When iron is produced from the ore in a blast furnace, it is rich in carbon and also contains many impurities. It then has to be converted into steel which has controlled quantities of carbon and much reduced concentrations of impurities.
Bessemer realised that forcing air through molten steel would oxidise impurities in the pig iron and raise the metal temperature without the need for additional fuel, i.e., `The manufacture of malleable iron and steel without fuel'.
His invention nearly failed in that some of the iron ores used by those who applied the idea found the iron to contain larger quantities of sulphur and phosphorus, leading to very poor properties. However, this was solved by Robert Forester Mushet who decided to first burn off most of the impurities along with carbon, and then top up the carbon by adding ferromanganese to the melt.
The nitrogen concentration of steel produced using the Bessemer process tended to be greater than desirable because of the use of air (which is mostly nitrogen).
This problem was solved in the Linz-Donawitz process (Linz and Donowitz are both parts of Austria). In this, high-pressure oxygen is blown at the molten pig iron using a water-cooled lance. A slag of lime and dolomite is used. The process is fast because of the use of commercially pure oxygen, and generates large quantities of heat which can be exploited to add scrap steel to the melt. The basic nature of the flux helps remove phosphorus so there is a greater tolerance to the raw materials used in the process.
The following photographs are provided by Professor Nirupam Chakroborti (IIT Kharagpur) who is shown next to the converter with his colleague Dr Ewald Linder. They are taken at the Voest-Alpine steel plant in Austria.
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