Transcript Extraction of metals

Extraction of
metals
Only some unreactive metals such
as silver, gold and platinum can
occur freely in nature. Most metals
react with other elements to form
ores.
Major steps in extraction of metal

Ore concentration
– Ore is purified and concentrated, unwanted
rocks removed

Reduction to crude metal
– Metal oxides to be reduced to metals, resulting
in a mixture of metals collected

Refining to obtain pure metal
– To obtain a specific metal, purify and remove
unwanted metal impurities
the extraction of metals
Method of extraction depends on the position of the metal in
the reactivity series.
 extraction of metal involves:
o
o
getting rid of the unwanted rock to obtain concentrated
form of the mineral
obtaining pure metal from the mineral by chemical
reactions
the extraction of metals
Metals at the top of the reactivity series are very reactive:
 bonds in their compounds are very strong
 must be extracted by decomposing their compounds with
electricity in an expensive process called electrolysis
 aluminium is extracted from aluminium oxide by passing an
electric current through it
2Al2O3
4Al + 3O2
Ways of Extraction




Extraction by
electrolysis of
molten Al2O3
dissolved in
cryolite










Potassium
Sodium
Calcium
Magnesium
Aluminium
Zinc
Iron
Tin
Lead
Copper
Mercury
Silver
Gold
Platinum
K
Na
Ca
Mg
Al
Zn
Fe
Sn
Pb
Cu
Hg
Ag
Au
Pt
Extracted by
electrolysis of
molten chlorides
Extraction by
reduction of
oxides using
carbon
Roasting ore by
heating alone
Extraction of Iron
Raw materials of extraction of Iron
 Iron
Ore
– eg haematite ore [iron(III) oxide,
Fe2O3]
 Coke
– carbon, C
 Hot
air
– for the O2 in it
 Limestone
– calcium carbonate, CaCO3
Stage 1 – Production of carbon
dioxide
 The
coke is ignited at the base and hot air
blown in to burn the coke (carbon) to form
carbon dioxide
– C(s) + O2(g)  CO2(g)
 The
limestone is decomposed by heat to
produce carbon dioxide & quicklime
– CaCO3(s)  CaO(s) + CO2(g)
Stage 2 – Production of carbon
monoxide
 At
high temperature, the carbon dioxide
formed reacts with more coke (carbon) to form
carbon monoxide
– CO2(g) + C(s)  2CO(g)
Stage 3 – Reduction of haematite
 The
carbon monoxide removes the oxygen
from the iron oxide ore.
 This frees the iron, which is molten at the high
blast furnace temperature, and flows down to
the base of the blast furnace.
 Fe2O3(s) + 3CO(g)  2Fe(l) + 3CO2(g)
 Other possible ore reduction reactions are ...
– Fe2O3(s) + 3C(s)  2Fe(l) + 3CO(g)
– 2Fe2O3 (s) + 3C(s)  4Fe(l) + 3CO2 (g)
Stage 3 – Reduction of haematite
 Waste
gases escape through the top of the
furnace
 Eg. Carbon monoxide, carbon dioxide,
nitrogen…
Stage 4 – Removal of Impurities

The original ore contains silica (SiO2, silicon
dioxide). These react with limestone to form a molten
slag of e.g. calcium silicate in 2 stages
– CaCO3  CaO + CO2
– CaO + SiO2  CaSiO3
The molten slag forms a layer above the more dense
molten iron and can be separately, and regularly,
drained away. The iron is cooled and cast into pig
iron ingots / transferred directly to a steel producing
furnace
 Slag can be used for road surfacing

http://www.bbc.co.uk/history/games/blast/blast.shtml
Why Steel?
Steel is iron that has most of the
impurities removed. Steel also has a
consistent concentration of carbon
throughout (0.5 percent to 1.5 percent)
 Impurities like silica, phosphorous and
sulphur weaken steel tremendously, so
they must be eliminated
 The advantage of steel over iron is greatly
improved strength

Pig Iron to Steel Using Basic
Oxygen Furnace
Pear-shaped furnace, lined with refractory
bricks, that refines molten iron from the
blast furnace and scrap into steel
 Scrap is dumped into the furnace vessel
 Followed by the hot metal from the blast
furnace.
 A high-pressure stream of oxygen is blown
into it to cause chemical reactions that
separate impurities as fumes or slag
 Once refined, the liquid steel and slag are
poured into separate containers

