Blast Furnace Plant

BLAST FURNACE PLANT Blowing Engines.—Fig. I indicates the essential steps where by the energy contained in the furnace gases is utilized to generate hot blast and auxiliary power for pumping, hoisting and lighting. Cleaned gas is used to drive gas engines direct connected with compressor cylinders. Compressed air goes to the furnace by way of a stove, therein acquiring a temperature of some 550° C, and on through a brick lined hot blast main to a bustle pipe circling the furnace. From the bustle pipe gooseneck connections deliver blast to the furnace through the tuyere openings. The required amount of air is fixed by the size of the furnace and the pressure, which varies from i2 to 20 lb. per sq. in. with oper ating conditions, and must be delivered without fail. About 50,000 cu. ft. of air per min. is necessary for a furnace making 600 tons of pig iron per day (see BLOWING ENGINES).

Dry Blast.

One auxiliary (not shown in fig. I) more com monly installed in Great Britain, although proposed by the Amer ican James Gayley, reduces the humidity of the blast. Gayley showed that water vapour entering the tuyeres, amounting in summer to about 25o gals. per hour, is dissociated into its elements by the white hot coke, this action absorbing heat not usefully regained within the furnace. Variations in moisture content are especially undesirable, for these produce variations in the tempera ture at the focus, and consequently change the chemical compo sition of the pig iron. To produce a low and uniform humidity in the air blast, the compressors may suck air through an adequate refrigeration system. (See AIR CONDITIONING.) Silica gel has been proposed as a more proper medium to dry the air blast.

Stoves.

Four stoves are usually installed to heat the blast. These are steel cylinders up to I oo ft. high and 22 ft. diameter. Each has a vertical combustion chamber, either at side or centre; the burned gases then return to the bottom through a multitude of narrow flues made of fire brick (see COWPER STOVE). In the passage they give much of their heat to the brick work, and flow off through a chimney. In operation (fig. I), stoves B, C, and D may at any hour be filled with burning gas, thus acquiring heat ; the cold blast passes through one of them, stove A; when this one cools off, valves are changed in a certain order, to prevent accumulation of explosive gas-air mixtures ; stove A will then be on gas, i.e., heating ; stove B will be on wind, i.e., giving up its heat to the air passing through it enroute from engines to furnace. Usually valves are changed every hour. In a four-stove installa tion, therefore, each stove is heating for three hours, and cooling for one. Many designs have been proposed for the method of building the stove interior. A two-pass stove is simplest, and most favoured in America, where the trend is to use cleaner gas, injected with a regulated amount of air for combustion, through carefully designed burners. Cleaner gas permits much smaller chequer-work, higher heat capacity, longer life before the openings are plugged with fused dust, frequently enabling satisfactory fur nace operation with only three stoves. A three-pass stove, where the gas is introduced at the bottom, goes up, down, up again and out a short stack built on top the stove, is favoured by some designers. (See HOT BLAST.) Valves for handling these furnace gases are of special construc tion. On the cleaning system a water valve is favoured. As shown in the diagram of the blast furnace department, it is nothing much but a large pot with a diaphragm extending from top toward the bottom. To close this valve it is only necessary to fill the pot with water until the bottom of the diaphragm is submerged several inches ; draining the pot opens the valve. Valves for handling hot blast are more difficult to construct and to maintain in good con dition. Also, since the valve seat is exposed to the corrosive action of hot gases, it requires occasional replacement. This can be done by loosening all the turnbuckle clamps, wedging the flue sections apart, and sliding the old seat out sideways. The valve disc is also a hollow copper casting with spherical bottom; cooling water enters the hollow stem as shown. To open the valve it is only necessary to lift the disc up until it strikes the bricked dome. All blast furnace stoves have four valves. Two are shown on stove A (fig. I) ; stove A is on wind, the chimney valve and the gas humor nnPnincr hPincr elncArl CtnvP R is nn crac • the r•h;mnnv x7n1x7c.

is open, the gas burner in place, and the valves into the cold and hot blast mains are closed.

Blast Furnace Operation.

