OCCURRENCE AND EXTRACTION OF METALS, Exercises of Chemistry

Download OCCURRENCE AND EXTRACTION OF METALS and more Exercises Chemistry in PDF only on Docsity! 1 MODULE - 6Occurrence and Extraction of Metals Notes Chemistry of Elements Metals and their alloys are extensively used in our day-to-day life. They are used for making machines, railways, motor vehicles, bridges, buildings, agricultural tools, aircrafts, ships etc. Therefore, production of a variety of metals in large quantities is necessary for the economic growth of a country. Only a few metals such as gold, silver, mercury etc. occur in free state in nature. Most of the other metals, however, occur in the earth's crust in the combined form, i.e., as compounds with different anions such as oxides, sulphides, halides etc. In view of this, the study of recovery of metals from their ores is very important. In this lesson, you shall learn about some of the processes of extraction of metals from their ores, called metallurgical processes. Objectives After reading this lesson, you will be able to :  differentiate between minerals and ores;  recall the occurrence of metals in native form and in combined form as oxides, sulphides, carbonates and chlorides;  list the names and formulae of some common ores of Na, Al, Sn, Pb ,Ti, Fe, Cu, Ag and Zn;  list the occurrence of minerals of different metals in India;  list different steps involved in the extraction of metals; * An alloy is a material consisting of two or more metals, or a metal and a non-metal. For example, brass is an alloy of copper and zinc; steel is an alloy of iron and carbon. OCCURRENCE AND EXTRACTION OF METALS 18 2 MODULE - 6 Chemistry Notes Chemistry of Elements  list and explain various methods for concentration of ores (gravity separation, magnetic separation, froth floatation and chemical method);  explain different metallurgical operations : roasting, calcination and smelting with suitable examples;  choose the reducing agent for a given ore;  differentiate between flux and slag, and  explain different methods for refining of metals : poling, liquation, distillation and electrolytic refining. 18.1 Occurrence of Metals Metals occur in nature in free as well as combined form. Metals having low reactivity show little affinity for air, moisture, carbon dioxide or other non-metals present in nature. Such metals may remain in elemental or native (free) state in nature. Such metals are called "noble metals" as they show the least chemical reactivity. For example gold, silver, mercury and platinum occur in free state. On the other hand, most of the metals are active and combine with air, moisture, carbon dioxide and non-metals like oxygen, sulphur, halogens, etc. to form their compounds, like oxides, sulphides, carbonates, halides and silicates. i.e., they occur in nature in a combined state. A naturally occurring material in which a metal or its compound occurs is called a mineral. A mineral from which a metal can be extracted economically is called an ore. An ore is that mineral in which a metal is present in appreciable quantities and from which the metal can be extracted economically. The main active substances present in nature, expecially in the atmosphere are oxygen and carbon dioxide. In the earth's crust, sulphur and silicon are found in large quantities. Sea-water contains large quantities of chloride ions (obtained from dissolved sodium chloride). Most avtive metals are highly electropositive and therefore exist as ions. It is for this reason that most of the important ores of these metals occur as (i) oxides (ii) sulphides (iii) carbonates (iv) halides and (v) silicates. Some sulphide ores undergo oxidation by air to form sulphates. This explains the occurrence of sulphate ores. Ores are invariably found in nature in contact with rocky materials. These rocky or earthy impurities accompanying the ores are termed as gangue or matrix. Some important ores and the metals present in these ores are listed in Table 18.1 5 MODULE - 6Occurrence and Extraction of Metals Notes Chemistry of ElementsPulverization can also be carried out in a ball mill. The crushed ore is taken in a steel cylinder containing iron balls. The cylinder is set into revolving motion. The striking balls pulverize the crushed ore into fine powder. Rotating cam shaft Crushed ore DieScreen Stamp Fig. 18.2 : The Stamp mill 18.2.2 Concentration or Dressing of the Ore Generally, the ores are found mixed with earthy impurities like sand, clay, lime stone etc. These unwanted impurities in the ore are called gangue or matrix. The process of removal of gangue from powdered ore is called concentration or ore dressing. There are several methods for concentrating the ores. The choice of method depends on the nature of the ore. Some important methods are : (i) Gravity separation (Hydraulic washing) : In this method, the light (low specific gravity) earthy impurities are removed from the heavier metallic ore particles by washing with water. It is therefore, used for the concentration of heavier oxide ores, like haematite  2 3Fe O tinstone  2SnO and gold (Au). In this method, as shown in the Fig. 18.3 the powdered ore is agitated with water or washed with a strong current of water. The heavier ore settles down rapidly in the grooves and the lighter sandy and earthy materials (gangue particles) are washed away. Jet of water Denser ore particles Water Gangue particles Vibrating table with grooves Fig. 18.3 : Gravity Separation (Hydraulic Washing) 6 MODULE - 6 Chemistry Notes Chemistry of Elements Fig. 18.5 : Froth floatation (ii) Magnetic separation method : By this method, those ores can be concentrated which either contain impurities which are magnetic or are themselves magnetic in nature. For example, the tin ore, tin stone  2SnO itself is non-magnetic but contains magnetic impurities such as iron tungstate  4FeWO and manganese tungstate  4MnWO . Ore Non-magetic wheel Magnetic wheel Non-magnetic gangue Magnetic ore Fig. 18.4. : Magnetic Separation The finely powdered ore is passed over a conveyer belt moving over two rollers, one of which is fitted with an electromagnet (Fig. 18.4). The magnetic material is attracted by the magnet and falls in a separate heap. In this way magnetic impurities are separated from non-magnetic material. (iii) Froth floatation method : This method is especially applied to sulphide ores, such as galena (PbS), zinc blende (ZnS), or copper pyrites  2CuFeS . It is based on the different wetting properties of the surface of the ore and gangue particles. The sulphide ore particles are wetted preferentially by oil and gangue particles by water. In this process, finely powdered ore is mixed with either pine oil or eucalyptus oil. It is then mixed with water. Air is blown through the mixture with a great force. Froth is produced in this process which carries the weted ore upwards with it. Impurities (gangue particles) are left in water and sink to the bottom from which these are drawn off (Fig. 18.5). (iv) Chemical method : In this method the ore is treated with a suitable chemical reagent which dissolves the ore leaving behind insoluble impurities. The ore is then recovered from the solution by a suitable chemical method. This is applied for extraction of aluminium from bauxite  2 3 2Al O .2H O . Bauxite is contaminated with iron (III) oxide  2 3Fe O , titanium (IV) oxide  2TiO and silica  2SiO . These impurities are removed by digesting the powdered ore with aqueous solution of sodium hydroxide at 420 K 7 MODULE - 6Occurrence and Extraction of Metals Notes Chemistry of Elementsunder pressure. Aluminium oxide dissolves in sodium hydroxide, whereas, iron (III) oxide, silica and titanium (IV) oxide remain insoluble and are removed by filtration. 2 3 3 3 2Al O 6NaOH 2Na AlO 3H O   Sodium aluminate Sodium aluminate is diluted with water to obtain precipitate of aluminium hydroxide. It is filtered and ignited to obtain pure alumina. 3 3 2 3Na AlO 3H O Al(OH) 3NaOH   3 2 3 22Al(OH) Al O 3H O  Intext Questions 18.1 1. Write the names of eight important metals. Give an example of one important ore of each metal. ............................................................................................................................. 2. What is the difference between an ore and a mineral? ............................................................................................................................. 3. Name some important methods of concentrating the ores. ............................................................................................................................. 4. Which method of concentration is applied in the following cases? (i) Magnetic ores (ii) Sulphide ores (iii) Bauxite ore ............................................................................................................................. 5. Which metal is extracted from the mineral zinc blene ? ............................................................................................................................. 18.2.3 Calcination and Roasting of the Ore The concentrated ore is converted into metal oxide by calcination or roasting. (A) Calcination : Calcination involves heating of the concentrated ore in a limited supply of air so that it loses moisture, water of hydration and gaseous volatile substances. The ore is heated to a temperature so that it does not melt. Two examples of calcination are given below: (i) Removal of water of hydration 2 3 2 2 3 2Al O .2H O Al O 2H O  (ii) Expulsion of 2CO from carbonate 3 2ZnCO ZnO 2CO  10 MODULE - 6 Chemistry Notes Chemistry of Elements (iii) Self-reduction : This is applied to the sulphide ores of copper, mercury and lead . The ores are heated in air, a part of these sulphide ores is changed into the oxide or sulphate which then reacts with the remaining part of the sulphide ore to give the metal and sulphur dioxide. The reactions showing their extraction are given below : (1) 2 2 2 22Cu S 3O 2Cu O 2SO   Copper glance 2 2 22Cu O Cu S 6Cu SO   Copper produced at this stage is called Blister copper. The evolution of sulphur dioxide produces blisters on the surface of solidified copper metal. (2) 2 22HgS 3O 2HgO 2SO   Cinnabar 22HgO HgS 3Hg SO   (3) 2 22PbS 3O 2PbO 2SO   Galena 2 4PbS 2O PbSO  2PbS 2PbO 3Pb SO   4 2PbS PbSO 2Pb 2SO   (B) Reduction of concentrated ores by other methods: Some metals cannot be obtained from their ores by using common reducing agents such as C, CO, 2H etc. Other methods of reduction are used for such cases. (i) Reduction by precipitation : Noble metals like silver and gold are extracted from their concentrated ores by dissolving metal ions in the form of their soluble complexes. The metal ions are then regenerated by adding a suitable reagent. For example, concentrated argentite ore  2Ag S is treated with a dilute solution of sodium cyanide (NaCN) to form a soluble complex : 2 2 2Ag S 4NaCN 2Na [Ag(CN) ] Na S   This solution is decanted off and treated with zinc to precipitate silver, 2 2 42Na[Ag(CN) ] Zn Na [Zn(CN) ] 2Ag    (ii) Electrolytic Reduction : Active metals like sodium, potassium and aluminium etc., are extracted by the electrolysis of their fused (molten) salts. For example, sodium is obtained by the electrolysis of fused sodium chloride (Down's process). The reactions taking place in the electrolytic cell are : NaCl Na Cl  11 MODULE - 6Occurrence and Extraction of Metals Notes Chemistry of Elements Na ions move towards the cathode and Cl ions move towards the anode. Following reactions take place at the electrodes : +Na e Na  (metal)(Negative electrode) At the Cathode (Reduction) Cl Cl e   (Positive electrode) At the Anode (Oxidation) 2Cl Cl Cl  Aluminium is extracted from molten alumina (Al 2 O 3 ) by electrolysis. The melting point of alumina is quite high (2323K) which is inconvenient for electrolysis. It dissolves in molten cryolite (Na 3 AlF 6 ) at around 1273 k. The reactions which take place in the cell are:. At the Cathode 3Al 3e Al (metal)   At the Anode 2 2C 2O CO 4e    Intext Questions 18.2 1. Explain the following terms : Calcination, Roasting, Smelting, Flux and Slag. ................................................................................................................................. 2. Which is the cheapest and most abundant reducing agent employed in the extraction of metals ? ................................................................................................................................. 3. Name the process which is used for converting sulphide ores into oxides. ................................................................................................................................. 4. How are oxide ores reduced ? ................................................................................................................................. 5. Name few materials which are used as flux in metallurgical processes. ................................................................................................................................. 6. What happens to (i) Bauxite and (ii) Calamine ores during calcination ? ................................................................................................................................. 18.2.5 Refining of Metals Except in the electrolytic reduction method, metals produced by any other method are generally impure. The impurities may be in the form of (i) other metals (ii) unreduced oxide of the metal (iii) non-metals like carbon, silicon, phosphorus, sulphur etc. and (iv) 12 MODULE - 6 Chemistry Notes Chemistry of Elements Green stick Molten metal Scum flux or slag. Crude metal may be refined by using one or more of the following methods : (i) Liquation : Easily fusible metals like tin, lead etc. are refined by this process. In this method, the impure metal is poured on the sloping hearth of a reverberatory furnace (Fig.18.6) and heated slowly to a temperature little above the melting point of the metal. The pure metal drains out leaving behind infusible impurities . Infusible Impurities Outlet For Molten Pure Metal Molten Pure Metal Impure Metal Hearth Fig. 18.6 : Liquation (ii) Poling : Poling involves stirring the impure molten metal with green logs or bamboo. The hydrocarbons contained in the pole reduce any metal oxide present as impurity. Copper and tin are refined by this method (Fig. 18.7). (iii) Distillation : Volatile metals like zinc and mercury are purified by distillation. The pure metal distils over, leaving behind non-volatile impurities. (iv) Electrolytic Refining : A large number of metals like copper, silver, zinc, tin etc. are refined by electrolysis. A block of impure metal is made the anode and a thin sheet of pure metal forms the cathode of the electrolytic cell containing suitable metal salt solution which acts as an electrolyte (Fig. 18.8). On passing current, pure metal deposits at the cathode sheet while more electropositive impurities are left in solution. Less electropositive metals do not dissolve and fall away from the anode to settle below it as anode mud. Fig. 18.8 : Electrolytic Refining For example, in the electrolytic refining of crude copper (blister copper), a large impure metal anode Electrolyte anode mud Pure metal cathode Fig. 18.7 : Polling