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Methods of Extraction

These methods are divided into three groups:

  • Pyrometallurgy

  • Hydrometallurgy

  • Electrolytic Reduction

Methods of Extraction

Pyrometallurgy

It involves the use of elevated temperature and changes in the chemical composition of the entire body of ore. Basic processes of pyrometallurgy are    Calcination      Roasting      Smelting      Distillation

  • Calcination
  • Roasting
  • Smelting
  • Distillation

Calcination

It is a process in which the ore is subjected to the action of heat but is confined to those operations only in which matter is simply expelled or the physical structure altered when heat is applied.

CaCO3 CaO + CO2↑­ (Calcined ore is left)
(Lime stone)

AI2O3.2H2O AI2O3 + 2H2O­↑
(Bauxite)

CuCO3.Cu(OH)2 2CuO + H2O↑­ + CO2­↑
(Malachite)

MgCO3 MgO + CO2­↑
(Magnesite)

2Fe2O3.3H2O 2Fe2O3 + 3H2O­↑
(Limonite)

Roasting

In the process, the ore is heated either alone or in the presence of some substances of that volatile impurities are removed and some chemical changes also take place during this process. It can be of two types.

Oxidised roasting: Ores when heated in presence of Oget converted into their oxides and impurities are converted into their volatile form, which do escape.

4FeS2 + 11O2 2Fe2O3 + 8SO2

2PbS + 3O2 2PbO + 2SO2

 

HgS + O2 Hg + SO2

2ZnS + 3O2 2ZnO + 2SO2

2Cu2S + 3O2 2Cu2O + 2SO2

Roasting or calcinations can be carried out in a reverberatory furnace.

Chlorinating roasting : This is done especially in the case of silver ore.

Ag2S + NaCI 2AgCI + Na2S

Smelting

The process of extracting metal from its fused (molten) state is called smelting. The roasted or calcined ore containing metal oxide is missed with a reducing agent and heated to a high temperature. In this case, a less electropositive metal ore of Pb, Zn, Fe etc. are treated with powerful reducing agent such as C, H2, CO etc. Depending upon the nature of the oxide and metal, the extraction of metal can be carried out by the following reducing agents.

Carbon reduction process: Carbon, the cheapest available reducing agent usually in the form of coke is employed in the extraction of the lead, zinc, iron and tin etc. The oxides of the metals (either naturally occurring or obtained by calcinations of the naturally occurring carbonates or roasting of the sulphies) are mixed with coke and heated in a suitable furnace. Carbon or carbon monoxide reduces the oxide to free metal. For example,

ZnO + CZn + CO

ZnO + CO Zn + CO2

PbO + C Pb + CO

PbO + CO Pb + CO2

Fe2O3 + 3C 2Fe + 3CO

Fe2O3 + 3CO 2Fe + 3CO2

MnO2 + 2C Mn + 2CO

Mn2O3 + 3C 2Mn + 3CO

SnO + C Sn + CO

SnO + CO Sn + CO2

Calcium cannot be extracted from CaO by carbon reduction process. Explain why?

During reduction, additional substance called flux is also added to the ore. It combined with impurities to form easily fusible product known as slag.

Impurities + FluxFusible product (slag)

Flux is a substance that is added to the ore during smelting (a) to decrease the melting point (b) to make the ore conducting and (c) to remove all the impurities (basic and acidic).

FeO             +             SiO2             →        FeSIO3 (fusible slag)
Impurity                    acidic flux

SiO2             +              CaO            →        CaSIO3 (fusible slag)
Impurity                      basic flux

Alumina is a bad conductor of electricity but when cryolite (flux) is added, it becomes a good conductor and the melting point is decreased. Hence, CaF2, KF, cryolite etc are neutral flux.

If FeO is present in the ore as impurity, then what type of flux is added to remove it?

Reduction by another metal (Aluminium):

  • If the temperature needed for carbon to reduce an oxide is too high for economic or practical purposes, the reduction may be effected by another highly electropositive metal such as aluminium, which liberates a large amount of energy (1675 kJ mol–1) on oxidation to AI2O3.

