Unit 11.5 Metals and Non-metals Topic 3: Extraction of metals and corrosion In the previous two Topics we looked at the physical and chemical properties of metals. In Topic 3 we now examine how metals are extracted from the raw ore that is dug from the earth. Considering the importance of mining in PNG, this Topic is of particular relevance. It deals with: Reactivity of metals related to their extraction from ores. Methods of extraction used to obtain metals. Corrosion of metals. Extraction of metals from their ores Metals were discovered in reverse order of their chemical activity: The least active metals (ie gold and silver) were discovered first. The less active metals (ie iron, zinc, lead, copper) were discovered next. The highly active metals (ie sodium, lithium, calcium, magnesium, aluminium) were discovered later. Extraction of metals from metal oxides The less active metals either have minerals that are oxides, eg iron, or the oxide can be readily obtained from the mineral (often the metal sulfide), eg zinc, lead and copper. The metal is extracted by thermal reduction of the metal oxide using carbon (in the form of carbon/coke) as the reducing agent. Iron Iron oxide, present as the mineral titanomagnetite (Fe 3 O 4.TiO 2 ), occurs in abundance in New Zealand in the iron sand on the west coast of the North Island. At the local steel works, a rotary kiln is used to produce iron from iron sand. (The titanium oxide (TiO 2 ) in titanomagnetite is not affected by the thermal reduction process that produces iron titanium is too reactive a metal.) mixture of iron sand and coal are heated as they move down kiln, by force of gravity kiln rotates coal, iron sand and air enter the kiln iron and impurities leave the kiln rotary kiln Commercial production of iron by reduction of iron oxide
234 Unit 11.5 Metals and Non-metals The reactions occurring in the rotary kiln are: Oxygen, from the air, combines with the carbon in the coal to produce carbon dioxide and heat: C(s) + O 2 (g) CO 2 (g). Carbon dioxide is reduced by heated carbon (coal) to produce carbon monoxide: CO 2 (g) + C(s) 2CO(g). The iron oxide is reduced by hot carbon and/or carbon monoxide to produce iron: Fe 3 O 4 (s) + 2C(s) 3Fe(s) + 2CO 2 (g) and, Fe 3 O 4 (s) + 4CO(g) 3Fe(s) + 4CO 2 (g). The iron is then converted into steel, and the impurities (slag) are stockpiled as a source of titanium. Zinc, lead and copper Zinc, lead and copper metals can be obtained from their minerals by thermal reduction in a two-stage process. Stage 1 the mineral is roasted in air to produce the metal oxide. Metal Mineral Reactions Zinc Zinc blende, ZnS 2ZnS(s) + 3O 2 (g) 2ZnO(s) + 2SO 2 (g) Lead Galena, PbS 2PbS(s) + 3O 2 (g) 2PbO(s) + 2SO 2 (g) Copper Malachite, CuCO 3.Cu(OH) 2 CuCO 3.Cu(OH) 2 2CuO(s) + CO 2 (g) + H 2 O(l) Stage 2 the metal oxide is reduced by carbon (in the form of coke), or by carbon monoxide (CO), to the metal. Metal oxide Reactions Zinc oxide ZnO(s) + C(s) Zn(s) + CO(g), or ZnO(s) + CO(g) Zn(s) + CO 2 (g). Lead oxide PbO(s) + C(s) Pb(s) + CO(g), or PbO(s) + CO(g) Pb(s) + CO 2 (g). Copper oxide CuO(s) + C(s) Cu(s) + CO(g), or CuO(s) + CO(g) Cu(s) + CO 2 (g). The sulfur dioxide formed in the Stage 1 reaction for zinc and lead is converted to sulfuric acid and sold as a by-product. The extraction of zinc, lead and copper from their ores by thermal reduction
