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METALS AND THEIR REACTIONS by shuifanglj

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									                                            METALS AND THEIR REACTIONS

Throughout human history, cultural developments have been closely associated with the discovery of materials and the
ability to use them in a variety of ways. The discovery of different metals, and the techniques needed to extract them and
shape them into weapons and tools, has had a profound effect on the history of civilisation. Today, due to advances in
chemistry and technology, many more metals are available for our use than in ancient times and they play important roles in
our lives. We use them in the construction of buildings, cars, trains, kitchen utensils, electrical wires, food cans and
computers, to name just a few. The uses we make of metals depend on their properties, their availability and the ease and
cost of their extraction.

History of Metals

The Earth provides us with an abundance of resources to meet our needs. In prehistoric times, before there was any written
record, early humans made most of their tools and weapons from stone, bone and wood. This period is called the Stone Age.
During this time, people led a nomadic existence and metals were unknown.

Early metal extractions

The first metals to be used were those that are least reactive—silver, gold and copper. These were probably found as
nuggets of metal and they were used for jewellery and ornaments. Samples found in ancient tombs indicate that copper and
gold were in use by about 8000 BC and silver by about 4000 BC. The use of metals coincided with settlement in many parts
of the world. In settled societies, specialist craft skills could develop and people gradually became skilled potters, weavers,
stonemasons and metalsmiths.

Copper is thought to have been the first metal extracted from an ore. This probably happened first around 6000 BC, the
beginning of the Age of Copper. Copper was extracted by heating the ore with charcoal, most likely in pottery kilns. The
temperature needed to fire clay is high enough to melt the copper ore and the low-oxygen, high-carbon environment in the
kiln would convert the copper ore to copper. This process of heating an ore in the presence of carbon is called smelting. The
ore would have been ground by hand before being placed in a furnace such as that shown in Figure 3.2.

Civilisations in ancient times developed the technological skills to enable them to smelt copper from its ores and work the
copper to produce weapons and tools. Copper replaced stone for some purposes, especially when it was found that it could
be hardened by hammering. However, it was too soft to replace most stone tools and it also had the disadvantage of being
difficult to work due to its high melting point.

By 3000 BC bronze, an alloy of copper and tin, had replaced copper and the Bronze Age had begun. Of course, this did not
happen simultaneously all over the world. While the Bronze Age had started in some parts of Asia and Egypt, America and
Europe were still in the Copper Age and some parts of Africa and the Pacific region were still in the Stone Age.

Bronze may have been first discovered accidentally when ores containing both metals were heated together. Once its
advantages were realised, people began to mix them deliberately. Bronze was better for tools and weapons as it was harder
and had a lower melting point than pure copper, making it easier to shape and sharpen to produce a better cutting edge. It
was also less brittle than stone.
By 2500 BC tin and copper were being mined on a large scale and the use of metals was widespread.

The Romans worked copper mines in Spain and used the copper for such purposes as water piping. Copper's resistance to
corrosion and its ability to be hammered into sheets led to the use of copper in cooking pots and even as roofing material.




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Figure 3.2 An ancient copper kiln (a) The kiln is lined with stones and clay with air blown in from goatskin bellows (b) Slag
floating on copper is removed (c) Copper solidifies in the kiln

Some other metals were known and used in ancient times. Two of these are lead and zinc. Lead was extracted about 5000
BC, probably being first discovered in the ashes of fires that had been burning over lead-bearing rocks. This could have
produced the lead beads worn as jewellery around that time. Some authorities believe that lead was discovered about the
same time as copper and maybe even earlier in some areas. The Romans used lead for coins, plumbing and some ornaments
and it was used to glaze pottery. Later lead was used for roofing and lead oxide was added to glass to make crystal glass.
Lead was the first metal mined in Australia, and we are still one of the world's largest producers. Today the main use of lead
is in car batteries, although its high density and resistance to corrosion also make it useful as a noise barrier and for
protection against X-rays and nuclear radiation. Computer and television screens have lead added to the glass for this
reason. Many of its uses as additives in petrol and paints have been phased out because it is now known to be toxic.

