Extraction of Metals

Question 1

Why is roasting necessary?

Answer 1

Sulphide ores cannot be converted directly into the metal. Instead they must be converted to the oxide.

Question 2

Equations of production of SO2 from Pb and Zn ore

Answer 2

2ZnS + 3O2(g)  2ZnO(s) + 2SO2(g)

2PbS + 3O2(g)  2PbO(s) + 2SO2(g)

Question 3

Why does roasting cause problems?

Answer 3

Acid rain but if the sulphur dioxide can be collected before being released into the atmosphere, it can be used to make sulphuric acid.

Question 4

Outline the extraction of Iron

Answer 4

1. Heating the furnace

The coke reacts with the oxygen at the base of the furnace to produce carbon dioxide. This is an exothermic reaction and keeps the furnace hot:
C(s) + O2(g) -> CO2(g)

2. Making the reducing agent

The carbon dioxide rises up the furnace and reacts with more coke to produce carbon monoxide. This is an endothermic reaction:
C(s) + CO2(g) -> 2CO(g)

3. Reducing the iron oxide

The carbon monoxide is a reducing agent and reduces the haematite to iron:
Fe2O3(s) + 3CO(g) -> 2Fe(l) + 3CO2(g)
The liquid iron sinks to the base of the furnace and is tapped off.

Some of the haematite is reduced directly by the carbon:
Fe2O3(s) + 3C(s) -> 2Fe(s) + 3CO(g)

Question 5

Outline the extraction of manganese and copper

Answer 5

Blast furnace

MnO2(s) + 2CO -> Mn(l) + 2CO2(g)
MnO2(s) + 2C(s) -> Mn(l) + 2CO(g)

CuO(s) + CO(g) -> Cu(l) + CO2(g)
CuO(s) + C(g) -> Cu(l) + CO(g)

Question 6

Advantages of using a blast furnace

Answer 6

1. the raw materials are very cheap
2. the reaction is exothermic, so less energy is needed to keep the furnace hot
3. the purity of the iron is sufficient for most steel-making purposes
4. it is a continuous process which means iron can be made more efficiently

Question 7

Disadvantages of using a blast furnace

Answer 7

1. the use of carbon does mean carbon dioxide is released (it is a greenhouse gas)
2. metals which are higher than carbon in the reactivity series (eg aluminium) cannot be extracted in this way
3. metals which react with carbon to form carbides (eg titanium and tungsten) cannot be extracted in this way
4. metals that need to be very pure cannot be produced in this way, as the metal produced always contains significant quantities of carbon

Question 8

Outline the extraction of aluminium

Answer 8

Once the bauxite has been purified, electrolysis can start.

Al2O3 has a melting point of 2000oC, which is too high to enable it to be electrolysed feasibly. It is thus dissolved in molten cryolite, Na3AlF6, and the solution is electrolysed at 900oC.

Oxygen is liberated at the anode, which is made of graphite:
2O2- -> O2(g) + 4e

Aluminium is liberated at the cathode, which is also made of graphite:
Al3+ + 3e -> Al(l)

The overall cell reaction is thus as follows:
2Al2O3 -> 4Al(l) + 3O2(g)

Question 9

Advantages of electrolysis

Answer 9

1. It is a continuous process, so is efficient

2. It makes the metal in pure form

Question 10

Disadvantages of electrolysis

Answer 10

1. The cost of melting the aluminium and supplying the energy for electrolysis is very high
2. It only works for ionic oxides

Question 11

Why isn’t coke used for tungsten and titanium?

Answer 11

In many cases, carbon cannot be used to reduce the metal oxide to the metal as the metal reacts with carbon to form the carbide instead. This is the case with tungsten and titanium:
TiO2(s) + 3C(s) -> TiC(s) + 2CO(g)
2WO3(s) + 9C(s) -> W2C3(s) + 6CO(g)

Question 12

Why is titanium ore first converted to a chloride?

