Extraction of Metals.

Occurrence of Metals in the Earth Crust

Metals are found in the earth’s crust. The least reactive metals are found native as elements e.g. silver, gold, but most are found in compounds, usually oxides or sulphides.

The occurrence in nature of metals will affect the cost of extracting and engineering them. Most metals are found as ores usually either as an oxide or a sulphide.

Sulphides are even rarer than oxides as these were formed when the earth’s crust was solidifying and there was plenty of sulphur from volcanic activity. Oxides became present later in the earth’s history when oxygen became more abundant.

Redox reductions are used to obtain most of the metals that we all take for granted.

Most metals are found occurring naturally in rocks called ores. They are in compounds, chemically bonded to other elements. However, there are metals with very low reactivity that can be found as the elements themselves.

We say these metals are found native in the ground.

Examples of metals found natively are copper, silver, gold and platinum. Copper and silver are also mined as ores.

Gold is expensive as it is rare and difficult to separate from the waste rock. All metals above copper in the Reactivity series are found as ores.

Rocks that contain a high enough percentage of a metal to be extracted commercially are known as ores (what percentage is required depends on the value of the metal).

Aluminium and iron are the most abundant metals in the earth’s crust, and they are widely distributed around the earth’s crust, which explains why they are so widely used.

There are some other commonly used metals, such as nickel and copper that are scarce in the earth’s crust, but are in high grade ores in a few locations.

Key: Non-metals

Metals

Most compounds are found as oxides or sulphides, but the sulphides are usually converted to oxides before extraction. Most metals are extracted using reduction reactions usually of metal oxides.

This reduction of the metal compound is usually done in one of three main ways:

by heating with carbon (in the form of coke)
by heating with a more reactive metal (active metal)
by electrolysis

Method to be used depends on:

the energy requirements – extraction uses large amounts of energy (electricity and or heat)
the cost of reductant (reducing agent) – carbon in the form of coke, is cheap and widely used but sometimes more reactive metals are required which are very expensive.
the metal purity required – the higher the required purity, the greater the cost in obtaining that purity

Elements that are low in the Reactivity series can be found native as the element themselves or can be displaced by a more reactive element.

Metals of medium reactivity, that is all above copper in the series, are only found as ores, either as a sulphide or an oxide. A sulphide, such as lead sulphide, will be roasted to convert it into lead oxide + sulphur dioxide (used to make sulphuric acid).

2PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g)

Then the lead oxide can be reduced to obtain the lead metal. Carbon can be used as a reducing agent as carbon is more reactive than lead.

C(s) + 2PbO(s) CO2(g) + 2Pb(s)

Highly reactive metals are the hardest to extract since carbon will not displace metals of higher reactivity, so electrolysis is used. This uses a lot of energy, so highly reactive metals are the most expensive to extract. Therefore there is a link between a metal’s place in the Reactivity series and how easy it is to extract. The more reactive a metal is, the more difficult it is to extract chemically from its ore.

Environmental Factors and Recycling

The extraction of metals involves the consumption of large amounts of energy, and often produces a range of harmful, pollution causing gases as a by-product, which can be toxic.

Many ores contain sulphides, which form sulphur oxides in the extraction process. If these sulphur oxides escape into the atmosphere they react with the moisture in the air to form acid-rain, which can cause detrimental damage to the environment.

Carbon has been used as a reductant for decades, it reacts with hot air which results in the formation of carbon monoxide ...

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Environmental Factors and Recycling

The extraction of metals involves the consumption of large amounts of energy, and often produces a range of harmful, pollution causing gases as a by-product, which can be toxic.

Carbon has been used as a reductant for decades, it reacts with hot air which results in the formation of carbon monoxide and carbon dioxide. Carbon monoxide is a toxic gas, carbon dioxide is a ‘greenhouse’ gas and is thought to increase global warming.

Some extraction processes use dangerous chemicals that could cause serious health problems if they were released into the atmosphere e.g. the extraction of titanium uses large quantities of chlorine.

A lot of metals are now being recycled. This helps to reduce the exploitation of valuable and finite mineral resources. Recycling also helps to reduce the requirement of energy. The two most commonly used metals are aluminium and iron, both metals are now extensively recycled.

Examples of how metals are recycled:

‘scrap iron’ is used in the manufacturing of steel, as long as it is high quality. The scrap iron is mixed with the iron from the blast furnace and is converted into steel.
It is really sensible to recycle used aluminium since the extraction of aluminium by electrolysis uses a great deal of energy. Aluminium cans can be recycled by using the collection point of recyclable material, where they are collected, then crushed and melted down to be re-used.

Reduction of Metal Oxides with Carbon

Carbon is a non-metal, but it can be placed in the Reactivity series of metals. It fits in between aluminium and zinc. This means that carbon can displace any metal below aluminium in the Reactivity series.

The carbon comes from coal which is cheap and there is plenty of it. Thus making metals extracted using carbon relatively inexpensive, as long as the ore is readily available.

