The Extraction of Metals

Question 1

How are metals found?

Answer 1

Metals are usually found in ores; usually as:

• sulphides (e.g. lead sulphide, zinc sulphide)
• or oxides (e.g. titanium dioxide and iron(III) oxide).

Sulphide ores are converted to oxides via roasting in the air.

2ZnS + 3O₂ → 2ZnO + 2SO₂

Question 2

What are the environmental problems associated with the conversion of sulphides into oxides?

Answer 2

The conversion produces sulphur dioxide as a product; causes acid rain; detrimental to plants, aquatic life and damages buildings consisting of limestone.

Question 3

What can the sulphur dioxide product from roasting be used for?

Answer 3

Sulphur dioxide can be collected and converted to H₂SO_<b>4</b>(sulphuric acid) in controlled conditions; can then be sold for a variety of purposes (chemical and manufacturing processes), making the industrial process more economical.

Question 4

Metal oxides are reduced to extract the metal. What is used to reduce iron, manganese and copper? Why?

Answer 4

Carbon (as coke; an impure solid fuel made from coal) and carbon monoxide are used as reducing agents; are more reactive than the metal, taking the oxygen away and reducing the oxide.
They are cheap and effective reducing agents; acts as a fuel to provide heat for the process too!

2C + O₂ → 2CO
C + CO₂ → 2CO

Question 5

What is the equation for the extraction of iron(III) oxide by carbon monoxide? (inc. conditions)

Answer 5

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Carbon monoxide is the main reducing agent.

Conditions:

• 700°C (at least)
• blast furnace; heat as above
• carbon monoxide/carbon

Fe₂O₃ + 3C → 2Fe + 3CO

In the hotter parts of the furnace, carbon itself acts as the reducing agent too. At these temperatures, the by-prouduct is CO, not CO₂.

Question 6

What is the equation for the extraction of manganese(IV) oxide by carbon? (inc. conditions)

Answer 6

MnO₂ + 2CO → Mn + 2CO₂

MnO₂ + C → Mn + CO₂

Conditions:

• Conducted in a blast furnace
• At least 1200°C; higher than iron(III) oxide.

Question 7

What is the equation for the extraction of copper(II) oxide by carbon? (inc. conditions)

Answer 7

Copper carbonate from malachite core is heated first until it decomposes, where the oxide is then reduced with carbon.

CuCO₃ → CuO + CO₂
2CuO(s) + C(s) → 2Cu(l) + CO₂(g)

Another method includes h eating the copper ore of malachite containing CuCO₃ directly with carbon will reduce the copper.

2CuCO₃ + C → 2Cu + CO₂

Question 8

Why is carbon not used for the extraction of titanium, aluminium and tungsten?

Answer 8

Titanium
Extracting titanium by carbon reduction results in a titanium carbide product; brittle and shitty, ruins it.
Carbon left in the metal as an impurity.

Aluminium
The temperature needed for carbon reduction is too high to be economic (it’s too reactive; C is below Al in the reactivity series) - so a different method has to be used; electrolysis.

Tungsten
Pure tungsten can’t be obtained by reducing tungsten(VI) oxide using carbon, because it reacts with carbon to make tungsten carbide (like titanium). Instead, the reducing agent is hydrogen.

Question 9

1. ) How is aluminium manufactured from purified bauxite (half-equations at both electrodes)?
2. ) Why do the postive electrodes need frequent replacement?

Answer 9

1.) Aluminium is extracted from purified bauxite ore - consists of largely aluminium oxide, Al₂O₃.

1. Impurities are removed
2. Dissolved in molten cryolite, Na₃AlF₆ (sodium aluminium fluoride) which lowers the melting point from 2345K to 1240K.
   (reduces operating costs)
3. Solution is electrolysed in rows of cells using currents up to 300000A.

Aluminium is produced at the negative electrode (cathode).
Negative electrode/cathode (steel casing of the cell):
Al₃⁺ + 3e^- → Al

Positive electrode/anode (made of carbon):
2O^2- → O₂ + 4e^-

Overall Reaction:
Al₂O₃ → 2Al + 1½ O₂

2.)
Carbon / graphite / the electrodes oxidise
OR
Carbon / graphite / the electrodes burn in / react with the oxygen
formed
OR
carbon dioxide / CO2 is formed

Question 10

How is titanium extracted; what is the conversion of titanium(IV) oxide to titanium(IV) chloride?
(equations and conditions)

What is the reduction of titanium(IV) chloride?

Answer 10

Titanium is produced by reducing titanium(IV) chloride using a more reactive metal; sodium or magnesium.

• *Conversion of titanium oxide to titanium chloride.**
• The ore is converted to titanium(IV) chloride by reacting it with coke and chlorine at 1173k.*

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

This takes place at 1173K.
The liquid TiCl₄ is purified by fractional distillation.

Reduction of titanium chloride.
Titanium chloride is reduced with molten sodium under an inert argon atmosphere at 1300K.
The argon is required to prevent the metals from reacting with nitrogen and oxygen in the air.

TiCl₄(l) + 4Na(l) → Ti(l) + 4NaCl(l)

Magnesium can also be used. This is a batch process and so is generally less effective than a continuous process.

TiCl₄(l) + 2Mg(l) → Ti(l) + 2MgCl₂(l)

Question 11

How is tungsten(VI/6) oxide extracted?

(equation, conditions and risk)

Answer 11

Powdered tungsten(VI) oxide (WO₃) is heated to temperatures in the range 550 - 850°C in a stream of hydrogen.

WO₃ + 3H₂ → W + 3H₂O

An excess of hydrogen is used, and this carries away the steam produced during the reaction. The hydrogen is dried and recycled.

Great care obviously has to be taken to keep the whole system free of air to avoid explosion risks with the hydrogen at these high temperatures.Hydrogen is a flammable gas so is only used as a last resort.

Hydrogen is more expensive, but worth the extra cost to get pure tungsten.

Question 12

What are the economical/enviromental advantages and disadvantages of recycling scrap metals compared with extracting metals?

Answer 12

Advantages

• Saves raw materials; ores are finite.
• Saves energy; recycling takes less energy than extraction; saves money.
• Reduces waste sent to landfill
• Mining damages the landscapes; visual pollution.
• Produces less CO₂ emissions.

Disadvantages

• Collecting and sorting metals from other waste can be difficult and expensive
• Purity of recycled metal varies - usually impurities mixed in.
• May not produce a consistent supply to meet demand.

Question 13

What are the environmental advantages of using scrap iron to extract copper from aqueous solutions compared with the high-temperature carbon reduction of copper oxide?

What type of ore is it used for?

Answer 13

• Much cheaper than carbon reduction; don’t need high temperatures.
• No CO₂ produced; more environmentally friendly.
• Reduces the scarp iron that is discarded in landfil.
• Has already been extracted from its ore.
• Also easily seperated from other materials as it’s magnetic.

This process produces copper more slowly than carbon reduction and has a lower yield; not used on ores with high copper content.

Mainly used for low grade ore - ore that only contains a small percentage of copper.