Methods of Extraction

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.

Distillation

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 803^oC to 505^oC.

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.

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 E^o < – 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 E^o > + 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 = ΔG^o (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.

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 3000^oC 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

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