Iron

Iron

Iron is the most important element in the industry. It is the second most abundant element on the earth crust after aluminium, but often occurs as a free metal.

The common ores are haematite found in united states, Australia and USSR. It can also occur as impure iron (III) oxide (Fe₂O₃), Magnetite or magnetic iron ore (Fe₃O₄) is found in Sweden and in North America.

Siderite or spathic iron ore, (FeCO₃), found in Great Britain. Iron also occurs as iron pyrites (FeS₂) and limonite (Fe₂O₃.3H₂O).

Iron is widely present as trioxosilicate (IV) in clay soils. Iron ore is available in Itakpe, Ajaokuta, Jebba and Lokoja all in Kwara State (Nigeria).

Extraction of Iron

The extraction of iron from iron ore (haematite), using coke, limestone and air in a blast furnace

Haematite is basically iron oxide, and the oxygen must be removed to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions. Since carbon is more reactive than iron, it can displace the iron from its oxide. Hence the method for extraction of iron is called ‘reduction by carbon’.

Coke is impure carbon, and it burns in the hot air blast to form carbon dioxide. This is a strongly exothermic reaction which makes it an important reaction, as it helps heat up the blast furnace. The iron ore, coke and limestone enter the blast furnace at the top. The hot waste gases at the top of the furnace are piped away and used to heat the air blast at the bottom.

C (s) + O₂ (g) ——> CO₂ (g)

At high temperatures in the furnace, the carbon dioxide is reduced by more carbon to give carbon monoxide.

CO₂ (g) + C (s) ——> 2CO (g)

It is the carbon monoxide which is the main reducing agent in the furnace-especially in the cooler parts.

Assuming that the iron ore is haematite, Fe₂O₃:

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

Due to the high temperatures, the iron produced melts and flows to the bottom of the furnace, where it can be tapped off.

In the hotter parts of the furnace, some of the iron oxide is also reduced by carbon itself.

Fe₂O₃ (s) + 3C (s) ——> 2Fe (l) + 3CO (g)

Notice that carbon monoxide is formed, rather than carbon dioxide, at these temperatures.

However some use this equation instead:

iron oxide + carbon    →    iron + carbon dioxide

2Fe₂O₃ + 3C    →    4Fe + 3CO₂

The limestone is added to the furnace to remove impurities in the ore which would otherwise clog the furnace with solid material.

The furnace is hot enough for the limestone (calcium carbonate) to undergo thermal decomposition. It splits up into calcium oxide and carbon dioxide. This is an endothermic reaction (it absorbs heat) and it is important not to add too much limestone to avoid cooling the furnace.

CaCO₃ (s) ——> CaO (s) + CO₂ (g)

Calcium oxide is a basic oxide, and its function is to react with acidic oxides such as silicon dioxide, SiO₂. Silicon dioxide is the main constituent of sand, and is typical of the sort of impurities that need to be removed from the furnace.

CaO (s) + SiO₂ (s) —–> CaSiO₃ (l)

The product is calcium silicate. This melts and trickles to the bottom of the furnace as a molten slag, which floats on top of the molten iron as it is less dense, and can be tapped off separately. Slag is used

Uses of iron

• Cast iron

Molten iron straight from the furnace can be cooled rapidly and solidified by running it into sand moulds. This is known as pig iron. If the pig iron is remelted and cooled under controlled conditions, cast iron is formed. This is very impure iron, containing about 4% carbon as its main impurity. Although cast iron is very hard, it is also very brittle and tends to shatter if it is hit hard. It is used for things like manhole covers, gutterings and drainpipes, and cylinder blocks in car engines.

• Mild steel

Mild steel is iron containing up to about 0.25% carbon. This small amount of carbon increases the hardness and strength of the iron. It is used for (among other things) wire, nails, car bodies, ship building, girders and bridges.

• Wrought iron

This is pure iron. It was once used to make decorative gates and railings but has now been largely replaced by mild steel. The purity of the iron makes it very easy to work because it is fairly soft, but the softness and lack of strength mean that it isn’t useful for structural purposes.

• High-carbon steel

High carbon steel is iron containing up to 1.5% carbon. Increases the carbon content makes the iron harder, but at the same time it gets more brittle. High-carbon steel is used for cutting tools and masonry nails. Masonry nails are designed to be hammered into concrete blocks or brickwork where a mild steel nail would bend. If you miss-hit a masonry nail, it tends to break into two bits because of its increased brittleness.

• Stainless steel

Stainless steel is an alloy of iron with chromium and nickel. Chromium and nickel form strong oxide layers in the same way as aluminium, and these oxide layers protect the iron as well. Stainless steel is therefore very resistance to corrosion.

Obvious uses include kitchen sinks, saucepans, knives and forks, and gardening tools. But there are also major uses for it in the brewing, dairy and chemical industries where corrosion- resistant vessels are essential

Physical Properties of Iron

1. Pure iron is a grey metal with density 7.8g cm^-3
2. It melts at 1535^oC and boils at 2800^oC
3. It is very malleable and ductile
4. It is a good conductor of heat and electricity
5. It can easily be magnetized

Chemical Properties of Iron

• Reaction With Air: It burns in air to form reddish hydrated iron (III) oxide of variable composition

4Fe(s) + 3O₂(g) + 2xH₂O(l) ——-> 2Fe₂O₃.xH₂O

• When finely divided iron is heated in air it burns at high temperature to form magnetic iron oxide, which behaves like a compound oxide

3Fe(s) + 2O₂ (g) ——> Fe₃O₄ (s)

• Reaction With Steam: When steam is passed over red-hot iron filings, tri iron tetraoxide and hydrogen are produced and the reaction is reversible

Fe(s) + 4H₂0 <——> Fe₃O₄(s) + 4H₂ (g)

The metal compound is:

1.           Melted, then
2.           Has electricity passed through it

ASSESSMENT (POST ANSWERS BELOW USING THE BOX)

1. What are the uses of the following Iron
   – Cast Iron
   – Stainless steel
   – Wrought Iron
2. Stainless steel is an alloy of iron with ………. and ………
3. Mention 2 common ores of iron ……… and ………