An Elementary Study of Chemistry

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~The family.~ The elements iron, cobalt, and nickel form a group in the
eighth column of the periodic table. The atomic weights of the three are
very close together, and there is not the same gradual gradation in the
properties of the three elements that is noticed in the families in
which the atomic weights differ considerably in magnitude. The elements
are very similar in properties, the similarity being so great in the
case of nickel and cobalt that it is difficult to separate them by
chemical analysis.

The elements occur in nature chiefly as oxides and sulphides, though
they have been found in very small quantities in the native state,
usually in meteorites. Their sulphides, carbonates, and phosphates are
insoluble in water, the other common salts being soluble. Their salts
are usually highly colored, those of iron being yellow or light green as
a rule, those of nickel darker green, while cobalt salts are usually
rose colored. The metals are obtained by reducing the oxides with
carbon.

IRON

~Occurrence.~ The element iron has long been known, since its ores are
very abundant and it is not difficult to prepare the metal from them in
fairly pure condition. It occurs in nature in many forms of
combination,--in large deposits as oxides, sulphides, and carbonates,
and in smaller quantities in a great variety of minerals. Indeed, very
few rocks or soils are free from small amounts of iron, and it is
assimilated by plants and animals playing an important part in life
processes.

~Metallurgy.~ It will be convenient to treat of the metallurgy of iron
under two heads,--Materials Used and Process.

~Materials used.~ Four distinct materials are used in the metallurgy of
iron:

1. _Iron ore._ The ores most frequently used in the metallurgy
of iron are the following:

Hematite Fe_{2}O_{3}.
Magnetite Fe_{3}O_{4}.
Siderite FeCO_{3}.
Limonite 2Fe_{2}O_{2}.3H_{2}O.

These ores always contain impurities, such as silica,
sulphides, and earthy materials. All ores, with the exception
of the oxides, are first roasted to expel any water and carbon
dioxide present and to convert any sulphide into oxide.

2. _Carbon._ Carbon in some form is necessary both as a fuel
and as a reducing agent. In former times wood charcoal was used
to supply the carbon, but now anthracite coal or coke is almost
universally used.

3. _Hot air._ To maintain the high temperature required for the
reduction of iron a very active combustion of fuel is
necessary. This is secured by forcing a strong blast of hot air
into the lower part of the furnace during the reduction
process.

4. _Flux._ (a) _Purpose of the flux._ All the materials which
enter the furnace must leave it again either in the form of
gases or as liquids. The iron is drawn off as the liquid metal
after its reduction. To secure the removal of the earthy matter
charged into the furnace along with the ore, materials are
added to the charge which will, at the high temperature of the
furnace, combine with the impurities in the ore, forming a
liquid. The material added for this purpose is called the
_flux_; the liquid produced from the flux and the ore is called
_slag_.

(b) _Function of the slag._ While the main purpose of adding
flux to the charge is to remove from the furnace in the form of
liquid slag the impurities originally present in the ore, the
slag thus produced serves several other functions. It keeps the
contents of the furnace in a state of fusion, thus preventing
clogging, and makes it possible for the small globules of iron
to run together with greater ease into one large liquid mass.

(c) _Character of the slag._ The slag is really a kind of
readily fusible glass, being essentially a calcium-aluminium
silicate. The ore usually contains silica and some aluminium
compounds, so that limestone (which also contains some silica
and aluminium) is added to furnish the calcium required for the
slag. If the ore and the limestone do not contain a sufficient
amount of silica and aluminium for the formation of the slag,
these ingredients are added in the form of sand and feldspar.
In the formation of slag from these materials the ore is freed
from the silica and aluminium which it contained.

[Illustration: Fig. 85]

~Process.~ The reduction of iron is carried out in large towers called
blast furnaces. The blast furnace (Fig. 85) is usually about 80 ft. high
and 20 ft. in internal diameter at its widest part, narrowing somewhat
both toward the top and toward the bottom. The walls are built of steel
and lined with fire-brick. The base is provided with a number of pipes
T, called tuyers, through which hot air can be forced into the
furnace. The tuyers are supplied from a large pipe S, which circles
the furnace as a girdle. The base has also an opening M, through which
the liquid metal can be drawn off from time to time, and a second
opening P, somewhat above the first, through which the excess of slag
overflows. The top is closed by a movable trap C and C, called the
cone, and through this the materials to be used are introduced. The
gases produced by the combustion of the fuel and the reduction of the
ore, together with the nitrogen of the air forced in through the tuyers,
escape through pipes D, called downcomer pipes, which leave the
furnace near the top. These gases are very hot and contain combustible
substances, principally carbon monoxide; they are therefore utilized as
fuel for the engines and also to heat the blast admitted through the
tuyers. The lower part of the furnace is often furnished with a water
jacket. This consists of a series of pipes W built into the walls,
through which water can be circulated to reduce their temperature.

