Extraction and Uses of Metals

Very few metals are found naturally in any quantity as the pure, uncombined element. In fact, it is generally only gold that is found this way. Most metals are found in nature as compounds, usually as metal oxides. For example, aluminium is found as aluminium oxide (also known as alumina), Al2O3, in a rock called bauxite. Iron is found as iron(III) oxide, Fe2O3, in rocks such as haematite and magnetite. A naturally occurring compound from which a metal can be extracted is known as an ore. In order to obtain the pure metal, it is necessary to extract the metal from its ore. This usually involves removal of oxygen from the metal oxide, which is a process known as reduction. That is, the extraction of metals involves reduction of their ores.

There are many ways in which a metal oxide can be reduced to the metal. The method actually chosen for each metal on an industrial scale depends on several factors:

Clearly, the overall cost of an extraction process is of prime importance. A process which requires less energy (e.g. operates at a lower temperature) will usually have an economic advantage over a process which requires a greater energy input. A reducing agent that is naturally available such as carbon (e.g. from coal) or hydrogen (from natural gas) is cheaper than one such as sodium, which has to be prepared by a separate (and often costly) process.

If a metal is required with very high purity (and in small quantities) for a particular use, then a relatively expensive process may be used, the extra expense being needed to satisfy the demand for high purity and being justified by the marketable value of the product. If high purity is not an essential requirement, then the cheapest route providing a saleable metal will be used.

The way in which a metal is extracted from its ores is related to its position in the reactivity series. In particular, the method depends on whether the metal is more or less reactive than carbon and hydrogen. Metals that are less reactive than carbon or hydrogen (which are cheap) can be reduced by them. Metals that are more reactive than carbon have to be reduced by a very reactive metal such as sodium or by a process such as electrolysis; both alternatives are more expensive. The reactivity series for a selection of metals (as well as the non-metals, carbon and hydrogen) is given below:

potassium> sodium> magnesium> aluminium> (carbon)> zinc> iron> lead> (hydrogen)> copper

K > Na > Mg > Al > (C) > Zn > Fe > Pb > (H2) > Cu

Zinc, iron, lead and copper can be extracted cheaply from their ores using carbon but potassium, sodium, magnesium and aluminium have to be extracted from their ores by more expensive methods such as electrolysis. Copper could be extracted from its ores using hydrogen. In fact, copper needs to be extremely pure (over 99.95%) when it is used to make electrical wires. In this case, the high purity is achieved by electrolysis of impure, cheaply extracted copper and the extra cost is incorporated in the market price of the purified copper.


Extraction of iron from iron ore

Iron is the second most abundant metal in the Earth's crust. Iron is extracted form its ore in a blast furnace. Four substances are required:

     Iron ore. The chief ore is haematite. It is mainly iron(III) oxide (Fe2O3) mixed with sand (SiO2).

     Limestone. This is a form of calcium carbonate (CaCO3) and is used to remove the sand from the iron ore.

     Coke. Derived from coal, this is mostly carbon and is used in the reduction of iron oxide to iron.

     Oxygen. The coke burns in pre-heated air, which also helps to heat the furnace to about 1200°C.

A blast furnace is like a giant chimney, at least 30m tall. It is made of steel and is lined with fireproof bricks. Iron ore, limestone and coke (the ‘charge’) are added through the top. Hot air is blasted through the bottom, making the charge glow white hot. The following reactions take place:

1. The coke reacts with oxygen in the air, giving carbon dioxide.

                  C(s)  +  O2(g)      CO2(g)

2. The limestone decomposes to give calcium oxide and carbon dioxide.

                  CaCO3(s)       CaO(s)  + CO2(g)

3. The carbon dioxide reacts with more coke, giving carbon monoxide.

                  CO2(g)  + C(s)      2CO(g)

4. Carbon monoxide reacts with iron oxide in the ore, giving liquid iron. The liquid iron trickles to the bottom of the furnace.

