155. Iron Ore.—An ore is a portion of the earth’s substance containing metal for which it is mined and worked; the class to which it belongs depends upon the amount and variety of the metal it contains. Any ore that is to be used for the extraction of a certain metal must contain the metal in sufficient amounts to make the operation profitable.

Iron, ordinarily, does not occur in a native state or in a condition suitable for use in the arts and manufactures. The iron in meteors, frequently called native iron, is the nearest possible approach to it. Meteorites, commonly known as falling stones or shooting stars, are solid masses that have fallen from high regions of the atmosphere and are only occasionally found in different parts of the world. They are considered more valuable as a curiosity than as material for manufacturing purposes. The metallurgist, chemist, or geologist can readily distinguish them from other masses, because they invariably contain considerable nickel, which seldom appears in any of the ordinary iron ores. They are usually found in a mass containing crystals and are nearly always covered with a thin coating of oxide, which protects the metal from further oxidation. Several large meteors have been found, one in Germany weighing 3300 pounds and a larger one in Greenland weighing 49,000 pounds. The largest one known was discovered by Lieutenant Peary in the Arctic regions. It weighs 75,000 pounds. He brought it to New York City, where it can now be seen at the American Museum of Natural History.

Pure iron is obtainable only as a chemical, and as such it is used in the preparation of medicines. As a commercial product, such as is used in the arts and manufactures and by the smith, it is always combined with other substances, such as carbon, silicon, and phosphorus.

Iron is distributed through the earth very widely, but not always in sufficient quantities to make its extraction from the ore profitable; consequently the ores used for the extraction of iron are somewhat limited. There are four general grades of iron ore, which are known by the following names: magnetite, red hematite, limonite, and ferrous carbonate. These are subdivided and classified according to the particular composition of each.

156. Magnetite when pure contains about 72 per cent of iron, and so is the richest ore used in the manufactures. It is black, brittle, and generally magnetic, and leaves a black streak when drawn across a piece of unglazed porcelain. It sometimes occurs in crystals or in a granular condition like sand, but generally in a massive form. It is found principally in a belt running along the eastern coast of the United States, from Lake Champlain to South Carolina. There are considerable quantities of it in New Jersey and eastern Pennsylvania, but the greatest deposits are found in Missouri and northern Michigan; some is mined also in eastern Canada. It is a valuable ore in Sweden.

A mineral known as franklinite, which is closely allied to magnetite, is a mixture of magnetite and oxides of manganese and zinc. In appearance it resembles magnetite, but is less magnetic. In New Jersey, where it is found quite abundantly, it is treated for the extraction of the zinc, and the residue thus obtained is used for the manufacture of spiegeleisen, which is an iron containing a large amount of manganese, usually from 8 to 25 per cent.

157. Red hematite is found in earthy and compact forms. It varies in color from a deep red to a steel gray, but all varieties leave a red streak on unglazed porcelain. It is found also in a number of shapes or varieties, such as crystalline, columnar, fibrous, and masses of irregular form. Special names have been given to these. The brilliant crystalline variety is known as specular ore, the scaly foliated kind as micaceous ore, and the earthy one as red ocher. Each one of this class contains about 70 per cent of iron, and on account of the abundance, the comparative freedom from injurious ingredients, and the quality of iron it produces, it is considered the most important of all the ores in the United States.

Until the discovery of large deposits of this ore in the Lake Superior district it was chiefly obtained from a belt extending along the eastern coast of the United States just west of the magnetite deposits and ending in Alabama. Some of this ore is found in New York, but there is not a great amount of it north of Danville, Pennsylvania. At present the greatest quantities that are used come from the Lake Superior district. There, ore of almost any desired composition may be obtained, and the enormous quantities, the purity, the small cost of mining, and the excellent shipping facilities have made it the greatest ore-producing section of the United States.

158. Limonite or brown hematite contains about 60 per cent of iron and is found in both compact and earthy varieties. Pipe or stalactitic and bog ore belong to this grade. The color varies from brownish black to yellowish brown, but they all leave a yellowish brown streak on unglazed porcelain. It is found in a belt lying between the red hematite and magnetite ores in the eastern United States. Formerly there was considerable of this mined in central Pennsylvania, Alabama, and the Lake Superior district.

