Nor is it necessary that the air be blown through the metal. Air blown upon its surface accomplishes practically the same purpose, and in many of the steel foundries of to-day smaller converters of this “surface-blown” type are used for producing steel for castings. The large steel plants, however, use the larger “bottom-blown” converter. Two or three of these vessels, working with proper metal from the “mixer,” produce an immense tonnage of steel each 24 hours.
The “mixer” is quite necessary. It is a large vessel or furnace holding and keeping hot from 75 to 300 or more tons of metal from the blast furnace. It mixes and equalizes irons of various compositions, so that the converters have the advantage of uniform and hot metal with which to work.
In addition it is made to perform a “refining” service. By mixing into the metal a quantity of manganese, considerable of the sulphur present (a deleterious substance) is removed.
The fifteen or twenty minute blowing of 15 tons of metal in the big egg-shaped converters of a steel plant presents a spectacle which, when once observed, will never be forgotten.
One sees a little “dinky” engine come shooting into the converter building with its ladle of molten iron from the “mixer.” With America’s time saving routine not a single minute is lost while emptying the metal into the converter, now in a horizontal position. Almost before the ladle is out of the way, the converter swings to the upright position with the blast already on, for otherwise the metal would flow into the tuyère holes at the bottom.
Comparison of Ingots
A. From Four Pot Crucible Furnace: Each Heat 400 Pounds in 4 Hours or 100 Pounds an Hour, Each Heat Pours 4 Ingots 3 × 3 × 36″. B. From Fifteen Ton Bessemer Converter: 30,000 Pounds in 20 Minutes or 90,000 Pounds an Hour, Each Heat Pours 6 Ingots 19 × 20 × 62″. C. From Fifty Ton Open-Hearth Furnace, 100,000 Pounds in 8 Hours or 12,500 Pounds an Hour, Each Heat Pours 6 Ingots 24 × 32 × 72″.
Reddish-brown smoke and a shower of sparks come from the converter. These gradually develop into a flame.
The blast shows considerable partiality in selecting for its first attention the metalloids silicon and manganese, in preference to the iron itself or any other of the metalloids present. After from three to five minutes half of the silicon and manganese have been burned out. If the temperature of the metal and other conditions have become right the carbon then begins to burn. This gives a change in the nature of the flame which becomes large and of a dazzling whiteness.
The metal is hot—very hot—so much so that pieces of cold steel often must be dropped in to cool it somewhat. This is known as “scrapping” the charge.