Extraction of Metals and Recycling

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Extraction of Metals and

Recycling


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Extraction of metals

Metals occur in nature as ores which arecombinations of metals with otherelements.

Metals are extracted from their ores inthree steps - concentration of ores,extraction of metals from ores and refiningthe crude metal to get the pure metal.

Concentration of ores involves removingthe earth, sand, stones etc which areunwanted materials.


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Extraction of metal from ores can becarried out in two ways

(i) by using electricity to break up the moltenore to metal and other products.

(ii) by heating the ore with carbon to reduce themetal oxide to metal

The position of the metal in the reactivityseries helps to select the method for itsextraction.

The more reactive the metal is, theharder it is to extract it from its ore.


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Reactive metals at the top of the seriessuch as potassium, sodium, calcium

and magnesium form compounds (ores)which are very difficult to split into theirelements. These metals cannot be

extracted by heating their oxides withcarbon. They are extracted by usingelectricity (electrolysis) to break up theircompounds.


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The metals lower in the reactivity series arenot very reactive. They can be extracted by

heating the ores containing metal oxides withcarbon.


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Most reactive Potassium extracted by usingSodium electricity to break upCalcium the metal compoundsMagnesium in molten ores

Aluminium

Zinc extracted by heatingIron its oxides with carbonLead (to reduce the metalCopper oxides to metal)Silver

Gold found naturally as the

Least reactive uncombined element


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Extraction of iron fromHaematite - using Blast furnace

Haematite is the main ore of iron andcontains iron(III) oxide mixed withimpurities such as clay and sand.

A blast furnace is used to extract iron fromhaematite.


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Iron ore (haematite), coke (carbon) andlimestone (calcium carbonate) are added

from the top of the furnace. Limestone isrequired for removing the impurites likesand and clay.

Hot air is blown into the furnace from thebottom. Impurities in the iron ore form a compound

called slag which flows out of a tap at thebottom of the furnace. Molten iron flows out from another tap at

the bottom of the furnace.


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Chemical reactions that take placein the blast furnace

1. Formation of carbon dioxide from cokeCarbon in coke burns in hot air to producecarbon dioxide and a lot of heat.

carbon + oxygen carbon dioxide

C(s) + O 2(g) CO 2(g)


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2. Formation of calcium oxide from limestoneLimestone (calcium carbonate ) is

decomposed by heat to give calcium oxide(quicklime) and carbon dioxide.

calcium carbonate calcium oxide + carbon(limestone) heat dioxide

CaCO 3(s) CaO(s) + CO 2(g)heat


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3. Formation of carbon monoxide fromcarbon dioxide

Carbon dioxide moves up and reacts withmore coke to form carbon monoxide .

carbon + carbon dioxide carbon monoxideheat

C(s) + CO 2(g) 2CO(g)heat


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iron(III) oxide + carbon monoxide molten + carbonheat iron dioxide

Fe 2O 3(s) + 3CO 2Fe(l) + 3CO 2(g)heat


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5. Removal of impurities as slaglmpurities like sand and clay are mainly

silicon(lV) oxides. They react with calciumoxide to form a molten slag which iscalcium silicate . The slag is lighter than

molten iron and floats on top. Both aretapped off separately at the bottom of thefurnace.

calcium oxide + silicon(lV) oxide calcium silicate (slag)heat

CaO(s) + SiO 2(s) CaSiO 3(l)

heat


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Rusting or corrosion of iron

When iron is exposed to moisture and airfor sometime, it reacts to form a hydratediron(IIl) oxide which is known as rust. lt isfound as a reddish brown substance onthe surface of the iron metal. This type ofreaction is known as oxidation since iron

metal is oxidised to iron(III) oxide .


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The rusting of iron in the presence of airand moisture can be represented by the

chemical equation,

iron + water + oxygen hydrated iron(lll) oxide( rust )4Fe(s) + 3O 2(g) + 2xH 2O(l) 2Fe 2O 3.xH 2O(s)


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From experiments done by chemists,evidence shows that

(a)both oxygen(air) and water are requiredfor rusting.

(b)presence of salt (sodium chloride) speeds

up the rate of rusting.(c)rust can be prevented by using a

protective layer of substances such as oil. The protective layer prevents the air and

moisture from coming into contact withthe surface of iron.


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The various methods used for preventingrust are shown in the table below.

Method to prevent rust Uses (examples)i greasing moving parts in

machines andtools

ii painting large objects likecars and ships

iii plastic coating cloth hangers,wire racks etc

iv coating with a film of zinc(galvanising)

kitchen sinks,buckets etc

v coating with a film of tin (which

is a less reactive metal)

food cans


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In addition to salt, other substances alsocan increase the speed of rusting. For

example, rusting is faster in industrialareas because acidic substances such ascarbon dioxide and sulphur dioxide speed

up the rate of rusting.


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Recycling of metals

Metals are finite resources because theamount of metals available on earth arelimited.

As the natural resources of metals getused up, the supply of some metals,especially lead and tin, becomes muchless. Hence, there is a need to recover themetals by recycling .


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For example, old metal objects can becrushed, melted and purified before

reusing. Aluminium can be recycled from cans and

food containers. Large amounts of iron and steel can be

recycled from scrap metal. Lead can be recovered from car batteries.


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Environmental issues of recycling

Advantages of recycling are: Pollution to the environment can be reduced

by using recycled metals. Air pollution, waterpollution and water usage are much lesscompared to the extraction from ores.

Big unsightly piles of dumped metals can be

avoided, reducing the need for land refills.


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Recycling metals ensures that the mineralores are conserved and will last longer.

Recycling metals like aluminium can save alot of energy and cost when compared toextracting them from the metal ores.


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Economic issues of recycling

A disadvantage of recycling is that recyclingcan be costly because

Different scrap metals have to be sorted outand cleaned before recycling.

The scrap metals have to be transported to theprocessing plant.


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Social issues of recycling

Extraction of metals from ores as well asrecycling metals produce waste products, butrecycling can reduce the amount of such wastesand help to maintain a cleaner environment forpeople to live in.

Increasing human population requires more landfor agriculture and housing. Recycling can makemore land available for these needs.

However, recycling has to be made more cost-effective and economical and every one needsto play an important role by conserving ournatural resources and avoiding wastage.

Extraction of Metals and Recycling

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