Topic 3: Extraction of metals and corrosion Extraction of metals by electrolysis The high chemical activity of sodium, lithium, calcium, magnesium and aluminium metals makes them difficult to separate from other element(s) in the mineral. As a result, these metals have been discovered only in the last two hundred years. The process of electrolysis using electrical energy to produce chemicals by passing an electric current through an electrolyte provides the high energy required to extract the metal in the mineral. The metals sodium, lithium, calcium and magnesium are obtained commercially by electrolysis of their molten metal chloride (the electrolyte). Magnesium production is by far the largest of these metals. Extraction of magnesium Sea water contains a large number of dissolved minerals, including magnesium as magnesium chloride. The magnesium is separated from the other minerals by being precipitated as magnesium hydroxide. The magnesium hydroxide is converted into magnesium chloride, which is then electrolysed. Extraction of aluminium Aluminium occurs as a mineral, bauxite a mixed oxide of aluminium and iron. The mineral is mined and then purified to produce alumina, Al 2 O 3. The alumina is then converted to aluminium metal by electrolysis: carbon positive electrode carbon negative electrode molten aluminium forming +ve molten alumina and cryolite Extraction of aluminium metal ve steel pot aluminium run off to form ingots The process of electrolysis is summarised in the equation: 2Al 2 O 3 (s) 4Al(s) + 3O 2 (g). The oxygen released causes the carbon positive electrodes to burn, so they then need to be replaced. The cryolite is present to lower the working temperature of the cell to 1000 o C (the melting point of aluminium oxide is 2072 o C). 235
236 Unit 11.5 Metals and Non-metals Unit 11.5 Activity 3A: Extraction of metals 1. State the name of a metal that occurs: a. As the element. b. As a metal oxide. c. As a metal sulfide. 2. State the name of a mineral that contains: a. Aluminium. b. Zinc. 3. Explain the meaning of the terms: a. Thermal reduction. b. Electrolysis. 4. Explain why copper metal can be obtained commercially by thermal reduction, whereas aluminium metal can only be obtained commercially by electrolysis. 5. Complete the following equations: a. C(s) + O 2 (g) b. + CO 2 (g) CO(g) c. Fe 3 O 4 (s) + CO(g) Fe(s) + d. 2PbS(s) + 2PbO(s) + 2SO 2 (g) e. ZnO(s) + C(s) + f. Al 2 O 3 (s) 4Al(s) + 6. a. Arrange the following metals in their order of discovery. Place the earliest discovery at the top of the list. i. Aluminium. ii. Copper. iii. Gold. b. Explain why the metals were discovered in this order. Corrosion of metals Corrosion is a chemical process in which a metal is either eaten away or its surface is changed in appearance. The atmosphere contains oxygen, water vapour and carbon dioxide, and in industrial areas, other acidic gases (eg sulfur dioxide and nitrogen dioxide). All these chemical agents can cause metals to corrode. Sodium, lithium and calcium The very active metals sodium, lithium, calcium react so rapidly with the chemicals in the atmosphere that they are of little commercial use except as laboratory or industrial chemicals. Sodium, lithium and calcium are kept under oil to protect them from atmospheric attack.
