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1. Introduction
to the extraction of metals - the theoretical and practical background
ideas
What
methods can be used in extracting metals from mineral ores?
Why can one method be used to extract one metal, but
not another?
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The Earth's crust contains many different
rocks.
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Rocks are a mixture of minerals and from some we
can make useful
substances.
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A mineral can be a solid metallic
or non–metallic element or a compound found naturally in the
Earth's crust.
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Mineral ores are naturally occurring rocks
that provide an economic starting
point for the extraction and manufacture of metals for a huge variety of
purposes ie a metal ore is rock containing sufficient metal to be worth
extracting the metal from it.
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The simplest definition of an ore is a
mixture of a metal containing mineral and other materials ('minerals')
from the surrounding rocks, which can be described as impurities with
respect to what you want from the ore.
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Metal ores are obtained by mining and that
this may involve digging up and processing large amounts of rock.
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Most ores are mined have to
be concentrated before the metal is extracted and purified.
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This often
results in lots of waste material that must be dealt with from an
environment of view.
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The
social, economic and environmental impacts of
exploiting metal ores are discussed on a separate page.
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The rock must contain enough
of the metal compound, hence enough of the metal, to be worth exploiting
the ore reserve and extract the metal by physical and chemical
processes.
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Low grade ores are usually uneconomic
to mine and process, but high grade ores have a high enough metal
content to be worth mining and extracting the metal.
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The metal ore, a mineral or mixture of
minerals from which economically viable amounts of metal can be extracted, i.e. its got to have enough of the metal, or one of its
compounds, in it to be worth digging out!
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Ores are often oxides, carbonates or sulphides
because these non-metals readily combine with many metals.
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e.g. iron + oxygen
===> iron oxide ores
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Fe + O2(air)
===> Fe2O3 or Fe3O4
(not balanced equations, just the idea!)
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This the typical chemistry involved
in metal ore formation.
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In extracting metals from ores you
have to reverse the process e.g. by heating the ore with carbon (coke or
charcoal) in a very hot smelting furnace e.g. in a blast furnace to make
iron ...
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iron oxide ore + carbon
===> free iron + carbon dioxide waste gas
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Fe3O4
+ 2C ===> 3Fe + 2CO2
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The iron oxide is reduced (oxygen
loss) and the carbon reducing agent is oxidised (oxygen gain).
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This is typical extraction chemistry
for less reactive metals.
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For details see sections
2.
Extraction of Iron and Steel Making and
4.
Extraction and Purification of Copper
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These ores are all
finite resources so we should use them wisely!
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Any ore must contain
enough of the metal to make it worthwhile to mine and then extract the
metal.
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High grade ores will
therefore be the most economical to exploit BUT over time the better
quality–richer sources will decrease, especially with the power of the
global economy and powerful and growing economies of Brazil, China,
India and other Asian economies.
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The economics of metal
extraction are not only dependent on the quality of the ore and the
cost of extraction (ie richer ores lead to cheaper production), but also
depend on the market price and demand.
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If demand is high the metal price
rises and may off–set the price of mining lower grade ores, but if
demand is low, the metal price falls and inefficient mines and smelters
will go out of business ie its not worth extracting the metal.
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Of course
it is possible to improve the technology of metal extraction and enable
companies to produce more metal from the ore than was previously
possible and even utilise low grade ores previously discarded and not
considered worthwhile mining or processing waste from mining
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Since the majority of
metals are found combined with non–metals like oxygen (oxide ion) or
sulfur (sulfide ion) or the carbonate ion, chemical reactions are needed
to free the metal from its mineral source.
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In order to extract a
metal, the ore or compound of the metal must undergo a process
called REDUCTION to free the metal i.e.
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The oxide/sulfide loses oxygen/sulfur, to form the free
metallic atoms, or
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the positive metal ion gains
negative electrons to form the neutral metal atom.
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The chemical
that removes the oxygen from an oxide is called the reducing
agent i.e. carbon, carbon monoxide or sometimes
hydrogen.
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 Order of decreasing reactivity
related to the earliest know date of extraction and use:
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(BP means before present time)
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francium (1939, very
radioactive), caesium (1860, ?), rubidium (1861, ?)
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potassium (1807, 1855 from electrolysis),
sodium (1807, from electrolysis)
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lithium (1817, electrolysis?),
calcium (1808, from electrolysis)
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magnesium (1755, 1808 from electrolysis),
aluminium
(1825, by electrolysis)
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zinc (before 1500, ), iron (extracted with charcoal before
3000 BP)
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tin (~4500 BP, used to make
bronze)
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lead (over 9000
BP, archaeologist have found lead beads 9000 years old, used by the
Romans for plumbing well over 2000 years ago)
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copper (~11000 BP extracted
via charcoal from ores >4000 years ago, found 'native' and was beaten
out of rocks and into a useful shape!)