Zinc was used to make brass (a copper/zinc alloy) about 150() BC in Egypt. Initially the addition of zinc may only have
been accidental, due to its natural presence in the copper ore used as the source of copper. The more zinc added, the harder
and stronger the copper alloy becomes. Rocks containing zinc were mined in India about 500 BC. However, zinc metal was
not produced commercially until relatively recent times. Today it has many uses, although its main application is to coat
iron and steel. When coated with zinc, or a zinc/aluminium alloy, iron and steel are said to be galvanised. Australia uses
more coated steel per person than any other country.

lron is the most abundant metal but it was not used until after copper because it is more difficult to smelt. Iron was used by
the Egyptians about 4000 BC, but this came mainly from meteor fragments. As iron is more reactive, and thus harder to
extract from its ores, it was not smelted until around 1500 BC, beginning the Iron Age. The technology to achieve the
higher temperatures needed to extract iron was probably first developed in Asia, spreading from there to the Middle East
and Europe. It did not change much until 200 years ago. An open fire cannot achieve the high temperatures needed, so a
furnace must be built and air pumped in. The furnace was probably made of stones and lined with clay. A mixture of

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charcoal, crushed copper ore and iron ore was added and ignited. The charcoal probably lined the furnace and covered the
top. Air would have been blown in by pipes or pumped in by bellows, operated by hand or foot. Blowing in air increased the
temperature of combustion, making it hot enough to convert the iron oxide to iron. The copper ore was added to act as a
flux, a substance that would combine with waste rock and form slag. After smelting, the less dense slag would be floating
on top so it could be run off, leaving relatively pure iron at the base of the kiln.

Early attempts to smelt iron produced an impure form, which still contained some slag. This iron was not as hard as bronze,
and bronze had the added advantage that it did not rust, so bronze was preferred. Iron was known for a long time before it
became used extensively. By 1200 BC, blacksmiths were heating iron with charcoal (carbon) and then quenching it with
cold water to make the first steel alloys. These were heated red hot to burn off some of the carbon and hammered to push
out the impurities, thus producing a harder, less brittle product. The development of these improved techniques, and the
realisation that iron was much more plentiful than copper and thus cheaper, meant that by about 1000 BC iron had come
into general use and replaced bronze for tools and weapons. Today, iron is still the most widely used and useful metal in the
world.

Relatively active metals, such as aluminium, which form more stable compounds are more difficult and require more energy
to extract. They could not be used until the modern era as technologies involving electricity had first to be developed.
Aluminium was not produced until the mid 1 800s and at that stage it was expensive and not in demand, being used mainly
for jewellery and medical instruments. The invention of the generator (dynamo) in 1867, which allowed for cheaper
production of electricity, and the development in 1886 of better electrolytic techniques using molten cryolite allowed more
aluminium to be produced at a cheaper price.

Modern era and future developments

Many metals are used extensively today, their uses depending on their availability, cost and their physical and chemical
properties (Chapter 1). Early metals such as copper, silver and gold are still used extensively and their uses are being
extended.

Copper's main use is in electrical appliances and wiring, although it also has many applications in the areas of construction,
engineering and transport. Properties that contribute to its usefulness include its good conductivity of heat and electricity,
resistance to corrosion and pleasing appearance. Its toxicity to marine organisms, combined with its resistance to corrosion,
has made it very good as a sheathing material for ship hulls and offshore platforms. In the future it may be used more
extensively in superconductors, computer chips and electric vehicles. Silver is the best conductor of electricity, so it is
widely utilised in electric circuits. It also has other uses, for instance, a type of glass being developed uses silver salts to
absorb UV light. However, availability of silver is limited by the short supply of silver ore. Gold is used not only in
jewellery and for decorative purposes, but also in such applications as microchips, electronics and for coating astronauts'
visors.

The number of metals available for us to use has increased during the last 200 years because of the development of
chemistry and new extraction techniques. Only in modern times have industrial skills been developed to isolate and refine
metals such as aluminium, manganese, magnesium and titanium. Although these metals have only become commercially
available in relatively recent times, they are already being used extensively and will have many more applications in the
future.

Aluminium is well known and has many applications. Its useful properties, such as low density, strength, availability and
ease of recycling, may lead to its increased importance in the future, extending its uses in transport, packaging, construction,
electric cables and as a solid fuel in powdered form. Magnesium is used in many industries including the production of
cement, rayon, fertiliser and glass. It is also alloyed with aluminium to make it stronger and more resistant to corrosion so
that it is suitable for construction of cars, planes, sporting goods and ladders. Manganese is used in the manufacture of
batteries, glass and dry-cell batteries as well as an additive in steel. Titanium, present as oxides in beach sands, is a strong,
light, silvery metal with many uses ranging from construction of aeroplanes and surgical implants to paint pigment
manufacture.




