The blast furnace is the real heart of the iron and steel industry; it is the most important device which mankind yet has developed. The continued operation of the furnaces is related to our daily life in a similar sense as is the daily rising of the sun we could get along without either for a while, but soon our whole condition of existence would change.
Fig. 16. Iron-One Washing Plant to Treat Twenty Thousand Tons ■ Day This plant is located at Coloraine, Minnceola.
The blast furnace for smelting iron ores is one of our largest machines and also one of the most complicated. At the top is charged in the iron ore, the limestone to flux the gangue, and the coke as the fuel. Near the top exit continuously the gases from the combustions and reactions, as well as the flue dust. The gas has considerable fuel value and is used partly to reheat the blast and partly to drive the engines for furnishing the blast, while a further portion is left over to use as desired in the plant. Through the tuyeres near the bottom of the furnace a great quantity of hot air is blown in to burn the coke and to maintain the smelting temperature of some 1600° C. Iron is tapped out every six hours from the bottom of the furnace, and slag is tapped more frequently from the cinder notch a little higher up but still below the tuyeres. As the charge settles through the furnace the iron ore is reduced to metallic iron. The limestone is calcined to lime; when the charge has settled far enough, the lime and silica of the gangue combine to make the slag, while the iron saturates with all of the available carbon, silicon, manganese, and phosphorus, and at last, wholly molten, trickles into the crucible to await casting time.
Fig. 17. Vertical Section of Iron Blast Furnacs Couttesy of "Engineering and Mining Journal.
To accomplish the reduction fully and to maintain the high and constant heat necessary, a large and immensely strong furnace is required. The base is a massive foundation; the crucible is water-cooled and is bound in with thick steel bands; the tuyere nozzles are clamped solidly; the boshed fusion zone is of the best firebrick held with heavy thick steel bands and is cooled thoroughly with bronze water blocks; the shaft is a massive steel shell lined with first-quality firebrick.
Fig. 18. Casting Iron at a Modern Furnace.
Fig. 17 is a section through a large iron blast furnace. This cut admits of a close inspection of the lines of the furnace: the cmcible, tuyeres, and boshed zone each are indicated clearly. Above the boshed zone the main upright truncated cone of the shaft extends upward; note how this shaft is supported on special columns, and see the thickness of the walls. Figure out how the bell mechanism at the top allows the charge to get inside the furnace without letting the gases escape. The inside dimensions should be copied and the shape should be redrawn on a larger scale. Follow the iron ore from the ore bin, and explain which part comes out through the dust catcher, which part through the iron tap, and which part through the cinder notch.
In the halftone from the photograph of the furnace, Fig. 18, we have a fair view of what the inside of the casting room looks like when the pig iron is running from the furnace. The hot metal is pouring out in a thick stream and runs through the iron and sand channels into the ladle at one side and lower than the floor of the casting room. Men are on hand to keep the flow of metal clear of obstructions. The big bustle pipe around the bosh is much in evidence, as are also the pipes connecting to the tuydres which are seen inserted between the steel bands around the furnace. Just over the runway, which is immediately above the bustle pipe, can be seen the many overflows of the cooling water which has circulated throughout all the lower section of the furnace inside the cooling blocks; each of these overflows is in plain sight from the floor. A big annular drain box collects all the water, which then flows away through the big pipe leading down over the men at the right.
The usual furnace of today makes about 500 tons of pig iron a day. Each ton of iron produced requires some 2 tons of ore and a ton of coke in the charge, besides the necessary limestone, while for this same tonnage of iron some 4 tons of hot air must be driven through the tuyeres under a pressure of from 15 to 30 pounds per square inch. At the top a double bell-valve arrangement is necessary to drop in the frequently arriving skips of charge without letting out the gases which are carefully conserved and, after leading down and through the dust catcher, are conducted to the other parts of the plant.