General Principles and Processes of Isolation of Elements Revision Notes

Ruhul Amin Alig

Terms used in Metallurgy

Minerals: They are naturally occurring chemical substances in the earth's crust which includes metal and the impurities.
Ores: The Minerals from which metal can be extracted profitably are called ores. Eg: Copper glance ( $C u_{2} S$ ), Haematite ( $F e_{2} O_{3}$ )
Gangue: Undesired materials which are present along with the metal in the earth crust are known as gangue.
Metallurgy: The entire scientific and technological process used for isolation of the metal from its ores is known as metallurgy

Important ores

The extraction and isolation of an element from its combined form involves some steps and these steps are as follows.

A) Concentration of the ore (or) Purification of the Ore

B) Isolation of the metal from its concentrated ore

C) Purification of the metal (Or) Refining of the Metal.

Concentration of ores

Concentration of ore

The process of removal of the gangue/impurities from Ore is known as Concentration or Dressing or Benefaction.

Different methods are used to concentrate ores

i) Hydraulic washing:

An upward stream of running water is used to wash the powdered ore. During this process the lighter impurities which are adhered to the ore float over the water which can be removed and the heavier ore particles are left behind.

ii) Electromagnetic Separation:

Electromagnetic separation is based on the magnetic properties of the ore. If either the ore or the gangue is attracted towards the magnetic flux, then the separation is carried out by this method.

In this process, the ore is passed through a belt that moves over the magnetic roller. The ore particles that are magnetic in nature are attracted by the magnetic field, and fall almost near it. The impurities which are non-magnetic aren't attracted by the magnetic wheel and fall faraway from the magnetic wheel.

iii) Froth Floatation Process:

In this process, a suspension of the powdered ore is formed with water. To it, collectors and froth stabilizers are added. A rotating paddle agitates the mixture and draws air in it. As a result, froth is made which carries the mineral particles. The froth is light and is skimmed off. It's then dried for recovery of the ore particles. Collectors: The chemical substances which enhance the non-wettability of the mineral particles. Eg.: pine oils, fatty acids, xanthates

Froth stabilizers: The chemical substances stabilize the froth. Eg.: cresols, aniline.

IV) Leaching:

It is often used if the ore is soluble in some suitable solvent. It is a chemical method used for purification of ore.

Leaching of Alumina from Bauxite:

The ore consists of two chemical impurities a) Iron oxide (

F e_{2} O_{3}

) b) Silica (

S i O_{2}

The ore is treated with Sodium hydroxide solution (NaOH), in which Iron oxide (

F e_{2} O_{3}

) impurity does not react with it and can be removed by filtration.

A l_{2} O_{3} (s) + 2 N a O H + 3 H_{2} O \to 2 N a [A l (O H)_{4}] (a q)

The aluminate in solution is neutralised by passing

C O_{2}

gas and hydrated

A l_{2} O_{3}

is precipitated.

2 N a [A l (O H)_{4}] (a q) + C O 2 (g) \to A l_{2} O_{3} . x H_{2} O (s) + 2 N a H C O_{3}

A l (O H)_{3}

is ignited to get pure Bauxite called Alumina.

A l_{2} O_{3} . x H_{2} O (s) H e a t A l_{2} O_{3} (s) + 3 H_{2} O (g)

Isolation of the metal from its concentrated ore

To extract metal from concentrated ore, it must be converted to a form which is suitable for reduction to metal. Isolation of metals from concentrated ore involves two major steps

Conversion to oxide, and
Reduction of the oxide to metal.

Conversion of Ore into Metal oxide

Calcination:

It is the process of the heating of ore to a high temperature for the purpose of removing volatile substances

F e_{2} O_{3} . x H_{2} O (s) △ F e_{2} O_{3} (s) + x H_{2} O (g)

Z n C O_{3} (s) △ Z n O (s) + C O_{2} (g)

C a C O_{3} . M g C O_{3} (s) △ C a O (s) + M g O (s) + 2 C O_{2} (g)

Roasting:

The process in which the ores are heated in presence of excess of oxygen. This process is mainly used for Sulphide ores.

