General Principles and Processes of Isolation of Elements(CLASS-12)

Transcription

1 General Principles and Processes of Isolation of Elements(CLASS-12) Topic Name : Earth as a Source of Elements In the combined state, Metals may exist as oxides, carbonates, sulphates, halides, sulphides, etc. This variation in occurrence is due to the difference in chemical reactivity of the elements. Elements Iron Aluminium Copper Zinc Gold, Platinum Nature of occurrence Iron oxide (Fe2O3) (Haematite) Iron oxide (Fe2O3, FeO2) (Magnetite) Iron sulphide (FeS) (Pyrites) Aluminium oxide hydrate (Al2O3, 2H2O) (Bauxite) Copper pyrite( CuFeS2) Zinc sulphide (ZnS) (Zinc blende) Native Topic Name : Extration of Metals During the formation of the earth, the elements showed a tendency to combine together to form compounds. Minerals The natural materials in which the metals or their compounds occur in the earth are called minerals. Ores Those minerals from which the metals can be extracted profitably and conveniently are called ores. Ores may be most commonly present as oxides, sulphides, carbonates or halides as shown below: Metallurgy The various processes involved in the extraction of a metal from its ores and its refining constitute metallurgy of that metal. The non-metallic impurities of earth and rocks associated with ores are called gangue or matrix. The major component of gangue is silica (SiO2) present as quartz. We do follow some general steps which are below: Main process 1. Concentration of the ore (removal of unwanted metals and gangue to purify the ore). Sub process i. Gravity separation or levigation or hydraulic washing ii. Froth floatation. iii. Chemical separation or leaching iv. Magnetic separation 2. Production of metals. i. Conversion of concentrated ore into metal oxide. a. Calcination for carbonate or hydrated oxide ore. b. Roasting for sulphide ore ii. Reduction of metal oxide to metal. a. Roasting - for example, Cu is produced by roasting copper pyrite (CuFeS2) in air. (Reduction by heat alone) b. Electrolytic reduction of highly reactive metals c. Chemical reduction Smelting i.e. reduction by carbon Aluminothermite process 3. Refining of impure metals into pure metals Electrolytic refining. Zone refining Distillation Vapour phase refining Liquation Chromatographic methods DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI Page 1

2 Concentration of the ore The process of removal of unwanted impurities (gangue) from the ore is called concentration of the ore. The first step would be crushing. Crushing of the ore breaks the huge lumps of ore into smaller pieces or even fine powder. This helps in easy separation of gangue from the useful mineral. The following processes are then followed individually or in combination to concentrate the ore: i. Gravity separation or levigation This method is used when the ore particles are heavier than the gangue particles. The finely powdered ore is washed down through a stream of water over a vibrating sloped table with long wooden stripes called riffles (forming grooves). The heavier ore particles are obstructed by the riffles and settle down in the grooves. They move to one side because of the motion of the table, and are removed. The lighter gangue particles are washed away. ii. iii. Froth floatation process This method is based on the difference in the wetting properties of ore and gangue with oil and water (preferential wetting). It is used for the extraction of those metals in which the ore is preferentially wetted by oil, and the gangue is wetted by water. Collectors:- Enhance the non-wettability of sulphide ore. Pine oil, xanthates and fatty acid are used as collectors Forth stabilizers:- Help in stabilization of the forth. Cresol and aniline are used as froth stabilizers. Depressant:- Froth floatation method can also be used for separating one sulphide ore from another by depressant. Depressants are used to prevent certain types of particles from forming the froth. NaCN can be used as a depressant in the separation ZnS and PbS. Leaching In this method, the difference in some chemical properties of the mineral and the gangue particles is used. The powdered ore is treated with certain reagents such that the mineral is soluble but other impurities remain insoluble. Leaching of Bauxite ore(bayer s Process):- Leaching is used to concentrate ores of aluminium, silver, gold, etc. In the Bayer's process of extraction of aluminium, bauxite (Al2O3, 2H2O) is treated with hot sodium hydroxide solution. Water soluble sodium aluminate (NaAlO2) formed is filtered to remove the undissolved gangue particles as sludge. The dissolved metal is recovered by agitating dilute filtrate (of sodium aluminate) for 2-3 hours to give aluminium hydroxide precipitate which on heating gives alumina (aluminium oxide). Al2O3 2H2O + 2NaOH 2NaAlO2 + 3H2O Bauxite ore Sodium aluminate NaAlO2 + 2H2O Al(OH)3 + NaOH Aluminium hydroxide Al(OH)3 Al2O3 + 3H2O Leaching of Gold and Silver from its ore(hydrometallurgy):- Leaching is also used for some precious metals such as gold and silver. Here, the silver and gold ores are treated with aqueous sodium or potassium cyanide. Cyanide complexes of gold and silver are formed. These are soluble in water and are easily separated from the gangue by filtration. The filtrate (cyanide complex solution) is treated with zinc to get free metallic silver. This process is called MAC-ARTHER FOREST PROCESS. for example For Argentite (Ag2S) ore of silver 4Au + 8KCN + 2H2O + O2 4K[Au(CN)2] + 4KOH iv. 2K[Au(CN)2] + Zn K2 [Zn(CN)4] + 2Au Magnetic separation process This process is used in the extraction of metals which exhibit magnetic properties. For example: in the extraction of iron, crushed magnetite ore (iron) particles are separated using their magnetic property. DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI Page 2

