Fluxes
Limestone, the rock which is ordinarily used for fluxing purposes, needs no introduction to any of us. As the marble of statuary, the material of which oyster and other sea shells and the white tombstones of our cemeteries are composed, it is well known. Any of these varieties of the material may be used for fluxing purposes, but usually it is limestone which is quarried for the purpose or obtained as chippings or spalls from building blocks.
Coke Going from Quenching Car to Bins
Loading Coke in Box Car
The active agent, which produces the chemical or fluxing action in the blast furnaces, is carbonate of calcium (lime) of which limestone contains about 98 per cent. Dolomite is a mixture of carbonates of lime and magnesium, about 53 per cent of the former and 45 per cent of the latter, and is sometimes used in place of limestone. Fluor spar, a rock composed of calcium and fluorine, is used in small quantities in some of the metallurgical processes. It is a very powerful flux.
CHAPTER IV
THE BLAST FURNACE
Up the dark tower shoots the elevator with its “buggy” of coke. Its speed is not conditioned to the comfort of man, who is not supposed to be a passenger, except the occasional laborer whose duty as buggy-pusher requires his presence on twelve-hour shifts at the top. So we, whose exploratory proclivities have led us at the office to sign away our lives for grant of a pass to the blast furnace, find our breath about taken from us with the first mad dash into the darkness of the climb. That stone tower had looked much more innocent from below.
But now the rickety elevator has as suddenly emerged into the light again and stopped abruptly at the charging floor which extends across the chasm to the top of the furnace.
As the smoke-begrimed buggy-pushers rush the buggy of coke across to the furnace bell, we have opportunity to notice that we are a full hundred feet above ground. Just here, seemingly so close that we can put our hands on them, in a row, are the round steel tops of the four stoves which are for the purpose of preheating the blast. The huge pipes, dust arresters, tanks, and buildings, all so necessary to the plant, look almost like a tangled mass from our high station, while the charging floor upon which we stand, the shoulder-high steel fence around it, the furnace top, the adjacent stoves and in fact everything for a half mile around us, is colored yellow-red with iron dust. We understand the reason for this when the buggies of ore which have succeeded the coke are dumped into the funnel-shaped depression around the conical bell at the center. As the huge bell is lowered and the charge slides in there is considerable blowing out of the fine ore dust, which, in fact, continually “oozes” out of all crevices under the heavy pressure of the blast inside.
Hand-fed Blast Furnace
Day and night, month in and month out, during the life of the fire-brick lining of the furnace, this routine of charging, first coke, next the theoretically correct charge of analyzed iron ores, then limestone, in rotation goes on. From 6 A.M. to 6 P.M. and from 6 P.M. to 6 A.M. on twelve-hour shifts, alternating gangs of laborers push the buggies across to the furnace top, dump and return them to the elevator already up with another load.
The incessant quiver of the iron plates beneath our feet with the rumbling and groaning from the inside of this monster are disquieting and the thought constantly recurs: “What if this powerful creature should just now rebel, as quite occasionally occurred in the old days when all of its moods had not been so well understood?” For this king of metallurgical devices, though gentle and obedient as a lamb under proper treatment, is a domineering fury when it has dyspepsia as occurs whenever its attendants are remiss in their attentions to its diet. “Those explosion doors just below the furnace top—are they in working order and would they be adequate?” But whether, as in recorded instances, the whole furnace top is torn off as evidence of its wrath, or its displeasure is exhibited in a milder way, we much prefer to be absent. The thought is disquieting and we are glad to leave.
Cinder or Slag Flowing into Ladles
“Fireworks” at the Cinder-Notch
Unwilling to test again the elevator for the downward trip, we take to the narrow iron stairway which leads from the top of the furnace to the ground. But this is worse than the elevator, for the stair treads are very narrow and made only of three slender iron rods. To our palpitating hearts they seem to give very insecure foothold and the gaps show that there is nothing but earth beneath us, and that a hundred feet below. To make matters worse, before we creepingly get half way down some visitors below have stopped to watch our slow and trembling steps and our nervous clutch on the low “stingy” hand rail. We hear them innocently inquire of one another why we move so slowly. We wish that we could appear brave, especially before the women in the party, but we could not move with greater alacrity if our lives depended upon it.
