Basic oxygen steelmaking
Basic oxygen steelmaking (BOS), also known as the Linz-Donawitz (LD) process, is a primary steelmaking method that refines molten pig iron from a blast furnace, along with scrap steel, into high-quality steel by injecting high-purity oxygen at supersonic speeds through a water-cooled lance into a pear-shaped converter vessel, thereby oxidizing and removing impurities such as carbon, silicon, phosphorus, and manganese while generating heat from exothermic reactions to sustain the process without external fuel.[1][2] The process begins with charging the converter—typically a refractory-lined vessel holding 100 to 400 metric tons—with approximately 70-80% molten hot metal (pig iron) and 20-30% steel scrap, followed by the addition of fluxes like burnt lime to form slag that captures non-metallic impurities.[2][3] Oxygen blowing lasts 15-25 minutes, reducing carbon content from about 4% in pig iron to below 0.1% in steel, with the entire cycle completing in 30-45 minutes, enabling high productivity of up to 400 tons per hour in modern plants.[1][3] After refining, the molten steel is tested for composition and temperature, alloys are added if needed, and the steel is tapped into a ladle while slag is separated for reuse in applications like cement production.[2] Invented in post-World War II Austria, the BOS process originated from experiments by Voest (Vereinigte Österreichische Eisen- und Stahlwerke) and Österreichisch-Amerikanische Magnesium Gesellschaft (ÖAMG) in Linz and Donawitz, with the first successful trial on June 25, 1949, using vertical oxygen blowing into a 2-ton converter and larger 15-ton vessel trials in October 1949, and the first industrial heat produced on November 27, 1952, at the Voest-Linz plant in a 30-ton converter.[3] This breakthrough, patented in 1953, rapidly replaced slower methods like the open-hearth process due to its efficiency, leading to global licensing through Brassert Oxygen Technik and widespread adoption by the 1960s, with converter sizes scaling from 30 tons to over 350 tons and innovations like bottom-stirring (OBM) and combined blowing enhancing scrap utilization and impurity removal.[4][3] Today, BOS accounts for approximately 70% of global crude steel production, totaling 1,885 million metric tons annually as of 2024, primarily via the blast furnace-basic oxygen furnace (BF-BOF) route, underscoring its dominance in integrated steel mills despite growing emphasis on sustainability challenges like CO2 emissions from cokemaking.[5] Key advantages include precise control over steel chemistry, low nitrogen content in the final product, and the ability to recycle up to 30% scrap, though ongoing developments focus on increasing scrap rates to 50% or more, integrating direct reduced iron (DRI), and capturing off-gases for energy recovery to reduce environmental impact.[1][3]Overview
Definition and principles
Basic oxygen steelmaking (BOS), also known as the basic oxygen process (BOP) or Linz-Donawitz process, is a primary method of steel production that refines molten pig iron from a blast furnace into steel by blowing high-purity oxygen at supersonic speeds onto the surface of the molten charge to oxidize and remove impurities such as carbon, silicon, phosphorus, and manganese.[6][1] The process typically involves a charge consisting of 70-80% hot metal (molten pig iron) and 20-30% scrap steel, processed in batches ranging from 100 to 400 tons, with most modern converters handling 100-250 tons per heat.[7] The core principles of BOS rely on exothermic oxidation reactions, similar to those in the thermite process, where the injected oxygen reacts vigorously with impurities in the molten bath, generating sufficient heat to maintain the liquid state without external energy input.[8][6] These reactions produce a basic slag—primarily calcium silicate—from added fluxes like lime, which absorbs and separates the oxidized impurities; the vessel is lined with basic refractories, such as magnesia, to withstand the alkaline slag and high temperatures exceeding 1600°C.[9] Oxygen, with purity greater than 99%, is delivered through a water-cooled lance positioned 1-2 meters above the bath surface, ensuring efficient mixing and reaction kinetics while minimizing excessive splashing.[10] Developed in the mid-20th century as a faster alternative to the labor-intensive open-hearth process, which required 10-12 hours per batch, BOS achieves a full heat in 30-40 minutes, enabling higher productivity and scalability in large-scale steel production.[11][9]Comparison to other steelmaking methods