Nearly all metals can be extracted in this way, but many are not because:

Metal carbides can be formed (e.g. TiC, WC or ZrC) giving metal unwanted properties
The temperature required is excessively high for reactive metals

Extraction of IRON in the Blast Furnace

Carbon is needed in the extraction of iron. To do this a giant blast furnace is used to create a continuous process of extracting iron from its ore. The raw materials are fed into the top of the furnace, these are:

Iron ore (mainly haematite, iron (III) oxide – Fe2O3)
Coke (a cheap form of carbon, made from coal)
Limestone (to get rid of sandy impurities)
Hot air (is blasted into furnace to increase the temperatures and react with Carbon to make CO2)

Coke (carbon) is burnt in hot air, producing CO2 and a lot of heat (needed for all the other reactions).

C(s) + O2(g) 2CO(g)

In high temperatures in the furnace, more coke is reacted with the carbon dioxide produced, forming CO.

C(s) + CO2(g) 2CO(g)

The iron oxide is reduced to iron by the carbon monoxide and some coke (due to the heat the iron is molten, and so can be run off).

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

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

The limestone (calcium carbonate) decomposes (splits up into simpler substances) in the heat.

CaCO3(s) CaO(s) + CO2(g)

The calcium oxide (an alkali) reacts with acidic impurities such as sand (silicon dioxide) producing a molten mixture of compounds, mainly calcium silicate, called slag which floats on the iron.

Slag is used in construction (e.g. road making and cement manufacture).

CaO(s) + SiO2(s) CaSiO3(s)

Pollution problems arise from using carbon, giving CO2 (greenhouse gas) and CO (toxic). Also the roasting of sulphide ores to give oxides leads to SO2 pollution, resulting in acid rain.

Steel Making

For this process the iron from the blast furnace is used to make steel. The iron obtained contains many impurities, up to 3 to 4% carbon plus some other non-metals.

The impure iron is very brittle – it cracks easily – and is called ‘cast’ or ‘pig’ iron (after the name given to the moulds).

Most of this will be turned into steel, which is much tougher.

Steel is mainly iron (over 98%) with a small amount of carbon left in it. Other metals can also be added to improve its properties for particular requirements.

To do this we must remove the carbon and other impurities. This can be done by blowing oxygen onto the molten pig iron using a Basic Oxygen Converter.

The impurities in pig iron:

carbon 3% to 5%

silicon 1% to 2%

phosphorous (0.05% to 1.5%)

sulphur (0.05% to 1%)

The carbon burns in the oxygen and

escapes as carbon dioxide:

C(s) + O2(g) CO2(g)

The sulphur can also be oxidised and escape

from the pig iron in the form of sulphur

dioxide gas:

S(l) + O2(g) SO2(g)

These gases, however, cause environmental problems since CO2 is a greenhouse gas and SO2 is known to cause Acid Rain.

The waste product gases can be ‘scrubbed’ before being released into the atmosphere. This involves the gases being passed through ‘scrubbers’ containing basic compounds to react with the acidic gases given off.

To avoid the expense of these gas scrubbers and the production of pollutants which are harmful to the environment, magnesium powder can be passed/injected into the molten pig iron.

This reacts vigorously to produce magnesium sulphide which floats on top and can be scraped from the surface.

Mg(s) + S(l) MgS(s)

This reaction, like the other redox reactions in the converter, is exothermic. Giving out the heat needed to keep the pig iron molten while most of the impurities are removed.

The phosphorus and silicon are removed in the converter by adding lime (calcium oxide). The phosphorous (V) oxide (P4O10) and silicon dioxide (SiO2) – the products of the reaction – are removed as slag.

6CaO + P4O10 2Ca3(PO4)2

calcium phosphate slag

Scrap steel is added into the converter(to recycle it and to lower the temperature in the converter).

This process is conducted one converter full at a time, so it is called a Batch Process (non-continuous).

Adding Other Metals:

Small amounts of other metals can be added to steel to give it special and specific properties. For example, stainless steel is made by alloying iron with a little chromium and nickel.

This steel will not rust or react with many things, so makes it ideal for instruments used by surgeons, as they can easily be sterilised. Stainless steel is also used for cutlery and in kitchens as it is a strong resistant metal.

Reduction of Metal Oxides by Electrolysis of Melts

Electrolysis is used to extract metals like aluminium from its ore, bauxite (Al2O3). Electrolysis is usually used where carbon reduction is not suitable (e.g. Aluminium is more reactive than carbon), but not if the metal has to be very pure.

The metal ions in a salt (e.g. sodium chloride) can be reduced in electrolysis, i.e. they gain electrons at the negative electrode (cathode).

At the cathode:

Na+(l) + e- Na(l)

Sodium is extracted from sodium chloride in a Downs cell (right).