  • This process of reduction of a metal oxide to metal with the help of aluminium powder is called aluminothermy or Goldschmidt Aluminothermic Process or Thermite process.

  • This process is employed in the case of those metal, which have very high melting points and are to be extracted from their oxides.

  • A mixture of concentrated oxide ore and aluminium powder, commonly called as thermite is taken in a stell containing magnesium powder and barium peroxide.

  • During the reaction, aluminium gets oxidized to AI2O3while metal oxides releases metals.

3Mn3O4 + 8AI            →        9Mn + 4AI2O3

3MnO2 + 4AI             →        3Mn + 2AI2O3

B2O3 + 2AI                 →        2B + AI2O3

Cr2O3 + 2AI                →        2Cr + AI2O3

Sr and Ba are obtained by the reduction of their oxides by aluminium in vacuum.

Magnesium is used in a similar way to reduce oxides. In certain cases where the oxide is too stable to reduce, electropositive metals are used to reduce metal halides. Titanium (for supersonic aircrafts) zirconium (used in atomic reactors) are obtained by the reduction of their tetrachlorides with metallic sodium or magnesium.

TiCI4 + 2Mg     Ti + 2MgCI2

TiCI4 + 4Na → Ti + 4NaCI
 

Self-Reduction Process

The cations of the less electropositive metals like Pb, Hg, Sb and Cu may be reduced without the use of any additional reducing agent. Elevated temperature and anion or anions associated with the metal may bring about this change.

For example, in the extraction of mercury, the sulphide ore (cinnabar) is heated in a current of air when the following reactions take place.

2HgS + 3O2 2HgO + 2SO2

2HgO + HgS 3Hg + SO2     (self reduction reaction)

Similarly, in the extraction of copper, the sulphide and the oxide interact at an elevated temperature to give the metal.

Cu2S + 2Cu2O SO2+ 6CU    (self reduction reaction)

Similar reactions take place in the self-reduction process for the extraction of lead.

2PbS + 3O2 2PbO + 2SO2

PbS + 2PbO 3Pb + SO2     (self reduction Reaction)

Reduction of Oxides with Hydrogen

Certain metallic oxides can be reduced using molecular hydrogen. Because of inflammable nature of hydrogen, it is used in very few cases. Molybdenum and tungsten are obtained by reducing their oxides by hydrogen at elevated temperatures.

Co3O4 + 4H2 3Co + 4H2O

GeO2 + 2H2 Ge + 2H2O

2NH4[MoO4] + 7H2 2Mo + 8H2O + 2NH3

2NH3[WO4] + 7H2 2W + 8H2O + 2NH3

This method is not widely used because many metals react with hydrogen at elevated temperature, forming hydrides. There is also a risk of explosion from hydrogen and oxygen present in the air.

Illustration

Question:

Why is it advantageous to roast a sulphide ore to the oxide before reduction?

Solution:

2MS + C→ CS2 + 2M

MS + H2 → H2S + M

The free energies of formation of most sulphides are greater than those for CS2 and H2S. So, the net reactions have unfavorable free energy of formations. Thus, neither carbon nor hydrogen is a suitable reducing agent for metal sulphides. Aluminium cannot be used, as reaction with metal sulphide would produce aluminum sulphide, which is not so exothermic. So, sulphide ores are preferentially converted to oxide ores as any reduction method (reduction by C, H2, AI etc.) could then be conveniently used.

 

Distillation

It is a process by which the metal or its chemical compounds are evaporated to liberate them from the non-volatile components of the charge. The vapours of the metal or its compounds are then condensed in a more or less pure state.

Hydrometallurgy

It is based on dissolving the metal sought in aqueous solutions of acids or alkalis and subsequent precipitation. Basic processes of hydrometallurgy are

Electrolytic Reduction

The strongest possible reducing agent is an electron. Any ionic material may be electrolysed and reduction occurs at the cathode. This is excellent method and gives very pure products but electricity is quite expensive. Electrolysis may be performed in aqueous solution provided that the products do not react with water.

