IRON AND ITS COMPOUNDS.
Examine magnetite, hematite, limonite, siderite, pig-iron,
wrought-iron, steel.
265. Ores and Irons.--As Fe occurs native only in meteorites and
in small quantities of terrestrial origin, it is obtained from
its ores. There are four of these ores--magnetite (Fe3O4),
hematite (Fe2O3), limonite (2 Fe2O3 + 3 H2O), and siderite
(FeCO3). Which is richest in Fe? Compute the proportion. FeCO3
occurs mostly in Europe. The reduction of these ores, as well as
of other metallic oxides, consists in removing O by C at a high
tempera- ture. As ordinarily classified there are three kinds of
iron,--pig- or cast-iron, steel, and wrought-iron.
Study this table, noting the purity, the fusing-point, and the
per cent of C in each case.
Per Cent Fe Fusibility. Per Cent
(general). C.
Pig......... 90 1200 degrees 2-6
Steel........ 99 1400 degrees 0.5-2
Wrought....... 99.7 1500 degrees Fraction.
Pure iron melts at about 1800 degrees. Pig-iron is obtained from
the ore by smelting, and from this are made steel and wrought-
iron.
266. Pig-Iron.--The ore is reduced in a blast furnace (Fig. 47),
in some cases eighty or one hundred feet high, and having a
capacity of about 12,000 cubic feet. The reducing agent is either
charcoal, anthracite coal, or coke,bituminous coal being too
impure. Charcoal is the best agent, and is used in preparing
Swedish iron; but it is too expensive for general use.
Fig. 47. Blast furnace. F, entrance of tuyeres, or blast-pipes.
E, F, hottest part. C, conductor for gases, which are
subsequently used to heat the air going into the tuyeres. G,
upper portion, slag, lower portion, melted iron.
Were ores absolutely pure, only C would be needed to reduce them.
Complete: Fe3O4 + 4 C =? Fe3O4 + 2C=?
Much earthy material--gangue--containing silica and silicates is
always found with iron ores. These are infusible, and something
must be added to render them fusible. CaO forms with SiO2 just
the flux needed. See page 132. Ca0 + Si02 = ? Which of these is
the basic, and which the acidic compound? CaO results from
heating CaCO3; hence the latter is employed instead of the
former. In what case would Si02 be used as the flux?
Into the blast furnace are put, in alternate layers, the fuel,
the flux, and the ore. The fire, once kindled, is kept burning
for months or years. Hot air is driven in through the tuyeres
(tweers). O unites with C of the fuel, forming CO2 and CO. The C
also reduces the ore. Fe2O3 + 3 C = ? CO accomplishes the same
thing. 3 CO + Fe2O3 = ? The intense heat fuses CaO and SiO2 to a
silicate which, with other impurities, forms a slag; this, rising
to the surface of the molten mass, is drawn off. The iron is
melted, falls in drops to the bottom, and is drawn off into sand
molds. See Figure 47. This is pig-iron. It contains as
impurities, C, Si, S, P, Mn, etc. If too much S or P is present
in an ore, it is worthless. This is why the abundant mineral FeS2
cannot be used as a source of iron. From the top of the furnace
N, CO, CO2, H2O, etc., escape. These gases are used to heat the
air which is forced through the tuyeres, and to make steam in
boilers.
267. Steel.--The manufacture of steel and wrought-iron consists
in removing most of the impurities from pig-iron. It will be seen
that the most common compounds of C, S, Si, and P, are their
oxides, and these are for the most part gases. Hence these
elements are removed by oxidation.
Bessemer steel is prepared by melting pig-iron and blowing hot
air through it. A converter (Fig. 48) lined with siliceous sand,
and holding several tons, is partially filled with the molten
metal; blasts of hot air are driven into it, and the C and other
impurities, together with a little of the Fe, are oxidized. The
exact moment when the process has gone far enough, and most of
the impurities have been removed, is indicated by the appearance
of the escaping flame. It usually takes from five to ten minutes.
The blast is then stopped, and the metal has about the
composition of wrought-iron; it contains some uncombined O. A
white pig-iron (spiegeleisen), which contains a known quantity of
C and of Mn, is at once added. Mn removes part of the extra O,
and, though it remains, does not injure the metal. The C is
"dissolved" by the Fe, which is then run into molds (ingots).
This process, the Bessemer, invented in 1856, has revolutionized
steel manufacture. No less than ten tons of iron have been
converted into steel, in five minutes, in a single converter.