Electrolysis is used for the most reactive metals as carbon will not displace them. The most important of the reactive metals is aluminium. Aluminium has many useful properties, it conducts heat and electricity well, it has a low density and it does not corrode because it is protected by an oxide layer on its surface.

To extract Aluminium, you need:

The raw materials. For the extraction of aluminium these are purified aluminium oxide (AL2O3) and cryolite (Na3AlF6)
The aluminium oxide must be molten or dissolved for the ions to conduct electricity, and so is dissolved in molten cryolite (this requires a lower temperature than using molten aluminium oxide).
The electrodes are made of graphite. The positive electrode burns due to the oxygen being produced there (forming carbon dioxide), and so the positive electrode has to be replaced frequently.
This process involves a lot of energy, so saving costs by dissolving aluminium oxide instead of melting it and being situated near to a cheap supply of electricity (e.g. hydroelectric power station) is necessary to reduce what would amount to massive costs.

Reduction of Metal Halides, by active metals

Carbon is a really cheap reducing agent, but is not suitable to extract all metals below itself in the Reactivity Series.

Doing so would result in the formation of Carbides.

For example, tungsten extracted using carbon would form tungsten carbide, W2C, as a stable product.

The tungsten used to make the filament in light bulbs is extracted by reduction using hydrogen gas.

heat

WO3(s) + 3H2(g) W(s) + 3H2O(g)

Titanium is unable to be extracted using carbon because titanium carbide (TiC) is formed not titanium (similar reactions occur for vanadium, tungsten and molybdenum).

This does not follow the trend in the extraction of metal, being more difficult to extract if it is a more reactive metal.

Titanium has many uses for its expensive high-performance alloys. The expense mainly comes from the fact that it has to be extracted using a more reactive metal as the reducing agent (e.g. magnesium or sodium) and is made in Batches (an inefficient, costly method of reduction).

This arises from the fact that the reactive metals being used as reducing agents have already had to be extracted from their ores, despite that its main ore (ilmenite – a mixed oxide of iron and titanium) is actually fairly common.

Titanium oxide (IV) is separated from the ore to form rutile, then converted to titanium (IV) chloride.

≈ 1000°C

TiO2(s) + C(s) + 2Cl2(g) TiCl4(l) + CO2(g)

rutile coke

Notice that carbon is used in this stage, but not the reaction below. TiCl4 is a simple molecular liquid. The TiCl4 is purified by fractional distillation in an Argon atmosphere, then reduced with magnesium or sodium.

heat

TiCl4(l) + 4Na(l) Ti(s) + 4NaCl(l)

This process is called the Kroll process.

Titanium is a very useful metal as it is abundant, has a low density and is corrosion resistant - it is used for making strong, light alloys for use, for example, in aircraft.
Titanium has to be very pure to have these useful properties and so electrolysis is not suitable. The metal is extracted by reaction with a more reactive metal (e.g. Mg, Na.).

This process is expensive due to the heat, the Na/Mg & the argon, but titanium metal is so useful it is used in limited quantities.

Oxide and Sulphide Ores

Most metals are found as ores. Usually either as an oxide or a sulphide. Sulphides are more rare than oxides, as these were formed when the earth’s crust was solidifying and there was plenty of sulphur from volcanic activity.

Roasting Process:

It is the process of beating the concentrated ore in the presence of excess air. Volatile impurities will be expelled. Zinc sulphide in its ore, zinc blende (ZnS) is roasted to obtain zinc oxide (ZnO)

2ZnS(s) + 3O2(g) 2ZnO(s) +2So2(g)

from air

Reduction:

The conversion of metal oxide into metal (by removal of oxygen) is called Reduction. Generally, the 3 methods used are:

Reduction by heating the oxide
Chemical reduction
Electrolytic reduction

Reduction by Heating the Oxide (Heating Process):

The oxides of metals that are low in the Reactivity series can be reduced to obtain the metals by heating their ore. For example, mercuric oxide (HgO), obtained from its ore mercuric sulphide (HgS), when heated to about 300 degrees C, forms mercury metal.

Heat

2HgO 2Hg + O2

(Reduction) mercury

Roasting and reduction processes are carried out simultaneously.

Chemical Reduction Process:

Under this process the oxide of metals that are in the middle of the Reactivity Series are reduced to free metals using chemical reducing agents such as carbon, aluminium, sodium or calcium.

Electrolytic Reduction Process:

The oxides (or chlorides) of highly reactive metals (such as sodium, magnesium, aluminium and calcium) cannot be reduced by using carbon or aluminium. Electrolytic reduction is used for the above metals. The cathode acts as a powerful reducing agent by supplying electrons to reduce the metal ions into metal. Fused alumina (molten aluminium oxide) is electrolysed in a carbon lined iron box. The box itself is the cathode. The aluminium ions are reduced by the cathode.

At the cathode:

electrolysis

Al 3+ + 3 e- Al

Aluminium ion aluminium atom

This concludes my investigation on the extraction of metals.

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