159. Ferrous carbonate contains about 50 per cent of iron. It also is found in several varieties, called spathic ore, clay ironstone, and blackband. Spathic ore when quite pure has a pearly luster and varies in color from yellow to brown. The crystallized variety is known as siderite; this ore frequently contains considerable manganese and in some places is used for the production of spiegeleisen. When siderite is exposed to the action of the air and water, brown hematite is formed.

Clay ironstone is a variety that is found in rounded masses or irregular shapes and sometimes in layers or lumps, usually in the coal measures. It varies in color from light yellow to brown, but the light-colored ore rapidly becomes brown when exposed to the atmosphere. Like the former it also contains considerable manganese.

Blackband is also a clay ironstone, but it is so dark in color that it frequently resembles coal; hence the name. The ore is not very abundant in this country nor extensively used; it is generally found with bituminous coal or in the coal measures, therefore it is mined to some extent in western Pennsylvania and Ohio. It is an important ore in England.

160. The Value of Ores.—Ores are valued according to the amount of iron they contain, the physical properties, the cost of mining, the cost of transportation to the furnace, and their behavior during reduction. The ores of the Mesaba range in the Lake Superior district are very rich, and free from many impurities; they are soft and easily reduced, and as they are found near the surface, they can be mined with steam shovels. These are great advantages, but the greatest disadvantage is the fact that the ore is fine and some of it blows out of the furnace with the escaping gases; this also fouls the heating stoves and clogs the boiler flues.

161. Preparation of Ores.—Most of the ores are used just as they come from the mines, but in some cases they are put through a preliminary treatment. This is sometimes done as an advantage and at other times as a necessity. This treatment is very simple and consists of weathering, washing, crushing, and roasting.

162. Weathering is a common process. Sometimes ores that have been obtained from the coal measures and others that may contain pyrites or similar substances are left exposed to the oxidizing influence of the weather. This separates the shale and the pyrites. The former can easily be removed, and the latter is partly oxidized and washed away by the water or rain falling upon it. The ore piles shown in Fig. 149 are exposed to the atmosphere and partly weathered before being used.

163. Washing is also done for the purpose of removing substances that would retard the smelting process. For instance, the limonite ores, which are generally mixed with considerable clay or earthy compositions, are put through an ore washer to remove those substances before they are charged into the smelting furnace.

164. Crushing is done with machinery to reduce to a uniform size such refractory ores as are mined in rather large lumps. If the ore were charged into the furnace as mined, the coarseness would allow the gases to pass through the ore too readily without sufficient action upon it. Smaller pieces will pack more closely together, thus offering greater resistance to the blast, and hastening the reduction.

165. Roasting or calcination is done to desulphurize ore which contains an excess of sulphur. It is done also to expel water, carbon dioxide, or other volatile matter which it may contain. Ore, made more porous by roasting, exposes a larger surface to the reducing gases. In the case of magnetic ores, roasting converts the ferrous oxide into ferric oxide, which lessens the possibility of the iron becoming mixed with the slag, thereby preventing considerable loss of metal.

Ore is frequently calcined in open heaps, but in more modern practice stalls or kilns are employed. Where fuel is cheap and space is abundant, the first process may be used. A layer of coal a few inches thick is laid on the ground, and a layer of ore is spread upon it; then coal and ore are laid in alternate layers until the pile is from 4 to 9 feet high. The coal at the bottom is then ignited, and the combustion extended through the entire mass. If at any time during the operation the combustion proceeds too rapidly, the pile is dampened with fine ore and the burning allowed to proceed until all the coal is consumed. Blackband ore frequently contains enough carbonaceous matter to accomplish roasting without the addition of any fuel except the first layer for starting the operation.

When the ore is calcined in stalls, it is placed in a rectangular inclosure with walls on three sides; these are from 6 to 12 feet high and are perforated to allow a thorough circulation of air. This method is very much like that of roasting in open heaps, but less fuel is necessary, for the draft is under better control and a more perfect calcination is accomplished.

