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. Na3[AlF6], at 1000°C 2Al2O3 4Al + 3O2 Sodium NaCl (sea water) Electrolysis of molten chlorides Also for calcium, magnesium, and 2NaCl 2Na + Cl2 other active metals in groups IA and IIA
Carbon reduction method : Smelting - The roasted or calcinated ore is mixed with suitable quantity of coke, the reducing substance and the flux and heated to a high temperature above the melting point of the metal. The metal oxide reduces to metal and the flux combines with impurities (gangue) to form easily fusible product known as slag. Slag is not soluble in molten metal. It is lighter and can be easily skimmed off from the surface of the molten metal. This process is done in reverberatory furnace or blast furnace. SnO2 + 2C Sn + 2CO Tin oxide Tin PbO (Lead oxide) + C Pb + CO ZnO (Zinc oxide) + C Zn + CO Flux : Metal ore contains some acidic or alkaline impurities which can form fusible products by combining with some substances. These substances are added during smelting and are called flux. The fusible product is called slag. For acidic impurity basic flux is used and for basic impurity acidic flux is used. CaO + SiO2 CaSiO3 Basic flux Acidic impurity Calcium silicate slag MnO + SiO2 MnSiO3 Basic impurity Acidic flux Manganese silicate slag FeO + SiO2 FeSiO3 Basic impurity Acidic flux Iron silicate slag Slag is generally a silicate or phosphate and melt before the melting point of the metal. Electrolytic method : If metal oxides, chlorides or hydroxides in fused state are electrolyzed the metals deposit at cathode. The reduction of active elements such as alkali metals, alkaline earth metals and the oxides of Al with carbon is difficult because these form carbides with carbon at high temperature. (D) Purification : Metals can be purified by following methods (a) Distillation : The metals with low boiling points can be purified by distillation. Ex. Zn, Cd, Hg etc. (b) Liquation : This process is used when the impurity is less fusible than the metal itself. The impure metal is placed on the slopping floor of the furnace and heated. The metal melts and drains away leaving behind the impurities. For e.g. tin. Gyaan Sankalp 3 General principles and processes of isolation of elements (c) Oxidation method : This method is employed when the impurity have greater affinity with oxygen as compared to metal. The impurities are oxidized to form scum. The scum is skimmed off. Some times metal oxides are used as oxidizing agents. For example, copper oxide is added to impure copper. (d) Thermal Decomposition methods : (i) Carbonyl method. It is used for the refining of Ni and Fe etc., The impure Ni is heated with CO. Nickel Carbonyl thus formed is then decomposed to get pure Ni and CO. The process is called Mond’s process.
Ni + 4CO Ni(CO)4 Ni + 4CO (ii)Van Arkel method: The crude metal is converted into volatile compound, while impurities remain unaffected during com- pound formation. The resulting compound is then decomposed to get pure metal . The method is used for refining of Ti , Zr , Be Heat and Si. Ti + 2I2 TiI4 Ti + 2I2
(e) Electrolytic Refining : Most of the metals like Ag, Au, Pb, Ni, Sn, Zn, etc., are refined by this method. The impure metal, in the form of blocks, is made the anode and thin plates of pure metal form the cathode. A solution of a salt of the metal is used as the electrolyte. On passing the electric current, pure metal from the anode dissolves and is deposited on the cathode. The soluble impurities pass into the solution while the insoluble impurities settle down below the anode as anode mud. (f) Zone refining method : This method is employed for obtaining metals in ultrapure state. It is based on the principle that the impurities which lower the melting point of a metal remain preferentially dissolved in the liquid phase. The solid phase will consist of pure metal. In actual practice, a cylinder of impure metal is kept in a tubular furnace and the heater is made to move at a very slow speed in a direction. As the heater moves along the tube, the solid material crystallizes and the advancing zone contains liquid with higher impurity content. The process is repeated number of times till the desired level of purity is obtained. (g) Chromatographic methods : This method is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. The mixture is put in a liquid or gaseous medium which is moved through the adsorbent. Different components are adsorbed at different levels on the column. Later the adsorbed components are removed (eluted) by using suitable solvents (eluant). This is very useful for purification of the elements which are available in minute quantities and the impurities are not very different in chemical properties from the element to be purified.
THERMODYNAMICS PRINCIPLES OF METALLURGY : To understand the variation in the temperature requirement for thermal reductions (pyrometallurgy) and to predict which element will suit as the reducing agent for a given metal oxide (MxOy), Gibbs energy interpretations are made. For any chemical process, G = H – TS and G° = – RT ln K For a process/reaction to be spontaneous, G should be negative, and therefore the equilibrium constant (K) should be greater than one. For a system consisting of reactants and products of two reactions, the overall reaction will occur only when the free energy change for the coupled (or combined) reaction is negative. When the metal MxO(s) is reduced, oxygen is removed from it by the reducing agent. This can be visualised decomposition of metal oxide into metal and oxygen. 1 M O(s) ––––x M (solid or liquid) + O (g) ; – G° (M O) ..... (1) x 2 2 f x
The reducing agent, say carbon, may get oxidised to CO or CO2 with corresponding free energy changes as described below : 1 C(s) + O (g) ––––CO (g) ; G° (CO) ..... (2) 2 2 f 1 1 1 1 C(s) + O (g) –––– CO (g) ; G° (CO ) ..... (3) 2 2 2 2 2 2 f 2 When carbon monoxide is formed : Adding eq. (1) and (2) MxO(s) + C(s) ––––x M (solid or liquid) + CO(g) ; rG° = [fG° (CO) – fG°(MxO)] So, if rG° (= fG° (CO) – fG° (MxO) is negative, carbon will be able to reduce metal oxide to metal with the formation of carbon monoxide (CO). When carbon dioxide is formed : Adding eq. (1) and (3) 1 1 1 M O(s) + C(s) ––––x M(s) + CO (g) ; G° = [ G° (CO ) – G°(M O)] x 2 2 2 r 2 f 2 f x
4 Gyaan Sankalp General principles and processes of isolation of elements
1 So, if G°( = G° (CO ) – G° (M O) is negative, carbon will be able to reduce metal oxide to metal with the formation of r 2 f 2 f x carbon dioxide (CO2). Should carbon be oxidised to CO or CO2 depends upon the relative change in free energy. The reaction having larger decrease in free energy (i.e., more negative value of rG°) will be preferred over the other one. rG° values are temperature-dependent. Therefore, the temperature at which the sum of rG° in the two combined redox processes is negative is chosen for carrying out the reaction. In rG° vs T plots, this temperature is that corresponds to the point of intersection of the two curves/lines, (the curve for metal oxide, and the other for the oxidation of the reducing agent). The difference in the two rG° values after that point determines if the reduction of the oxide of the upper line/curve is feasible by the element represented by the lowest line/curve. Ellingham diagrams helps us in predicting the feasibility of the thermal reduction of an ore. These diagrams also helps us in making a choice for a suitable reducing agent for the reduction of oxides.
–100
–200 Cu2O 4Cu+O2 eO 2F –300 +O2 2Fe C+O2 CO2 –400 O 2 C 2C
2 + +O2 O2 O –500 O C nO 2C f 2 2Z O o
1 –600 +O 2 – n l 2Z o m –700 J A k / ° –800 G O3 Al2 –900 2/3
l +O2 3 A O –1000 4/ 2Mg
g+O2 –1100 2M –1200
0°C 400°C 800°C 1200°C 1600°C 2000°C 273K 673K 1073K 1473K 1873K 2273K Temperature
Figure : Gibbs energy (G ) vs T plots (schematic) for the formation of some oxides (Ellingham diagram)
Ellingham diagrams are the plots of free energy of formation (fG°) for the oxides, sulphides and halides as a function of temperature. Each plot is a straight line except when any phase change occurs. The phase change, if any, is indicated by an abrupt change in the slope of the line/curve. Due to a decrease in entropy (S is –ve) during oxide formation, fG° becomes less negative. As a result, the fG° – T curves/lines have positive slope. There is a point on each Ellingham plot below which MxO is stable and above it, it is unstable. Applications : (a) Iron oxides ores are reduced by using coke (carbon-reduction method) in a blast furnace. The net reaction for the reduction of FeO by carbon is FeO (s) + C(s) ––––Fe(s) + CO (g) The change in the free energy for this reaction is given by
rG f G (CO) f G (FeO)
The reduction reaction will be feasible if rG is negative.
From Ellingham diagram f G (C, CO) vs T plot goes downwards, and f G (FeO, FeO) vs T plot goes upwards. Gyaan Sankalp 5 General principles and processes of isolation of elements At temperature above 700K, the C, CO line falls below Fe, FeO line. Therefore above, 700K, carbon should be able to reduce FeO to Fe, and itself gets oxidised to CO. From the Ellingham plots it can be seen that below 980K, carbon monoxide (CO) is a better reducing agent and above it, haematite is reduced by carbon. (b) From the rG° vs T plots the Cu – Cu2O plot is almost the top. Both C – CO and C – CO2 plots are much lower particularly beyond 500-600 K. Therefore, Cu2O can be easily reduced by heating with coke. (c) From the rG° vs T plot, it can be seen that the Zn – ZnO curve lies above the C – CO line beyond 1100 K. Therefore ZnO can be easily reduced to Zn by carbon.
ELECTROCHEMICAL PRINCIPLES OF METALLURGY : Highly reactive metals cannot be obtained from their compounds (or ores) by carbon reduction method. This is because these metals have more affinity for oxygen than carbon has. Such highly electropositive metals can be obtained by electrolytic reduction method. For an electrochemical reaction, G° = – nFE° where n is the number of electrons involved in the reactions, E° is the standard electrode potential of the redox pair. For a reaction to become spontaneous, E° should be positive. For more reactive metals, E° are highly negative. To make the net emf positive, an external emf greater than E° is applied in the direction opposite to that of the cell. Thus the ions in the solution or melt can be reduced by applying external emf, that is, by electrolysis, (or by electrolytic reduction method). In electrolytic reduction, the ions of the metal get reduced to the corresponding metal at the cathode, (–ve electrode).