Topic 3: Extraction of metals and corrosion Magnesium The surface of magnesium is rapidly oxidised in the atmosphere to magnesium oxide, which develops into magnesium hydroxide and magnesium carbonate, and forms a protection for the metal. These compounds of magnesium protect the metal from further corrosion but make the appearance of the metal dull. Aluminium Aluminium metal reacts vigorously with oxygen in the air to form a strong, thin, coating of aluminium oxide on its surface. This oxide film protects the aluminium from further attack by oxygen, water and dilute acids present in air thus, aluminium metal appears not to corrode in air. Aluminium metal does not need further protection from the chemicals in air. Iron Aluminium window frames are often coloured for cosmetic effect, and not for protection. Corrosion of iron the rusting process The commonest example of corrosion is the rusting of iron metal the term rust applies only to the corrosion product(s) of the metal iron. The corrosion of iron is dependent upon the activity of the metal and the fact that the oxide formed, iron(iii) oxide, does not bond with the remaining iron underneath the rust flakes off. Rust is hydrated iron(iii) oxide. Iron will rust when in the presence of oxygen (air), water vapour and carbon dioxide it will not rust if any one of these three factors (oxygen (air), water vapour or carbon dioxide) is excluded. The rusting process can be prevented by protecting the iron from the components of air by: Coating the iron with zinc metal (galvanising), paint or tin. Adding to the iron a high percentage (about 25%) of chromium and nickel to produce stainless steel. Placing a thin film of oil or grease on an iron tool when the tool is not in use. Copper Copper is slowly corroded by the chemicals of the atmosphere to produce the oxide, the hydroxide and, finally, a green insoluble coating of basic copper carbonate. Copper roofs on European monasteries and cathedrals may be four hundred years old and still in good condition due to the insoluble copper carbonate coating protecting the copper underneath. 237
238 Unit 11.5 Metals and Non-metals clean, shiny iron nail water in contact with air over a period of a few days the iron nail corrodes and a deposit of brown rust is observed lumps of anhydrous calcium chloride (a drying agent) plug of cotton wool clean, shiny iron nail Iron will corrode in the presence of water, oxygen and carbon dioxide water that has been boiled and contains no dissolved gases clean, shiny iron nail plug of cotton wool over a period of a few days soda-lime (to absorb carbon dioxide) tightly fitting bung to stop air entering over a long period of time Iron will not corrode in the presence of water but in the absence of oxygen and carbon dioxide No corrosion if oxygen and carbon dioxide are present, but water is absent plug of cotton wool to prevent dust entering the test tube clean, shiny iron nail the iron nail shows no sign of corrosion over a period of a few days the iron nail shows no sign of corrosion The conditions necessary for iron to corrode (rust) the iron nail shows no sign of corrosion
Topic 3: Extraction of metals and corrosion Zinc and lead Zinc and lead react with the components of air oxygen, water and carbon dioxide to build up a layer of metal carbonate and hydroxide which protects the metal from further corrosion. Thus, these metals do not appear to corrode in air, except that they become dull in appearance. Neither of these metals needs further protection from the chemicals in air. Zinc is alloyed with copper to form brass, which is resistant to corrosion and retains an attractive appearance. Gold and silver Gold has always been valued, partly because of its scarcity, but also for its yellow shiny appearance, which is not affected in any way by the chemicals in air over very long periods. When discovered, the tomb of Tutankhamen who died in 1352 BC (3500 years ago) contained a large quantity of shiny gold. Gold is sometimes used for teeth repair because of its appearance and its resistance to corrosion. Silver can tarnish, especially in regions where sulfur is present the surface of the metal becomes black silver sulfide. The high lustre of silver can be easily restored by removing the surface tarnishing, either by chemical means or by abrasion. Unit 11.5 Activity 3B: Corrosion of metals 1. Identify the chemicals in air that can cause corrosion. 2. Identify which of the following sets of conditions would allow a shiny iron nail to rust: a. In a beaker of tap water. b. Vacuum-packed in a plastic envelope. c. Left on the moon by a visiting astronaut. d. Dropped into a shallow swimming pool. e. At the bottom of the deep blue sea. 3. Identify the change of appearance that would be noticed if each of the following metals in bright, shiny form were left in the presence of air for a year or more. For each metal, explain what new substance(s) is/are formed. a. Gold. b. Aluminium. c. Lead. d. Copper. 239
240 Unit 11.5 Metals and Non-metals 4. State a use for each of the following metals that involves the metal being in contact with the air. Describe the changes in appearance that would occur to the metal as it remained in contact with air for a long time (several years). Explain what chemicals are causing these changes and what new chemicals are being formed. a. Magnesium. b. Zinc. c. Silver. 5. A can that contains lemonade, or some other beverage, is made from aluminium although it could be made from steel. State two reasons why aluminium is preferred to steel.