3.3 Occurrence and Extraction of Metals, and Corrosion

The Iron Pillar in Delhi has stood in the open for about $1600$ years without rusting much. Gold ornaments handed down for generations look as bright as when they were made. Yet an iron gate left in the rain rusts within a year, and a silver spoon turns black. Metals behave very differently, and so does the effort needed to get them. Gold is found shining in river sand; sodium has never been found as a metal in nature, because it reacts with everything; aluminium is the most abundant metal in the Earth's crust, but it was a rare, costly metal until about $1890$ because it is so hard to separate from its compounds.

This section follows metals from the ground to the workshop. You will learn the language of ores and how they are cleaned up; how the position of a metal in the reactivity series (Section 3.1) decides the method used to extract it — heating, reduction with carbon or aluminium, or electrolysis; how impure copper is refined electrically; why metals corrode and how galvanising, painting and alloying protect them; and why most everyday metal objects are alloys.

Occurrence of Metals: Minerals, Ores and Enrichment

The Earth's crust is the main source of metals. Seawater also holds soluble salts such as sodium chloride and magnesium chloride. Elements or compounds that occur naturally in the crust are called minerals. A few minerals contain so much of a metal that extracting it is worthwhile.

Very reactive metals are always found combined with other elements; metals at the bottom of the series — gold, platinum, sometimes silver and copper — can be found free. The ores of most metals are oxides, sulphides or carbonates, so metal extraction almost always means removing the non-metal part of the compound.

The first step is enrichment (concentration, dressing): removing the gangue by a method that depends on the properties of the ore, such as washing with water (hydraulic washing, for heavy ores), magnetic separation, or froth flotation for sulphide ores. The purified ore then goes forward to extraction.

Extraction: the Method Follows the Reactivity

To obtain a metal from its compound we must reduce it — the metal ion has to gain electrons and become a neutral atom. How hard that is depends on how eagerly the metal holds on to its compound, that is, on its position in the reactivity series. Metals fall naturally into three groups.

Metals low in the series. Cinnabar, $\mathrm{HgS}$, is heated in air. The sulphur burns off as sulphur dioxide, leaving mercury(II) oxide, which breaks up on further heating into mercury and oxygen: $$\mathrm{2HgS(s) + 3O_{2}(g) \rightarrow 2HgO(s) + 2SO_{2}(g)}$$ $$\mathrm{2HgO(s) \xrightarrow{\text{heat}} 2Hg(l) + O_{2}(g)}$$ Copper from copper glance, $\mathrm{Cu_{2}S}$, is obtained in the same way: heating it in air gives copper(I) oxide, which reacts with more of the sulphide to give the metal: $$\mathrm{2Cu_{2}S(s) + 3O_{2}(g) \rightarrow 2Cu_{2}O(s) + 2SO_{2}(g)}$$ $$\mathrm{2Cu_{2}O(s) + Cu_{2}S(s) \rightarrow 6Cu(s) + SO_{2}(g)}$$

Metals in the middle of the series. Sulphides and carbonates are first changed into oxides by roasting or calcination, because it is much easier to obtain a metal from its oxide. Zinc oxide is then reduced by heating it with carbon (coke); carbon takes the oxygen: $$\mathrm{ZnO(s) + C(s) \rightarrow Zn(s) + CO(g)}$$ Lead is obtained from galena in the same way ($\mathrm{PbO + C \rightarrow Pb + CO}$), and iron from haematite can be obtained by heating $\mathrm{Fe_{2}O_{3}}$ with carbon. In the language of Chapter 1 the oxide loses oxygen and is reduced while carbon gains oxygen and is oxidised: a redox reaction.

Carbon is not the only possible reducing agent. A more reactive metal can displace a less reactive one from its oxide, just as it does from a salt solution. Aluminium is used to obtain manganese, chromium and iron from their oxides, and the reactions are violent: $$\mathrm{3MnO_{2}(s) + 4Al(s) \rightarrow 3Mn(l) + 2Al_{2}O_{3}(s) + \text{heat}}$$

Metals high in the series. Carbon cannot reduce the oxides of sodium, magnesium, calcium and aluminium: these metals have more affinity for oxygen than carbon has. They are obtained by electrolytic reduction. An electric current is passed through the molten chloride (or oxide). The positive metal ion moves to the cathode (negative electrode), where it gains electrons and is deposited as the metal; the negative ion goes to the anode (positive electrode) and loses electrons. For molten sodium chloride: $$\text{cathode: }\mathrm{Na^{+} + e^{-} \rightarrow Na}\qquad\text{anode: }\mathrm{2Cl^{-} \rightarrow Cl_{2} + 2e^{-}}$$ $$\mathrm{2NaCl(l) \xrightarrow{\text{electricity}} 2Na(l) + Cl_{2}(g)}$$ Aluminium is obtained from molten aluminium oxide ($\mathrm{2Al_{2}O_{3} \rightarrow 4Al + 3O_{2}}$), and magnesium and calcium from their chlorides.

Refining, Corrosion and Alloys

Refining. The metals produced by reduction are not pure and must be refined. The most widely used method for refining a metal is electrolytic refining, used for copper, zinc, tin, nickel, silver and gold. For copper, the electrolytic cell is set up as follows:

Corrosion. Many metals are slowly eaten away by the air and moisture around them (Section 1.3 introduced rusting). Silver goes black because it reacts with sulphur compounds in the air to form silver sulphide: $\mathrm{4Ag + 2H_{2}S + O_{2} \rightarrow 2Ag_{2}S + 2H_{2}O}$. Copper slowly acquires a green coat of basic copper carbonate when it reacts with moist carbon dioxide of the air: $\mathrm{2Cu + O_{2} + H_{2}O + CO_{2} \rightarrow CuCO_{3}\cdot Cu(OH)_{2}}$. Iron is covered by reddish-brown, flaky hydrated iron(III) oxide, $\mathrm{Fe_{2}O_{3}\cdot xH_{2}O}$, which needs both air (oxygen) and water; the flakes fall off and expose fresh iron. Aluminium is protected by its thin oxide layer, and gold and platinum do not corrode.

Alloys. A pure metal is often too soft, too weak, or too easily corroded for use. Pure iron is soft and easily stretched, but adding about $0.05\%$ of carbon makes it hard and strong. Pure gold ($24$-carat) is so soft that jewellery is made from $22$-carat gold, in which gold is mixed with copper or silver.

Alloys conduct electricity less well than pure metals, so pure copper and aluminium are used for electric wires, while alloys with high resistance such as nichrome are used for heating elements. A purity of $22$ carat means that $22$ parts out of $24$ by mass are gold.

Exercises