When the same operation is performed in kilns, it is more economical in regard to fuel and labor than either of the two methods explained above. The process is under better control, and a more uniform product is obtained. The kilns are built in a circular form of iron plates, somewhat like a smelting furnace and lined with about 14 inches of fire brick. The most common size of the kilns is about 14 feet in diameter at the bottom, 20 feet at the widest part, and 18 feet at the top; the entire height is about 30 feet. They are capable of receiving about 6000 cubic feet of ore.

166. Fuels.—A variety of fuels may be used in the blast furnace reduction process, but the furnace should be modified to suit the particular quality of fuel. In this country the fuels most used are coke, charcoal, and anthracite coal. Coke is the most satisfactory and is more generally used than either of the others. Charcoal is used to a certain extent on account of its freedom from impurities and because it is generally considered that iron produced with charcoal is better for some purposes than that made by using other fuels. Anthracite coal is used principally in eastern Pennsylvania because the coal mines are near at hand, and it is therefore the cheapest fuel available. In some instances a mixture of anthracite and coke is used.

167. Fluxes.—The materials that are charged into the furnace with the ore, to assist in removing injurious elements that it may contain, are called fluxes. They collect the impurities and form a slag which floats on top of the molten iron and which is tapped off before the metal is allowed to run out. The fluxes also assist in protecting the lining of the furnace by thus absorbing the impurities which would otherwise attack the lining and destroy it.

Limestone is almost universally employed as a flux, although dolomite is used also to some extent. The value of limestone as a flux depends upon its freedom from impurities, such as silicon and sulphur.

Sulphur and phosphorus are two elements which must be kept out of the product. When there is too much sulphur, the iron is exceedingly brittle at a dull red heat, although it can be worked at a higher or lower temperature. It is called red-short iron and makes welding difficult. With steel, sulphur diminishes the tensile strength and ductility. If there is too much phosphorus combined with iron, the metal will crack when hammered cold. Iron of this kind is called cold-short iron. This metal can be worked, however, at a higher temperature than can the red-short iron just described.

168. The Blast.—The air blown into the furnace to increase and hasten combustion is called the blast. Formerly when a cold blast was used, considerable extra fuel was required to heat the air after it entered the furnace, but a hot blast is used now almost exclusively. The air is heated by passing through large stoves built for that purpose. The stoves are heated by burning the waste gases which are generated in the furnace and which are conducted from the top of the furnace through a pipe leading into the stoves. Four of these stoves are shown in Fig. 149 at the left of the picture.

169. The Reduction or Blast Furnace.—Fig. 145. The reduction or blast furnace is almost universally used for the reduction of iron ore. It is a large barrel-shaped structure, the exterior of which is formed of iron plates about ¹/₂ inch thick, bent and riveted together like the outer shell of a boiler. This is lined with brickwork or masonry, the inner portion being made of fire brick to protect the furnace from the intense heat. Figure 146 shows a sectional view of a furnace of this kind.

The stack D is supported on a cast-iron ring, which rests on iron pillars. The hearth K and the boshes E are beneath the stack and are built independent of it, usually after the stack has been erected. This is done so that the hearth can be repaired or relined whenever it becomes injured. The hearth is also perforated for the introduction of the tuyÈres t, through which the blast enters the furnace from the blast main B. The opening to the downcomer or pipe leading to the stoves is shown at A.

Figure 147 shows the mechanical arrangement at the top of the furnace, called the bell and hopper, for receiving and admitting the ore flux and fuel. By lowering the bell C the material is allowed to drop into the furnace.

The fuel, ore, and flux are charged into the furnace at the top in alternate layers, as previously explained; the iron settles down through the boshes, is melted, and drops to the bottom or hearth. The slag is drawn off at the cinder notch c, Fig. 146, after which the iron is tapped off at the iron notch g. Hollow plates p for water circulation are inserted in the boshes to protect the lining from burning out too rapidly.

