IRON


IRON


4. Iron, Chromium and Manganese (ISBN 1-869860-24-1)

Contents: Introduction; Calcium; Crystals containing calcium; Limestone; Dissolving limestone; Caves and caverns; Calcium carbonate; Building stone; Calcium oxide; Using calcium oxide; Calcium hydroxide; Limewater; Calcium in the soil; Calcium sulphate; Bones; Magnesium; Protecting with magnesium; Hard water; Softening water; Antacids; Key facts about calcium and magnesium; The Periodic Table; Understanding equations; Glossary of technical terms; Index

Iron cover


Iron is one of the world's workhorse elements, found in most of the structures we make, from bridges and skyscrapers to computers and fencing wire.

Iron is cheap to obtain, easy to shape and very strong. For all these reasons it is used more widely than any other metal. But all of the advantages of iron have to be balanced against one major disadvantage: it is a fairly reactive element, prone to rust when exposed to damp air.

Iron (whose chemical symbol, Fe,comes from the Latin ferrum) is now one of the most commonly used metals of modern times. But it was not always so. Although the first use of iron dates back some three thousand years to the period of archaeology called the Iron Age, until the last century iron was difficult to find and work and expensive to use.

Pure iron is a soft silvery-grey metal. It can be bent and stretched at room temperature, and at 1535°C it will melt. This temperature is much higher than the temperature at which wood burns. Earlier civilisations found iron so difficult to use because they could not produce such high temperatures to work iron.

In fact iron has only been widely available since the Industrial Revolution of the 18th and 19th centuries. At this time, inventors like Abraham Darby learned how to obtain large quantities of iron economically using coke. With iron at last cheap and plentiful, the people of the 19th century led the world into the engineering age.

Iron is not just found as a metal. Compounds of iron, for example, are found in almost all living things: iron compounds are a vital nutrient for all plants and animals, and they make our blood appear red in colour.

Iron compounds are also the basis of a rich variety of natural colours, both in rocks and in nature.

Iron, along with very few other elements, possesses the property of magnetism. This property makes iron essential in compasses and inside every electric motor. Indeed, this makes iron one of the most versatile of all the elements.

Iron is a dense, silvery-grey metal. It is the most widely used of all the metal elements because it is common, is cheap to produce from its ores, can be bent while cold, and is strong.

However, some of its properties are less welcome. For example, it is quite reactive, especially with air and water, a process called corrosion.

The reactivity of iron

Some metals are more reactive than others. Gold, for example, hardly reacts at all, which is why it stays bright and does not tarnish (develop a dull oxide coating). On the other hand, some metals, like potassium, react vigorously with oxygen (corrode) as soon as they are placed in the air.

Metals can be placed in order of how vigorously they react, in a list called a reactivity series (see right).

Copper is near the bottom of the reactivity series, while iron is higher up. Therefore iron will always corrode when placed in a solution containing a copper compound such as copper sulphate. On the other hand, iron is below magnesium, which is why magnesium will always corrode when placed next to iron. This is shown in the picture on the right, where a small strip of magnesium has been wrapped around the shank of an iron nail and placed in a bottle containing water and an

Properties of iron

A soft, silver­grey metal, chemical symbol Fe
Essential for plant growth
Fourth most plentiful element at the surface of the Earth
Has no smell
Rusts easily
Forms the colouring of most rocks
Has no taste
Makes up over 5% of the Earth's crust
Magnetic
Found most commonly as iron oxide in rocks
Easily forms alloys with other metals
Atomic number 26, atomic weight about 56

Reaction of iron and copper

1 Iron filings (the dark material) are placed in a small pile in a dish. Copper sulphate (the blue solution) is added.
The red colour is caused by an indicator in the water. The clear water shows a neutral reaction. As the magnesium corrodes it produces magnesium hydroxide, an alkali, which turns the region near the corroding magnesium red.
2 After a while a number of changes have occurred because the iron filings have reacted with the copper sulphate. There is no acid or other chemical here.

The solution becomes a very pale green. The edges of the iron filings have changed to a reddish-brown (coppery) colour, looking a little like a reef fringing a coral island.

The reason for the changes is that a chemical reaction has occurred. Some of the iron has gone into solution. The copper "reef" has been deposited (precipitated) out of the solution onto the iron filings.

The copper in the copper sulphate makes the solution blue; iron sulphate solutions are green, so the green colour shows that the copper and iron have "swapped", and the solution is now iron sulphate.

However, not all of the iron has reacted. There is far more iron than copper, so a complete swap cannot be achieved. If the iron were placed in a huge vat of copper sulphate it would eventually react entirely and disappear.

EQUATION: Reaction between iron and copper sulphate

Iron + copper sulphate = iron sulphate + copper

Fe(s) + CuSO4(aq) = FeSO4(aq) + Cu(s)

Corrosion of iron: rust

The environment is a hazardous place for materials. Even in "clean" country air, materials will slowly show signs of surface change because of the effects of water and gases in the air. The change in the surface of materials is a reaction with oxygen in the air. It is called corrosion and produces a coating called an oxide.

Iron is particularly susceptible to corrosion, known as rusting, in damp air or oxygen-rich water because it is a reactive material.

The materials that most often show signs of corrosion are metals. The oxide coating that develops on the surface of some metals is so thin it is invisible.

Look at a clean iron or steel nail and the surface looks unaffected because the oxide layer is so thin. When the oxide coating is thicker, it may appear as a discolouring, or tarnishing, of the surface.

However, unlike some other metals such as aluminium and copper, iron's oxide coating is not able to keep water and oxygen out. On the contrary, it is a porous coating, which is why it rusts.

Steel nails need water and oxygen from the air to rust quickly. Water alone is a poor corroding agent. However, when oxygen is present, such as when water is left uncovered, nails left in water will rust (oxidise) very rapidly.

The chemistry of smelting

The processes that occur inside the blast furnace are quite complex, and different reactions happen at the top and bottom of the furnace.

The objective is to ensure that the right chemical reactions occur in each part of the furnace so that controlled and continuous processing is achieved.

Iron oxide sinks down the furnace where it reacts with carbon monoxide gas coming up from the bottom of the furnace. The two then react to produce carbon dioxide gas and liquid iron.

After the last reaction, the iron that flows from the bottom of the furnace into the moulds (known as pigs) is about 95% iron with various proportions of carbon and other impurities.

EQUATION: Oxidation of coke

carbon + oxygen = carbon dioxide

C(s) + O2(g) Í CO2(g)
EQUATION: Carbon dioxide reduced

carbon dioxide + carbon = carbon monoxide

CO2(g) + C(s) Í 2CO(s)
EQUATION: Iron oxide is reduced

Iron oxide + carbon monoxide = iron metal + carbon dioxide

Fe2O3(s) + 3CO(g) Í 2Fe(l) + 3CO2(g)

Gases pass up through the mixture in the furnace. Liquids descend through the mixture in the furnace. Carbon monoxide reacts with iron ore to produce molten iron and carbon dioxide gas. The molten iron sinks to the bottom of the furnace. The waste gases pass out through pipes at the top of the furnace. The oxygen reacts with the coke to produce carbon dioxide gas, which in turn reacts with more of the coke to produce carbon monoxide.Cold iron ore, coke and limestone are added as a charge into the top of the furnace.Oxygen gas is pumped in.The molten iron, which is densest, is tapped off as pig iron. The slag, less dense than the iron, is tapped off.