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silver (~7000 BP, used by ancient
civilisations)
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gold (~8000 BP, used by ancient civilisations,
e.g. Egyptian civilisations, found 'native' in streams and extracted by
'panning')
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platinum (~1735, recognised as
a rare metal
but known to ancient South American civilisations
before Europeans arrived in the 15th century, brought to Europe ~1750)
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I've tried to indicate the earliest
date of extraction and use, however impure the metal might be, like
bronze age copper and iron age iron!).
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The less reactive metals could
be found as the element or relatively easily extracted using
charcoal (like coke, it has a high % of carbon in it).
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The date is quoted as the
'normal' year
(BCE/AD) or BP meaning years before present year (I've not used
BC).
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'native' seems a politically
incorrect term, but it is a term that means finding the element in its
free chemically uncombined state e.g. gold in alluvial deposits washed
into mountain streams or 'native' copper was beaten out of rocks over
6000 years ago.
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The understanding of electricity and
the development of d.c. electrical supplies e.g. using simple voltaic
batteries meant that reactive metals could then be extracted by
electrolysis.
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Once more reactive metals could be
produced in larger quantities by electrolysis, these metals themselves
were (and still are) used to extract other metals e.g. chromium, which
were often difficult to extract by conventional smelting furnaces using
carbon.
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Generally speaking the more
reactive the metal the later it is extracted and used as the technology
of metal extraction improves.
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Detailed notes on the 'Reactivity Series of Metals'
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As described above, historically as
technology and science have developed the methods of extraction
have improved to the point were all metals can be produced. The
reactivity is a measure of the ease of compound formation and
stability. The more reactive the metal, the more readily the
metal forms a stable compound eg with oxygen or sulfur, and
therefore because of this greater compound stability, the more
difficult it is to reduce the compound to the metal.
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The
position of a metal in the reactivity series of metals (right
diagram) has important implications for the method used to extract
metals.
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In 'general' to summarise before
a more general discussion and detailed notes page:
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The most reactive metals K, Na,
Ca, Mg and Al cannot be extracted using carbon or hydrogen, as the
reducing agent and must be obtained by costly electrolysis.
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You should realise that in order
to extract a metal from its oxide, you have to remove the oxygen!
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The more reactive a metal, the
strongly it is chemically bound to oxygen (or sulfur) in its
oxide (or sulfide).
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Therefore, the more reactive
the metal, the more difficult it is to extract the metal and
usually more energy is needed, and up go the costs too!
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Metals below aluminium can be
extracted using carbon or carbon monoxide as the reducing agents,
those above zinc can't.
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The carbon displaces the metal
and is cheaper than electricity!
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Charcoal is made by baking wood
at a high temperature in an oven with a limited oxygen supply.
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Coke is made by heat coal to a
high temperature. Both contain a high % of the element carbon.
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A reducing agent is a
chemical agent that typically removes oxygen from a metal
oxide ore to leave the free metallic element.
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In the process the metal oxide is
reduced by oxygen loss, which also equates to the metal ion gaining
electrons - reduction e.g. CuO ==> Cu or Cu2+
+ 2e- ==> Cu
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A reducing agent like carbon (C)
or carbon monoxide (CO) can remove the oxygen from the oxide
to leave the free metal (and forming carbon dioxide, CO2,
in the process)
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i.e. the reducing agent is oxidised by
oxygen gain - so C ==> CO or CO2, or CO ==> CO2.
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Metals below lead can be
extracted using hydrogen as the reducing agent, those above copper
can't.
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The least
reactive (unreactive) metals such as gold, silver and copper have been used for
the past 10000 years because the pure metal was found
naturally.
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Their lack of chemical
reactivity allows them to exist as the uncombined element.
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Conversely, the more
reactive a metal, the stronger it bonds to other elements like
oxygen and sulfur. Therefore it is far less likely such a metal is
found as the uncombined element AND the more reactive a metal the
more difficult it is to extract from its ores AND more costly to extract it
from the compounds the metal had formed over geological time.
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 Moderately reactive metals like copper, iron, lead, tin
have been extracted using carbon based smelting for the past 2000–4000
years.
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This is possible because
carbon is sufficiently reactive to displace these less reactive
metals.
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Any metal below carbon can
theoretically be extracted from its oxide by heating with carbon
(coke or charcoal).
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Compounds of these less
reactive are reduced by heating with carbon e.g. copper and
silver.
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Reduction is the loss of
oxygen from a compound.
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BUT it is only in the last 200 years that
very reactive metals
like sodium or aluminium have been extracted by electrolysis.
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In other words, our
exploitation of metal mineral resources as developed and
expanded as the technology of metal extraction has also
developed and improved.
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Therefore the economics
of extraction may change over time with eg reduced costs by
technological advances or increased by depletion of high grade ore
reserves.
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The depletion of high
grade ore reserves has resulted in technology research increasingly looking at ways of
extracting metals from low grade ores which were previously
uneconomic to use.
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The crucial point is that
generally speaking, the
method of extraction depends on the metals position in the reactivity
series.
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The reactivity series of
metals can be presented to include two non–metals, carbon and
hydrogen, to help
predict which method could be used to extract the metal.
