CBSE NOTES CLASS 12 CHEMISTRY CHAPTER 6

GENERAL PRINCIPLES AND PROCESSES OF ISOLATION OF ELEMENTS

Occurrence of metals

Earth crust is the source of many elements. Out of these elements, 70% are metals. Aluminium is the most abundant metal on earth crust followed by iron.

Many gemstones are impure forms of Al2O3.

Impurity of Cr in Al2O3 results in ‘ruby’.

Impurity of Co in Al2O3 results in ‘sapphire’.

Metals occur in two forms in nature (i) in native state (ii) in combined state, depending upon their chemical reactivity.

Native state

Elements which have low chemical reactivity or noble metals having least electropositive character are not attacked by oxygen, moisture and CO2 of the air. These elements, therefore, occur in the free-state or in the native state, e.g., Au, Ag, Pt, S, O, N, noble gases, etc.

Combined state

Highly reactive elements such as F, Cl, Na, K, etc., occur in nature in combined form as their compounds such as oxides, carbonates, sulphides, halides etc.

Hydrogen is the only non-metal which exists in oxidised form only.

Minerals

The naturally occurring substances in the form of which the metals occur in the earth crust are called minerals.

Ores

The mineral from which the metal is economically and conveniently extracted is called an ore. Hence all ores are minerals but all minerals are not ores.

Metals and their ores

 Metal Ores Composition Aluminium Bauxite AlOx(OH)3-2x [where 0 < x < 1] Kaolinite (a form of clay) [Al2(OH)4Si2O5] Iron Haematite Fe2O3 Magnetite Fe3O4 Siderite FeCO3 Iron pyrites FeS2 Copper Copper pyrites CuFeS2 Malachite CuCO3.Cu(OH)2 Cuprite Cu2O Copper glance Cu2S Zinc Zinc blende or Sphalerite ZnS Calamine ZnCO3 Zincite ZnO

Types of ores

 Oxides Carbonates Halides Sulphides Sulphates Zincite (ZnO) Marble or limestone (CaCO3) Fluorspar (CaF2) Zinc blende (ZnS) Galena (PbS) Anglesite (PbSO4) Haematite (Fe2O3.xH2O) Magnetite (Fe3O4) Calamine (ZnCO3) Cryolite (Na3AlF6) Iron pyrites (FeS2) Baryl (BaSO4) Bauxite (Al2O3.2H2O) Siderite (FeCO3) Horn Silver (AgCl) Cinnabar (HgS) Gypsum (CaSO4.2H2O) Cuprite (Cu2O) Magnesite (MgCO3) Rock salt (NaCl) Epsom salt (MgSO4.7H2O)

Gangue or matrix

Impurities associated with ores are called gangue or matrix.

Metallurgy

The entire scientific and technological process used for isolation of the metal from its ores is known as metallurgy.

Types of metallurgical processes

1. Pyrometallurgy: Extraction of metals takes place at very high temperature. Cu, Fe, Zn, Sn, etc are extracted by this method.

2. Hydrometallurgical process: In this method, metals are extracted by the use of their aqueous solution. Ag and Au are extracted by this method.

3. Electrometallurgical process: Na, K, Li, Ca, etc., are extracted from their molten salt solution through electrolytic method.

STEPS INVOLVED IN METALLURGY

• Crushing of the ore

• Concentration of ore,

• Isolation of the metal from its concentrated ore, and

• Purification of the metal.

Crushing of ore

The big lumps of ore are crushed into smaller pieces with the help of jaw-crushers. The process of grinding the crushed ore into fine powder with the help of the stamp mills is called pulverisation.

Concentration of ores

Removal of unwanted materials (e.g., sand. clays, etc.) from the ore is known as ore concentration, ore dressing or ore benefaction. It can be carried out by various ways depending upon the nature of the ore.

Hydraulic washing/gravity separation or Levigation

The process by which lighter earthy impurities are removed from the heavier ore particles by washing with water is called levigation. The lighter impurities are washed away. This method is based on the difference in the densities (specific gravities) of ore and gangue.

This method is commonly used for oxide ores such as haematite, tin stone and native ores of Au, Ag, etc.

