12 Class Chapter 6- General principle and process of isolation of the elements

General Principle and Process of Isolation of Elements

Minerals: - 

               These are naturally occurring chemical substances in the in the earth crust obtainable by mining.

Ores   : - 

       only few minerals are used as sources of that metal. Such minerals are known as ores.

Gangue: -

            ores are usually contaminated with earthly or undersides materials known as gangue.

Metallurgy: - 

                 The entire scientific and technological process used for isolation of the metal from its ores is known as metallurgy.

Concentration of the ore: - 

                                   The removal of earthy and siliceous impurities (i.e. gangue or matrix) from the ores is called concentration of ores.

(i)                Hydraulic washing: -

                                                 The processes by which lighter earthy particles are free from the heavier ore particles by washing with water.

(ii)             Magnetic separation: - 

                                         This method of concentration is  employed  when  either the ore or impurities associated with it are magnetic in nature.

Example: - chromate.

(iii)           Froth floatation: - 

                                       This method is based upon the fact that surface of sulphide ores is preferentially wetted by oils while that of gangue is preferentially wetted by water.

(iv)           Leaching: - 

                               This process consists in treating the powdered ore with a suitable reagent which are selectively dissolved the ore but not the impurities.

1)     Leaching of Alumina from bauxite: - 

                                                                   ore of aluminum, bauxite usually contain  SiO₂, iron oxide, Tio₂, as impurities .the concentration is carried out by digesting the powdered ore with the concentrated solution of  NaOH  at 473 to 523 K and 35-36 bar pressure. This way Al₂O₃ is leached out as sodium aluminates behind.

·        3H₂O+Al₂O₃+2NaOH   ---->  2Na[Al(OH)₄]

The aluminates in solution is neutralized by passing Co₂ gas and hydrated  Al₂O₃ is precipitated.

·        2Na[Al(OH)₄]+CO₂  ---->  Al₂O₃*xH₂O + 2NaHCO₃ 

NaHCO₃ remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al₂O₃.

2)     Other Examples: -    

                                   In the metallurgy of silver and that of gold , the respective  metal is leached with a dilute solution of NaCN or KCN  in the presence of air (for O₂) from which the metal is obtained later by replacement.

Where M = Ag or Au.

4[M(CN)₂]^- + Zn  ---->  [Zn(CN)₄]^2- + 2M

Extraction of crude metal from concentrated Ore :-

The concentrated are must be converted into a form which is suitable for reduction. The process used to obtain metals in Free State from concentrated ores is called extraction. It involves two chemical processes-

1.      Conversion of the ore into metallic oxide or de-electronation of ores: - 

                                  Metal usually present as hydroxides, carbonates and sulphides depending upon the nature of the minerals present in the ore, the following two present in the ore, the following two methods are used for conversion of ore into their respective oxides.

(a)  Calcinations: - 

                          It is the process of converting on ore into is oxide by heating it strongly below its melting point either in absent or limited supply of air during calcinations following change occur

(i)                Moisture is driven out

(ii)             Volatile impurities of  S, As and P  are removed as their volatile oxides

(iii)           Water is remove from hydrated oxides and hydroxide ores

(iv)           Carbonate ore are converted into their respective oxides by lose of  carbon dioxide

(b)  Roasting: - 

                   It is the process of converting an ore into its metallic oxide by heating strongly at temperature insufficient to melt in excess of air. This process is usually used for sulphide ores. Following change occur during roasting.

(i)                Moisture  is removed

(ii)             Organic matter   is destroyed

(iii)           Non metallic impurities like that of S, P and Ar are oxidized and removed   as volatile gases.

S₈ + 8O₂  ---->  8SO₂­ (Sulphur dioxide)

P₄ + 5O₂  ---->  P₄O₁₀­ (Phosphorus pentaoxide)

4As + 3O₂   ---->  2As₂O₃­ (Arsenious oxide)

(iv)           Ores are generally converted into metallic oxides

2ZnS (Zinc sulphide) + 3O₂   ---->   2ZnO (Zinc oxide) + 2SO₂­

2PbS (Lead oxide) + 3O₂    ---->    2PbO (Lead oxide) + 2SO₂­

2Cu₂S (Cuprous sulphide) + 3O₂    ---->  2Cu₂O (Cuprous oxide) + 2SO₂­

Like calcinations, roasting is also carried out in a reverberatory furnace

Thermodynamic principles of metallurgy: - 

                                                                     For any process, Gibbs free energy change (∆G) is given by

                                    ∆G = ∆H - T∆S

When ∆H is the enthalpy change and ∆S is the entropy change and T is the absolute change for any reaction

∆G^ϴ = -RT ln K

Where, k is the equilibrium constant of the reactant – product system at the temperature T, A negative ∆G implies a +ve K in equation and this can happen only when reaction proceeds towards product.