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 Although
most metals occur as compounds, some metals are so
unreactive that they do not readily combine with oxygen in the air
or any other element
present in the Earth's crust, and so can be found as the metal
itself (sometimes referred to as 'native' metal).
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For example, a metal, most frequently found as the
uncombined metal
is gold (and sometimes copper and silver)
and no chemical separation is needed.
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In fact all the
metals below hydrogen can be found as the 'free' or 'native' element,
though they occur mainly as compounds combined with non–metals like
oxygen (oxide ion) or sulfur (sulfide ion) or the carbonate ion in
their ores.
A
summary diagram of important ideas to do with the reactivity series of
metals!
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At
this point we need to say more about oxidation and reduction and redox
reactions.
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One definition of
oxidation is oxygen gain.
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Somewhere in the past
aluminium combined with oxygen to form aluminium oxide, the main
compound in bauxite ore from which aluminium is extracted.
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aluminium + oxygen ==>
aluminium oxide.
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We would therefore say the
aluminium was oxidised by oxygen gain.
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Therefore to extract
aluminium we must reduce it and remove the oxygen.
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One definition of
reduction is oxygen loss.
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Copper ores are processed to
give copper oxide from which copper may be obtained by a reduction
process to remove the oxygen (reduction). This can be done by heating
with carbon.
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copper oxide + carbon ==>
copper + carbon dioxide.
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Here the copper oxide is
reduced by oxygen loss, and the carbon is the 'oxygen remover' and
is referred to as the reducing agent.
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Similarly in a blast
furnace, the iron oxide is reduced with carbon to free the iron from
oxygen.
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The method employed to
extract a metal by a reduction process largely depends on its reactivity
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See a separate page for a detailed
discussion of oxidation - reduction ('redox') reactions
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Metals below carbon
in the reactivity series (see table on right) can be extracted by heating the oxide with carbon or carbon monoxide. The non–metallic elements
carbon will displace
the metals less reactive than carbon in a smelter or blast furnace
e.g. iron or
zinc and metals lower in the series.
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Therefore metals
like iron, copper, tin, lead, zinc can readily be extracted by
reaction–reduction of their e.g. oxides using cheap carbon (i.e.
coke made from coal).
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Iron ore is
used to make iron and steel and iron is produced in a blast
furnace by reducing iron oxides with carbon and it is the carbon
that removes the oxygen from the iron oxides – the carbon is
known as the reducing agent.
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The metal copper can be easily extracted
BUT
copper–rich ores are becoming scarce so new methods of extracting copper are
being developed to exploit low grade ores.
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Metals below
hydrogen will not displace hydrogen from acids. Their oxides
are easily reduced to the metal by heating in a stream of
hydrogen, though this is an extraction method rarely used in
industry.
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In fact most metal oxides below carbon can be reduced
when heated in hydrogen, even if the metal reacts with acid.
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Metals above
carbon in the reactivity series
cannot usually be extracted
with carbon or carbon monoxide.
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So, metals more reactive
than carbon are usually extracted by electrolysis
of the purified molten ore or other suitable compound.
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Electrolysis is the
process of breaking down a compound using electrical energy and is
needed to extract the most reactive metals.
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The process of
electrolysis uses of large amounts of energy in the extraction
of these reactive metals and makes them expensive to produce.
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The metal ions in the
ore compound are forced by electrical energy into accepting
electrons and producing free metal atoms.
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Aluminium is a very useful metal but expensive to produce.
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e.g.
aluminium
from molten aluminium oxide or
sodium from molten sodium chloride.
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The ore or
compound must be molten or dissolved in a solution in an
electrolysis cell to allow
free movement of ions (electrical current). The conducting melt
or solution is called the electrolyte. Theory given in the
appropriate sections.
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Because these reactive
metals cannot be obtained by relatively cheap carbon reduction
methods, their extraction tends to be more costly due to more
specialised stages in the extraction process, more energy is
needed (maybe costly electricity) and more costly specialist
chemicals like a more reactive metal or chlorine (remember
carbon–coke is relatively cheap e.g. as used in the blast
furnace extraction of iron).
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Other methods are used in
special cases using the displacement rule.
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A more reactive
metal can be used to displace and extract a less reactive metal but these are
costly processes since the more reactive metal also has to be produced
in the first place!
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Titanium is another very useful metal but expensive to produce.
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Sometimes electrolysis
is used to purify less reactive metals which have previously been
extracted using carbon or hydrogen (e.g. see extraction of
copper
or zinc).
Electrolysis is also used to plate one metal with another.
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The demand for raw
materials does have
social, economic and environmental implications
e.g.
conservation of mineral resources by recycling metals, minimising
pollution etc.
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Metals can be mixed together
to make alloys to improve the metal's properties
to better suit a particular
purpose.
WHERE NEXT?
Other
associated KS4 Science GCSE/IGCSE chemistry web pages on this site
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| Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
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gap–word–fill exercise on METAL EXTRACTION
Reactivity
of metals
: all the reactions of metals with oxygen (air), water and acids,
displacement reactions