Froth floatation

This method is used for the concentration of sulphide ores. This method is based on the preferential wetting of ore particles by oil and that of gangue by water. As a result, the ore particles become lighter and rise to the top in the form of froth while the gangue particles become heavy and settle down. Preferential adsorption is involved in this method.

The froth can be stabilised by the addition of stabilisers (aniline or cresols).

Activators activate the floating property of one of the component of the ore and help in the separation of different minerals present in the same ore. CuSO4 is used as activator in recovery of ZnS.

Depressants are used to prevent certain types of particles from forming the froth with air bubbles, e.g., NaCN can be used as a depressant in the separation of ZnS and PbS ores. It selectively prevents ZnS from floating. KCN is another depressant.

Collectors increase the non-wettability of ore particles by water, e.g., pine oils and fatty acids.

Magnetic separation

This method of concentration is employed when either the ore or the impurities associated with it are magnetic in nature. The ground ore is carried on a conveyer belt which passes over a magnetic roller. The magnetic material is thus separated from non-magnetic one as shown in the figure.

Electrostatic separation

This method is used for the separation of lead sulphide (good conductor) which is charged immediately in an electrostatic field and is thrown away from the roller from zinc sulphide (poor conductor) which is not charged and hence, drops vertically from the roller.

Chemical method of separation or Leaching

Leaching is the process in which the ore is concentrated by chemical reaction with a suitable reagent which dissolves the ore but not the impurities. For example,

• Red bauxite is leached with concentrated solution of NaOH at 473 – 523 K and 35 – 36 bar pressure, which dissolves aluminium while other oxides (Fe2O3, TiO2, SiO2), remain undissolved.

Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4] (aq)

The impurities are then filtered and the solution is neutralized by passing CO2 gas. In this process, hydrated Al2O3 gets precipitated and sodium hydrogencarbonate remains in the solution.

2Na [Al(OH)4] (aq) + CO2 (g) → Al2O3.xH2O (s) + 2NaHCO3 (aq)

Hydrated alumina thus obtained is filtered, dried, and heated to give back pure alumina (Al2O3).

Al2O3.xH2O (s) Al2O3 + xH2O (g)

• Noble metals (Ag and Au) are leached with a dilute aqueous solution of NaCN or KCN in the presence of air.

4M(s) + 8CN-(aq) + 2H2O(aq) + O2(g) → 4[M(CN)2)]-(aq) + 4OH-(aq)

2[M(CN)2]-(aq) + Zn(s) → [Zn(CN)4)]2-(aq) + 2M(s)

For example,

4Au(s) + 8CN-(aq) + 2H2O(aq) + O2(g) → 4[Au(CN)2)]-(aq) + 4OH-(aq)

2[Au(CN)2]-(aq) + Zn(s) → [Zn(CN)4)]2-(aq) + 2Au(s)

EXTRACTION OF CRUDE METALS FROM CONCENTRATED ORE

The concentrated ore is usually converted to oxide before reduction, as oxides are easier to reduce. Thus, isolation of crude metal from concentrated ore involves two major steps, namely conversion to oxide and reduction of the oxides to metal.

Conversion of ores to oxides

1. Calcination: It is the process of converting an ore into its oxides by heating it strongly, below its melting point in a limited supply of air or in absence of air.

During calcination, volatile impurities as well as organic matter and moisture are removed.

Fe2O3.xH2O(s) Fe2O3 + xH2O(s)

ZnCO3(s) ZnO(s) + CO2(g)

CaCO3.MgCO3(s) CaO(s) + MgO(s) + 2CO2(g)

Calcination is used for metal carbonates and hydroxides and is carried out in reverberatory furnace, a furnace in which the roof and walls are heated by flames from another chamber and radiate heat on to material in the centre of the furnace.

Reverberatory Furnace

1. Roasting: It is the process of converting an ore into its metallic oxide by heating it strongly below its melting point in excess of air. This process is commonly used for sulphide ores and is carried out in blast furnace or reverberatory furnace. Roasting helps to remove the non-metallic impurities and moisture.

ZnS(s) + 3O2(g) 2ZnO(s) + 2SO2(g)

PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g)

Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)

The furnaces used in calcination and roasting employ refractory materials which resist high temperature and do not become soft.

SO2 produced is used in manufacturing of H2SO4

Reduction of oxides to metal

The roasted or the calcinated ore is converted to the free metal by reduction. Reduction method depends upon the activity of metal.