Types of Steel
Steel
Percentage of carbon
Mild carbon steel
Up to 0.25%
High carbon steel
0.45% - 1.50%
Stainless steel – alloy Little carbon, with
chromium & nickel
Properties of Steel
 Can
be changed by the use of
controlled additives
 Eg. Carbon, chromium, nickel,
manganese, silicon etc…
Uses of Steel
Steel
Mild carbon steel –
strong, hard &
malleable
High carbon steel –
strong but brittle
Stainless steel –
does not rust
Uses
Make steel parts in
car bodies ,
machineries
Make knives,
hammer, cutting
tools
Pipes & tanks in
chemical plants,
making cutlery,
surgical instruments
Alloy
 Mixture
of a metal with other
elements
 Element in the largest proportion is
the base metal
 Elements in smaller proportions are
the alloying elements
Metals
 Soft
 Low
resistance to corrosion
 High m.p
 Easy to shape
Alloys
 Have
different physical properties
compared to their constituent
elements
 Produce mainly for:
– Improving strength and hardness
– Improving resistance towards corrosion
– Improving appearance of metal
– Lower m.p of metal
Extraction of Aluminium from
Bauxite

Raw materials
– Bauxite: ore containing hydrated aluminium
oxide Al2O3.2H2O
 M.p:
~2000C
– Molten Cryolite aka sodium aluminium fluoride
Na3AlF6
used to lower m.p to ~900C
– Carbon electrodes

http://www.patana.ac.th/parents/curriculu
m/Chemistry/units/LR803.html
Extraction of Aluminium
 Cryolite
is added to lower the melting
point & to dissolve the ore & bauxite
ore of aluminium oxide is
continuously added
 When p.d is applied,
– Al3+ is attracted to the negative cathode
– O2- is attracted to the positive anode
Extraction of Aluminium

At the cathode,
– Al3+ gains 3 electrons from the cathode to
form molten aluminium, which is tapped off
– Al3+(l) + 3e-  Al (l)

At the anode,
– O2- loses 2 electrons to the anode to form
oxygen
– 2O2-(l)  O2(g) + 4e– Oxygen released attacks carbon anode, to
form Carbon monoxide/dioxide. Carbon anode
dissolved. Needs to be replaced regularly
Anodising
 Form
of electroplating using oxygen,
used commonly for aluminium
 Aluminium when exposed in air
forms a thin protective coat of
aluminium oxide
 For better protection, a thicker coat
is made
 Through the process: Anodising
Anodising
Make aluminium the anode in sulphuric
acid bath
 Oxygen produced at the anode then
combines with aluminium to form a
protective porous layer aluminium oxide
1000 times thicker, compared when
exposed to air
 Pores can be sealed by dipping into hot
water or coloured by using dyes which can
be absorbed into it

Uses of Aluminium
Uses
Properties
Overhead
electric cables
Low density, light
Resistant to corrosion
(protected by aluminium oxide)
Good electrical conductivity
Food containers Non-toxic
Resistant to corrosion
Good conductor of heat
Aircraft body
Low density, light
High tensile strength
Resistant to corrosion
Conditions for
Corrosion of Iron
Presence of oxygen
 Presence of water
 Presence of sodium
chloride/acidic pollutants
speed up rusting

4Fe(s)
+
3O
(g)
+
2
Rusting is an exothermic
redox reaction where iron
2xH
O(l)
2
is oxidized to form
hydrated iron(III) oxide
 2Fe2O3.xH2O (s)
Prevention of rusting
 Use
of protective layer
 Painting – Used in cars, ships,
bridges
 Greasing – Tools & machine parts
 Zinc plating(Galvanising) – Zinc
roofs
 Tin plating – Food cans
 Creates barrier around the metal
preventing contact with oxygen
and water
Sacrificial protection
 More
reactive metal, eg, Magnesium
or zinc is attached to iron or steel
 Protects by sacrificing itself, corrodes
first since it is more reactive
 Iron will not rust in the presence of a
more reactive metal
 Used in underground pipes, ships,
steel piers
Alloying
 Addition
of nickel and chromium to
iron
 Chromium (III) oxide Cr2O3 on the
surface protects iron from corrosion
 Used in cutlery, surgical instruments,
pipes & tanks in chemical plants
Finite Resource
 Metal
ores – finite resource, will be
used up
 Need to recycle metals
 Save resources and solves litter
disposal
 Saves energy
 Saves costs