Iron

Coating iron with zinc is called galvanization. The zinc layer keeps air and moisture away from the iron (barrier protection). Zinc is more electropositive than iron, so even when the coating is scratched, zinc is oxidised in preference to iron and acts as a sacrificial anode: \(Zn\rightarrow Zn^{2+}+2e^-\). In this way the iron is protected from rusting.

Coke burns in the hot air blast to give a large amount of heat, which keeps the furnace hot enough to melt the iron and the slag: \(2C+O_2\rightarrow2CO+\text{heat}\), \(2CO+O_2\rightarrow2CO_2+\text{heat}\). This makes it the fuel. The \(CO_2\) formed is reduced by hot coke to carbon monoxide, \(CO_2+C\rightarrow2CO\), and the carbon monoxide reduces the ore, \(Fe_2O_3+3CO\rightarrow2Fe+3CO_2\). So coke is also the reducing agent.

Pure iron is soft, malleable and ductile, and it bends easily, so it does not have enough hardness and tensile strength for tools or for load-bearing structures. When a little carbon is added, the iron becomes hard and strong, so it is used as steel (alloy of iron with 0.2 to 2% carbon) or as cast iron. Pure iron also rusts easily.

Rusting is an electrochemical process. Water (with the dissolved gases) works as the electrolyte that carries the ions between the anodic and the cathodic regions. Oxygen is the oxidising agent that takes up the electrons at the cathode (\(O_2+4H^++4e^-\rightarrow2H_2O\)) and it further oxidises \(Fe^{2+}\) to rust. Iron in dry air or in air-free water does not rust.

Spiegeleisen (German, mirror iron) is an alloy of iron, manganese and carbon, containing roughly 15 to 30% manganese and 4 to 5% carbon. In steel making a calculated amount of this mixture of \(Fe\), \(Mn\) and \(C\) is added after the impurities have been oxidised away.

Importance:

1. It restores the required amount of carbon (recarburiser) to the purified iron, so steel of the desired quality is obtained.

2. Manganese removes the dissolved oxygen and oxides (it works as a deoxidiser), and also removes sulphur as slag: \(Mn+FeO\rightarrow MnO+Fe\).

Rusting is the slow corrosion of iron in moist air, in which a reddish brown layer of hydrated ferric oxide, \(Fe_2O_3\cdot xH_2O\) (rust), is formed on the surface of iron.

Two methods of prevention:

1. Galvanization: coating the iron with a thin layer of zinc, which protects it as a barrier and as a sacrificial metal.

2. Barrier protection: painting, oiling, greasing or enamelling the surface to keep out air and moisture. (Alloying with chromium and nickel to make stainless steel is another method.)

\(Fe_2O_3\) (ferric oxide).

\(Fe_3O_4\) (ferroso-ferric oxide).

\(2Fe_2O_3\cdot3H_2O\) (brown haematite).

\(FeCO_3\) (ferrous carbonate).

Prediction: the iron vessel will be attacked and the solution will be spoiled. Iron lies above copper and silver in the electrochemical series, so iron is more reactive and it displaces both metals from their salts (displacement reaction). The vessel itself dissolves as \(Fe^{2+}\) and the less reactive metals are deposited.

Observations: the blue colour of the copper sulphate solution fades and the solution becomes pale green because of ferrous sulphate. A reddish brown deposit of copper and a grey deposit of silver form on the inner wall of the vessel, which gets thinner and may develop holes.

Modification: the solution should be stored in a vessel of glass, plastic or porcelain, which does not react with the metal salts. A metal vessel less reactive than copper and silver (for example one lined with an inert material) can also be used.

The coating is rust, hydrated ferric oxide, \(Fe_2O_3\cdot xH_2O\). It is formed when iron is in contact with oxygen of air and moisture. Small voltaic cells form on the surface, the iron is oxidised to \(Fe^{2+}\), and this is further oxidised to rust: \(4Fe+3O_2+2xH_2O\rightarrow2Fe_2O_3\cdot xH_2O\).