EXTRACTION OF ALUMINIUM Aluminium is found in nature only in combined state. Its important ores are : (a) Felspar - KAISi3O8 (b) Mica (Abhrak) - KH2Al2(SiO3)4 (c) Kaolin - Al2O3.2SiO3.2H2O (d) Corundum - Al2O3 (e) Diaspore - Al2O3.H2O (f) Bauxite - Al2O3 . 2H2O (g) Cryolite-Na3AIF6 (h) Alunite or alumstone – K2SO4. Al2(SO4)34Al(OH)3 (i) Thrquoise - AlPO4.Al(OH)3- H2O (j) Gibbsite-Al2O3.3H2O (k) Beryl - 3BeO. Al2O3. 6SiO2. Wholler prepared aluminium by the reaction of potassium metal with anhy. AlCl3 Bunsen and Deville prepared it from the reaction of NaAlCl4 with sodium and also from the electrolysis of NaAlCl4 Charles M. Hall prepared aluminium from fused cryolite. Purification of bauxite ore : (a) Baeyer’s process : This process is mainly applied to bauxite ore containing ferric oxide as chief impurity. The colour of such ore is usually red and hence called red bauxite. Al2O3.2H2O + 2NaOH 2NaAlO2 + 3H2O Sodium metaaluminate (soluble) NaAlO2 + 2H2O NaOH + Al(OH)3 Precipitate (b) Hall’s process : Bauxite is fused with sodium carbonate. Al2O3 combines with sodium carbonate forming sodium metaaluminate . The fused mass is extracted with water where Fe2O3 and SiO2 remain as precipitate. Al2O3 + Na2CO3 2NaAlO2 + CO2 2NaAlO2 + CO2 + 3H2O 2Al(OH)3 + Na2CO3 (c) Serpeck’s process : This process is used when silica is present in considerable amounts in bauxite ore. The ore is mixed with coke and heated at 1800 ºC in presence of nitrogen, where AlN(aluminium nitride) is formed. Al2O3 + 3C + N2 2AlN + 3CO SiO2 + 2C Si + 2CO Calcination of aluminium hydroxide : The aluminium hydroxide precipitate obtained from the above process is calcined at 1500ºC in a rotatory kiln to obtain pure alumina (Al2O3). 1500ºC 2Al(OH)3 Al2O3 + 3H2O Electrolytic reduction of alumina- Pure. Al2O3 on electrolytic reduction gives aluminium. The melting point of Al2O3 is about 2000°C. This is reduced by mixing fused cryolite (Na3AlF6) + fluorspar (CaF2) to it. The melting point of this mixture is about 900°C. This mixture is good conductor of electricity. The inner carbon lining of the cell is used as cathode and the carbon rods as anode. The fused mixture of alumina + Cryolite + Fluorspar is taken in the cell whose temperature is maintained at about 900 to 1000°C. This mixture is covered by carbon powder which maintains the temperature of the cell. The carbon anodes are dipped in the mixture. Aluminium gets free at cathode while oxygen at anode. Aluminium thus obtained is about 99.8% pure.
6 Gyaan Sankalp General principles and processes of isolation of elements Alloys of aluminium :
Name of alloy Approximate composition Uses 1. Magnalium 95 % Al , 5 % Mg In the construction of airships, balances and pistons of motor engines. 2. Duralumin 95 % Al , 4 % Cu , Aeroplanes and automobile parts as its strength of 0.5 % Mg , 0.5 % Mn toughness is comparable to mild steels 3. Aluminium Bronze 90 % Cu , 9.5 % Al , For making utensils, cheap artificial jewelry , Photo 0.5 % Sn frames coins, golden paints 4. Nickel alloy 95 % Al , 4 % Cu ,1 % Ni Aircraft parts. 5. Y-alloy 93 % Al , 4 %Cu , For making pistons and machinery parts 2 % Ni , 1 % Mg
EXTRACTION OF COPPER, Cu : Copper is found in nature in both free and combined state. The Important ores of copper are (a) Cuprite or ruby copper or cuprite red Cu2O (b) Copper pyrite (Chalcopyrites) CuFeS2 or Cu2S.Fe2S3 (c) Copper glance Cu2S (d) Malachite Cu(OH)2.CuCO3 (e) Azurite Cu(OH)2.2CuCO3 ; 76% of the copper is extracted from the ore copper pyrites. Extraction of copper from copper pyrites : (a) Concentration : The ore is concentrated by froth flotation method. . (b) Roasting : This process is done in reverberatory furnace. The Impurities of arsenic and antimony are removed as volatile oxides. Sulphur oxidises to SO2. 2CuFeS2 + O2 Cu2S + 2FeS + SO2 (Main) ; 2Cu2S + 3O2 2Cu2O + 2SO2 (Side) ; 2FeS + 3O2 2FeO + 2SO2 (Side) (c) Smelting : The roasted ore is mixed with coke and silica and transferred to a small blast furnace. The mixture is heated in the presence of excess of air. The following changes occur in the blast furnace. (i) The cuprous oxide reacts with ferrous sulphide. FeS + Cu2O FeO + Cu2S (ii) Most of the iron sulphide is oxidised to ferrous oxide. 2FeS + 3O2 2FeO + 2SO2 (iii) Ferrous oxide combines with silica and forms ferrous silicate. By this reaction most of the iron is removed as slag. FeO + SiO2 FeSiO3 Ferrous silicate (Slag) (d) Bessemerisation : Molten matte i.e. mixture of Cu2S and FeS is transferred to a Bessemer converter. Bessemer converter is a pear shaped furnace in which the inner lining is of CaO or MgO. A blast of air mixed with sand is blown through the molten matte. The following reactions take place 2Cu2S + 3O2 2Cu2O + 2SO2 ; Cu2S + 2Cu2O 6Cu + SO2 The molten copper thus obtained is cast into ingots. It is about 98% pure copper. It is called blister copper because of blistered appearance due to escape of SO2 gas during the cooling of molten copper. This copper contains about 2% mixture of Ag, Au, Ni, Zn, Pb and Sn. etc. (e) Refining : Poling : The molten crude metal sample is stirred with poles of the green wood. The organic matter (hydrocarbon) in the green poles acts as a reducing agent and reduces the oxide impurities. Extraction of copper from oxide or carbonate ores : (a) Concentration : Gravity separation method is employed. (b) Calcination : On calcination the volatile impurities are removed and the following reactions take place CuCO3 CuO + CO2 ; Cu(OH)2 CuO + H2O (c) Smelting : Calcinated ore is smelted by mixing the suitable flux Cu2O + C 2Cu + CO ;CuO + C Cu + CO (d) Purification : Electrolytic refining is used for purification. Alloys of Copper (i) Brass - Cu, 60-80% and Zn, 40-20% (ii) Bronze - Cu, 75-90% and Sn, 25-10% (iii) Aluminium bronze– Cu 90% and Al 10% (iv) Monel Metal– Cu 30%, Ni, 67% and Fe + Mn, 3% (v) Bell Metal – Cu, 80% Sn, 20% (vi) Gun metal– Cu, 87%: Sn, 10%; Zn, 3% (vii) German Silver – Cu, 50%, Zn 25%; Ni 25% (viii) Phosphorus bronze– Cu 85%, Sn 13%, P 2% Gyaan Sankalp 7 General principles and processes of isolation of elements EXTRACTION OF ZINC, Zn : Zinc does not occur in the native form since it is a reactive metal . The chief ores of zinc are (i) Zinc blende (ZnS) (ii) Calamine or zinc spar (ZnCO3) and (iii) Zincite (ZnO). Zinc blende, after concentration by Froth floatation process, is roasted in air to convert it into ZnO. In case of calamine, ore is calcined to get ZnO. The oxide thus obtained is mixed with crushed coke and heated at 1673 K in fire clay retorts (Belgian Process) when ZnO gets reduced to metallic zinc. Being volatile at this temperature, that metal distills over and is condensed leaving behind Cd, Pb and Fe as impurities. The crude metal is called spelter. The metal may be refined either by electrolysis or by fractional distillation. Alloys of Zinc : (i) Brass (Cu = 80 % , Zn = 20 %) is used for making utensils, machinery parts and wires. (ii)German Silver (Cu = 60 % , Zn = 20 % and Ni = 20 %) is used for making silver ware and silver plating.
EXTRACTION OF IRON, Fe : The important ores of iron are : (a) Haematite-Fe2O3 (b) Magnetite-Fe3O4 (c) Limonite-Fe2O3. 3H2O (d) Siderite - FeCO3 (e) Iron pyrites - FeS2 (f) Copper pyrites - CuFeS2 (g) Arsenickle pyrites - FeAsS (h) Vivianite- Fe3(PO4)2. 8H2O Various forms of iron : (a) Cast iron or pig iron- It is least pure form of iron containing about 2.2 to 4.4% carbon. It is brittle and can not be hammered. (b) Wrought or malleable iron - It is the purest form of iron containing about 0.1 to 0.25% carbon. It is malleable. (c) Steel- It comes in between cast iron and wrought iron and contains about 0.25 to 2.0% carbon. It is malleable. Its properties depend upon the quantity of carbon present. Extraction of cast iron : (i) Concentration of ore – Iron ores are concentrated by magnetic separation method. (ii) Hematite and magnetite after concentration can be used directly. Sulphide ore is first roasted and carbonate ore is calcined. (iii)Smelting- In calcined or roasted ore, coke (reductant) and lime stone (Flux) is mixed in appropriate proportion. This mixture is smelted in a blast furnace. The following reactions take place At 500-800K in the reduction region, the following reactions occur : 3Fe2O3 + CO 2Fe3O4 + CO2 ; Fe3O4 + 4CO 3Fe + 4CO2 ; Fe2O3 + CO 2FeO + CO2 Iron thus obtained is in solid state and becomes porous. It is known as spongy iron. At 900-1500K, C + CO2 2CO ; FeO + CO Fe + CO2 White cast iron - If the molten iron obtained from blast furnace is suddenly cooled, the carbon is present in the form of cementite (Fe3C) and the white coloured cast iron is obtained. Grey cast iron - If the molten iron obtained from blast furnace is cooled slowly, the most part of carbon is present in the form of graphite and the grey coloured cast iron is obtained. Cast iron is not rusted. Its melting point is about 250ºC less than the pure iron. Wrought iron : It is obtained by purifying cast iron by the process known as pudding. The cast iron is heated on the hearth of a reverberatory furnace lined with hematite (Fe2O3) or magnetite (Fe3O4). The impurities of C, Si, P, Mn, etc. are oxidised. 3C + Fe2O3 2Fe + 3CO ; 3Si + 2Fe2O3 4Fe + 3SiO2 ; 3Mn + Fe2O3 2Fe + 3MnO 6P + 5Fe2O3 10Fe + 3P2O5 P2O5 forms slag ferric phosphate with hematite. P2O5 + Fe2O3 2FePO4 After separation of slag, the iron is taken out in the form of balls and the slag squeezed out by hammering. Wrought iron thus obtained retains about 0.2% C and traces of P and Si etc. in the form of slag. The presence of slag gives strength and toughness to metal and make it resistant towards rusting.
