Iron

Ores of iron

1. Haematite: Fe₂O₃
2. Magnetite: Fe₃O₄
3. Iron pyrite: FeS₂
4. Siderite: FeCO₃
5. Limonite: Fe₂O₃.3H₂O

Extraction of iron from Haematite

1. Crushing and pulverization

Big lumps of ores are crushed using jaw crushers to get crushed ore which is pulverized using a pulverizer or stamp mill to get powdered ore.

2. Concentration by gravity separation

The powdered ore is washed with an upward stream of water. Heavier particles settle down washing the lighter impurities.

*If iron is to be extracted by iron pyrite, the froth floatation process is used.

3. Calcination

The concentrated ore is heated with coke. Following changes are observed:
i. Moisture, carbon, sulphur, phosphorous and arsenic impurities are removed.
ii. If the iron carbonate is present, it is decomposed into FeO and further converted into Fe₂O₃.

\begin{align*} FeCO_{3} &\rightarrow FeO + CO_{2}\\ 4FeO + O_{2} &\rightarrow 2Fe_{2}O_{3} \end{align*}

4. Smelting

The calcined ore is mixed with coke and limestone by a smelting process in a blast furnace. The blast furnace is a cylindrical tower of 30-40 meters in height. The upper and lower parts are narrow known as the body and the middle part is wider known as bosh. The furnace is lined externally with fire bricks. Hot air is passed through tuyers.
The mixture of ore, coke and limestone(8:4:1) is fed from the top of the furnace by cup and cone arrangement.

On the basis of the range of temperature and different chemical reactions, the regions of the blast furnace can be classified into the following zones:

1. Zone of combustion (1300-1500°C): This is the hottest part of furnace. Combustion of carbon takes place and being exothermic, large amount of heat is produced.

C + O_{2} \rightarrow CO_{2} + 97 Kcal

The hot CO₂ rises up which combines with coke to give CO.

CO_{2} + C \rightarrow 2CO - 58 Kcal

2. Zone of fusion (1200-1300°C): This lies above the zone of combustion. The spongy iron formed in the zone of reduction slides down to this zone, then fuses to give molten iron. The molten iron forms cementite with coke.

3Fe + C \rightarrow Fe_{3}C (Cementite)

3. Zone of slag formation (700-1000°C): This lies middle of furnace. Here, limestone is decomposed.

CaCO_{3} \rightarrow CaO + CO_{2}

Quick lime reacts with silica to form slag.

\underset{Flux}{CaO} + \underset{Impurity}{SiO_{2}} \rightarrow \underset{Slag}{CaSiO_{3}}

4. Zone of reduction (400-700°C): This lies near the top of furnace. CO coming from bottom of furnace reduces ferric oxide to iron. This obtained iron is spongy iron.

\begin{align*} 3Fe_{2}O_{3} + CO &\rightarrow 2Fe_{3}O_{4} + CO_{2}\\ Fe_{3}O_{4} + CO &\rightarrow 3FeO + CO_{2}\\ FeO + CO &\rightarrow Fe + CO_{2} \end{align*}

The molten iron and slag fall into the lower part of the furnace. Molten iron being heavier forms the lower layer and slag floats over it. Both are collected separately.
The molten iron is allowed to solidify in a big lump called pig iron. The pig iron is remelted into the desired shape called cast iron. Cast iron is not a pure form of iron. Wrought iron is the purest form of iron. Steel is an alloy containing 0.2-2% carbon intermediate between cast and wrought iron.

Different forms of iron

Stainless steel

It is a special type of alloy steel. It contains 12% chromium, 2% nickel and 0.2% carbon. It resists corrosion, heat and action of organic acid. It is used to manufacture household and hospital utensils, automobile parts, etc.

Properties of Iron

Physical properties

1. Pure iron is white soft solid. Presence of carbon makes it hard. Ordinary iron (impure) is grey and crystalline.
2. It is malleable, ductile and good conductor of heat and electricity.
3. It’s melting point is 1538°C and boiling point is 2862°C.
4. It shows magnetic property (ferromagnetism) and loses its property if heated at 760°C.

Chemical properties

1. Action with air: Iron is not affected by dry air in normal conditions. But when exposed to moist air, it gets covered with a reddish-brown layer of rust due to the formation of hydrated ferric oxide.

4Fe + 3O_{2} + 3H_{2}O \rightarrow \underset{\substack{hydrated\ ferric\ oxide\\ (rust)}}{2Fe_{2}O_{3}.3H_{2}O}

When iron is strongly heated in the air, the ferrosoferric oxide is formed.

3Fe + 2O_{2} \rightarrow Fe_{3}O_{4}\ or\ FeO.Fe_{2}O_{3}

2. Action with steam: Iron is not attacked with water at normal conditions. But when heated iron is passed over steam, Fe₃O₄ and H₂ are formed.

