How to extract crude metals from concentrated ores write full details

Extraction of crude metals from concentrated ores :

The process used to obtain metals in free state from concentrated ores is called extraction.

     Since many metals can be easily obtained from their respective oxides by reduction (electronation), therefore, the extraction of metals actually involves the following two chemical processes :

     1. conversion of the ore into metal oxide, i.e., de-electronation of ore.

     2. Reduction or electronation of the metal oxide to the free metal.

Conversion of the ore into metal oxide or de-electronation of ores :

Metals are usually present in ores as hydrated oxides (hydroxides), carbonates, and sulfides. Depending upon the nature of the minerals present in the ores, the following two methods are used to convert ores into their respective oxides.

(a) Calcination.

Calcination is the process of converting an ore into its oxide by heating it strongly below its melting point either in absence or limited supply of air.

    This method is commonly used to convert metal carbonates and hydroxides to their respective oxides. During the process of calcination, the following chemical changes occur :

(i) Moisture is driven out.

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

(iii) Water is removed from hydrated oxides and hydroxide ores.

            Al₂O₃.2 H₂O(Bauxite) → Al₂O₃ + 2 H₂O(Alumina) ;       Fe₂O₃.3 H₂O(Limonite)→ Fe₂O₃ + 3 H₂O

(iv) Carbonate ores are converted into their respective oxides by loss of carbon dioxide.

  CaCO₃(Limestone)→ CaO + CO₂(Calcium oxide) ;        CaCO₃.MgCO₃(Dolomite)→ CaO + MgO + 2 CO₂ ;

  CuCO₃.Cu(OH)₂(Malachite)→ 2CuO + H₂O + CO₂ ;          ZnCO₃(Clamine) → ZnO + CO₂

(v) It makes the ore porous and hence easily workable in subsequent stages. Calcination is usually carried out in a reverberatory furnace.

The concentrated ore is placed on the hearth of the furnace and heated by flames deflected from the roof.

(b) Roasting.

Roasting 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 commonly used for sulfide ores. The following changes occur during roasting :

(i) Moisture is removed.

(ii) Organic matter is destroyed.

(iii) Non-metallic impurities like sulfur, phosphorus, and arsenic are oxidized and are removed as volatile gases.

                    S₈ + 8 O₂ → 8 SO₂ (Sulphur dioxide)

                    P₄ + 5 O₂ → P₄O₁₀ (Phosphorus pentoxide)

                   4 As + 3 O₂ → 2 As₂O₃ (Arsenous oxide)

(iv) Ores are generally converted into metallic oxides.

       2 ZnS + 3 O₂ (Zinc sulphide) → 2 ZnO + 2 SO₂ (Zinc oxide) ;          2 PbS + 3 O₂ (Lead sulphide) → 2 PbO + 2 SO₂ (Lead oxide)

           2 Cu₂S + 3 O₂ (cuprous sulphide) → 2 Cu₂O + 2 SO₂ (cuprous oxide)

The SO₂ thus produced is used for manufacture of H₂SO₄.

(v) It makes the ore porous and hence easily workable in subsequent stages.

     Like calcination, roasting is also carried out in a reverberatory furnace. During roasting, air vents are kept open, while during calcination, air vents are either partially or completely closed.

   Flux. If the calcined or the roasted ore still contains non-fusible impurities of earthy matter, an additional substance called the flux is usually added during the reduction process. Thus,

Flux is a substance that chemically combines with gangue (earthy impurities) which may still be present in the roasted or the calcined ore to form an easily fusible material called the slag.

                                            Flux + Gangue → Slag

      The slag thus formed melts at the temperature of the furnace. It is insoluble in the molten metal and, being lighter, floats over the surface of the molten metal from where it can be skimmed off from time to time.

Types of fluxes. Depending upon the nature of the impurities present in the ore, fluxes are classified into the following two types :

(i) Acidic fluxes. For basic impurities like lime or metallic oxides (CaO, FeO, MnO, etc.) present in the ore, acidic fluxes like silica (SiO₂) and borax (Na₂B₄O₇.10 H₂O), etc. are used.

                      CaO            +               SiO₂                     →                 CaSiO₃

                      FeO            +               SiO₂                     →                 FeSiO₃                                                                (Acidic impurities)              (Basic flux)                                 (Fusible slag)

(ii) Basic fluxes. For acidic impurities like silica (SiO₂), phosphorus pentoxide (P₄O₁₀), etc., present in the ore, basic fluxes like limestone (CaCO₃), magnetize (MgCO₃), hematite (Fe₃O₄), etc. are used.

                    SiO₂                +            CaCO₃              →                  CaSiO₃         +            CO₂

                    SiO₂             +            MgCO₃              →                 MgSiO₃         +            CO₂                                       (Acidic impurities)             (Basic flux)                            (Fusible slag)