STEEL : It can be made as follows (i) By reducing the amount of carbon in cast iron. (ii) By adding some carbon to wrought iron. (iii) By mixing the required amounts of cast iron and wrought iron. (iv) Directly from ore. Generally steel is prepared from cast iron. Production of steel from cast iron : (i) Acidic Bessemer process : (a) The Bessemer is lined with silica. (b) In the acidic Bessemer, hot air oxidise Mn, Si and C impurities present in the cast iron. 2Mn + O2 2MnO ; Si + O2 SiO2 ; 2C + O2 2CO (c) MnO combines with SiO2 to give slag MnSiO3 which is separated. Carbon is oxidised to carbon monoxide which burns with a blue flame at the mouth of converter. 8 Gyaan Sankalp General principles and processes of isolation of elements (d) When the whole of the carbon is oxidised the blue flame suddenly stops. The requisite amount of carbon is then added usually in the form of an alloy of Fe + Mn + C, called spiegel. The process is called spiegeleisen. The resulting product is manganese steel which is harder and possesses much greater tensile strength. (ii) Basic Bessemer process : (a) This process is used when cast iron contains comparatively the higher impurity of phosphorus. (b) The Bessemer is lined with CaO or MgO. (c) Cast iron and lime stone are taken in the Bessemer and hot air is passed through them. Mn, Si, C and, P oxidise as 2Mn + O2 MnO ; Si + O2 SiO2 ; MnO + SiO2 MnSiO3 Slag 4P + 5O2 2P2O5 ; 2C + O2 2CO ; 3CaO + P2O5 Ca3(PO4)2 Slag Ca3(PO4)2 is known as Thomas slag and is used as valuable fertilizer. (d) Iron thus obtained undergoes spiegeleisen and steel is obtained. Alloys of steel : Alloys Composition 1. Chrome steel 2-4%Cr, 98-96% Fe 2. Tungsten steel 10-20% W, 90-80% Fe 3. Stainless steel 12-14% Cr, 88-86% Fe or 2-4% Ni + 86-82% Fe 4. Manganese steel 10-18% Mn, 90-82% Fe 5. Nickel steel 3-5% Ni, 97-95% Fe 6. Vanadium steel 0.2-1% V 7. Invar 36% Ni, 64% Fe Heat Treatment of Steels: The hardness and elasticity of steel can be controlled by proper heat treatment. Annealing : If steel is heated to redness and then allowed to cool slowly it becomes malleable and soft. This operation is called annealing. Hardening : If steel is heated to redness and is suddenly quenched by plunging into water or oil, it becomes extremely hard and brittle. This process is called hardening. Tempering : Hard steel is heated up to 270°C and then allowed to cool slowly. It becomes’ comparatively less hard and less brittle. Example 1 : Explain the followings: (i) Why is chalcocite roasted and not calcined in the extraction of copper? (ii) Magnesium oxide is used for the lining of steel making furnace. Sol. (i) Air is necessary to convert impurities such as sulphur, arsenic and antimony into volatile oxides and for partial oxidation of the pyrite ore. Calcination does not use oxygen while roasting is done in presence of air. Thus, roasting and not calcination is done in the metallurgy of copper. (ii)MgO is basic in nature. It removes acidic impurities present is cast iron used for making steel. MgO SiO MgSiO 2 3 ; 3MgO P O Mg (PO ) Slag 2 5 3 4 2 MgO is also a refractory material as it can tolerate very high temperature of the furnace. Example 2 : Excess of carbon is added in zinc metallurgy. Explain. Sol. Carbon play two roles in the metallurgy of zinc. (a) It reduces zinc oxide to zinc.
ZnO C Zn CO ; ZnO CO Zn CO2 (b) It reduces CO2 into CO, which is used as a fuel.
CO2 C 2CO Example 3 : What is the significance of leaching in the extraction of aluminium? Sol. Leaching is significant as it helps in removing the impurities like SiO2, Fe2O3, etc. from the bauxite ore. Example 4 : Although thermodynamically feasible, in practice, magnesium metal is not used for the reduction of alumina in the metallurgy of aluminium. Why ? Sol. Temperatures above the point of intersection of Al2O3 and MgO curves, magnesium can reduce alumina. But the temperature required would be so high that the process will be uneconomic and technically difficult. Gyaan Sankalp 9 General principles and processes of isolation of elements Example 5 : At a site, low grade copper ores are available and zinc and iron scraps are also available. Which of the two scraps would be more suitable for reducing the leached copper ore and why ? Sol. Zinc being above iron in the electrochemical series (more reactive metal is zinc), the reduction will be faster in case zinc scraps are used. But zinc is costlier metal than iron so using iron scraps will be advisable and advantageous.
TRY IT YOURSELF Q.1 The purpose of smelting of an ore is (A) to oxidise it (B) to reduce it (C) To remove volatile impurities from it (D) to obtain an alloy Q.2 The main function of roasting is (A) to make slag (B) Oxidation (C) Reduction (D) Removal of volatile substance Q.3 Smelting is a process in which (A) Ore is heated in presence of air (B) Ore is cooled (C) Ore is heated in absence of air (D) Ore is melted Q.4 Which of the following ores is concentrated .by froth flotation method - (A) Bauxite ore (B) Haematite ore (C) Cryolite ore (D) Sulphide ore Q.5 In calcination (A) Ore is oxidised in the presence of air. (B) Ore is heated in the absence of air below its melting point (C) Ore is reduced with carbon (D) None of these Q.6 Slags are generally (A) Silicates and Phosphates (B) Metallic oxides (C) Pure fused metal (D) Rocky stones Q.7 The following process is necessary to obtain purest form of copper metal (A) Carbon reduction (B) Hydrogen reduction (C) Electrolytic process (D) Thermite process Q.8 Iron can be extracted by the following process (A) Smelting process (B) Roasting (C) Aluminothermite process (D) Electrolytic reduction of fused iron oxide Q.9 Which has the highest percentage of carbon? (A) Wrought iron (B) Soft steel (C) Mild steel (D) Hard steel ANSWERS (1) (B) (2) (B) (3) (D) (4) (D) (5) (B) (6) (A) (7) (C) (8) (A) (9) (D)
USEFUL TIPS 1. Important ores : Oxide Ore Sulphide Ore ZnO Zincite ZnS Zinc Blende MnO2 Pyrolusite HgS Cinabar SnO2 Cassiterite PbS Galena Cu2O Cuprite Ag2S Argentite or Silver glance Fe2O3 Hematite FeS2 Iron pyrites Al2O3.2H2O Bauxite CuFeS2.CuS.FeS Copper pyrites FeO.Cr2O3 Chromite Cu2S. Ag2S Copper silver glance Fe3O4 Magnetite Ag2S.Sb2S3 Pyrargyrites, Rubi silver Fe2O3.3H2O Lymonite Ag5,SbS4 Stifenite 3BeO.Al2O3.6SiO2 Beryl FeAsS Arsenickel Pyrites Halide Ore Carbonate Ore NaCl Rock Salt MgCO3 Magnesite AgCl Horn Silver CaCO3 Lime stone CaF2 Flour Spar MgCO3.CaCO3 Dolomite A;F3.3NaF Cryolite ZnCO3 Calamine KCl MgCl2.6H2O Camelite PbCO3 Cerusite FeCO3 Siderite CuCO3.Cu(OH)2 Malachite 2CuCO3.Cu(OH)2 Azurite SrCO3 Strontianite 10 Gyaan Sankalp General principles and processes of isolation of elements Sulphate Ore Silicate Ore BaSO4 Barytes LiAl(SiO3)2 Spodumene PbSO4 Anglisite KAl Si3O8 Felsper CaSO4.2H2O Gypsum Al2O3.2SiO2.2H2O Caolin (or China-Clay) MgSO4.7H2O Eypsomite 3BeO.Al2O3.6SiO2 Beryl SrSO4 Celestine CaO.3MgO.4SiO2 Asbestus K2SO4.Al2(SO4)3.24H2O Alum K2O.3Al2O3.6SiO22H2O Mica K2SO4.Al2(SO4)3.4AI (OH)3 Alunite or Alumstone KH2.Al3.(SiO3)4 Nitrate Ore Phosphate Ore NaNO3 Chili-Salt Peter Ca3(PO4)2 Rock Phosphate KNO3 Salt peter or Indian salt peter Fe3(PO4)2.8H2O Vivianite AlPO4.Al(OH)3.H2O Terguoise 2. Important Alloy 1. - Iron - Fe (100%) 16. 14 Carat Gold - 54% Au + Ag (14 to 30%) + Cu (12–28%) 2. Bronze - Cu(75–90%) + Sn (10 – 25%) 17. 24 Carat Gold - 100% Au 3. Brass - Cu (60 – 80%) + Zn (20 – 40%) 18. Genny gold or coil of gold - Au + Cu (11 : 1) 4. Aluminium Bronze - Cu(90%) + Al (10%) 19.Solder - Pb + Sn 5. Gun metal - (Cu + Zn + Sn) (87 : 3 : 10) 20. Delta metal - Cu + Zn + Fe 6. German Silver - Cu + Zn + Ni (2 : 1 : 1) 21. Coin of Silver - Ag (92.5%) + Cu (7.5%) 7. Bell metal - Cu (80%) + Sn (20%) 22. Coin of aluminium - Mg + Al 8. Monel Metal - Cu : Ni : (Fe + Mn) (30 : 67 : 3) 23. Coin of Steel - 3.5% Ni in steel 9. Nichrome - (Ni + Cr + Fe) (24 : 26 : 28) 24. Magnellium - Mg (10%) + Al(90%) 10. Alnico - (Al, Ni, Co) 25. Duralumin - (Al + Mn + Cu) 11. Britania Metal - (Sn + Pb) 26. Y-alloy - Al(93%) + Cu(4%) + Ni(2%) + Mg (1%) 12. Babbit metal - Sn + Pb + Cu 27. Dutch metal - Cu (80%) + Zn (20%) 13. Munz metal - Cu + Zn (60 – 62%) – (68 – 40%) 28. Artificial Gold - Cu(90%) + Al(10%) 14. Type metal - Pb + Sn + Sb 29. Constantan - Cu(60%) + Ni(40%) 15. Alloys of steel 30. Rinman Green - CoZnO2 Vanadium steel V (0.2 – 1%) Chromium steel Cr (2 — 4%) Nickel Steel Ni (3 – 5%) Manganese steel Mn (10 – 18%) Stainless steel Cr (12 – 14%) & Ni (2 – 4%) Tungston steel W (10–20%) Invar Ni (36%) 3. % of Carbon in different type of Iron Name % of C Wrought iron/Meleable 0.1 to 0.25 Steel 0.25 to 2.0 Pig Iron 2.3 to 4.6 Cast Iron 2.6 to 4.3 4. Sulphide ores are concentrated by froth floatation method. 5. Oxide and carbonate ores are concentrated by gravitational method. 6. Liquation process is used for the concentration of ores which are having lesser melting point than impurities 7. Calcination is the process in which concentrated ore is heated in the absence of air. 8. Calcination and roasting are carried out in reverberatory furnace. 9. Smelting is carried out in blast furnace. 10. Matte is a mixture of Cu2S containing little FeS. 11. Zone refining is used to obtain metals of very high purity.
MISCELLANEOUS SOLVED EXAMPLES Example 1 : Lead can also be obtained by reduction of roasted ore with coke. Out line the process. roasting Sol. PbS PbO, PbSO4 O2
This mixture is smelted with coke, SiO2 and CaCO3 Gyaan Sankalp 11 General principles and processes of isolation of elements
SiO2 (PbO + PbSO4) PbSiO3 PbO + C Pb + CO
PbO + CO Pb + CO2
CaCO3 CaO + CO2
CaO + PbSiO3 CaSiO3 + PbO (slag) Example 2 : Describe a method for refining nickel. Sol. Nickel can be refined by Mond’s process. In this process, the impure nickel is heated in a stream of carbon monoxide forming a volatile complex nickel tetracarbonyl, which at higher temperature decomposes to give pure nickel. 330 350K 450 470K Ni(s) + 4CO(g) Ni(CO)4(g) Ni(s) + 4CO(g) Example 3 : A student suggested that the calcium should be made if CaO is reacted with Aluminium powder. Was the student correct? 0 0 Given Gf (CaO) = – 604.2 kJ/mol ; Gf (Al2O3) = –1582 kJ/mol Sol. 2Al + 3CaO Al2O3 + 3Ca o 0 G° = Gf (Al2O3) – 3Gf (CaO) = –1582 – 3 × (–604.2) = –1582.0 + 1812.6 = + 130.6 G° = +ve reaction is non spontaneous hence not possible. So student was wrong. Example 4 : How is ‘cast iron’ different from ‘pig iron” ? Sol. The iron obtained from the blast furnace is called pig iron. Pig iron contains about 4% carbon, and many impurities such as S, P, Si, Mn etc. Cast iron is made by melting pig iron with scrap iron and coke using hot air blast. Cast iron contains about 3% carbon and is extremely hard and brittle. Example 5 : Why is the reduction of a metal oxide easier if the metal formed is in liquid state at the temperature of reduction? Sol. The entropy is higher if the metal is in liquid state than when it is in solid state. The value of entropy change (S) of the reduction process is more on +ve side when the metal formed is in liquid state and the metal oxide being reduced is in solid state. Thus the value of G° becomes more on negative side and the reduction becomes easier. Example 6 : Predict conditions under which Al might be expected to reduce MgO. Sol. From the fGº vs T plots for MgO and Al2O3, the point of intersection is found to be around 1575 K. Therefore aluminium is expected to reduce MgO above 1575 K. Example 7 : The reaction, Cr2O3 + 2Al Al2O3 + 2Cr ( G 421 kJ) is thermodynamically feasible as is apparent from the Gibbs energy value. Why does it not take place at room temperature? Sol. Certain amount of activation energy is essential even for such reactions which are thermodynamically feasible, therefore heating is required. Example 8 : Is it true that under certain conditions, Mg can reduce SiO2 and Si can reduce MgO? What are those conditions? Sol. Yes, below 1350°C Mg can reduce Al2O3 and above 1350°C, Al can reduce MgO. This can be inferred from G vs T plots. Example 9 : Why copper matte is put in silica lined converter ? Sol. During bessemerigation, copper matte is put into the convertor lined with lime and magnetic lining. Example 10 : –1 –1 The value of fGº for formation of Cr2O3 is –540 kJ mol and that of Al2O3 is –827 kJ mol . Is the reduction of Cr2O3 possible with Al ? Sol. Reduction of Cr2O3 with Al can be described by the equation heat Cr2O3(s) + 2Al(s) 2Cr(s) + Al2O3(s) Then, rGº = fGº (products) – fGº (reactants) rGº = (fGº (Al2O3) + 2 fGº(Cr) – (fGº (Cr2O3) + 2fGº (Al)) –1 –1 –1 rGº = (–827 kJ mol + 0) – (–540 kJ mol + 0) = – 287 kJ mol Since rGº for the reduction of Cr2O3 with aluminium is negative, hence the reduction of Cr2O3 with aluminium is possible. 12 Gyaan Sankalp General principles and processes of isolation of elements Example 11 : Name the process shown by the following equation . 1400 1450ºC 2CuFeS2 + O2 Cu2S + Fe2O3 + SO2 How do we extract blister copper from Cu2S ? Explain with the help of equations. O2 Sol. CuFeS2 1400 1450ºC Cu2S + Fe2O3 + 3SO2 This is a roasting process of copper pyrites. In this process, sand is added to remove the iron as iron silicate slag, Fe2(SiO3)3, which floats on the surface. Air is blown through the liquid matte of Cu2S with FeS and silica, causing partial oxidation.
2Fes + 3O2 2FeO + 2SO2 ; Fe + SiO2 Fe2(SiO3)3 ; Cu2S + O2 Cu2O + SO2 After sometime the air is turned off and self-reduction of oxide and sulphide occurs, giving impure ‘blister copper’ which is 98-
99% pure. Cu2S + 2Cu2O 6Cu + SO2 Example 12 : Out of C and CO, which is a better reducing agent for ZnO ? Sol. Carbon is a better reducing agent for ZnO. Example 13 : When iron ore is reduced in a blast furnace, some of the SiO2 impurity is also reduced by reaction with carbon to give elemental silicon and carbon monoxide. The silicon is subsequently reoxidised in basic oxygen process, and the resulting SiO2 reacts with CaO, yielding slag, which is then separated from the molten steel. Write balanced equations for the three reactions involving SiO2.
Sol. SiO2 (s) 2C(s) Si(s) 2CO(g)
Si(s) O(g)2 SiO(s) 2
CaO(s) SiO2 (s) CaSiO 3 ( ) (slag) Example 14 : Name the process from which chlorine is obtained as a by product. What will happen if an aqueous solution of NaCl is subjected to electrolysis ? Sol. Chlorine is obtained as a byproduct in the following processes : (i) Castner-Kellner process for the manufacture of caustic soda (NaOH). (ii) Down’s cell for the extraction of sodium by the electrolysis of a fused (molten) mixture of sodium chloride (40%) and calcium chloride (60%) at 1123 K. The products obtained during the electrolysis of an aqueous solution of NaCl depend on the nature of cathode used in the electrolysis and on the concentration of sodium chloride in solution. For a concentrated solution ( 20% NaCl). Using mercury electrode (cathode) : Na is obtained which with Hg forms an amalgam. Anode : Cl2 is obtained. Example 15 : Why is zinc not extracted from zinc oxide through reduction using CO ? Sol. Through the entire temperature range, the fGº (ZnO) is much lower than the fGº (CO CO2). That is ZnO is more stable than CO2 (the product of oxidation of CO). Example 16 : What is the role of graphite rod in the electrometallurgy of aluminium ? Sol. Graphite rods are used as anodes during the electrometallurgy of aluminium. These graphite (in fact carbon) anodes react with oxygen produced producing CO and CO2. Example 17 : The choice of a reducing agent in a particular case depends on thermodynamic factor. How far do you agree with this statement? Support your opinion with examples. Sol. The choice of a reducing agent in the extraction of a metal from its ore or any suitable salt of that metal depends upon the energetics and economics of the process. The direction of any reaction/process at any temperature and pressure depends upon the change in the Gibb’s free energy (G) during the process. For a favourable process, G should be negative i.e., the free energy of the system must decrease. During the reduction of haematite to iron, carbon is used. At temperature below 983 K, haematite is reduced by CO, whereas at higher temperatures carbon is a better reducing agent.
Gyaan Sankalp 13 General principles and processes of isolation of elements Example 18 : Carbon monoxide is more effective reducing agent than carbon below 983 K but above this temperature, the reverse is true. How would you explain this ? Sol. The ellingham diagrams for the formation of CO2 and CO from C and oxygen are shown. From the diagram, it is clear that below 710ºC (983 K), CO curve lies below the carbon curve, whereas above 983 K, CO oxidation curve lies above the carbon oxidation curve. Thus, CO is more effective reducing agent than carbon below 983 K, and above this temperature the reverse is true.
(g) O 2 C
2 + O 2
O
CO
f 2
o C(s)+O2 CO(g) 2
1 –
l –400 o 2C + m 710ºC O J 2
k / (983K) C
º –600 O
G
f –800
Temperature Example 18 : A1 and A2 are two ores of metal M. A1 on calcination gives black precipitate (C), CO2 and water. With KI and HCl, it gives I2 and a precipitate(D) is formed. A2 on roasting gives the metal (M) and a gas (G) is given out. The gas when passed through an acidified K2Cr2O7 solution, it turns green. Identify A1, A2, C, D, M and G. Sol. Calcination Black solid + CO2 + H 2 O roasting (C) A2 Metal + Gas A1 (M) (G) dil. HCl ; K2 Cr 2 O 7 + H 2 SO 4 I2 + Precipitate KI (D) Green colour
From given information, A1 appears to be a basic carbonate ore of a metal that gives black oxide. So the metal M is copper, because copper oxide is black. Thus, A1 is CuCO3.Cu(OH)2 The reactions are :
calcination CuCO.Cu(OH)3 2 2CuO(s) CO(g) 2 HO(g) 2 (A)1 (black)(C) copper (II) oxide
CuCO3 .Cu(OH) 2 4HCl 2CuCl 2 (aq) CO 2 (g) 3H 2 O( )
CuCl2 (aq) 4KI(aq) Cu 2 I 2 (s) 4KCl(aq) I 2 (D)
The ore A2 undergoes self-reduction on roasting and the gas (G) turns acidified dichromate solution green. Therefore, the ore is copper glance (Cu2S). roasting Cu2 S 3O 2 (g) 2Cu 2 O 2SO 2 (g) (A2 ) (G)
Cu2 S 3Cu 2 O 6Cu 2SO 2 (g) (M) (G)
K22724 Cr O H SO 3SO 2 (g) K 24 SO (aq) Cr 243 (SO ) (aq) 4H 2 O (green)
14 Gyaan Sankalp General principles and processes of isolation of elements QUESTION BANK
EXERCISE - 1 ONLY ONE OPTION IS CORRECT Q.14 A mineral is called ore if : Q.1 A substance which reacts with gangue to form fusible ma- (A) The metal present in the mineral is costly terial is called- (B) A metal can be extracted from it (A) Flux (B) Catalyst (C) A metal can be extracted profitably from it (C) Ore (D) Slag (D) A metal cannot be extracted from it. Q.2 Identify the statement which is not correct for copper sul- Q.15 Tin plague means- phate : (A) Tin plating (A) It reacts with NaOH and glucose to give Cu2O (B) Conversion of white tin to grey tin (B) It reacts with KCl to give Cu2Cl2 (C) Emission of sound while bending a tin plate (C) It gives CuO on strong heating (D) Conversion of stannous salts to stannic salts. (D) It reacts with KI to give I2 Q.16 The process in which, Thermal reduction of Mg by Fe or Si Q.3 The ores that are concentrated by floatation method are- is called- (A) Carbonates (B) Sulphides (A) Krup-renn process (B) Kroll’s process (C) Oxides (D) Phosphates (C) Pidgeon process (D) Van Arkel process Q.4 Of the following which can be extracted by the electrolytic Q.17 If Bauxite consists of SiO2 as impurity, this process is em- reduction of their salts ? ployed- (A) Ag (B) Mg and Al (A) Hall’s process (B) Baeyer’s process (C) Ca (D) All (C) Hoope’s process (D) Serpeck’s process Q.5 Magnetic separation is used for increasing concentration Q.18 High purity copper is obtained by- of the following- (A) Zone refining (B) Poling (A) Horn silver (B) Calcite (C) Electrolytic refining (D) Cupelling (C) Hamatite (D) Magnesite Q.19 Roasting is carried out prior to leaching in- Q.6 In blast furnace, iron oxide is reduced by- (A) Baeyer’s process (B) Hall’s process (A) Silica (B) CO (C) Serpeck’s process (D) In all (C) C (D) Lime stone Q.20 When FeCr2O4 (chromite) is reduced with ‘C’ in an electric Q.7 Steel consists of ...... percentage of carbon. arc furnace : (A) 3.1 - 4.5% (B) 2.2 - 3.1% (A) Cr and Fe2O3 are formed (B) Fe and Cr2O3 are formed (C) 0.15 - 0.28% (D) 0.15 - 1.5% (C) Fe and Cr (Ferrochrome) are formed Q.8 The flux used in the smelting of copper ores is : (D) FeCrO4 is formed (A) Lime stone (B) Magnesia Q.21 In the pidgeon process, Mg is produced by- (C) Silica (D) Coke (A) Electrolysis of fused MgCl2 Q.9 Lead is not affected by dil. HCl in the cold because- (B) Reducing calcined dolomite with ferrosilicon at high (A) A protective coating of PbCl2 is formed on the metal. temperature under pressure. (B) Pb is less electropositive than H2 (C) Both are correct (D) None is correct (C) PbO film is formed which resists chemical attack by the Q.22 Which of the following is not used for extraction of Alu- acid. minium- (D) PbO2 film is always present on the metal which resists (A) Van arkel process (B) Serpeck’s process chemical attack. (C) Baeyer’s process (D) Hall-Heroult’s process Q.10 Tetraethyl lead is a – Q.23 Britannia metal contains : (A) Poisonous compound present in petrol (A) Lead (B) Tin (B) Antiknock compound (C) Graphite (D) Silicon. (C) Component of ethyl fluid Q.24 Which of the following is not an ore- (D) Propellant (A) Malachite (B) Calamine Q.11 Which of the following is not a concentration technique- (C) Stellite (D) Cerussite (A) Levigation (B) Froth floatation Q.25 On igniting Fe2O3 at 1400ºC, in presence of carbon the (C) Leaching (D) Calcination product obtained is- Q.12 Major ore of lead is- (A) Fe2O3 melt (B) FeO (A) Anglesite (B) Galena (C) Fe3O4 (D) Metallic iron (C) Cerussite (D) Azurite Q.26 Loss of Tin in the form of SnSiO3 can be reduced by add- Q.13 In Dow’s process, the source of magnesium is- ing ...... in smelting stage- (A) Magnesite (B) Dolamite (A) CaO (B) Fe2O3 (C) Asbestos (D) Sea water (C) Fe (D) Cu2S
Gyaan Sankalp 15 General principles and processes of isolation of elements Q.27 Removal of Fe, Cu, W from Sn metal after smelting is by Q.40 In the extraction of Cu, the metal is formed in the bessemer ...... because ...... converter due to the reaction :
(A) Poling; of more affinity towards oxygen for impurities (A) Cu2S + 2Cu2O 6Cu + SO2 (B) Selective oxidation; of more affinity towards oxygen (B) Cu2S 2Cu + S for impurities (C) Fe + Cu2O 2Cu + FeO (C) Electrolytic refining; impurities undissolved in electro- (D) 2Cu2O 4Cu + O2 lyte Q.41 Slag obtained during smelting stage, consists of- (D) Liquation; Sn having low melting point compared to (A) CaSiO (B) FeSiO impurities. 3 3 (C) CaSiO3 + SnSiO3 (D) B & C Q.28 In the Down’s process of obtaining sodium, CaCl2 is added- Q.42 In very cold weather, tin crumbles to a powder due to- (A) To get high yield of sodium (A) Expansion of the crystal lattice. (B) To improve the quality of sodium obtained (B) Formation of SnO at low temperature (C) To lower the operating temperature (C) Non-metallic properties of white tin at low temperature. (D) To act as a reducing agent. (D) The transition from white tin to the grey form which is Q.29 Zinc blende on roasting in air gives- amorphus. (A) Zinc carbonate (B) SO2 and ZnO Q.43 Ag is refined by cupellation process. The process removes (C) ZnS and ZnSO4 (D) CO2 and ZnO the impurity of : Q.30 Al2O3 formation involves evolution of a large quantity of (A) Cu (B) Au heat, which makes it useful in: (C) Pb (D) Pt (A) Deoxidiser (B) Indoor photography Q.44 Matte is obtained after this step- (C) Cofectionary (D) Thermite welding (A) Froth floatation (B) Roasting Q.31 In the extraction of Al, the function of cryolite is to : (C) Smelting (D) Refining (A) Lower the melting point of alumina Q.45 Which of the following metals will not form an amalgam ? (B) Increase the melting point of alumina (A) Gold (B) Silver (C) Remove impurities from alumina (C) Zinc (D) Iron (D) Minimise the anodic effect Q.46 Ag can be separated from Pb by : Q.32 During extraction of silver, which of the following is formed (A) Fractional crystallisation (B) Amalgamation (A) Na [Ag(CN)2] (B) Na2 [Ag(CN)2] (C) Filtration (D) Addition of Zn (C) Na4Ag(CN)2 (D) None of these Q.47 Precious metals are obtained as by products during this Q.33 In the alumino-thermite process, Al acts as- operation- (A) An oxidising agent (B) A flux (A) Concentration (B) Smelting (C) Solder (D) A reducing agent (C) Screening (D) Refining Q.34 Aluminium is obtained from Al O by this method- 2 3 Q.48 In the extraction of silver, Ag2S is dissolved in - (A) Thermal reduction (A) HCl (B) HNO (B) Hydro metallurgical method 3 (C) KCN (D) H2SO4 (C) Electrolytic reduction Q.49 The function of flux during the smelting of the ore is : (D) Reduction by Iron. (A) To make the ore porous (B) To remove gangue Q.35 Thermite process is used to extract metals : (C) To facilitate reduction (D) To facilitate oxidation. (A) When their oxides cannot be reduced by carbon Q.50 Of the following metals the one which cannot be obtained (B) When their carbonates do not yield oxides by