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Building America and Making Steel – The Bessemer Process

Building America and Making Steel – The Bessemer Process
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The Bessemer process of steel making was the first low cost industrial process for the mass-production of general steel from molten pig iron. Its inventor, Henry Bessemer, took out a patent on the process in 1855. The process was also independently figured out in 1851 by William Kelly. The process had also been used in other countries for hundreds of years, but not in industrial use. The primary principle involved is removal of impurities from the iron by oxidation through the use of air being forced through the molten iron. The oxidation also raises the temperature of the iron mass and maintains it’s molten state.

The Basic Bessemer process: A certain amount of lime is charged with the iron, and the process is conducted in practically the same way as the acid until the flame drops. Here, instead of turning the vessel down, the blowing is continued (sometimes a little more lime is added) for about four to five minutes, during which time the phosphorus and some of the sulphur are removed. The end of the operation is determined by taking a test ingot which is forged down, quenched in water, and broken; the fracture and also the malleability indicate the state of the process. The period before the drop of the flame is called the fore blow, the latter one the after blow (rarely over blow). When ready, the vessel is turned down, as much as possible of the slag poured off, and the metal decarburized in the vessel or in the ladle. As a small percentage of phosphorus always passes back into the metal from the slag, the heat must be blown down, i.e., the phosphorus reduced to a lower percentage than that required in the steel. A heating takes about 20 to 25 minutes, and one vessel can produce about 1000 to 5000 tons per day, depending on the size, which varies from about 15 to 25 tons

Modifications of the Bessemer process have been concerned almost entirely with basic practice, as will be seen below. The Champin pneumatic process was a modification for making wrought iron of which the advantages would appear to be very doubtful. Pig iron was blown in a converter in the usual way, and the blown metal was then distributed, by means of a ladle, into heated revolving cylinders, called ballers, and when it had solidified in balls, it was taken to a squeezer, and afterward given a wash heat, and then worked down as usual into the desired product. Flohr adds (basic process) briquets of iron ore, scale, etc., bound together with about 10% of slaked lime, and claims that thereby the slag is made thinner, and the dephosphorization much more rapid.

Harmet’s process consists in treating the molten pig, first in an acid vessel to eliminate the silicon, and then in a basic vessel to remove the phosphorus and the remaining impurities, care being taken that none of the silicious slag from the first vessel goes into the second; this is also termed “repouring” process, or transfer process. The Massenez modification of the basic process for treating high silicon metal consists in charging only part of the lime at the commencement of the process, and when the carbon flame appears (showing the silicon has been eliminated) as much as possible of the slag is poured off; the balance of the lime is then added, and the blowing completed. In Pettitt’s process (basic) iron oxide was used to effect the removal of the phosphorus, irrespective of the lining of the converter. Rochussen and Daelen’s process consisted in charging rich iron ore into the converter (acid) with the molten pig, and in lining the converter with it previous to the blow. The claim was made that the amount of blast required was largely diminished while the amount of steel was increased, but the excessive corrosion which must take place would more than offset any such advantages. Schiebler’s process consists in charging the metal and part of the lime in the vessel, and both the metal and slag are poured out when the first or phosphoric slag is formed. The treatment is completed in a furnace having a basic or a neutral hearth, with the addition of the balance of lime or flux.

In the United States, commercial general steel production using this method stopped in the late sixties. It was replaced by processes such as the Linz-Donawitz process, which offered better control of the final chemistry of the steel.