Thomas-Gilchrist Discovery

THOMAS-GILCHRIST DISCOVERY Control of Phosphorus.—This so-called acid process demands low-phosphorus ores and pig, and such material in Europe is relatively scarce and costly. Early efforts to remove phosphorus in the converter failed. Arguing from an analogy in the puddling furnace, where a slag rich in iron oxide will carry off phosphorus oxides, it was thought that if the converter blast was carried on after the carbon was gone, much iron oxide would go to the slag, and it would then be in a condition to absorb the phosphorus. When this was tried, however, the iron wasted away, ate into and ruined the lining, but unfortunately had no effect on the phos phorus. Not until 1876 did the Englishmen, Sidney G. Thomas and Percy G. Gilchrist discover that if the converter were lined with a basic material like burned limestone, dolomite or magnesite, and if burned lime were charged with the pig iron, then a basic slag would form and be maintained in such condition that it would absorb and hold phosphorous oxides produced during the blow. This is the basic Bessemer process now so widely used on the Continent for the production of so-called Thomas steel.

Basic Bessemer Process.

A modern basic plant is arranged much like its counterpart using the acid process. Converter shells are lined with different materials. Dolomite is freshly calcined in a cupola, ground with hot dehydrated tar, and the sticky mass tamped between the shell and a centre form. The perforated bot toms are lined two ft. thick, yet must be replaced daily.

In its modern development a heat of Thomas steel is made thus: Into a r o-ton converter II tons of freshly burned lime are charged and then ro tons of pig iron. The blast is turned on, the converter tipped up, and the blow proceeds exactly like an acid heat already described, but it continues after the carbon flame drops. In this last stage the following reaction occurs : 5Fe0+2P=5Fe+P205 and the phosphorus joins with excess lime in the slag to form a calcium phosphate. There is no flame to indicate occurrences in the converter during this period. The duration of the after blow is judged from a knowledge of the phosphorus contained in the original pig iron. When the process is thought to be com plete, the converter is tipped to its side, the blast turned off, and a spoonful of metal cast into an iron mould. This test ingot is chilled, hammered into a bar, and nicked, bent and broken. If the metal is brittle and coarsely crystalline, the air blast is re sumed for a few seconds, and another test taken, and repeated until the metal is tough and fine grained. This finishes the blow. Next the slag must be skimmed off as closely as possible, because phosphorus has a trick of returning to the metal on slight pre text ; then the spiegel or other recarburizer and degasifier is added, and the steel poured into the ladle. About the only additional

equipment in the converter house needed beyond that found in an acid Bessemer plant is some handy means of caring for the large volume of slag produced. This slag, by the way, is ground and sold as fertilizer, it contains up to 20% of soluble phos phoric acid.

Thomas Steel.

In this Thomas-Gilchrist process, the pig iron must have relatively low silicon to limit the amount of slag formed. Phosphorus is the principal heat producing element, and it may be as high as 3%. It is doubly essential that a mixer be used to store pig iron, in order that phosphorus may never show wide fluctuations in analysis, heat to heat. Percentages in average analyses are as follows: with hearth and side walls of sintered dolomite—lime and mag nesia oxide—and to refine the steel under a high lime slag. J. H. Darby built the first of these basic open-hearth furnaces at Brymbo, Wales in 1884. The most available ores in both England and America smelt into pig iron well suited for such basic open hearth refining; furthermore the product is distinctly superior in quality to Bessemer steel. Consequently at the present time the latter is used in America for the production of low-carbon steels for non-engineering purposes. Basic open-hearth steel, however, makes boiler and ship plate, structural steel, rails and all the higher carbon machine steels (and even alloy steels), such as shafting and forging billets. Production by processes is shown in the following table—it will be observed that the basic open hearth is competing with the basic converter even on the Con tinent.

The process was introduced in Germany in 1879, and at the present time is used to refine the bulk of the pig iron produced from the Lorraine deposits in France, Germany, Luxembourg and Belgium.

Thomas steel provides satisfactory material for all classes of tonnage steels used on the Continent, such as bars, boiler plate, pipe, structural steel and rails. But despite the fact that the Thomas-Gilchrist process was proposed to solve the problem of high phosphorus British ores, Thomas steel is not popular among English engineers, and hardly any of it is now made in England. This is probably due to the fact that the home ores do not smelt into a low-silicon high-phosphorus pig iron necessary for the economical operation of a basic converter, and further that the basic open-hearth furnace has meanwhile been developed to refine such phosphoric pig irons as are made.