The oxidation process removes and skims off impurities such as silicon, manganese, and carbon in the form of oxides. These oxides either escape as gas or form a solid slag. The refractory lining of the converter also plays a role in the conversion—the clay lining is used in the acid Bessemer, in which there is low phosphorus in the raw material. Dolomite is used when the phosphorus content is high in the alkaline Bessemer (limestone or magnesite linings are also sometimes used instead of dolomite)—this is also known as a Gilchrist-Thomas converter, named after its inventor, Sidney Gilchrist Thomas. In order to give the steel the desired properties, other substances could be added to the molten steel when conversion was complete, such as spiegeleisen (a ferromanganese alloy).
Managing the processWhen the required steel has been formed, it is poured out into ladles and then transferred into moulds while the lighter slag is left behind. The conversion process called the "blow" is completed in around twenty minutes. During this period the progress of the oxidation of the impurities is judged by the appearance of the flame issuing from the mouth of the converter: the modern use of photoelectric methods of recording the characteristics of the flame has greatly aided the blower in controlling the final quality of the product. After the blow, the liquid metal is recarburized to the desired point and other alloying materials are added, depending on the desired product.
A Bessemer converter can treat a heat (batch of hot metal) of 5 to 30 tonnes at time . They usually are operated in pairs; one being blown while another being filled or tapped.
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