1. Minerals: The naturally occurring chemical substances in the earth’s crust which are obtained by mining.
2. Ore: The mineral from which the metal is conveniently and economically extracted.
3. Gangue: The earthy materials associated with the ores.
4. Occurrence of metals: Metals which have low chemical reactivity generally occur in free state. For example, gold and platinum. Metals which are chemically reactive occur in combined state. For example, aluminium, iron and zinc.
Table 6.1: Principal Ores of Some Important Metals
Metal | Ores | Composition |
Aluminium | Bauxite Kaolinite (a form of clay) | AlOx(OH)3–2x [where 0 < x < 1] [Al2(OH)4Si2O5] |
Iron | Haematite Magnetite Siderite Iron pyrite | Fe2O3 Fe3O4 FeCO3 FeS2 |
Copper | Copper pyrite Malachite Copper glance Cuprite | CuFeS2 CuCO3.Cu(OH)2 Cu2S Cu2O |
Zinc | Zinc blende or Sphalerite Calamine Zincite | ZnS ZnCO3 ZnO |
5. Metallurgy: The scientific and technological process used for isolation of the metals from its ore.
6. Concentration: The process of removal of unwanted earthy and silicious impurities (gangue) from the ore.
Some of the important concentration methods are:
(a) Hydraulic washing: It is based on the differences in densities or gravities of the ore and the gangue particles. In one such process, an upward stream running water is used to wash the powdered ore. The lighter gangue particles are washed away and the heavier ores are left behind. The oxide ores are generally concentrated by this method.
(b) Magnetic separation: This method is based on differences in magnetic properties of ore and gangue (Fig. 6.1). For example, magnetic ores, magnetite (Fe3O4) and haematite (Fe2O3) are separated from the non-magnetic gangue by this method.
(c) Froth floatation: This method is based on preferential wetting of ore particles by oil and gangue particles with water. The sulphide ores of zinc, copper and lead are usually concentrated by this method.
In the froth floatation process, a suspension of the powdered ore is made with water. To it, collectors and froth stabilisers are added. The suspension is violently agitated by the rotating paddle which draws in air causing frothing (Fig. 6.2). The ore particles which are preferentially wetted by oil stick to the air bubbles, rise to the surface along with the froth while gangue particles which are preferentially wetted by water settle at the bottom. The froth is skimmed off. It is allowed to collapse and finally dried to get the concentrated ore.
l Collectors: Substances like pine oil, fatty acids, xanthates, which give water-repellant properties to the surface of the ore particles to be floated.
l Froth stabilisers: Substances like cresols and aniline, which stabilise the froth.
l Depressants: These are the substances which selectively prevent certain type of particles from forming the froth with the bubbles. For example, NaCN is added as a depressant for the separation of an ore containing ZnS and PbS. NaCN acts as a depressant for ZnS but not for PbS. NaCN forms a layer of zinc complex Na2[Zn(CN)4] on the surface of ZnS, thereby, preventing it from forming froth.
(d) Leaching: This method consists of treating the powdered ore with a suitable reagent which can selectively dissolve the ore but not the impurities. The impurities are filtered out and ore recovered from solution. For example, bauxite ore containing SiO2, iron oxide and titanium oxide as impurities are concentrated by this method.
Leaching of aluminium from bauxite: Finely powdered bauxite ore is digested with an aqueous solution of sodium hydroxide at 473–523 K and 35–36 bar pressure. Al2O3 is leached out as sodium aluminate (and SiO2 too as sodium silicate) leaving impurities behind.
Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4] (aq)
The aluminate in solution is neutralised by passing CO2 gas and hydrated Al2O3 is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al2O3 which induces the precipitation.
2Na[Al(OH)4] (aq) + CO2 (g) → Al2O3 . xH2O (s) + 2NaHCO3 (aq)
The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to get back pure Al2O3.
7. Extraction of Crude Metal from Concentrated Ore: The process used to obtain metals in free state from the concentrated ore is called extraction. It involves the following two major steps:
(a) Conversion of the ore into metal oxide and
(b) Reduction of the oxide to metal.
(a) Conversion of the ore into metal oxide: The following two methods are used for conversion of ores into their respective oxides.
(i) Calcination: It is the process of heating an ore below its melting point either in the absence or limited supply of air. During calcination,
n Moisture present in the ore is expelled.
n Hydrated ores become anhydrous. For example,
Carbonates are converted into their respective oxides.
(ii) Roasting: It is the process of heating the ore below its melting point in excess of air. The following changes occur during roasting:
n Moisture is driven away.
n Organic matter is destroyed.
n Non-metallic impurities such as sulphur, phosphorus and arsenic are oxidised and are removed as volatile gases.
n Sulphide ores are converted into metallic oxides.
n The ore becomes porous and hence easily workable in subsequent stages.
(b) Reduction of oxide to metal: The role of reducing agent is to provide DG negative and large enough to make the sum of DG of oxidation of reducing agent and reduction of metal oxide negative.
The free energy change, DG is related with other thermodynamic quantities by the expression;
∆G = ∆H – T∆S
where DH = enthalpy change, DS = entropy change, and T = temperature in kelvin
As heating, i.e., increase in T, favours a negative value of DrG, therefore, the temperature is chosen such that sum of DrG in two combined redox processes is negative.
Some of the common methods used for the reduction are given below.
(i) Auto-reduction: In this method, inactive metals can be reduced simply by heating the ore in air. Extraction of copper, lead, antimony, mercury, etc. have been carried out by this process.
2Cu2S + 3O2 → 2Cu2O + 2SO2↑
Cu2S + 2Cu2O → 6Cu + SO2↑
2PbS + 3O2 → 2PbO + 2SO2↑
2PbO + PbS → 3Pb + SO2↑
2HgS + 3O2 → 2HgO + 2SO2↑
2HgO + HgS → 3Hg + SO2↑
(ii) Smelting: In this process, metal oxide is reduced to metal with C or CO.
ZnO + C → Zn + CO↑
SnO2 + 2C → Sn + 2CO↑
(iii) Aluminothermic reduction: The process of reduction of metal oxide by aluminium is known as aluminothermic reduction. Metals like manganese and chromium are extracted by thermite process.
3MnO4 + 8Al → 4Al2O3 + 3Mn
Cr2O3 + 2Al → Al2O3 + 2Cr + Heat
(iv) Reduction with hydrogen: Hydrogen is an efficient reducing agent for metal oxides. For this purpose, the roasted ore is heated in a current of hydrogen when metal oxide is reduced to metal. For example, oxides of W, Mo, etc. are reduced with hydrogen.
WO3 + 3H2 → W + 3H2O
(v) Hydrometallurgy: The process of extraction of metal by dissolving the ore in a suitable reagent followed by precipitation or displacement of the metal by a more electropositive metal is known as hydrometallurgy.
For example, when native silver or gold is treated with a dilute solution (0.5%) of sodium or potassium cyanide, they go into the solution forming a soluble complex. From this soluble complex, metal is precipitated by adding zinc.
4Ag + 8NaCN + O2 + 2H2O → 4Na[Ag(CN)2] + 4NaOH
2Na[Ag(CN)2] + Zn → Na2[Zn(CN)4] + 2Ag↓
(vi) Electrometallurgy: The process of extraction of metal by electrolysis of their fused salt is known as electrometallurgy. This method is used for the extraction of highly reactive metals like Na, Ca, Al, etc.
In reduction of highly reactive metals, chemical reduction is not feasible, therefore electrolytic reduction is to be carried out. In reduction of molten metal salt, electrolysis is done. This method is based on electrochemical principles which can be understood with the help of equation
DG° = –nE°F
where ‘n’ is the number of electrons and Eo is electrode potential of the redox couple formed in the system.
More reactive metals have large negative values of Eo therefore, DGo becomes +ve and their reduction is difficult.
If the difference in two values of Eo of redox couple is +ve, then DGo will be –ve and less reactive metal can be obtained from its salt by more reactive metal.
Cu2+(aq) + Fe(s) → Cu(s) + Fe2+(aq)
In electrolysis, metal ions are discharged at negative electrode (cathode) and deposited there. Sometimes a flux is added for making the molten mass more conducting.
8. Flux: A flux is a substance which when mixed with calcinated or roasted ore, chemically combine with impurities present to form an easily fusible material called slag.
Flux + Impurity → Slag
The slag is insoluble in molten metal, and being lighter, floats over the surface of molten metal.
Fluxes are of two types:
(i) Acidic fluxes: For basic impurities like lime present in the ore, acidic fluxes like silica are used.
(ii) Basic fluxes: For acidic impurities like silica present in the ore, basic fluxes like limestone are used.
9. Refining: The process of purifying the impure metals is called refining. Various methods are available for refining of impure metals. The choice of purification method depends on the nature of the metal and the impurities present. Following methods are in general, employed for refining metals.
(a) Distillation: This method is used to remove the non-volatile impurities from volatile metals like zinc, cadmium and mercury. The impure metal is heated in a retort when the pure metal vaporises and condenses separately leaving behind the non-volatile impurities.
(b) Liquation: This is very useful for low melting metals like tin and lead. The impure metal is heated on the sloping hearth of a furnace when the molten metal flows away from the infusible impurities.
(c) Electrolytic refining: In this method, impure metal is made to act as anode. A strip of same metal in pure form is used as cathode. They are put in an electrolytic bath containing soluble salt of same metal. On passing electric current, metal ions from the electrolyte solution are deposited at the cathode in the form of pure metal while an equivalent amount of metal dissolves from the anode and goes into the electrolyte solution as metal ions, i.e.,
At cathode: Mn+(aq) + ne– → M(s)
At anode: M(s) → Mn+(aq) + ne–
The voltage applied for the electrolysis is such that the impurities of more electropositive metals remain in the solution as ions whereas impurities of less electropositive metals settle down under the anode as anode mud. A large number of metals such as copper, gold, silver, zinc, aluminium, etc., are refined by this method.
(d) Zone refining: It is based on the principle that the impurities are more soluble in the molten state than in the solid state of the metal. The impure metal is heated with the help of a circular mobile heater at one end. This results in the formation of a molten zone or melt. As the heater is moved along the length of the rod, the pure metal crystallises out of the melt and impurities pass into the adjacent molten zone. This process is repeated several times till the impurities are completely driven to one end of the rod which is then cut off and discarded. This method is very useful for producing semiconductor and other metals of very high purity, e.g., silicon, germanium, boron and gallium.
(e) Vapour phase refining: In this method, the metal is converted into its volatile compound and collected elsewhere. It is then thermally decomposed to get the pure metal.
Following examples will illustrate this technique:
(i) Mond process: In this process, nickel is heated in a stream of carbon monoxide to form a volatile complex, nickel tetracarbonyl.
Ni + 4CO
The vapour of nickel carbonyl is taken to decomposer chamber maintained at 450–470 K where it decomposes to give pure nickel.
Ni(CO)4
(ii) van Arkel method: This method is very useful for removing all the oxygen and nitrogen present in the form of impurity in certain metals like Zr and Ti. Impure metal is heated with iodine in an evacuated vessel and the resultant tetraiodide is decomposed on a tungsten filament to get the pure metal.
(f) Chromatographic methods: It is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. The mixture to be separated 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 (eluent). There are several chromatographic techniques such as paper chromatography, column chromatography, gas chromatography, etc. Chromatography is used for the 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.
(g) Oxidative refining: In this method, the impure metal is heated to a high temperature and then exposed to air when the impurities such as carbon, phosphorus and arsenic, etc., are oxidised along with outgoing gases. Foreign metals present as impurities also form their oxides and float on the surface of molten metal from where they are skimmed off periodically. Oxidative refining of iron is carried out in the Bessemer converter by blowing air through the molten metal.
10. Flow-Sheet Diagram for Metallurgical Process:
11. Thermodynamic Principles of Metallurgy: For any process, at any specified temperature, Gibbs free energy change (DG) is given by the equation:
DG = DH – TDS
where DH is the enthalpy change and DS is the entropy change for the process. The change in free energy is also related to the equilibrium constant K of the reactant product system through the equation:
DG = – RT ln K
A –ve DG implies a +ve K in equation. And this can happen only when reaction proceeds towards products. From these facts we can draw the following conclusions:
(i) The criterion of feasibility of a reaction at any temperature is that the change in free energy (DG) must be negative. If DS is +ve on increasing the temperature (T), the value of TDS will increase and when DH < TDS, DG will be negative and the reaction will proceed towards products.
(ii) A reaction with positive DG can still occur when it is coupled with another reaction having large negative DG so that the net DG of the two reactions is negative.
12. (a) Ellingham Diagram: Ellingham diagram is the graphical representation of Gibbs energy. It provides a basis for considering the choice of reducing agent in the reduction of oxides and helps us in predicting the feasibility of thermal reduction of an ore.
Some salient features of Ellingham diagram are:
(i) Ellingham diagram normally consists of plots of Df G° vs T for formation of oxides of elements, i.e., for the reaction.
2xM(s) + O2(g) → 2MxO(s)
In this reaction, DS is –ve. Due to this DG shifts towards higher side despite rising T.
(ii) Each plot is a straight line unless some change in phase (s → liq or liq → g) takes place. The temperature at which such change occurs, is indicated by an increase in the slope on +ve side (e.g., in the Zn, ZnO plot, the melting is indicated by an abrupt change in the curve).
(iii) There is a point in curve below which DG is negative, i.e., MxO is stable. Above this point, MxO will decompose on its own.
(iv) In an Ellingham diagram, the plots of DG° for oxidation (and therefore reduction of the corresponding species) of common metals and some reducing agents are given. The values of Df G°, etc. (for formation of oxides) at different temperature are depicted which make interpretation easy.
(v) Similar diagrams are also constructed for sulphides and halides and it becomes clear why reduction of MxS is difficult. Therefore, the Df G° of MxS is not compensated.
(b) Choice of reducing agent using Ellingham diagram: Any metal can reduce the oxides of the other metals which lie above it in the Ellingham diagram. This is due to the fact that DG becomes more negative by an amount equal to the difference between the two graphs at that temperature.
(c) Ellingham diagram and temperature: As heating (i.e., increasing temperature) favours a negative value of Dr G°. Therefore, the temperature is chosen such that the sum of Dr G° in the two combined reactions is negative. In Dr G° vs T plots, this is indicated by the point of intersection of two curves (curve for MxO and that for the oxidation of the reducing substance). After that point, the Dr G° value becomes more negative for the combined reaction including the reduction of MxO. The difference in two Dr G° values after that point determines whether reductions of the oxide of the upper line is possible by element represented by the lower line. Larger the difference, easier is the reduction.
(d) Limitations of Ellingham diagram
(i) Ellingham diagram simply indicates whether a reaction is possible or not. It does not say about the kinetics of the reduction process, i.e., how fast a reaction could be under given thermodynamic conditions.
(ii) The interpretation of DG° is based on K (DG° = – RT ln K). Thus, it is presumed that the reactants and products are in equilibrium. This is not always true because the reactant/ product may be solid.
13. (a) Extraction of iron from its oxides: Oxide ores of iron, after concentration through calcination or roasting are mixed with limestone and coke and fed into a blast furnace from its top. Thermodynamics help us to understand how coke reduces the oxide and why blast furnace is chosen.
FeO + C → Fe + CO
It can be seen as a couple of two simpler reactions.
Adding (i) and (ii), we get
DG(C, CO) + DG(FeO, Fe) = DrG
If DG is –ve, the reaction will take place.
In DG° vs T plot representing reaction (i) goes upward and that representing C → CO goes downward. At temperatures above 1073 K approx., the C, CO line comes below Fe, FeO line [DG(C, CO) < DG(Fe, FeO)]. So in this case coke will be reducing FeO to Fe and itself be oxidised to CO.
In a similar way, the reduction of Fe3O4 and Fe2O3 at relatively lower temperatures by CO can be explained on the basis of lower lying points of intersection of their curves with (CO, CO2) curve in Ellingham diagram.
Reduction of iron oxide in blast furnace: Reduction of iron oxides takes place in different temperature ranges. Coke is burnt to give temperature up to about 2200 K at lower portion itself.
The reactions can be summarised as follows:
At 500–800 K (lower temperature range in the blast furnace):
3Fe2O3 + CO → 2Fe3O4 + CO2↑
Fe3O4 + 4CO → 3Fe + 4CO2↑
Fe2O3 + CO → 2FeO + CO2↑
At 900 – 1500 K (higher temperature range in the blast furnace):
C + CO2 → 2CO(g)
FeO + CO → Fe + CO2↑
Limestone is also decomposed to CaO which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron.
(b) Extraction of zinc from zinc oxide: The reduction of zinc oxide is done using coke. The temperature in this case is higher than that in case of copper. For the purpose of heating, the oxide is made into brickettes with coke and clay.
ZnO + C
The metal is distilled off and collected by rapid chilling.
14. Extraction of Copper from Cuprous Oxide [copper (I) oxide]: In the graph of Dr G° vs T for formation of oxides (Ellingham diagram), the Cu2O line is almost at the top. So, it is quite easy to reduce Cu2O both with C and CO at 500 to 600 K.