Once below again with our breath regained, things are more interesting. The red-hot molten slag which has just been tapped out is running from the furnace along a long trough into a ladle six feet high resting upon a car on the railway track alongside the “cast house,” as the huge structure which houses the lower part of the furnace is called. This smoking, molten slag stream gives off a powerful sulphur smell and throws a lurid glare over everything round about.
The Slag Dump
The furnace superintendent is just explaining to the other party that No. 2 furnace has been “hanging” for a couple of days and is still dangerous. “If you realize,” said he, “the great weight of coke, ore, and limestone in that furnace, you can see what a splash it would make if the clogged, bridged part with all above it should fall suddenly into the molten pool in the ‘hearth’ of the furnace. Two years ago No. 1 broke out and the molten metal caught and burned to death one of our men and injured several others. ‘Hanging’ does not occur when the furnace is working right, but failure of the charge to come down evenly is very serious sometimes. A blast furnace is like a coquette; she has to be handled just so, and even then you cannot always be sure what she is going to do next.
“Oh yes,—where does the iron come from? Well, we often read nowadays of a man and his ‘affinity.’ Now the chemist has used that word ‘affinity’ for years to describe the liking or attraction of one chemical ‘element’ for another; in fact, that is where this recent colloquial use of the word originated. Iron ore is nothing more nor less than metallic iron, the element, chemically combined with oxygen, another element which constitutes one-fifth of the air we breathe. Under conditions produced in the blast furnace, though for centuries wedded to iron, oxygen deserts him for her ‘affinity’ carbon, which is best known to you as coke, coal, or charcoal. The iron, now free, becomes molten at the high temperature encountered (about 2800° to 3000° F.) and descends into the ‘hearth’ or bottom of the furnace, while oxygen and her new partner escape out of the top of the furnace in gaseous form. When molten iron has accumulated in the hearth to the extent desired it is tapped out as you will soon see.
Diagrammatic Sketch of Blast Furnace
“The limestone charged has ‘affinity’ for dirt and certain other impurities of the ore which it removes in molten form as the ‘cinder’ or ‘slag’ which is there running into the ladle. When cold, this cinder is a dark greenish-black, glassy substance, which of recent years has come to be used to a certain extent in the manufacture of Portland cement but is mainly used for filling-in purposes. Some is crushed and utilized in concrete mixtures and for road building. In Chicago considerable land has been ‘made’ by dumping slag into the lake, and South Chicago is reported as standing on a swamp which has been filled in with slag from the steel works there.
Old-Fashioned Pig Bed
“This immense furnace is simply a strong steel shell lined two or three feet thick with fire bricks. At the ‘bosh,’ which is the region where the greatest heat is produced, hollow bronze plates are inserted among the bricks of the lining through which circulating cold water keeps the bricks from being fused.
“The hot gases which are led from the upper part of the furnace through that brick-lined ‘downcomer’ are burning in three of those ‘stoves’ to heat them, while cold, clean air from the blowing engines is coming through the other stove, which, ten minutes ago, when it was put on blast, was the hottest of the four. It is now giving up part of its accumulated heat to the blast on its way to the furnace. Switching the cold incoming air every little while from a partially cooled stove through a hotter one while allowing the former to reheat, provides continuous blast of a temperature of 800° to 1400° F. The ‘hot blast’ idea originated about 1830 with James Neilson, a gas engineer of England, and its introduction revolutionized the blast furnace industry and made the highly efficient modern practice possible.
Furnace and Sand Bed Ready for Iron
“This big pipe above our heads which encircles the furnace is the ‘bustle pipe.’ It also has to be lined with fire bricks. The hot blast is distributed by this ‘bustle pipe’ to the tuyères here—these L-shaped pipes—which shoot it directly into the furnace. Through the peep-holes in the tuyères you can get a glimpse of the dazzling interior of the furnace. The blast of heated air is causing the coke to burn fiercely there so that it melts the iron, which farther up in the furnace has been forced to part from the oxygen, as I explained to you.”
Layout of Blast Furnace Plant
But now six men with a long steel bar are starting to break through the two or three feet of clay with which the “tap hole” of the monster furnace is plugged, and our informant hurries away.
