Learning Outcomes

The learners will be able to

• Explain the terms minerals, ores, calcination, roasting, refining, etc.

• Understand the principles of oxidation & reduction in isolation of elements

• Apply the uses of thermodynamics parameters like Gibbs energy & entropy in extraction of metals.

Minerals

The compounds of metals in which metals occur in nature are called minerals.

Ores

The minerals from which metal can be conveniently and economically extracted are called ores. While all ores are minerals but all minerals are not ores.

Gangue

An ore is usually contaminated with undesired materials known as gangue.

Metallurgy

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

The extraction & isolation of metals from ores involve the following major steps:

• Concentration of the ore.

• Isolation of the metal from its concentrated ore

• Purification of the metal

Classification of Ores

Ores may be divided into following types

• Oxides: Haematite, $\mathrm{Fe} _{2} \mathrm{O} _{3}$; Limonite, $\mathrm{Fe} _{2} \mathrm{O} _{3} \cdot 3 \mathrm{H} _{2} \mathrm{O}$; Magnetite or Loadstone, $\mathrm{Fe} _{3} \mathrm{O} _{4}$; Bauxite, $\mathrm{Al} _{2} \mathrm{O} _{3} .2 \mathrm{H} _{2} \mathrm{O}$; Cassiterite or Tinstone, $\mathrm{SnO} _{2}$; Corundum, $\mathrm{Al} _{2} \mathrm{O} _{3}$; Diaspore, $\mathrm{Al} _{2} \mathrm{O} _{3} . \mathrm{H} _{2} \mathrm{O}$; Pyrolusite, $\mathrm{MnO} _{2}$; Zincite, $\mathrm{ZnO}$; Rutile, $\mathrm{TiO} _{2}$; Cuprite or Ruby copper, $\mathrm{Cu} _{2} \mathrm{O}$

• Carbonates: Magnesite, $\mathrm{MgCO} _{3}$; Dolomite, $\mathrm{CaCO} _{3}$. $\mathrm{MgCO} _{3}$; Cerussite, $\mathrm{PbCO} _{3}$; Calamine or Smithosonite, $\mathrm{ZnCO} _{3}$; Siderite, $\mathrm{FeCO} _{3}$; Malachite or Basic copper carbonate, $\mathrm{CuCO} _{3}$. $\mathrm{Cu}(\mathrm{OH}) _{2} ;$ Azurite, $2 \mathrm{CuCO} _{3}$. $\mathrm{Cu}(\mathrm{OH}) _{2}$; Limestone or Calcite (Marble, Chalk, Slate), $\mathrm{CaCO} _{3}$

• Sulphides : Iron pyrites, $\mathrm{FeS} _{2}$; Galena, PbS; Zinc blende, ZnS ; Cinnabar, $\mathrm{HgS}$; Chalcopyrites or Copper pyrites, CuFeS; Silver glance orArgentite, $\mathrm{Ag} _{2} \mathrm{~S}$; copper glance, $\mathrm{Cu} _{2} \mathrm{~S}$; Pentlandite, (Ni, Fe) S ,

• Halides : Sylvine, $\mathrm{KCl}$; Camallite, $\mathrm{KCl}$. $\mathrm{MgCl} _{2} .6 \mathrm{H} _{2} \mathrm{O}$; Common salt or Rock salt, $\mathrm{NaCl}$; Horn silver, $\mathrm{AgCl}$; Cryolite, $\mathrm{Na} _{3} \mathrm{AlF} _{6}$; Fluorite or Fluorspar, $\mathrm{CaF} _{2}$

• Sulphates: Anglesite, $\mathrm{PbSO} _{4}$; Gypsum, $\mathrm{CaSO} _{4}$. $2 \mathrm{H} _{2} \mathrm{O}$; Barytes, $\mathrm{BaSO} _{4}$; Epsomite or Epsom salt, $\mathrm{MgSO} _{4} \cdot 7 \mathrm{H} _{2} \mathrm{O}$; Kieserite, $\mathrm{MgSO} _{4}$. $\mathrm{H} _{2} \mathrm{O}$.

• Silicates : Asbestos, $\mathrm{Mg} _{3}\left(\mathrm{Si} _{2} \mathrm{O} _{5}\right)(\mathrm{OH}) _{4}$; Talc, $\mathrm{Mg} _{2}\left(\mathrm{Si} _{2} \mathrm{O} _{5}\right) _{2} \cdot \mathrm{Mg}(\mathrm{OH}) _{2}$; Willemite, $\mathrm{Zn} _{2} \mathrm{SiO} _{4}$; Felspar, $\mathrm{KAlSi} _{3} \mathrm{O} _{8} ;$ Mica, $\mathrm{K} _{2} \mathrm{O} .3 \mathrm{Al} _{2} \mathrm{O} _{3} .6 \mathrm{SiO} _{2} .2 \mathrm{H} _{2} \mathrm{O}$

I) Concentration of ores:

Removal of unwanted impurities (gangue) e.g. sand, clay etc. from the ore is known as concentration, benefaction or dressing of the ore

1) Levigation.

It is the process of separating the lighter gangue particles from the heavier ore by washing in a current of water. This is also called gravity separation or hydraulic washing and is generally used for oxide ores and carbonate ores. This method is based on the difference in densities of the ore particles and impurities. For example, haematite, tin stone and native ores of $\mathrm{Au}, \mathrm{Ag}$ etc. which are usually concentrated by this method.

2) Froth-Floatation

This process is used for the concentration of sulphide ores and is based upon preferential wetting of the ore particles by the oil used as a foaming agent (pine oil and fatty acids) and gangue particles by water. As a result, the ore particles become light and rise to the top in form of froth while the gangue particles become heavy and settle down. Thus adsorption phenomenon is involved in this method. Cresols and aniline are used as froth stabilizers.

