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