14.1 Metal ores and metal extraction Cree Ue eee LT eee Cee ng Creer aoe f Cerner tind The composition of Earth's crust ‘We get our metals from the rock Earth’s crust - the hard outer skin of Earth that we live on Rock is mixture of substances. They are tly Compounds. But unreactive elements such as silver, mercury, platinum, and gold occur native, or uncombined, If you could break the rock in Earth’s crust into its elements, you would find it is almost fF oxygen, Amazing! This shows its composition JA oo ea Ss Note that © oxygen and silicon make up nearly three-quarters of the crust ‘They occur together in compounds such as silican(IV) oxide (siliea), Oxygen is also found in metal oxides and carbonates. ‘© just six metals - aluminium to potassium in the pie chart — make up ing metals isis the world’s ‘over a quarter of the crust. Aluminium is the most abundant of these, & and iron next. All six occur as compounds, because they are reactive. #999 argest copper mine, in Utah, Metal ores USA. It was started in 1906, The rock in an area may contain a large amount of one metal compound, andi ‘or metal. So it may be worth digging the rock up to extract the metal. km wide. ‘The rocks from which we obtain metals are called ores. Look at these: ee Hematite, a common ore of __ Bauxite, the main alun Gold. It is unreactive, so is found iron. It is mainly iron(II) oxide. ore. It is mostly alumin as seams of t Jement, in rock smExtraction Qo oes metal ore. And now you have to extract the metal from it, Reduction of metal ores y + Remember, you can define How will you do this? It depends on the metal's reactivity. parerapy © Where a metal occurs as the element ~ for example, gold and silver= joss of oxygen all you need to do is separate it from sand and other impurities. Fe,0, — 2Fe © But most metals occur as compounds in their ores. A chemical © oF gain of electrons reaction is needed to reduce them to the metal: AM 4 3e= > Al rico metal compound "metal Ether way, the ore is reduced ‘© The compounds of the more reactive metals are very stable, and fate eeet need electrolysis to reduce them. This is a powerful method. But it is expensive, because it needs a lot of electricity. '® The compounds of the less reactive metals are less stable. They can be reduced to the metal by heating with a suitable reactant. Extraction and the reactivity series So the method of extraction is linked to the reactivity series. Look: on lO 4 4 calcium electrolysis metals more ron magnesium eee difficult to of extraction _of extraction ‘aluminium decompose more powerful more expensive ‘occur naturally as elements, 0 reduction is not needed gold Note that the table includes heating with earbon monoxide, as well as carbon. Why? Because when carbon is heated in a furnace, it reacts ‘with the limited supply of oxygen present, to give carbon monoxide gas (CO). This then brings about the reduction. Iron and aluminium are the two most common metals in Earths crust, and the two we use most. As the table shows, different methods are used to extract them. We look more closely at these in the next two units. 1A. No need to reduce gold14.2 Extracting iron Cone Ce eee eet Prete nee ee mre ter nner Serre cn Ta ern noir The blast furnace Iron is the world’s mo: ccly important, It is carried out by reducing iron ore using carbon monoxide, in a blast furnace. This is an oven shaped like a chimney 4 hopper for loading charge 30 EE led the charge, containing the iron ore, is added through ‘op of the furnace. Hot air is blasted in through the bottom. After a eries of reactions, liquid iron collects at the bottom of the furnace What's in the charge? The charge contains three things: 1 Iron ore. The chief ore of iron is hematite. This is J oth 2 Limestone, This common rock is mainly calcium carbonate, CaCO, iron(II) oxide, Fe,0,, mixed with sand a mpounds. | & Mining h 3 Coke. This is made from coal, and is almost pure carbon , 374The reactions in the blast furnace ‘Stage 1: The coke burns to give carbon dioxide and heat ‘The blast of hot air starts the coke burning: ‘carbon + oxygen —> carbon dioxide So the carbon is oxidised - it gains oxygen. ‘Stage 2: Carbon dioxide is reduced to carbon monoxide ‘The carbon dioxide reacts with more coke, like this: carbon monoxide Here the carbon dioxide is reduced - it loses oxygen ‘Stage 3: Carbon monoxide reduces the iron(ti) oxide to iron ‘This is the stage where the extraction of the metal occurs. iron(ii) oxide + carbon monoxide —» iron + carbon dioxide So the ironiill) oxide is reduced. ‘The iron trickles to the bottom of the furnace. Stage 4: Impurities are removed as slag ‘There are two reactions. | First the limestone from the charge breaks down in the heat of the furnace. This is a thermal decomposition: calcium carbonate “"S calcium oxide + carbon dioxide ii Then the calcium oxide reacts with sand, which is the main impurity in iron ore, Sand is mainly silicon(V) oxide. The reaction is: calcium oxide + silicon(tV) oxide —> calcium silicate ‘The calcium silicate forms slag which runs down the furnace and floats on the iron. It is drained off. When it solidifies itis sold, ‘mostly for road building. ‘The waste gases: hot carbon dioxide, and nitrogen from the air, exit from the top of the furnace. Heat is transferred from them ‘to heat the incoming blast of air. Where next? Most of the impurity from the iron ore is removed in the blast furnace ~ but not all. The molten iron contains carbon, and some sand too. Some of it is run into moulds to make east iron. This is hard, but its high carbon content makes it brittle. So it is used only for things for bottled gas, and drain covers, that are not put under much stress. But mo CO) + 0,@) > C0, 0) ‘Thisis a redox reaction. tis highly exothermic. The heat given out heats the furnace. €G) + CO, @) + 20 Another redox reaction. It is endothermic - intakes in heat from the furnace. That helps, because stage 3 needs a lower temperature Fe,0, (3) + 3CO (9) > 2Fe(/) + 3CO,<9) Another redox reaction. Carbon monoxide is the reducing agent. Itis itself oxidised to carbon dioxide. caco, “5 ca0 6) + €0; (9) €20 (8) + SiO, (5) —+ CaSiO, (5) Calcium oxide is a basic oxide. Silicon(V) oxide is an acidic oxide. Calcium silicate is a salt. ce canisters 4 Aaastiron drain cover. tt was once rock. is turned into steels. You can find out about steels in Unit 14.5.