From the foregoing description it may be surmised that the blast furnace superintendent's task is primarily to harmonize operations of various large mechanical elements. But the furnace itself is his greatest care, even though the principal reactions within it are in mobile equilibrium. Slight variations from normal are self-adjusting—that is to say, if for some reason a surplus of heat is generated, the action within the furnace changes in the direction which will absorb that extra quantity of heat. Nevertheless the skilled operator is quick to observe clues which indicate deviations from the normal, and he immediately makes such adjustments in the operating conditions as experience tells him will avoid any serious irregularity. A hundred years ago his problem was simpler. His furnace plant consisted only of stack and blower; his ore was pure and rich, carefully selected by hand ; his fuel was charcoal, ideal in char acter; the driving rate was slow; under such circumstances his pig iron was relatively free from impurities. Even to-day some of this

charcoal pig iron is made in small furnaces with either cold blast or hot blast, and is favoured by foundrymen for castings which require a dense, tough and strong body, or by manufacturers of high grade tool steels. Rich ores and forests were very soon exhausted, however, and the progress of metallurgy has been asso ciated with the ability to recognize the specific effects of each dis turbing element and to devise ways and means to utilize the impure ores and coke fuel remaining, for many of the minerals entering the furnace top with the iron ore are broken up and the elements therefrom alloy with the pig iron, often to its detriment.

Control of Sulphur.—Sulphur, largely from iron pyrite contained in coke ash, is the worst offender; if left in the iron causes it to be very tender at red heat (red short). High sulphur castings often crack in the moulds after cooling, and high sulphur steels may break when being worked in rolls or forging. Since no sulphur is removed in the converter or open-hearth steel making processes, it is essential that this element be eliminated from the pig iron. This is done in the blast furnace by charging limestone so the slag shall be distinctly basic, and thus have a higher solubility for sulphur compounds. Such a slag requires more coke, and German works now smelt with minimum heat and desulphurize the pig iron with sodium carbonate. Once in the iron, its harm ful effects may be mitigated in castings if two or three times as much manganese is present ; an innocuous manganese sulphide results. In steel, zirconium has been found to have the same ef fect. Sulphur may be eliminated in electric furnace steel by re fining at a high temperature under a "carbide" slag very high in lime on which floats some coke. Pig iron for grey iron castings must therefore have less than o.Io% sulphur; structural steel must contain less than o.o6% ; many alloy steels are specified to contain less than 0.04% sulphur.

Effect of Silicon on Pig Iron.—A blast furnace working very hot tends to reduce silicon and manganese from any minerals con taining them present in the charge. Manganese is now considered beneficial in all subsequent processes, but silicon must be sharply limited to pass some specifications. Silicon in pig iron causes much of the carbon present to separate and collect into tiny flakes of graphite, permeating the entire metallic structure ; this graphite is responsible for the characteristic grey fracture of castings for machinery ; its presence also makes for easy machineability. The carbon would otherwise exist in the alloy as a compound of iron, a carbide, called cementite, and form a much harder, stronger alloy. Consequently in such things as car wheels or in white iron castings, which are later to be converted into malleable iron (q.v.) by annealing, the silicon must be strictly limited. Sili con in pig for steel making must also be under close control, as we shall presently see. Therefore for those irons where both sul phur and silicon are limited, the ore and fuel must be specially selected for a low content of sulphur.

Grey and White Iron.—Carbon is another element in pig iron which will vary somewhat. It may be from 3.o to 4.25%, depend ing upon furnace conditions. Generally if either silicon or phos phorus runs unduly high, the carbon will be low. Manganiferous irons, on the other hand, are usually high in carbon. The colour of a freshly broken piece of pig iron is due to the way the carbon is held, rather than its total quantity ; in white iron, nearly all the carbon is combined with iron in the carbide cementite ; in grey iron, seven-eighths of it is in the form of graphite ; intermediate proportions are called mottled irons.

Phosphoric Ores and Iron.—Phosphorus, until about the turn of the century, was the most undesirable element entering the blast furnace. Whereas much sulphur can be slagged off, between 90 and i 00% of the phosphorus entering with the ore and the fuel ash comes out with the iron, and an excess makes both iron castings and steel brittle. For some types of thin castings requiring fluid iron e.g., stove plates, from about r.o to i•5% of phosphorus may be permitted, but in steel it is kept as low as sulphur, namely from 0.02 to 0.07%; in general, the lower the better the steel. Conse quently the phosphoric iron ores of Lorraine were of small value— in fact of no value for steel-making until the Thomas-Gilchrist basic process was suggested (1878). Subsequent developments have converted the phosphorus in the ore to a pronounced eco nomic asset. In view of all these facts, the furnace operator selects and mixes the ores and fuels which are available to him, and then adjusts his flux and controls his hearth temperature, by varying the amount of coke charged and the temperature of the blast, so the desired kind of iron will issue from the tap hole (see CAST IRON). Further notes on the varieties required for the various conversion processes will be given later.