This process is mainly used in the extraction of alkali and alkaline earth metals. In the case of highly electropositive metals, isolation by chemical agents is extremely difficult. In such cases, the metal is obtained by electrolysis of fused salts. Under such conditions, the ions readily mover to the oppositely charged electrodes and are distinguished and discharged over there. Some other salts may have to be added to lower the melting point of the compound taken.

  • In other solvent: Electrolysis can be carried out in solvents other than water. Fluorine reacts violently with water, and it is produced by electrolysis of KHF2 dissolved in anhydrous HF. (The reaction has many technical difficulties (i) HF is corrosive (ii) hydrogen produced at the cathode must be kept separate from the fluorine produced at the anode otherwise explosion may occur (iii) water must be rigorously excluded (iv) fluorine produced attacks the anode and the reaction vessel).

  • In fused melts: Elements that react with water are often extracted from fused melts of their ionic salts. These melts are frequently corrosive and involve large fuel bills to maintain the high temperature required. Aluminium is obtained by electrolysis of a fused mixture of AI2O3 and cryolite. Na3[AIF6]. Both sodium and chlorine are obtained from the electrolysis of fused NaCI: In this case up to two-thirds by weight of CaCI2 is added as an impurity to lower the melting point from 803oC to 505oC.

An example of this is the manufacture of sodium by electrolysis of a fused mixture of sodium and calcium chlorides (Down’s process).

The cell and electrodes used should not be effected by the electrolyte or the products. Hence a steel cell, a graphite anode an iron cathode are employed. The various reactions that take place are,

On fusion, NaCI ↔ Na+ + CI (ions become mobile) 

On electrolysis,

  • At the cathode (negative electrode): Na+ + e → Na (reduction)

  • At the anode (positive electrode) : CI → CI + e (oxidation); CI + CI → CI2

The products obtained react readily, hence a suitable arrangement has to be made to keep them separate.
Down’s process

Thermodynamics of Metallurgy

The method employed for extracting a metal from its ores depends on the nature of the metal and that of the ore and may be related to the position of the metal in the electrochemical series.

In general, metals with Eo < – 1.5 volt yield compounds which are very difficult to reduce and electricity is usually used for the isolation of such metals.

One the other hand, noble metals with Eo > + 0.5 volt form easily reducible compound. A metal higher up in the electrochemical series should be more difficult to reduce to metallic form. As we move down, the reduction becomes more and more easy.

Standard electrode potential of a metal provides some idea regarding the selection of an appropriate method for extracting the metal from its compounds.

However, the free energy changes (ΔG) occurring during the reduction processes are of more importance and help in deciding the suitable method.

In order that the reduction of an oxide, halide or sulphide or by an element may take place spontaneously at a given temperature and pressure, it is essential that there is a decrease in the free energy of the system (negative ΔG). As a matter of fact, the more the negative value of ΔG, the higher is the reducing power of an element.

The free energy change (ΔG) is related to the heat change (Δ) as well as to the product of temperature (T) and the entropy change by an expression

ΔG = ΔH – TΔS                                 …… (1)

When all the reacting substances are at unit activity, ΔG = ΔGo (standard free energy change).

For a reaction such as the formation of an oxide,

2M + O2 → 2MO                               ……. (2)

ΔG becomes smaller with the increase in temperature. This is because the gaseous reactant oxygen is consumed in the reaction leading to the decrease in randomness or entropy (S) of the system and consequently ΔS becomes negative. With further increase in temperature, TΔS acquires more negative value. Since the term TΔS is subtracted from ΔH, ΔG will become increasingly less negative with increase in temperature.

Factors influencing the choice of extraction process

The type of process used commercially for any particular element depends on a number of factors.

  • Is the element unreactive enough to exist in the free state?
  • Are any of its compounds unstable to heat?
  • Does the element occur as sulphide ores, which can be roasted or oxide ores, which can be reduced using carbon, is the cheapest whilst the use of Mg, AI and Na as reducing agents is more expensive.
  • If all other methods fail, electrolysis usually works for ionic materials, but is expensive. If the element is stable in water, electrolyzing aqueous solution is cheaper than using fused melts.