Z n S + O_{2} △ Z n O + S O_{2}

Reduction of metal oxide to metal

Reduction of the metal oxide usually involves heating it with a reducing agent, for example, C, or CO, or even another metal. The reducer(e.g., carbon) combines with the oxygen of the metal oxide.

M_{x} O_{y} + y C \to x M + y C O

Thermodynamic principle of metallurgy

Some metal oxides get reduced easily while others are very difficult to be reduced. In any case, heating is required. To understand the variation in the temperature requirement for thermal reductions (pyrometallurgy) and to predict which element will suit as the reducing agent for given metal oxide (MxOy), Gibbs energy interpretations are made. The change in Gibbs energy, G for any process at any specified temperature is described by the equation:

△ G = △ H - T △ S

where

△ H

is that the enthalpy change and

△ S

is that the entropy change for the method.

1. If the valueof

△ G

is negative then only then the reaction will proceed. If

△ S

is positive, on increasing the temperature (T), the value of

T △ S

would increase (

△ H

T △ S

), and then

△ G

will become -ve.

2. If reactants and products of two reactions are put together in a system and the net value of

△ G

of the two possible reactions is -ve, the reaction will occur. So, the process of interpretation involves coupling the two reactions, getting the sum of their

△ G

, and looking for its magnitude and sign. Such coupling is easily understood through Gibbs energy (

△ G

) vs T plots for the formation of the oxides in Ellingham Diagram.

Ellingham Diagram provides an appropriate reducing agent for the reduction of oxides. Such diagrams help us in predicting the feasibility of the thermal reduction of ore.

Extraction of some metals from their metal oxides

Iron from Iron oxide:

a) Removal of chemical impurity:

Silica (

S i O_{2}

) is a chemical impurity which is acidic in nature present along with the ore.

A basic flux like quicklime(CaO) is added to get rid of Silica.

CaO is obtained from Limestone (

C a C O_{3}

), which decomposes to CaO and

C O_{2}

in the furnace due to intense heat.

C a C O_{3} (s) \to C a O (s) + C O_{2} (g) .

The impurity

S i O_{2}

, combines with CaO to form calcium silicate (

C a S i O_{3}

) called Slag.

C a O (s) + S i O_{2} (s) \to C a S i O

b) Extraction of Iron:

The pure haematite ore, ferric oxide (

F e_{2} O_{3}

) is reduced to iron in presence of a reducing agent coke (carbon).

Coke (C) combines with

C O_{2}

to form carbon monoxide which is a very good reducing agent and reduces

F e_{2} O_{3}

in to Iron (Fe).

C (C o k e) + C O_{2} (g) \to 2 C O (g)

F e_{2} O_{3} (s) + 3 C O (g) \to 2 F e (m o l t e n) + 3 C O_{2} (g)

Slag, calcium silicate (

C a S i O_{3}

), is lighter than molten Iron and floats over it. This prevents the molten Iron to oxidize in presence of oxygen in the furnace.

The slag and the molten Iron are collected separately

Aluminium from Alumina (Aluminium oxide):

Electrolytic Reduction: Hall's Process:

Purified alumina undergoes electrolysis to give aluminium. In the process, alumina is mixed with a substance called Cryolite (

N a_{3} A l F_{6}

) to lower the melting point of the mixture and to increase conductivity.

The electrolysis process is carried out in an electrolytic cell using carbon electrodes as shown in the diagram.

Cathode reaction :

A l^{+ 3} + 3 e \to A l

Anode Reaction :

2 O^{2 -} - 4 e \to O_{2} (g)

The oxygen gas liberated at anode reacts with graphite anode and form

C O_{2}

gas, so anode is to be replaced from time to time.

C (g r a p h i t e) + O_{2} (g) \to C O_{2} (g) .