thermal by electrolysis of the aqueous solution of the salt- decomposition (A) Ag (B) Cu (C) When their sulphides cannot be converted into oxides (C) Mg (D) Au by roasting. Q.51 Which of the following metals is obtained by leaching the (D) When their melting points are very high. ore with dilute sodium cyanide solution ? Q.36 Copper glance is ...... type of ore- (A) Silver (B) Titanium (A) Carbonate (B) Sulphide (C) Vanadium (D) Zinc. (C) Oxide (D) Sulphate Q.52 German silver is an alloy of- Q.37 Which of the following is extracted by air reduction. (A) Ag and Cu (B) Cu, Sn and Ag (A) Graphite (B) Silicon (C) Cu, Zn and Ni (D) C, Ni, Fe and Zn (C) Tin (D) Lead Q.53 Calcination is the process in which- Q.38 Most suitable method of extraction of magnesium is by- (A) Heating the ore in presence of air (A) Pyrometallurgy (B) Hydro metallurgy (B) Heating the ore in presence of sulphur (C) Electro metallurgy (D) All (C) Heating the ore in absence of air Q.39 In the extraction of Cu from copper pyrites, iron is removed (D) Heating the ore in presence of chlorine as : (A) FeSO4 (B) FeSiO3 (C) Fe3O4 (D) Fe2O3 16 Gyaan Sankalp General principles and processes of isolation of elements Q.54 The element which could be extracted by electrolytic re- Q.66 Percentage of gold in 20 karat gold- duction of its oxide dissolved in a high temperature melt is- (A) 80% (B) 83.33% (A) Sodium (B) Magnesium (C) 86.66% (D) 96% (C) Fluorine (D) Aluminium Q.67 In steel making the impurities are removed by- Q.55 Tin reacts with conc. HNO3 and gives- (A) Oxidation (B) Reduction (A) Metastannic acid (B) Stannic nitrate (C) Converted into high melting compounds by addition (C) Tinstone (D) None of these of some elements (D) Gasification Q.56 Plumbo-solvency means dissolution of lead in- Q.68 Verdigis is- (A) Ordinary water (B) Hot water (A) Basic copper acetate (B) Basic lead acetate (C) Acids (D) Alkalies (C) Basic lead (D) None Q.57 When silver chloride is treated with potassium cyanide- Q.69 The liquefied metal expanding on Solidification is- (A) Silver is precipitated (A) Ga (B) Al (B) A complex ion is formed (C) Zn (D) Cu (C) Double decomposition takes place Q.70 Which of the following is used to remove lead impurities (D) No reaction takes place. from silver ? Q.58 Copper is extracted from its ore by- (A) Leaching with dilute NaCN solution (A) Electrolytic reduction (B) Auto reduction (B) Parke’s process (C) Cyanide process (D) Magnetic separation. (C) Leaching with dilute NaCN solution in presence of air Q.59 Which of the following metals burns in air at high tempera- (D) Solvay’s process. ture with the evolution of much heat ? Q.71 Froth flotation process is based on- (A) Cu (B) Hg (A) Specific gravity of the ore particles (C) Pb (D) Al (B) Magnetic properties of the ore particles Q.60 Aluminothermy used for on the spot welding of large iron (C) Wetting properties of the ore particles structure is based on the fact that- (D) Electrical properties of the ore particles (A) As compared to iron, aluminium has greater affinity for Q.72 Bell metal is an alloy of- oxygen. (A) Cu + Pb (B) Cu + Sn (B) As compared to aluminium, iron has greater affinity for (C) Cu + Zn (D) Cu + Ni. oxygen. Q.73 The soldiers of Napoleon army while at Alps during freez- (C) Reaction between aluminium and oxygen is endothermic. ing winter suffered a serious problem as regards the tin (D) Reaction between iron and oxygen is endothermic. buttons of their uniforms. White metallic tin buttons got Q.61 Principle in the dow’s process is- converted into grey powder. This transformation is related (A) MgO is reduced with ‘C’ in clay retorts to : (B) MgCl2 electrolysis (A) An interaction with nitrogen of the air at very low tem- (C) MgO is reduced with Fe/Si in clay retorts perature. (D) None of these (B) A change in the crystalline structure of tin. Q.62 In the extraction of lead by air reduction process, the re- (C) A change in the partial pressure of oxygen in the air. ducing agent is- (D) An interaction with water vapour contained in the hu- (A) PbS (B) O2 mid air. (C) C (D) Al Q.74 The mass of carbon anode consumed (giving only Q.63 925 fine silver means an alloy of- carbondioxide) in the production of 270 kg of aluminium (A) 7.5% Ag and 92.5% Cu (B) 92.5% Ag and 7.5% Cu metal from bauxite by the Hall process is (C) 9.25% Cu and 90.75% Ag (D) 9.25% Ag and 90.75% Cu (Atomic mass Al = 27) Q.64 Heating of Ag with conc. HNO3 gives- (A) 180 kg (B) 270 kg (A) NO (B) NO2 (C) 540 kg (D) 90 kg (C) N2O (D) N2O3 Q.75 Purple of cassias is- Q.65 MgO can be reduced by C above this temperature- (A) Pure gold (B) Solid solution of gold (A) 1150ºC (B) 1500ºC (C) Gold (I) hydroxide (D) Gold (III) Chloride (C) 1750ºC (D) 2000ºC EXERCISE - 2 ONE OR MORE THAN ONE CHOICE MAY (C) To lower the temperature of the melt BE CORRECT (D) To decrease the rate of oxidation of carbon at anode. Q.2 Off the following, the metals that cannot be obtained by Q.1 The major role of fluorspar (CaF2) which is added in small quantities in the electrolytic reduction of alumina dissolved electrolysis of the aqueous solution of their salts are : (A) Ag (B) Mg in fused cryolite (Na3AlF6) is- (A) As a catalyst (C) Cu (D) Al (B) To make the fused mixture very conducting Gyaan Sankalp 17 General principles and processes of isolation of elements Q.3 Which of the following common element(s) is/are present MATCH THE COLUMN TYPE QUESTIONS in the anode mud in electrolytic refining of copper – (Q.12-Q.14) (A) Selenium (B) Tellurium Each question contains statements given in two columns (C) Silver (D) Gold which have to be matched. Statements Q.4 Which of the following statement(s) is (are) true – (A, B, C, D) in column I have to be matched with state- (A) In process of the precipitation of silver from sodium ments (p, q, r, s) in column II. dicyano argentate (I), the zinc acts as reducing agent as Q.12 Match the column correctly – well as complexing agent. Column I Column II (B) In process of the roasting, the copper pyrites is con- (A) Aluminium (p) use depends on carbon verted into a mixture of Cu2S and FeS which, in turn, are content partially oxidised. (B) Copper (q) yellow pigment (C) Limonite, haematite and magnesites are ores of iron (C) Zinc (r) electroplating (D) Tin and lead both are extracted from their ores by self- (D) Iron (s) thermite welding reduction. Q.13 Match the column correctly – Q.5 In which of the following pair(s), the minerals are con- Column I Column II verted into metals by self-reduction process – (A) Cyanide process (p) Extraction of Al (A) Cu2S, PbS (B) PbS, HgS (B) Self reduction (q) Extraction of Ag (C) PbS, ZnS (d) Ag2S, Cu2S (C) Electrolytic reduction (r) Extraction of Pb (D) Carbon reduction (s) Extraction of Sn ASSERTION AND REASON QUESTIONS Q.14 Match the column correctly – (Q.6-Q.11) Column I Column II Note : Each question contains STATEMENT-1 (Assertion) (A) Bessemer’s converter (p) Copper and STATEMENT-2 (Reason). Each question has 5 choices (B) Blast furnace (q) Iron (A), (B), (C), (D) and (E) out of which ONLY ONE is cor- (C) Alumumium thermic (r) Chromium rect. process (A) Statement-1 is True, Statement-2 is True; Statement-2 (D) Magnetic separation (s) Tin is a correct explanation for Statement-1. (B) Statement-1 is True, Statement-2 is True; Statement-2 PASSAGE BASED QUESTIONS is NOT a correct explanation for Statement-1. Passage 1- (Q.15-Q.17) (C) Statement -1 is True, Statement-2 is False. Castner-Keller cell is the common cell in which mercury is (D) Statement -1 is False, Statement-2 is True. used as cathode. The advantage of using Hg as a cathode (E) Statement -1 is False, Statement-2 is False. is that the discharge potential of Na+ ions is less than that Q.6 Statement 1 : Copper can be extracted by hydrometallurgy of H+ ions. Na+ ions get discharged on mercury and sodium but not zinc. so deposited combines with mercury to form sodium Statement 2 : In hydrometallurgy, the less reactive metal is amalgam. The cell consists of a large rectangular through displaced from the solution by a more reactive (or more divided into three compartments by slate partitions which electropositive) metal. do not touch the bottom of the cell but dipping in mercury. Q.7 Statement 1 : Metals do not occur as nitrates in nature. When electricity is circulated, sodium chloride in the outer Statement 2 : The nitrates of almost all metals are soluble compartments is electrolysed. Chlorine is evolved at the in water. graphite anode white Na+ ions are discharged at the Hg cathode. The liberated sodium forms amalgam with mercury. Q.8 Statement 1 : Aluminium cannot be extracted by reducing NaCl Na+ + Cl– alumina with carbon. At anode 2Cl– 2Cl + 2e Cl Statement 2 : Aluminium has more affinity for oxygen than 2 At cathode Na+ + e Na carbon has. Na + Hg Amalgam Q.9 Statement 1 : Limestone is added to the blast furnace in The sodium amalgam thus formed comes in the inner the extraction of iron from haematite. compartment due to rocking. Here, the sodium amalgam Statement 2 : Limestone at higher temperature give CaO acts as the anode and iron rods acts as cathode. (lime) An anode Na-amalgam Na+ + Hg + e Q.10 Statement 1 : Silver ores and native gold have to be leached At cathode 2H O + 3e H + 2OH– with metal cyanides. 2 2 Q.15 In Castner-Keller cell, anode is Statement 2 : Silver and gold do not form complex ion with (A) Graphite (B) amalgam cyanide ion. (C) Both A and B (D) Mercury Q.11 Statement 1 : Copper and silver are found in the combined Q.16 The NaOH is produced in: state as sulphides in nature. (A) Sider compartment (B) Centre compartment Statement 2 : Copper and silver are below hydrogen in (C) Both (D) NaOH is not produced electrochemical series. 18 Gyaan Sankalp General principles and processes of isolation of elements
Q.17 The wheel is used in Castner-Keller cell is useful to help and another gas (G2) which turns lead acetate solution in- black and also reacts with gas (G1) to precipitate colloidal (A) Transfer of amalgam from sider compartment to centre sulphur in presence of moisture. compartment. The MS, [X] and [Y] gives greenish blue flame. (B) Transfer of Hg from centre compartment to sider Q.18 The metal ores [X] and [Y] are respectively : compartment. (A) Carbonate and sulphide ores (C) Both A and B (D) None of these (B) Sulphide and carbonate ores Passage 2- (Q.18-Q.20) (C) Carbonate and hydroxide ores Amongst the various ores of a metal (M) (sulphide, car- (D) Carbonate and oxide ores bonates, oxides, hydrated or hydroxides) two ores [X] and Q.19 Which of the following statements is correct about [Y] ? [Y] show the following reactivity. (A) [Y] is converted to metal (M) by self reduction. (i) [X] on calcination gives a black solid (S), carbon dioxide (B) Carbonate extract of [Y] gives yellow ppt with suspen- and water. sion of CdCO3. (ii) [X] Dissolved in dil. HCl on reaction with KI gives a (C) [Y] is chalcocites or chalcopyrites white ppt. (P) and iodine. (D) All of these (iii) [Y] on roasting gives metal (M) and a gas (G1) which Q.20 The gas (G1) acts as turns acidified K2Cr2O7 solution green. (A) Oxidising agent (B) reducing agent (iv) [Y] on reaction with dil. HCl gives a white ppt. (MS) (C) oxidising and reducing agent (D) fluxing agent EXERCISE - 3
SUBJECTIVE QUESTIONS by passing CO2 through it. Write an equation for the reac- Q.1 Write down the chemical equations for metallurgical pro- tion that occurs. cesses to represent : Q.7 How Ag is extracted from silver coin ? (A) roasting of galena (PbS) Q.8 Tin stone is amphoteric but not amphiprotic. Explain- Q.9 Copper can be extracted by hydrometallurgy but not zinc. (B) Reduction of Cu2O using charcoal as a reducing agent (C) Deposition of pure silver from an aqueous solution of Explain. Ag+ Q.10 Out of C and CO, which is a better reducing agent at 673 K? Q.2 Name the common elements present in the anode mud in Q.11 State the role of silica in the metallurgy of copper. electrolytic refining of copper. Why are they so present ? Q.12 Dolamite (MgCO3.CaCO3) can also be treated to get MgCl2 Q.3 Why is chalconite roasted and not calcinated during which in turn is electrolysed to get Mg. (Dow natural Brine recovery of copper ? process). Give reactions of this process. 2+ Q.4 It is possible to identify on “Iron cycle” of the same kind Q.13 To prevent the air oxidation of aqueous solution of Sn to 4+ as a water or nitrogen cycle. Suggest some stages in the Sn , some times metallic tin is kept in contact with the 2+ cycle. Sn . Suggest how this helps to prevent the oxidation ? Q.14 Give briefly the isolation of magnesium from sea water by Q.5 SnO2 can be precipitated as Sn(OH)4. Can this be used to concentrate tin stone ? the Dow process. Give equations for the steps involved. Q.15 Why is the extraction of copper from pyrite difficult than HO2 High temp. SnO2 Sn(OH) 4 SnO 2 that from its oxide ore through reduction ? Q.16 High purity metals can be obtained by zone refining method. Q.6 In the purification of bauxite ore as preliminary step in the Why ? – production of Al, [Al(OH)4 ] can be converted to Al(OH)3 EXERCISE - 4 PREVIOUS YEAR IIT-JEE QUESTIONS Q.1 The chemical composition of ‘slag’ formed during the smelt- Q.4 Extraction of zinc from zinc blende is achieved by : [2007] ing process in the extraction of copper is : [2001] (A) electrolytic reduction (A) Cu2O + FeS (B) FeSiO3 (B) roasting followed by reduction with carbon (C) CuFeS2 (D) Cu2S + FeO (C) roasting followed by self-reduction Q.2 Which of the following process is used in the extractive (D) roasting followed by reduction with another metal metallurgy of magnesium ? [2002] Q.5 Match the conversions in Column I with the type(s) of (A) fused salt electrolysis reaction(s) given in Column II. [2008] (B) self reduction Column I Column II (C) aqueous solution electrolysis (A) PbS PbO (p) roasting (D) thermite reduction (B) CaCO3 CaO (q) calcination Q.3 Which ore contains both iron and copper ? [2005] (C) ZnS Zn (r) Carbon reduction (A) Cuprite (B) Chalcocite (D) Cu2S Cu (s) self reduction (C) Chalcopyrite (D) malachite
Gyaan Sankalp 19 General principles and processes of isolation of elements HINTS & SOLUTIONS
EXERCISE - 1 Q 1 2 3 4 5 6 7 8 9 10 11 A D B B B C B D C A B D Q 12 13 14 15 16 17 18 19 20 21 22 A B D C B C D C A C B A Q 23 24 25 26 27 28 29 30 31 32 33 A B C D C D C B D A A D Q 34 35 36 37 38 39 40 41 42 43 44 A C A B D C B A D A C B Q 45 46 47 48 49 50 51 52 53 54 55 A D D D C B C A C C D A Q 56 57 58 59 60 61 62 63 64 65 66 A A B B D A B A B B D B Q 67 68 69 70 71 72 73 74 75 A A A A B C B B D B (1) (D). Flux combines with gangue to form a low melting sub- very pure metal. In this process the impure metal is con- stance called slag. verted into volatile iodides, which are again dissociated to (2) (B). Copper sulphate does not react with KCl. get pure metal. (3) (B). In floatation process, the ore particles should have (23) (B). Britannia metal is an alloy of Sn, Sb and Cu. aerofillic in preference to gangue particles. Sulphide ores (24) (C). Stellite is an alloy of Co, Cr, W & F and is extremely having this character. hard, which is used in drill bits for rock drilling. (4) (B). Magnesium and aluminium can be obtained by the (25) (D). At this high temperature Fe2O3 is directly reduced by electrolytic reduction of their ores respectively. carbon to give Fe and CO2 (5) (C). Haematite ore is having magnetic property, can be (27) (D). Liquation is the principle based on difference in melt- separated by magnetic separation. ing points. (8) (C). In the smelting of copper ore SiO2 is used as a flux as (28) (C). Melting point of NaCl is 803ºC, After mixing with CaCl2, it forms FeSiO2 as fusible slag. melting point of mixture comes to 600ºC. (9) (A). A protective layer of PbCl is formed when lead reacts 2 (29) (B). ZnS + O ZnO + SO with cold and dilute HCl. 2 2 (10) (B). Tetraethyl lead is used as an antiknock additive to (30) (D). Large amount of heat is liberated in thermite welding. petrol. (31) (A). Cryolite lowers the melting point of bauxite in the ex- (11) (D). Concentration is the method employed to remove traction of aluminium. gangue materials by mechanical separation. In calcination, (32) (A). During the extraction of silver by cyanide process volatile impurities are removed. [NaAg(CN)2] is formed. (12) (B). Because of its availability and consists of high per- (33) (D). Aluminium is more electropositive than iron hence centage of Pb compared to other lead ores. acts as a reducing agent. (14) (C). An ore is a mineral if a metal is extracted from it profit- (34) (C). For all metals with high electropositive nature, electro- ably. lytic reduction is best method. (15) (B). Conversion of white tin to grey tin is called tin plague. (35) (A). Thermite process is used to extract metals when their (18) (C). After electrolytic refining, purity of copper is 99.99% oxides cannot be reduced by carbon. which is enough for electrical applications. (36) (B). Copper glance chemical formula is Cu2S. (20) (C). Iron and chromium both will get reduced to its metallic (37) (D). Lead is extracted by air reduction. state and forming an alloy ferrochrome, which is used for alloying additions during alloy steel making. (38) (C). Being electro positive in nature and low reduction nature, electro metallurgy is suitable for extraction. (21) (B). In Pidgeon process, MgO obtained from MgCO3 is reduced with Fe/Si to form Mg vapours, which are con- (39) (B). In the extraction of copper from copper pyrites iron is densed to get Mg metal. removed as FeSiO3 (slag.) (22) (A). Van Arkel process is one the refining process to get (40) (A). In the Bessemer’s converter copper is formed by the action of Cu2O on Cu2S. (42) (A). In very cold weather white tin, due to expansion 20 Gyaan Sankalp General principles and processes of isolation of elements crumbles into powder forming grey tin. (64) (B). 