THE ALUMINIUM FAMILY
~The family.~ The element aluminium is the most abundant member of the
group of elements known as the aluminium family; indeed, the other
members of the family--gallium, indium, and thallium--are of such rare
occurrence that they need not be separately described. The elements of
the family are ordinarily trivalent, so that the formulas for their
compounds differ from those of the elements so far studied. Their
hydroxides are practically insoluble in water and are very weak bases;
indeed, the bases are so weak that their salts are often hydrolyzed into
free base and free acid in solution. The salts formed from these bases
usually contain water of crystallization, which cannot be driven off
without decomposing them more or less.
The trivalent metals, which in addition to aluminium include also iron
and chromium, are sometimes called the _earth metals_. The name refers
to the earthy appearance of the oxides of these metals, and to the fact
that many earths, soils, and rocks are composed in part of these
substances.
ALUMINIUM
~Occurrence.~ Aluminium never occurs in the free state in nature, owing to
its great affinity for oxygen. In combined form, as oxides, silicates,
and a few other salts, it is both abundant and widely distributed, being
an essential constituent of all soils and of most rocks excepting
limestone and sandstone. Cryolite (Na_{3}AlF_{6}), found in Greenland,
and bauxite, which is an aluminium hydroxide usually mixed with some
iron hydroxide, are important minerals. It is estimated that aluminium
composes about 8% of the earth's crust. In the industries the metal is
called aluminum, but its chemical name is aluminium.
[Illustration: Fig. 82]
~Preparation.~ Aluminium was first prepared by Wöhler, in 1827, by heating
anhydrous aluminium chloride with potassium:
AlCl_{3} + 3K = 3KCl + Al.
This method was tried after it was found impossible to reduce the oxide
of aluminium with carbon. The metal possessed such interesting
properties and promised to be so useful that many efforts were made to
devise a cheap way of preparing it. The method which has proved most
successful consists in the electrolysis of the oxide dissolved in melted
cryolite.
~Metallurgy.~ An iron box A (Fig. 82) about eight feet long and
six feet wide is connected with a powerful generator in such a
way as to serve as the cathode upon which the aluminium is
deposited. Three or four rows of carbon rods B dip into the
box and serve as the anodes. The box is partially filled with
cryolite and the current is turned on, generating enough heat
to melt the cryolite. Aluminium oxide is then added, and under
the influence of the electric current it decomposes into
aluminium and oxygen. The temperature is maintained above the
melting point of aluminium, and the liquid metal, being heavier
than cryolite, sinks to the bottom of the vessel, from which it
is tapped off from time to time through the tap hole C. The
oxygen in part escapes as gas, and in part combines with the
carbon of the anode, the combustion being very brilliant. The
process is carried on at Niagara Falls.
The largest expense in the process, apart from the cost of
electrical energy, is the preparation of aluminium oxide free
from other oxides, for most of the oxide found in nature is too
impure to serve without refining. Bauxite is the principal ore
used as a source of the aluminium because it is converted into
pure oxide without great difficulty. Since common clay is a
silicate of aluminium and is everywhere abundant, it might be
expected that this would be utilized in the preparation of
aluminium. It is, however, very difficult to extract the
aluminium from a silicate, and no practical method has been
found which will accomplish this.
~Physical properties.~ Aluminium is a tin-white metal which melts at 640°
and is very light, having a density of 2.68. It is stiff and strong, and
with frequent annealing can be rolled into thin foil. It is a good
conductor of heat and electricity, though not so good as copper for a
given cross section of wire.
~Chemical properties.~ Aluminium is not perceptibly acted on by boiling
water, and moist air merely dims its luster. Further action is prevented
in each case by the formation of an extremely thin film of oxide upon
the surface of the metal. It combines directly with chlorine, and when
heated in oxygen burns with great energy and the liberation of much
heat. It is therefore a good reducing agent. Hydrochloric acid acts upon
it, forming aluminium chloride: nitric acid and dilute sulphuric acid
have almost no action on it, but hot, concentrated sulphuric acid acts
upon it in the same way as upon copper:
2Al + 6H_{2}SO_{4} = Al_{2}(SO_{4})_{3} + 6H_{2}O + 3SO_{2}.
Alkalis readily attack the metal, liberating hydrogen, as in the case of
zinc:
Al + 3KOH = Al(OK)_{3} + 3H.
