General Principles And Processes Of Isolation Of Elements

The ore can be concentrated by the process of magnetic separation, only if either the ore or the gangue can be attracted in presence of magnetic field. In table 6.1, the ores of iron such as haematite (Fe₂O₃), magnetite (Fe₃O₄), siderite (FeCO₃), and iron pyrite (FeS₂) can be separated by the process of magnetic separation.

The method of leaching consists of treating the powdered ore with a suitable reagent which can selectively dissolve the ore but not the impurities. The impurities are filtered out and are recovered from the solution. For example, bauxite ore containing SiO₂, iron oxide, and titanium oxides impurities are concentrated by this method.

Leaching is significant as it helps in removing the impurities like SiO₂, FeO₂, TiO₂, etc from the bauxite ore.

The reaction,

Cr₂O₃ + 2 Al → Al₂ O₃ + 2 Cr (ΔG⁰ = – 421 kJ)

Is thermodynamically feasible as is apparent from Gibbs energy value. The change in Gibbs free energy is related to the equilibrium constant, k as

ΔG = - RT ln K

A certain amount of energy activation is required even for such reactions which are thermodynamically feasible, therefore heating is required.

ΔG = ΔH + S

Increasing the temperature increases the value of TΔS, making the value of ΔG more and more negative. Therefore, the reaction becomes more and more feasible as the temperature is increased.

Copper can be extracted by hydrometallurgy but not zinc as per the below-given reason:

The E^o of zinc ( Zn²⁺/Zn = - 0.76V) is lower than that of copper (Cu²⁺/Cu = 0.34V). this means that zinc is a stronger reducing agent and can displace copper from a solution of Cu²⁺ ions.

Zn (s) + Cu²⁺ (aq) → Zn²⁺ + Cu (s)

In order to displace zinc by hydrometallurgy, we need stronger reducing agent like K ( E^o_K+/K = - 2.93V)

2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g)

As a result, these metals cannot be used in hydrometallurgy to extract zinc. Hence, copper can be extracted by hydrometallurgy but not zinc.

In the froth floatation method, the role of depressants is to prevent certain types of particles from forming the froth with the air bubbles. For example, NaCN is used as a depressant to separate lead sulfide ore (PbS) from zinc Sulphide ore (ZnS).NaCN forms a zinc complex Na₂[Zn(CN)₄] on the surface of ZnS thereby preventing it from the formation of froth.

The Gibbs free energy formation ( Δ_fG) of Cu₂S is less than that of H₂S and CS₂. Therefore, H₂ and C cannot reduce Cu₂S to Cu.

On the other hand, the Gibbs free energy formation of Cu₂O is greater than that of CO. hence, C can reduce Cu₂O to Cu.

C (s) + Cu₂O (s) → 2Cu (s) + CO (g)

Hence, the extraction of copper from its pyrite ore is difficult than from its oxide ore through reduction.

Zone refining method is based on the principle that impurities are more soluble in the molten state of metal (the melt) than in the solid state. The impure metal is heated with the help of a circular mobile heater at one end. This results in the formation of the molten zone or melt. As the heater is removed along with the length of the rod, the pure metal crystallizes out of the melt and impurities pass into the adjacent molten zone. This process is repeated several times till the impurities are completely driven to the end of the rod which is then cut off and discarded. The method is very useful for semiconductor and other metals of very high purity, e.g., silicon, germanium, boron, and gallium.

It is based on the principle that different components of a mixture are differently adsorbed on an adsorbent. The mixture to be separated is put in a liquid or gaseous medium which is moved through the adsorbent. Different components are adsorbed at different levels on the column. Later, the adsorbed components are removed (eluted) by using suitable solvents (eluents). There are several chromatographic techniques such as paper chromatography, column chromatography, gas chromatography, etc. chromatography is used for the purification of elements which are available in minute quantities and the impurities are not very different in chemical properties from the element to be purified.

At 673K, the value of G_(CO,Co2) is less than that of G_(C,Co). Therefore, CO can be reduced more easily to Co₂ than C to CO. hence, CO is a better reducing agent than C at 673K.

The common elements present in anode mud in electrolytic refining are antimony, selenium, tellurium, silver, gold, and platinum. These elements being less reactive, are not affected by CuSO₄ + H₂SO₄ solution and hence settle down under anode as anode mud.

During the extraction of iron, the reduction of iron oxides takes place in the blast furnace. In the process, hot air is blown from the bottom of the furnace and coke is burnt to raise the temperature up to 2200K in the lower portion itself. The temperature is lower in the upper part. Thus, it is the lower part where the reduction of iron oxides (Fe₂O₃ and Fe₃O₄) takes place.