In the rainy season the tools stay wet and the air is very humid. Water is the electrolyte which is needed for the electrochemical cells on the iron surface, so more water means faster rusting. Dissolved gases such as \(CO_2\) in the rain water also supply \(H^+\) ions and speed up the process.

1. Clean and dry the tools after use and apply oil or grease on them (barrier protection).

2. Paint the tools or use galvanized (zinc coated) tools or tools of stainless steel.

First remove the old rust and dirt and dry the surface, then apply a continuous, uniform coat (a primer and then the paint). The coating must have no gaps or scratches. If it is chipped, moisture enters, the exposed iron acts as the anode, rust spreads under the paint and the paint flakes off. So the paint should be inspected and renewed from time to time.

(With warm dilute nitric acid nitric oxide is given off: \(3Fe+8HNO_3\rightarrow3Fe(NO_3)_2+2NO\uparrow+4H_2O\).)

Spongy iron is obtained, which melts lower down in the zone of fusion.

According to this theory, a tiny voltaic cell develops on the surface of iron because the surface is uneven or has impurities or stains. The part with the lower reduction potential acts as the anode and the part with the higher reduction potential acts as the cathode. The moisture on the iron, with dissolved oxygen, carbon dioxide and other gases, works as the electrolytic solution.

At the anode iron atoms are oxidised to ferrous ions and the electrons move through the metal to the cathode:

At the cathode hydrogen ions (from water or from carbonic acid, \(CO_2+H_2O\rightarrow H_2CO_3\rightleftharpoons2H^++CO_3^{2-}\)) take up the electrons, and dissolved oxygen is reduced to water:

Overall cell reaction:

Formation of rust: the ferrous ions are further oxidised by atmospheric oxygen and precipitate as hydrated ferric oxide:

Iron is extracted from haematite, \(Fe_2O_3\), by pyrometallurgy. The steps are:

1. Crushing and pulverization: big lumps of ore are broken into small pieces by a jaw crusher or hammer and powdered in a ball mill.

2. Concentration: the powdered ore is washed with a stream of running water (gravity separation). The heavy ore particles settle and the lighter earthy impurities (gangue) are washed away. If magnetic impurities are present, magnetic separation is used.

3. Calcination: the concentrated ore is heated with a little coal in a limited supply of air in a shaft furnace or a rotary kiln. Moisture is removed, ferrous carbonate is changed to ferric oxide, and the impurities \(P\), \(S\), \(As\), \(Sb\) go away as volatile oxides:

4. Reduction (smelting) in the blast furnace: the calcined ore, coke and limestone are mixed in the ratio 8 : 4 : 1 (this mixture is the charge) and fed from the top of the blast furnace. Hot air is blown in through the tuyeres at the bottom. The following changes occur in the different zones, from the bottom to the top:

(i) Zone of combustion (1300 to 1500 degrees Celsius): coke burns and a large amount of heat is produced, \(2C+O_2\rightarrow2CO+\text{heat}\); \(2CO+O_2\rightarrow2CO_2+\text{heat}\).

(ii) Zone of fusion (1200 to 1300 degrees Celsius): the \(CO_2\) from the zone below is reduced by hot coke to \(CO\) (endothermic), \(CO_2+C\rightarrow2CO-\text{heat}\). The fall in temperature is enough to melt the spongy iron, which absorbs carbon, \(3Fe+C\rightarrow Fe_3C\), and also \(P\), \(Si\) and \(S\). Any remaining \(Fe_2O_3\) is reduced, \(Fe_2O_3+3C\rightarrow2Fe+3CO\).

(iii) Zone of slag formation (700 to 1000 degrees Celsius): limestone decomposes, \(CaCO_3\rightarrow CaO+CO_2\), and the lime combines with silica to form slag, \(CaO+SiO_2\rightarrow CaSiO_3\).

(iv) Zone of reduction (400 to 700 degrees Celsius): the carbon monoxide reduces the ore to spongy iron, \(Fe_2O_3+3CO\rightarrow2Fe+3CO_2\).

The molten iron collects at the hearth below the lighter molten slag, and both are tapped out separately. The iron which solidifies in big lumps is pig iron (about 4% carbon with some \(Si\), \(P\), \(S\) and \(Mn\)).

