Extraction of Metals

• Introduction to General Principles of Extraction of Metals
• Extraction of Metals:Principlesand Procedures involved inMetal Extraction.
• Guide for students to solve theExtraction of Metalsproblem

 Extraction of metals from its ores is known as isolation of crude metal. Basically particular method depends on the nature of metal ore (sulphide ore, carbonate ore, bicarbonate ore, sulphide ore etc.). Metallurgy of a metal involves main processor-                             

Ø  Concentration of ore: First crush ore into small pieces with the help of crushers and then finally into a fine powder, known as Pulverisation. Ores usually contain undesired impurities called Gangue or matrix so the removal of these  impurities from the ores is known as concentration or dressing of ore. Ore crushed into pieces and then removal of impurities has to be done by different separation methods.

(A) Gravity Separation

(B)  Electromagnetic Separation

(C)  Froth Floatation Process

(D) Leaching                    

(A) Gravity Separation- Basically this method is based on the difference in the specific gravity of gangue. In this method powdered ore is agitated with water (by running water). By stream of water heavy ore particles settle down and separate it from lighter impurities.

(B)   Electromagnetic Separation- When one of them in between ore and impurity is magnetic in nature By using magnetic separator , consist of two rollers (one of which is magnetised strongly), separate magnetic particles from non-magnetic particles.

(C)  Froth Floatation Process-This method basically used for sulphide ores. Separation of ore from gangue based on the wettabilities by a liquid solution. Powdered ore is added into water having frothing agent (Pine oil) and collecting agent (Sodium ethyl xanthate) . By passing stream of air , there is the separation of sulphide particles in the form of froth from gangue particles which sink to the bottom.

(D) Leaching- A chemical separation which involves treatment of ore by using a suitable reagent to make it soluble. When insoluble impurities are separated from soluble ore , it can be filtered.

(Bayer's Process ) Bauxite  ore of Aluminium contains ferric oxide , titanium oxide and silica as impurities. When powdered ore is treated with Aqueous solution of NaOH under pressure, then alumina is converted into sodium aluminates by leaving ferric oxide (Fe₂O₃), TiO₂ and Silica as insoluble part.

Al₂O₃.2H₂O + 2NaOH + H₂O → 2Na[Al(OH)₄ ]

                                             Sodium aluminates (soluble)

Now separate the soluble part from insoluble. Filtrate is neutralized by passing CO₂ gas.

2Na[Al(OH)₄ ] + 2CO₂ → Al₂O₃.xH₂O + 2NaHCO₃

Hydrated alumina is dried by heating after filtration to get pure alumina.

Al₂O₃.xH₂O → Al₂O₃ + xH₂O

Ø  Isolation of metal : On the basis of the nature of ore there are two methods -

(A) Conversion of ore into metal oxides

(B)  Formation of free metal by the reduction of the metallic oxide

(A) Conversion of ore into metal oxides- Involves following steps-

(i)  Calcinations (fusion of ore  in absence of air )

(ii) Roasting (heating in the presence of air)

(i)  Calcination - Involves heating of the hydrated oxide and carbonate ore in absence of air.

Al₂O₃.2H₂O  → Al₂O₃ + 2H₂O

 Alumina

CaCO₃   → CaO + CO₂

            Calcium oxide

(ii)Roasting - This step involves heating (below the m.p. of ore) in presence of air.

Sulphide ores are converted into metallic oxides by oxygen.

 2ZnS + 3O₂ →  2ZnO + 2SO₂

                        Zinc oxide

In the extraction of copper from copper pyrites. Copper is obtained in the form of Cu₂S.

2CuFeS₂ → Cu₂S + 2FeS + S

Most of them FeS is converted into FeO.

(B)  Formation of free metal by the reduction of the metallic oxide- By using suitable reducing agents on the basis of reactivity of the metal, calcinated and roasted ore is converted into crude metal. Few methods are used to get free metal-

(i)                 Smelting - At high temperature roasted ore is converted into free metal. For the extraction of less electropositive metal like Pb, Zn, Fe etc., powerful reducing agent like C, H₂, CO are used.

PbO + C → Pb + CO

Fe₂O₃ + 3C → 2Fe + 3CO

Ores are also contaminated with gangue even ater concentration. Now to remove gangue, it is mixed with a substance to form fusible material (insoluble in molten metal) , known as Flux and fusible material is known as Slag( lighter  so floats on the surface of molten metal). Flux are of two types -Acidic and Basic Flux ( basic difference is only the separation medium).

(ii) Thermite Process-  Those metal, which having high m.p., are extracted by mixing it with aluminium powder, called thermite. 

Cr₂O₃ +2Al → 2Cr + Al₂O₃

(iii)             Self Reduction process- No external reducing agent is used. Less electropositive metals like Pb, Cu etc., are heated in air to convert into its ore into oxide and sulphide, finally it is converted into metal.

Extraction of lead from Galena ore

PbS + 3O₂ → 2PbO + 2SO₂

2PbO + PbS → 3Pb +SO₂

(iv) Electrolytic Reduction -

Highly electropositive metals oxide cannot be reduced by using carbon at moderate temperature. These metals are extracted by using electrolysis of their oxides, chlorides and hydroxides.

Sodium is extracted by electrolysis of NaCl and CaCl₂ (Down's process) and by fused NaOH (Castner's process).

NaCl  ↔ Na⁺  + Cl^-

Na⁺ + e → Na (at Cathode)

Cl^- -  e  → Cl

2Cl  → Cl₂ ( at Anode )

Ø  Purification - Impure metal is converted into pure metal by different refining process like Liquefaction, Distillation and electrolytic refining. 

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