3 The pulverized ore is moved on a conveyor belt. Electro-magnetic wheel of the conveyor attracts only the magnetic particles into a separate heap. Only the magnetic particles are attracted by the magnetic wheel. These particles fall separately into a different heap as shown in the diagram. Production of metals This mainly consists of two steps: i. Conversion of concentrated ore to metal oxide. ii. Reduction of metal oxide to metal. iii. Displacement method i. Conversion of concentrated ore to metal oxide It is easier to obtain metals from their oxides rather than from carbonates or sulphides. So, the ore is first converted into an oxide. A carbonate ore or hydrated oxide ore is converted into oxide by calcinations. A sulphide ore is converted into oxide by roasting. a. Calcination Calcination is a process of heating the ore strongly either in a limited supply of air or in absence of air. Water from hydrated oxides is removed. Carbonates decompose to oxides with expulsion of carbon dioxide. Volatile impurities are also removed. For eg: CaCO3 CaO + CO2 Al2O3.2H2O Al2O3 + 2H2O b. Roasting Roasting is a process of heating the ore strongly in the presence of excess air. Sulphide ores are oxidized to oxides. Volatile impurities and moisture are also expelled. 2HgS + 3O2 2HgO + 2SO2 Calcination and roasting are selective processes as they cannot be used for chloride ores. Calcination Roasting 1. It converts carbonate ores into oxides. 1. It converts sulphide ores into oxides. 2. Ore is heated in the absence of air. 2. Ore is heated in the presence of air. ii. Reduction of metal oxide to metal The roasted or calcinated ore is reduced to its metallic form. Depending on the type of ore, various methods for reduction may be used. a. Reduction by heating alone (Roasting) The oxides of metals, low in the reactivity series, can be reduced to obtain the metals by heating the ore. Copper obtained from its ore copper pyrite (CuFeS2), by roasting, when heated to high temperature, forms copper metal. Here, the sulphide itself serves as a reducing agent for the oxide.this method is also called auto-reduction. 2Cu2S + 3O2 2Cu2O + 2SO2 2Cu2O + Cu2S 6Cu + SO2 b. Chemical reduction process Under this process, the metal oxides of metal in the middle of the reactivity series are reduced to free metal using chemical reducing agents such as carbon, aluminium or magnesium. The basic principle is to remove the oxygen of the metal oxides by reaction with carbon, carbon monoxide or other metals for which oxygen has greater affinity. Reduction by Carbon (Smelting) Oxides of metals such as zinc, iron, copper, nickel, tin and lead are mixed with a suitable amount of carbon and heated to a high temperature (above the melting point of the metal). The oxide is reduced to metal. DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI Page 3

4 ZnO + C Zn + CO Carbon is used only if it has a greater affinity for oxygen than for the metal, for example, carbon can reduce copper oxide to copper, but it cannot reduce calcium oxide. During reduction, a flux is also added to the ore. The flux combines with the impurities to form an easily fusible, removable slag. SLAG AND FLUX Impurities + Flux Slag The type of flux depends upon the nature of the impurities. For basic impurities such as CaO, FeO, etc., acidic flux like SiO2 is used. FeO + SiO2 FeSiO3 For acidic impurities such as SiO2, a basic flux like CaO is used. SiO2 + CaO CaSiO3 Reduction by aluminium Metals, which are too active to be obtained by reduction of their oxides with carbon, are reduced by using aluminium, a powerful reducing agent. The reaction is highly exothermic and is known as the Alumino-thermite process. Manganese and chromium oxides are reduced by this process. Cr2O3 + 2Al 2Cr + Al2O3 Chromium metal Fe2O3 + 2Al 2Fe + Al2O3 c. Electrolytic reduction The oxides (or chlorides) of highly reactive metals such as sodium, potassium, magnesium, aluminium and calcium cannot be reduced by using carbon or aluminium. Electrolytic reduction is the process used to extract the above metals. The oxides (or chlorides) are taken in the molten form. Since these metals are highly reactive, care should be taken to remove them from the electrolytic cell after their formation (to avoid further reaction). Further, electrodes used should be inert. Refining of metals This process ensures the separation of even the residual impurities from the extracted metals. Refining methods are different for different metals. The methods depend upon the purpose for which the metal is to be used. The two main categories of refining are: Physical methods Liquation Distillation Zone refining (Fractional refining) Chemical methods Oxidative refining Electrolytic refining Van Arkel method (Vapour phase refining) Chromatographic method Physical methods Liquation method Readily fusible metals (low melting points) such as tin, lead and bismuth are purified by liquation. The impurities do not fuse and are left behind. In this process, the block of impure metal is kept on the sloping floor of a hearth and heated slowly. The pure metal liquefies (melts) and flows down the furnace. The non-volatile impurities are infusible and remain behind. Distillation method In this process, metals with low boiling points such as zinc, calcium and mercury are vapourized in a DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI Page 4