Charges consisting of coke (or anthracite coal), ore, and flux in proper
proportions are introduced into the furnace at intervals through the
trap top. The coke burns fiercely in the hot-air blast, giving an
intense heat and forming carbon monoxide. The ore, working down in the
furnace as the coke burns, becomes very hot, and by the combined
reducing action of the carbon and carbon monoxide is finally reduced to
metal and collects as a liquid in the bottom of the furnace, the slag
floating on the molten iron. After a considerable amount of the iron has
collected the slag is drawn off through the opening P. The molten iron
is then drawn off into large ladles and taken to the converters for the
manufacture of steel, or it is run out into sand molds, forming the bars
or ingots called "pigs." The process is a continuous one, and when once
started it is kept in operation for months or even years without
interruption.

It seems probable that the first product of combustion of the
carbon, at the point where the tuyers enter the furnace, is
carbon dioxide. This is at once reduced to carbon monoxide by
the intensely heated carbon present, so that no carbon dioxide
can be found at that point. For practical purposes, therefore,
we may consider that carbon monoxide is the first product of
combustion.

~Varieties of iron.~ The iron of commerce is never pure, but contains
varying amounts of other elements, such as carbon, silicon, phosphorus,
sulphur, and manganese. These elements may either be alloyed with the
iron or may be combined with it in the form of definite chemical
compounds. In some instances, as in the case of graphite, the mixture
may be merely mechanical.

The properties of iron are very much modified by the presence of these
elements and by the form of the combination between them and the iron;
the way in which the metal is treated during its preparation has also a
marked influence on its properties. Owing to these facts many kinds of
iron are recognized in commerce, the chief varieties being cast iron,
wrought iron, and steel.

~Cast iron.~ The product of the blast furnace, prepared as just described,
is called cast iron. It varies considerably in composition, usually
containing from 90 to 95% iron, the remainder being largely carbon and
silicon with smaller amounts of phosphorus and sulphur. When the melted
metal from the blast furnace is allowed to cool rapidly most of the
carbon remains in chemical combination with the iron, and the product is
called white cast iron. If the cooling goes on slowly, the carbon
partially separates as flakes of graphite which remain scattered through
the metal. This product is softer and darker in color and is called gray
cast iron.

~Properties of cast iron.~ Cast iron is hard, brittle, and rather easily
melted (melting point about 1100 deg.). It cannot be welded or forged into
shape, but is easily cast in sand molds. It is strong and rigid but not
elastic. It is used for making castings and in the manufacture of other
kinds of iron. Cast iron, which contains the metal manganese up to the
extent of 20%, together with about 3% carbon, is called spiegel iron;
when more than this amount of manganese is present the product is called
ferromanganese. The ferromanganese may contain as much as 80% manganese.
These varieties of cast iron are much used in the manufacture of steel.

~Wrought iron.~ Wrought iron is made by burning out from cast iron most of
the carbon, silicon, phosphorus, and sulphur which it contains. The
process is called _puddling_, and is carried out in a furnace
constructed as represented in Fig. 86. The floor of the furnace F is
somewhat concave and is made of iron covered with a layer of iron oxide.
A long flame produced by burning fuel upon the grate G is directed
downward upon the materials placed upon the floor, and the draught is
maintained by the stack S. A is the ash box and T a trap to catch
the solid particles carried into the stack by the draught. Upon the
floor of the furnace is placed the charge of cast iron, together with a
small amount of material to make a slag. The iron is soon melted by the
flame directed upon it, and the sulphur, phosphorus, and silicon are
oxidized by the iron oxide, forming oxides which are anhydrides of
acids. These combine with the flux, which is basic in character, or with
the iron oxide, to form a slag. The carbon is also oxidized and escapes
as carbon dioxide. As the iron is freed from other elements it becomes
pasty, owing to the higher melting point of the purer iron, and in this
condition forms small lumps which are raked together into a larger one.
The large lump is then removed from the furnace and rolled or hammered
into bars, the slag; being squeezed out in this process. The product has
a stranded or fibrous structure. _The product of a puddling furnace is
called wrought iron._

[Illustration: Fig. 86]

~Properties of wrought iron.~ Wrought iron is nearly pure iron, usually
containing about 0.3% of other substances, chiefly carbon. It is tough,
malleable, and fibrous in structure. It is easily bent and is not
elastic, so it will not sustain pressure as well as cast iron. It can be
drawn out into wire of great tensile strength, and can also be rolled
into thin sheets (sheet iron). It melts at a high temperature (about
1600 deg.) and is therefore forged into shape rather than cast. If melted,
it would lose its fibrous structure and be changed into a low carbon
steel.

