Basic oxygen steelmaking

To put it simply, basic oxygen steelmaking is a means of producing steel by using oxygen. The word "basic" refers to the pH of the refractories lining the ladle interior, which are normally composed of calcium oxide or magnesium oxide.4 This process is one of the worlds leading methods for producing steel, and since its implementation, many methods have developed which better utilize oxygen steelmaking for improved efficiency and quality. Current processes can produce around 350 tons of steel in less than 40 minutes.

The basic oxygen furnace was first developed in 1952 and was called the LD (Linz-Donawitz) process. This process utilized similar mechanics to the Bessemer process except the LD process used oxygen as fuel, which was introduced into the top of the ladle. The process would reach many parts of world and develop into many variations. Presently three general classifications for oxygen steelmaking have been established and are denoted as the top-blown, bottom blown, and combined blowing processes.

The first process, top-blown, is named for the way in which oxygen is introduced into the system. This process is also known as the Linz-Donawitz process (LD), Basic Oxygen Furnace process (BOF), or the Basic Oxygen Process (BOP). In this process, oxygen is blown at supersonic speeds though a water-cooled lance that enters the top or "roof" of the ladle. The lance can range from 60-70 feet in length and contains 3-5 nozzles. The introduction of oxygen in this way is important because it allows for the formation of the slag emulsion, as well as, sustaining the reactions required to produce steel. This remains the most common method of oxygen steelmaking. Figure 1 depicts the construction of the top-blown vessel, also referred to as a ladle, and the reactions/compositions within the vessel during operation.

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An advantage to the top-blown process is the fact that the refractories wear more evenly upon design than other processes which reduces the amount of down-time. This process is also able to most effectively use slag splashing. Bottom blowing or bottom stirring operations can experience clogging due to the deposition of slag onto gas entryways. In some operations, the vessels have been converted back to top-blown processes to alleviate clogging.

The bottom-blown oxygen process did not become useful until the early 1970's when shrouded tuyeres were developed that could withstand the heat and wear associated with transporting oxygen into the vessel. This process is commonly known as an Oxygen Bottom Maxhutte process (OBM) or the Quick-Quiet Basic Oxygen Process (Q-BOP) among other names and uses several tuyeres, typically 8-12, which run along the bottom of the ladle in evenly dispersed rows. Figure 2 shows an OBM process with tuyeres and a cross-section showing their location. Each tuyere is composed of two concentric pipes. An inner pipe carries the oxygen to the charge material while an outer pipe carries the coolant that protects the tuyere from overheating. Methane (natural gas) or propane is typically used as coolant, although some use fuel oil. These fuel choices can vary depending upon availability, price, and the process.1 As the gas breaks down, causing an endothermic reaction, it forms "mushroom" accretions which protect the tuyere from wear. After the oxygen blow is completed, nitrogen gas is blown through the tuyeres so that plugging does not occur. To further improve process efficiency, however, some bottom blowing processes have introduced a stationary top lance into the system which allows for less wear to the bottom refractories.

The charging of bottom blowing poses some great advantages over top blowing operations due to its ability to melt scrap up to 2 feet in size. This allows the process to utilize many types of scrap which lowers preparation costs. In addition, solid scrap does not remain within the vessel after converting it to liquid. Dolomitic lime can be charged into the ladle through bins located at the top of the vessel or as a mixture with burnt lime through tuyeres. The remaining raw charge materials are similar to the BOF process, however.

Another main advantage found in bottom blowing is that the oxygen reacts directly with carbon and silicon within liquid iron to form a stronger reaction. This stronger reaction results in fewer oxides remaining in the metal following the blow. Higher residual manganese can also be attained through bottom blowing. These improved reaction rates allow steels with 0.015-0.020% to require less bath and slag oxidation as well as no vacuum decarburization. Overall, similar steel grades can be produced in comparison to the top-blown processes as well.

The bottom refractories and tuyeres require frequent replacing as one unit and an increased amount of down time in relation to the top-blown process. The design of the process vessel is more complicated due to the need for changing out the bottom refractories, as well as, the nozzles.