Ethyl xanthate and potassium ethyl xanthate are used as collectors (to make the ore water repellant) and copper sulphate is used as an activator while sodium and potassium cyanides are used as depressants. In case of sulphide minerals containing two or more metals such as galena (PbS) is separated from sphalerite (ZnS) and iron pyrites $\left(\mathrm{FeS} _{2}\right.$ ) by froth floatation process in presence of sodium cyanide and alkali as depressants and potassium ethyl xanthate as collector. The former compounds depress the floatation property of $\mathrm{ZnS}$ and $\mathrm{FeS} _{2}$ particles by forming their cyanide complexes $\mathrm{Na} _{2}\left[\mathrm{Zn}(\mathrm{CN}) _{4}\right]$ and $\mathrm{Na} _{4}\left[\mathrm{Fe}(\mathrm{CN}) _{6}\right]$ and only PbS particles go into the froth.

3) Electromagnetic Separation

This method is used when either the ore or the impurities associated with it are magnetic in nature. For example, cassiterite $\left(\mathrm{SnO} _{2}\right.$, tinstone) an ore of tin (nonmagnetic) is separated from of ferrous tungstate (wolframite, $\mathrm{FeWO} _{4}$ ) (magnetic).

4) Chemical Method_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, in Baeyer’s process bauxite is leached with a hot concentrated solution of $\mathrm{NaOH}$ to dissolve aluminium and $\mathrm{SiO} _{2}$ while other oxides $\left(\mathrm{Fe} _{2} \mathrm{O} _{3}, \mathrm{TiO} _{2}\right.$ ) remain undissolved and noble metals $(\mathrm{Ag}$ and $\mathrm{Au}$ ) are leached with a dilute aqueous solution of $\mathrm{NaCN}$ or $\mathrm{KCN}$ in presence of air.

II) Isolation of the metal from its concentrated ore

The concentrated ore must be converted into a form which is suitable for reduction. Oxides are easier to reduce, thus non-oxide ores are first converted into an oxide ore and then it reduce to metal by suitable agent.

a) Conversion to oxide:

i) Roasting

Roasting is the process in which the ore is heated strongly, below its melting point, in presence of excess of air. This process is used for the conversion of sulphide ores to their respective metal oxides (de-electronation of ores).

$$ \begin{aligned} & 2 \mathrm{ZnS}+3 \mathrm{O} _{2} \longrightarrow 2 \mathrm{ZnO}+2 \mathrm{SO} _{2} \\ & \mathrm{ZnS}+2 \mathrm{O} _{2} \longrightarrow \mathrm{ZnSO} _{4} \end{aligned} $$

ii) Calcination

Calcination is the process in which the ore is heated strongly, below its melting point, either in absence or in a limited supply or air. This process is used for the conversion of the carbonates and hydrated oxide ores to their respective oxides.

$$ \mathrm{CaCO} _{3} \longrightarrow \mathrm{CaO}+\mathrm{CO} _{2} $$

b) Reduction of oxide to the metal:

i) Reduction by Carbon-Smelting

Smelting is the process of extraction of a metal from its roasted or calcined ore by heating it with powdered coke in presence of a flux.

$$ \mathrm{ZnO}+\mathrm{C} \longrightarrow \mathrm{Zn}+\mathrm{CO} $$

ii) Flux is a substance which combines with gangue (earthy impurities) still present in the roasted or the calcined ore to form a fusible product called slag.

$$ \text { Flux + Gangue } \longrightarrow \text { Slag } $$

Acidic fluxes: For basic impurities like those of lime, or oxides of iron present in the ore, acidic fluxes like silica $\left(\mathrm{SiO} _{2}\right)$, borax $\left(\mathrm{Na} _{2} \mathrm{~B} _{4} \mathrm{O} _{7}, 10 \mathrm{H} _{2} \mathrm{O}\right)$ are used.

$$ \underset{\text { Gangue }}{\mathrm{FeO}}+\underset{\text { Acidic flux }}{\mathrm{SiO} _{2}}\longrightarrow \underset{\text { Slag }}{\mathrm{FeSiO} _{3}} $$

Basic fluxes:: For acidic impurities like those of silica $\left(\mathrm{SiO} _{2}\right)$, phosphorus pentoxide $\left(\mathrm{P} _{4} \mathrm{O} _{10}\right)$, etc. present in the ore, basic fluxes like limestone $\left(\mathrm{CaCO} _{3}\right)$, magnesite $\left(\mathrm{MgCO} _{3}\right)$, haematite $\left(\mathrm{Fe} _{2} \mathrm{O} _{3}\right)$ etc. are used.

$$ \underset{\text { Gangue }}{\mathrm{SiO} _{2}}+\underset{\text { basic flux }}{\mathrm{CaCO} _{3}} \xrightarrow[\text{Slag}]{} \mathrm{CaSiO} _{3}+\mathrm{CO} _{2} $$

Thus silica $\left(\mathrm{SiO} _{2}\right)$ acts both as a flux as well as a gangue.

iii) Reduction by Aluminium-Goldschmidt Aluminothermic Process

Aluminothermic process involves the reduction of oxides $\left(\mathrm{Fe} _{2} \mathrm{O} _{3}, \mathrm{Cr} _{2} \mathrm{O} _{3}, \mathrm{Mn} _{3} \mathrm{O} _{4}\right.$, etc.) by aluminium power.

$$ \begin{aligned} & \mathrm{Fe} _{2} \mathrm{O} _{3}+2 \mathrm{Al} \longrightarrow \mathrm{Al} _{2} \mathrm{O} _{3}+2 \mathrm{Fe} \\ & \mathrm{Cr} _{2} \mathrm{O} _{3}+2 \mathrm{Al} \longrightarrow \mathrm{Al} _{2} \mathrm{O} _{3}+2 \mathrm{Cr} \\ & 3 \mathrm{Mn} _{3} \mathrm{O} _{4}+8 \mathrm{Al} \longrightarrow 4 \mathrm{Al} _{2} \mathrm{O} _{3}+9 \mathrm{Mn} \end{aligned} $$

The mixture of metallic oxide such as $\mathrm{Fe} _{2} \mathrm{O} _{3}$ and aluminium powder in the ratio of $3: 1$ is known as thermite.

iv) Reduction by Precipitation (Hydro-metallurgy)

Hydrometallurgy is the process of dissolving the metal or its ore by the action of a suitable chemical reagent followed by recovery of the metal either by electrolysis or by the use of a suitable precipitating agent (displacement method). For example,

$$ \begin{aligned} & 4 \mathrm{Au}+8 \mathrm{KCN}+2 \mathrm{H} _{2} \mathrm{O}+\mathrm{O} _{2} \longrightarrow 4 \mathrm{~K}\left[\mathrm{Au}(\mathrm{CN}) _{2}\right]+4 \mathrm{KOH} \\ & 2 \mathrm{~K}\left[\mathrm{Au}(\mathrm{CN}) _{2}\right]+\mathrm{Zn} \longrightarrow 2 \mathrm{Au}+\mathrm{K} _{2}\left[\mathrm{Zn}(\mathrm{CN}) _{4}\right] \end{aligned} $$

v) Reduction by Mg

This method is applicable for the reduction of $\mathrm{TiCl} _{4}$ (Krolls process).

$$ \mathrm{TiCl} _{4}+2 \mathrm{Mg} \longrightarrow 2 \mathrm{MgCl} _{2}+\mathrm{Ti} $$

III) REFINING OF CRUDE METALS

Refining or purification of crude metals.

Refining of metals is the process of obtaining the metals in the pure state from crude metals. Refining is done by the following methods:

1) Liquation

Liquation is used for the refining of such metals as are readily fusible (i.e. having low melting points) such as $\mathrm{Pb}, \mathrm{Sn}, \mathrm{Bi}$ and $\mathrm{Hg}$ as compared to impurities (less fusible).

2) Distillation

Distillation is used for the refining of such metals which have low boiling points such as $\mathrm{Hg}, \mathrm{Zn}, \mathrm{Cd}$, etc., i.e., volatile metals.

3) Poling

This method is employed when the impure metal contains impurities of its own oxide. For example, $\mathrm{Cu} _{2} \mathrm{O}$ in blister copper and $\mathrm{SnO} _{2}$ in impure $\mathrm{Sn}$.

4) Cupellation

Cupellation is used for the refining of such metals which contain impurities of other metals which form volatile oxides.

5) Electro-refining

Metals like $\mathrm{Cu}, \mathrm{Ag}, \mathrm{Au}, \mathrm{Cr}$, $\mathrm{Zn}$, Ni etc. are purified by this method. In this method, impure metal forms the anode while the cathode is a rod or sheet of pure metal. The electrolytic solution consists of a soluble salt of the metal. On passing electricity, the pure metal gets deposited on the cathode while the insoluble impurities settle down below the anode as anode mud or anode sludge.

6) Zone-refining

This method is based upon the principle of fractional crystallization, ie. difference in solubilities of impurities in molten (more soluble) and solid state of metal Semiconductors like silicon, germanium and gallium are purified by this method.

7) Van Arkel Method - Vapour Phase Refining

This method is used for preparing ultrapure metals required in space technology in this method, the impure metal is first converted into a volatile stable compound

generally iodide at lower temperature (leaving behind the impurities) which is then decomposed at a higher temperature to give the pure metal. Metals like titanium, zirconium, thorium and uranium are purified by this method.

$$\underset{\begin{array}{l} \text { Impure } \\ \text { metal } \\ \text { (stable) } \end{array}}{\mathrm{Ti}+2 \mathrm{I}_2} \xrightarrow[\substack{\text { Volatile } \\ \text { compound }}]{500 \mathrm{~K}} \mathrm{TiI}_4 \xrightarrow{1700 \mathrm{~K}} \underset{\text{Pure metal}}{\mathrm{Ti}}+2 \mathrm{I}_2$$

This process is also called vapour-phase refining.

$$ \begin{aligned} & \text { Mond’s process - For refining nickel } \\ & \underset{\text { Impure }}{\mathrm{Ni}+4 \mathrm{CO}} \xrightarrow{330-350 \mathrm{~K}} \underset{\text { Volatile }}{\mathrm{Ni}(\mathrm{CO}) _{4}} \xrightarrow{450-470 \mathrm{~K}} \underset{\text { Pure }}{\mathrm{Ni}}+4 \mathrm{CO} \end{aligned} $$

8) Chromatography

The method is based on the principle that the different components of a mixture are adsorbed to different extents on an adsorbent.

9) Electrolytic Reduction-Electrometallurgy

Electrometallurgy is the process of extracting highly electropositive (active) metals such as $\mathrm{Na}, \mathrm{K}, \mathrm{Ca}, \mathrm{Mg}$, Al etc. by electrolysis of their oxides, hydroxides or chlorides in fused state.

For example, Na is obtained by the electrolysis of fused NaCl in Down’s cell and Al by the electrolysis of fused $\mathrm{Al} _{2} \mathrm{O} _{3}$. The metal is liberated at the cathode and electrons here serve as the reducing agent. The process of electrolysis has been used to carry out the reduction of molten metal salts. The electrochemical principles of this method can be understood in terms of the following equation

$$ \Delta G^{\circ}=-n F E^{\circ} $$

where $\mathrm{n}$ is the number of electrons involved in the reduction process, $\mathrm{E}^{\circ}$ is the electrode potential of the redox couple $\left(\mathrm{M} / \mathrm{M}^{\mathrm{n}}\right)$ present in the system. More reactive metals have large negative values of electrode potentials and hence are difficult to reduce. If the difference in $E^{\circ}$ values of two redox couples is positive, then $\Delta G^{\circ}$ in the above equation would be negative and thus, the more reactive metal will displace the less reactive metal from the solution. In other words, less reactive metal will come out of the solution and the more reactive metal will go into the solution. For example,

$$ \mathrm{Cu}^{2+}(\mathrm{aq})+\underset{\substack{\text{More reactive} \\ \text{metal}}}{\mathrm{Fe(s)}} \longrightarrow \underset{\substack{\text { Less reactive } \\ \text { metal }}}{\mathrm{Cu(s)}+\mathrm{Fe}}^{2+}(\mathrm{aq}) $$

$$ \text { or } \mathrm{Cu}^{2+}(\mathrm{aq})+\mathrm{H} _{2}(\mathrm{~g}) \longrightarrow \mathrm{Cu}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) $$

Since $\mathrm{E}^{\circ}$ of $\mathrm{Fe}^{2+} / \mathrm{Fe}(-0.44 \mathrm{~V})$ or that of $\mathrm{H}^{+} / \mathrm{H} _{2}(0.0 \mathrm{~V})$ redox couple is lower than that of $\mathrm{Cu}^{2+} / \mathrm{Cu}(+0.34 \mathrm{~V})$, therefore, Fe or $\mathrm{H} _{2}$ displaces $\mathrm{Cu}$ from $\mathrm{Cu}^{2+}$ ions.

The above reaction is made use of in the extraction of copper from low grade ores and scraps.

Varieties of Iron

• Pig iron.

It is the impurest form of iron. It is obtained directly from the blast furnace and contains about $4 \%$ carbon besides impurities of $\mathrm{Mn}, \mathrm{Si}, \mathrm{S}$ and $\mathrm{P}$. Due to the presence of impurities its $\mathrm{mp}$. is low ( $1473 \mathrm{~K}$ ).

• Cast iron

It contains 3\% carbon. It is extremely hard but brittle. Its melting point is the same as that of pig iron.

• Wrought iron

It is the purest form of iron. Carbon is partly present as graphite and partly as iron carbide.

• Steel

The carbon content of various steels lies between those cast iron and wrought iron.

The choice of the reducing agent, however, depends upon the reactivity of the metal. Oxides of very reactive metals like $\mathrm{K}, \mathrm{Na}, \mathrm{Ca}, \mathrm{Mg}$, Al, etc. can be reduced only by the electrolytic method. Oxides of less reactive metals such as zinc, iron, copper, lead, tin, manganese, chromium can be reduced by a number of reducing agents such as carbon (coke), carbon monoxide or even another metal. The process of extracting the metal by heating the metal oxide with a suitable reducing agent is called pyrometallurgy. To predict which element or compound will suit as a reducing agent for a given metal oxide and at what optimum temperature, the basic concepts of thermo-dynamics are quite useful as explained below: -

Extraction of Cast Iron

Cast iron is usually extracted from haematite. The ore after concentration by gravity separation process is calcined to remove moisture, impurities of $\mathrm{S}, \mathrm{P}$ and $\mathrm{As}$ as volatile oxides and to ferrous oxide to ferric oxide (which prevents the loss of iron slag during smelting)

$$ 4 \mathrm{FeO}+\mathrm{O} _{2} \longrightarrow 2 \mathrm{Fe} _{2} \mathrm{O} _{3} $$

The ore becomes sintered (porous) and hence is more suitable for reduction to metallic state. In case of carbonate ore (siderite), during calcination, it is converted into ferric oxide

$$ \begin{aligned} & \mathrm{FeCO} _{3} \longrightarrow \mathrm{FeO}+\mathrm{CO} _{2} \\ & 4 \mathrm{FeO}+\mathrm{O} _{2} \longrightarrow 2 \mathrm{Fe} _{2} \mathrm{O} _{3} \end{aligned} $$

However, in case of sulphide ore (iron pyrites), concentration is carried out by roasting:

$$ 4 \mathrm{FeS} _{2}+11 \mathrm{O} _{2} \longrightarrow 2 \mathrm{Fe} _{2} \mathrm{O} _{3}+8 \mathrm{SO} _{2} $$

The next step is smelting where the calcined ore ( 8 parts) is mixed with limestone ( 1 part) and coke (4 parts) and is then fed from the top into a blast furnace while preheated air at about $1000 \mathrm{~K}$ is passed into the furnace through a number of nozzles provided near the bottom of the furnace. The added coke serves both as a fuel as well as a reducing agent while limestone acts as the basic flux.

Thermodynamic Principles of Metallurgy

For any process, Gibbs free energy change $(\Delta G)$ is given by the equation,

$$ \Delta \mathrm{G}=\Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S} $$

where $\Delta \mathrm{H}$ is the enthalpy change, $\Delta \mathrm{S}$ is the entropy change and $\mathrm{T}$ is the absolute temperature. The free energy change is also related to the equilibrium constant $\mathrm{K}$ of the reactant-product system at temperature $\mathrm{T}$ by the following equation

$$ \Delta \mathrm{G}=-\mathrm{R} T \ln \mathrm{K} $$

If $\Delta G$ is -ve, then $K$ will be positive meaning thereby that the reaction will proceed towards products. From these facts, we can draw the following conclusion:

The criterion of feasibility of a reaction at any temperature is that the $\Delta G$ of the reaction must be negative.

A reaction with $\Delta G$ positive can be still made to occur by coupling it with another reaction having large negative $\Delta G$ so that the net $\Delta G$ of the two reactions is negative. Such couplings can be easily understood through Gibbs free energy of formation $\Delta _{\mathrm{r}} \mathrm{G}^{\circ} \mathrm{vs} \mathrm{T}$ plots for formation of oxides known as Ellingham diagram. Such diagram helps us in predicting the feasibility of thermal reduction of an oxide ore.

Each plot is a straight line except when some change in phase (solid $\longrightarrow$ liquid or liquid $\longrightarrow$ gas) takes place. The temperature at which such a change occurs is indicated by an increase in the slope on the +ve side. For example, in the Zn-Zn0 plot, the boiling point of zinc at $180 \mathrm{~K}$ is indicated by an abrupt increase in the +ve slope of the curve. The feasibility of a reduction process can be predicted simply by looking at the Ellingham diagram. Metals for which free energy of formation $\left(\Delta _{\mathrm{f}} \mathrm{G}^{\circ}\right)$ of their oxides is more negative can reduce those metal oxides for which the free energy of formation of their respective oxides is less negative. For example, at any given temperature, any metal will reduce the oxides of other metals which is above it in the Ellingham diagram because the free energy change $\left(\Delta _{\mathrm{t}} \mathrm{G}^{\circ}\right)$ for the overall redox reaction becomes more negative by an amount equal to difference between the free energy of formation $\left(\Delta _{\mathrm{r}} \mathrm{G}^{\circ}\right)$ of the two oxides at that temperature. Further, greater the difference, easier is the reduction. For example, $\mathrm{Al}$ reduces $\mathrm{ZnO}, \mathrm{FeO}$ and $\mathrm{Cu} _{2} \mathrm{O}$ more readily than $\mathrm{Zn}$ reduces $\mathrm{FeO}$ and $\mathrm{Cu} _{2} \mathrm{O}$.

Reducing behaviour of carbon: When carbon reacts with dioxygen, three types of reactions are possible:

$\begin{aligned} & \mathrm{C}(\mathrm{s})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{CO}_2(\mathrm{g}) ……\text{(i)} \\ & 2 \mathrm{C}(\mathrm{s})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow 2 \mathrm{CO}(\mathrm{g}) ……\text{(ii)} \\ & 2 \mathrm{CO}(\mathrm{g})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow 2 \mathrm{CO}_2(\mathrm{g}) ……\text{(iii)} \end{aligned}$

For reaction (i) the volume of $\mathrm{CO} _{2}$ formed is almost equal to the volume of $\mathrm{O} _{2}$ consumed so $\Delta \mathrm{S}$ does not change significantly and Ellingham plot is almost horizontal. For reaction (ii) the plot is downward and unlike other reactions, $\Delta _{t} G^{\circ}$ value becomes more negative as temperature is increased. It is due to positive value of $\Delta \mathrm{S}$ because here two volumes of $\mathrm{CO}$ are produced for one volume of $\mathrm{O} _{2}$ consumed, thus $\Delta \mathrm{S}$ is positive. The third reaction produces two volumes of $\mathrm{CO} _{2}$ for every three volumes of reactants used. Thus, $\Delta S$ is -ve and hence $\Delta _{\mathrm{f}} \mathrm{G}^{\circ}$ becomes increasingly +ve as the temperature increases. Consequently, $\mathrm{CO}, \mathrm{CO} _{2}$ plot slopes upwards.

The three lines cross at $983 \mathrm{~K}$. Below this temperature, formation of $\mathrm{CO} _{2}$ from $\mathrm{C}$ as well as from $\mathrm{CO}$ is energetically more favourable and above this temperature formation of $\mathrm{CO}$ from $\mathrm{C}$ is energetically more favourable. In other words, below $983 \mathrm{~K}$, both $\mathrm{C}$ and $\mathrm{CO}$ can act as reducing agents but since $\mathrm{CO}$ can be more easily oxidised to $\mathrm{CO} _{2}$ than $\mathrm{C}$ to $\mathrm{CO} _{2}$, therefore, below $983 \mathrm{~K}, \mathrm{CO}$ is a more effective reducing agent than $\mathrm{C}$. However, above $983 \mathrm{~K}, \mathrm{CO}$ is more stable and hence its oxidation to $\mathrm{CO} _{2}$ is less rapid than that of $\mathrm{C}$ to $\mathrm{CO} _{2}$. Therefore, above $983 \mathrm{~K}, \mathrm{C}$ is a better reducing agent than $\mathrm{CO}$.

Effect of Temperature on the Free Energy Change $\left(\Delta _{\mathrm{f}} \mathrm{G}^{\circ}\right)$ of the overall reduction process

We know that for any process,

$\Delta \mathrm{G}=\Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S}$

Since on increasing the temperature, the values of $\Delta \mathrm{H}$ and $\Delta \mathrm{S}$ nearly remain constant, therefore, the value of $\Delta _{\mathrm{f}} \mathrm{G}^{\circ}$ becomes more negative. This means that if a particular reduction process does not occur at a lower temperature, it may occur, at a higher temperature but for that we are to select the temperature in such a way that the $\Delta _{\mathrm{f}} \mathrm{G}^{\circ}$ of the overall redox reaction becomes -ve. In the Ellingham diagram, this temperature is indicated by the intersection of the two curves (curve for formation of metal oxide and the curve for the oxidation of the reducing agent). For example, the temperature at the intersection point ’ $\mathrm{A}$ ’ of the $\mathrm{Al} \longrightarrow \mathrm{Al} _{2} \mathrm{O} _{3}$ and $\mathrm{Mg} \longrightarrow \mathrm{MgO}$ curves is approx. $1623 \mathrm{~K}$. Therefore, below $1623 \mathrm{~K}, \mathrm{Mg}$ can reduce $\mathrm{Al} _{2} \mathrm{O} _{3}$ to $\mathrm{Al}$ but above $1623 \mathrm{~K}$, $\mathrm{Al}$ can reduce $\mathrm{MgO}$ to $\mathrm{Mg}$

Theory of reduction process:

Thermodynamics helps us to understand how coke reduces iron oxide to metal. One of the main reduction steps in the process is

$$ \mathrm{FeO}(\mathrm{s})+\mathrm{C}(\mathrm{s}) \longrightarrow \mathrm{Fe}(\mathrm{s} / \mathrm{l})+\mathrm{CO}(\mathrm{g}) \ldots \ldots .(\mathrm{i}) $$

This redox reaction can be divided into the following two half reactions one involving reduction and the other oxidation.