~Steel.~ Steel, like wrought iron, is made by burning out from cast iron a
part of the carbon, silicon, phosphorus, and sulphur which it contains;
but the process is carried out in a very different way, and usually,
though not always, more carbon is found in steel than in wrought iron. A
number of processes are in use, but nearly all the steel of commerce is
made by one of the two following methods.

[Illustration: Fig. 87]

1. _Bessemer process._ This process, invented about 1860, is by far the
most important. It is carried out in great egg-shaped crucibles called
converters (Fig. 87), each one of which will hold as much as 15 tons of
steel. The converter is built of steel and lined with silica. It is
mounted on trunnions T, so that it can be tipped over on its side for
filling and emptying. One of the trunnions is hollow and a pipe P
connects it with an air chamber A, which forms a false bottom to the
converter. The true bottom is perforated, so that air can be forced in
by an air blast admitted through the trunnion and the air chamber.

White-hot, liquid cast iron from a blast furnace is run into the
converter through its open necklike top O, the converter being tipped
over to receive it; the air blast is then turned on and the converter
rotated to a nearly vertical position. The elements in the iron are
rapidly oxidized, the silicon first and then the carbon. The heat
liberated in the oxidation, largely due to the combustion of silicon,
keeps the iron in a molten condition. When the carbon is practically all
burned out cast iron or spiegel iron, containing a known percentage of
carbon, is added and allowed to mix thoroughly with the fluid. The steel
is then run into molds, and the ingots so formed are hammered or rolled
into rails or other forms. By this process any desired percentage of
carbon can be added to the steel. Low carbon steel, which does not
differ much from wrought iron in composition, is now made in this way
and is replacing the more expensive wrought iron for many purposes.

~The basic lining process.~ When the cast iron contains
phosphorus and sulphur in appreciable quantities, the lining of
the converter is made of dolomite. The silicon and carbon burn,
followed by the phosphorus and sulphur, and the anhydrides of
acids so formed combine with the basic oxides of the lining,
forming a slag. This is known as the basic lining process.

2. _Open-hearth process._ In this process a furnace very similar to a
puddling furnace is used, but it is lined with silica or dolomite
instead of iron oxide. A charge consisting in part of old scrap iron of
any kind and in part of cast iron is melted in the furnace by a gas
flame. The silicon and carbon are slowly burned away, and when a test
shows that the desired percentage of carbon is present the steel is run
out of the furnace. _Steel may therefore be defined as the product of
the Bessemer or open-hearth processes._

~Properties of steel.~ Bessemer and open-hearth steel usually contain only
a few tenths of a per cent of carbon, less than 0.1% silicon, and a very
much smaller quantity of phosphorus and sulphur. Any considerable amount
of the latter elements makes the steel brittle, the sulphur affecting it
when hot, and the phosphorus when cold. This kind of steel is used for
structural purposes, for rails, and for nearly all large steel articles.
It is hard, malleable, ductile, and melts at a lower temperature than
wrought iron. It can be forged into shape, rolled into sheets, or cast
in molds.

~Relation of the three varieties of iron.~ It will be seen that wrought
iron is usually very nearly pure iron, while steel contains an
appreciable amount of alloy material, chiefly carbon, and cast iron
still more of the same substances. It is impossible, however, to assign
a given sample of iron to one of these three classes on the basis of its
chemical composition alone. A low carbon steel, for example, may contain
less carbon than a given sample of wrought iron. The real distinction
between the three is the process by which they are made. The product of
the blast furnace is cast iron; that of the puddling furnace is wrought
iron; that of the Bessemer and open-hearth methods is steel.

~Tool steel.~ Steel designed for use in the manufacture of edged tools and
similar articles should be relatively free from silicon and phosphorus,
but should contain from 0.5 to 1.5% carbon. The percentage of carbon
should be regulated by the exact use to which the steel is to be put.
Steel of this character is usually made in small lots from either
Bessemer or open-hearth steel in the following way.

