extraction of metals only some unreactive metals such as silver, gold and platinum can occur freely...
TRANSCRIPT
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 metalsthe extraction of metals
extraction of metal involves:o getting rid of the unwanted rock to obtain concentrated
form of the mineralo obtaining pure metal from the mineral by chemical
reactions
o getting rid of the unwanted rock to obtain concentrated form of the mineral
o obtaining pure metal from the mineral by chemical reactions
Method of extraction depends on the position of the metal in the reactivity series.
the extraction of metalsthe 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 Potassium K Sodium Na Calcium Ca Magnesium Mg Aluminium Al Zinc Zn Iron Fe Tin Sn Lead Pb Copper Cu Mercury Hg Silver Ag Gold Au Platinum Pt
Extracted by electrolysis of molten chlorides
Extraction by reduction of oxides using carbon
Extraction by electrolysis of molten Al2O3 dissolved in cryolite
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
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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
Properties of Steel
Can be changed by the use of controlled additives
Eg. Carbon, chromium, nickel, manganese, silicon etc…
Extraction of Aluminium from Bauxite
Raw materials– Bauxite: ore containing hydrated aluminium
oxide Al2O3.2H2O M.p: ~2000C
– Molten Cryolite aka sodium aluminium fluoride Na3AlF6
used to lower m.p to ~900C
– Carbon electrodes
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
Uses of AluminiumUses Properties
Overhead electric cables
Low density, lightResistant to corrosion (protected by aluminium oxide)Good electrical conductivity
Food containers Non-toxicResistant to corrosion Good conductor of heat
Aircraft body Low density, lightHigh tensile strengthResistant to corrosion
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
Conditions for Corrosion of Iron
Presence of oxygen Presence of water Presence of sodium
chloride/acidic pollutants speed up rusting
Rusting is an exothermic redox reaction where iron is oxidized to form hydrated iron(III) oxide
4Fe(s) + 3O2(g) +
2xH2O(l)
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
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
Uses of SteelSteel UsesMild carbon steel – strong, hard & malleable
Make steel parts in car bodies , machineries
High carbon steel – strong but brittle
Make knives, hammer, cutting tools
Stainless steel – does not rust
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