Skip Hoist at Work. Skip Dumping into Hopper
For ten minutes with strong sledge blows the tappers struggle to break through the plug of burned clay. Meanwhile the monotonous whistle of the heavy blast into and through the bustle pipe and tuyères goes on and the discharge pipes of the water-cooling plates empty into the gutter around the furnace bottom the water which has been circulating to keep the inner bricks from fusing.
SECTION OF BLAST FURNACE SHOWING FILLING ARRANGEMENT, BINS AND ORE BRIDGE
And now a shout and the strong red glow throughout the cast house tell us that the tap hole is open and the iron is running down the main channel of the sand bed. Past the plugged entrances to the lateral branches runs the molten iron stream to the end of the cast house nearly one hundred feet distant where it divides, filling the laterals on each side and from them running into many open molds arranged like the teeth of a comb. Each of these molds is about five feet long. They form the “pigs,” and the laterals what are known as “sows.” As each lateral and its molds fill, the lateral ahead of it is opened and the process repeated, laterals and pigs filling up simultaneously on opposite sides of the main channel till the whole cast house floor is filled nearly up to the furnace with the red smoking metal.
Sand Cast Pig Iron
There are few sights more glorious than the cast house with bed just filling with metal, and especially is it so at night. The strong yellow-red light of the flaming metal issuing from the furnace and the intense glow of that already in the bed illuminates everything in and about the building. But already, before the furnace is empty, the workmen are spraying with water the earliest cast pigs. Covering them with a light layer of sand they venture upon them with thick-soled shoes and break the “pigs” from the “sows” with sledge hammers.
This is the old-fashioned “sand cast” pig iron. After remelting in the cupola furnaces of neighboring towns and casting into stove parts or other forms it is known as “cast iron”; through “puddling” in reverberatory or special furnaces it becomes “wrought iron”; after decarbonizing treatment in steel furnaces of various design it is changed into the wonderful material called “steel.”
Pig iron is thus the intermediate or semi-raw material from which practically all of our various iron and steel products are made and the transition product through which they pass.
Machine Cast Pig Iron
But the romantic period of the hand-fed furnace and the gloriously beautiful pig beds at casting time are rapidly passing; in fact, are almost past. Modern “skip-hoists” carrying automatically dumped buckets or cars charge the furnace more economically than even low-waged laborers can do it. The two charging cars alternate, one filling at the bottom in the stock house while the other is dumping through the double bell at the top of the furnace. Furnaces are now tapped by power driven drills which make quick work of a formerly difficult operation. Instead of running it into the sand bed, the molten iron from the furnace is nowadays run into ladles alongside the cast house as is the cinder which was described above. If the metal is to be made into pigs it goes to the pig casting machine, where the traveling iron molds very quickly convert the entire cast into “chilled cast” pigs. At the top of the incline these pigs which have been cooling under sprays of water fall from the traveling molds into railroad cars below which deliver them to the consumer.
How Pig Iron Is Now Cast into Traveling Molds
Upper End of Casting Machine Where Pigs Are Dumped from Traveling Molds into Railway Cars
In large steel works the greater part of the molten iron is not cast into pigs at all but while yet molten is directly charged into the open-hearth or Bessemer furnaces which convert it at once into steel of which the greater part is made into plate, rails, or other shapes before being allowed to cool. Even the gas is recovered nowadays. Its journey through the “dust arrester” rids it of most of its dust, after which filters and washers clean it thoroughly. That not required for the heating of the stoves is used for firing the steam boilers about the plant and as fuel for batteries of huge gas engines which in large plants have been installed to generate low-priced electric current.
It should be noted that in modern practice iron is mined, loaded, transported to the furnaces, unloaded, charged and made into pigs or converted into steel and even into the finished products with practically no hand labor, all operations being performed by machinery.
