How can metals be made more useful? e.g. iron
See also Extraction
Metals can be mixed together to make alloys to improve the metal's properties
to better suit a particular
purpose. Quite often the presence of
different atoms stops the layers of the metal sliding over each other when
stressed so making the metal tougher (see
for more details).
Pure copper, lead, gold, iron and aluminium are too soft for many
uses and so are mixed with small amounts of similar metals to make them
harder for everyday use. These mixtures are called alloys and have
range of properties that can be tailored to use for specific purposes i.e.
match the properties of the alloy to its function.
can be made more
resistant to corrosion by a process called anodising. Iron can be made
more useful by mixing it with other substances to make various types of steel
alloys. Many metals can be given a coating of a different metal, or
painted, to protect
them or to improve their appearance.
is theoretically a reactive
metal but it is resistant to corrosion. This is because aluminium
reacts in air to form a layer of aluminium oxide which then protects the
aluminium from further attack.. doesn't reaction with water and very slow
reacts with acids.
For some uses of
aluminium it is desirable to increase artificially the thickness of
the protective oxide layer in a process
is called anodising.
This involves removing the oxide layer by
treating the aluminium sheet with sodium hydroxide solution.
The aluminium is then placed in
dilute sulphuric acid as the positive electrode (anode) used in the
electrolysis of the acid.
Oxygen forms on the surface of the aluminium
and reacts with the aluminium metal to form a thicker protective oxide layer
Aluminium can be
alloyed to make 'Duralumin'
by adding copper (and smaller amounts
of magnesium, silicon and iron), to make a stronger alloy used in
aircraft components (low density = 'lighter'!), greenhouse and window
frames (good anti-corrosion properties), overhead power lines (quite a
good conductor and 'light'), but steel strands are included to make the
'line' stronger and poorly electrical conducting ceramic materials are
used to insulate the wires from the pylons and the ground.
The properties of iron
can be altered by adding small quantities of other metals or carbon to
Steels are alloys since they are mixtures of iron with
other metals or with non-metals like carbon or silicon.
Most metals in everyday use are alloys.
Iron from the
furnace contains about ~96% iron with ~4% of impurities including
carbon, silica and phosphorus.
In this state the cast iron
is too hard and too brittle for most purposes.
Cast iron is hard and can be
used directly for some purposes, eg manhole covers, because of its
strength in compression.
However, if all the
impurities are removed, the resulting very pure iron is too soft for any
Therefore, strong useful
steel is made by controlling the amount of carbon and selected metals to
produce an alloy mixture with the right physical properties fit for a
particular application e.g. steel for car bodies, chrome stainless
steel, extremely hard and tough tungsten-iron steel alloys etc.
The real importance of alloys is
that they can be designed
to have properties for specific uses.
steels are easily shaped for car bodies, high-carbon steels are
hard, and stainless steels are resistant to corrosion etc.
Although the metals used in
construction are strong, in some situations they may become dangerously weak
If iron or steel becomes
badly corroded, there is no strength in rust!, and, the thicker the rust
layer, the thinner and weaker the supporting iron or steel layer, hence
the possibility of structural failure. Therefore, most iron and steel
structures exposed to the outside weather are maintained with a good
coating of paint.
and may lead to
a very dangerous situation of mechanical failure of the structure.
Also, if metal structures
e.g. in aircraft or bridges, are continually strained under stress, the
crystal structure of the metal can change so it becomes brittle. This
effect is called
So it is important develop
alloys which are well designed, well tested and will last the expected
lifetime of the structure whether it be part of an aircraft (eg titanium
aircraft frame) or a part of a bridge (eg steel suspension cables).
See notes on
of Metals and Rust Prevention
(1) Molten iron from the
blast furnace is mixed with recycled scrap iron
Then pure oxygen is
passed into the mixture and the non-metal impurities
such as silicon or phosphorus are then converted into
acidic oxides (oxidation process) ..
Calcium carbonate (a
base) is then added to remove the acidic
oxide impurities (in an acid-base reaction). The salts produced by this reaction form a slag which can
be tapped off separately.
produce pure iron.
of carbon and/or other metallic elements such as titanium, manganese
or chromium are then added to make a wide range
of steels with particular properties.
Because of the high
temperatures the mixture is stirred by bubbling in unreactive
Economics of recycling
scrap steel or ion: Most steel consists of >25% recycled
iron/steel and you do have the 'scrap' collection costs and
problems with varying steel composition* BUT you save enormously
because there is no mining cost or overseas transport costs AND
less junk lying around! (NOTE: * some companies send their own
scrap to be mixed with the next batch of 'specialised' steel they
order, this saves both companies money!)
Different steels for
High % carbon steel is
strong but brittle.
Low carbon steel or mild
steel is softer and is easily shaped and pressed e.g. into a motor car body.
alloys contain chromium and nickel and are tougher and more resistant to corrosion.
strong steels can be made by alloying the iron with titanium or
Steel can be
galvanised by coating in zinc, this is physically done by
dipping the object into a bath of molten zinc. On removal and
cooling a thin layer of zinc is left on. The zinc chemically bonds
to the iron via the free electrons of both metals - its all the
same atoms to them! It can also be done by electroplating
Steel (and most metals)
can be electroplated.
The steel object to
be plated is made the negative electrode (cathode) and placed in a solution containing ions of the plating metal.
positive electrode (anode) is made of the pure plating metal (which dissolves
and forms the fresh deposit on the negative electrode).
Nickel, zinc, copper,
and gold are examples of plating metals.
details of copper
purification amount to copper
plating, so all you have to do is swap the pure
negative copper cathode with the metal you want to coat (e.g. Ni, Ag
or Au or any material with a conducting surface). Swap the impure positive copper anode with a pure block
of the metal you want to form the coating layer. The electrodes dip
into a salt solution of nickel, zinc, copper, silver or gold ions etc.
and a low d.c. voltage passed through. If M = Ni, Cu, Zn ....
At the positive
anode, the process is an oxidation, electron
loss, as the metal atoms dissolve to form metal(II) ions.
==> M2+(aq) + 2e-
at the negative
cathode, the process is a reduction, two electron
gain by the attracted metal(II) ions to form neutral metal
atoms on the
surface of the metal being coated.
+ 2e- ==> M(s)
plating it is Ag+, Ag and a single electron change.
Any conducting (usually
metal) object can be electroplated with copper or
silver for aesthetic reasons or steel with zinc or
chromium as anti-corrosion protective layer.
Many other metals have
countless uses e.g. zinc
Zinc is used to make
the outer casing of zinc-carbon-weak acid batteries.
Zinc is alloyed with
copper to make the useful metal brass (electrical plug
pins). Brass alloy is stronger and more hardwearing than copper
AND not as brittle as zinc.
extraction/recycling of iron
and extraction/recycling of
manufacture and uses are described below in section 5. below
notes on metals: