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General Principles and Processes of Isolation of Elements

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General Principles and Processes of Isolation of Elements General principles and processes of isolation of elements CHAPTER GENERAL PRINCIPLES AND PROCESSES 7 OF ISOLATION OF ELEMENTS LEARNING OBJECTIVES (i) Explain the terms minerals, ores, concentration, benefaction, calcination, roasting, refining, etc. (ii) Understand the principles of oxidation and reduction as applied to the extraction procedures. (iii) Apply the thermodynamic concepts like that of Gibbs energy and entropy to the principles of extraction of Al, Cu, Zn and Fe. (iv) Explain why reduction of certain oxides like Cu2O is much easier than that of Fe2O3. (iv) Explain why CO is a favourable reducing agent at certain temperatures while coke is better in some other cases. (v) Explain why specific reducing agents are used for the reduction purposes. INTRODUCTION : Metals occur in nature, sometimes in free or native state but mostly in combined state i.e., in the form of their stable compounds associated with gangue or matrix. The earthy impurities such as sand, clay, rocks etc. associated with ore are collectively known as gangue or matrix. Metals like Cu, Ag, Au and Pt occur in the native form (metallic state). The natural material in which the metallic compounds (in the combined state) occur in the earth’s crust is known as mineral. A mineral may be a single compound or a complex mixture. Those minerals from which the metal can be readily and economically extracted are called ores. Thus all ores are minerals but all minerals are not ores. For eg. , bauxite (Al2O3 . 2H2O) and clay (Al2O3 . 2 SiO2 . 2H2O) are two minerals of aluminium, but aluminium can be profitably extracted only from bauxite and not from clay. Hence bauxite is an ore of aluminium, while clay is a mineral. Metallurgy is the science and technology of extracting metals from their natural sources and preparing them for practical use. It usually involves several steps : (1) mining, (2) concentrating the ore or otherwise preparing it for further treatment, (3) reducing the ore to obtain the free metal, (4) refining or purifying the metal, and (5) mixing the metal with other elements to modify its properties. This last process produces an alloy, a metallic material that is composed of two or more elements. STEPS IN EXTRACTION OF METALS : The following steps are involved in the extraction of metals : (A) Concentration of ore (B) Conversion of concentrated ore into metal oxide (C) Conversion of metal oxide into metals (D) Purification of metal. (A) Concentration of ore : The metal ores obtained from mines are generally mixed with non metals, sand, lime, clay and rock materials. These impurities are known as gangue or matrix. The removal of impurities from the ore is called concentration of the ore. The common methods of concentration are : (i) Gravity separation : This separation is based on the fact that specific gravities of metallic ore and the earthy impurities are different. The finely powered ore is agitated with water or washed with a running stream of water on a wilfley table. The lighter impurities are washed away leaving behind ore particles on the table. (ii) Froth flotation process : This method is generally applied to sulphide ores because sulphide ores are preferentially wetted by oils. The finely powdered ore is introduced into water to which small quantity of a suitable oil is added. Water is then agitated with a stream of air. The ore which is preferentially wetted by oil rises to the surface along with the foams while the gangue which is preferentially wetted by water remains in the water in the bottom. The foam along with ore is separated. This process is called froth flotation process. In froth flotation two types of substances are used - Frothing agent and flotation agent. Pine oil and eucalyptus oil are generally used as frothing agents and sodium ethyl xanthate or sodium amyl xanthate as flotation agents. (iii) Electromagnetic separation : The method is used when either the ore or the impurities are magnetic in nature e.g., chromite FeO, Cr2O3 being magnetic, can be separated from the gangue. Similarly, wolframite, FeWO4 (magnetic) and cassiterite, SnO2 (non-magnetic) as well as rutile TiO2 (Magnetic) and chlorapatite, 3Ca3(PO4)2.CaCl2 (non-magnetic ) are separated by magnetic separators. (iv) Chemical method or leaching : This method depends upon the chemical nature of the ore. In this method, the ore is treated with a suitable reagent which preferentially dissolves the active component of the ore. The ore in the concentrated form is then recovered from the solution by a suitable chemical method. For example bauxite ore of aluminium contains the impurities of iron oxide, titanium oxide and silica etc. Bauxite is heated in a solution of NaOH at 150-200°C. It dissolves in the solution forming a soluble complex while the impurities remain insoluble which are removed by filtration. Al2O3 + 6NaOH 2Na3AlO3 + 3H2O Sodium aluminate Gyaan Sankalp 1 General principles and processes of isolation of elements The filtrate is boiled with water, aluminium hydroxide gets precipitated which on heating at high temperature changes to alumina. Na3AlO3 + 3H2O Al(OH)3 + 3NaOH ; 2Al(OH)3 Al2O3 + 3H2O Leaching process is also employed in the recovery of some precious metals like silver and gold. In the metallurgy of silver and that of gold, the respective metal is leached with a dilute solution of NaCN or KCN in the presence of air (for O2) from which the metal is obtained later by replacement : – – – 4 M(s) + 8CN (aq) + 2H2O (aq) + O2 (g) ––––4[M(CN)2] (aq) + 4OH (aq) (M = Ag or Au) – 2– 2[M(CN)2] (aq) + Zn(s) ––––[Zn(CN)4] (aq) + 2M(s) (B) Conversion of concentrated ore into metal oxide : (i) Calcination : It is a process in which the concentrated ore is heated to expel the organic matter and moisture present in the ore. The ores are heated in the absence of air below their melting point. Ores are converted to metal oxide. This process is also used to remove water from hydrated hydroxide ores and carbon dioxide from carbonate ores. The process is generally carried out in reverberatory furnace. It renders the ore porous. CaCO3 (Lime stone) CaO + CO2 Al2O3.2H2O (Bauxite) Al2O3 + 2H2O ; 2Fe2O3.3H2O (haematite) 2Fe2O3 + 3H2O (ii) Roasting : This process is generally done with sulphide ores. Sulphide ores alone or in the presence of other substance are heated in the presence of air below their melting points. The impurities like sulphur, arsenic, phosphorus etc. are removed in the form of their volatile oxides. This process is generally carried out in reverberatory furnace or blast furnace. 2ZnS (Zinc blende) + 3O2 2ZnO + 2SO2 ; 2PbS (Galena) + 3O2 2PbO + 2SO2 (C) Conversion of metal oxide into metal : Reduction of the free metals : The method used for reduction, or smelting, of metal ores to the free metals depends on how strongly the metal ions are bonded to anions. When bonding is stronger, more energy is required to reduces the metals. This makes reduction more expensive. The most active metals usually have the strongest bonding. The least reactive metals occur in the free state and thus require no reduction. Examples include Au, Ag and Pt. This is why gold and silver have been used as free metals since prehistoric times. Some less active metals, such as Hg, can be obtained directly from their sulfides ores by roasting. This reduces metal ions to the free metals by oxidation of the sulfide ions. HgS(s) O(g)2 SO(g) 2 Hg(g) cinnabar from air obtained as vapour,later condensed Roasting more active metal sulfides produces metal oxides, but no free metals. 2NiS(s) + 3O2 (g) 2NiO(s) + 2SO2(g) The resulting metal oxides are then reduced to free metals with coke or CO. If C must be avoided, another reducing agent, such as H2, Fe, or Al, is used. SnO2 + 2C(s) Sn ( ) + 2CO(g) ; WO3(g) + 3H2(g) W(s) + 3H2O (g) The very active metals, such as Al and Na, are reduced electrochemically, usually from their anhydrous molten salts. If H2O is present, it is reduced preferentially. Reduction processes for some metals : Metal ion Typical reduction process Lithium, Li+ Potassium, K+ 2+ Calcium, Ca Electrolysis of molten salt Sodium, Na+ Magnesium, Mg2+ s 3+ l Aluminium, Al a t e m 2+ f Manganese, Mn o 2+ y Zinc, Zn t i Reaction of oxide with coke (carbon) v i Chromium, Cr2+ , Cr 3+ t or carbon monoxide (CO) c a 2+ 3+ Iron, Fe , Fe g n i s a 2+ e Lead, Pb r c 2+ n Copper, Cu I Silver, Ag+ 2+ Element occurring free, or easily obtained Mercury, Hg by roasting the sulfide or oxide ore Platinum, Pt2+ Gold, Au+ 2 Gyaan Sankalp General principles and processes of isolation of elements Some specific reduction processes : Metal Compound (ore) Reduction process Comments Mercury HgS (cinnabar) Roast reduction : heating of ore in air HgS + O2 Hg + SO2 Copper sulfides such as Cu2S Blowing of oxygen through purified Preliminary ore concentration (chalcocite) molten Cu2S and purification steps required Cu2S + O2 2Cu + SO2 to remove FeS impurities. Zinc ZnS (sphalerite) Conversion to oxide and reduction with Process also used for the carbon production of lead from galena, PbS 2Zns + 3O 2ZnO + 2SO 2 2 ZnO + C Zn + CO Iron Fe2O3 (hematite) Reduction with carbon monoxide 2C (coke) + O2 2CO Fe2O3 + 3CO 2Fe + 3CO2 Titanium TiO2 (rutile) Conversion of oxide to halide salt and Also used for the reduction of reduction with an active metal UF4 obtained from UO , 2 TiO2 + 2Cl2 + 2C TiCl4 + 2CO pitchblende TiCl4 + 2Mg Ti + 2MgCl2 Tungsten FeWO4 (wolframite) Reduction with hydrogen Used also for molybdenum WO3 + 3H2 W + 3H2O Aluminium Al2O3.nH2O (bauxite) Electrolytic reduction (electrolysis) in molten cryolite.
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