Extraction Of Metals: Metallurgy

The process of extracting a metal in pure form from its ore is known as metallurgy. The process of treatment depends on upon the nature of the ore, impurities, and the metal.

From the extraction point of view, the metals can be broadly classified into five categories.

Category I :Highly electropositive metals such as alkali metals are extracted by the electrolytic method from their molten/fused salts.

Category II :Electropositive metals such as alkaline earth metals are extracted by electrolytic method form their molten salt.

Category III : Reactive metals of high valency are extracted by reduction and electrolytic method.

1. Concentration or Benefaction of the ore
2. Extraction of crude metal from the concentrated ore
3. Refining of the crude metal.

A. Concentration or Benefaction of the Ore

This is a very important in metallurgy. The ore is obtained from the earth and usually contain dust, stone, quartz, feldspar and other foreign materials.These impurities are known as gangue or matrix. The process of separation of gangue from ore is known as concentration. The important process of concentration is discussed below .

1. Crushing and grinding of the ore

   Big lumps of ore are first broken into small pieces by the means of mechanical crusher or by hammering. The crushed ore is then ground in a ball mill or a roller mill or a stamp mill. The powdered ore is passed through sieves of the particular size and remains of the sieves are further pulverized in a ball mill or a stamp mill to get ultimately fine dust like powder of theor.

2. Gravity separation or Levigation

   This method depends on the specific gravity of metallic ore and the earthy impurities. The powdered ore is washed with an upward stream of running water. The lighter gangue particles are washed away while the heavier ore particles settle down and are removed from the base. Generally, oxides like the tin stone (SnO₂), haematite (Fe₂O₃) are concentrated by this method.
   
   
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4. Magnetic Separation

   If the ore is attracted by the magnet (i.e ferromagnetic ores, as for example of iron ) then it is separated from the non-magnetic impurities by the means of magnetic separator. The mechanism of the magnetic separation is shown in the following figure :
   
   
   

   

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

5. Froth Floatation Process

   This is the method based on the principle that different solids have different behavior towards oil and water. Thus certain metal sulphides such as PbS, ZnS, CuFeS₂, etc. are wetted by certain oils like pine or eucalyptus oil or various organic sulphur compounds and not by only water. In this process, the powder ore is added to the mixture of pine oil and water and agitated by blowing air. The ore which is weighted by oil from froth and come to the surface while impurities settle below.This method is called froth and floatation method as in the figure :
   
   

   

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

6. Liquation :

   This method is used to concentrate ore having the lower melting point than the impurities. For example, stibnite, an ore of antimony which has low melting point, can be concentrated by this method. The powdered ore is heated on the slopping floor of a furnace and the temperature is adjusted to be just above the melting point of the ore. The ore melts and flows while the impurities are left behind .
   

   

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

7. Leaching :

   It is a chemical process in which powder ore is treated with suitable reagent. The ore gets dissolve which impurities ore dissolve. This process is called Leaching process.
   For example- Bauxite is treated with conc. NaOH solution , aluminium oxide is dissolved leaving behind insoluble impurities which are removed by filtration.
   
   Al₂O₃.2H₂O + 2NaOH→ 2NaAlO₂ + 3 H₂O
   
   NaAlO₂ + 2H₂O→ Al(OH)₃↓ + NaOH
   
   2Al(OH)₃→ Al₂O₃ + 3H₂O

B. Extraction of Crude metal from the concentrated ore:

The extraction of crude metals from the concentrated ore involves the following steps:

1. Calcination :

   In this process the concentrated ore is heated in the absence of air to a temperature below the melting points of metal. Calcination is done in a reverberatory furnace. In Calcination,
   a. Volatile orgainc matter is driven off.
   b. Moisture present on the ore is expelled
   c. The ore becomes porous
   d. Water is removed from the hydrated oxides ores.
   
   Al₂O₃.2H₂O (bauxite) → (clacination) Al₂O₃ + 2H₂O↑
   
   Fe₂O₃.3H₂O ( Limonite )→ (calcination) Fe₂O₃ + 3H₂O↑
   e. Carbonate ores are decomposed to their oxides
   ZnCO₃ (Zincite)→ ZnO + CO₂↑
   MgCO₃ (Magnesite)→ MgO + CO₂↑
   CaCO₃ (limestone)→ CaO + CO₂↑

2. Roasting

   In this process , the ore is heated alone or mixed with other materials under regular supply of air at a temperature below the melting point of the metal. Roasting is usually applied to sulphide ore. During roasting
   a. Moisture is driven away
   b.Volatile impurities are removed
   c. Organic matter is destroyed
   d. Sulphur, arsenic and other volatile substance present in the free or combined states are oxidized to volatile oxides.
   2Zns + 3O₂ → 2ZnO + 2SO₂↑
   4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂↑
   2HgS + 3O₂→ 2HgO + 2SO₂↑
   Sometimes ore is mixed with suitable material and then subjected to roasting in the absence of air,
   Ag₂S + 2NaCl → 2AgCl + Na₂S
   AgCl + 2Hg → AgHg + HgCl
   Roasting is generally carried out in a reverberatory furnace or in a blast furnace.

Fluxes

The ore even after concentration and calcination, still contain certain impurities which have been removed. There are certain substance known as flux which when mixed with calcinated ore combine with impurities to form easily fusible materials known as slag.

Impurity + Flux → Slag
These slag are not soluble. They are lighter than the molten metal and hence floats over its surface. These are two types of flux .

1. Acidic Flux
2. Basic Flux

C. Refining of Crude Metals

The metals obtained by any of these methods described before are seldom pure and hence need further purification. The choice of the purification method depends on the nature of metals and the impurities present. Some purification methods are discussed below.

1. Oxidative Refining :-

   This is usually where a metals has been prepared by reduction methods because other elements present have also been reduced at the same time. In this method, the impure metal is heated to a high temperature and then exposed to air when the impurities such as carvon , ohosphorous and arsenic etc. are oxidized to their volatile oxides which escape along with outgoing gases.

2. Distillation :-

   This is employed for volatile metals ( low boiling point ) like zinc ( 407ºC), mecury (356.58ºC) and cadmium (765ºC) etc . The impure metal is heated iin a retort and its vapour are separately condensed in a receiver. The pure metal distils over and the impurities (non-volatile) are left behind in the retort.

3. Liquation :-

   This method is used for the extraction of metals with low melting points eg. Tin and Lead. The impure metal is heated on the sloping hearth of a furnace when the molten metal flows away from the infusible impurities.

4. Electrolytic Refining:

   This method of purifying the impure metals is employed in the refining of chromium, copper, tin, zine, silver, lead, gold, aluminium, etc.
   In this method, the cathode is made of pure metal and impure metal is used as anode as in figure below :
   
   
   

   

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   Here, copper is refined by this process as
   Cu→ Cu⁺² + 2e^- ( at anode)
   Cu⁺² + 2e^-→ Cu ( at cathode )

References :-

Ghosh, A.K. Chemical Calculations. 15th Edition. India: Scientific Book Company, 1991.

M.L Sharma & P.N. Chaudhary. Advanced Level Chemistry. 2nd Edition . Vol. I. Kathmandu, Nepal: Ekta Books, 2011.

Palak, K.R. Fundamentals of Chemistry. Kathmandu, Nepal: Ratna Pustak Bhandar, 2000.

Pathak, Prof. Dr. Tulsi Prasad. Rectified Chemistry. Kalikasthan, Kathmandu : Airawati Prakashan (P.) Ltd., 2014.