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Construction materials and main structural elements
Published in Pere Roca, Paulo B. Lourenço, Angelo Gaetani, Historic Construction and Conservation, 2019
Pere Roca, Paulo B. Lourenço, Angelo Gaetani
Steel represents the technological evolution of wrought and cast iron. It is an alloy of iron and other elements, primarily carbon, widely used in construction because of its high tensile strength (and low cost). Although steel has been produced in bloomery furnaces for thousands of years, its use expanded extensively starting from the 18th century, with the invention of more efficient production methods. An important breakthrough was represented by the invention of the Bessemer process in the mid-19th century, marking the start of a new era. With the Siemens-Martin and then Gilchrist-Thomas process, mild steel replaced wrought iron.
Historical overview on the development of converter steelmaking from Bessemer to modern practices and future outlook
Published in Mineral Processing and Extractive Metallurgy, 2019
As pointed out earlier, the Bessemer process could not accept hot metal rich in phosphorus. The reason was that the converter had an acidic silica lining, which made it impossible to make basic slag and refine high phosphorus hot metal, common from British as well as continental blast furnaces that time. It was known that phosphorus could be removed by using basic lime-rich slag but that rapidly destroyed the acid lining. The problem was investigated in the 1870s by calcining limestone (CaCO3), magnesite (MgCO3) or dolomite (Ca,Mg)CO3 at high temperature to produce calcia, magnesia or doloma material, which was then pressed into bricks for lining the converter. Problems arose due to the hydration sensitivity of these materials. S. G. Thomas and P. C. Gilchrist, who were cousins, succeeded in developing basic doloma lining, which achieved readiness for industrial application in 1879 (Barracglough 1990). The decisive inventions were dead burning of dolomite at 1200°C and hot tar binder in brick ramming, which gave good mechanical strength and hydration resistance. Metallurgical results of industrial trials were most promising. Hot metal for the Gilchrist-Thomas process could contain up to 3% P. By using lime-rich slag (∼50% CaO), phosphorus could be effectively removed into the slag as calcium phosphate. It is noteworthy that most dephosphorisation took place only in the final stage of the blowing; i.e. at low carbon level. In order to reach low P contents, two minutes ‘overblow’ was continued after the carbon flame had died down. A comparison of the acid and basic processes is shown in Figure 3 (Barth 1942). The new process had another advantage. Due to high phosphorus burden the Thomas slag, which formed in the process, was very rich in calcium phosphate (around 5wt % P in the slag) and was thus a valuable by-product suitable as fertiliser. Basic Thomas converters gradually gained a foothold especially in Central Europe where high-P ores were utilised in larger amounts.