THE MODERN BLAST FURNACE
Blast furnaces are towering cylindrical structures of steel lined with fire brick. They are loaded from the top with alternate layers of coke and ore. Limestone also is added to act as a flux for the earthy matter, as explained above. Running around the furnace near the base there is a large pipe known as the “bustle” pipe. Through this air is fed to a set of “tuyeres” which lead to the base of the furnace and admit blasts of air requisite to maintain combustion at an intense heat. The gases of combustion are not allowed to escape freely into the atmosphere. The top of the furnace through which the ore and fuel is admitted is closed by an air lock and the flaming hot gases are led into “stoves” where they give up a large part of their heat to preheat the air which is pumped to the blast furnace. The gases being mainly composed of carbon monoxide are further combustible and may be used for heat, light, and power purposes. In fact, they are commonly used to drive the air compressors which feed the blast furnaces.
The blast furnace has two openings, one above the other. Through the upper one slag is drawn off while the molten iron which trickles down and collects at the bottom of the furnace is tapped off through a hole near the base of the furnace. The fiery stream pours out into a lot of small trough-shaped molds and is thus formed into “pigs.” These pigs are all connected to the main body of the metal stream and must be broken off. To save the time of cooling and of breaking off the pigs a machine is used which consists of a series of molds connected to form an endless belt. The molten iron is poured into these molds which in their course dip into a trough of water. Here the iron is cooled and solidified. The molds then run up an incline and finally dump the pigs directly into railway cars which haul them away.
BURNING OUT THE CARBON
The production of steel economically and on a large scale dates back to the inventions of Henry Bessemer. While searching for an improved method of making big guns, Bessemer hit upon the idea of forcing a blast of air through the molten iron and thus burning away carbon, silicon, and manganese in the cast iron. No fuel was supplied except the carbon and silicon in the iron itself. In burning out this carbon sufficient heat was generated to keep the metal fluid.
When Bessemer made the announcement of his new process before the British Association in 1856, his paper met with skepticism, but he was able to demonstrate by actual experiment that cast iron could be converted into malleable iron in this way. However, when several firms operating under licenses from the inventor endeavored to reproduce his experiment on a commercial scale they were unsuccessful, and after costly experiments the process was given up as a failure. Bessemer, however, persisted in his efforts and succeeded eventually in producing malleable iron of a quality equal if not superior to that on the market. But iron makers after the failure of the first experiments would have nothing to do with the new process until Bessemer began to turn out quantities of iron at $100 a ton below the prevailing market price. Then iron makers woke up and Bessemer had no difficulty in placing his process with numbers of firms on a very profitable royalty basis.
This process pertained to the making of iron and not steel. When Bessemer tried to produce steel he was confronted with serious difficulties. The steel he obtained was very brittle. He tried purer ores with little better success. Then a solution of his problem was offered by Robert F. Mushet, who discovered a compound which would be added to the molten metal to purify it. This compound which is known as “spiegeleisen” is composed of iron, carbon, and manganese. It removes the oxide of iron and the sulphur and regulates the amount of carbon in the steel.
A Bessemer converter furnishes by far the most spectacular operation in steel manufacture. The converter consists of a large bottle-shaped vessel lined with refractory material. In the bottom of the vessel there are openings through which the air blast is admitted. The molten metal is poured into the flask and then the air blast is turned on. The metal begins to boil violently. A dazzlingly brilliant blast of flame and sparks comes roaring out of the mouth of the converter. Bubbles of metal are thrown high into the air where they burst into showers of sparks. The effect is similar to that of a volcanic eruption. In from ten to twenty minutes the eruption subsides and then a quantity of spiegeleisen is added. The converter is mounted on trunnions so that when the operation is completed the vessel is tilted over and the charge of molten metal now converted into steel is poured out.