Minerals and Ores

PYQ-2023 General Principles and Processes of Isolation of Metals Q6

The compound of a metal found in nature is called a mineral. The minerals from which metal can be economically and conveniently extracted are called ores. An ore is usually contaminated with earthy or undesired materials known as gangue.

(a) Native ores contain the metal in free state. Silver, gold, platinum etc. , occur as native ores.

(b) Oxidized ores consist of oxides or oxysalts (e.g. carbonates, phosphates, sulphates and silicates ) of metals.

(c) Sulphurised ores consist of sulphides of metals like iron, lead, zinc, mercury etc.

(d) Halide ores consist of halides of metals.

Metallurgy :

PYQ-2023 General Principles and Processes of Isolation of Metals Q4, PYQ-2023 General Principles and Processes of Isolation of Metals Q9

The scientific and technological process used for the extraction/isolation of the metal from its ore is called as metallurgy.

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

(A) Crushing and Grinding : The ore is first crushed by jaw crushers and ground to a powder.

(B) Concentration :

The removal of unwanted useless impurities from the ore is called dressing, concentration or benefaction of ore.

(i) Hydraulic washing or Gravity separation or Levigation method :

It is based on the difference in the densities of the gangue and ore particles. This method is generally used for the concentration of oxide and native ores.

(ii) Electromagnetic separation :

It is based on differences in magnetic properties of the ore components. Chromite ore $(FeO.Cr_2O_3)$ is separated from non-magnetic siliceous impurities and cassiterite ore $(SnO_2)$ is separated from magnetic Wolframite $(FeWO_4 + MnWO_4)$.

(iii) Froth floatation process. This method is commonly used for the concentration of the low grade sulphide ores like galena, PbS (ore of Pb ); copper pyrites $Cu_2S.Fe_2S_3$ or $CuFeS_2$ (ore of copper) ; zinc blende, $\mathrm{ZnS}$ (ore of zinc) etc., and is based on the fact that gangue and ore particles have different degree of wettability with water and pine oil; the gangue particles are preferentially wetted by water while the ore particles are wetted by oil. In this process one or more chemical frothing agents are added.

(iv) Leaching: Leaching is often used if the ore is soluble in some suitable solvent, e.g. , acids, bases and suitable chemical reagents.

(C) Extraction of crude metal from concentrated ore :

The isolation of metals from concentrated ore involves two major steps as given below.

(i) Conversion to oxide :

Calcination:

It is a process of heating the concentrated ore strongly in a limited supply of air or in the absence of air. The process of calcination brings about the following changes :

(a) The carbonate ore gets decomposed to form the oxide of the metal.

(b) Water of crystallization present in the hydrated oxide ore gets lost as moisture.

(c) Organic matter, if present in the ore, gets expelled and the ore becomes porous. Volatile impurities are removed.

Roasting :

It is a process of heating the concentrated ore (generally sulphide ore) strongly in the excess of air or $\mathrm{O}_{2}$ below its melting point. Roasting is an exothermic process once started it does not require additional heating.

Smelting :

(i) Slag formation : In many extraction processes, an oxide is added deliberately to combine with other impurities and form a stable molten phase immiscible with molten metal called a slag. The process is termed smelting.

The principle of slag formation is essentially the following :

Nonmetal oxide (acidic oxide) + Metal oxide (basic oxide) $\longrightarrow$ Fusible (easily melted) slag

Removal of unwanted basic and acidic oxides: For example, $\mathrm{FeO}$ is the impurity in extraction of $\mathrm{Cu}$ from copper pyrite.

Matte also contains a very small amount of iron(II) sulphide.

To remove unwanted acidic impurities like sand and $P_4O_{10}$, smelting is done in the presence of limestone.

$CaCO_3 \longrightarrow CaO+CO_2$

$CaO + SiO_2 \longrightarrow CaSiO_3(\text{fusible slag})$

$6CaO + P_4O_{10} \longrightarrow 2Ca_3(PO_4)_2 (\text{fusible slag - Thomas slag})$

(ii) Reduction of a metal oxide :

The free metal is obtained by reduction of a compound, using either a chemical reducing agent or electrolysis.