The pure aluminium (Al) is collected from the cathode from time to time.

Extraction of Copper

By Auto reduction/ Self Reduction:

Process takes place in a reverberatory furnace. Hot air is blown to convert the

C u_{2} S

C u_{2} O

. The

C u_{2} O

formed is converted to metallic copper in the presence of remaining

C u_{2} S

which acts as a reducing agent.

2 C u_{2} S + 3 O_{2} \to 2 C u_{2} O + 2 S O_{2}

2 C u_{2} O + C u_{2} S \to 6 C u + S O_{2}

The solidified copper obtained has blistered appearance due to the evolution of

S O_{2}

and so it is called blister copper.

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 briquettes with coke and clay.

Z n O + C (c o k e) \to Z n + C O a t 673 K

The metal is distilled off and collected by rapid chilling.

Purification of the metal

A metal extracted by any method is typically 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 present. Some of them are listed below.

a) Distillation

b) Liquation

c) Electrolysis

d) Zone refining

e) Vapour phase refining

f) Chromatographic methods

(a.) Distillation:

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

(b.) Liquation:

In this method, a low melting metal like tin is made to flow on a sloping surface. As it flows through the sloping surface, it is separated from the higher melting impurities.

(c.) Electrolytic refining: In this method

The impure metal is the 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:

A n o d e : M \to M^{n +} + n e^{-}

C a t h o d e : M^{n +} + n e^{-} \to M

(d.) Zone refining :

The principle behind zone refining is that the impurities are more soluble in the melt than in the solid state of the metal.

The circular mobile heater is fixed at one end of a rod of the impure metal. The molten zone moves along side 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 method 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 extremely 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. It is then decomposed to give pure metal. So, the two requirements are:

(i) The metal should form a volatile compound with an available reagent

ii) The volatile compound should be easily decomposable, so that the recovery is easy.

Mond Process for Refining Nickel:

In this process, nickel is heated in presence of a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl.

N i + 4 C O \to N i (C O)_{4} a t 330 - 350 K .

The carbonyl is subjected to higher temperature such that it's decomposed to form the pure metal.

N i (C O)_{4} \to N i + 4 C O a t 450 - 470 K

Van Arkel Method for Refining Zirconium or Titanium:

This method is extremely useful for removing all the oxygen and nitrogen present in the form of impurity in certain metals like Zr and Ti. The crude metal is heated in an evacuated vessel with iodine.

The metal iodide being more covalent, volatilizes.

Z r + 2 I_{2} \to Z r I_{4}

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

Z r I_{4} \to Z r + 2 I_{2}

(e) Chromatographic methods:

The principle of this method is that the different components of a mixture are differently adsorbed on an adsorbent.

The mixture is put in a liquid medium which is moved through the adsorbent. Different components are adsorbed at different levels on the column. Later the adsorbed components are removed by using suitable solvents.

Column chromatography: In this method, the column of

A l_{2} O_{3}

is prepared in a glass tube and the moving medium containing a solution of the components is in liquid form. This is very useful for purification of the elements which are available in minute quantities and the impurities are not very different in chemical properties from the element to be purified.

Uses of Metals

Uses of Aluminium

Aluminium foils are used to wrap food materials.
The fine dust of the metal is used in paints and lacquers.
Aluminium it 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.

2. Uses of Copper

It is used to make electrical wire and steam pipes.
It is also used in alloys like brass, bronze and coinage

3. Uses of Zinc

It is used to galvanize iron
It is used in cells
Zinc is also used in many alloys such as brass, and german silver.
Zinc is used as a reducing agent

4. Uses of Iron

It is used to make machinery and tools, as well as vehicles, hulls of ships, structural elements for buildings, bridges and aircraft.
Iron to create steel, often used in manufacturing and civil engineering.
Stainless steel, which is highly resistant to corrosion, is commonly used in kitchen cutlery, appliances and cookware - it's also used for hospital equipment.

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