Furnaces used in metallurgy

Some of the principal furnaces used are the following:

Kilns

These are the structures of enclosures in which the materials are mixed with proper fuel, free access of air is permitted but no fusion takes place. The kilns are sometimes heated by gas or by the waste heat from other furnaces.

Blast Furnaces

These are tall structures with gate at the bottom and openings at the top. An air blast is supplied to the furnace by means of blow fans or blowing engines through nozzles provided at the bottom; the nozzles are called tuyers. The materials to be treated are charged into the furnace mixed with fuel and as the substances melt, they run down to the bottom and accumulate in the space below the tuyers known as hearth. When sufficient material has accumulated into the space a hole is tapped into the furnace and the molten matter is allowed to flow out in a separate receiver. Such blast furnaces are obviously utilized for fusions of reducing character, in which the carbonaceous matter of the fuel acts as the reducing agent. In them, the combustion takes place near the region at which the air is blown in and the ascending stream of gases is cooled by the material in the upper part of the furnace. The extent of cooling depends on the rate of ascent and the height of the column.

Reverberatory Furnace

These are the furnaces in which the fuel is burnt in a separate part of the structure, the flame and got gases only coming into contact with the material treated. The chamber in these furnaces is horizontal and is divided into two equal parts by a bridge like partition. The smaller part is the fireplace, closed with fire-bars below.

The larger portion is the laboratory of the furnace, the bottom of which is known as the bed or hearth. The materials are placed on this bed for treatment. Opposite to the fireplace at the other end, is the fine bridge which communicates with stack or chimney. The root of the furnace gradually takes a bend towards the flue end and the whole concave bend deflects or reverberates the flame and hot gases from the fire downwards, the roof comprising the concave bend becomes heated and radiates heat on the bed. As the fuel does not come directly in contact with the material, the reverberatory furnace can be utilized both for reduction and oxidation processes. If reduction be desired the material is mixed with a reducing agent.
Reverberatory Furnace

Muffle Furnaces

It is sometimes described for certain reasons to exclude the products of combustion as well as the fuel and this is accomplished in muffle furnaces. The muffle is a chamber surrounded by the fire or by flues through which the products of combustion and hot gases from the fire pass. These furnaces are used for annealing and gold and silver assaying.

Regenerative Furnaces

The heat carried away to the flues by the escaping gases is again utilized in these furnaces. A flowing column of air is heated by the hot flue gases, the air is then brought back to the fire and returned to the furnace. This means an economy of fuel. Most of the furnaces are fitted up with regenerative systems.

Electric Furnaces

Such furnaces are largely used where cheap power is available and very high temperatures are required and also for electrolytic reductions. The furnaces may be classified as (a) Induction furnaces, in which the charge lying on the furnace bed or in a crucible constitutes the secondary coil of an induction unit, and the induced current produced by making and breaking the primary circuit, heat up the material, (b) Resistance furnaces in which the heat generated by resistance I the circuit is utilized. In some of the furnaces, the body of the furnace itself is made of a resistance material. Small furnaces may be prepared by heat is generated by arcs and thereby, a temperature of over 3000oC current and the arc is struck between them and the charge.

Besides these, there are many types of furnaces such as Bessemer converter, Heroult’s furnace etc. which are used in metallurgy.

Question 1: The process of extracting metal from its fused (molten) state is called

a) Smelting

b) Electrolysis

c) Distillation

d) Roasting

Question 2: Sr and Ba are obtained by the reduction of their oxides by

a) Al

b) Au

c) Hg

d) Na

Question 3: Chlorinating roasting is done especially in the case ore of

a) gold

b) silver

c) copper

d) calcium

Question 4: Chemical method of concentration of ores is?

a) froth floatation process

b) magnetic separation

c) hydraulic washing

d) leaching

Q.1

Q.2

Q.3

Q.4

a

a

d

d

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