Alloys and their uses



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Metals can be used in their pure form, for example aluminium (aluminium foil) and copper (electricity wires). However,
metals are increasingly being used as alloys rather than as pure metals. This allows the production of a substance with the
exact properties required for a particular use.

An alloy is a mixture containing a metal and one or more other elements. Usually the other elements are metals; however,
non-metals can be used, such as in carbon steel. The substances to be alloyed are melted, mixed and allowed to cool. Alloys,
being mixtures, can vary in composition—they do not have constant composition or properties. For example, solder is an
alloy of tin and lead but the tin content varies from 30 to 60 per cent depending on the properties required for the solder.
Electrician's solder has more tin and sets faster. Plumber's solder has less tin and this allows time while it is setting when it
can be worked to allow the adjustment of pipes.

Adding small amounts of a metal to another metal can change its properties. For example, adding chromium to iron
produces stainless steel, which resists rusting; adding nickel to iron makes it harder; and adding silicon makes iron easily
magnetised and de-magnetised and thus suitable to use as electromagnets. Adding a metal with different-sized particles
makes a stronger alloy—layers cannot slide over each other as easily if they are different sizes (Figure 3.6).




Figure 3.6 Alloys containing different-sized atoms (a) Atoms of about the same size can substitute for existing atoms (b)
Atoms that are much smaller can fit between existing atoms.

Examples of some alloys and their uses are shown in Table 3.1.
Table 3.1 Composition and uses of alloys

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Name of alloy           Approximate composition             Properties of alloy                 Uses
Stainless steel         70-80% iron                         Resistant to corrosion Hard,        Cutlery
                        10-18% chromium                     resists abrasion Shiny              Building
                        1-8% nickel
Tungsten steel          70-75% iron                         Hard, even when heated              Tools for cutting and
(high speed steel)      15-25% tungsten                                                         grinding- blades, drill bits
                        5% chromium
Alnico                  62% iron                            Improves magnetic properties        Permanent magnets
                        21% nickel
                        12% aluminium
                        5% cobalt
Zinc-aluminium          45% zinc                            Resists corrosion                   Coating on steel for roofs
                        55% aluminium                                                           Wall cladding
Zinc- aluminium         96% zinc                            Can be pressure die-cast in steel   Carburettors
                        4% aluminium                        moulds to make accurate             Door handles
                                                            complex shapes with high            Zippers
                                                            strength
Brass                   60-70% copper                       Strong Easily worked Resists        Boats
                        30-40% zinc                         corrosion, including effects of     Musical instruments
                                                            salt water Can be polished          Decorative uses, Screws and
                                                                                                bolts
Cupro-nickel            75% copper                          Resists corrosion Shiny, silvery    Coins—50c, 20c, 10c and 5c
                        25% nickel                          appearance                          coins

Bronze                  95% copper                          Durable Lower melting point         Statues
                        5% tin                              than copper Easy to cast Harder     Medals
                                                            than pure copper                    Ship propellers
Aluminium bronze        92% copper                          Gold colour                         $1 and $2 coins
                        6% aluminium                        Relatively low density
                        2% nickel
Nitinol                 50% nickel                          Shape memory                        Heat-sensitive switches
                        50% titanium                                                            Keeping arteries open
Solder                  40-70% lead                         Relatively low melting point of     Joins metals, especially in
                        30-60% tin                          lead Adheres to other metals        electronics and plumbing
                                                            when molten


                              Modern Extractions, Recycling and Economic Issues
Some metals are extracted from their ores by modifications of standard techniques used to separate mixtures. Thus gold can
be separated from quartz by first crushing the quartz and then using gold panning or other techniques to separate the two
using the difference in density. If the metals occur as a compound, then the metal is extracted by the chemical
decomposition of that compound. The extraction of a metal from its ore generally involves the stages shown in Figure 3.8.
                                                             Mining


                                  Separation of mineral from the rest of the ore (gangue)


                                          Extraction of the metal from the mineral


                                      Purification of the metal and/or alloy production

Figure 3.8 Extracting metals from their ores
Extraction of copper



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Copper ores
You may remember from your earlier studies in Science that:

•        A rock is a mixture of mineral grains or crystals solidified together.

•        A mineral is a naturally formed material with a definite chemical composition and distinctive physical properties,
         such as hardness and colour. It is a crystalline solid, with a definite composition, that occurs in the Earth's crust.

•        An ore is a rock that contains enough of one or more minerals to make it profitable to mine.

