Chemistry Of Elements
Occurrence and Extraction of MetalsMODULE - 6Chemistry of Elements18NotesOCCURRENCE ANDEXTRACTION OF METALSMetals and their alloys are extensively used in our day-to-day life. They are used formaking machines, railways, motor vehicles, bridges, buildings, agricultural tools, aircrafts,ships etc. Therefore, production of a variety of metals in large quantities is necessary forthe 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 crustin 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 fromtheir ores, called metallurgical processes.ObjectivesAfter 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, Agand 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. Forexample, brass is an alloy of copper and zinc; steel is an alloy of iron and carbon.1
MODULE - 6Chemistry of ElementsNotesChemistry 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 withsuitable 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 andelectrolytic refining.18.1Occurrence of MetalsMetals occur in nature in free as well as combined form. Metals having low reactivityshow 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 arecalled "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, carbondioxide and non-metals like oxygen, sulphur, halogens, etc. to form their compounds, likeoxides, sulphides, carbonates, halides and silicates. i.e., they occur in nature in a combinedstate.A naturally occurring material in which a metal or its compound occurs is called amineral. A mineral from which a metal can be extracted economically is called anore.An ore is that mineral in which a metal is present in appreciable quantities andfrom which the metal can be extracted economically.The main active substances present in nature, expecially in the atmosphere are oxygenand 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 sodiumchloride). Most avtive metals are highly electropositive and therefore exist as ions. It is forthis 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 airto form sulphates. This explains the occurrence of sulphate ores.Ores are invariably found in nature in contact with rocky materials. These rocky orearthy impurities accompanying the ores are termed as gangue or matrix.Some important ores and the metals present in these ores are listed in Table 18.12
Occurrence and Extraction of MetalsTable 18.1 Some Important OresType of OreMetals (Common Ores)Native MetalsGold (Au), silver (Ag)Oxide oresIron (Haematite, Fe2O3 ); Aluminium (Bauxite, Al2O3 . 2H2O); Tin(Cassiterite, SnO2); Copper (Cuprite, Cu2O); Zinc (Zincite, ZnO);Titanium (Ilmenite, FeTiO3, Rutile, TiO2)Sulphide oresZinc (Zinc blende, ZnS); Lead (Galena, PbS); Copper (Copperglance, Cu2S); Silver (Silver glance or Argentite, Ag2S); Iron (IronMODULE - 6Chemistry of ElementsNotespyrites, FeS2)Carbonate oresIron (Siferite, FeCO3 ); Zinc (Calamine, ZnCO3) , Lead(Cerrusite, PbCO3)Sulphate oresLead (Anglesite, PbSO4)Halide oresSilver (Horn silver, AgCl); Sodium (Common salt or Rock salt,NaCl); Aluminium (Cryolite, Na3AlF6)Silicate oresZinc (Hemimorphite, 2ZnO.SiO2.H2O)18.1.1 Mineral Wealth of IndiaIndia possesses large deposits of minerals of some important metals such as iron,manganese, aluminium, magnesium, chromium, thorium, uranium, titanium and lithium.They constitute one-quarter of the world’s known mineral resources.Mineral fuels (likecoal ,petroleum and natural gas) constitute more than 80% while metallic minerals constituteonly about 10% of the total volume of mineral production in a year. In this section, we shalllist some of the important minerals of a few common metals such as Fe, Cu, Ag, Zn, Ti, AlSn, Pb and Na and their locations in India.Important Ores of Some Common ElementsIronIron ore reserves in the country are estimated at 1750 crore tonnes.Major iron ore mining is done in Goa, Madhya Pradesh, Bihar, Karnataka,Orissa and Maharashtra.AluminiumIts chief ore, bauxite is available in India in abundance. The annualproduction of bauxite is estimated to be more than 2 million tonnes.Important deposits of bauxite in the country occur in Bihar, Goa, MadhyaPradesh, Maharashtra, Tamil Nadu, Gujarat, Karnataka, Orissa, UttarPradesh, Andhra Pradesh, Jammu & Kashmir and Rajasthan.CopperIndia does not have rich copper ore deposits. The total reserves of theore in the country are estimated around 60 crore tonnes. Major oreproducing areas are Singhbum district (Bihar), Balaghat district (MadhyaPradesh) and Jhunjhunu district (Rajasthan).Zinc and Lead India has no significant deposits of lead ores. Lead and Zinc have beenlocated at Zawar mines near Udaipur (Rajasthan) and at Hazaribagh3