Reduction:

$$ \mathrm{FeO}(\mathrm{s}) \longrightarrow \mathrm{Fe}(\mathrm{s})+1 / 2 \mathrm{O} _{2}(\mathrm{~g}) ; \Delta \mathrm{G} _{(\mathrm{FeO}, \mathrm{Fe})} $$

Oxidation:

$$ \mathrm{C}(\mathrm{s})+1 / 2 \mathrm{O} _{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}(\mathrm{g}) ; \Delta \mathrm{G} _{(\mathrm{C}, \mathrm{co})} $$

The net free energy change of these two combined reactions is

$$ \Delta \mathrm{G} _{(\mathrm{C}, \mathrm{CO})}+\Delta \mathrm{G} _{(\mathrm{FeO}, \mathrm{Fe})}=\Delta _{\mathrm{r}} \mathrm{G} $$

Naturally, the resultant reaction, i.e., Eq. (i) will take place only when the $\Delta _{\mathrm{f}} \mathrm{G}$ is -ve.

From the Ellingham diagram, it is evident that on increasing the temperature. the curve for the reaction $\mathrm{Fe} \longrightarrow \mathrm{FeO}$ ( $\mathrm{Fe}, \mathrm{FeO}$ line) goes upwards while that for the reaction, ( $\mathrm{C} \longrightarrow \mathrm{CO}$ ( $\mathrm{C}, \mathrm{CO}$ line) goes downwards. At approx. $1073 \mathrm{~K}$ or above, the $\mathrm{C}, \mathrm{CO}$ line is much below the $\mathrm{Fe}, \mathrm{FeO}$ line. This means that, $\Delta \mathrm{G} _{\mathrm{Fe}, \mathrm{FeO}}>\Delta \mathrm{G} _{\mathrm{C} \text {, co }}$ and hence $\Delta _{\mathrm{r}} \mathrm{G}$ is -ve. In other words, at $1073 \mathrm{~K}$ or above coke will reduce $\mathrm{Fe} \mathrm{O} \longrightarrow \mathrm{Fe}$ and itself will be oxidised to $\mathrm{CO}$.

In contrast, at temperatures below $1073 \mathrm{~K}$, the $\mathrm{CO}, \mathrm{CO} _{2}$ line lies below $\mathrm{Fe}, \mathrm{FeO}$ line. Therefore, below $1073 \mathrm{~K}, \mathrm{CO}$ reduces the oxides of iron (i.e., $\mathrm{Fe} _{2} \mathrm{O} _{3}, \mathrm{Fe} _{3} \mathrm{O} _{4}$, etc.) to metal. Thus, in the blast furnace, reduction of iron oxides takes place in different temperature ranges as summarised below:

At 500-800 K (lower temperature range in the blast furnace)

$$ \begin{aligned} & 3 \mathrm{Fe} _{2} \mathrm{O} _{3}+\mathrm{CO} \longrightarrow 2 \mathrm{Fe} _{3} \mathrm{O} _{4}+\mathrm{CO} _{2} \\ & \mathrm{Fe} _{3} \mathrm{O} _{4}+4 \mathrm{CO} \longrightarrow 3 \mathrm{Fe}+4 \mathrm{CO} _{2} \end{aligned} $$

$$ \mathrm{Fe} _{2} \mathrm{O} _{3}+\mathrm{CO} \longrightarrow 2 \mathrm{FeO}+\mathrm{CO} _{2} $$

At 900-1500 K (higher temperature range in the blast furnace)

$$ \mathrm{C}+\mathrm{CO} _{2} \longrightarrow 2 \mathrm{CO} $$

$$ \mathrm{FeO}+\mathrm{CO} \longrightarrow \mathrm{Fe}+\mathrm{CO} _{2} $$

Reactions taking place in the furance.

Near the bottom of the furnace (zone of combustion, $2170 \mathrm{~K}$ ), coke first combines with air to form $\mathrm{CO} _{2}$ which then combines with more coke (zone of heat absorption, $1425 \mathrm{~K}$ ) to form $\mathrm{CO}$. The $\mathrm{CO}$ thus produced acts as the reducing agent and reduces iron oxide to spongy iron near the top of the furnace (zone of reduction, $823 \mathrm{~K}$ )

$$ \begin{aligned} & \mathrm{C}+\mathrm{O} _{2} \longrightarrow \mathrm{CO} _{2} ; \Delta \mathrm{H}=-393.3 \mathrm{~kJ} \text { (Exothermic) } \\ & \mathrm{CO} _{2}+\mathrm{C} \longrightarrow 2 \mathrm{CO} ; \Delta \mathrm{H}=+163.2 \mathrm{~kJ} \text { (Endothermic) } \\ & \mathrm{Fe} _{2} \mathrm{O} _{3}+\mathrm{CO} _{2} \xrightarrow{823 \mathrm{~K}} 2 \mathrm{FeO}+\mathrm{CO} _{2} \\ & \mathrm{Fe} _{3} \mathrm{O} _{4}+\mathrm{CO} \xrightarrow{823 \mathrm{~K}} 3 \mathrm{FeO}+\mathrm{CO} _{2} \end{aligned} $$

But the further reduction of $\mathrm{Fe} 0$ to $\mathrm{Fe}$ by $\mathrm{CO}$ occurs around $1123 \mathrm{~K}$.

$$ \mathrm{FeO}+\mathrm{CO} \xrightarrow{1123 \mathrm{~K}} \mathrm{Fe}+\mathrm{CO} _{2} $$

However, direct reduction of iron ores (haematite, magnetite etc.) left unreduced around $823 \mathrm{~K}$, occurs completely to iron by carbon above $1123 \mathrm{~K}$.

$$ \begin{aligned} & \mathrm{Fe} _{2} \mathrm{O} _{3}+3 \mathrm{C} \xrightarrow{>1123 \mathrm{~K}} 2 \mathrm{Fe}+3 \mathrm{CO} \\ & 2 \mathrm{Fe} _{2} \mathrm{O} _{3}+3 \mathrm{C} \xrightarrow{>1123 \mathrm{~K}} 4 \mathrm{Fe}+3 \mathrm{CO} _{2} \end{aligned} $$

Limestone which acts as flux, decomposes at $1123 \mathrm{~K}$ (zone of slag formation) to form $\mathrm{Ca} 0$ which then combines with silica to form slag

$$ \begin{aligned} & \mathrm{CaCO} _{3} \xrightarrow{1123 \mathrm{~K}} \mathrm{CaO}+\mathrm{CO} _{2} \\ & \mathrm{CaO}+\mathrm{SiO} _{2} \longrightarrow \mathrm{CaSiO} _{3} \text { (slag) } \end{aligned} $$

At the lower part of the furnace (zone of fusion, 1423-1673 K) the spongy iron melts and dissolves some carbon, $\mathrm{S}, \mathrm{P}, \mathrm{SiO} _{2}$, $\mathrm{Mn}$, etc.

