Extraction of Elements and Metallurgy

EXTRACTION OF ELEMENTS AND METALLURGY

Metals can be categorised into three parts on the basis of their reactivity: Most reactive, medium reactive and least reactive.

 

The three major steps involved in the extraction of a metal from its ore are

1.     Concentration or enrichment of ores.

2.     Conversion of concentrated ore into crude metal and,

3.     Refining of impure or crude metal.

 

Occurrence of Metals

Metals are majorly found in the earth’s crust although salts such as magnesium chloride and sodium chloride are found in seawater.

Metals occur in nature in two states:

Free state

·         Metals which do not react easily with moisture, oxygen, or other reagents are called Native or Free.

·         It is in a free or native state when it’s available in a pure form.

For example, gold, platinum, silver, etc.

Combined state

·         The reactive metals react easily with oxygen, moisture, or other reagents.

·         They are found in the combined state in the form of compounds.

·         Metals usually occur in combination with non-metallic elements.

For example, sodium, potassium, calcium, etc.

 

Mineral

The naturally occurring substances in the form of which the metals occur in the earth’s crust are called minerals.

Ore

The mineral from which  a metal can be extracted profitably and conveniently is called an ore.

 

Gangue

The earthy impurities like sand, rock, etc., that surround the worthy mineral in a ore, are called called gangue.

 

Concentration of ore

The process of removal of unwanted materials from the ore is called concentration or benefaction of the ore.

It can be carried out by various steps as stated below:

Hydraulic Washing

In this method, the lighter earthy impurities are washed away from the heavier ore particles.  Thus this method of concentration of ore is based on the difference in specific gravities of the ore and gangue particles.

 

Magnetic Separation

 This is based on differences in magnetic properties of the ore components. If either the ore or the gangue is attracted towards magnetic field, then the separation is carried out by this method. For example iron ores are attracted towards magnet, hence, non–magnetic impurities can be separted from them using magnetic separation. The powdered ore is dropped over a conveyer belt which moves over a magnetic roller Magnetic substance remains attracted towards the belt and falls close to it.

 

 

Froth Floatation Method

It is based on the principle that sulphide ores are preferentially wetted by the pine oil or fatty acids or xanthates etc., whereas the gangue particles are wetted by the water.

Collectors are added to enhance the non-wettability of the mineral particles.
Froth stabilizers such as cresols, aniline etc., are added to stabilize the froth.
If two sulphide ores are present, it is possible to separate the two sulphide ores by adjusting proportion of oil to water or by adding depressants.
For example, in the case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to froth but allows PbS to come with the froth.

 

Leaching

It is a process in which ore is treated with suitable solvent which dissolves the ore but not the impurities.

Purification of Bauxite by leaching ( Baeyer’s process):

a) Step 1:

b) Step 2:

c) Step 3:

d) Concentration of Gold and Silver Ores by Leaching:

Where [M =Ag or Au]

 

Extraction  of Crude Metal from Concentrated Ore

 

Concentrated ore is usually converted to oxide before reduction, as oxides are easier to reduce. Thus, extraction of crude metal from concentrated ore involves two major steps:

(i) Conversion to oxide.

(ii) Reduction of the oxides to metal

 

Conversion to oxide

It can be carried out by following two methods:

·        Calcination

·        Roasting

Calcination

It is the process of converting an ore into its oxide by heating it in a limited supply of air or in absence of air, below its melting point. The volatile matter is burnt away and the oxide of the metal is obtained. This process is useful for metal carbonates and hydroxides. For example: CaCO3 → CaO + CO2  and  Al2O3.2H2O → Al2O3 + 2H2O.

 

Roasting

It is the process of converting an ore into its oxide by heating the ore in excess of oxygen (air). This process is commonly used for suiphide ores.

For example: 2PbS + 3O2 → PbO + 2 SO2 and 2 Cu2S + 3O2 → 2 Cu2O + 2 SO2

 

Reduction of oxide to the metal

The metal present in metal oxide can be converted from cationic form to free by supplying electrons, i.e., by reduction of metal oxide. The nature of reducing agent used depends upon the activity of metal.