The main copper ores (Australia and worldwide) contain chalcopyrites (CuFeS2) and other sulfides such as copper(II)
sulfide (CuS) and copper(l) sulfide (Cu2S). These sulfides provide most of the copper produced around the world and
occupy deeper parts of deposits that have not been exposed to weathering. Near the Earth's surface, sulfides are altered by
chemical reactions with air and water to the native metal, oxides and carbonates, especially malachite (Cu2(OH)2CO3) and
azurite (Cu3(OH)2(CO3)2) with some cuprite (Cu2O) and tenorite (CuO).

Mining sites in Australia

Mt Isa (Queensland) and Olympic Dam (South Australia) contain over 90% of Australia's identified economic resources of
copper (Figure 3.9). Mt Isa is one of the largest underground mines in the world, producing about 39 000 tonnes of copper,
silver, lead, and zinc ores annually. Olympic Dam produces ores containing copper, gold, silver and uranium oxide. Copper
ore is mined, smelted and electrolytically refined at Olympic Dam.

Figure 3.9 Major copper mines in Australia




Copper is also produced in Australia at sites such as Mount Morgan (Queensland), Mount Lyell (Tasmania) and Tennant
Creek (Northern Territory). These mines were originally established as gold mines. However, profits from gold declined as
deeper ores were reached, so mining switched to copper.

Separation processes

Crushing and grincling

Copper minerals are present in rock as fine grains, so to separate them out the rock must first be crushed to particles less
than 10 mm in diameter. It is then taken by conveyor to the roll and ball mil1 where it is ground to a fine powder.

Concentration



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Ores contain useful minerals, such as copper sulfide, as particles mixed with useless rock material. The mineral has to be
separated out from the waste rock, called gangue, before the metal can be obtained from it. Froth flotation is a method used
to separate the components of a mixture when they vary in their

ability to cling to bubbles. Froth flotation is used in mining to separate sulfide and phosphate ore minerals from gangue and
from each other. It is also being used in coal mining.

Figure 3.10 Froth flototion The rotating impeller and air cause the detergent to form bubbles. The ore particles stick to the
bubbles and rise to the top, leaving the rock particles at the bottom.




In this process, crude ore is crushed and ground to a powder so the useful particles can be separated out. The crushed ore is
mixed with water and frothing reagents and fed into banks of flotation cells or tanks (Figure 3. 10). Air is blown in through
the mixture so that it forms a thick froth. Particles of the metal mineral, for example, copper sulfide, cling to the air bubbles
and rise to the surface with the froth bubbles. The froth is skimmed off. This goes through more cells where the water and
chemicals are removed, leaving concentrated mineral, which is called a metal concentrate. The metal mineral has been
concentrated by the removal of unwanted rock.

The useless rock (gangue) does not cling to the bubbles; it sinks to the bottom of the tank and is removed. Gangue is often
stored in a dam, called a tailings dam, or returned underground to fill in used mines.

Extraction

The copper mineral concentrate must then be treated chemically to extract copper from the compounds in which it occurs.
In a modern Australian method, the mineral concentrate (CuFeS 2) is first mixed with sand (SiO2) and coal (for heat) and
placed into a furnace (Figure 3.11). Oxygen is then blown in through a lance and the intense heat generated causes the ore to
be smelted, separating the iron and copper. The SiO2 reacts with the iron oxide formed to produce FeSiO3 (slag) which
floats on the copper(I) sulfide and is discarded. Two very simplified versions of the chemical reactions involved are:




Smelting:

                                       2CuFeS2(s) + 402(g)  Cu2S(l) + 2FeO(s) + 3SO2(g)

                                                  FeO(s) + SiO2(S)  FeSiO3(l)




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The copper(l) sulfide is then heated in a converter (furnace) with oxygen blown through to extract the copper from the
copper sulfide (Figure 3.11). Sand is also added to remove any remaining iron oxide. Two very simple versions of the
reactions involved are:

Converter:                           Cu2S(s) + O2(g)  2Cu(l) + SO2(g)

                                     FeO(s) + SiO2(s)  FeSiO3(l)

The overall reaction is:

                                     2CuFeS2(s) + 5O2(g)  2Cu(l) + 2FeO(s) + 4SO2(g)



Australian-based research has developed other new and efficient methods to extract copper. For example, the process used
at the Olympic Dam operation is called flash smelting. The copper concentrate is dried, mixed with chemicals called fluxes,
such as sand (SiO2), and fed into a heated smelter with oxygen-enriched air. Here the fine concentrate reacts (flashes)
instantly, forming metallic copper, sulfur dioxide gas and molten waste material called slag. Typical reactions are:

Cu2S(s) + O2(g)  2Cu(l) + SO2(g)

2CuFeS2(s) + 5O2(g)  2Cu(l) + 2FeO(s) + 4SO2(g)

FeO(s) + SiO2(s)  FeSiO3(l) (iron silicate—slag)

Molten slag, which contains the waste rock combined with the fluxes, falls to the bottom of the furnace where it floats on
top of the molten copper. This slag contains about 20% copper. The slag is removed by tapping it off, allowed to cool, then
treated by grinding and froth flotation to recover more copper.

Sulfur dioxide goes out the top of the smelter and is piped to a sulfuric acid plant. Here it is converted to sulfur trioxide,
which is then dissolved in water to make sulfuric acid.

2SO2(g) + O2(g)  2SO3(g)

SO3(g) + H2O(l)  H2SO4(aq)

The copper metal produced is molten and falls to the bottom of the furnace where it can be tapped off. This copper is called
blister copper because bubbles of escaping sulfur dioxide look like blisters. It is about 98% pure, but contains about 0.6%
sulfur as well as other impurities such as gold and silver and has to be purlfied (refined) so it can be used for such purposes
as conducting electricity.




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Figure 3.11 Producing copper In the Isasmelt process, copper ore is first smelted in a modern furnace before being




Figure 3.12 Refining copper Electrolysis purifies blister copper


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Refining

Electrolysis is used to produce very pure copper (Figure 3.12). You may study the process in detail in the HSC book.
Originally the process used very pure copper cathodes. Australian refineries have developed a new method that uses
stainless steel cathodes, and this method is now used around the world.

The impure molten copper is cast into slabs, which will act as the positive electrodes (anodes). The negative electrodes
(cathodes) are made of stainless steel (Figure 3.12). These electrodes are suspended in tanks containing an electrolyte
(substance that conducts electricity). When electricity is applied, the blister copper (anode) goes into solution and pure
copper is deposited on the cathode. Impurities, including gold and silver, fall to the bottom of the tank as 'sludge'. After
several days the cathode is removed from the tank and the pure copper stripped off the stainless steel cathodes and shipped
to markets. The 'sludge' is collected and treated to extract gold and silver bullion.

Environmental issues

In the past, physical changes and damage to the environment as a result of mining have been extensive. Today a great deal
of care and money goes into preventing pollution and restoring the environment to its original condition. Mining in
Australia uses less than 0.02% of the land surface and most developments are in areas that are not heavily populated, but
stringent controls are still in place to ensure the safety of people, native plants and native animals. Some problems
associated with copper mining include the release of sulfur dioxide gas, pollution of water and the disposal of unused rock
material.
Sulfur dioxide

Sulfur dioxide is formed by the heating of sulfides. This has harmful effects on both plants and animals. It affects the
respiratory system, causing asthma in some people. Sulfur dioxide also dissolves in rain water, forming acid rain. To
prevent sulfur dioxide reaching the atmosphere, exhaust gases are passed through scrubbers, to dissolve the sulfur dioxide in
water. It is then used to produce sulfuric acid. Remaining gases are discharged through chimney stacks and concentrations
at ground level are continually monitored.

Water

Mining uses large volumes of water. Many processes, such as froth flotation, require water—water is sprayed on roads and
stockpiles to reduce dust and of course it must be available for drinking and washing. Water used at copper mines is
recycled wherever possible. Any water that may be contaminated is kept in retention ponds, which are lined to prevent
contaminated water entering the water supply. Bores and dams on surrounding properties are monitored.
Tailings

Waste rock material must be disposed of in an acceptable manner. In the past, such tailings have been dumped along rivers
and lakes and poisonous heavy metals that were not extracted have been able to leach into the water supply. Today the
waste rock is generally used to help restore the area to its original condition. The area is revegetated to prevent I dust and
erosion. Hills, plants and animals in the area j must all be restored after mining has ceased.

Economic issues

In the last 50 years we have used an increasing amount and variety of metals. We are using twice as much iron, almost three
times as much copper and more than six times as much aluminium as we were using 50 years ago.