                  Fe2O3(s) + 3CO(g)      2Fe(l)  +  3CO2(g)

5. Calcium oxide from step 2 reacts with sand in the ore to form calcium silicate (CaSiO3), also known as slag. The slag, which is a liquid at the temperatures in the furnace, runs down the furnace and floats on the iron.

                  CaO(s)  +  SiO2(s)      CaSiO3(l)

The waste gases, mainly nitrogen and oxides of carbon, escape from the top of the furnace. They are used in a heat exchange process to heat incoming air and so help reduce the energy cost of the process. The liquid slag and the liquid iron are drained from the bottom of the furnace. When the slag solidifies it is sold, mostly for use as foundations in road building and for the construction of breeze-blocks.

The iron obtained from this process is known as ‘pig’ or cast iron and contains about 4% carbon (as well as some other impurities). The name pig iron arises from the fact that if it is not subsequently converted into steel it is poured into moulds called pigs. Because of its brittle and hard nature, the iron produced by this process has limited use. Gas cylinders are sometimes made of cast iron, since they are unlikely to get deformed during their use. The majority of the iron produced in the blast furnace is converted into different steel alloys such as manganese and tungsten steel as well as stainless steel.


The Blast Furnace

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[D.R.B1] 

 

 


 

Alloys

The majority of the metallic substances used today are alloys. Alloys are mixtures of two or more metals and are formed by mixing molten metals thoroughly. It is generally found that alloying metals produces a metallic substance that has more useful properties than the original pure metals it was made from.

Steel

The ‘pig iron’ obtained from the blast furnace contains between 3% and 5% of carbon and other impurities such as sulphur, silicon and phosphorus. These impurities make the iron hard and brittle. In order to improve the quality of the metal, most of the impurities must be removed and in doing this, steel is produced.

The impurities are removed in the basic oxygen process. In this process, molten pig iron from the blast furnace is poured into another, bucket-shaped furnace. A water-cooled lance is then lowered into the furnace and oxygen at 5- 15 bar pressure is blown onto the surface of the molten metal. Most of the carbon is oxidised to carbon dioxide gas:

C(s)  +  O2(g)      CO2(g)

At the same time, sulphur is oxidised to sulphur dioxide (SO2, a gas), silicon to silicon dioxide (SiO2, a solid) and phosphorus to phosphorus pentoxide (P2O5, a solid). The solid oxides are removed from the melt by adding calcium oxide (lime). Lime combines with the oxides to form slag, which floats on the surface. The slag is regularly skimmed or poured off the surface of the melt. Samples of the melt are taken continuously and when the required amount of carbon has been reached the blast of oxygen is turned off.

There are various types of steel that differ only in their carbon content. The differing amounts of carbon present confer different properties on the steel and they are used for different purposes. If other types of steel are required then other metals can be added. For example, when nickel and chromium are added to the melt, stainless steel is produced. The chromium prevents the steel form rusting while the nickel makes it harder. Tungsten (W) toughens and hardens steel, even at high temperatures.

 

Steel

Typical composition

Properties

Uses

Mild steel

99.5% Fe, 0.5% C

Easily worked

Lost most of brittleness

 

Car bodies, large structures

Hard steel

99% Fe, 1% C

Tough and brittle

Cutting tools, chisels, razor blades

 

 

Manganese steel

87% Fe, 13% Mn

Tough, springy

 

Drill bits, springs

 

 

Stainless steel

74% Fe, 18% Cr, 8% Ni

Tough, does not corrode

Cutlery, kitchen sinks, surgical instruments

 

 

Tungsten steel

95% Fe, 5% W

Tough, hard, even at high temperatures

Edges of high speed cutting tools

 

 


 

Factors affecting the siting of an industrial plant

When deciding on the best site for the large-scale production of a chemical, industries have to decide on a number of social, economic and environmental factors.

Social factors

 

 

 

 

 

 

Economic factors

 

 

 

 

 

 

Environmental factors

 


Blast furnace

p.161 Ramsden

Chemistry for GCSE

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