Chemical reduction method :

Reduction with carbon :

$$ \mathrm{PbO}+\mathrm{C} \longrightarrow \mathrm{Pb}+\mathrm{CO} \text { (extraction of lead) } $$

Reduction with $\mathrm{CO}$ : In some cases $\mathrm{CO}$ produced in the furnace itself is used as a reducing agent.

$$ Fe_2O_3 + 3CO \longrightarrow 2Fe + 3CO_2 $$

Reduction by other metals :

Metallic oxides ( $\mathrm{Cr}$ and $\mathrm{Mn}$ ) can be reduced by a highly electropositive metal such as aluminium that liberates a large amount of energy (1675 $\mathrm{kJ} / \mathrm{mol}$ ) on oxidation to $Al_2 O_3$. The process is known as Goldschmidt or aluminothermic process and the reaction is known as thermite reaction.

$$ Cr_2O_3 + Al\rightarrow 2Cr(l) + Al_2O_3 $$

Magnesium reduction method : Magnesium is used in similar way to reduce oxides. In certain cases where the oxide is too stable to reduce, electropositive metals are used to reduce halides.

$TiCl_4 + 2 Mg \xrightarrow[1000-1150^circC] {\text{ Krollprocess }} Ti + 2 MgCl_2$

$TiCl_4 + 4 Na \xrightarrow { \text{IMI process }} Ti + 4 NaCl$

Self-reduction method :

This method is also called auto-reduction method or air reduction method. If the sulphide ore of some of the less electropositive metals like $\mathrm{Hg}, \mathrm{Cu}$, $\mathrm{Pb}, \mathrm{Sb}$, etc. are heated in air, a part of these is changed into oxide or sulphate then that reacts with the remaining part of the sulphide ore to give its metal and $\mathrm{SO}_{2}$.

$Cu_2S + 3O_2 \longrightarrow 3Cu_2O + 2SO_2$

$2Cu_2O + Cu_2S \longrightarrow 6Cu + SO_2$

Electrolytic reduction :

It presents the most powerful method of reduction and gives a very pure product. As it is an expensive method compared to chemical methods, it is used either for very reactive metals such as magnesium or aluminum or for production of samples of high purity.

(i) In aqueous solution : Electrolysis can be carried out conveniently and cheaply in aqueous solution that the products do not react with water. Copper and zinc are obtained by electrolysis of aqueous solution of their sulphates.

(ii) In fused melts : Aluminum is obtained by electrolysis of a fused mixture of $Al_2O_3$ and cryolite $Na_3[AlF_6]$.

Extraction of Aluminium:

It involves the following processes

(a) Purification of bauxite :

(b) Electrolytic reduction (Hall-Heroult process) :

$$ 2Al_2O_3 + 3C \longrightarrow 4Al + 3CO_2 $$

Cathode :

$$ \mathrm{Al}^{3+}(\mathrm{melt})+3 \mathrm{e}^{-} \longrightarrow \mathrm{Al}(\mathrm{l}) $$

Anode :

$$ \begin{aligned} & \mathrm{C}(\mathrm{s})+\mathrm{O}^{2-}(\text { melt }) \longrightarrow \mathrm{CO}(\mathrm{g})+2 \mathrm{e}^{-} \\ & \mathrm{C}(\mathrm{s})+2 \mathrm{O}^{2-}(\mathrm{melt}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{e}^{-} \end{aligned} $$

Ellingham Diagram

PYQ-2023 General Principles and Processes of Isolation of Metals Q11

Metallurgy of some important metals

1. Extraction of iron from ore haematite

Reactions involved :

At $500-800 \mathrm{~K}$ (lower temperature range in the blast furnace)

$3Fe_2O_3 + CO \longrightarrow 2Fe_3O_4 + CO_2$

$Fe_3O_4 + CO \longrightarrow 3Fe + 4CO_2$

$Fe_2O_3 + CO \longrightarrow 2FeO + CO_2$

At $900-1500 \mathrm{~K}$ (higher temperature range in the blast furnace):

$C + CO_2 \longrightarrow 2 CO ; \quad FeO + CO \longrightarrow Fe + CO_2$

Limestone is also decomposed tom $\mathrm{CaO}$ which removes silicate impurity of the ore as slag. The slag is in molten state and separates out from iron.