Though a “direct” process for converting the ore into wrought iron or steel has been long sought, a method has never been found, except that used in the very small way followed by the old iron-workers with their crude furnaces. It has always proved commercially advantageous to make pig iron in the blast furnace as an intermediate step and then by a second step convert it into wrought iron, steel, etc. So the ore is brought from the mines to the furnace, the coke and limestone arrive from another region, and batteries of huge blast furnaces through the country make from them the pig iron.
| Blast Furnace Data and Annual Pig Iron Production | ||||||
|---|---|---|---|---|---|---|
| Year | Average Height[[1]] of Blast Furnace | Average Cu. Ft. Capacity of Blast Furnace | Average Daily Output in Tons Each | Tons Pig Iron Produced During Year | ||
| United States | Great Britain | Germany and Luxemberg | ||||
| 1850 | 30′ | 2000 | 29 | 565,000 | ||
| 1860 | 920,000 | |||||
| 1870 | 1,865,000 | 5,963,000 | 1,391,000 | |||
| 1880 | 70′ | 8200 | 117 | 3,835,000 | 7,749,000 | 2,729,000 |
| 1890 | 90′ | 18200 | 360 | 9,000,000 | 7,904,000 | 4,658,000 |
| 1900 | 100′ | 24000 | 600 | 13,790,000 | 9,003,000 | 7,550,000 |
| 1905 | 90 to 100′ | 24000 | 600 | 23,000,000 | 9,746,000 | 10,988,000 |
| 1910 | 90 to 100′ | 24000 | 600 | 27,300,000 | 10,380,000 | 14,495,000 |
| 1912 | 90 to 100′ | 24000 | 600 | 30,000,000 | 9,037,000 | 17,869,000 |
| 1913 | 90 to 100′ | 24000 | 600 | 31,000,000 | 10,654,000 | 19,292,000 |
[1]. No advantage has been found in furnaces having a height of over 110 feet.
The chemical laboratory plays a very important rôle in iron making. Analyses made of each car or boat load of ore by the furnace chemists representing the buyer must check very closely the analyses made by the mine chemists for the seller, as the price of every ton of ore is based on its iron and its phosphorus contents and the percentages of certain other constituents present. In calculating the “burden” or charges for the furnace, each of these constituents is estimated in pounds actually present per charge, losses or gains during the journey through the furnace are allowed for, and by combining the various ores the charges are so made up that the resulting iron will be of certain desired composition. The closeness of actual composition obtained to calculated results is startling.
Truly this is an age of efficiency.
Though technical information and statistics are not to be inflicted upon our readers to any extent in this series of articles, it may not be amiss to give one table of interesting figures which show the increase in height and capacity of furnaces from 1850 to the present time. During this period the annual production of pig iron in the United States has risen from 565,000 to 31,000,000 long tons (2240 pounds). It will be noted that since 1887, when we passed Great Britain, the United States has been the champion pig iron producer of the world.
The record production of a single blast furnace to date was that of one of the United States Steel Corporation’s furnaces at Duquesne, Penn., which produced 900 tons of pig iron in one day.
CHAPTER V
A GENERAL GLIMPSE AHEAD
We have arrived at the parting of the ways. From the vast beds of iron ore, the coal fields and coke ovens, and from the quarries of limestone, all roads have led to the blast furnace. This we have visited and we now know how pig iron is made.
From this point the several paths diverge. One by one we are to follow them to get acquainted with the interesting country which they traverse and the regions to which they lead. However, before choosing any one of these paths for our first trip, it will be to our advantage to pause, to study for a moment our position and get a general view of the country ahead of us. We should know the relative locations and importance of the places we are going to visit, for only by getting a comprehensive idea of the general plan of this ferrous (meaning iron) world, can we understand to the best advantage the position of each of the main products, wrought iron, steel, cast iron, malleable iron, etc., and acquire a satisfactory knowledge of them. In the last chapter pig iron was called the intermediate stage between the ore and the finished iron product, and such the sketch given shows it to be. It is only as an intermediate product that pig iron has value, for nowhere in the commercial world has it a purpose except as a material to be chemically and structurally transformed into other materials which may themselves be used without further transformation. It is desirable that we fully realize the position of this very important semi-raw material, pig iron, before passing on to the study, one at a time, of the refined ferrous products which are susceptible of direct use in our commercial life.
Electric Arc, Microscope, and Camera, Part Of Metallographic Apparatus
From Table A (page [71]) it will be seen that from blast furnace metal several well-known products are formed by the various processes of refinement. Each of these methods of purification may be said to result in a certain general composition and structure which give to the material formed its character and properties.