Basic oxygen steelmaking (BOS) represents a significant advancement over the open-hearth furnace (OHF) process, primarily due to its superior efficiency in both time and energy consumption. While the OHF required 6-12 hours per batch to refine molten iron and scrap through prolonged heating and oxidation, BOS completes a heat in approximately 40-50 minutes by directing a high-velocity stream of pure oxygen onto the melt surface.[12][9] This rapid cycle enables much higher throughput, with BOS facilities producing up to 300-400 tons per heat compared to the OHF's slower output. Additionally, BOS is far less energy-intensive, consuming 0.7-1.0 GJ per tonne of steel versus 3.9-5.0 GJ per tonne for the OHF, largely because it relies on the exothermic oxidation reactions within the melt rather than external fuel combustion.[13] However, unlike the more versatile OHF that could accommodate higher scrap charges, BOS predominantly requires hot metal from a blast furnace as its primary input, limiting its flexibility in scrap utilization.[14] In contrast to electric arc furnace (EAF) steelmaking, BOS follows the primary route, converting pig iron or hot metal from blast furnaces into steel, which accounts for the majority of global production. As of 2023, BOS (integrated with blast furnace-basic oxygen furnace, or BF-BOF) comprised about 71% of worldwide crude steel output, enabling large-scale, continuous production suited for high-volume applications like construction and automotive sectors.[15] EAF, the secondary route, primarily melts scrap steel using electric arcs, offering greater flexibility for recycling and adaptation to fluctuating scrap availability, but it is more energy-intensive per tonne when relying solely on scrap due to the need for electrical melting.[16] While EAF's total primary energy use can be lower overall in scrap-based operations (around 8-12 GJ per tonne versus over 20 GJ per tonne for BF-BOF), the steelmaking step requires significant electrical input.[14][17] The EAF share is growing, projected to reach 36% by 2030, driven by increasing scrap recycling and decarbonization efforts.[15] Compared to the earlier Bessemer process, BOS addresses key limitations in impurity control and product quality. The Bessemer converter, which blew air through molten pig iron to oxidize carbon, introduced nitrogen from the air, resulting in brittle steel unsuitable for many structural uses, and struggled with phosphorus removal without specialized linings.[18] BOS, as a refined descendant, employs pure oxygen to avoid nitrogen pickup entirely, producing tougher, more ductile steel.[19] Furthermore, BOS utilizes a basic refractory lining (typically magnesia or dolomite) that facilitates effective phosphorus removal by forming stable phosphates in the slag, enabling the processing of high-phosphorus ores that challenged the original Bessemer method.[20] These improvements made BOS a dominant technology by the mid-20th century, supplanting the Bessemer process which had largely faded by the early 1900s due to its inefficiencies.[18]History
Invention and early development
The development of basic oxygen steelmaking (BOS), also known as the Linz-Donawitz (LD) process, originated in Austria during the 1940s amid efforts to modernize steel production following World War II. Swiss metallurgist Robert Durrer played a pivotal role in establishing the theoretical foundations, drawing on earlier experiments with oxygen blowing in small-scale converters conducted in Switzerland as early as 1948. These initial tests utilized high-purity oxygen supplied from nearby industrial plants, such as those operated by Linde, to refine pig iron by top-blowing oxygen into a basic-lined vessel. Durrer's work emphasized the potential for rapid oxidation and impurity removal, addressing limitations of traditional open-hearth methods.[21][22] In collaboration with Austrian steelmakers, Durrer advised Voestalpine (then VÖEST) in Linz and the steelworks in Donawitz on practical implementation, leading to pilot test runs from 1949 to 1952. These trials began with a 2-ton converter in June 1949 at Linz, scaling to a 15-ton unit by October 1949, where hundreds of heats demonstrated the process's viability despite early issues with oxygen penetration and lance positioning. The LD process had patents first applied for in 1950 by Voestalpine, with the key patent granted in 1953 amid inventor disputes involving key Austrian engineers like Theodor Suess, Hubert Hauttmann, Herbert Trenkler, and Rudolf Rinesch. A core innovation was the use of a water-cooled lance delivering 99% pure oxygen at controlled pressures for "soft blowing," enabling efficient decarburization without excessive refractory wear.[21][23][22] Early challenges centered on scaling from laboratory and pilot stages to industrial production, including optimizing oxygen flow to avoid splashing and ensuring consistent steel quality. The first commercial LD plant at Voestalpine's Linz works was commissioned on November 27, 1952, with official opening on January 5, 1953, featuring a 30-ton converter lined with basic refractories primarily composed of dolomite to withstand the alkaline slag environment, later expanded to additional units. A second plant followed in Donawitz by May 1953. Initially, the Linz facility achieved an annual production capacity of around 200,000 tons of steel, reaching 100,000 tons within its first seven months of operation. This marked the breakthrough for BOS, proving its speed and efficiency in industrial settings.[21][24][22]Global adoption and evolution
