© as 080 10-jul-12. ores most metals are found in compounds in the earth’s crust. a rock...
TRANSCRIPT
© www.chemsheets.co.uk AS 080 10-Jul-12
ORES
• Most metals are found in compounds in the Earth’s crust.
• A rock containing enough of the metal for extraction to be profitable is called an ore.
iron ore haematite
Fe2O3
copper ore malchite
CuCO3
titanium ore rutileTiO2
aluminium ore
bauxiteAl2O3
lead ore galena
PbS
EARTH’S CRUST
Metals have to be extracted by chemical
reactions from compounds found in the
crust.
EXTRACTION METHODS
Methods
1) Reduction by heating with carbon
2) Reduction by electrolysis of melt
3) Reduction by active metal
Which method is used depends on
1) Energy requirements (heat, electricity)
2) Cost of reductants (C / active metal / electricity)
3) Purity of metal required
4) Reduction by hydrogen
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ROASTING SULPHIDE ORES
Ore is roasted in air
e.g. ZnS + 3/2 O2 ZnO + SO2
Produces SO2 – causes acid rain
To prevent this, SO2 is collected to stop it entering the atmosphere (and then used to make some H2SO4)
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Branches from a tree in Germany's Black Forest show needle loss and yellowed boughs caused by acid rain
Kills fish
Damages some stones (e.g. limestone)
REDUCTION WITH CARBON
Most metals can be extracted by heating with carbon. Many are not because:
1) Metal carbides may be formed (e.g. with Ti, W, Al)
2) Temperature required is excessively high (energy costs)
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BLAST FURNACE
1) C + O2 → CO2
2) C + CO2 → 2 CO
3) Fe2O3 + 3 CO → 2 Fe + 3 CO2
Fe2O3 + 3 C → 2 Fe + 3 CO
4) CaCO3 → CaO + CO2
5) CaO + SiO2 → CaSiO3
BLAST FURNACE
Pollution issues
1) CO2 formed – greenhouse gas
2) CO formed – toxic
3) SO2 formed (roasting of sulphide ores) – acid rain
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STEEL MAKING
Impurities: C, P, S, Si
1) Inject powdered Mg to remove S (forms MgS – floats on surface and scraped off)
2) Scrap steel added to recycle
3) O2 blown through molten metal (converts C, P, Si to oxides, CO2 bubbles off)
4) Limestone added (decomposes to CaO) which reacts with P4O10 and SiO2 to form slag
5) Other metals and/or C added to make steel
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REDUCTION WITH CARBON
Mn MnO2 + C Mn + CO2
Cu 2 CuO + C 2 Cu + CO2
Malachite oreCuCO3 CuO +
CO2
ELECTROLYSIS OF MELTS
graphite negative electrode
Al3+ + 3 e- Al
graphite positive electrode
2 O2- - 4 e- O2 (then C + O2 CO2)
molten mixture of aluminium oxide
and cryolite
molten aluminium
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ACTIVE METAL REDUCTION
Titanium is
Replacement hip joints
Nuclear reactors
• Low density• Strong• Corrosion
resistant
Aircraft
ACTIVE METAL REDUCTION
• Titanium must be very pure.
• Reduction with C forms TiC which is brittle.
• Electrolysis does not make it pure enough.
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Ti EXTRACTION
1) TiO2 + 2 Cl2 + 2 C → TiCl4 + 2 CO
2) Purify TiCl4 by fractional distillation
3) TiCl4 + 4 Na → Ti + 4 NaCl
TiCl4 + 2 Mg → Ti + 2 MgCl2
Expensive due to:
• High temperatures (high energy)
• Use of Ar
• Cost of Na/Mg (made by electrolysis)© www.chemsheets.co.uk AS 080 10-Jul-12
HYDROGEN REDUCTION
Wolframite (contains WO3) Tungsten
WO3 + 3 H2 W + 3 H2O
C reduction would form WC (tungsten carbide)
RECYCLING METALS
Advantages
• Lower energy cost
• Saves resources
• Less waste
• Less pollutants (e.g. CO2, SO2, CO)
Disadvantages
• Preventing people throwing away
• Transport cost
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COPPER RECYCLING & EXTRACTION
1) Covert waste Cu to Cu2+(aq) – add H2SO4 or enzyme
2) Extract Cu from solution with Fe:
Cu2+(aq) + Fe(s) Cu(s) + Fe2+(aq)
Useful for recycling scrap extraction from low grade ores