268. Wrought-Iron.--The chemical principle involved in making
wrought-iron is the same as that in making steel, but the process
is different. Impurities are burned out from pig-iron in an open
reverberatory furnace, by constantly stirring the metal in
contact with air. This is called puddling. A reverberatory
furnace is one in which the fuel is in one compartment, and the
heat is reflected downward into another, that holds the substance
to be acted upon (Fig. 49).
Steel may also be made by carburizing wrought-iron. Iron and
charcoal are packed together and heated for days, without
melting, when it is found that, in some unknown way, solid C has
penetrated solid Fe. The finer kinds of steel are made in this
way, but they are very expensive.
Wrought-iron may also be made directly from the ore in an open
hearth furnace, with charcoal. This was the original mode.
269. Properties.--The varying properties of pig-iron, steel, and
wrought-iron are due in part to the proportion of C and of other
elements present, either as mixtures or as compounds, and in part
to other causes not well understood. Wrought-iron is fibrous, as
though composed of fine wires, and hence is ductile, malleable,
tough, and soft, and cannot be hardened or tempered, but it is
easily welded. Pig-iron is crystalline, and so is not ductile or
malleable; it is hard and brittle, and cannot be welded. On
account of its low melting-point it is generally employed for
castings. Steel is crystalline in structure, and when suddenly
cooled from red heat by plunging into cold water, becomes hard
and brittle. The tempering can be varied by afterwards heating to
any required degree, indicated by the color of the oxide formed
on the exterior. The higher temperatures give the softer steel.
270. Salts of Iron.--Examine FeSO4, FeS, FeS2.
Fe has a valence of 2 or 4. This gives rise to two kinds of
salts, ferrous and ferric, as in FeCl2 and Fe2Cl6 The valence of
Fe in ferric salts is 4. Ferrous sulphate is FeSO4; ferric
sulphate, Fe2(SO4)3. Write the symbols for ferrous and ferric
hydrate; for the oxides; for the nitrates. Write the graphic
symbols for each.
271. Colors.--The characteristic color of ferrous salts is green,
as in FeSO4. These salts give the green color to the chlorophyll
in leaves and grass, and bottle glass owes its green color to
ferrous silicate. Ferric salts are a brownish red, as shown in
hematite and limonite, and in some bottles. Red sandstone, and
most soils and earths, are illustrations of this coloring action.
The blood of vertebrates owes its color to ferric salts. Bricks
are made from a greenish blue clay in which iron exists in the
ferrous state. On being heated, ferrous salts are oxidized to
ferric, and their color is changed to red. Iron rust is hydrated
ferric oxide, Fe2O3 and Fe2(OH)6.
272. Change of Valence.
Experiment 122.--Dissolve 2 g. of iron filings in diluted HCl.
Filter or pour off the clear liquid, divide it into two parts,
and add NH4OH to one part till a ppt. occurs. Notice the greenish
color of Fe(OH)2. Oxidize the other part by adding a few drops of
HNO3 and boiling a minute. Now add NH4OH, and observe the reddish
color of the ppt., Fe2(OH)6.
Solutions of ferrous salts will gradually change to ferric, if
allowed to stand, thus showing the greater stability of the
latter. In changing from FeCl2 to Fe2Cl6 oxidation does not
consist in adding O, but in increasing the negative element or
radical. This is possible only by changing the valence of Fe from
2 to 4. Hence oxidation, in its larger sense, means increasing
the valence of the positive element. To oxidize FeSO4 is to make
it Fe2(SO4)3, changing the valence of Fe as before. Reduction or
deoxidation diminishes the valence of the positive element.
Illustrate this by the same iron salts. Illustrate it by PbO and
Pb02; AuCl and AuCl3; Sb2S3 and Sb2S5. In this sense define an
oxidizing agent. A reducing agent.
273. Ferrous Sulphate.
Experiment 123.--Dissolve a few iron filings in dilute H2SO4, and
slowly evaporate for a few minutes. Write the equation.
Ferrous sulphate, green vitriol, or copperas, FeSO4 + 7 H2O, is
the source of what acid? See page 66. It is also one of the
ingredients in many writing inks. On being heated, or exposed to
the air, it loses its water of crystallization and becomes a
white powder. It is prepared as above, or by oxidizing moistened
FeS2 by exposure to the air.
Ferrous sulphide, protosulphide of iron, FeS, is how prepared?
See Experiment 6. State its use. See Experiment 108. It also
occurs native.
Ferric sulphide, pyrite, FeS2, occurs native in large quantities.
What is its use? See page 65.