Answer 12

Magnesium forms an alloy with the titanium

Question 13

Outline the extraction of Titanium

Answer 13

1. Conversion of ore into chloride

TiO2 is heated in a stream of chlorine, in the presence of coke, to produce TiCl4.
TiO2(s) + 2C(s) + 2Cl2(g) -> TiCl4(g) + 2CO(g)

The TiCl4 is covalent and volatile. It can be separated from the other products by fractional distillation.

2. Reduction of the chloride

The Ti is extracted from its chloride by reduction with magnesium or sodium:
TiCl4(g) + 2Mg(l) -> Ti(s) + 2MgCl2(l)
TiCl4(g) + 4Na(l) -> Ti(s) + 4NaCl(l)

These reactions are exothermic and keep the reaction vessel at a very high temperature.
This process is known as the Kroll process. An inert atmosphere of argon is used to prevent the magnesium or sodium reacting with oxygen.

Question 14

Advantage of extraction of Titanium

Answer 14

It produces very pure titanium

Question 15

Disadvantages of extraction of Titanium

Answer 15

1. It is a batch process, which means the titanium is not produced continuously. This adds to the cost of the process.
2. The sodium/magnesium are expensive
3. The energy costs are very high

Question 16

Why is steel usually used instead of Titanium?

Answer 16

Due to the high cost of this process, titanium is not widely used, despite is useful properties. Steel is used instead unless the special properties of titanium are specifically needed.

Question 17

Outline the extraction of Tungsten

Answer 17

Tungsten is reacted with hydrogen gas at a high temperature:

WO3 + 3H2 -> W + 3H2O

Question 18

Advantages of using H2 to extract Tungsten

Answer 18

1. it produces very pure tungsten

2. hydrogen is a cheap reagent

Question 19

Disadvantages of using H2 to extract Tungsten

Answer 19

1. The energy cost are high

2. using a flammable gas such as hydrogen at high temperatures is very dangerous

Question 20

Reasons for recycling

Answer 20

1. Recycling generally uses less energy than extraction
2. Mines and quarries are unsightly
3. The extraction process produces polluting gases like sulphur dioxide and carbon dioxide
4. Disposing of scrap metal is difficult
5. Most scrap contains a high percentage of the desired metal, so the process can be quite efficient

Question 21

Problems with recycling

Answer 21

1. collection of scrap metal

Scrap metal needs to be collected from wherever it is used. There are costs involved in doing this.

2. removing the desired metal from the rest of the scrap

Once collected, the desired metal needs to be separated from the other metals. Unless the metal is magnetic, this needs to be done manually. There are costs involved in doing this.

3. purifying the desired metal

One separated, the metal needs to be returned to the purity required for it to be useful. There are costs involved in doing this.

Question 22

Methods of recycling Aluminium

Answer 22

1. Most of the recycled aluminium is from drinks cans and foil.
2. The cans are collected at recycling points.
3. The cans are shredded and the paint is removed.
4. The metal is melted down into large ingots
5. The ingots are rolled into sheet metal from which more cans are cut.

Question 23

Methods of recycling Iron

Answer 23

1. Most of the recycled iron is from old trains, ships and cars.
   Because of the large size of these objects, the collection is done by specialist metal reclaiming companies.
2. The objects are cut into a manageable size.
3. The iron is separated from the other metals using a large magnet.
4. The scrap is melted and purified in the Basic Oxygen Process.

Question 24

Why is tungsten and titanium not recycled?

Answer 24

The purity required for the commercial use of titanium and tungsten means that recycling is not commercially viable.

Question 25

Outline extraction of copper from scrap iron

Answer 25

1. copper ores are converted into solutions containing Cu2+ ions using dilute sulphuric acid and a specialised bacterium
2. the copper ions are then reduced to copper using scrap iron:
   Cu2+(aq) + Fe(s)  Cu(s) + Fe2+(aq)

Question 26

Advantages of using scrap iron to extract copper

Answer 26

1. it uses very little energy (although energy would have been used to make the iron)
2. it works on very low grade copper ores, including copper mining waste
3. iron is much cheaper than copper