The molten slag being less dense floats over the surface of the molten iron. The molten iron is tapped off from the furnace and is then solidified to give blocks of iron called Cast iron or Pig iron.

Solved Examples

1) In view of the sign of $\Delta _{\mathrm{f}} \mathrm{G}^{\ominus}$ following reactions:

$$ \begin{aligned} & \mathrm{PbO} _{2}+\mathrm{Pb} \rightarrow 2 \mathrm{PbO} ; \Delta _{\mathrm{f}} \mathrm{G}^{\ominus}<0 \\ & \mathrm{SnO} _{2}+\mathrm{Sn} \rightarrow 2 \mathrm{SnO} ; \Delta _{\mathrm{f}} \mathrm{G}^{\ominus}>0 \end{aligned} $$

Which oxidation states are more characteristic for lead and tin?

1. For lead +2 , for tin +2

2. For lead +4 , for tin +4

3. For lead +2 , for tin +4

4. For lead +4 , for tin +2

2) Which of the following metal is leached by cyanide process?

3. $\mathrm{Ag}$

4. $\mathrm{Al}$

5. $\mathrm{Na}$

6. $\mathrm{Cu}$

3) Which of the following ore is best concentrated by froth floatation method?

1. Magnetite

2. Galena

3. Cassiterite

4. Malachite

4) During the process of electrolytic refining of copper some metals present as impurity settle as amide mud. These are

1. $\mathrm{Sn}$ and $\mathrm{Ag}$

2. $\mathrm{Pb}$ and $\mathrm{Zn}$

3. $\mathrm{Ag}$ and $\mathrm{Au}$

4. $\mathrm{Fe}$ and $\mathrm{Ni}$

5) Heating $\mathrm{Cu} _{2} \mathrm{O}$ and $\mathrm{Cu} _{2} \mathrm{~S}$ will give:

1. $\mathrm{Cu}+\mathrm{SO} _{2}$

2. $\mathrm{CuO}+\mathrm{CuS}$

3. $\mathrm{Cu}+\mathrm{SO} _{3}$

4. $\mathrm{Cu} _{2} \mathrm{SO} _{3}$

PRACTICE QUESTIONS

Question 1- Roasting process is applied to which of the following ores?

a) Galena

b) Iron pyrites

c) Copper glance

d) All

Question 2- Which of the following metal can be extracted by smelting?

a) $\mathrm{Al}$

b) $\mathrm{Fe}$

c) $\mathrm{Mg}$

d) All

Question 3- The chemical reagent used for leaching of gold and silver ores is

a) sodium hydroxide

b) potassium cyanide

c) potassium cyanate

d) sodium thiosulphate.

Question 4- The most abundant ore of iron is

a) haematite

b) limonite

c) magnetite

d) siderite.

Question 5- The chemical composition of ‘slag’ formed during smelting process in the extraction of copper is

a) $\mathrm{Cu} _{2} \mathrm{O}+\mathrm{FeS}$

b) $\mathrm{FeSiO} _{3}$

c) $\mathrm{CuFeS} _{2}$

d) $\mathrm{Cu} _{2} \mathrm{~S}+\mathrm{FeO}$

Question 6- The incorrect statement among the following is

a) hydrogen is used to reduce $\mathrm{NiO}$

b) zirconium is refined by van Arkel method

c) the sulphide ore galena is concentrated by froth floatation process

d) in the metallurgy of iron flux used is $\mathrm{SiO} _{2}$

Question 7- Which of the following is used in thermite welding ?

a) $\mathrm{TiO} _{2}+4 \mathrm{Na} \longrightarrow \mathrm{Ti}+2 \mathrm{Na} _{2} \mathrm{O}$

b) $2 \mathrm{Al}+\mathrm{Fe} _{2} \mathrm{O} _{3} \longrightarrow \mathrm{Al} _{2} \mathrm{O} _{3}+2 \mathrm{Fe}$

c) $\mathrm{SnO} _{2}+2 \mathrm{C} \longrightarrow \mathrm{Sn}+2 \mathrm{CO}$

d) $\mathrm{Al} _{2} \mathrm{O} _{3}+2 \mathrm{Cr} \longrightarrow \mathrm{Cr} _{2} \mathrm{O} _{3}+2 \mathrm{Al}$

Question 8- Zinc metal is refined by

a) fractional crystallization

b) fractional distillation

c) electrolysis

d) both by (b) and(c).

Question 9- The froth floatation process is based upon

a) magnetic properties of gangue

b) specific gravity of ore particles

c) preferential wetting of ore particles by oil

d) preferential wetting of gangue particles by oil

Question 10- in Goldschmidt aluminothermic process, reducing agent used is .

a) coke

b) Al powder

c) $\mathrm{Na}$

d) $\mathrm{Ca}$

Question 11- Cupellation process is used in the metallurgy of

a) $\mathrm{Cu}$

b) $\mathrm{Ag}$

c) $\mathrm{Al}$

d) $\mathrm{Fe}$

Question 12- The method used for refining of iron is called

a) bessemerisation

b) electrolysis

c) cupellation

d) Liquation

Question 13- When copper pyrites is roasted in excess of air, a mixture of $\mathrm{CuO}+\mathrm{Fe} \mathrm{O}$ is formed. $\mathrm{FeO}$ is present as impurities. This can be removed as slag during reduction of $\mathrm{CuO}$. The flux added to form slag is

a) $\mathrm{SiO} _{2}$, which is an acidic flux

b) lime stone, which is a basic flux

c) $\mathrm{SiO} _{2}$, which is a basic flux

d) $\mathrm{CaO}$, which is a basic flux.