5 vessel. The pure vapours are condensed into pure metal in a different vessel. The non-volatile impurities are not vapourized and so are left behind. Zone refining (Fractional crystallization) This process is used to obtain metals of very high purity, for example, the refining of germanium. It is based on the property that when an impure molten metal is cooled gradually, only the pure crystals of the metal are formed. The impurities are left in the remaining part of the molten metal. A circular heater fitted around an impure germanium rod is slowly moved from one end to the other. The heater melts a band of metal and as the heater moves ahead, the pure metal crystallizes out of the melt. As the heater moves ahead, the impurities are swept forward in the molten zone. Finally, all impurities reach the other end of the rod. This end is cut and discarded. Silicon and gallium (used as semi-conductors) are also zone refined. Chemical methods Zone refining Oxidation method In this process, the impurities are oxidized instead of the metal itself. Air is passed through the molten metal. Impurities such as phosphorus, sulphur, silicon and manganese get oxidized and rise to the surface of the molten metal, which are then removed. Electrolytic refining method The process of electrolysis is used to obtain very highly purified metals. It is very widely used to obtain refined copper, zinc, tin, lead, chromium, nickel, silver and gold. In this process, The impure slab of the metals acts like the anode. A pure thin sheet of metal acts like the cathode. A salt solution of the metal is used as the electrolyte. Electrolytic refining On passing an electric current, pure metal from the electrolyte is deposited on the cathode. The impure metal dissolves from the anode and goes into the electrolyte. The impurities collect as the anode mud below the anode. Van Arkel method (Vapour phase refining) This method is based on the thermal decomposition of metal components. The process of decompositon of a compound into different substances due to the supply of heat is called thermal decomposition. The two requirements of the this methods are: (i) (ii) The metal should form a volatile compound with an available reagent. The volatile compound should be easily decomposable, so that the recovery is easy. DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI Page 5

6 Titanium metal is purified by this method. When the impure titanium metal is heated with iodine at a temperature of 250 o C, volatile titanium tetraiodide (TiI4) is formed. The impurities are left behind, as they do not react with iodine. The titanium tetraiodide vapour is passed over a hot tungsten filament at 1400 o C. The vapour is decomposed and pure titanium is deposited on the filament and is removed. The iodine is reused. Ti + 2I2 TiI4 Titanium tetraiodide TiI4 Ti + 2I2 Tungsten filament Mond s process for Refining Nickel:- In this method, nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl K Ni + 4CO Ni(CO) 4 The carbonyl is subjected to higher temperature so that it is decomposed giving the pure metal K Ni(CO) 4 Ni + 4CO Chromatographic method This method is based on the difference in the adsorbability of the metal and the impurities.in column chromatography a glass column is packed with an adsorbent like alumina(al2o3).the mixture of metal and the impurity is poured over this adsorbent followed by an eluent. The component which is weakly adsorbed comes out first followed by the next strongly adsorbed component. This method is specifically useful for the separation of the elements available in very small amounts. Hydrometallurgy:- Extraction of metal involving aqueous solution is known as hydrometallurgy. Silver, Gold etc are extracted by this method. Pyrometallurgy:- Extraction of metal using heat is called pyrometallurgy. Cu, Hg etc. which are found in the form of their oxides, carbonates and sulphides are extracted by this method. Electrometallurgy:- Extraction of highly reactive metals such as Na, Mg, Ca etc. by carrying out electrolysis from their molten or fused ore. Pig iron:- Iron which is obtained from the blast furnace is highly impure contains 4% carbon. Cast iron:- It is made from pig iron contains 3% carbon is hard and brittle. Wrought iron:- This type iron is the malleable and purest form of iron which contains 0.5% - 1.5% carbon. Copper matte:- It is mixture of Cu2S and FeS. Blistter copper:- Solidified copper after extraction has blistered appearance due evolution of SO2 is called blister copper. Extraction of Cu, Zn, Fe, Al from their respective ores.in c.w. notebook. DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI Page 6