The reactions taking place in the lower temperature range (500K – 800K ) in the blast furnace are:-

3 Fe₂O₃ + CO → 2 Fe₃O₄ + CO₂

Fe₃O₄ + 4 CO → 3 Fe + 4 CO₂

Fe₂O₃ + CO → 2 Fe₃ + CO₂

The reactions taking place in the higher temperature range ( 900 – 1500K ) in the blast furnace are:-

C + CO₂→ 2 CO

FeO + CO → Fe + CO₂

The silicate impurity of the ore is removed as a slag by calcium oxide ( CaO), which is formed by the decomposition of limestone (CaCO₃).

CaCO₃→ CaO + CO₂

CaO + SiO₂→ CaSiO₃ (Calcium silicate/ slag)

The steps involved in the extraction of zinc from zinc blende (ZnS) are as followed:-

1) Concentration: the ore is crushed and then concentrated by froth floatation process.

2) Roasting: the concentrated ore is heated in the presence of an excess of air at about 1200K to form zinc oxide.

ZnS + 3 O₂→ 2 ZnO + 2 SO₂

3) Reduction: ZnO obtained above is mixed with powdered coke and heated to 1673K in a fireclay resort.

4) Electrolytic Refining: Zinc is refined by the process of electrolytic refining. In this process, impure zinc is made the anode and a pure copper strip is made the cathode. The electrode used in an acidified solution of zinc sulphate (ZnSO₄). Electrolysis results in the transfer of zinc in pure form from the anode to the cathode.

During the roasting of pyrite ore, a mixture of FeO and Cu₂O is obtained.

2 CuFeS₂ + O₂→ Cu₂S + 2 FeS + SO₂

2 Cu₂S + 3 O₂ → 2 Cu₂O + 2 SO₂

2 FeS + 3 O₂ → 2 FeO + 2 SO₂

The role of silica in the metallurgy of copper is to remove the iron oxide obtained during the process of roasting as ‘slag’. If the sulphide ore of copper iron, then silica (SiO₂) is added as a flux before roasting. Then, FeO combines with silica to form silicate, FeSiO₃ (slag).

FeO + SiO₂→ FeSiO₃ (slag)

Chromatography is a term used for numerous laboratory techniques for the separation of mixtures. The term is derived from Greek word ‘chroma’ meaning ‘color’ and ‘graphein’ meaning ‘to write’. Chromatographic techniques are based on the principles that different components are adsorbed differently on an adsorbent. There are various chromatographic techniques used such as paper chromatography, column chromatography, gas chromatography, etc.

The components selected for the stationary phase should have different solubility’s in the phase, only then it is selected as the stationary phase. Hence, different components have different rates of movement through the stationary phase, as a result, can be separated from each other.

Nickel is refined by Mond’s process. In this process, nickel is heated in the presence of carbon monoxide to form nickel tetracarbonyl, which is a volatile complex.

To separate alumina from silica in bauxite ore associated with silica, firstly, the powdered ore is digested with a concentrated NaOH solution at a temperature of 473 – 523K and 35 – 36 bar pressure. This results in the leaching out of alumina (Al₂O₃) as sodium aluminate and silica (SiO₂) as sodium silicate leaving the impurities behind.

Al₂O₃ (s) + 2NaOH (aq) + 3H₂O (l) → 2 Na[Al(OH)₄] (aq)

Alumina

SiO₂ + 2NaOH (aq)→ Na₂SiO₃ (aq) + H₂O (l)

Silica

After this, CO₂ gas is passed through the resultant solution to neutralize the alumina present in the solution, this results in the precipitation of hydrated samples of alumina. To include precipitation, the solution is seeded with freshly prepared samples of hydrated alumina.

During this process, sodium silicate remains in the solution. The hydrated alumina obtained is then filtered, dried, and heated to get back pure alumina.

The difference between roasting and calcination is:

The iron obtained from blast furnaces is known as pig iron. It contains around 4% carbon and many impurities such as S, P, Si, Mn in smaller amounts.

Cast iron is obtained by melting pig iron and coke using a hot air blast. It contains a lower amount of carbon (3%) than pig iron, cast iron is extremely hard and brittle.

The naturally occurring substances in the form of which metals occur in the earth’s crust along with impurities are called as minerals. The mineral from which the metal can be extracted conveniently and profitably is called as an ore. Thus, all ores are minerals but all minerals are not ores. For example, aluminium occurs in earth’s crust in the form of bauxite and clay. Out of these two minerals, aluminium can be conveniently and economically extracted from the bauxite. Therefore, bauxite is the ore of aluminium. Similarly, Zinc can be obtained from blende (ZnS), calamine (ZnCO₃), Zincite (ZnO) etc.