2Ag + 4HNO 2NO + 2AgNO + 2H O (43) (C). In the refining of silver by cupellation process lead is 3 2 3 2 removed as its oxides. (conc.) (45) (D). Iron does not form an amalgam. If dil. HNO3 is used, NO is produced. (46) (D). Silver is 300 times more soluble in zinc than in lead in (65) (D). MgO + C 2000ºC Mg + CO. the molten state. On cooling Zn – Ag alloy separates leav- Mg, so formed is highly reactive, so backward reaction ing behind molten lead. taking place. To avoid this, shock cooling of Mg has to be (48) (C). KCN is to dissolve Ag2S. done (49) (B). Flux removes gangue as a fusible slag. (66) (B). 20 karat gold means it consists of 20 parts gold out of (50) (C). Since magnesium has lower standard reduction po- 24 parts and four parts of copper. tential (S.R.P.) than hydrogen, on electrolysis of its aque- 20 ous salt solution hydrogen will be discharged at the cath- % of gold = × 100 = 83.33% ode. Ag, Au, and Cu all have higher S.R.P. than hydrogen, 24 as such these metal ions will be discharged at the cathode (67) (A). In steel making, pure O2 is purged into molten iron, when their aqueous salt solutions are electrolysed respec- where all impurities like C, S, Si, Mg converted into oxides tively. then forming as slag, which is removed afterwards. (51) (A). Silver metal is obtained by leaching the ore with conc. (68) (A). Verdigris is Cu(OH)2.(CH3COO)2Cu. NaCN solution. (69) (A). Gallium expands on solidification. (52) (C). German silver is composed of Cu, Zn and Ni. (70) (B). Lead is removed from silver by Parke’s process. (53) (C). Calcination is the process in which moisture and vola- (71) (C). Froth flotation process is based on the wetting prop- tile impurities are removed and process is carried in ab- erty of the one particles. sence of air. (72) (B). Bell metal is an alloy of copper and tin. (54) (D). The metals Na & Mg are reduced from their chlorides (73) (B). Tin shows enantiotropy in its three crystalline forms, below 18ºC it forms grey tin which has the least specific and fluorine. But for Aluminium source is Al2O3. gravity. (55) (A). Tin forms metastannic acid with conc. HNO3. (56) (A). Dissolution of lead in ordinary water is called plumbo (74) (D). solvency. 2Al2O3 + 3C 4Al + 3CO2 (57) (B). A complex compound [KAg(CN) ] is formed. 2 2 [2 × 27 + 3 × 16] 3 × 12 4 × 27 3 × 44 (58) (B). Auto reduction is the process used for the extraction 204 36 108 132 of copper. 108 kg Al 36 kg carbon. (59) (D). Aluminium burns at high temperature. The reaction is highly exothermic. 36 270 270 kg Al =90 kg. 108 3Al + 3O2 2Al2O3 (60) (A). Aluminium has greater affinity for oxygen and the re- (75) (B). It is obtained by treating gold (III) chloride solution action is highly exothermic. with SnCl2 (61) (B). In Dow’s process, the source is either sea water or 2AuCl3 + SnCl2 2Au + 3SnCl4 Dolomite, Mg from these sources converted into MgCl 2 SnCl4 so formed hydrolyses as follows and followed by fused salt electrolysis to get pure Mg SnCl + 4H O Sn(OH) + 4HCl metal. 4 2 4 (63) (B). Silver being soft is alloyed with copper. The composi- stannic acid tion of a silver alloy is expressed as its fineness i.e., the The colloidal precipitate of stannic acid absorbs colloidal amount of Ag in 1,000 parts of the alloy. 925 fine silver particles of gold. This has beautiful purple colour and is means an alloy of 92.5% Ag and 7.5% Cu. known as purple of cassius. It is used for colouring glass (ruby red) and pottery.
EXERCISE - 2 (1) (BC). Fluorspar reduced the melting point of bauxite. (5) (AB). (2) (BD) Cu2S + 2Cu2O 6Cu + SO2 (3) (ABCD). Selenium, tellurium, silver, gold and platinum are PbS + 2PbO 2Pb + SO2 the metals present in anode mud. This is because these are HgS + 2HgO3Hg + SO2 less reactive than copper. (6) (A). In hydrometallurgy, the less reactive metal is displaced (4) (AB). from the solution by a more reactive (or more (C) Magnesite (MgCO3) is ore of magnesium. electropositive) metal. To displace zinc, we need more (D) Tin is extracted by carbon reduction. electropositive metals such as magnesium etc. Such metals SnO2 + 2C Sn + 2CO are more expensive and therefore such displacement will (A) and (B) are correct statements. not be economical from commercial point of view. Gyaan Sankalp 21 General principles and processes of isolation of elements (7) (A). Metals do not occur as nitrates in nature because the in the combined state as sulphides in nature because both nitrates of almost all metals are soluble in water. Therefore, copper and silver are weakly electropositive, and react with any nitrate salt present in the soil/rocks will get washed larger, more polarizable anions (such as S2– ion) which are away by rain water into the sea. basic in nature. (8) (A). Aluminium cannot be extracted by reducing alumina (12) (A) s, (B) r, (C) q, (D) p with carbon because aluminium has more affinity for oxygen (13) (A) q, (B) r, (C) p, (D) s than carbon has. (A) Cyanide process Ag, (B) Self reduction Pb (9) (A). Limestone at higher temperature give CaO (lime) which (C) Electrolytic reduction Al, (D)Carbon reductionSn removes silica impurities present in the ore in the form of (14) (A) p, q, (B) q, (C) q, r, (D) s liquid slag. (15) (C) (16) (B) (17) (C) (10) (C). Silver and gold do not form complex ion with cyanide (18) (A). [X] = CuCO3 Cu(OH)2 or 2CuCO3 . Cu(OH)2 ; ion Such complex ions are soluble in water. Thus, the gold [Y] = Cu2S or CuFeS2 and silver portion (which is generally in small amounts), of (19) (D). It is sulphide ore (Cu2S or CuFeS2) & is called as the ore can be brought into the solution thereby removing chalcopyrites or chalcocites ; S2– gives yellow ppt. of CdS all the undesirable material. with CdCO3. (11) (B). Copper and silver are below hydrogen in (20) (C). G1 = SO2 ; can increase & decrease its oxidation state. electrochemical series and yet copper and silver are found EXERCISE - 3
(1) (A) 2PbS + 3O2 2PbO + 2SO2 (9) In hydrometallurgy, the less reactive metal is displaced 8PbS + 2O2 PbSO4 from the solution by a more reactive (or more electroposi- + – (B) Cu2O + C 2Cu + CO ; (C) Ag + e Ag. tive) metal. To displace zinc, we need more electropositive (2) The elements present in the anode mud during the electro- metals such as magnesium etc. Such metals are more ex- lytic refining of copper are, antimony, selenium, tellurium, pensive and therefore such displacement will not be eco- silver, gold and platinum. These elements do not get nomical from commercial point of view. oxidised at the potential copper is oxidised. (10) At 673 K, CO is a better reducing agent than carbon. (3) Air (used during roasting) is necessary for converting (11) Silica removes iron, present as FeO in the copper ore, as slag. chalcocite (a sulphide ore) to oxide. Calcination does not FeO + SiO2 FeSiO3 use oxygen. Iron (II) oxide silica slag (4) Iron oxide reduction Fe metal forming 800ºC (12) MgCO3.CaCO3 MgO + CaO + 2CO2 iron and steel products MgO + CaO + HCl MgCl2 + CaCl2 O , moisture CO2 2 MgCl2 + CaCl2 MgCl2 + CaCO3 Iron oxide (rusting) CaCO gets separated as white ppt. leaving MgCl in the (5) This process will not precipitate impurities hence precipi- 3 2 solution. MgCl2 is electrolysed to get Mg & Cl2 tation of Sn(OH)4 and subsequent heating will enrich SnO2. Cl2 gas is used to manufacture (6) Aq. solution of CO2 is acidic. Hence CO2 when passed – HCl. Cl + H O 2HCl + (O) into [Al(OH4)] , solution makes it acidic hence Al(OH)3 is 2 2 precipitated. (13) Sn4+ + Sn 2Sn2+. Sn reduces Sn4+ to Sn2+. + – CO2 + 2H2O H3O + HCO3 (14) In sea water Mg exists as MgCl2 – + [Al(OH4)] + H3O Al(OH)3 + 2H2O On treating sea water with slaked lime Mg(OH)2 is obtained. MgCl + Ca(OH) Mg (OH) + CaCl (7) Silver coin (Cu + Ag) Conc. HNO3 (Cu2+ + Ag+) NO – 2 2 2 2 3 in sea water slaked lime NaCl On reacting Mg(OH)2 with HCl , MgCl2 is obtained Mg (OH) + 2 HCl MgCl + H O AgCl (White ppt) 2 2 2 From MgCl , Mg as obtained by reduction of MgCl with CaC . 2AgCl + Na CO Ag CO + 2NaCl 2 2 2 2 3 2 3 MgCl2 + CaC2 Mg + CaCl2 + 2 C Ag CO 2Ag + CO + (1/2) O 2 3 2 2 (15) In the case of reduction of sulphides, the f Gº of MxS or use cyanide process with AgCl. (metal sulphide) is not compensated. (8) SnO2 is soluble in acid and alkali both, hence amphoteric (16) Zone refining is based on the difference in solubility of SnO2 + 4HCl SnCl4 + 2H2O impurities in molten and solid state of the metal. This method basic is used for obtaining metals of very high purity. Ge, Si and SnO2 + 2NaOH Na2SnO3 + H2O Ga used as semi-conductors are refined in this manner. + Amphiprotic substance give or accept H ion e.g. H2O, NH3. EXERCISE - 4 (1) (A) (2) (A) (5) (A) p, (B) q, (C) p, r, (D) p, s (3) (C) (4) (B)
22 Gyaan Sankalp