The 1980's brought the introduced of the combination blowing processes including the LBE (Lance Bubbling Equilibrium) and K-BOP (Kawasaki) processes. These processes, along with many others, are composed of a retractable supersonic lance like those used in the top-blown processes and tuyeres or porous plugs similar to the bottom-blown process which are embedded within the refractory at the bottom of the ladle and are used for stirring. This modified top-blown configuration results in a lower operating cost due to stirring of the liquid steel from the bottom. This is because the bottom stirring reduces the occurrence of FeO in the slag, resulting in a greater Fe yield within the steel. Bottom stirring also increases the formation of the slag layer which provides better thermal insulation for the steel bath. The slag layer can further be increased with the injection of lime into the bath.

Three prominent configurations of combination blowing operations exist. The first configuration consists of a top lance with the addition of permeable elements or porous plugs (LBE process). An advantage to this process is the fact that steel can not intrude the pores even while gas pressure is not present. A problem arises, however, when a lime/slag agglomeration can cover the plugs and inhibit adequate stirring. An application of this process is the injection of nitrogen through the plugs to stir the bath and bring it closer to equilibrium.2

The second configuration found in combined blowing operations is the use of a top lance with the addition of cooled bottom tuyeres and is also known as the Kawasaki Basic Oxygen Process (K-BOP). The process varies from the first configuration in that the bottom permeable elements or porous plugs are replaced by cooled tuyeres which are similar to that of the bottom-blown process. This configuration proves to be more reliable than first configuration due to the reduced maintenance required by the tuyeres. To assure that blockage does not occur, the gas flow must remain throughout the process.

The third configuration consists of a top lance with the addition of uncooled bottom tuyeres. This process was designed to introduce large quantities of inert gas through each nozzle. This alleviates the need for cooling and produces strong stirring at the bottom of the vessel. A drawback to this system, however, is that air and oxygen can not be fed through the tuyeres due to a major decrease in their lifespan.

Like the bottom-blown processes, the combined blowing processes suffer from the need to change out the bottom refractories and tuyeres at a greater rate than the other refractories. They also suffer from the top-blown process's need for larger vertical space due to the oxygen lance height.

With the evolution of oxygen steelmaking throughout the years, it is clear that many choices are present to fill the needs of different situations. In either case, the need for greater process quality and efficiency will always be present. And with the fulfillment of these needs, new processes will develop that will continue to improve the way steel is produced using oxygen.

References

Technical Papers

1. C.P. Manning (2007) Emerging technologies for iron and steelmaking. Journal of the Minerals, Metals and Materials Society, 53(4), 36-43 DOI 10.1007/s11837-001-0054-3
2. P. E. Queneau (1996) Oxygen Pyrometallurgy at Copper Cliff- A Half Century of Progress. Journal of the Minerals, Metals and Materials Society, 48 (1), 14-21 Retrieved from http://www.tms.org/pubs/journals/JOM/9601/Queneau-9601.html#ToC8
3. Katsukiyo Marukawa, Isao Yamazaki, Syoji Anezaki, Tsutomu Kajimoto, "Production of carbon steel and low-alloy steel with bottom blowing basic oxygen furnace" Patent 4280838 July 28, 1981
4. Alper et al, "Refractory and Furnace Lining" Patent 3293053 December 1966
5. Carloien Kattenbelt(2008) Oxygen Steelmaking Using Measured Step Responses. Metallurgical and Materials Transactions B, 39 (5), 764-769 DOI 10.1007/s11663-008-9184-0

Books

• Fruehan, Richard J. The Making, Shaping and Treating of Steel 11th Edition Steelmaking and Refining Volume, Pittsburgh, PA The AISE Steel Foundation 1998

Webpages

• http://www.steeluniversity.org/content/html/eng/default.asp?catid=24&pageid=2081272110
• http://www.srmbsteel.com/srmb-steel.pdf