\underset{red\ hot}{3Fe} + \underset{steam}{4H_{2}O} \xrightarrow{850\degree C}Fe_{3}O_{4} + 4H_{2}

3. Action with non-metal

\begin{align*} 3Fe + C &\rightarrow \underset{Iron\ carbide}{Fe_{3}C}\\ 2Fe + 3Cl_{2} &\rightarrow \underset{Ferric\ chloride}{2FeCl_{3}}\\ Fe + S &\rightarrow \underset{Ferrous\ sulphide}{FeS} \end{align*}

4. Action with acids

• Action with HCl: It reacts with dil and conc. HCl to give hydrogen gas.

Fe + 2HCl \rightarrow FeCl_{2} + H_{2}

• Action with H₂SO₄

\begin{align*} Fe + \underset{dil.}{H_{2}SO_{4}} &\rightarrow FeSO_{4} + H_{2}\\ Fe + \underset{conc.}{2H_{2}SO_{4}} &\rightarrow FeSO_{4} + SO_{2} + 2H_{2}O \end{align*}

• Action with HNO₃
  – with conc. HNO₃: Conc. HNO₃ reacts with iron to form a layer of ferrosoferric oxide on the surface of iron. Then the reaction stops. This is called passivity of iron. Such iron is called passive iron.

2HNO_{3} \rightarrow 2NO_{2} + H_{2}O + [O] ] \times 4\\ 4 + 3Fe \rightarrow Fe_{3}O_{4}\\ \overline{3Fe + 8HNO_{3} \rightarrow 8NO_{2} + Fe_{3}O_{4} + 4H_{2}O}

– with moderate conc. HNO₃:

Fe + 6HNO_{3} \rightarrow Fe(NO_{3})_{3} + 3NO_{2} + 3H_{2}O

– with very dilute HNO₃

4Fe + 10HNO_{3} \rightarrow 4Fe(NO_{3})_{2} + NH_{4}NO_{3} + 3H_{2}O

5. Action with carbon monoxide: Iron is heated with CO at 120°C and 120 atm to form iron pentacarbonyl.

Fe + 5CO \xrightarrow{120\degree C} \underset{Iron\ pentacarbonyl}{Fe(CO)_{5}}

*Above 150°C, iron carbonyl decomposes to give back Fe and CO.

6. Displacement reaction

\begin{align*}Fe + CuSO_{4} &\rightarrow FeSO_{4} + Cu\\ Fe + 2AgNO_{3} &\rightarrow Fe(NO_{3})_{2} + 2Ag\end{align*}

Uses of Iron

• It is used to carry oxygen in through our blood (also known as haemoglobin).
• It is used to make steel.
• It is used in construction, container, packing, shipping, machines, rail transportation, oil and gas industries, electrical equipment, appliances and utensils.

Manufacture of steel by

A. Open Hearth process

This is the modern process for the manufacture of steel. For the manufacture of steel, 70-80% cast iron, 20-30% scrap iron and a little haematite is used. The charge is heated by passing producer gas (CO + N₂ in 1:2) and the process works on a regenerative system of heat economy. Oxidation is carried by haematite. Depending upon the nature of impurities, the open-hearth process may be acidic or basic.

i. Acidic process: If the cast iron used for the manufacture of steel consists of basic impurities such as Mn or Sn or phosphorous content is low, an acidic process is used. In this process, the internal lining of the furnace is carried out by an acidic material such as SiO₂.

\begin{align*} 3Mn + Fe_{2}O_{3} &\rightarrow 3MnO + 2Fe\\ MnO + SiO_{2} &\rightarrow MnSiO_{3} (slag)\\ 3C + Fe_{2}O_{3} &\rightarrow 2Fe + 3CO \end{align*}

After the impurities are removed, a calculated quantity of spiegeleisen is added to obtain the desired quality of steel.

ii. Basic process: If the cast iron used for the manufacture of steel consists of acidic impurities or phosphorous content is high, the basic process is used. In this process, the internal lining of the furnace is carried out by a basic material such as dolomite (CaCO₃.MgCO₃).

\begin{align*}3P_{4} + 10Fe_{2}O_{3} &\rightarrow 20Fe + 6P_{2}O_{5}\\ CaCO_{3} &\rightarrow CaO + CO_{2}\\ 3CaO + P_{2}O_{5} &\rightarrow Ca_{3}(PO_{4})_{2} (slag)\\ 3C + Fe_{2}O_{3} &\rightarrow 2Fe + 3CO \end{align*}

After the impurities are removed, a calculated quantity of spiegeleisen is added to obtain the desired quality of steel.

B. Basic oxygen method

This process was developed in 1948 by Swiss engineer Robert Durrer and commercialized in 1952-1953 by the Austrian steel making company. The name Linz-Donawitz (LD) converter was kept after the name of Austrian towns Linz and Donawitz (a district of Leoben).

