3.2.7 Extraction of Metals - Principles of metal extraction

Specification

The metals industry

Metals are one of the most commonly used resource in the world. Very few metals are found native, i.e. uncombined in nature, they must be mined, and extracted from their ores before use. Taken as a whole the most important driving force behind production methodology is price. The raw materials must be as cheap as possible, the processes must be cost efficient and the metal must be profitable to manufacture.

All phases of production are analysed with these considerations in mind.

What follows is a look at some of the most important metals and their manufacturing processes

The whole process depends very much on the type of metal being extracted, but can be broken down into roughly the same steps:

• 1 Mining of ore containing rock
• 2 Separation, purification or preparation of useful ore
• 3 Extraction
• 4 Purification of metal

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1. Mining of ore containing rock

The composition of rock around the world varies greatly and locations with metal bearing ore have been sought ever since man was able to extract metals. Nowadays the search is still going on for important deposits of rock with high percentages of the mineral in question. This search is now taking place under the sea and in other inhospitable environments.

Recently, for example, rock containing an appreciable percentage of rare earth elements has been discovered under the pacific ocean. This is a particulaly important discovery as virtually 99% of known working deposits are in china and rare earths are essential in the manufacture of the strong neodymium magnets needed for the computer industry

2. Separation, purification or preparation of useful ore

Very few metal ores occur in a pure enough form to be used directly in the extraction process. The first stage is to separate the useful ore from the rock. This may not be necessary in some cases, for example, the extraction of iron, but essential in the extraction of aluminium.

This separation may be physical, such as floatation, or chemical such as digestion of the required compound in a strong base or acid followed by reprecipitation and filtration.

Most ores are either oxides or sulfides. The sulfides are usually converted to oxides by roasting in air. This tends to release sulfur in the form of sulfur(IV) oxide, a pollutant and acidic gas. However, it is also a useful gas in that it is used for the manufacture of sulfuric acid by the contact process.

3. Extraction of metal from ore

Metals are all electropositive and need to be reduced to become metallic elements. Hence, all extraction processes use reduction. For the less reactive metals chemical reduction suffices, but for the more reactive metals electrochemical reduction is needed.

4. Purification of metal

Metals that are extracted by reductive processes usually need to be further processed to make them industrially useful.

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Iron, manganese and copper

These three common metals exemplify the use of cheap raw materials. Both manganese and copper production are similar to that of iron.

The blast furnace (Bessemer) process for iron manufacture uses iron ore, coke, air and limestone.

The iron ore is heated with coke in a blast of hot air, which converts the coke to carbon monoxide, which in turn reduces the iron ore to iron. The limestone is added to remove the silicon dioxide impurities as slag (calcium silicate)

1 Coke reacts with air to form carbon monoxide

2C(s) + O₂(g) 2CO(g)

2 Carbon monoxide reduces the iron oxide

3CO(g) + Fe₂O₃(s) 3CO₂(g) + 2Fe(l)

3 Limestone decomposes at high temperature to calcium oxide

CaCO₃(g) CaO + CO₂(g)

4 Calcium oxide (basic) reacts with silicon dioxide (acidic)

CaO(s) + SiO₂(s) CaSiO₃(l)

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Aluminium

It is not practicable to make it by reducing the ore with coke, since too high a temperature would be required , > 2000ºC. This would be expensive, dangerous and technically difficult to prevent re-oxidation to alumina.

Aluminium is the most abundant metallic element in the earth's crust, 8.3% by mass. Al cannot be extracted from aqueous solution by electrolysis as hydrogen is obtained. It is not practical to make it by reducing the ore with coke, as aluminium is too reactive

Mg reduction is too expensive, as Mg is more expensive than Al - less demand for Mg

The most common ore is BAUXITE which is 40-60% aluminium oxide (Al₂O₃). Bauxite is purified to pure alumina (Al₂O₃) at the mine to save on transportation costs. The factory or smelter needs to be near a deep water port to take the large cargo ships.

