Metal extraction

Metal extraction
Objective: The main objective of this project work is to show us the application of
electrochemistry in metal extraction and purification industry.
So here in our project work we see in detail the application of electrochemistry in metal
extraction and purification industries. This will be discussed by taking examples or we will see
how metals like (potassium, sodium, lithium, calcium, magnesium
and aluminium) are extracted and how metal like (copper and zinc) purified using electrolysis
process.
1 Introduction
Metals are very useful. Ores are naturally occurring rocks that contain metal or metal
compounds in sufficient amounts to make it worthwhile extracting them. For example, iron ore
is used to make iron and steel. Copper is easily extracted, but ores rich in copper are becoming
more difficult to find. Aluminum and titanium are metals with useful properties, but they are
expensive to extract. Most everyday metals are mixtures called alloys.
A solid element or compound which occurs naturally in the Earth's crust is called a mineral. A
mineral which contains a high enough percentage of a metal for economic extraction is called a
metal ore. Economic extraction means that the cost of getting the metal out of the ore is
sufficiently less than the amount of money made by selling the metal.
The most common metal ores are oxides and sulfides. Sulfides are the oldest ores, formed in the
Earth's history when there was a lot of sulfur from volcanic activity. Oxides formed later when
photosynthesis in plants released large amounts of oxygen into the atmosphere.
Metal ore deposits are a finite resource (there are only a certain amount of them) and non-
renewable (once used, they are gone and will not be replaced).
Many metals are obtained today from recycling (smelting and refining) scrap metals.
1.1 How is a Metal Extracted from its Ore?
The best method to use depends on a number of factors:
- Will the method successfully extract the metal?
This depends on the reactivity of the metal
- How much do the reactants cost?
Raw materials vary widely in cost
- What purity is needed, and are the purification methods expensive?
Some metals are not useful unless very pure, others are useful impure
- How much energy does the process use?
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High temperatures and electrolysis use a lot of energy
- How efficiently, and in what quantities, can the metal be made?
Continuous processes are more efficient than batch processes
- Are there any environmental considerations?
Some processes produce a lot of pollutants
The method used to extract a metal from its ore depends on where the metal is in the reactivity
series.
Metals above Carbon in the Reactivity Series.
A metal above carbon in the reactivity series (potassium, sodium, lithium, calcium, magnesium
and aluminium) can be extracted by electrolysis.
Extraction of the metal from its ore involves reduction of the metal ions. Electrons are able to
reduce any metal ion.
Metal ions + electrons metal atoms (reduction).
Mon-metal ions - electrons non-metal atoms (oxidation).
Metals above carbon in the reactivity series could also be reduced by reaction with a more
reactive metal but this is more expensive than electrolysis and is only used on a commercial scale
for the extraction of titanium.
Metals below Carbon in the Reactivity Series.
A metal below carbon in the reactivity series (zinc to silver) may be extracted by heating the
metal ore with carbon. During the reaction, the metal in the ore is displaced from its non-metal
anion because carbon is more reactive than the metal. Carbon is used because it is cheap and
readily available (coke and charcoal are both carbon). This form of extraction is less expensive
than electrolysis. The metal in the ore is said to be reduced by reaction with carbon.
Hydrogen can also be used to reduce metals that are lower than itself in the reactivity series, but
since hydrogen is more expensive than carbon it is only used on a large scale for the extraction of
tungsten. This avoids tungsten reacting with carbon to form tungsten carbide.
Gold and platinum occur in the Earth as native metal. This means that they are found as the
element, not the compound, and so do not need to be reduced. Silver and copper may also be
found as native metal.
2 Application of electrochemistry in metal extraction industries
The method used to extract metals from the ore in which they are found depends on their
reactivity. For example, reactive metals such as aluminium are extracted by electrolysis, while a
less-reactive metal such as iron may be extracted by reduction with carbon or carbon
monoxide.
Electrolysis is the breakdown of chemical compounds by the passage of an electric current,
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with decomposition occurring at the electrodes. An electrode is a conductor placed in a liquid
electrolyte (Liquid that conducts electricity) One of the electrodes is negatively charged-the
cathode and the other is positively charged- the anode. Reduction occurs at the cathode and
oxidation at the anode.
The process of electrolysis uses of large amounts of energy in the extraction of these reactive
metals and makes them expensive to produce. The metal ions in the ore compound are forced by
electrical energy into accepting electrons and producing free metal atoms.
o Another definition of reduction is electron gain.
o E.g. during the electrolysis of molten aluminium oxide the following reaction
happens.
 Al3+
(aq) + 3 e –
==> Al(s)
 Al3+
+ 3e–
==> Al
 So, once again the metal ore compound is reduced to the metal.
Aluminium is a very useful metal but expensive to produce.
 E.g. aluminium from molten aluminium oxide or sodium from molten sodium chloride.