Thus these minerals are called ores of zinc.

Copper matte contains Cu₂S and FeS. When a blast of hot air is passed through molten matte took in a silica lined convertor, FeS present in matte is oxidized to FeO which combines with Silica (SiO₂) to form FeSiO₃, slag.

When the whole of iron has been removed as slag, some of the Cu₂S undergoes oxidation to form Cu₂O which then reacts with more Cu₂O to form copper metal.

Cryolite (Na₃AlF₆) has two roles in the metallurgy of aluminium:

1) It lowers the melting point of the mixture to about 1140K.

2) It increases the electrical conductivity of the mixture.

For the group of low-grade copper ores, leaching is carried out using acid or bacteria in the presence of air or oxygen. In this process, copper goes into the solution as Cu²⁺ ions.

The resulting solution is then treated with scrap iron or H₂ to obtain metallic copper.

The standard Gibbs free energy formation of ZnO from Zn is lower than that of CO₂ from CO. That is why CO cannot reduce ZnO to Zn. thus, Zn is not extracted from ZnO through reduction using CO.

The value of Δ _fG⁰ for the formation of Cr₂O₃ is – 540 kJ mol^-1 which are higher than that of Al₂O₃ is – 827 Kjmol^-1. Thus, Al can reduce Cr₂O₃ to Cr. Hence, the reduction of Cr₂O₃ with Al is possible.

The free energy formation ( Δ_fG^o ) of CO from C becomes lower at temperatures above 1120K whereas that of CO₂ from C becomes lower above 1323K than Δ_fG⁰ of ZnO. However, Δ _fG⁰ of CO₂ from CO is always higher than that of ZnO. Therefore, C can reduce ZnO to Zn but not CO. therefore, out of C and CO, C is a better reducing agent than CO for ZnO.

Thermodynamic factors help us in choosing a suitable reducing agent for the reduction of a particular metal state as described below.

From Ellingham diagram, it is evident that metals for which the standard free energy of formation of their oxides is more negative can reduce those metal oxides for which the standard free energy of formation of their respective oxides is less negative. In other words, any metal will reduce the oxides of other metals which lie above in the Ellingham Diagram because the standard free energy change of the combined redox reaction will be negative by any amount of equal to the difference in Δ _fG⁰ of the two metal oxides. Hence, both Al, Zn can reduce FeO to Fe, but Fe cannot reduce Al₂O₃ to Al and ZnO to Zn.

In the electrolysis of NaCl by Down’s process, chlorine is obtained as a by-product. This process involves the electrolysis of a fused mixture of NaCl and CaCl₂ at 873K. during electrolysis, sodium is liberated at the cathode and Cl₂ is liberated at the anode.

If an aqueous solution of NaCl is electrolysed, H₂ is evolved at the cathode and Cl₂ is obtained at the anode. the reason being that E⁰ of Na⁺/Na redox couple is much lower (E⁰ = - 2.71 V) than that of H₂O (E⁰_H2O/H2) = - 0.83V ) and hence water is reduced to H₂ in presence of Na⁺ ions. However, NaOH is obtained in the solution.

Graphite rod acts as anode and graphite lined iron acts as a cathode in the electrometallurgy of aluminium. Carbon reacts with oxygen liberated at anode producing CO and CO₂ otherwise oxygen liberated at the anode may oxidize some of the liberated aluminium back to Al₂O₃.

(i) Zone refining Is the method based on the principle that the impurities are more soluble in the melt than in the solid state of the metal.

(ii) Electrolytic refining works on the principle of refining impure metals by the use of electricity. In Electrolytic refining, the impure metal is made the anode and a strip of pure metal is made as the cathode. A solution of a soluble salt of the same metal is taken as the electrolyte. When an electric current is passed, metal ions from the electrolyte are deposited at the cathode as a pure metal and the impure metal from the anode dissolves into the electrolyte in the form of ions. The impurities present in the impure metal gets collected below the anode.

This is called as the anode mud.

(iii) Vapour refining is the process of refining metal by converting it into its volatile compound and then, decomposing it to obtain a pure metal. To carry out this process,

i) the metal should form a volatile compound with an available reagent, and

ii) The volatile compound should be easily decomposable so that the metal can be easily recovered.

iii) Nickel, Zircinium. And titanium are refined using this method.

Above 1350^oC, the standard Gibbs free energy formation of Al₂O₃ from Al is less than that of MgO from Mg. Therefore, above 1350^oC, Al can reduce MgO.