5. Refining: pig iron is converted to cast iron, to wrought iron (the purest form, 99.5% iron) or to steel by oxidising its impurities.

Quenching of steel: it is the process of heating steel to a high temperature (red hot) and then cooling it rapidly by dipping it in water or oil. It is done to increase the hardness of the steel, but quenched steel is also very brittle.

Tempering of steel: it is the process of reheating the quenched steel to a lower temperature (about 200 to 300 degrees Celsius) and then cooling it slowly. It is done to reduce the brittleness and to improve the toughness of the steel, while it keeps enough hardness.

Steel is an alloy of iron which contains 0.2 to 2% carbon and traces of other elements like \(S\), \(P\), \(V\), \(Cr\), \(Ni\), \(Mn\) and \(Co\). If only carbon is present it is plain steel, and if \(Ni\), \(Mn\), \(Cr\) etc. are also present it is special steel (for example stainless steel, which contains \(Fe\), \(C\), \(Cr\) and \(Ni\)).

Principle of the open hearth process: the impurities (\(C\), \(Si\), \(S\), \(P\), \(Mn\)) of the charge, which is a mixture of about 70 to 80% pig (cast) iron, 20 to 30% scrap iron, haematite and lime, are oxidised by haematite (and by air) on a shallow open hearth, which is heated by burning producer gas. The oxides of carbon and sulphur escape as gases and the oxides of silicon, phosphorus and manganese combine with the flux and are removed as slag:

When the impurities have been removed, a calculated amount of spiegeleisen (a mixture of \(Fe\), \(Mn\) and \(C\)) is added to get steel with the desired carbon content.

The basic oxygen process (BOP), also called the LD (Linz-Donawitz) process, was discovered by Robert Durrer. It is a modified version of the Bessemer process. Molten pig iron from the blast furnace is poured into a vessel called the LD converter, which has a basic lining. A water-cooled lance is lowered into it and extremely pure oxygen is blown through the lance at high pressure.

Theory: the carbon of the pig iron is oxidised to carbon monoxide and carbon dioxide, and silicon, phosphorus and the other impurities are oxidised to their oxides:

After some time the supply of oxygen is stopped and a basic flux, quicklime (\(CaO\)), is added. It combines with the acidic oxides to form slag, and the sulphur, if present, dissolves in the slag and is removed:

The oxidation reactions are exothermic, so the process needs no external fuel (it is autogenous). When the steel has the required amount of carbon, the vessel is tilted and the molten steel is tapped off below the level of the slag, and then the slag is poured out by tilting the vessel to the other side. The whole process is completed in 40 to 45 minutes.

1. It can produce high quality steel on a large scale.

2. It allows good control over the carbon content and the composition of the steel, because the process is slow and samples can be taken.

3. It uses scrap iron as well as pig iron as raw materials.

4. It removes impurities such as \(Si\), \(P\) and \(S\) effectively, and different grades of steel can be produced.

1. It can produce high quality steel on a large scale within a short time of 30 to 45 minutes.

2. The steel is of better quality than that of the open hearth process (free from nitrogen, because pure oxygen is used).

3. The process is autogenous, that is, the heat required is obtained from the oxidation reactions, so no fuel is needed.

4. It causes very less air pollution and most of the steel of the world is made by this method.

The metal is iron (\(Fe\)) and its oxide ore is haematite, \(Fe_2O_3\).

This zone (700 to 1000 degrees Celsius) lies in the middle part of the furnace. The limestone decomposes into quicklime (a basic flux) and the quicklime combines with silica, the acidic gangue, to form the fusible slag:

The metal \(M\) is iron (\(Fe\)): atomic number 26, atomic mass 55.85 amu, 3d series, period 4 and group 8 (melting point 1530 degrees Celsius, boiling point 2750 degrees Celsius, specific gravity 7.86).

The chief ore is haematite, \(Fe_2O_3\).