Concentration of ore: Cu2S is concentrated by froth floatation process.
Roasting: Sulphide ore (Copper glance) is heated in the presence of oxygen to form Cu2O and sulphur dioxide is formed.
2Cu2S + 3O2 → 2Cu2O + 2SO2↑
Reduction: The oxide can be reduced to metallic copper using coke as reducing agent.
Cu2O + C → 2Cu + CO
Smelting: In actual process, the ore is mixed with SiO2 and heated in a reverberatory furnace. Iron oxide reacts with SiO2 to form iron silicate as slag.
Copper is produced in the form of copper matte. This contains Cu2S and FeO. Copper matte is charged into silica lined convertor. Some silica is also added and hot blast air is blown to convert the remaining FeS into FeO and Cu2S/Cu2O to metallic copper.
2FeS + 3O2 → 2FeO + 2SO2↑
2Cu2S + 3O2 → 2Cu2O + 2SO2↑
2Cu2O + Cu2S → 6Cu + SO2↑
The solidified copper obtained has blistered appearance due to the evolution of SO2 and so it is called blister copper.
Purification of copper: Copper is purified by electrolytic refining by taking impure Cu as anode, pure Cu as cathode and acidified CuSO4 solution as electrolyte. The net result of electrolysis is the transfer of copper in pure form from the anode to the cathode.
At anode: Cu → Cu2+ + 2e–
At cathode: Cu2+ + 2e– → Cu
Impurities from the blister copper deposit as anode mud which contains antimony, selenium, tellurium, silver, gold and platinum; recovery of these elements may meet the cost of refining.
15. Extraction of Aluminium from Bauxite: Aluminium is extracted from bauxite Al2O3.2H2O.
(i) Concentration of bauxite is done by leaching as explained in Basic Concepts Point 6.
(ii) Electrolytic reduction (Hall–Heroult Process): Purified Al2O3 is mixed with Na3AlF6 or CaF2 which lowers the melting point of mixture and brings electrical conductivity. Fused mixture is electrolysed using graphite rods as anode and carbon lining as cathode.
The graphite anode is useful for reduction of metal oxide to metal. The overall electrolytic reactions are:
Al2O3
Cathode: Al3+(melt) + 3e– → Al (l)
Anode: C(s) + O2– (melt) → CO(g) + 2e–
C(s) + 2O2– (melt) → CO2(g) + 4e–
The overall reaction may be given as:
2Al2O3 + 3C → 4Al + 3CO2↑
16. Copper from Low Grade Ores and Scraps: Copper is extracted by hydrometallurgy from low-grade copper ores and scraps. It is leached out by using acid or bacteria.
Reduction: The solution containing Cu2+ is treated with scrap iron or H2.
Cu2+(aq) + H2(g) → Cu(s) + 2H+(aq)
Cu2+(aq) + Fe(s) → Fe2+(aq) + Cu(s)
17. Production of Chlorine: Chlorine can be obtained from electrolysis of brine solution (saturated solution of sodium chloride).
2Cl–(aq) + 2H2O(l) → 2OH– (aq) + H2(g) + Cl2(g)
DG° = + 422 kJ, E° = – 2.2 V
It means external e.m.f. greater than 2.2 V is required to carry out electrolysis.
18. Table 6.2: A Summary of the Occurrence and Extraction of Some Common Metals
Metal | Occurrence | Common Method of Extraction | Remarks |
Aluminium | 1. Bauxite, Al2O3.xH2O 2. Cryolite, Na3AlF6 | Electrolysis of Al2O3 dissolved in molten Na3AlF6. | For the extraction, a good source of electricity is required. |
Iron | 1. Haematite, Fe2O3 2. Magnetite, Fe3O4 | Reduction of the oxide with CO and coke in Blast furnace. | Temperature approaching 2170 K is required. |
Copper | 1. Copper pyrites, CuFeS2 2. Copper glance, Cu2S 3. Malachite, CuCO3.Cu(OH)2 4. Cuprite, Cu2O | Roasting of sulphide partially and reduction. | It is self reduced in a specially designed converter. The reduction takes place easily. Sulphuric acid leaching is also used in hydrometallurgy from low grade ores. |
Zinc | 1. Zinc blende or Sphalerite, ZnS 2. Calamine, ZnCO3 3. Zincite, ZnO | Roasting followed by reduction with coke. | The metal may be purified by fractional distillation. |
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