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. Because these reactive metals cannot be obtained by relatively cheap carbon
reduction methods, their extraction tends to be more costly due to more specialized stages in the
extraction process, more energy is needed (may be 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).
Sometimes electrolysis is used to purify less reactive metals which have previously been
extracted using carbon or hydrogen (e.g. copper and zinc).
2.1 What is Electrolysis?
Electrolysis is the process where an electric current is passed through a liquid that conducts
electricity. A liquid will only conduct electricity if it contains ions.
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The electrodes are often made from graphite. The liquid that conducts electricity is called the
electrolyte. The amount of electricity needed to produce a particular mass of metal (or non-
metal) can be calculated.
The negative electrode, called the cathode, will attract positively charged metal ions.
The metal ions collect electrons from the cathode (this is called reduction)
and are discharged as metal atoms.
The positive electrode, called the anode, will attract negatively charged non-metal ions.
The non-metal ions lose electrons to the anode (this is called oxidation) and are discharged as
non-metal atoms which often combine to form molecules.
In this way, elements that are present in ionic compounds can be separated by electrolysis. This
method is used for the extraction of some metals from their ore. See for example lead bromide,
magnesium chloride, potassium chloride, sodium chloride and zinc chloride.
3 Potassium production/extraction
K is obtained either directly by electrolysis of K salts or by reaction of Na with K salts. K is
relatively easy to distill once it has been produced by either method, though "crude" commercial
potassium is often sold directly after electrolysis.
Typical electrolysis reaction would involve:
2K2CO3 ---> 4K + 2CO2 + O2
Whereas Na reduction would involve
K2CO3 + 2Na ---> 2K + Na2CO3
Obviously, both processes require inert temperature. Electrolysis requires high temperatures, as
K salts have to be molten to conduct electricity.
Electrolysis of Potassium Chloride.
Potassium chloride must be heated until it is molten before it will conduct electricity. Electrolysis
separates the molten ionic compound into its elements.
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The reactions at each electrode are called half equations. The half equations are written so that
the same number of electrons occur in each equation.
2K+
+ 2e-
2K (potassium metal at the (-) cathode).
2Cl-
- 2e-
Cl2 (chlorine gas at the (+) anode).
Potassium ions gain electrons (reduction) to form potassium atoms. Chloride ions lose electrons
(oxidation) to form chlorine atoms. The chlorine atoms combine to form molecules of chlorine
gas.
The overall reaction is
2K+
Cl-
(l) 2K(s) + Cl2 (g)
4 Electrolysis of Lead Bromide
Lead bromide must be heated until it is molten before it will conduct electricity. Electrolysis
Pb2+
+ 2e-
Pb (lead metal at the (-) cathode).
2Br-
- 2e-
Br2 (bromine gas at the (+) anode).
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Lead ions gain electrons (reduction) to form lead atoms. Bromide ions lose electrons (oxidation)
to form bromine atoms. The bromine atoms combine to form molecules of bromine gas.
PbBr2 (l) Pb(s) + Br2 (g)
5 Electrolysis of Magnesium Chloride
Magnesium chloride must be heated until it is molten before it will conduct electricity.
Electrolysis separates the molten ionic compound into its elements.
The half equations are
Mg2+
+ 2e-
Mg (magnesium metal at the (-) cathode).
2Cl-
- 2e-
Magnesium ions gain electrons (reduction) to form magnesium atoms. Chloride ions lose
electrons (oxidation) to form chlorine atoms. The chlorine atoms combine to form molecules of
chlorine gas.
MgCl2 (l) Mg(s) + Cl2 (g)
6 Electrolysis of Sodium Chloride
Sodium chloride must be heated until it is molten before it will conduct electricity. Electrolysis
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What is a Half Equation?
2Na+
+ 2e-
2Na (sodium metal at the (-) cathode).
2Cl-
- 2e-
Sodium ions gain electrons (reduction) to form sodium atoms. Chloride ions lose electrons
(oxidation) to form chlorine atoms. The chlorine atoms combine to form molecules of chlorine
gas.
2Na+
Cl-
(l) 2Na(s) + Cl2 (g)
7 Electrolysis of Zinc Chloride
Zinc can be extracted from zinc oxide by heating with carbon or from zinc chloride by
electrolysis.
Zinc chloride must be heated until it is molten before it will conduct electricity. Electrolysis
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The half equations are
Zn2+
+ 2e-
Zn (zinc metal at the (-) cathode).
2Cl-
- 2e-
Zinc ions gain electrons (reduction) to form zinc atoms. Chloride ions lose electrons (oxidation)
to form chlorine atoms. The chlorine atoms combine to form molecules of chlorine gas.
ZnCl2 (l) Zn(s) + Cl2 (g)
8 Extraction of aluminium
Aluminium is the most abundant metal on Earth. Despite this, it is expensive, largely because of
the amount of electricity used up in the extraction process.
Aluminium ore is called bauxite. The bauxite is purified to yield a white powder, aluminium
oxide, from which aluminium can be extracted.