Salt solutions, such as sea water, corrode the metal rapidly. It alloys
readily with other metals.
~Uses of aluminium.~ These properties suggest many uses for the metal. Its
lightness, strength, and permanence make it well adapted for many
construction purposes. These same properties have led to its extensive
use in the manufacture of cooking utensils. The fact that it is easily
corroded by salt solutions is, however, a disadvantage. Owing to its
small resistance to electrical currents, it is replacing copper to some
extent in electrical construction, especially for trolley and power
wires. Some of its alloys have very valuable properties, and a
considerable part of the aluminium manufactured is used for this
purpose. Aluminium bronze, consisting of about 90% copper and 10%
aluminium, has a pure golden color, is strong and malleable, is easily
cast, and is permanent in the air. Considerable amounts of aluminium
steel are also made.
~Goldschmidt reduction process.~ Aluminium is frequently employed as a
powerful reducing agent, many metallic oxides which resist reduction by
carbon being readily reduced by it. The aluminium in the form of a fine
powder is mixed with the metallic oxide, together with some substance
such as fluorspar to act as a flux. The mixture is ignited, and the
aluminium unites with the oxygen of the metallic oxide, liberating the
metal. This collects in a fused condition under the flux.
An enormous quantity of heat is liberated in this reaction, and a
temperature as high as 3500° can be reached. The heat of the reaction is
turned to practical account in welding car rails, steel castings, and in
similar operations where an intense local heat is required. A mixture of
aluminium with various metallic oxides, ready prepared for such
purposes, is sold under the name of _thermite_.
[Illustration: Fig. 83]
~Preparation of chromium by the Goldschmidt method.~ A mixture of
chromium oxide and aluminium powder is placed in a Hessian
crucible (A, Fig. 83), and on top of it is placed a small
heap B of a mixture of sodium peroxide and aluminium, into
which is stuck a piece of magnesium ribbon C. Powdered
fluorspar D is placed around the sodium peroxide, after which
the crucible is set on a pan of sand and the magnesium ribbon
ignited. When the flame reaches the sodium peroxide mixture
combustion of the aluminium begins with almost explosive
violence, so that great care must be taken in the experiment.
The heat of this combustion starts the reaction in the chromium
oxide mixture, and the oxide is reduced to metallic chromium.
When the crucible has cooled a button of chromium will be found
in the bottom.
~Aluminium oxide~ (Al_{2}O_{3}). This substance occurs in several forms in
nature. The relatively pure crystals are called corundum, while emery is
a variety colored dark gray or black, usually with iron compounds. In
transparent crystals, tinted different colors by traces of impurities,
it forms such precious stones as the sapphire, oriental ruby, topaz, and
amethyst. All these varieties are very hard, falling little short of
the diamond in this respect. Chemically pure aluminium oxide can be made
by igniting the hydroxide, when it forms an amorphous white powder:
2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O.
The natural varieties, corundum and emery, are used for cutting and
grinding purposes; the purest forms, together with the artificially
prepared oxide, are largely used in the preparation of aluminium.
~Aluminium hydroxide~ (Al(OH)_{3}). The hydroxide occurs in nature as the
mineral hydrargyllite, and in a partially dehydrated form called
bauxite. It can be prepared by adding ammonium hydroxide to any soluble
aluminium salt, forming a semi-transparent precipitate which is
insoluble in water but very hard to filter. It dissolves in most acids
to form soluble salts, and in the strong bases to form aluminates, as
indicated in the equations
Al(OH)_{3} + 3HCl = AlCl_{3} + 3H_{2}O,
Al(OH)_{3} + 3NaOH = Al(ONa)_{3} + 3H_{2}O.
It may act, therefore, either as a weak base or as a weak acid, its
action depending upon the character of the substances with which it is
in contact. When heated gently the hydroxide loses part of its hydrogen
and oxygen according to the equation
Al(OH)_{3} = AlO·OH + H_{2}O.
This substance, the formula of which is frequently written HAlO_{2}, is
a more pronounced acid than is the hydroxide, and its salts are
frequently formed when aluminium compounds are fused with alkalis. The
magnesium salt Mg(AlO_{2})_{2} is called spinel, and many other of its
salts, called aluminates, are found in nature.
When heated strongly the hydroxide is changed into oxide, which will not
again take up water on being moistened.
~Mordants and dyeing.~ Aluminium hydroxide has the peculiar
property of combining with many soluble coloring materials and
forming insoluble products with them. On this account it is
often used as a filter to remove objectionable colors from
water. This property also leads to its wide use in the dye
industry. Many dyes will not adhere to natural fibers such as
cotton and wool, that is, will not "dye fast." If, however, the
cloth to be dyed is soaked in a solution of aluminium compounds
and then treated with ammonia, the aluminium salts which have
soaked into the fiber will be converted into the hydroxide,
which, being insoluble, remains in the body of it. If the fiber
is now dipped into a solution of the dye, the aluminium
hydroxide combines with the color material and fastens, or
"fixes," it upon the fiber. A substance which serves this
purpose is called a _mordant_, and aluminium salts,
particularly the acetate, are used in this way.
~Aluminium chloride~ (AlCl_{3}·6 H_{2}O). This substance is prepared by
dissolving the hydroxide in hydrochloric acid and evaporating to
crystallization. When heated it is converted into the oxide, resembling
magnesium in this respect:
2(AlCl_{3}·6 H_{2}O) = Al_{2}O_{3} + 6HCl + 9H_{2}O.
The anhydrous chloride, which has some important uses, is made by
heating aluminium turnings in a current of chlorine.
~Alums.~ Aluminium sulphate can be prepared by the action of sulphuric
acid upon aluminium hydroxide. It has the property of combining with the
sulphates of the alkali metals to form compounds called _alums_. Thus,
with potassium sulphate the reaction is expressed by the equation
K_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
= 2(KAl(SO_{4})_{2}·12H_{2}O).
Under similar conditions ammonium sulphate yields ammonium alum:
(NH_{4})_{2}SO_{4} + Al_{2}(SO_{4})_{3} + 24H_{2}O
= 2(NH_{4}Al(SO_{4})_{2}·12H_{2}O).
Other trivalent sulphates besides aluminium sulphate can form similar
compounds with the alkali sulphates, and these compounds are also called
alums, though they contain no aluminium. They all crystallize in
octahedra and contain twelve molecules of water of crystallization. The
alums most frequently prepared are the following:
Potassium alum KAl(SO_{4})_{2}·12H_{2}O.
Ammonium alum NH_{4}Al(SO_{4})_{2}·12H_{2}O.
Ammonium iron alum NH_{4}Fe(SO_{4})_{2}·12H_{2}O.
Potassium chrome alum KCr(SO_{4})_{2}·12H_{2}O.
An alum may therefore be regarded as a compound derived from two
molecules of sulphuric acid, in which one hydrogen atom has been
displaced by the univalent alkali atom, and the other three hydrogen
atoms by an atom of one of the trivalent metals, such as aluminium,
iron, or chromium.
Very large, well-formed crystals of an alum can be prepared by
suspending a small crystal by a thread in a saturated solution
of the alum, as shown in Fig. 84. The small crystal slowly
grows and assumes a very perfect form.
[Illustration: Fig. 84]
~Other salts of aluminium.~ While aluminium hydroxide forms fairly stable
salts with strong acids, it is such a weak base that its salts with weak
acids are readily hydrolyzed. Thus, when an aluminium salt and a soluble
carbonate are brought together in solution we should expect to have
aluminium carbonate precipitated according to the equation
3Na_{2}CO_{3} + 2AlCl_{3} = Al_{2}(CO_{3})_{3} + 6NaCl.
But if it is formed at all, it instantly begins to hydrolyze, the
products of the hydrolysis being aluminium hydroxide and carbonic acid,
Al_{2}(CO_{3})_{3} + 6H_{2}O = 2Al(OH)_{3} + 3H_{2}CO_{3}.
Similarly a soluble sulphide, instead of precipitating aluminium
sulphide (Al_{2}S_{3}), precipitates aluminium hydroxide; for hydrogen
sulphide is such a weak acid that the aluminium sulphide at first formed
hydrolyzes at once, forming aluminium hydroxide and hydrogen sulphide:
3Na_{2}S + 2AlCl_{3} + 6H_{2}O = 2Al(OH)_{3} + 6NaCl + 3H_{2}S.