For example: If the metal is very reactive like Na, K, electrolytic reduction method is used whereas the less reactive metals like Cu, Sn, Fe can be reduced by chemical reducing agents like CO, H2, etc.

Reduction by carbon (Smelting):

The process of using carbon in form of coke, charcoal, CO to reduce metal oxides to respective metals, is termed as smelting.

For example: Fe2O3 + 3 CO → 2 Fe + 3CO2

Reduction by hydrogen:

Because of highly inflammable nature of H2 it is used as a reducing agent especially for oxides of Tungsten (W) and Molybdenum (Mo).

For example: MO3 + 3H2 → M + 3H2O

Reduction by Aluminium:

Reduction of metal oxides to respective metals by using aluminium is known as alumino thermite or Gold Schmidt thermite process. It is mainly used to reduce Cr2O3 or Fe2O3

For example: Cr2O3 + 2Al → Al2O3 + 2Cr

 

Thermodynamic principle of metallurgy

Some basic concepts of thermodynamics help in understanding the conditions of temperature and selecting suitable reducing agent in metallurgical processes:

Gibbs free energy change at any temperature is given by ΔG = ΔH – TΔS where ΔG is free energy change, ΔH is enthalpy change and ΔS is entropy change.

The relationship between and K is = –2.303 RT log K where K is equilibrium constant. R = 8.314 JK-1 mol-1, T is temperature in Kelvin.

A negative  means +ve value of K i.e., products are formed more than the reactants. The reaction will proceed in forward direction.

If ΔS is +ve, on increasing temperature the value of increases so that >  and  will become negative.

 

Ellingham Diagram

A Ellingham diagram is a graphical plot of the relative stability of various oxidation states of a metal. The diagram is named after the chemist Max Ellingham, who first described the concept in a scientific paper in 1945.

The diagram is a plot of the Gibbs free energy of formation of a metal ion in various oxidation states against the standard reduction potential of the metal in those oxidation states. The most stable oxidation state is at the bottom of the diagram, and the least stable oxidation state is at the top.

The stability of a metal ion can be affected by its surrounding ligands. For example, the stability of a metal ion in an oxidation state of +2 can be affected by the presence of ligands that can donate electrons to the metal ion, such as oxygen or nitrogen. In this case, the metal ion would be more stable in an oxidation state of +3, because it would have more electrons to stabilize it.

The Ellingham diagram can be used to predict the stability of a metal ion in different oxidation states. For example, if the standard reduction potential of a metal ion in an oxidation state of +3 is more negative than the standard reduction potential of a metal ion in an oxidation state of +2, then the metal ion in the oxidation state of +3 is more stable.

Salient Features of Ellingham Diagram

The salient features of an Ellingham diagram are:

1. It is used to predict the stability of a compound.

2. The stability of a compound is determined by its Gibbs energy.

3. The Gibbs energy is represented on the y-axis and the stability of the compound is represented on the x-axis.

4. The stability of a compound increases as the Gibbs energy decreases.

5. The Ellingham diagram can be used to predict the stability of a compound in both the gas and the liquid phases.

Applications of Ellingham Diagram

There are many different applications of Ellingham diagrams. A few examples are given below.

1. To predict the stability of a particular compound

2. To predict the products of a chemical reaction

3. To understand the mechanism of a chemical reaction

4. To understand the factors that influence the stability of a compound

Limitations of Ellingham Diagram

The Ellingham diagrams are useful for predicting the stability of metal complexes, but there are some limitations to their use.

1. The diagrams are limited to predicting the stability of metal complexes. They cannot be used to predict the stability of other types of compounds.

2. The diagrams are also limited to predicting the stability of compounds in their neutral form. They cannot be used to predict the stability of compounds in their ionic form.

3. The diagrams are also limited to predicting the stability of compounds at room temperature. They cannot be used to predict the stability of compounds at other temperatures.

Predicting spontaneity of a reaction

The spontaneity of a reaction is decided by the Gibbs energy change ΔG, which is given as, ΔG = ΔH ‒ TΔS

Where, ΔH  is enthalpy change and ΔS is entropy change.