MODULE - 6ChemistryChemistry of Elements(Jharkhand), Sargipalli mines in Orissa and Bandalamottu lead project inAndhra Pradesh. Some reserves have been located in Gujarat andSikkim.The total reserves are estimated at about 360 million tonnes in whichlead content is estimated to be around 5 million tonnes and zinc to be around16 million tonnes.TinDeposits of tinstone SnO2 are found in Hazaribagh (Jharkhand ) andOrissa.SilverIndia does not possess rich silver deposits. Gold from Kolar fields andHutti gold fields (Karnataka) and lead-zinc ores of Zawar mines(Rajasthan) contain some silver.TitaniumIlmenite FeTiO3 is recovered from beach sands of Kerala and TamilNadu. The estimated reserves are around 100 to 150 million tonnes.Sodium :Tincal or Native borax Na 2 B4 O7 .10H 2 O is found in Ladakh andKashmir.Notes18.2. General Principles of Extraction of MetalsThe process of extracting the metals from their ores and refining them is called metallurgy.The choice of the process depends upon the nature of the ore and the type of the metal.The metal content in the ore can vary depending upon the impurities present and chemicalcomposition of the ore. Some common steps involved in the extraction of metals fromtheir ores are :(i)Crushing and pulverization(ii)Concentration or dressing of the ore(iii) Calcination or roasting of the ore(iv) Reduction of metal oxides to free metal(v) Purification and refining of metal.18.2.1. Crushing and PulverizationOre-stoneThe ore is generally obtained as big rock pieces. These biglumps of the ore are crushed to smaller pieces by using jawcrushers and grinders. It is easier to work with crushed ore.The big lumps of the ore are brought in between the plates ofa crusher forming a jaw. One of the plates of the crusher isstationary while the other moves to and fro and the crushedpieces are collected below (Fig. 18.1).Crushed ore The crushed pieces of the ore are then pulverized (powdered)in a stamp mill shown in Fig. 18.2. The heavy stamp rises andFig. 18.1 : Jaw Crusher falls on a hard die to powder the ore. The powdered ore isthen taken out through a screen by a stream of water.4
Occurrence and Extraction of MetalsPulverization can also be carried out in a ball mill. The crushed ore is taken in a steelcylinder containing iron balls. The cylinder is set into revolving motion. The striking ballspulverize the crushed ore into fine powder.Rotating cam shaftMODULE - 6Chemistry of ElementsNotesCrushed oreStampScreenDieFig. 18.2 : The Stamp mill18.2.2 Concentration or Dressing of the OreGenerally, 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 orore dressing.There are several methods for concentrating the ores. The choice of method depends onthe nature of the ore. Some important methods are :(i) Gravity separation (Hydraulic washing) : In this method, the light (low specificgravity) earthy impurities are removed from the heavier metallic ore particles bywashing with water. It is therefore, used for the concentration of heavier oxide ores,like haematite Fe 2 O3 tinstone SnO2 and gold (Au). In this method, as shown inthe Fig. 18.3 the powdered ore is agitated with water or washed with a strong currentof water. The heavier ore settles down rapidly in the grooves and the lighter sandyand earthy materials (gangue particles) are washed away.Jet of waterDenser oreparticlesVibratingtable withgroovesWaterGangueparticlesFig. 18.3 : Gravity Separation (Hydraulic Washing)5
MODULE - 6Chemistry of ElementsChemistry(ii) Magnetic separation method :By this method, those ores can be concentrated which either contain impurities whichare magnetic or are themselves magnetic in nature.For example, the tin ore, tin stone SnO2 itself is non-magnetic but contains magneticimpurities such as iron tungstate FeWO 4 and manganese tungstate MnWO 4 .NotesOreMagneticwheelNon-magneticgangueMagnetic oreNon-mageticwheelFig. 18.4. : Magnetic SeparationThe finely powdered ore is passed over a conveyer belt moving over two rollers, oneof which is fitted with an electromagnet (Fig. 18.4). The magnetic material is attractedby the magnet and falls in a separate heap. In this way magnetic impurities areseparated 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 CuFeS2 . It is based on the different wetting propertiesof the surface of the ore and gangue particles. The sulphide ore particles are wettedpreferentially by oil and gangueparticles by water. In this process,finely powdered ore is mixed witheither pine oil or eucalyptus oil. Itis then mixed with water. Air isblown through the mixture with agreat force. Froth is produced inthis process which carries theweted ore upwards with it.Impurities (gangue particles) areleft in water and sink to the bottomfrom which these are drawn off(Fig. 18.5).(iv) Chemical method :In this method the ore is treatedwith a suitable chemical reagentwhich dissolves the ore leaving behind insoluble impurities. The ore is then recoveredfrom the solution by a suitable chemical method. This is applied for extraction ofaluminium from bauxite Al2 O3 .2H 2 O . Bauxite is contaminated with iron (III) oxide Fe 2 O3 , titanium (IV) oxide TiO2 and silica SiO2 . These impurities are removedby digesting the powdered ore with aqueous solution of sodium hydroxide at 420 KFig. 18.5 : Froth floatation6
Occurrence and Extraction of Metalsunder pressure. Aluminium oxide dissolves in sodium hydroxide, whereas, iron (III)oxide, silica and titanium (IV) oxide remain insoluble and are removed by filtration.MODULE - 6Chemistry of ElementsAl2 O3 6NaOH 2Na 3 AlO3 3H 2 OSodium aluminateSodium aluminate is diluted with water to obtain precipitate of aluminium hydroxide. It isfiltered and ignited to obtain pure alumina.Na 3 AlO3 3H 2 O Al(OH)3 3NaOHNotes 2Al(OH)3 Al2O3 3H 2OIntext Questions 18.11.Write the names of eight important metals. Give an example of one important oreof 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 OreThe 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 losesmoisture, water of hydration and gaseous volatile substances. The ore is heated to atemperature so that it does not melt. Two examples of calcination are given below:(i) Removal of water of hydrationAl2 O3 .2H 2 O Al2O3 2H 2O(ii) Expulsion of CO2 from carbonateZnCO3 ZnO 2CO 27
MODULE - 6Chemistry of ElementsChemistry(B) Roasting :Roasting is a process in which the concentrated ore is heated in a free supply of air at atemperature insufficient to melt it. The following changes take place during roasting :(i) Drying of the ore.(ii) Removal of the volatile impurities like arsenic, sulphur, phosphorus and organic matter.Notes4As 3O2 2As 2O3 (g)S O2 SO2 (g)4P 5O2 P4O10 (g)(iii) Conversion of the sulphide ores into oxides2PbS 3O2 2PbO 2SO22ZnS 3O2 2ZnO 2SO2Calcination and roasting are generally carried out in a reverberatory furnace or in a multiplehearth furnace.18.2.4 Reduction of the Metal Oxides to Free MetalThis process is carried out after calcination or roasting of ores. In this process calledsmelting, the oxide ores are converted into the metallic state by reduction.(A) Smelting :Smelting is a process in which the oxide ore in molten state is reduced by carbon orother reducing agents to free metal.(i)by using carbon as a reducing agent :This method is used for the isolation of iron, tin and zinc metals from their respectiveoxides. The oxide ores are strongly heated with charcoal or coke. Reduction occursby the action of carbon and/or carbon monoxide which is produced by the partialcombustion of coke or charcoal.Fe2 O3 3C 2Fe 3COFe2 O3 CO 2FeO CO 2FeO CO Fe CO 2SnO2 2C Sn 2COZnO C Zn COAlthough the ore has been concentrated in an earlier step, it is still contaminated withsome gangue material which is finally removed in the reduction process by the additionof flux during smelting.8
Occurrence and Extraction of MetalsFlux is a chemical substance which combines with gangue at higher temperaturesto form easily fusible material called slag which is not soluble in the molten metal.Flux are of two types :Basic Flux :On heating, lime stone is converted into calcium oxide used as basic flux whichcombines with acidic impurities like silica in metallurgy of iron and forms fusiblecalcium silicate CaSiO3 .CaCO3(Limestone)CaO(Basic flux) MODULE - 6Chemistry of ElementsNotes CaO CO 2 (g)SiO 2(Acidic gangue) CaSiO3(Slag)Acidic flux :SiO2 is used as acidic flux to remove basic impurity of FeO in metallurgy of Cu.SiO 2 FeO(Basic gangue)(Acidic flux) FeSiO3(Slag)The fusible slag, such as calcium silicate formed during smelting floats over themolten metal and is thus easily removed. Another advantage is that the slag providesa covering to the molten metal thus preventing it from getting oxidized by air.(ii) Other reducing agents :Oxide ores which cannot be reduced by carbon or metals which show affinity tocarbon by forming metal carbides, are reduced by reducing agents like aluminium,sodium, magnesium or hydrogen. Oxide like chromium oxide Cr2 O3 or manganeseoxide Mn3O4 are reduced by aluminium powder is a highly exothermic reaction.This process is known as Goldschmidt's Alumino-thermite reduction method.Cr2 O3 2Al 2Cr Al2O3 Heat3Mn 3O4 8Al 9Mn 4Al2O3 HeatHeat is generated in the process due to the formation of Al2 O3 which is a highlyexothermic reaction.Titanium is obtained by the reduction of TiCl4 (produced by the action of carbon andchlorine on TiO2) by Mg in an inert atmosphere of argon (Kroll process).HeatTiCl4 2Mg Ti 2MgCl 21103 KTitanium can also be obtained by the reduction of TiO2 by sodiumTiO2 4Na Ti 2Na2OTungsten and molybedenum can be obtained by the reduction of their oxides byhydrogen,MoO3 3H2 Mo 3H2O9
MODULE - 6Chemistry of ElementsChemistry(iii) Self-reduction :This is applied to the sulphide ores of copper, mercury and lead . The ores are heatedin air, a part of these sulphide ores is changed into the oxide or sulphate which thenreacts with the remaining part of the sulphide ore to give the metal and sulphurdioxide. The reactions showing their extraction are given below :(1)Notes2Cu 2S 3O 2 2Cu 2 O 2SO 2Copper glance2Cu 2 O Cu 2S 6Cu SO 2Copper produced at this stage is called Blister copper. The evolution of sulphurdioxide produces blisters on the surface of solidified copper metal.(2)2HgS 3O 2 2HgO 2SO 2Cinnabar2HgO HgS 3Hg SO2(3)2PbS 3O2 2PbO 2SO2GalenaPbS 2O 2 PbSO 4PbS 2PbO 3Pb SO2PbS PbSO 4 2Pb 2SO 2(B) Reduction of concentrated ores by other methods:Some metals cannot be obtained from their ores by using common reducing agents suchas C, CO, H 2 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 bydissolving metal ions in the form of their soluble complexes. The metal ions are thenregenerated by adding a suitable reagent. For example, concentrated argentite ore Ag2S is treated with a dilute solution of sodium cyanide (NaCN) to form a solublecomplex :Ag 2S 4NaCN 2Na [Ag(CN)2 ] Na 2SThis solution is decanted off and treated with zinc to precipitate silver,2Na[Ag(CN)2 ] Zn Na 2 [Zn(CN)4 ] 2Ag (ii)Electrolytic Reduction :Active metals like sodium, potassium and aluminium etc., are extracted by theelectrolysis of their fused (molten) salts. For example, sodium is obtained by theelectrolysis of fused sodium chloride (Down's process). The reactions taking placein the electrolytic cell are : NaCl Na Cl10
Occurrence and Extraction of MetalsNa ions move towards the cathode and Cl ions move towards the anode. FollowingMODULE - 6Chemistry of Elementsreactions take place at the electrodes :At the Cathode Na e (Negative electrode)At the AnodeNa(metal)Cl Cl e (Positive electrode)Cl Cl (Reduction)(Oxidation)NotesCl 2Aluminium is extracted from molten alumina (Al2O3) by electrolysis. The melting point ofalumina is quite high (2323K) which is inconvenient for electrolysis. It dissolves in moltencryolite (Na3AlF6) at around 1273 k. The reactions which take place in the cell are:.At the Cathode Al3 3e At the AnodeC 2O 2 Al (metal)CO 2 4e Intext Questions 18.21.Explain the following terms :Calcination, Roasting, Smelting, Fl
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
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