~Alum baking powders.~ It is because of the hydrolysis of aluminium
carbonate that alum is used as a constituent of some baking powders. The
alum baking powders consist of a mixture of alum and sodium hydrogen
carbonate. When water is added the two compounds react together, forming
aluminium carbonate, which hydrolyzes into aluminium hydroxide and
carbonic acid. The carbon dioxide from the latter escapes through the
dough and in so doing raises it into a porous condition, which is the
end sought in the use of a baking powder.
~Aluminium silicates.~ One of the most common constituents of rocks is
feldspar (KAlSi_{3}O_{8}), a mixed salt of potassium and aluminium with
the polysilicic acid (H_{4}Si_{3}O_{8}). Under the influence of
moisture, carbon dioxide, and changes of temperature this substance is
constantly being broken down into soluble potassium compounds and
hydrated aluminium silicate. This compound has the formula
Al_{2}Si_{2}O_{7}·2H_{2}O. In relatively pure condition it is called
kaolin; in the impure state, mixed with sand and other substances, it
forms common clay. Mica is another very abundant mineral, having varying
composition, but being essentially of the formula KAlSiO_{4}.
Serpentine, talc, asbestos, and meerschaum are important complex
silicates of aluminium and magnesium, and granite is a mechanical
mixture of quartz, feldspar, and mica.
~Ceramic industries.~ Many articles of greatest practical
importance, ranging from the roughest brick and tile to the
finest porcelain and chinaware, are made from some form of
kaolin, or clay. No very precise classification of such ware
can be made, as the products vary greatly in properties,
depending upon the materials used and the treatment during
manufacture.
Porcelain is made from the purest kaolin, to which must be
added some less pure, plastic kaolin, since the pure substance
is not sufficiently plastic. There is also added some more
fusible substance, such as feldspar, gypsum, or lime, together
with some pure quartz. The constituents must be ground very
fine, and when thoroughly mixed and moistened must make a
plastic mass which can be molded into any desired form. The
article molded from such materials is then burned. In this
process the article is slowly heated to a point at which it
begins to soften and almost fuse, and then it is allowed to
cool slowly. At this stage, a very thin vessel will be
translucent and have an almost glassy fracture; if, however, it
is somewhat thicker, or has not been heated quite so high, it
will still be porous, and partly on this account and partly to
improve its appearance it is usually glazed.
Glazing is accomplished by spreading upon the object a thin
layer of a more fusible mixture of the same materials as
compose the body of the object itself, and again heating until
the glaze melts to a transparent glassy coating upon the
surface of the vessel. In some cases fusible mixtures of quite
different composition from that used in fashioning the vessel
may be used as a glaze. Oxides of lead, zinc, and barium are
often used in this way.
When less carefully selected materials are used, or quite thick
vessels are made, various grades of stoneware are produced. The
inferior grades are glazed by throwing a quantity of common
salt into the kiln towards the end of the first firing. In the
form of vapor the salt attacks the surface of the baked ware
and forms an easily fusible sodium silicate upon it, which
constitutes a glaze.
Vitrified bricks, made from clay or ground shale, are burned
until the materials begin to fuse superficially, forming their
own glaze. Other forms of brick and tile are not glazed at all,
but are left porous. The red color of ordinary brick and
earthenware is due to an oxide of iron formed in the burning
process.
The decorations upon china are sometimes painted upon the baked
ware and then glazed over, and sometimes painted upon the glaze
and burned in by a third firing. Care must be taken to use such
pigments as are not affected by a high heat and do not react
chemically with the constituents of the baked ware or the
glaze.
EXERCISES
1. What metals and compounds studied are prepared by electrolysis?
2. Write the equation for the reaction between aluminium and
hydrochloric acid; between aluminium and sulphuric acid (in two steps).
3. What hydroxides other than aluminium hydroxide have both acid and
basic properties?
4. Write equations showing the methods used for preparing aluminium
hydroxide and sulphate.
5. Write the general formula of an alum, representing an atom of an
alkali metal by X and an atom of a trivalent metal by Y.
6. What is meant by the term polysilicic acid, as used in the discussion
of aluminium silicates?
7. Compare the properties of the hydroxides of the different groups of
metals so far studied.
8. In what respects does aluminium oxide differ from calcium oxide in
properties?
9. Supposing bauxite to be 90% aluminium hydroxide, what weight of it is
necessary for the preparation of 100 kg. of aluminium?