Haematite is crushed and powdered, concentrated by gravity separation (washing with water), and calcined with a little coal in a limited supply of air to remove moisture and volatile impurities. The calcined ore is mixed with coke and limestone (8 : 4 : 1) and smelted in the blast furnace with a blast of hot air. Coke burns to give heat and carbon monoxide, which reduces the ore:

Limestone gives \(CaO\), which removes silica as slag (\(CaO+SiO_2\rightarrow CaSiO_3\)). The molten iron is tapped from the bottom as pig iron, which is refined to cast iron, wrought iron or steel.

With dilute sulphuric acid iron liberates hydrogen and forms ferrous sulphate (iron lies above hydrogen in the electrochemical series):

With concentrated sulphuric acid it gives sulphur dioxide:

(With excess of hot concentrated acid the product is ferric sulphate: \(2Fe+6H_2SO_4\rightarrow Fe_2(SO_4)_3+3SO_2\uparrow+6H_2O\).)

Pure iron is not affected by dry air, but when iron is exposed to moist air for a long time, a reddish brown layer of hydrated ferric oxide (rust) forms on its surface by electrochemical corrosion:

1. Iron is used as a construction material for roads, bridges, buildings, ships, pipes and vehicles, and it is used to make steel and other alloys.

2. Iron is an essential ingredient of haemoglobin in blood for the transport of oxygen, and it is used to make magnetic tapes, hard drives and cooking pots and pans.

The suitable ore is the iron oxide, haematite, \(Fe_2O_3\), which is the chief ore of iron (the iron carbonate present is siderite, \(FeCO_3\), which is changed to the oxide during calcination). The impurities (gangue) present are silica, \(SiO_2\), and alumina, \(Al_2O_3\).

Gravity separation (hydraulic washing). The powdered ore is washed with a stream of running water: the heavy ore particles settle down and the lighter impurities of silica and alumina are washed away. If magnetic impurities are present, magnetic separation is used.

1. Crushing and pulverization: by jaw crusher and ball mill.

2. Concentration: gravity separation with running water.

3. Calcination: heating with a little coal in a limited supply of air. Moisture is removed, siderite is decomposed, and ferrous oxide is oxidised to ferric oxide (this prevents the loss of iron as ferrous silicate slag, \(FeO+SiO_2\rightarrow FeSiO_3\)):

4. Smelting in the blast furnace: the charge (calcined ore : coke : limestone = 8 : 4 : 1) is fed from the top, and hot air is blown from the tuyeres.

In the zone of combustion (1300 to 1500 degrees Celsius) coke burns, \(2C+O_2\rightarrow2CO+\text{heat}\). In the zone of fusion (1200 to 1300 degrees Celsius) \(CO_2+C\rightarrow2CO\), and the spongy iron melts and takes up carbon. In the zone of slag formation (700 to 1000 degrees Celsius) \(CaCO_3\rightarrow CaO+CO_2\) and \(CaO+SiO_2\rightarrow CaSiO_3\); alumina is also taken into the slag. In the zone of reduction (400 to 700 degrees Celsius) \(Fe_2O_3+3CO\rightarrow2Fe+3CO_2\).

The molten pig iron and the slag are tapped from different outlets at the hearth.

Steel (0.2 to 2% carbon) lies between the two: it is hard and strong and also malleable.

Iron is coated with zinc (galvanization) to protect it from rusting. The zinc layer keeps out air and moisture, and zinc, being more electropositive than iron, acts as a sacrificial anode. If the coating is scratched, zinc is oxidised first (\(Zn\rightarrow Zn^{2+}+2e^-\)) and the iron is not corroded.

The impurities (\(C\), \(Si\), \(S\), \(P\), \(Mn\)) of the pig iron are oxidised by haematite (and air) on an open hearth heated by burning producer gas. The oxides of carbon and sulphur escape as gases, and the oxides of silicon, phosphorus and manganese combine with the flux and are removed as slag. Finally a calculated amount of spiegeleisen is added to get the desired quality of steel.

Pig (cast) iron (70 to 80%) and scrap (waste) iron (20 to 30%), haematite as the oxidising agent, lime or limestone as the flux, spiegeleisen (added at the end), and producer gas as the fuel. The furnace is lined with silica (acidic process) or dolomite (basic process).