The sign of ΔG depends on the sign of ΔH, ΔS and the temperature. When the value of ΔG is negative, only then the reaction will proceed. If ΔS is positive, on increasing the temperature (T), the value of TΔS would increase (ΔH < TΔS) and then ΔG will become –ve.

The graphical representation of Gibbs energy is known as Ellingham diagram. Such diagrams help us in predicting the feasibility of thermal reduction of an ore.

The height of the line in an Ellingham diagram indicates the instability of the oxide (or the sulphide ore) since the higher the line, the more positive the ΔG, the less spontaneous the formation of the oxide (or the sulphide).

Example: Consider the reduction of Cr2O3 by Al.

The two equations involved in the formation of respective oxides of Cr and Al, are expressed as follows:

 

Here formation of aluminium oxide is represented by the lower line, i.e., ΔG is more negative for this reaction which means the oxide formed in this reaction is more stable. Thus Al can be used to reduce Cr2O3 to form more stable oxide, Al2O3.  Cr2O3 + 2 Al → 2 Cr + Al2O3 

 

Applications of metallurgy

 

Extraction of iron from its oxides

The haematite ore (Fe2O3) is first calcined and then subjected to smelting. The charge consisting of calcined haematite, coke and limestone is fed into the blast furnace from its top. A blast of hot air is passed near the base of the blast furnace.

The coke undergoes combustion at the bottom of the furnace producing CO2 at about 1900 K. According to the Ellingham diagram, CO is more stable compared to CO2  to  CO only above 1000 K , thus at 1900 K , CO  is formed. It cools as it rises up the furnace and at a temperature of about 1000 K, the CO/CO2  line so that  CO is able to reduce the iron oxide to iron.

Various reactions taking place inside the furnace at 500 – 800 K range are:

3Fe2O3 + CO → 2Fe3O4 + CO2

Fe3O4 + 4 CO → 3 Fe + 4 CO2

Fe2O3 + CO → 2 FeO + CO2

Pig iron: Iron obtained from a blast furnace is called pig iron and contains about 4% carbon and other impurities such as S, P, Si and Mn.

Cast iron: It is the iron that contains about 3% carbon, extremely hard, cannot be welded and brittle.

Wrought iron: Also known as malleable iron, it is the purest form of iron. It is prepared by oxidising the impurities in cast iron in a reverberatory furnace lined with haematile.

 

Extraction of copper from cuprous oxide [copper(I) oxide]

Cu2O can be easily redued to Cu directly by heating with coke. But in case if Cu ores are sulphides and contain some iron, the following methods are applied:

(iFroth flotation of the sulphide ore

(iiRoasting of the sulphide ore

2 Cu2S + 3O2 → 2 Cu2O + 2 SO2

2 FeS + 3 O→ 2FeO + 2SO2

FeO can be removed by using SiO2 as flux

FeO + SiO2 → FeSiO3

(iiiThe  Cu2O above obtained is reduced to Cu by using Cu2S  (Auto reduction)

2 Cu2O + Cu2S → 6Cu + SO2

The solidified copper obtained has blistered appearance due to the evolution of SO2 and so it is called blister copper.

 

Extraction of zinc from zinc oxide

The decomposition of ZnO to Zn and O2 does not occur until over 2000 K. However, ZnO can be reduced to Zn using CO at around 1200 K, because above 1200 K, ΔG for the reaction 2Zn + O2 → 2ZnO is more negative than foe the reacton 2 Zn + O2 → 2 ZnO

 

Hall-Heroult process

Purified bauxite ore is mixed with cryolite () or  which lowers its melting point and increases electrical conductivity. Molten mixture is electrolysed using a number of graphite rods as anode and carbon lining as cathode.

The graphite anode is useful for reduction of metal oxide to metal.

 

 

At cathode:

At anode:

Graphite rods get burnt forming CO and CO2. The aluminium thus obtained is refined electrolytically using impure Al as anode, pure Al as cathode and molten cryolite as electrolyte.