Applications:-

(a)  Extraction of iron from its oxides - 

                                               Oxides ores of iron after concentration through calcinations/ roasting are mixed with limestone and coke and fed into a blast blast-furnace from its top. Here, the oxides is reduced to the metals

                                FeO + C    ---->   Fe + CO

And FeO is reduced and C is oxidized to CO.

FeO   ---->   Fe + 1/2O₂

C + 1/2O₂    ---->   CO

At 500-800 K

3Fe₂O₃ + CO   ---->   2Fe₂O₄ + CO₂

 Fe₃O₄ + 4CO  ---->   3Fe + 4CO₂

 Fe₂O₃ +CO   ---->  2FeO + CO₂

At 900-1500 K

 C + CO₂   ---->  2CO

 FeO +CO   ---->  Fe +CO₂

Limestone is also decompose to CaO which remove silicate impurities of the ore as a slag

                                  CaCO₃  ---->  CaO + CO₂

                                  CaO + SiO₂ ---->  CaSiO₃ (slag)

The iron obtained from blast furnace contains about 4% carbon and many impurities in smaller amount (e.g. S, P, Si, Mn) this is known as pig iron.

(b)  Extraction of copper from cuprous oxide: - 

                                                                           Most of the ores are sulphide and some may also contain iron . The sulphide ores are roasted/ melted to give oxides.

2Cu₂S + 3O₂  ---->    2Cu₂O + 2SO₂

The oxide then can be easily reduced to metallic copper using coke.

Cu₂O + C  ---->   2Cu + CO

And slag of iron silicate is produced

                        FeO + SiO₂ ---->  FeSiO₃ (slag)

Following reaction also takes place

                        2FeS + 3O₂ ---->  2FeO + 2SO₂

                        FeO + SiO₂ ---->  FeSiO₃

2Cu₂S + 3O₂  ----> 2Cu₂O + 2SO₂

2Cu₂O + 2Cu₂S  ----> 6Cu + SO₂

(c)  Extraction of zinc from zinc oxide: - 

                                                              The reduction of zinc oxide is done by using coke

            The metal is distilled off and collected by rapid chilling

(d)  Extraction of Aluminum: - 

                                               Alumina Al₂O₃ is mixed with Na₃AlF₆ or CaF₂ which lowers the melting point of the mixture and brings conductivity. Fused mixture is electrolyzed. Steel cathode and graphite anode is used. The graphite anode is useful for the reduction of metal, the overall reaction are as follow-

2Al₂O₃ + 3C ----> 4Al + 3CO₂

            The electrolytic reactions are –

                                    Cathode: Al³⁺ + 3e^- ---->   Al

                                    Anode: C + O^2-  ---->  CO + 2e^-

                                                    C + 2O^2- ---->  CO₂ + 4e^-

(e)  Extraction of  copper  low grad ores and  scraps  :- 

                                                                                         Copper  is extracted  by  hydro metallurgy from low  grade  ores it is leached out using acid or bacteria the sol containing Cu²⁺   is treated with scrape  iron or hydrogen.

Cu²⁺ + H₂ ---->   Cu + 2H⁺

Oxidation reduction: - 

                                 Besides reduction, some extraction are based on oxidation particularly for non-metals. For extraction of chlorine from brine.

2Cl^-  + 2H₂O ----> 2OH^- + H₂ + Cl₂

Refining: -   

                The process of purifying the crude metals is called refining. Some methods of refining are as follow.

a)     Distillation: - 

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

b)     Liquation: - 

                       In this method a low melting metal like tin can be made to flow on a sloping surface, in this way it is separated from higher melting impurities.

c)     Electrolytic refining: - 

                                          In this method the impure metal is made to act as anode a strip of some metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the some metals the more basic metal remains in the solution and the less basic ones go to the anode mud.

Anode:    M   ----> Mⁿ⁺ + ne^-

Cathode:   Mⁿ⁺ + ne^-   ----> M

d)     Zone refining: - 

                               In this method a circular mobile heater is fixed at one end of a rod or the impure metal, as heater more forward, the pure metal crystallises out of the melt and impurities pass on into the adjacent molten zone.

e)     Vapour phase refining  :- 

                                            The crude metals is free from  impurities by first converting it into a suitable volatile compound by heating it with specific reagent at a lower temperature and then decomposing the volatile compound at some higher temperature   to give the pure metal.
Ex: - (1) Mond Process and (2)  van arkel method

f)      Chromatographic method: - 

                                               The method is based upon the principle that the different components of a mixture are adsorbed to different extents of an adsorbent.

For example in column chromatography, different components of the mixture are adsorbed to different extents depending upon their polarity.