Prices of metals fluctuate continually. They are affected by a number of factors, including:

Difficulty and cost of extraction

To extract metals from ores energy is needed and the amount of energy needed is determined partly by the activity of the
metal. The least active metals are found as the element or as compounds that are relatively easy to decompose. Today most
silver is found as silver sulfide (argentite, Ag2S), which decomposes readily. More active metals require more energy to
decompose the compounds in which they occur.

Three metals that are used extensively in our society are copper, iron and aluminium. Copper can be extracted from its ores
by roasting and smelting— copper sulfide and copper oxides are reduced to copper by heating with carbon at relatively low
temperatures. Iron compounds are also reduced to iron by carbon, but this can only take place at higher temperatures. The

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higher temperatures are needed because iron is more active than copper, so more energy is needed to decompose its
compounds. Aluminium compounds require electrolysis at high temperatures to obtain the relatively active metal,
aluminium. The techniques used to extract metals have become increasingly sophisticated over time. The mining and
extraction techniques used today make it possible to mine much lower-grade deposits than previously.

Abundance of ores and the concentration of metals in the ores

The relative abundance of metals in the crust has influenced their value and extent of use. The most abundant metal in the
Earth's crust is aluminium (about 8% by weight), followed by iron (about 5%). Metals are not evenly distributed in the
crust. We mine only those rocks that contain a sufficiently high percentage of the metal to make them economic to mine.

Copper only makes up about 0.07% of the crust and the ores mined may contain from 0.2 to 3% copper, the remainder being
waste rock. Froth flotation concentrates the ore to about 25%. The minerals in the ore contain from 27% to 69% copper
depending on the particular minerals present (e.g. CuFeS2 or Cu2S).

Location of ores and cost of transport

Whether a metal can be extracted economically depends also on such factors as its location, costs of setting up and
operating the mine and costs of transporting raw materials, equipment, products and people. If a mine is located at a
considerable distance from towns these costs may be greater as facilities such as homes and schools will also have to be
provided.

Ability to recycle the metal

Metal ores are non-renewable. When we have used up the Earth's resources of these ores we cannot replace them. We need
to use our metal resources wisely and develop recycling techniques so we can re-use metals. Whether we recycle or not is
partly determined by the relative costs of producing more ore compared to recycling costs. The ease of recycling is also an
important factor. Copper and aluminium are both able to be recycled readily; iron is less easily recycled as it is so easily
corroded.

Producing metals by recycling represents a considerable saving in energy. Table 3.4 shows a comparison of the energy
needed to produce 1 kg of three different metals from their ores and by recycling. The figures include energy needed for
transport, making sodium hydroxide to extract the alumina from bauxite, heating the materials when needed—as well as the
energy needed to break chemical bonds. It is obvious that recycling represents a considerable cost savings.

Other techniques that could help to extend the life of our metal resources include

•        reducing corrosion so that metals last longer

•        using abundant metals where possible instead of those that are scarce, e.g. replacing high-voltage copper electrical
         cables with aluminium

•        finding alternative materials for some uses.

Australia should also be retaining ownership of our mines and selling manufactured products rather than raw materials.
Australia is the fifth largest producer of mined copper in the world, most of which is exported. In recent years there has
been a decrease in the amount of copper ore and concentrates exported from Australia, while the amount of refined copper
and manufactured products has increased. This value adding results in more jobs and greater income for Australians.



Aluminium and recycling

Aluminium is used extensively in our society, in transport, transmission cables, packaging and construction. Much of the
recent growth in aluminium use has come from aluminium replacing heavier metals in vehicle (car) manufacture. The
properties of aluminium that make it suitable for these applications include its low density, corrosion resistance, ductility,
electrical conductibility and ability to be cast in different shapes.



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Aluminium is mostly used as alloys. Adding small amounts of other metals to aluminium can vary its properties, making it
even more versatile. For example, adding silicon will lower its melting point by up to eighty degrees, which makes it flow
more readily into moulds. Some examples are shown in Table 3.5.
Aluminium makes up about 8% of the Earth's crust; it is the most abundant metallic element, occurring mostly as the oxide
alumina (aluminium oxide, Al2O3) in the ore bauxite. More than 100 million tonnes of bauxite are mined each year.
Although aluminium is relatively plentiful, it is difficult to extract from its ore, requiring a great deal of energy to break the
chemical bond between aluminium and oxygen. Recycling uses less energy, as the melting point of aluminium is relatively
low (660°C) and remelting aluminium to recycle it requires only about 5 MJ/kg compared to 95 MJ/kg needed to extract it
by electrolysis from alumina. These values do not include the energy needed for mining and transport (Table 3.4).




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