slag making stage, fluxes such as lime and fluorspar are added to form high-alkalinity slag (CaO/SiO₂ > 3), which absorbs impurities.

3. Endpoint Control

Modern BOF uses a flue gas analyzer to monitor the CO/CO₂ ratio in real time, combined with a mass-energy balance model to accurately predict the endpoint carbon content (error < 0.01%) and temperature (error < 17°C), minimizing human intervention.

III. Technical Advantages and Application Scenarios of Basic Oxygen Furnace (BOF)

1. Efficiency and Economy

A BOF can produce up to 400 tonnes per heat with a smelting cycle of only 40 minutes, significantly outperforming open hearth and electric arc furnaces, while also featuring low energy consumption and lower investment costs.

2. Steel Quality and Applications

It is suitable for producing high-strength low-alloy steel, construction steel, shipbuilding steel plates, and other high-value-added products, particularly excelling in processing high-phosphorus molten iron.

The steel's purity is high, with sulfur and phosphorus contents each controlled below 0.02%.

3. Process Variants

Top Blowing (LD Process): Invented by Austrian Steel Union in 1952, it has become the mainstream process.- Bottom Blowing (Q-BOP Process): Oxygen is injected from the furnace bottom, providing a more stable reaction, suitable for ultra-low carbon steel production.

Combined Blowing: Combines top and bottom oxygen blowing to enhance reaction uniformity and efficiency.

IV. Historical Evolution and Future Trends

1. Development Timeline

- 1879: Sidney Gilchrist Thomas invented the basic oxygen furnace, solving the phosphorus removal problem.

- 1952: The top-blowing oxygen furnace (LD process) was commercialized, leading to the phase-out of open hearth processes.

- 1970s: The emergence of bottom blowing and combined blowing technologies expanded the application scope of BOF.

2. Environmental Protection and Innovation

Current R&D focuses include hydrogen-based steelmaking (e.g., HYBRIT project), nuclear hydrogen production to replace coke, and intelligent control systems (e.g., NextGen® flue gas analysis) to reduce carbon emissions.

V. Conclusion

The basic oxygen furnace achieves revolutionary improvements in steelmaking efficiency and quality by combining alkaline furnace lining with strong oxidation technology.

Its core advantages lie in rapid decarburization, precise slag control, and large-scale production, making it the cornerstone of modern steelmaking. In the future, with breakthroughs in low-carbon smelting technologies, BOF will further evolve towards zero emissions and intelligent operations.