The extraction is done by electrolysis. But first the aluminium oxide must be made molten so
that electricity can pass through it. Aluminium oxide has a very high melting point (over
2,000°C), so it would be expensive to melt it. Instead, it is dissolved in molten cryolite, an
aluminium compound with a lower melting point than aluminium oxide. The use of cryolite
reduces some of the energy costs involved in extracting aluminium.
The steel container is coated with carbon (graphite) and this is used as the negative electrode
(cathode).
Aluminium oxide (Al2O3) is an ionic compound. When it is melted the Al3+
and O2-
ions are free
to move and conduct electricity.
Electrolysis of the alumina/cryolite solution gives aluminium at the cathode and oxygen at the
anode.
4Al3+
+ 12e-
4Al (aluminium metal at the (-) cathode) reduction.
6O2-
- 12e-
3O2 (oxygen gas at the (+) anode) oxidation.
Aluminium is denser than the alumina/cryolite solution and so it falls to the bottom of the cell
where it can be tapped off as pure liquid metal.
Aluminium oxide aluminium + oxygen.
2Al2O3 (l) 4Al(l) + 3O2 (g)
Oxygen is given off at the positive carbon anode. Carbon dioxide is also given off at the carbon
anode because hot oxygen reacts with the carbon anode to form carbon dioxide gas.
Carbon + oxygen carbon dioxide.
C(s) + O2 (g) CO2 (g)
The carbon anodes slowly disappear because each molecule of carbon dioxide which is given off
takes a little piece of carbon away with it. The carbon anodes need to be replaced when they
become too small.
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9 Extraction and purification of copper
Copper is less reactive than carbon, so it can be extracted from its ores by heating it with carbon.
For example, copper is formed if copper oxide is heated strongly with charcoal - which is mostly
carbon:
Copper oxide + carbon → copper + carbon dioxide
Copper is purified by electrolysis. Electricity is passed through solutions containing copper
compounds, such as copper sulphate. The anode - positive electrode - is impure copper. Pure
copper forms on the cathode - negative electrode.
The anode is a block of impure copper. The cathode is a thin piece of pure copper. The
electrolyte is copper sulfate.
When electricity is passed through the cell copper is dissolved at the anode by oxidation and Cu2+
ions go into solution.
Cu(s) - 2e-
Cu2+
(aq)
At the cathode, copper is deposited by reduction.
Cu2+
(aq) + 2e-
Cu(s)
As copper ions move from the anode to the cathode the anode gets smaller as the cathode gets
bigger.
10 Advantages and disadvantages of the electrolysis process
Advantages:
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- it is a continuous process, so is efficient
- it makes the metal in pure form
Disadvantages:
- the cost of melting the metal ore/ compound and supplying the energy for electrolysis is very
high
- it only works for ionic oxides
11 Conclusion & Discussion
All metals above iron in the reactivity series (that is, potassium, sodium, calcium, magnesium
and aluminum) are extracted by electrolysis. This is because they form too stable compounds
that are difficult to reduce using simple reducing agents like carbon monoxide. They have to be
extracted by the most powerful reducing agent which is none other than the cathode of the
electrochemical cell. Anyways, all other metals can also be extracted from their ores using
electrolysis but usually, if simple reduction is possible, it is preferred to electrolysis because of
the cost (electrolysis uses electricity, which is really expensive).
The demand for raw materials does have social, economic and environmental implications e.g.
conservation of mineral resources by recycling metals, minimizing pollution etc.
Metals can be mixed together to make alloys to improve the metal's properties to better suit a
particular purpose.
In general, when we select the method of extraction we should consider varies factors and the
extraction also seen in the angle of economy, environment and social. Thus these all factors
should be considered.
References
 http://www.gcsescience.com/ex5.htm
 http://www.madsci.org/posts/archives/1999-11/943199476.Ch.r.html
 http://www.docbrown.info/page04/Mextract.htm
 http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/chemicals/extracti
onmetalsrev
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Table of Contents
Page
Metal extraction...........................................................................................................................1
1 Introduction .............................................................................................................................................1
Many metals are obtained today from recycling (smelting and refining) scrap metals.
1.1 How is a Metal Extracted from its Ore?.............................................................................................1
2 Application of electrochemistry in metal extraction industries ...............................................................2
2.1 What is Electrolysis?..........................................................................................................................3
3 Potassium production/extraction.............................................................................................................4
4 Electrolysis of Lead Bromide.....................................................................................................................5
5 Electrolysis of Magnesium Chloride..........................................................................................................6
7 Electrolysis of Zinc Chloride......................................................................................................................7
8 Extraction of aluminium...........................................................................................................................8
9 Extraction and purification of copper.......................................................................................................9
10 Advantages and disadvantages of the electrolysis process....................................................................9
11 Conclusion & Discussion ......................................................................................................................10
References ................................................................................................................................................10

Metal extraction

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