lime to it. In the basic Bessemer process phosphorus is readily
removed by oxidation, because the product of its oxidation,
phosphoric acid, P2O5, in the presence of an excess of base forms
stable phosphates of lime and iron which pass into the slag,
making it valuable as an artificial manure. But this dephosphorization
by oxidation can be carried out only in the case slag
is basic. If it is acid, i.e. if it holds much more than 20% of
so powerful an acid as silica, then the phosphoric acid has so
feeble a hold on the base in the slag that it is immediately re-deoxidized
by the carbon of the metal, or even by the iron itself,
P2O5 + 5Fe = 2P + 5FeO, and the resultant deoxidized phosphorus
immediately recombines with the iron. Now in an acid-lined
converter the slag is necessarily acid, because even an initially
basic slag would immediately corrode away enough of the acid
lining to make itself acid. Hence phosphorus cannot be removed
in an acid-lined converter. Though all this is elementary to-day,
not only was it unknown, indeed unguessed, at the time of the
invention of the Bessemer process, but even when, nearly a
quarter of a century later, a young English metallurgical chemist,
Sidney Gilchrist Thomas (1850–1885), offered to the British Iron
and Steel Institute a paper describing his success in dephosphorizing
by the Bessemer process
with a basic-lined converter
and a basic slag, that body
rejected it.
 | |
| Fig. 16.—12-15 ton Bessemer Converter. | |
A, Trunnion-ring. |
O, Tuyere-plate. |
 |
| Fig. 17.—Bessemer Converter, turned down in position to receive and discharge the molten metal. |
85. In carrying out the acid Bessemer process, the converter, preheated to about 1200° C. by burning coke in it, is turned into the position shown in fig. 17, and the charge of molten pig iron, which sometimes weighs as much as 20 tons, is poured into it through its mouth. The converter is then turned upright into the position shown in fig. 16, so that the blast, which has been let on just before this, entering through the great number of tuyere holes in the bottom, forces its way up through the relatively shallow layer of iron, throwing it up within the converter as a boiling foam, and oxidizing the foreign elements so rapidly that in some cases their removal is complete after 5 minutes. The oxygen of the blast having been thus taken up by the molten metal, its nitrogen issues from the mouth of the converter as a pale spark-bearing cone. Under normal conditions the silicon oxidizes first. Later, when most of it has been oxidized, the carbon begins to oxidize to carbonic oxide, which in turn burns to carbonic acid as it meets the outer air on escaping from the mouth of the converter, and generates a true flame which grows bright, then brilliant, then almost blinding, as it rushes and roars, then “drops,” i.e. shortens and suddenly grows quiet when the last of the carbon has burnt away, and no flame-forming substance remains. Thus may a 20-ton charge of cast iron be converted into steel in ten minutes.[1] It is by the appearance of the flame that the operator or “blower” knows when to end the process, judging by its brilliancy, colour, sound, sparks, smoke and other indications.
86. Recarburizing.—The process may be interrupted as soon as the carbon-content has fallen to that which the final product is to have, or it may be continued till nearly the whole of the carbon has been burned out, and then the needed carbon may be added by “recarburizing.” The former of these ways is followed by the very skilful and intelligent blowers in Sweden, who, with the temperature and all other conditions well under control, and with their minds set on the quality rather than on the quantity of their product, can thus make steel of any desired carbon-content from 0.10 to 1.25%. But even with all their skill and care, while the carbon-content is still high the indications of the flame are not so decisive as to justify them in omitting to test the steel before removing it from the converter, as a check on the accuracy of their blowing. The delay which this test causes is so unwelcome that in all other countries the blower continues the blow until decarburization is nearly complete, because of the very great accuracy with which he can then read the indications of the flame, an accuracy which leaves little to be desired. Then, without waiting to test the product, he “recarburizes” it, i.e. adds enough carbon to give it the content desired, and then immediately pours the steel into a great clay-lined casting ladle by turning the converter over, and through a nozzle in the bottom of this ladle pours the steel into its ingot moulds. In making very low-carbon steel this recarburizing proper is not needed; but in any event a considerable quantity of manganese must be added unless the pig iron initially contains much of that metal, in order to remove from the molten steel the oxygen which it has absorbed from the blast, lest this make it redshort. If the carbon-content is not to be raised materially, this manganese is added in the form of preheated lumps of “ferro-manganese,” which contains about 80% of manganese, 5% of carbon and 15% of iron, with a little silicon and other impurities. If, on the other hand, the carbon-content is to be raised, then carbon and manganese are usually added together in the form of a manganiferous molten pig iron, called spiegeleisen, i.e. “mirror-iron,” from the brilliancy of its facets, and usually containing somewhere about 12% of manganese and 4% of carbon, though the proportion between these two elements has to be adjusted so as to introduce the desired quantity of each into the molten steel. Part of the carbon of this spiegeleisen unites with the oxygen occluded in the molten iron to form carbonic oxide, and again a bright flame, greenish with manganese, escapes from the converter.
87. Darby’s Process.—Another way of introducing the carbon is Darby’s process of throwing large paper bags filled with anthracite, coke or gas-carbon into the casting ladle as the molten steel is pouring into it. The steel dissolves the carbon of this fuel even more quickly than water would dissolve salt under like conditions.
88. Bessemer and Mushet.—Bessemer had no very wide knowledge of metallurgy, and after overcoming many stupendous
- ↑ The length of the blow varies very greatly, in general increasing with the proportion of silicon and with the size of charge. Thus the small Swedish charges with but little silicon may be blown in 5 minutes, but for a 20-ton charge the time is more likely to reach, or exceed 10 minutes, and sometimes reaches 20 minutes or even more.