A charge of melted steel is placed in a large crucible and the
calculated quantity of pure carbon is added. The carbon dissolves in the
steel, and when the solution is complete the metal is poured out of the
crucible. This is sometimes called crucible steel.

~Tempering of steel.~ Steel containing from 0.5 to 1.5% carbon is
characterized by the property of "taking temper." When the hot steel is
suddenly cooled by plunging it into water or oil it becomes very hard
and brittle. On carefully reheating this hard form it gradually becomes
less brittle and softer, so that by regulating the temperature to which
steel is reheated in tempering almost any condition of temper demanded
for a given purpose, such as for making springs or cutting tools, can be
obtained.

~Steel alloys.~ It has been found that small quantities of a number of
different elements when alloyed with steel very much improve its quality
for certain purposes, each element having a somewhat different effect.
Among the elements most used in this connection are manganese, silicon,
chromium, nickel, tungsten, and molybdenum.

The usual method for adding these elements to the steel is to first
prepare a very rich alloy of iron with the element to be added, and then
add enough of this alloy to a large quantity of the steel to bring it to
the desired composition. A rich alloy of iron with manganese or silicon
can be prepared directly in a blast furnace, and is called
ferromanganese or ferrosilicon. Similar alloys of iron with the other
elements mentioned are made in an electric furnace by reducing the mixed
oxides with carbon.

~Pure iron.~ Perfectly pure iron is rarely prepared and is not adapted to
commercial uses. It can be made by reducing pure oxide of iron in a
current of hydrogen at a high temperature. Prepared in this way it
forms a black powder; when melted it forms a tin-white metal which is
less fusible and more malleable than wrought iron. It is easily acted
upon by moist air.

~Compounds of iron.~ Iron differs from the metals so far studied in that
it is able to form two series of compounds in which the iron has two
different valences. In the one series the iron is divalent and forms
compounds which in formulas and many chemical properties are similar to
the corresponding zinc compounds. It can also act as a trivalent metal,
and in this condition forms salts similar to those of aluminium. Those
compounds in which the iron is divalent are known as _ferrous_
compounds, while those in which it is trivalent are known as _ferric_.

~Oxides of iron.~ Iron forms several oxides. Ferrous oxide (FeO) is not
found in nature, but can be prepared artificially in the form of a black
powder which easily takes up oxygen, forming ferric oxide:

2FeO + O = Fe_{2}O_{3}.

Ferric oxide is the most abundant ore of iron and occurs in great
deposits, especially in the Lake Superior region. It is found in many
mineral varieties which vary in density and color, the most abundant
being hematite, which ranges in color from red to nearly black. When
prepared by chemical processes it forms a red powder which is used as a
paint pigment (Venetian red) and as a polishing powder (rouge).

Magnetite has the formula Fe_{3}O_{4} and is a combination of FeO and
Fe_{2}O_{3}. It is a very valuable ore, but is less abundant than
hematite. It is sometimes called magnetic oxide of iron, or lodestone,
since it is a natural magnet.

~Ferrous salts.~ These salts are obtained by dissolving iron in the
appropriate acid, or, when insoluble, by precipitation. They are usually
light green in color and crystallize well. In chemical reactions they
are quite similar to the salts of magnesium and zinc, but differ from
them in one important respect, namely, that they are easily changed into
compounds in which the metal is trivalent. Thus ferrous chloride treated
with chlorine or aqua regia is changed into ferric chloride:

FeCl_{2} + Cl = FeCl_{3}.

Ferrous hydroxide exposed to moist air is rapidly changed into ferric
hydroxide:

2Fe(OH)_{2} + H_{2}O + O = 2Fe(OH)_{3}.

~Ferrous sulphate~ _(copperas, green vitriol)_ (FeSO_{4}.7H_{2}O). Ferrous
sulphate is the most familiar ferrous compound. It is prepared
commercially as a by-product in the steel-plate mills. Steel plates are
cleaned by the action of dilute sulphuric acid upon them, and in the
process some of the iron dissolves. The liquors are concentrated and the
green vitriol separates from them.

~Ferrous sulphide~ (FeS). Ferrous sulphide is sometimes found in nature as
a golden-yellow crystalline mineral. It is formed as a black precipitate
when a soluble sulphide and an iron salt are brought together in
solution:

FeSO_{4} + Na_{2}S = FeS + Na_{2}SO_{4}.

It can also be made as a heavy dark-brown solid by fusing together the
requisite quantities of sulphur and iron. It is obtained as a by-product
in the metallurgy of lead:

PbS + Fe = FeS + Pb.