$CaCO_3 \longrightarrow CaO + CO_2 ; \quad CaO + SiO_2 \longrightarrow CaSiO_3$

2. Extraction of copper :

PYQ-2023 General Principles and Processes of Isolation of Metals Q3, PYQ-2023 General Principles and Processes of Isolation of Metals Q7, PYQ-2023 General Principles and Processes of Isolation of Metals Q12

From copper glance / copper pyrite (self reduction) :

$2CuFeS_2 + 4O_2 \longrightarrow Cu_2S + 2FeO + 3SO_2$

$Cu_2S + FeO + SiO_2 \longrightarrow FeSiO_3$ (fusible slag) $+Cu_2S$ (matte)

$2FeS + 3O_2 \longrightarrow 2FeO + 2SO_2 \quad ; FeO + SiO_2 \longrightarrow FeSiO_3$

$2Cu_2S + 3O_2 \longrightarrow 2Cu_2O + 2SO_2$;

$2Cu_2O + Cu_2S \longrightarrow 6Cu + SO_2$ (self reduction)

3. Extraction of lead :

(i) $2PbS(s) + 3O_2$ (g) $\stackrel{\Delta}{\longrightarrow} 2PbO(s) \xrightarrow[\Delta]{+C} 2 Pb(\ell) + CO_2(g)$

(ii) $3PbS(s) \xrightarrow[\text { air }]{\text { heat in }} PbS(s) + 2PbO$ (s) $\xrightarrow [\text {absence of air}]{\text {Heat in }} 3Pb(\ell) + SO_2(g)$

4. Extraction of zinc from zinc blende :

The ore is roasted in presence of excess of air at temperature $1200 \mathrm{~K}$.

$$ 2ZnS + 3O_2 \longrightarrow 2ZnO + 2SO_2 $$

The reduction of zinc oxide is done using coke.

$$ \mathrm{ZnO}+\mathrm{C} \xrightarrow{\text { Coke,1673K }} \mathrm{Zn}+\mathrm{CO} $$

5. Extraction of tin from cassiterite :

The concentrated ore is subjected to the electromagnetic separation to remove magnetic impurity of wolframite.

$\mathrm{SnO}_{2}$ is reduced to metal using carbon at $1200-1300^{\circ} \mathrm{C}$ in an electric furnace. The product often contains traces of $\mathrm{Fe}$, which is removed by blowing air through the molten mixture to oxidize $\mathrm{FeO}$ which then floats to the surface.

6. Extraction of Magnesium :

From Sea water (Dow’s process) :

Sea water contains $0.13 %$ magnesium as chloride and sulphate. It involves following steps.

(a) Precipitation of magnesium as magnesium hydroxide by slaked lime.

(b) Preparation of hexahydrated magnesium chloride.

The solution on concentration and crystallization gives the crystals of $MgCl_2 \cdot 6H_2O$.

(c) Preparation of anhydrous magnesium chloride.

(d) Electrolysis of fused anhydrous $\mathrm{MgCl}_{2}$ in presence of $\mathrm{NaCl}$.

$$ \mathrm{MgCl}_{2} \rightleftharpoons \mathrm{Mg}^{2+}+2 \mathrm{Cl}^{-} $$

At cathode : $\quad \mathrm{Mg}^{2+}+2 \mathrm{e}^{-} \longrightarrow \mathrm{Mg} (99$ % pure );

At anode : $\quad 2 \mathrm{Cl}^{-} \longrightarrow \mathrm{Cl}_{2}+2 \mathrm{e}^{-}$

7. Extraction of gold and silver (MacArthur-Forrest cyanide process)

(a) From native ores : Extraction of gold and silver involves leaching the metal with $\mathrm{CN}^{-}$.

