Thomas-Gilchrist Process

(also basic Bessemer process), a method of converting molten pig iron into steel without the consumption of fuel (seeCONVERTER PRODUCTION). The process, developed in 1878 by S. G. Thomas, competed successfully with the Bessemer process because it made possible the conversion of pig iron containing up to 2 percent P. It came to be widely used, since it produced cheaper steel than other methods.

Initially, the Thomas process was mainly used in Germany, which had large reserves of high-phosphorus ores from Lorraine. The first melts were made in 1879. The process was introduced in Russia in the 1880’s at the Taganrog, Kerch’, and Mariupol’ steel mills. By the end of the 19th century, 25 percent of the world output of steel was produced using the Thomas-Gilchrist process, making it second to the Bessemer process.

Thomas steel contains somewhat larger amounts of nitrogen and phosphorus than open-hearth steel, which tends to increase the brittleness and cold-shortness of the metal and limits its use. In the early 20th century, the Thomas-Gilchrist process was surpassed in volume of production by the open-hearth process, and its use continued to decline over the succeeding years. By 1974, Thomas steel accounted for less than 2 percent of world steel production.

The course of the Thomas-Gilchrist process is determined primarily by the chemical composition of the phosphorus-rich pig iron used. The Thomas converter is similar in design to the Bessemer converter, except that it is larger; the essential difference between the two is in the lining. The basic lining of the Thomas converter, thoroughly calcined dolomite, makes it possible to charge the furnace with lime (about 12–15 percent of the volume of pig iron) for slagging and the removal of phosphorus. After charging, molten pig iron at a temperature of 1180°-1250°C is poured into the converter, which is tipped up to its vertical operating position and the blast is turned on. During the blow, Si and Mn are fully oxidized and Fe, C, and P are partially oxidized. The blow is continued until a carbon content of 0.05 percent is achieved, since intensive oxidation of phosphorus (up to 0.04–0.05 percent) does not begin until near the end of carbon oxidation. Sulfur is only partially removed from the metal. During the Thomas-Gilchrist process it is often necessary to cool the metal by adding ore, scale, or scrap. At the end of the heat, the metal is deoxidized and recarburized using coke, graphite, ther-moanthracite, or charcoal in paper packages. The yield of acceptable metal is 85–89 percent, and the yield of Thomas slag, which is used as phosphorus fertilizer, is 18–20 percent of the weight of the metal. For converters having a capacity of 18–70 tons, the blow lasts 16–22 min, and the total length of a heat is 25–40 min. The smelted steel is used for rolled stock, sheet, roofing iron, wire, and rails.

New variations of the Thomas-Gilchrist process developed during the 1950’s produced steels with lower nitrogen content using air blows enriched with oxygen, a steam-oxygen mixture, and a mixture of oxygen and carbon dioxide. By the mid-1970’s, however, the Thomas-Gilchrist process had been replaced, for all practical purposes, by the basic oxygen process. There are no Thomas converters in the USSR.