Metals which are low in the activity series (like Cu, Hg, Au) are obtained by heating their compounds in air.

Metals which are in the middle of the activity series (like Fe. Zn, Ni, Sn) are obtained by heating their oxides with carbon.

Metals which are very high in the activity series (e.g., Na, K, Ca, Mg, Al) are obtained by electrolytic reduction method.

1. Smelting (reduction with carbon)

The process of extracting the metal by fusion of its oxide ore with carbon (C) or CO is called smelting. It is carried out in a reverberatory furnace.

ZnO + C Zn + CO↑

Fe2O3 + CO 2FeO + CO2

2Fe2O3 + 3C 4Fe + 3CO2

Flux

During smelting a substance, called flux is added which removes the non-fusible impurities as fusible slag. This slag is insoluble in the molten metal and is lighter than the molten metal. So, it floats over the molten metal and is skimmed off.

Acidic flux

For basic impurities, acidic flux is added. e.g,

Basic flux

For acidic impurities, basic flux is added. e.g,

The slag is used in road making as well as in the manufacturing of cement and fertilizers.

2. Reduction by hydrogen

It is done for W or Mo oxide.

WO3 + 3H2 W + 3H2O

3. Reduction by aluminium

Aluminium powder is used in this process. It is known as alumino-thermic reduction or Gold Schmidt thermite process.

Cr2O3 + 2Al → Al2O3 + 2Cr

Mixture of the oxide and Al in the ratio of 3:1 is known as thermite and mixture of BaO2 + Mg powder acts as ignition powder.

4. Auto reduction

This is used for reduction of sulphide ores of Pb, Hg, Cu, etc. The sulphide ore is heated in a supply of air at 770-970 K when the metal sulphide is partially oxidised to form its oxide or sulphate which then reacts with the remaining sulphide to give the metal.

Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)

Cu2S(s) + 2Cu2O(s 6Cu(s) + 2SO2(g)

5. Reduction by Mg

TiCl4 + 2Mg → 2MgCl2 + Ti (Kroll’s process)

6. Electrolytic reduction or electrometallurgy

It is the process of extracting highly electropositive (active) metals such as Na, K, Ca, Mg, Al, etc by electrolysis of their oxides, hydroxides or chlorides in fused state, e.g., Mg is prepared by the electrolysis of fused salt of MgCl2 (Dow’s process).

THERMODYNAMICS OF METALLURGY

Change in Gibbs energy, ΔG for any process at any specified temperature, is described by

ΔG = ΔH – TΔS

where,

ΔH = enthalpy change

ΔG = Gibbs free energy

T = Temperature in K

ΔS = Entropy change

For any reaction,

ΔGΘ = – RT ln K,

where, K is the equilibrium constant of the ‘reactant – product’ system at the temperature T.

A negative ΔG implies a +ve ln K (K > 1) in this equation, which can happen only when reaction proceeds towards products.

1. When the value of ΔG is negative the reaction will proceed. If ΔS is positive, on increasing the temperature (T), the value of TΔS would increase (ΔH < TΔS), hence ΔG will become –ve.

2. If reactants and products of two reactions are put together in a system and the net ΔG of the two possible reactions is –ve, the overall reaction will occur. So the process of interpretation involves coupling of the two reactions, getting the sum of their ΔG and looking for its magnitude and sign.

ELLINGHAM DIAGRAM

The plot of ΔfG° against T is called Ellingham diagram.

Such diagrams help in predicting the feasibility of thermal reduction of an ore.

The criterion of feasibility is that at a given temperature, Gibbs energy of the reaction must be negative.

Characteristics of Ellingham diagram

1. Ellingham diagram normally consists of plots of ΔfG° vs T for formation of oxides of elements.

Let us consider the reaction,

2xM(s) + O2(g) → 2MxO(s)

In this reaction, the gaseous amount (hence molecular randomness) is decreasing from left to right due to the consumption of gases leading to a –ve value of ΔS which changes the sign of the second term in equation. Subsequently ΔG shifts towards higher side despite rising T (normally, ΔG decreases i.e., goes to lower side with increasing temperature). The result is +ve slope in the curve for most of the reactions for formation of MxO(s).

2. Each plot is a straight line except when some change in phase (s → l or l → 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).

3. There is a point in a curve below which ΔG is negative (So MxO is stable). Above this point, MxO will decompose on its own.

4. A metal will reduce the oxide of other metals which lie above it in Ellingham diagram, i.e., the metals for which the free energy of formationfG°) of their oxides is more negative can reduce those metal oxides which has less negative Δf

5. The decreasing order of the negative values of ΔGf°of metal oxides is

Ca > Mg (below 1773 K) > Al > Ti > Cr > C > Fe > Ni > Hg > Ag

Thus, Al reduces FeO, CrO and NiO in thermite reduction but it will not reduce MgO at temperature below 1773 K.

6. CO is more effective reducing agent below 1073 K but coke is better above 1073 K, e.g., CO reduces F2O3 below 1073 K but above it, coke reduces Fe2O3.

Coke reduces ZnO above 1270 K.

Limitations of Ellingham diagram

1. The graph simply indicates whether a reaction is possible or not i.e., the tendency of reduction with a reducing agent is indicated. This is so because it is based only on the thermodynamic concepts. It does not say about the kinetics of the reduction process (Cannot answer questions like how fast it could be?).

2. The interpretation of ΔGo is based on K (ΔGo = – RT ln K). Thus it is presumed that the reactants and products are in equilibrium:

MxO + AredxM + AOox

This is not always true because the reactant/product may be solid.

EXPLANATION OF ELLINGHAM DIAGRAMS

The reducing agent forms its oxide when the metal oxide is reduced. The role of reducing agent is to provide Δr negative and large enough to make the sum of ΔrG° of the two reactions (oxidation of the reducing agent and reduction of the metal oxide), negative.

During reduction, the oxide of a metal decomposes:

MxO(s) → xM (s or l) + $\frac{1}{2}$ O2 (g) …..(i)

This can be visualised as reverse of the oxidation of the metal.

xM(s or l) + $\frac{1}{2}$ O2(g) → MxO(s) [ΔG° (M, MxO)] ……(ii)

The oxidation of the reducing agent (e.g., C or CO) will be,

C(s) + $\frac{1}{2}$ O2(g) → CO(g) [ΔG(C, CO)] ……(iii)

CO(g) + $\frac{1}{2}$ O2(g) → CO2(g) [ΔG(CO, CO2)] ……(iv)

There may also be complete oxidation to CO2, from carbon,

$\frac{1}{2}$C(s) + $\frac{1}{2}$ O2(g) → $\frac{1}{2}$ CO2(g) [½ ΔG(C, CO2)] ……(v)

On subtracting equation (ii) [it means adding its negative or the reverse form of (ii)] from (iii)/(iv)/ (v), we get,

MxO(s) + C(s) → xM(s or l) + CO(g)

MxO(s) + CO(g) → xM(s or l) + CO2(g)

MxO(s) + $\frac{1}{2}$ C(s) → xM(s or l) + $\frac{1}{2}$ CO2(g)

The ΔrG° values for these reactions can be obtained by subtraction of the corresponding ΔfG° values.

Heating (i.e., increasing T) favours a negative value of ΔrG°. Therefore, the temperature is chosen such that the sum of ΔrG° in the two combined redox process is negative. Graphs of ΔrG° vs T are called Ellingham Diagram. The sum of ΔrG° in the two combined redox process is indicated by the point of intersection of the two curves (curve for MxO and that for the oxidation of the reducing substance).

After that point, the ΔrG° value becomes more negative for the combined process including the reduction of MxO. The difference in the two ΔrG° values after that point determines whether reductions of the oxide of the upper line is feasible by the element represented by the lower line.

• Larger the difference, easier is the reaction.

APPLICATIONS OF ELLINGHAM DIGRAMS

(A) Extraction of iron from its oxides

Oxide ores of iron, after concentration through calcination/roasting (to remove water, to decompose carbonates and to oxidise sulphides) are mixed with limestone and coke and fed into a Blast furnace from its top. Here, the oxide is reduced to the metal.

One of the main reduction step

FeO(s) + C(s) → Fe(s/l) + CO (g)

can be seen as a couple of two simple reactions. In one, the reduction of FeO is taking place

FeO(s) → Fe(s) + $\frac{1}{2}$ O2(g) [ΔG(FeO, Fe)]

and in the other, C is being oxidized to CO,

C(s) + $\frac{1}{2}$ O2 (g) → CO (g) [ΔG (C, CO)]

The net Gibbs energy change is,

ΔG (C, CO) + ΔG (FeO, Fe) = ΔrG

The resultant reaction will take place when the right hand side here is negative.

In ΔGo vs T plot for FeO(s) → Fe(s) + ½ O2(g), the plot goes upward and that representing the change C → CO goes downwards.

The points of intersection of curves of Fe3O4 and Fe2O3 with the CO, CO2 curve are at lower temperatures. Hence they are reduced by CO at relatively lower temperatures.

At temperatures above 1073K (approx.), the C, CO line comes below the Fe, FeO line [ΔG(C, CO) < ΔG(Fe, FeO)]. So in this range, coke will reduce the FeO and will itself be oxidised to CO.

In the Blast furnace, reduction of iron oxides takes place in different temperature ranges.

Hot air is blown from the bottom of the furnace and coke is burnt to give temperature upto about 2200K in the lower portion itself.

The CO and heat moves to upper part of the furnace. In upper part, the temperature is lower and the iron oxides (Fe2O3 and Fe3O4) coming from the top are reduced in steps to FeO.

Temperature in the lower part of the furnace is high and FeO is reduced by C in this area.

Thus, the reduction reactions taking place in the lower temperature range and in the higher temperature range, depend on the points of corresponding intersections in the ΔrGo vs T plots.

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

FeO + CO → Fe + CO2

Limestone is decomposed to CaO which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron.

The iron obtained from blast furnace contains about 4% carbon and many impurities in smaller amount (e.g., S, P, Si, Mn). This is known as pig iron and cast into variety of shapes.

Cast iron is made by melting pig iron with scrap iron and coke using hot air blast. It has carbon content of about 3% and is extremely hard and brittle.

Further Reductions

Wrought iron or malleable iron is the purest form of commercial iron and is prepared from cast iron by oxidising impurities in a reverberatory furnace lined with haematite. This haematite oxidises carbon to carbon monoxide,

Fe2O3 + 3C → 2Fe + 3CO

Limestone is added as a flux and sulphur, silicon and phosphorus are oxidised and passed into the slag. The metal is removed and freed from the slag by passing through rollers.

(B) Extraction of copper from cuprous oxide [copper(I) oxide]

In the graph of ΔrGo vs T for formation of Cu2O line is almost at the top. So it is easy to reduce oxide ores of copper directly to the metal by heating with coke (both the lines of C, CO and C, CO2 are at much lower positions in the graph particularly after 500 – 600K).

However most of the ores are sulphide and some may also contain iron. The sulphide ores are roasted/smelted to give oxides:

2Cu2S + 3O2 → 2Cu2O + 2SO2

The oxide can then be easily reduced to metallic copper using coke,

Cu2O + C → 2Cu + CO

The ore is heated in a reverberatory furnace after mixing with silica. Iron oxide ‘slags off’ as iron silicate and copper is produced in the form of copper matte, containing Cu2S and FeS.

FeO + SiO2 → FeSiO3 (Slag)

Copper matte is then charged into silica lined convertor. Some silica is also added and hot air blast is blown to convert the remaining FeS, FeO and Cu2S/Cu2O to the metallic copper.

2FeS + 3O2 → 2FeO + 2SO2

FeO + SiO2 → FeSiO3

2Cu2S + 3O2 → 2Cu2O + 2SO2

2Cu2O + Cu2S → 6Cu + SO2

The solidified copper obtained has blistered appearance due to the evolution of SO2 and is called blister copper.

(C) 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 Zn + CO

The metal is distilled off and collected by rapid chilling.

Electrochemical principles of metallurgy

These methods are based on electrochemical principles which states,

ΔGo = – n F Eo

here n is the number of electrons and E0 is the electrode potential of the redox couple formed in the system. More reactive metals have large negative values of the electrode potential. So their reduction is difficult.

If the difference of two E0 values corresponds to a positive E0 and consequently negative Δg, then the less reactive metal will come out of the solution and the more reactive metal will go to the solution, e.g.,

Cu2+ (aq) + Fe(s) → Cu(s) + Fe2+ (aq)

In simple electrolysis, the Mn+ ions are discharged at negative electrodes (cathodes) and deposited there. Materials of electrodes should be chosen keeping in mind the reactivity of the metals. Sometimes a flux is added for making the molten mass more conducting.

(D) Extraction of aluminium (Hall-Heroult process)

Purified Al2O3 is mixed with Na3AlF6 or CaF2 which lowers the melting point of the mix and brings conductivity. The fused matrix is electrolysed.

Cathode – Carbon coated steel container

Anode - Graphite

The oxygen liberated at anode reacts with the carbon of anode producing CO and CO2. This way for each kg of aluminium produced, about 0.5 kg of carbon anode is burnt away. The electrolytic reactions are:

Cathode: Al3+ (melt) + 3e → Al(l)

Anode: C(s) + O2– (melt) → CO(g) + 2e

and C(s) + 2O2– (melt) → CO2 (g) + 4e

The overall reaction is,

2Al2O3 + 3C → 4Al + 3CO2

• The graphite lining at the anode is used, as it is easier to forms CO and CO2 when oxidized with O2-. Now this reaction is easier than liberation of O2 at the anode, due to over-potential of oxygen.

(E) Extraction of copper from low grade ores and scraps

Copper is extracted by hydrometallurgy from low grade ores. It is leached out using acid or bacteria. The solution containing Cu2+ is treated with scrap iron or H2. See earlier equations for reactions of Fe.

Cu2+(aq) + H2(g) → Cu(s) + 2H+ (aq)

OXIDATION REDUCTION

Some extractions are based on oxidation particularly for non-metals.

• Extraction of chlorine from brine (chlorine is abundant in sea water as common salt),

2Cl(aq) + 2H2O(l) → 2OH(aq) + H2(g) + Cl2(g)

The ΔGo for this reaction is +422 kJ. When it is converted to E0 (using ΔGo = – n F Eo), we get Eo = – 2.2 V. Hence an external e.m.f. greater than 2.2 V, is required.

Cl2 is obtained by electrolysis giving out H2 and aqueous NaOH as byproducts.

In case electrolysis of molten NaCl, Na metal is produced instead of NaOH. This process is called Downs' process.

• Extraction of gold and silver involves leaching the metal with CN. This is also an oxidation reaction (Ag → Ag+ or Au → Au+).

The metal is later recovered by displacement method.

4Au(s) + 8CN(aq) + 2H2O(aq) + O2(g) → 4[Au(CN)2](aq) + 4OH(aq)

2[Au(CN)2](aq) + Zn(s) → 2Au(s) + [Zn(CN)4]2– (aq)

In this reaction zinc acts as a reducing agent.

REFINING OR PURIFICATION OF CRUDE METALS

A metal extracted by any method is contaminated with some impurity. For obtaining metals of high purity, several techniques are used depending upon the differences in properties of the metal and the impurity.

(a) Distillation (b) Liquation (c) Electrolysis (d) Zone refining

(e) Vapour phase refining (f) chromatographic methods

(a) Refining of metals by distillation

This is very useful for low boiling metals like zinc and mercury. The impure metal is evaporated to obtain the pure metal as distillate.

(b) Refining of metals by liquation

In this method a low melting metal like tin can be made to flow on a sloping surface. In this way it is separated from higher melting impurities.

(c) Refining of metals by electrolysis

In this method, the impure metal is made to act as anode. A strip of the same metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal.

The more basic metal remains in the solution and the less basic ones go to the anode mud.

The reactions are:

Anode: M → Mn+ + ne

Cathode: Mn+ + ne →M

Copper is refined using an electrolytic method. Anodes are of impure copper and pure copper strips are taken as cathode. The electrolyte is acidified solution of copper sulphate and the net result of electrolysis is the transfer of copper in pure form from the anode to the cathode:

Anode: Cu → Cu2+ + 2e

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.

Zinc may also be refined this way.

(d) Zone refining

This method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.

A circular mobile heater is fixed at one end of a rod of the impure metal. The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystallises out of the melt and the impurities pass on into the adjacent molten zone.

The process is repeated several times and the heater is moved in the same direction. At one end, impurities get concentrated. This end is cut off. This method is useful for producing semiconductor and other metals of very high purity, e.g., germanium, silicon, boron, gallium and indium.

(e) Vapour phase refining

In this method, the metal is converted into its volatile compound and collected elsewhere. It is then decomposed to give pure metal.

The two requirements for this are,

1. the metal should form a volatile compound with an available reagent,

2. the volatile compound should be easily decomposable, so that the recovery is easy.

Examples:

Mond process for refining nickel

Nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl,

Ni + 4CO Ni(CO)4

The carbonyl is subjected to higher temperature so that it is decomposed giving the pure metal,

Ni(CO)4 Ni + 4CO

van Arkel method for refining zirconium or titanium

For removing all the oxygen and nitrogen present in the form of impurity in Zr and Ti. The crude metal is heated in an evacuated vessel with iodine. The metal iodide being more covalent, volatilizes,

Zr + 2I2 → ZrI4

The metal iodide is decomposed on a tungsten filament, electrically heated to about 1800K. The pure metal is deposited on the filament.

ZrI4 → Zr + 2I2

(f) Chromatographic methods

Column chromatography is a technique used to separate different components of a mixture. It is a very useful technique used for the purification of elements available in minute quantities. It is also used to remove the impurities that are not very different in chemical properties from the element to be purified.

Chromatography is based on the principle that different components of a mixture are differently adsorbed on an adsorbent.

In chromatography, there are two phases: mobile phase and stationary phase. The stationary phase is immobile and immiscible. Al2O3 column is usually used as the stationary phase in column chromatography.

The mobile phase may be a gas, liquid, or supercritical fluid in which the sample extract is dissolved. Then, the mobile phase is forced to move over through the stationary phase. The component that is more strongly adsorbed on the column takes a longer time to travel through it than the component that is weakly adsorbed. The adsorbed components are then removed (eluted) using a suitable solvent (eluant).

Other chromatographic techniques are paper chromatography, gas chromatography, etc.

Uses of aluminium

Aluminium foils are used as wrappers for chocolates.

The fine dust of the metal is used in paints and lacquers.

Aluminium, being highly reactive, is also used in the extraction of chromium and manganese from their oxides.

Wires of aluminium are used as electricity conductors.

Alloys containing aluminium, being light, are very useful.

Uses of copper

Copper is used for making wires used in electrical industry and for water and steam pipes.

It is also used in several alloys that are rather tougher than the metal itself, e.g., brass (with zinc), bronze (with tin) and coinage alloy (with nickel).

Uses of zinc

Zinc is used for galvanising iron. It is also used in large quantities in batteries, as a constituent of many alloys, e.g., brass, (Cu 60%, Zn 40%) and german silver (Cu 25-30%, Zn 25-30%, Ni 40–50%). Zinc dust is used as a reducing agent in the manufacture of dye-stuffs, paints, etc.

Uses of iron

Cast iron, which is the most important form of iron, is used for casting stoves, railway sleepers, gutter pipes, toys, etc. It is used in the manufacture of wrought iron and steel.

Wrought iron is used in making anchors, wires, bolts, chains and agricultural implements.

Alloy steel is obtained when other metals are added to it.

Nickel steel is used for making cables, automobiles and aeroplane parts, pendulum, measuring tapes, chrome steel for cutting tools and crushing machines, and stainless steel for cycles, automobiles, utensils, pens, etc.

SUMMARY OF EXTRACTION METHODS

 Metal Ore Extraction Method Conditions/Remarks Aluminium Bauxite, Al2O3. x H2O Cryolite, Na3AlF6 Electrolysis of Al2O3 dissolved in molten Na3AlF6 For the extraction, a good source of electricity is required. Iron Haematite, Fe2O3 Magnetite, Fe3O4 Reduction of the oxide with CO and coke in Blast furnace Temperature approaching 2170 K is required. Copper Copper pyrites, CuFeS2 Copper glance, Cu2S Malachite,CuCO3.Cu(OH)2 Cuprite, Cu2O Roasting of sulphide partially and reduction It is self reduction in a specially designed converter. The reduction takes place easily. Sulphuric acid leaching is also used in hydrometallurgy from low grade ores. Zinc Zinc blende or Sphalerite, ZnS Calamine, ZnCO3 Zincite, ZnO Roasting followed by reduction with coke The metal may be purified by fractional distillation.