Question 14- The most convenient method for the extraction of silver from silver glance is

(a) leaching

(b) hydrometallurgy

(c) smelting

(d) roasting

Question 15- Carbon cannot reduce $\mathrm{Fe} _{2} \mathrm{O} _{3}$ to $\mathrm{Fe}$ at a temperature below $983 \mathrm{~K}$ because

(a) free energy change for the formation of $\mathrm{CO}$ is more negative than that of $\mathrm{Fe} _{2} \mathrm{O} _{3}$

(b) $\mathrm{CO}$ is thermodynamically more stable than $\mathrm{Fe} _{2} \mathrm{O} _{3}$

(c) carbon has higher affinity towards oxygen than iron

(d) iron has higher affinity towards oxygen than carbon

Question 16- From the Ellingham graphs on carbon, which of the following statements is false?

(a) $\mathrm{CO} _{2}$ is more stable than $\mathrm{CO}$ at less than $983 \mathrm{~K}$

(b) $\mathrm{CO}$ reduces $\mathrm{Fe} _{2} \mathrm{O} _{3}$ to $\mathrm{Fe}$ at less than $983 \mathrm{~K}$

(c) $\mathrm{CO}$ is less stable than $\mathrm{CO} _{2}$ at more than $983 \mathrm{~K}$

(d) $\mathrm{CO}$ reduces $\mathrm{Fe} _{2} \mathrm{O} _{3}$ to $\mathrm{Fe}$ in the reduction zone of blast furnace.

Question 17- Which is incorrect statement?

(a) Below $1073 \mathrm{~K}, \mathrm{CO}$ is more effective reducing agent

(b) At $1073 \mathrm{~K}, \mathrm{CO}$ is more effective reducing agent than carbon

(c) Above $1073 \mathrm{~K}$, coke is more effective reducing agent

(d) Above $1973 \mathrm{~K}, \mathrm{Mg}$ can reduce $\mathrm{SiO} _{2}$.

Question 18- Copper metal is refined by

(a) liquation

(b) cupellation

(c) bessemerisation

(d) poling

Question 19- The temperature of blast furnace to produce iron from its ore $\mathrm{Fe} _{2} \mathrm{O} _{3}$ varies from $500^{\circ} \mathrm{C}$ at the top of the furnace to about $1900^{\circ} \mathrm{C}$ at the bottom of the furnace. The reaction between the ore $\mathrm{Fe} _{2} \mathrm{O} _{3}$ and $\mathrm{CO}$ at the lowest temperature $\left(\sim 500^{\circ} \mathrm{C}\right.$ ) is

(a) $3 \mathrm{Fe} _{2} \mathrm{O} _{3}+\mathrm{CO} \longrightarrow 2 \mathrm{Fe} _{3} \mathrm{O} _{4}+\mathrm{CO} _{2}$

(b) $\mathrm{Fe} _{2} \mathrm{O}+\mathrm{CO} \longrightarrow 2 \mathrm{FeO}+\mathrm{CO} _{2}$

c) $\mathrm{Fe} _{2} \mathrm{O} _{3}+3 \mathrm{CO} \longrightarrow 2 \mathrm{Fe}+3 \mathrm{CO} _{2}$

d) $\mathrm{Fe} _{2} \mathrm{O} _{3}+\mathrm{CO}+\mathrm{CaCO} _{3} \longrightarrow \mathrm{Fe} _{2} \mathrm{O} _{3}+\mathrm{CO}+\mathrm{CO} _{2}+\mathrm{CaO}$.

Question 20- Which of the following factors is of no significance for roasting sulphide ores to the oxides and not subjecting the sulphide ores to carbon reduction directly?

a) $\mathrm{CO} _{2}$ is more volatile than $\mathrm{CS} _{2}$

b) Metal sulphides are thermodynamicaliy more stable than $\mathrm{CS} _{2}$

c) $\mathrm{CO} _{2}$ is thermodynamically more stable man $\mathrm{CS} _{2}$

d) Metal sulphides are less stable than the corresponding oxides.

Question 21- The process of extracting metals by electrolysis of their oxides; hydroxides or chlorides in the fused state is called

a) electrometallurgy

b) electro-refining

c) zone-refining

d) hydrometallurgy.

Question 22- According to Ellingham diagram, the oxidation reaction of carbon to carbon monoxide may be used to reduce which one of the following oxides at the lowest temperature?

a) $\mathrm{Al} _{2} \mathrm{O} _{3}$

b) $\mathrm{Cu} _{2} \mathrm{O}$

c) $\mathrm{MgO}$

d) $\mathrm{ZnO}$

Question 23- The reduction of zinc oxide with coke occurs at temperature.

a) greater than that for $\mathrm{CuO}$

b) less than that for $\mathrm{CuO}$

c) less than that for $\mathrm{Ag} _{2} \mathrm{O}$

d) equal to that for $\mathrm{CuO}$

Question 24- $\Delta G^{\circ}$ vs $T$ plot in the Ellingham’s diagram slopes downward for the reaction

a) $\mathrm{Mg}+1 / 2 \mathrm{O} _{2} \longrightarrow \mathrm{MgO}$

b) $2 \mathrm{Ag}+1 / 2 \mathrm{O} _{2} \longrightarrow \mathrm{Ag} _{2} \mathrm{O}$

c) $\mathrm{C}+1 / 2 \mathrm{O} _{2} \longrightarrow \mathrm{CO}$

d) $\mathrm{CO}+1 / 2 \mathrm{O} _{2} \longrightarrow \mathrm{CO} _{2}$