Extraction of metals by chemical reduction

Extraction of iron and steel

The main ores of iron are haematite, $Fe_2O_3$, and magnetite $Fe_3O_4$, iron(II) carbonate, $FeCO_3$ also occurs. The ore is heated to expel water. It is then loaded into the top of a blast furnace together with coke and limestone.

Stage 1

Near the base of the furnace where a hot air blast enters by the tubers, coke burns in air to produce carbon dioxide, providing the main source of heat for the process

Stage 2

A little higher in the furnace the carbon dioxide is reduced by hot coke to carbon monoxide

Stage 3

At higher stage still carbon monoxide reduces the ore to iron, some reduction may also occur directly by carbon

As it settles in the furnace the iron melts and drops into the bottom of the funnel

The function of the limestone ($CaCO_3$)

The limestone is heated and decomposes to quick lime (CaO) and $CO_2$

The quicklime (CaO) reacts with acid impurities in the ore to form molten slag

Slag and iron are tapped off separately at frequent intervals. The iron is cast into "pigs" and is known as pig iron or cast iron.

Example

Railings, water pipes, bases of Bunsen, cookers, stoves etc.

Wrought iron

It is the purest form of commercial iron. It contains 99% iron and less than 0.25% of carbon. To make wrought iron, cast iron is melted and stirred in a furnace. Carbon in the cast iron is oxidized away as gas (CO and $CO_2$) while other impurities such as $Mn$ and $Si$ are oxidized to a slag.

Function of limestone – to remove impurities

1. The wrought iron is strong and malleable. It can be shaped by hammering at very high temperature, $1000°C$.

2. It is used for making nails, sheets, chains, gates, farm machines etc.

Steel

This is an alloy of iron with 0.15% to 0.17% of carbon. Steel is obtained by oxidizing the impurities in molten pig iron using a jet or "tonnage oxygen" directly vertically down the metal. After removing of impurities a calculated mass of carbon and (Ni, Co, Cs, Mn, if needed) is added.

Properties of metals

Physical properties of metals as compared to non-metals

PROPERTY	METALS	NON-METALS
Thermal or electrical conductivity	High	Low
Lustre	High	Low
Sonority	High	Low
Ductility	High	Low
Tensile strength	High	Low
Malleability	High	Low

1. Metals are lustrous

   They can be polished and can be rolled into different shapes. That is why they are useful in manufacture of lightweight cans. For example, aluminium can be rolled into very thin foils which can be used even for wrapping sweets. Lead is soft and bendable. It's useful in protecting underground cables. Because of its ductility and malleability steel can be brought into different shapes.

2. Metals conduct heat

   Metals are used to make cooking utensils like kettles and saucepans because they are good conductors. Heat can be conducted from source to the food being cooked through them.

3. Metals have high melting points

   Although not all metals have high melting points, for example Na and K have low densities and float on water and their melting points are low. The use of tungsten in filaments; alloys of chromium and nickel in heating elements of electric fires and engines in cars and aeroplanes do utilize this property.

4. Metals have high tensile strength

   Metals can support heavy loads. Steel is useful in making girders, hawsers and chains. Because of this property the head of hammer is useful because it's hard and tough.

Chemical properties of metals

1. Physical strength and chemical strength

   Physical strength results from the way atoms are arranged in a substance. Metals of high tensile strength like Fe, Cu and Al and low tensile strength like Na and K. Chemical strength refers to the reactivity of the element. This depends on how the electrons are arranged in atom. K and Na have got very low tensile strength but chemically they are the strongest metals.

2. Reducing power of metals

   The reactivity depends on the easiness to donate electrons i.e. reducing power. A more reactive metal can displace a less reactive metal from its compound. For example, when we roast copper(I) sulphide we get Cu while when we roast lead sulphide, no lead is made.

3. Displacement reactions of metals and reactivity

   A more reactive metal can displace a less reactive metal from an aqueous solution or its oxide.

4. Reaction of metals with water

   As reducing agents, metals react with water to liberate $H_{2(g)}$. K and Na react violently with water while Ca and Mg react with steam and Al, Zn, Fe, Pb and Cu have no action on water.

5. Reaction of metals with dilute HCl

   The reaction between K, Na and Ca and dilute HCl to liberate $H_{2(g)}$ is violent and dangerous, while Mg, Al, Zn, Fe and Sn and Pb give off $H_{2(g)}$ very slowly. The acid needs to be warmed up. Cu, Ag and Au are not attacked at all.