Chapter 20 (James E. Brady – GENERAL CHEMISTRY, Principles & structures)
METALS AND THEIR COMPOUNDS;
THE REPRESENTATIVE METALS
20.1 OCCURRENCE AND METALLURGY
Metallurgy is the science and technology of extracting metals from the
earth and sea and bringing them to the point at which they can be put
to practical use. It is a subject with a long and rich history, dating from
the time when humans first fashioned tools and weapons from natural
deposits of metals such as copper. Modern metallurgy touches our
lives at every turn, from special alloys used in automobile engines to
stainless steels used in surgical implants.
Sources of Metals
Most metals are obtained from land-based deposits that are called
ores. An ore is a material that contains a particular constituent in a
sufficiently high concentration that its extraction from the ore is
economically worthwhile. Notice the stress on the economics of the
process. Aluminium, for example, is present in the form of
aluminosilicates in various kinds of rock, such as granite. However,
there is no economical way of recovering the aluminium from these
sources, so they are not considered aluminium ores. Instead, the ore of
aluminium is a mineral called bauxite, in which the aluminium occurs
as its oxide, Al2O3. As you learned in Chapter 18, aluminium can be
extracted from its oxide by electrolysis in the Hall process.
Some metals, such as gold, silver, and (occasionally) copper as
free metals in nature. However, most metals exist in nature in
compounds. Some are found in deposits of their carbonates or sulfates.
Limestone, for example, is primarily CaCO3, and gypsum is a hydrate
of calcium sulfate. Many important metal ores contain oxides. Two
examples are aluminium (Al2O3) and iron (Fe2O3 and Fe3O4). Sulfides
are the primary sources of lead (as PbS) and copper (as Cu2S).
The oceans also are a source of some metals, which exist there
primarily as soluble halides and sulfates. The principal source of
magnesium, for example;e, is sea water, even though MG2+ is present
at a concentration of only about 0.13 %. On the ocean floor, large
deposits of orange-sized metal-rich manganese nodules that are rich in
manganese (~25%) and iron (~15%)are found in certain locations.
There has been some interest expressed in attempting to recover them,
although at this time no commercial mining operations are under way.
Once a metal ore has been obtained from either the earth or the
sea, it must undergo a series of metallurgical processes that ultimately
provide the finished metal for commercial use. Because of the wide
variety of sources and the varying nature of the metal compounds in
the ores, no single method can apply to the production of all metals.
Nevertheless, it is possible to divide metallurgical processes into three
principal categories.
 Concentration. Ores that contain substantial amounts of impurities,
such as rock, must often be treated to concentrate the metal-bearing
constituent. Pretreatment of an ore is also carried out to convert
some metal compounds into substances that can be more easily
reduced.
 Reduction. In their compounds, metals nearly always exist in
positive oxidation states. Therefore, to obtain a metal from its ore it
must be reduced. The particular procedure employed for a given
metal depends on its ease of reduction to the free state.
 Refining. Often, during reduction, substantial amounts of impurities
become introduced into the metal. Refining is the process whereby
these impurities are removed and the composition of the metal
adjusted (alloys formed) to meet specific applications.
Concentration
Not all ores have to be subjected to a pre-treatment step prior to
reduction, although most of them must. These pretreatment procedures
involve the separation of the metal-bearing component of the ore from
unwanted or interfering impurities. This is particularly important for
low-grade ores in which the desired metal is present only in small
amounts.
As expected, different methods are applied to different ores,
depending on the specific properties of the impurities and the metal
compounds. We can divide these procedures into two classes: physical
separation, in which the chemical compositions of the constituents are
not altered, and chemical separations, which use the chemical
properties of the different substances in the ore.
As noted above, some metals, such as silver and gold, are found
in deposits as the free element, and their recovery simply involves
removing them from the rock and sand with which they are mixed.
One of the earliest forms of physical separation was used by the
“forty-niners” in panning for gold. A mixture of sand containing (it is
hoped!) particles of metallic gold is placed in a shallow pan with
water. The mixture is swirled about and the sand is washed over the
rim, leaving the gold dust in the bottom of the pan. The success of this
procedure is based on the fact that gold is about nine times as dense as
the sand and gravel impurities. As a result, the lighter impurities are
more easily washed away than the more dense metal.
Another way of removing metallic gold and silver from their
ores is to treat the mixture with metallic mercury, a liquid in which
silver and gold dissolve to form an alloy called an amalgam. The
silver and gold are later recovered by distilling away the mercury,
which is reclaimed and used again. You are probably familiar with
silver and gold amalgams as the material used by dentists to fill teeth.
A physical separation technique that can be applied to the sulfide
ores of zinc, copper, and lead is called flotation. In this process,
illustrated in Figure 20.1, the ore is pulverized and added to large vats
of a mixture of water and oil containing suitable additives. The finely
powdered metal-bearing sulfide particles of the ore become coated by
the oil while the unwanted material, called the gangue, is wetted by
the water. A stream of air is then blown through the mixture, and the
oil-covered mineral is carried to the surface by bubbles where it is
trapped in a froth that can be removed to recover the metal compound.
The gangue, on the other hand, simply settles to the bottom of the
apparatus and is later discarded.
Iron, as you are probably aware, is society’s most important
metal, and its ores have traditionally been rich deposits of reddishbrown hematite, composed primarily of Fe2O3. Through decades of
mining, hematite deposits in regions such as the Masabi range of
Minnesota have been nearly exhausted and other sources of iron ore
have been sought. One such ore being mined today is taconite.
Taconites have Fe3O4 as their iron-containing compound, and the
better of them contain 30 to 40 % iron by weight. The compound
Fe3O4 is called magnetite, and as its name suggests, it is magnetic.
This provides the basis for the enrichment of taconite ores by a
physical separation process. The rock-hard ore is first ground to give
particles of very small size and then the Fe2O3 is pulled out with a
powerful electromagnet.
Chemical methods of concentrating the metal-bearing
component of an ore vary considerably because of the variety of
chemical properties exhibited by the metals and their compounds. As
mentioned earlier, aluminium, whose electrolytic reduction was
described in Chapter 18, occurs in deposits of bauxite, a form of
Al2O3. In this case the ore is concentrated by taking advantage of the
amphoteric behavior of aluminium. The bauxite is treated with
concentrated base, which dissolves the Al2O3 to produce aluminate
ion, AlO2-.
Al2O3 + 2 OH- 2 AlO2- + H2O
After the solution is removed from the gangue, it is neutralized with
acid. This precipitates Al(OH)3, which yields pure Al2O3 when heated.
heat
2 Al(OH)3  Al2O3 + 3 H2O
This purified aluminium oxide serves as the raw material in the Hall
process discussed previously.
Another chemical pre-treatment, often given to a sulfide ore, is
called roasting. Here the ore is heated in air, converting the metal
sulfide to an oxide that is more conveniently reduced.
2PbS + 3 O2  2 PbO + 2 SO2
2 ZnS + 3 O2  2 ZnO + 2 SO2
The sulfur dioxide produced in the roasting process has been a severe
source of air pollution, as described in Chapter 23.