The interesting thing about the BOS process is that the original concept is actually from the 19th century. Recall that the Bessemer process that we studied earlier, works by blowing air through hot molten metal and the oxygen in the air burns off the impurities in the molten iron. Well, the reader is probably thinking that since air consists of a mixture of nitrogen, oxygen, carbon dioxide and other gases, and since only oxygen is needed in this process, wouldn't the process become more efficient if we directly blew pure oxygen over the molten metal? The same idea occurred to Henry Bessemer (allegedly suggested to him by his father as a joke) and he received a patent on October 5th, 1858 for this concept. Unfortunately for him, this idea was not practical in the 19th century, because bottled oxygen was not available at reasonable cost or in large quantities at that time.
Also recall when we studied the Bessemer process, there are two types: the acid bessemer process and the basic bessemer process. The "basic bessmer process" is called that, because it uses an alkaline (i.e. basic) lining in the vessel (as opposed to an acidic lining). The Basic Oxygen Steelmaking process is also called "basic" because it uses an alkaline lining (usually, Magnesium Oxide (MgO)) in the vessel. The purpose of this alkaline lining is to remove elements such as phosphorus and sulfur from the molten metal, as these elements are harmful to steel's properties.
The idea of using oxygen in the furnace was revisited in the 20th century and made practical during the late 1940s. Interestingly, the modern BOS process was developed, not by any large steel companies, but mainly due to the efforts of one man and the support of a few managers in a small company that he worked for. Our story starts with a Swiss metallurgist, Robert Durrer, who graduated from Aachen university in Germany in 1915 and remained there until 1943. He served as a professor of steelmaking in Berlin's Technishe Hochschule (Berlin Institute of Technology) between 1928 and 1943, where he performed many years of experiments using oxygen for steel refining. In 1943, he returned to Switzerland and joined a small Swiss company called Von Roll AG. Here, he continued his experiments in the town of Gerlafingen, with a German colleague, Dr. Heinrich Hellbrugge. In 1947, Durrer bought a small 2.5 ton converter from the US and with it, he reported his first success in the internal plant newspaper in May 1948:
"On the first day of spring, our "oxygen man", Dr. Heinrich Hellbrugge carried out the initial tests and thereby, for the first time in Switzerland, hot metal was converted into steel by blowing with pure oxygen... On Sunday, the 3rd of April 1948 ... results showed that more than half the hot-metal weight could be added in the form of cold scrap ... which is melted through the blast produced heat"
Soon after this, two Austrian steelmakers, VOEST and Alpine Montan AG (OAMG), got interested in these developments and worked with Von Roll to commercialize this process. Theodor Suess of VOEST's plant in Linz and the managers of the Alpine Montan plant in Donawitz organized the actual experiments and worked out all the technical issues and decided to construct two 30-ton furnaces in 1949. On November 27th 1952, the first steel was produced by this new type furnace. Since the VOEST plant in Linz and the Alpine Montan plant in Donawitz were instrumental in commercializing this technology, their version is called the Linz-Donawitz process.
Since oxygen containers became available in large quantities and low cost after the 1940s, this process was very efficient and cheap. Readers interested in history might be amused to learn that the reason that methods to produce low-cost oxygen at large volumes were developed was mainly because of the German V2 rocket program! After World War II, the Germans were not allowed to manufacture oxygen in large quantities, but the factories and equipment that they had pioneered were shipped off to other countries.
In the beginning, big steel manufacturers in the US paid no attention to this innovation by a small Central European company, whose total steel making output was less than one third that of a single US Steel factory! A smaller American company, McLouth Steel in Michigan, was the first to install BOS furnaces in the US in 1954. The larger American companies, such as US Steel and Bethlehem Steel only built their first BOS furnaces in 1964. However, the rest of the world quickly adopted this new technology and by 1970, 50% of the world's steel (and 80% of Japan's steel) came from BOS furrnaces. As recently as 2011, about 70% of the world's steel output was still made using this method.
A large container, called a ladle, is lined with refractory materials, such as magnesium oxide (MgO). The ladle is tilted about 45 degrees and is charged with scrap steel and then molten pig iron from a blast furnace is also added. The ratio is about 20-30% of scrap steel to about 70-80% of molten pig iron, based on the requirements of the final steel to be produced. This takes a couple of minutes. After this, fluxes such as magnesium or lime are added to remove sulfur and phosphorus. Then the vessel is turned back to the vertical position and a water-cooled lance with a copper tip is lowered down within a few feet of the bottom of the vessel. Through this lance, pure oxygen (greater than 99% pure) is blown over the hot metal at supersonic speeds (about 2x the speed of sound). The oxygen ignites the carbon in the molten iron, forming carbon monoxide and carbon dioxide. These reactions are exothermic (i.e. they produce heat), so the temperature of the molten iron increases even more. The magnesium burns with the sulfur, forming magnesium sulfide, which is also an exothermic reaction, contributing to the rise in temperature. Silicon combines with the oxygen forming silicon dioxide slag. The blowing of the oxygen also churns the molten metal and fluxes, which helps the refining process. The slag, being lighter than the molten steel, floats on top of it.
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The temperature of the furnace is closely monitored and after about 15-20 minutes, a small sample of the steel is taken and analyzed to make sure that its chemistry is correct. After that, the furnace is tilted horizontally and the molten steel is tapped out into another ladle. At this point, other alloying elements such as nickel, chromium etc. may be added. Sometimes, an inert gas, such as argon may be bubbled through the ladle, to mix the alloying elements properly into the steel. To prevent slag from being poured out with the steel at the end of the tapping process, various "slag stoppers" are used, but a human eye remains the best device to determine when to stop tapping the steel. After tapping the steel out, the vessel is turned upside down and the remaining slag is poured out into a separate slag pot. The vessel is examined to make sure its refractory lining is intact and more lining material is added if needed and the vessel is prepared for the next batch.
The entire process takes about 40 minutes, which is substantially faster than the 10-12 hours that the Open Hearth Process takes. This is why it quickly replaced the open hearth process in many places around the world. Using pure oxygen instead of air makes the process more efficient and it also avoids piping nitrogen and other undesirable gases in the air through the molten steel. The process can take about 250-350 tons of metal in one charge. Unlike the electric arc furnace, this is a primary steelmaking process (i.e.) it works mostly with pig iron rather than scrap steel. This process increases the productivity of steel making -- in fact, as this process became popular, the labor requirements of steel making went down by a factor of 1000. Instead of taking 3 man-hours per ton of steel produced, it now takes 0.003 man-hours per ton of steel. The only disadvantage of this over the open-hearth process is the reduced flexibility of the charge -- the open hearth process can use up to 80% scrap steel, whereas the BOS process can only use a maximum of about 30% of scrap steel. About 70% of the world's steel today is made by the BOS process.
In our next post, we will look at some finishing up processes for steel and after that, we will look at a factory producing rifle barrels at the beginning of the 20th century.