The impurities are carbon, sulphur, silicon, phosphorus and manganese. They are oxidised by haematite:

The gases \(SO_2\) and \(CO\) escape. In the acidic process the oxides are removed as slag by the flux, \(MnO+SiO_2\rightarrow MnSiO_3\). In the basic process (high phosphorus) lime is used as the basic flux: \(CaCO_3\rightarrow CaO+CO_2\) and \(P_2O_5+3CaO\rightarrow Ca_3(PO_4)_2\) (slag).

The open hearth process is slow, so the operator can take samples of the metal from time to time and analyse them. The oxidation can be stopped at the moment when the carbon content has fallen to the low value which is needed (below about 0.25%), and so there is good control over the carbon content. This gives soft, formable steel for vehicle body parts.

Merits: it produces high quality steel on a large scale, it gives control over the carbon content, it can use scrap iron as well as pig iron, it removes \(Si\), \(P\) and \(S\) effectively, and many grades of steel can be made.

Demerits: it is slow and time consuming, it needs a large amount of fuel and energy, the investment is high, and it is not eco-friendly because it gives harmful gases like \(CO\), \(CO_2\) and \(SO_2\).

Carbon-rich molten pig iron is poured into the LD converter and a jet of pure oxygen is blown on it through a water-cooled lance. The oxygen oxidises carbon, silicon, phosphorus and the other impurities. The oxides of carbon escape as gases and the other oxides are removed as slag by the basic flux \(CaO\). The oxidation is exothermic, so the process is autogenous.

Molten pig iron (the main raw material), a small amount of steel scrap, pure oxygen gas and quicklime (\(CaO\)) as the basic flux.

It causes very little air pollution. As pure oxygen is used instead of air, no nitrogen is blown through the metal. No external fuel is burnt, so there is less smoke, and the carbon monoxide that comes out can be collected through the fume hood and used as a fuel gas.

1. It produces high quality steel on a large scale within 30 to 45 minutes.

2. The steel is of better quality than that of the open hearth process.

3. It is autogenous, so the thermal energy is generated during oxidation and no fuel is needed.

4. It causes very less air pollution, and the majority of the world’s steel is manufactured by this method.

The patches are rust. They are caused by the corrosion (rusting) of iron when it is in contact with the moist air containing oxygen. Iron is oxidised by the oxygen in the presence of water, and the product forms a reddish brown flaky layer.

Hydrated ferric oxide, \(Fe_2O_3\cdot xH_2O\).

The air in coastal areas is very humid and carries sea salt (mainly sodium chloride). The salt dissolves in the film of moisture on the iron and makes it a good electrolyte, which increases its conductivity and lets the electrons and ions move faster between the anodic and the cathodic sites. So the electrochemical corrosion is much faster than in inland areas.

Figure: Electrochemical theory of rusting of iron

A tiny voltaic cell is formed on the iron surface. The part with lower reduction potential is the anode, \(Fe\rightarrow Fe^{2+}+2e^-\), and the electrons move through the metal to the cathodic region, where \(O_2+4H^++4e^-\rightarrow2H_2O\). The overall reaction is \(2Fe+4H^++O_2\rightarrow2Fe^{2+}+2H_2O\). The moist surface with dissolved gases works as the electrolyte. The ferrous ions are further oxidised, \(4Fe^{2+}+O_2+4H_2O\rightarrow2Fe_2O_3+8H^+\), and the product combines with water to give rust, \(Fe_2O_3\cdot xH_2O\).

1. Galvanization: coating the iron with a thin layer of zinc, which gives barrier as well as sacrificial (cathodic) protection. More electropositive metals like \(Zn\), \(Al\) and \(Mg\) in contact with the iron are corroded in place of it.

2. Forming alloys: alloying iron with \(Cr\), \(Ni\), \(Mn\) or \(Si\), for example stainless steel, which resists corrosion. Chromium is the most effective metal for this.

3. Forming a protective barrier: coating the surface with paint, grease, oil, enamel or varnish.

4. Bower-Barff process: iron is exposed to steam so that a thin protective layer of magnetic oxide forms on the surface.

5. Corrosion inhibitors: chemicals such as phosphates, chromates, amines and soaps, which neutralise the corrosive substances or form a protective layer.