$4Au / Ag(s) + 8CN^-(aq) + 2H_2O(aq) + O_2(g) \longrightarrow 4\left[Au / Ag(CN)_2\right]^-(aq) + $ $4 OH(aq)$

$2\left[Au/ Ag(CN)_2\right]^{-}(aq) + Zn(s) \longrightarrow 2Au / Ag(s) + \left[Zn(CN)_4\right]^{2-}(aq)$

(b) From argentite ore :

$Ag_2S$ (conc. ore) $ + 2NaCN \stackrel{\text { Air }}{\rightleftharpoons} 2AgCN + Na_2S$.

$4Na_2S + 5O_2 + 2H_2O \longrightarrow 2Na_2SO_4 + 4NaOH + 2S$

$AgCN + NaCN \longrightarrow Na\left[Ag(CN)_2\right]$ (soluble complex)

$2Na\left[Ag(CN)_2\right] + Zn$ (dust) $\longrightarrow 2Ag \downarrow + Na_2\left[Zn(CN)_4\right]$.

(D) Purification or Refining of metals :

PYQ-2023 General Principles and Processes of Isolation of Metals Q1, PYQ-2023 General Principles and Processes of Isolation of Metals Q2, PYQ-2023 General Principles and Processes of Isolation of Metals Q5, PYQ-2023 General Principles and Processes of Isolation of Metals Q8 PYQ-2023 General Principles and Processes of Isolation of Metals Q10

Physical methods:

These methods include the following processes:

(I) Liquation process : This process is used for the purification of the metal, which itself is readily fusible, but the impurities present in it are not, used for the purification of $\mathrm{Sn}$ and $\mathrm{Zn}$, and for removing $\mathrm{Pb}$ from $\mathrm{Zn}-\mathrm{Ag}$ alloy.

(II) Fractional distillation process : This process is used to purify those metals which themselves are volatile and the impurities in them are nonvolatile and vice-versa. $\mathrm{Zn}, \mathrm{Cd}$ and $\mathrm{Hg}$ are purified by this process.

(III) Zone refining method (Fractional crystallization method) : This process is used when metals are required in very high purity, for specific application. For example pure $\mathrm{Si}$ and $\mathrm{Ge}$ are used in semiconductors

Chemical methods :

These methods include the following methods:

(i) OXIDATIVE REFINING :

This method is usually employed for refining metals like $\mathrm{Pb}, \mathrm{Ag}$, $\mathrm{Cu}, \mathrm{Fe}$, etc. In this method the molten impure metal is subjected to oxidation by various ways.

(ii) POLING PROCESS :

This process is used for the purification of copper and tin which contains the impurities of their own oxides.

$\text { Green wood } \rightarrow \text { Hydrocarbons } \rightarrow CH_4$

$4CuO + CH_4 \rightarrow 4Cu\text { (pure metal) } + CO_2 + 2H_2O$

(iii) ELECTROLYTIC REFINING :

Some metals such as $\mathrm{Cu}, \mathrm{Ni}$, and $\mathrm{Al}$ are refined electrolytically.

(iv) VAPOUR PHASE REFINING :

(i) Extraction of Nickel (Mond’s process) : The sequence of reaction is

$H_2O(g )+ C \longrightarrow CO(g) + H_2$

$ Ni(s) + 4CO(s) $ $ \xrightarrow{50^{\circ} C}$ $ Ni(CO_4) (g) $

$ Ni(CO)_4 (g) \xrightarrow{200^{\circ} C} Ni + 4CO(g)$

(ii) Van Arkel-De Boer process :

$$\text { Impure } Ti + 2I_2 \xrightarrow{50-250^{\circ} \mathrm{C}} TiI_4 \xrightarrow[\text { Tungsten filament }]{1400^{\circ} \mathrm{C}} Ti + 2I_2 $$