The basic oxygen steelmaking (BOS) process rapidly expanded beyond its Austrian origins in the mid-1950s, driven by its superior efficiency over traditional methods. The first adoptions in the UK occurred in the late 1950s. This was followed by installations in the US at McLouth Steel in Trenton, Michigan, in 1954 and in Japan at the Yawata Works in 1957, where two 30-ton LD converters were commissioned. By 1960, BOS accounted for approximately 10% of global steel production, reflecting its quick international uptake. Global licensing through companies like Brassert Oxygen Technik facilitated rapid spread to other European countries in the mid-1950s.[22][25][22] Post-World War II reconstruction demands in Europe and Asia, combined with advancements in large-scale air separation for inexpensive high-purity oxygen, fueled BOS growth. These factors enabled shorter cycle times—typically 30-40 minutes per heat compared to hours for open-hearth furnaces—and higher productivity, making BOS economically attractive for scaling steel output to support industrial recovery and urbanization. By the 1970s, BOS had surpassed the open-hearth process worldwide, producing over 50% of global steel as it became the standard for primary steelmaking.[1][26][3] Technological evolution further solidified BOS dominance through incremental innovations. In the 1970s and 1980s, computer-based automation systems were introduced for precise control of oxygen blowing, lance positioning, and endpoint carbon detection, reducing variability and operator intervention. The 1990s saw significant efficiency gains, with furnace capacities expanding to around 300 tons per heat, allowing larger batches and lower per-ton costs. Entering the 2020s, BOS maintains a share of about 70% of global crude steel production, though this is declining modestly relative to electric arc furnace (EAF) methods amid rising emphasis on scrap-based and low-carbon alternatives.[27][28][29] China's post-2000 surge in BOS adoption represents a pivotal milestone, with state-driven investments building hundreds of blast furnace-BOS facilities to meet explosive demand from infrastructure and manufacturing booms. This rapid scaling positioned China as the largest BOS producer, accounting for over 50% of worldwide steel output by the early 2020s and amplifying BOS's global footprint.[30]Process
Raw materials and charging
The primary raw materials for basic oxygen steelmaking (BOS) are molten pig iron, also known as hot metal, sourced from the blast furnace, and steel scrap. Hot metal constitutes the majority of the metallic charge, typically comprising 65% to 90% of the total input, and arrives at temperatures between 1300°C and 1350°C to provide the necessary heat for the process.[31][8] Its typical composition includes 3.8% to 4.5% carbon, 0.5% to 1.5% silicon, 0.25% to 1.5% manganese, 0.05% to 0.15% phosphorus, and 0.03% to 0.08% sulfur, which influences the efficiency of subsequent refining steps.[31][8] Steel scrap serves as a coolant and source of recycled iron, making up 10% to 35% of the metallic charge to balance the exothermic reactions during oxygen blowing.[31][9] Scrap types include light materials like sheet shearings and heavy items like mill scale, with cleanliness essential to minimize tramp elements such as copper or tin that could degrade steel quality.[31] Additives, primarily fluxes like burnt lime (CaO) and calcined dolomite, are introduced to form slag for impurity removal; lime addition is roughly six times the silicon content in the hot metal by weight.[31][7] Charging occurs in a refractory-lined vessel tilted at approximately 45 degrees, with a typical total charge weight of 100 to 250 tons per heat.[7] The sequence begins with loading scrap via a hopper or charging box to protect the furnace lining, followed by pouring the hot metal from a ladle.[31][8] Fluxes such as lime are added early in the sequence or during initial oxygen blowing to ensure rapid slag formation, while other fluxes like fluorspar are used sparingly for phosphorus control.[31] Quality control emphasizes selecting hot metal with consistent composition to optimize yield and scrap that is properly sized—large pieces are often baled—to promote complete melting.[31]Oxygen blowing and refining
The oxygen blowing phase in basic oxygen steelmaking begins immediately after charging the furnace with molten hot metal, scrap, and fluxes, with the water-cooled lance positioned approximately 1.5 to 1.8 meters above the bath surface to ensure effective jet penetration without excessive wear.[32] High-purity oxygen (≥99.5%) is then introduced through the lance at supersonic velocities, typically at flow rates of 600 to 800 normal cubic meters per minute for a standard furnace charge, sustaining the blow for 15 to 20 minutes to achieve the desired refinement.[7] This procedure generates intense mixing and reaction conditions within the melt, optimizing impurity removal while minimizing refractory damage. The blowing process is generally divided into two main stages: the initial carbon removal phase, which accounts for about 70% of the oxygen consumption, and the subsequent finishing phase. During the early stage, the oxygen jet induces violent boiling of the bath, primarily through the rapid oxidation of carbon and silicon, which releases large volumes of carbon monoxide gas and drives the exothermic reactions forward.