Methods of extracting metals

The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To become useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in high enough concentration to make it economic to extract the metal.

Ores are mined. They may need to be concentrated before the metal is extracted and purified. The economics of using a particular ore may change over time. For example, as a metal becomes rarer, an ore may be used when it was previously considered too expensive to mine.

Metals are produced when metal oxides are reduced (have their oxygen removed). The reduction method depends on the reactivity of the metal. For example, aluminium and other reactive metals are extracted by electrolysis, while iron and other less reactive metals may be extracted by reaction with carbon or carbon monoxide.

Gold, because it is so unreactive, is found as the native metal and not as acompound. It does not need to be chemically extracted from its ore, but chemical reactions may be needed to remove other elements that might contaminate the metal.

The transition metals are placed in the periodic table in a large block between groups 2 and 3. Most metals (including iron, titanium and copper) are transition metals.

Common properties

The transition metals have these properties in common:

• they are metals
• they are good conductors of heat and electricity
• they can be hammered or bent into shape easily

The transition metals are useful as construction materials. They are also useful for making objects that need to let electricity or heat travel through them easily.

Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.

 Carbon is more reactive than iron, so it can push out or displace the iron from iron oxide. Here are the equations for the reaction:

iron oxide + carbon → iron + carbon dioxide

2Fe₂O₃ + 3C → 4Fe + 3CO₂

In this reaction, the iron oxide is reduced to iron, and the carbon is oxidised to carbon dioxide.

In the blast furnace, it is so hot that carbon monoxide will also reduce iron oxide:

iron oxide + carbon monoxide → iron + carbon dioxide

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Copper is soft and easily bent and so is a good conductor of electricity, which makes it useful for wiring. Copper is also a good conductor of heat and it does not react with water. This makes it useful for plumbing, and making pipes and tanks.

Copper ores

Some copper ores are copper-rich – they have a high concentration of copper compounds. Copper can be extracted from these ores by heating them in a furnace, a process called smelting. The copper is then purified using a process called electrolysis.

Electricity is passed through solutions containing copper compounds, such as copper sulfate. During electrolysis, positively charged copper ions move towards the negative electrode and are deposited as copper metal.

Phytomining, bioleaching and scrap iron

Some plants absorb copper compounds through their roots. They concentrate these compounds as a result of this. The plants can be burned to produce an ash that contains the copper compounds. This method of extraction is calledphytomining.

Some bacteria absorb copper compounds. They then produce solutions called leachates, which contain copper compounds. This method of extraction is calledbioleaching.

Copper can also be extracted from solutions of copper salts using scrap iron. Iron is more reactive than copper, so it can displace copper from copper salts. For example:

iron + copper sulfate → iron sulfate + copper

Aluminium and titanium are two metals with a low density. This means that they are lightweight for their size. They also have a very thin layer of their oxides on the surface, which stops air and water getting to the metal, so aluminium and titanium resist corrosion. These properties make the two metals very useful.Aluminium is used for aircraft, trains, overhead power cables, saucepans and cooking foil. Titanium is used for fighter aircraft, artificial hip joints and pipes in nuclear power stations.

Extraction

Unlike iron, aluminium and titanium cannot be extracted from their oxides by reduction with carbon. You do not need to know any details of how these metals are extracted, but existing methods are expensive because:

• the processes have many stages
• large amounts of energy are needed

Recycling

Aluminium is extensively recycled because less energy is needed to produce recycled aluminium than to extract aluminium from its ore. Recycling preserves limited resources and requires less energy, so it causes less damage to the environment.

The properties of a metal are changed by adding other elements to it. A mixture of two or more elements, where at least one element is a metal, is called analloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.

Pure copper, gold, iron and aluminium are too soft for many uses. They are mixed with other similar metals to make them harder for everyday use. For example:

• brass, used in electrical fittings, is 70 percent copper and 30 percent zinc
• 18 carat gold, used in jewellery, is 75 percent gold and 25 percent copper and other metals
• duralumin, used in aircraft manufacture, is 96 percent aluminium and 4 percent copper and other metals.

Pure iron is soft and easily shaped because its atoms are arranged in a regular way that lets layers of atoms slide over each other. Pure iron is too soft for many uses. Iron from the blast furnace is an alloy of about 96 percent iron, with carbon and some other impurities. It is hard, but too brittle for most uses, so most iron from the blast furnace is converted into steel by removing some of the carbon. 

Steel

Carbon is removed from molten iron by blowing oxygen into it. The oxygen reacts with the carbon, producing carbon monoxide and carbon dioxide, which escape from the molten metal. Enough oxygen is used to achieve steel with the desired carbon content. Other metals are often added, such as vanadium and chromium, to produce alloys with properties suited to specific uses.

There are many different types of steel, depending on the other elements mixed with the iron.