It is used in the laboratory in the preparation of hydrosulphuric acid:

FeS + 2HCl = FeCl_{2} + H_{2}S.

~Iron disulphide~ _(pyrites)_ (FeS_{2}). This substance bears the same
relation to ferrous sulphide that hydrogen dioxide does to water. It
occurs abundantly in nature in the form of brass-yellow cubical crystals
and in compact masses. Sometimes the name "fool's gold" is applied to it
from its superficial resemblance to the precious metal. It is used in
very large quantities as a source of sulphur dioxide in the manufacture
of sulphuric acid, since it burns readily in the air, forming ferric
oxide and sulphur dioxide:

2FeS_{2} + 11O = Fe_{2}O_{3} + 4SO_{2}.

~Ferrous carbonate~ (FeCO_{3}). This compound occurs in nature as
siderite, and is a valuable ore. It will dissolve to some extent in
water containing carbon dioxide, just as will calcium carbonate, and
waters containing it are called chalybeate waters. These chalybeate
waters are supposed to possess certain medicinal virtues and form an
important class of mineral waters.

~Ferric salts.~ Ferric salts are usually obtained by treating an acidified
solution of a ferrous salt with an oxidizing agent:

2FeCl_{2} + 2HCl + O = 2FeCl_{3} + H_{2}O,

2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.

They are usually yellow or violet in color, are quite soluble, and as a
rule do not crystallize well. Heated with water in the absence of free
acid, they hydrolyze even more readily than the salts of aluminium. The
most familiar ferric salts are the chloride and the sulphate.

~Ferric chloride~ (FeCl_{3}). This salt can be obtained most conveniently
by dissolving iron in hydrochloric acid and then passing chlorine into
the solution:

Fe + 2HCl = FeCl_{2} + 2H,

FeCl_{2} + Cl = FeCl_{3}.

When the pure salt is heated with water it is partly hydrolyzed:

FeCl_{3} + 3 H_{2}O <--> Fe(OH)_{3} + 3HCl.

This is a reversible reaction, however, and hydrolysis can therefore be
prevented by first adding a considerable amount of the soluble product
of the reaction, namely, hydrochloric acid.

~Ferric sulphate~ (Fe_{2}(SO_{4})_{3}). This compound can be made by
treating an acid solution of green vitriol with an oxidizing agent. It
is difficult to crystallize and hard to obtain in pure condition. When
an alkali sulphate in proper quantity is added to ferric sulphate in
solution an iron alum is formed, and is easily obtained in
large crystals. The best known iron alums have the formulas
KFe(SO_{4})_{2}.12H_{2}O and NH_{4}Fe(SO_{4})_{2}.12H_{2}O. They are
commonly used when a pure ferric salt is required.

~Ferric hydroxide~ (Fe(OH)_{3}). When solutions of ferric salts are
treated with ammonium hydroxide, ferric hydroxide is formed as a
rusty-red precipitate, insoluble in water.

~Iron cyanides.~ A large number of complex cyanides containing iron are
known, the most important being potassium ferrocyanide, or yellow
prussiate of potash (K_{4}FeC_{6}N_{6}), and potassium ferricyanide, or
red prussiate of potash (K_{3}FeC_{6}N_{6}). These compounds are the
potassium salts of the complex acids of the formulas H_{4}FeC_{6}N_{6}
and H_{3}FeC_{6}N_{6}.

~Oxidation of ferrous salts.~ It has just been seen that when a ferrous
salt is treated with an oxidizing agent in the presence of a free acid a
ferric salt is formed:

2FeSO_{4} + H_{2}SO_{4} + O = Fe_{2}(SO_{4})_{3} + H_{2}O.

In this reaction oxygen is used up, and the valence of the iron is
changed from 2 to 3. The same equation may be written

2Fe^{++}, 2SO_{4}^{--} + 2H^{+}, SO_{4}^{--} + O
= 2Fe^{+++}, 3SO_{4}^{--} + H_{2}O.

Hydrogen ions have been oxidized to water, while the charge of each iron
ion has been increased from 2 to 3.

In a similar way the conversion of ferrous chloride into ferric chloride
may be written

Fe^{++}, 2Cl^{-} + Cl = Fe^{+++}, + 3Cl^{-}.

Here again the valence of the iron and the charge on the iron ion has
been increased from 2 to 3, though no oxygen has entered into the
reaction. As a rule, however, changes of this kind are brought about by
the use of an oxidizing agent, and are called oxidations.

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