Metallurgy

The One Thing

Metallurgy obtains metals from ores through concentration, conversion into a suitable compound, reduction and refining. The required extraction method depends mainly on the metal’s position in the activity series, the nature of its ore and the impurities present.

Reactions, Processes and Experiments

Key Terms

• Mineral: A naturally occurring substance in the Earth’s crust containing a metal or its compounds.
• Ore: A mineral from which a metal can be extracted conveniently and economically.
• Gangue: Unwanted earthy or rocky impurities mixed with an ore.
• Metallurgy: The science and process of extracting metals from their ores and refining them.
• Concentration of ore: The removal of gangue from an ore to increase the percentage of the useful metal compound.
• Hydraulic washing: Separation using a stream of water in which lighter impurities are carried away while heavier ore particles settle.
• Magnetic separation: Separation using a magnetic field to remove magnetic ore particles from non-magnetic impurities.
• Froth flotation: Separation, mainly of sulphide ores, in which ore particles attach to froth and separate from heavier impurities.
• Calcination: Strong heating of an ore in limited or no air to remove moisture and volatile substances or convert carbonates into oxides.
• Roasting: Strong heating of an ore in excess air, usually to convert sulphide ores into oxides.
• Flux: A substance added during extraction to react with gangue and form an easily removable slag.
• Slag: A fusible substance formed when flux reacts with gangue.
• Reduction: Removal of oxygen from a metal oxide or gain of electrons by metal ions to form the metal.
• Electrolytic reduction: Extraction of a metal by passing electric current through its molten compound.
• Refining: Purification of an impure metal obtained after extraction.
• Bauxite: The chief ore of aluminium, consisting mainly of hydrated aluminium oxide with impurities such as iron(III) oxide and silica.
• Alumina: Pure aluminium oxide, Al2O3, obtained from bauxite before electrolytic extraction.
• Bayer process: The process used to purify bauxite and obtain pure alumina.
• Hall-Heroult process: The electrolytic process used to extract aluminium from molten alumina dissolved in molten cryolite.
• Cryolite: Sodium aluminium fluoride, Na3AlF6, added to alumina to lower its melting point and improve electrical conductivity.
• Anode effect: Consumption of carbon anodes because oxygen released at the anode reacts with carbon to form carbon monoxide and carbon dioxide.
• Red mud: The insoluble residue removed during the Bayer process, consisting mainly of iron(III) oxide and other impurities.
• Activity series: The arrangement of metals according to reactivity, which determines the suitable extraction method.

Easily Confused

• Mineral and ore: A mineral contains a metal or its compounds naturally; an ore is a mineral from which extraction is economically convenient.
• Gangue and slag: Gangue is naturally occurring impurity in an ore; slag is the fusible product formed when flux reacts with gangue.
• Calcination and roasting: Calcination uses limited or no air, whereas roasting uses excess air.
• Hydraulic washing and froth flotation: Hydraulic washing separates according to density using water; froth flotation separates mainly sulphide ore particles by their attachment to froth.
• Bayer process and Hall-Heroult process: The Bayer process purifies bauxite to produce alumina; the Hall-Heroult process electrolyses alumina to produce aluminium.
• Alumina and bauxite: Bauxite is the impure hydrated aluminium oxide ore; alumina is purified aluminium oxide, Al2O3.
• Reduction and refining: Reduction produces the metal from its compound; refining purifies the extracted metal.
• Cathode and anode in aluminium extraction: Aluminium ions are reduced at the cathode, while oxide ions are oxidised at the anode and the carbon anodes are consumed.

What Gets Asked

• Identify the appropriate extraction method from the metal’s position in the activity series. Highly reactive metals require electrolysis, moderately reactive metals generally undergo chemical reduction, and less reactive metals may be obtained by heating or simpler methods. The main error is ignoring reactivity when selecting the method.
• Distinguish calcination from roasting and reproduce the relevant equations. Calcination involves limited or no air and carbonate ores, whereas roasting involves excess air and sulphide ores.
• Describe the Bayer process using bauxite, hot concentrated sodium hydroxide, soluble sodium aluminate, red mud, aluminium hydroxide and alumina. Marks are lost by omitting the specific reactions or by confusing the insoluble iron(III) oxide impurities with the soluble aluminium compound.
• Explain why aluminium is extracted by electrolysis rather than carbon reduction. Aluminium is more reactive than carbon and forms a stable oxide, so carbon reduction is not economically suitable.
• Explain the functions of cryolite in the Hall-Heroult process. Cryolite lowers the melting point of alumina and increases electrical conductivity; omitting either function loses detail.
• Write the electrode reactions and overall reaction for aluminium extraction, and explain the anode effect. The carbon anodes are consumed because oxygen reacts with carbon to form carbon dioxide and some carbon monoxide.
• Explain the choice of flux in relation to gangue. An acidic flux removes basic gangue, while a basic flux removes acidic gangue.