IRON ORE, PREPARATION AND SMELTING
=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.
[Illustration: FIG. 145.—RUNNING METAL FROM THE BLAST FURNACE TO LADLES
FOR TRANSPORTING TO EITHER THE OPEN HEARTH FURNACE OR THE PIG MOLDER.]
=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.
[Illustration: FIG. 146.—SECTIONAL VIEW OF A BLAST FURNACE.]
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.
[Illustration: FIG. 147.—SECTIONAL VIEW OF THE BELL AND HOPPER.]
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.
[Illustration: FIG. 148.—PIG MOLDING MACHINE.]
=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.
[Illustration: FIG. 149.—GENERAL VIEW OF A SMELTING PLANT, SHOWING
BLAST FURNACE STOVES AND THE ORE PILE.]
QUESTIONS FOR REVIEW
What is ore? Name four grades of iron ore. What is native iron? What
is the difference between it and other iron ores? What class of ore
contains the largest percentage of iron? Red hematite contains less,
so why is it considered more valuable than the magnetite? What amount
of iron do the limonite and ferrous carbonate ores contain? What
determines the value of an ore? How is ore prepared for reduction?
What are the results of these preparations? What are fluxes used for?
What flux is most generally used? What effect does sulphur produce
in wrought-iron? What is the effect of phosphorus in wrought-iron?
How is air heated before it enters the blast furnace? What is the
difference between sand-molded and machine-molded pig iron? What is
the objection to machine-molded pig iron? Name the different classes
of pig iron and state the use of each. How is iron graded?