At anode:

At cathode:

Electrolysis of molten NaCl:

At cathode:

At anode:

Thus sodium metal is obtained at cathode and  (g) is liberated at anode.

 

Refining

It is the process of converting an impure metal into pure metal depending upon the nature of metal.

 

Distillation

It is the process used to purify those metals which have low boiling points, e.g., zinc, mercury, sodium, potassium. Impure metal is heated so as to convert it into vapours which changes into pure metal on condensation and is obtained as distillate.

 

Liquation

Those metals which have impurities whose melting points are higher than metal can be purified by this method. In this method, Sn metal can be purified. Tin containing iron as impurities heated on the top of sloping furnace. Tin melts and flows down the sloping surface where iron is left behind and pure tin is obtained.

 

Electrolysis

In this method, impure metal is taken as anode, pure metal is taken as cathode, and a soluble salt of metal is used as electrolyte. When electric current is passed, impure metal forms metal ions which are discharged at cathode forming pure metal.
At anode:
At cathode:

 

Electrolytic Refining of Copper-

·         Anodes are of impure copper and pure copper strips are taken as cathode. The electrolyte is acidified solution of copper sulphate and the net result of electrolysis is the transfer of copper in pure form from the anode to the cathode:

·         Anode: Cu → Cu2+ + 2 e–

·         Cathode: Cu2+ + 2e– → Cu

·         Impurities present in metal are settled near the bottom of anode in the solution. Settled impurities in the solution are called Anode Mud.

 

 

Zone refining

It is based on the principle that impurities are more soluble in the melt than in the solid state of the metal. The impure metal is heated with the help of circular heaters at one end of the rod of impure metal. The molten zone moves forward along with the heater with impurities and reaches the other end and is discarded. Pure metal crystallizes out of the melt. The process is repeated several times and heater is moved in the same direction. It is used for purifying semiconductors like B, Ge, Si, Ga and In.

 

Vapour phase refining

In this method, crude metal is made free from impurities by first converting it Into its volatile compound by heating with a chemical reagent at low temperature. After this, the volatile compound is decomposed by heating to some higher temperature to give pure metal. Ni , Zr and Ti are refined by this method.

Mond’s process 

In this process, nickel is heated in a stream of carbon monoxide forming a volatile complex named as nickel tetracarbonyl. This complex is decomposed at higher temperature to obtain pure metal

Van-arkel method 

This method is very 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, volatilises:


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

 

Chromatographic methods

It is based on the principle of separation or purification by chromatography which is based on differential adsorption on an adsorbent. In column chromatography,  is used as adsorbent. The mixture to be separated is taken in suitable solvent and applied on the column. They are then eluted out with suitable solvent (eluent). The weakly adsorbed component is eluted first. This method is suitable for such elements which are available only in minute quantities and the impurities are not very much different in their chemical behaviour from the element to be purified.

 

Uses of Aluminium,  Copper, Zinc and Iron

  • Aluminium is used as a wrapper for food materials, in paints and lacquers, and in the extraction of chromium and manganese from their oxides.
  • Aluminium wires are used as electricity conductors, and alloys containing aluminium are light and useful.
  • Copper is used for making wires used in the electrical industry and for water and steam pipes. It is also used in several alloys like brass, bronze, and coinage alloy.
  • Zinc is used for galvanising iron, in large quantities in batteries, and is a constituent of many alloys like brass and german silver. Zinc dust is used as a reducing agent in the manufacture of dye-stuffs, paints, etc.
  • Cast iron is used for casting stoves, railway sleepers, gutter pipes, and toys. It is used in the manufacture of wrought iron and steel.
  • Wrought iron is used in making anchors, wires, bolts, chains, and agricultural implements.
  • Steel finds numerous uses, and alloy steel is obtained when other metals are added to it.
  • Nickel steel is used for making cables, automobiles, and aeroplane parts, pendulum, measuring tapes.
  • Chrome steel is used for cutting tools and crushing machines, and stainless steel is used for cycles, automobiles, utensils, pens, etc.