A specially designed steel furnace is a refractory lined, tiltable converter (rotated through 360° in a verticle plane) into which a vertically moveable, water-cooled lance is inserted to blow oxygen through nozzles at supersonic velocity on the charge. The furnace is inner lined with heat resistant refractory materials consists of dolomite (MgCO₃/CaCO₃) and magnesite (MgO) which lasts for two weeks before they have to be replaced.
The raw materials of the LD converter are pig iron(70-75%) from the blast furnace, recycled steel or scrap steel (20-30%), slag formers and fluxes (lime or dolomite), oxygen blown through the lance, inert gas blown from the bottom. This method involves the manufacture of steel from molten pig/cast iron from blast furnace along with scrap iron by removing impurities like carbon, silicon, manganese, phosphorous, etc. through oxidation with pure oxygen blowing with the help of retractable steel lance till steel of the desired composition achieved followed by addition of flux to get steel of desired quality.
Molten pig iron obtained from blast furnace and scrap metals is fed into LD-converter. The vessel is kept in an upright position and pure oxygen is blown into the surface of the molten crude metal at high speed with the help of a water-cooled lance. Carbon present in the iron gets oxidized into carbon monoxide and carbon dioxide i.e. blowing pure oxygen into hot metal removes carbon from the metal. The reaction occurs as follows:

\begin{align*}C + O_{2} &\rightarrow CO_{2}\\ CO_{2} + C &\rightarrow 2CO\\ 2CO + O_{2} &\rightarrow 2CO_{2} \end{align*}

After passing oxygen for few minutes, the supply of oxygen is paused and suitable flux consisting of calcium fluoride and calcium oxide is added to remove impurities (Si, P, etc) in the form of fusible slag. Sulphur gets dissolved in the slag and is removed.

\begin{align*} CaCO_{3} &\rightarrow CaO + CO_{2}\\ P_{4} + 5O_{2} &\rightarrow 2P_{2}O_{5}\\ 6CaO + 2P_{2}O_{5} &\rightarrow 2Ca_{3}(PO_{3})_{2}\\ Si + O_{2} &\rightarrow SiO_{2}\\ CaO + SiO_{2} &\rightarrow \underset{Slag}{CaSiO_{3}} \end{align*}

After this, the blowing of oxygen through the lance is continued till the carbon content gets adjusted. For this, a small quantity of samples is analysed for carbon content from time to time. When molten metal achieves the required amount of carbon, then the vessel is tilted and the steel is tapped off from the furnace below the slag level. Finally, the slag is removed by tilting the vessel in the opposite direction. The whole process completes about 45 minutes. Most of the steel manufacturing company applies this method. This method is much faster than the open-hearth process. Another benefit of this process is that carbon dioxide is a major byproduct that is used as a fuel. This process also produces less air pollution.

Rusting of iron

When iron is exposed to moist air for a long time, a reddish-brown layer of hydrated ferric oxide is formed which is called rust and the process of forming rust is called rusting. Millions of iron are lost every year due to rusting. So more attempts have been made to prevent rusting. Among them, the electrochemical theory is the most efficient.

Electrochemical theory

This is modern theory. According to this, rusting of iron is electrochemical in nature. The attack of iron is basically a chemical reaction occurring by the flow of current.
Irons have non-uniform surfaces with impurities. Due to this, some portion of the iron surface is at lower reduction potential and acts as an anode while another portion of the iron surface at higher reduction potential acts as a cathode. Following reactions takes place on iron and produce rust.
At the anode, oxidation of iron takes place:

Fe \rightarrow Fe^{++} + 2e^{-}

The electrons thus liberated migrate towards the cathode, where they are utilized in the evolution of hydrogen or absorption of oxygen.
At cathode,

2H + 2e^{-} \rightarrow H_{2}

The hydrogen ion is obtained by the ionization of water or carbonic acid.

\begin{align*} H2O &\rightarrow H^{+} + OH^{-}\\ CO_{2} + H_{2}O &\rightarrow H_{2}CO_{3}\rightarrow H^{+} + HCO3^{-} \end{align*}

In neutral water, reduction of oxygen takes place as:

2H_{2}O + O_{2} + 4e^{-} \rightarrow 4OH^{-}

Fe⁺⁺ obtained from the anode and OH^– obtained from cathode combine to form ferrous hydroxide.

Fe^{++} + 2OH^{-} \rightarrow Fe(OH)_{2}

In excess of oxygen and water, ferrous hydroxide is oxidized to ferric hydroxide.

4Fe(OH)_{2} + O_{2} + 2H_{2}O \rightarrow 4Fe(OH)_{3}

Ferric hydroxide is unstable and loses water to give hydrated ferric oxide called rust.

2Fe(OH)_{3} \rightarrow \underset{Rust}{Fe_{2}O_{3}.3H_{2}O}

Prevention of rusting

1. Protective coating

• A coating of oil and grease
• A thin coating of paints and enamels
• By passing steam on red hot iron.

2. Application of corrosion inhibitor: Treating iron with certain solutions like potassium chromate, phosphoric acid, conc. nitric acid inhibits rusting.

3. Galvanization (cathodic protection): The process of coating zinc (electropositive metal) over the surface of iron is called galvanization. Zinc becomes anode and iron becomes the cathode. Anode sacrifices itself and prevents cathode from rusting.