At the smelter alumina is mixed with cryolite Na₃AlF₆. This reduces the temperature needed from the melting point of pure alumina from 2200ºC down to 960ºC, saving money on energy costs. The lower temperature also means a safer operation

The molten alumina and cryolite is electrolysed using a D.C. current of 150,000 amps at about 5V, meaning a power consumption of about 15,000 KWhr per tonne of Aluminium produced. This huge amount of electricity is the major cost and a major factor in siting an Al smelter.

The electrolytic cell used is a steel tank with a carbon base which is made the cathode. Multiple carbon anodes dip into the cell:

Aluminium is produced at the cathode (-):

Al³⁺ + 3e Al(l)

Oxygen is produced at the anode (+)

2O^2- O₂(g) + 4e-

The released oxygen reacts with the anode at 960ºC to form CO₂ ; the anodes wear out fast and have to be replaced every month. This is a major cost of the process.

The Aluminium is removed from the reactor by siphoning (sucking) the liquid into huge crucibles. These are emptied into moulds and cast in cylinders or blocks.

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Titanium

Titanium is a very important and valuable metal, due to its hardness, high melting point and strength. Titanium is lighter and stronger than steel and does not corrode. However, these actually cause problems in terms of industrial manufacture. The carbon that is used to reduce metals would actually react with titanium at the high temperature needed to reduce the metal ore.

Unfortunately titanium cannot be formed by the coke reduction of titanium(IV) oxide - TiO₂. When this is attempted a very high proportion of titanium carbide TiC is formed.

TiO₂(s) + 3C(s) TiC(s) + 3CO(g)

Titanium is thus prepared by reducing its chloride with magnesium or sodium.

Titanium cannot be obtained by coke reduction of TiO₂(s) , because TiC is formed and not Ti.

Instead TiO₂ is heated with carbon in a stream of chlorine gas. This converts the titanium to titanium chloride, which is a gas at the operating temperature.

TiO₂(s) + 2C(s) + 2Cl₂(g) TiCl₄(g) + 2CO(g)

The chorine is removed by heating the TiCl₄ with a reactive metal, magnesium or sodium are used. However, this cannot be done in air as the magnesium and sodium would react with oxygen and not the TiCl₄. An inert atmosphere of helium or argon is used

Heat in Argon

TiCl₄(g) + 2Mg(s) Ti(s) + 2MgCl₂(s)

This produces Ti with about 30 % impurities such as MgCl₂(s). This is removed by heating when the more volatile MgCl₂is driven off from Ti.

This process is a batch process, i.e. done in one container, and when the reaction is finished the container is cooled and emptied, and the reaction started up again. Because this is not done continuously, as in Al and Fe production, the rate of production is slower and more expensive.

The process is also costly because Magnesium is expensive; it is produced by electrolysis like Al, but since there is not such a big demand for Mg it is more expensive

Titanium is therefore expensive and has not yet displaced steel, despite its superior properties, (stronger, does not corrode, lighter). It is used for high value uses, such as the jet turbine blades and hip joints, where its cost is outweighed by its superior performance.

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Tungsten

Tungsten is very strong and has a high melting point, 3410ºC. Tungsten added to steel makes it incredibly hard.

Its major use is in light bulb filaments which heat up to 2000ºC, when many other metals would vaporise, particularly at the pressures found inside light bulbs. The scarcity of tungsten provided a stimulus for the invention of 'low energy' light bulbs and LEDs.

Like titanium, tungsten cannot be extracted by reduction of its ore tungsten(VI) oxide, WO₃, using carbon. Once again the carbide is formed

Extraction

The Ores are crushed and ground and the compounds containing Tungsten are separated by gravity/magnetic methods.

They are converted into WO₃, Tungsten oxide.

Heating with carbon only forms Tungsten Carbide which is hard and brittle and used to make machinery parts and tools.

W + C WC

Today Tungsten it is reduced by heating between 700-1000ºC with hydrogen.

WO₃ + 3H₂ W + 3H₂O

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