Extraction of metals

Extraction of Sodium – the Downs Cell The electrolyte is a molten mixture of sodium chloride and calcium chloride. The calcium chloride is added to reduce the melting point of the mixture Steel cathodes are used. The anodes are made of inert carbon Steel gauze, which separates the two electrode compartments, prevents the sodium formed from reacting with chlorine

Extraction of Sodium – the Downs Cell

Extraction of Sodium – the Downs Cell At the cathode, the reaction is Na+(l) + e- → Na(l) The anode reaction is Cl-(l) - e- → 1/2Cl2(g) The overall reaction in the Downs cell is NaCl (l) → Na(l) + 1/2Cl2(g) Chlorine is a useful by-product

Uses of sodium Sodium vapour is used in street lighting. Sodium, alloyed with potassium, is used as a coolant in some nuclear reactors

Extraction of Aluminium Alumina (aluminium oxide) is first extracted from the ore bauxite by chemical means. Aluminium is then extracted from alumina by electrolysis.

Alumina from bauxite Crushing, and mixing with hot NaOH solution Digestion: 2NaOH(aq) + Al2O3.3H2O(s) → 2NaAlO2 (aq) + 4H2O(l) Removal of impurities as red mud Precipitation: 2NaAlO2 (aq) + 4H2O(l) → 2NaOH(aq) + Al2O3.3H2O(s) Removal of water of crystallisation: Al2O3.3H2O(s) → Al2O3(s) + 3H2O(l)

Aluminium from alumina The electrolysis of alumina is carried out in large steel boxes lined with carbon. Carbon anodes and a carbon cathode are used. An electrolyte consisting of a mixture of molten cryolite (Na3AlF6) and aluminium fluoride (AlF3) is used. The aluminium oxide dissolves in the electrolyte. A low voltage is used in the electrolysis to avoid decomposing the electrolyte.

Aluminium from alumina

Aluminium from alumina During the electrolysis, the following reactions occur: Cathode: 2Al3+(l) + 6e- → 2Al(l)   Anode: 3O2-(l) - 6e- → 11/2O2(g) The molten aluminium formed settles at the bottom of the tank, and is siphoned from there.

Aluminium from alumina The electrolysis is carried out at a high temperature, at which the large carbon anodes are steadily burnt away by the oxygen liberated there, and have to be replaced at regular intervals. The electrolysis of alumina consumes enormous amounts of electricity, and is therefore normally carried out where electricity is relatively cheap.

Environmental aspects of aluminium production The extraction of aluminium oxide from bauxite results in the formation of very large amounts of red mud, which is unsightly. Huge amounts of electricity are used in the extraction of aluminium from alumina

Uses of aluminium Used to make strong, light, corrosion-resistant alloys, which are used in the construction of aeroplanes and ships. Aluminium is a good conductor of heat and so is used to make saucepans and as a cooking foil. It is an excellent electrical conductor, and so is used in electricity wires.

Recycling of aluminium The recycling of aluminium is an economically viable process. It is also desirable from an environmental point of view, saving energy and conserving natural resources. When aluminium is being recycled, the aluminium waste is melted. The impurities are either burned off or separated from the aluminium.

Anodising The purpose of anodising is to produce a very hard oxide coating on the surface of aluminium, or one that is very resistant to corrosion. In anodising, the aluminium is used as an anode in the electrolysis of dilute sulfuric acid. Graphite, platinum or another inert cathode can be used.

Anodising

H2O(l)  2H+(aq) + 1/2O2(g) + 2e- Anodising Anode reaction H2O(l)  2H+(aq) + 1/2O2(g) + 2e- Oxygen reacts with the aluminium anode forming a thicker tenacious coating of aluminium oxide on the surface of the metal. Cathode reaction: 2H+(aq) + 2e-  H2(g)

Properties of transition metals Transition metals have variable valencies They form coloured compounds They and their compounds have catalytic properties

Manufacture of iron The extraction of iron from its ores is carried out in a blast furnace, in which the iron ore is reduced to molten iron, using coke at a high temperature. This extraction is a chemical reduction process rather than an electrochemical process.

Manufacture of iron

Manufacture of iron The raw materials for the blast furnace process are: coke, limestone and iron ore (such as haematite). The ore, coke and limestone are fed regularly into the furnace from the top. Hot air is added from the bottom.

Manufacture of iron The ore is reduced by reaction with coke: Fe2O3(s) + 3C(s) → 2Fe (s) + 3CO(g) The carbon monoxide causes further reduction in the upper, cooler regions of the furnace: Fe2O3(s) + 3CO(g) → 2Fe (s) + 3CO2(g)

Manufacture of iron The limestone decomposes into calcium oxide and carbon dioxide. The calcium oxide reacts with impurities in the iron ore, forming slag: CaO + SiO2 → CaSiO3 CaO + Al2O3 → CaAl2O4 The less dense slag floats on the molten iron. They are separately tapped off

Manufacture of steel – Basic Oxygen Process Oxygen is blown onto the surface of the liquid iron at the base of the furnace. This oxidises silicon, manganese and some iron, and the products are removed as slag. Carbon is also oxidised and the gaseous products escape to the atmosphere.

Manufacture of steel – Basic Oxygen Process The molten iron, which is now sufficiently pure, is poured into a ladle and carefully measured amounts of carbon and other elements are added. Alloying carbon with iron to make steel increases the hardness and strength of the metal.

Properties of different steels Metals such as tungsten are used to harden steel. Corrosion-resistant steels are made using chromium and nickel. Manganese is used to make very tough steel.

Uses of iron and steel Steel is used in the manufacture of car bodies, and in the construction of bridges and buildings. Iron is used in manhole covers, chains and gates.

Electric arc process for steel manufacture

Electric arc process for steel manufacture In an electric arc furnace, steel is made from recycled steel, which is melted before it is refined and cast and rolled into steel products. There are five main stages in making steel using the electric arc furnace: charging (i.e. adding the scrap steel and lime), melting, refining, tapping and casting.

Electric arc process for steel manufacture In the refining stage, oxygen is blown onto the molten steel to oxidise non-acidic impurities to their oxides, which then react with lime to form silicates,components of slag. Excess carbon burns to form carbon monoxide and carbon dioxide Slag is less dense than liquid steel and floats on top, which means that it can be easily removed.

Environmental aspects of iron and steel production If iron is being extracted from a sulfide ore, some sulfur dioxide will be formed in the blast furnace. When coke (for use in blast furnace) is being produced from coal, smoke and sulfur dioxide emissions must be kept to a minimum. Fumes from a blast furnace must also be cleared of dust particles before being released into the atmosphere.

Environmental aspects of the electric arc process All emissions from the steelmaking plant have to be carefully monitored. Fumes are filtered to remove dust particles, and water used for cooling purposes is recycled. The use of natural gas for heating eliminates emissions of sulfur dioxide.