The melted iron runs from the tapping notch into a large groove made in sand. This groove is called the “sow.” It is connected with smaller grooves called the “pigs.” Into these the metal runs and forms pig iron. Considerable sand adheres to pigs thus formed, and as the sand is objectionable for foundry, Bessemer, and open-hearth purposes, and as an enormous amount of hand labor is required in breaking up and removing it, pig molding machines are used. Figure 148 shows one of these machines with a ladle pouring the metal into it.

The only objection to this method is that the metal is chilled rather suddenly by the water through which the molds are led. This sudden chilling causes a structure different from that found in the same quality of metal molded in the sand and allowed to cool off gradually, and most foundrymen as well as other users of iron judge the quality by the appearance of a fracture. On this account machine-molded pigs are objectionable. It is claimed, however, that some machines in use at present have overcome this difficulty.

The approximate dimensions of a modern coke-burning furnace are as follows (see Fig. 146): The hearth K is about 13 feet in diameter and about 9 feet high. The diameter of the portion above the hearth increases for about 15 feet to approximately 21 feet in diameter at the boshes E. From the top of the boshes the diameter gradually decreases until it is about 14 feet in diameter at the stock line. The throat, or top, where the fuel and ore are charged in through the bell and hopper, is about 70 feet above the hearth. On the brackets which are connected to the pillars, the blast main rests, completely surrounding the furnace, and at numerous places terminal pipes convey the blast to the tuyÈres. After the furnace has been charged, or “blown in,” as it is commonly called, it is kept going continually night and day, or until it becomes necessary to shut down for repairs.

A general view of a smelting plant is shown in Fig. 149. The four circular structures to the left with a tall stack between them are the stoves for heating the blast. Next to these in the center of the picture is the blast furnace, somewhat obstructed by the conveyor which carries the ore and fuel to the top for charging. The structural work to the right is the unloader, which takes the ore from the vessels and conveys it to the stock pile in the foreground, where the ore is allowed to drop.

170. Classification of Pig Iron.—The pig iron produced by the blast furnace is graded as to quality, and is known by the following names: Bessemer, basic, mill, malleable, charcoal, and foundry iron. This classification indicates the purpose for which each kind is best suited.

171. Bessemer iron is that used for making Bessemer steel. In this grade the amounts of sulphur and phosphorus should be as low as possible. Bessemer iron is generally understood to contain less than 0.1 per cent of phosphorus and less than .05 per cent of sulphur.

172. Basic iron is that which is generally used in the basic process of steel manufacture. It should contain as little silicon as possible, because the silicon will attack the basic lining of the furnace; therefore the surface of the pig iron used for this purpose should, if possible, be free from sand. By the basic process of making steel, most of the phosphorus in the pig iron is removed, consequently basic iron may contain considerably more phosphorus than if it were to be used in the Bessemer process.

173. Mill iron is that which is used mostly in the puddling mill for the manufacture of wrought iron. It should contain a low percentage of silicon. Therefore pig iron that has been made when the furnace was working badly for foundry iron is sometimes used for this purpose.

174. Malleable iron is that used for making malleable castings. It usually contains more phosphorus than Bessemer iron and less than foundry iron. The percentage of silicon and graphitic carbon is also very low in this class.

175. Charcoal iron is simply that which has been made in a furnace where charcoal has been used as the fuel. It is generally used as a foundry iron for special purposes.

176. Foundry iron is used for making castings by being melted and then poured into molds. For this purpose an iron that will readily fill the mold without much shrinkage in cooling is desired. Other properties of foundry iron will depend upon the character of the castings desired.

177. Grading Iron.—Iron is graded and classified according to its different properties and qualities by two methods; namely, chemical analysis and examination of fracture.

Grading by analysis, although not universally used at present, is no doubt the more perfect method, because the foreign substances contained in the metal can be accurately determined. Grading by fracture is more generally used, although it cannot be considered absolutely perfect, but when done by one who has had years of experience and has trained his eye to discover the different granular constructions and luster of the fractured parts, it is very nearly correct; unless the properties are to be known to an absolute certainty, grading by fracture is sufficiently accurate for all practical purposes.

Questions for Review