[7] As the blow progresses into the finishing stage, the focus shifts to precise control of residual elements, ensuring the melt reaches the targeted composition without over-oxidation. Endpoint control is critical to halt the blow at the optimal moment, typically when the carbon content is reduced to below 0.1%, preventing excessive iron loss or temperature overshoot. This is achieved through sublance sampling, which allows for quick analysis of bath temperature and composition about two minutes before the end of the blow, or via real-time off-gas analysis monitoring the carbon monoxide to carbon dioxide ratio for reaction kinetics.[7] Advanced monitoring often incorporates optical emission spectrometers to track elemental changes dynamically during the process. The heat balance during blowing relies on the exothermic nature of the oxidation reactions, which elevate the bath temperature from an initial 1250–1300°C to 1650–1700°C, sufficient to fully melt the scrap charge and homogenize the alloy.[7] Gas evolution from the reactions, combined with the impinging oxygen jet, provides vigorous stirring of the bath, enhancing reaction efficiency and uniformity. Overall, the process yields 90–95% metal recovery, reflecting efficient conversion with minimal losses to slag or fumes.[7]Tapping and casting
Once refining is complete, the basic oxygen furnace (BOF) is tilted to tap the molten steel, which is poured through the tap hole into a waiting ladle below. This process achieves a steel yield of approximately 95%, reflecting the high efficiency of impurity removal while minimizing metal losses. To prevent slag carryover, a slag dam or stopper is employed to retain the floating slag layer, and any residual slag is skimmed off the surface of the steel in the ladle.[33][2] During tapping, alloying elements such as ferromanganese (FeMn) are added directly to the ladle to adjust the final steel composition, ensuring it meets specifications for carbon, manganese, and other elements. The slag, primarily composed of calcium silicates and oxides, is then separated and collected; it is commonly recycled as a raw material in cement production or as aggregate for road construction, promoting resource recovery and reducing waste.[2][34] The tapped molten steel, maintained at around 1600–1650°C, is transported to the casting stage, where it is either continuously cast into semifinished shapes like slabs, billets, or blooms, or poured into molds to form ingots. Temperature control is critical during this transfer and casting to avoid solidification defects such as cracks or inclusions, with adjustments made as needed to optimize quality. Secondary refining in the ladle may follow, but the primary shaping occurs here.[2] The entire BOF cycle, from charging to tapping, typically lasts 40–50 minutes, enabling modern furnaces to produce 10–12 heats per day and supporting high-throughput operations in steel plants.Chemistry and reactions
Oxidation reactions
The oxidation reactions in basic oxygen steelmaking (BOS) primarily target the removal of impurities from molten pig iron through exothermic processes that also generate the necessary heat for refining. The main reaction involves carbon oxidation, where dissolved carbon reacts with oxygen to form carbon monoxide:\ce{[C] + 1/2 O2 -> CO}
with an enthalpy change of approximately -110 kJ/mol, serving as the dominant heat source due to the high carbon content (typically 4-5%) in the charge.[35][36] Silicon and manganese, present at levels of 0.5-1% and 0.5-2% respectively, undergo rapid oxidation early in the process:
\ce{[Si] + O2 -> SiO2}
\ce{[Mn] + 1/2 O2 -> MnO}
These reactions are highly exothermic, contributing significantly to the bath temperature rise, with silicon oxidation alone providing about 32 MJ per kg of Si oxidized.[8][36][37] Secondary oxidation involves phosphorus, which requires interaction with the basic slag for effective removal:
\ce{2[P] + 5/2 O2 + 3 CaO -> Ca3(PO4)2}
This process oxidizes phosphorus to P₂O₅ intermediate before incorporation into calcium phosphate, achieving low residual levels under basic conditions. Iron oxidation is intentionally minimized to less than 5% of the charge to avoid excessive metal loss, as the reaction \ce{2Fe + O2 -> 2FeO} forms temporary FeO that is later reduced back to metal.[8][36] The primary oxidation reactions produce substantial volumes of off-gas, predominantly CO, with typical evolution of 80-100 Nm³ of CO per tonne of steel, depending on charge composition and oxygen efficiency. Post-combustion of CO to CO₂ in the furnace space partially oxidizes the gas, yielding an off-gas CO₂/CO ratio that rises toward the blow endpoint (indicating carbon depletion) and is monitored for process control.[38][39] Thermodynamically, the feasibility and sequence of these oxidation reactions at the operating temperature of around 1600°C are governed by principles illustrated in Ellingham diagrams, which plot standard Gibbs free energy changes (ΔG⁰) against temperature for oxide formation. Lines for SiO₂, MnO, and FeO lie below that for 2C + O₂ → 2CO at this temperature, ensuring silicon and manganese oxidize preferentially over iron under typical oxygen partial pressures (>10⁻⁸ atm), while carbon oxidation to CO becomes favorable at lower partial pressures to minimize FeO formation. Phosphorus oxidation requires even more selective conditions due to its line position, emphasizing the role of slag basicity.[40]