EXTRACTION OF METALS

Attainment of pure metal from its compound is called extraction of metals. Some metals are found in earth’s crust in free state while some are found in the form of their compounds. The extraction and isolation of an element from its combined form involve various principles of chemistry.

METALLURGY

The entire scientific and technological process used for extraction and isolation of the metal from its ore, is known as metallurgy. This process can be three types:

1- Hydrometallurgy– The process of extraction of metals by dissolving in aqueous solution, is called hydrometallurgy.

2- Pyrometallurgy– The process of extraction of metals by heating with the suitable reducing agent, is called pyrometallurgy.

3- Electrometallurgy– The process of extraction of metals by using electricity, is called electrometallurgy.

TERMS USED IN METALLURGY

MINERALS– The naturally occurring chemical substances in the earth’s crust obtainable by mining, are called minerals. e.g., Bauxite and Kaolinite.

ORE– Minerals from which metals can be extracted through various steps conveniently and profitably, are termed as ores. All ores are minerals but all minerals are not ores. e.g., Iron pyrite is a mineral of iron but cannot be considered as its ore because extraction of iron from it, is very costly.

ORES OF SOME IMPORTANT METALS

SODIUM (Na)

– Common salt or Brine (NaCl)

– Sodium carbonate (Na₂CO₃ . 10H₂O)

– Chile Saltpeter or Sodium nitrate (NaNO₃)

ALUMINIUM (Al)

– Bauxite (Al₂O_{3 .} 2H₂O)

– Cryolite (Na₃AlF₆)

– Feldspar (KAlSi₃O₈)

– Corundum (Al₂O₃)

– Diaspore (Al₂O_{3 .} H₂O)

POTASSIUM (K)

 

– Potassium chloride (KCl)

– Carnalite (KCl . MgCl_{2 .} 6H₂O)

– Potassium carbonate (K₂CO₃)

– Potassium nitrate (KNO₃)

MAGNESIUM (Mg)

– Magnesite (MgCO₃)

– Dolomite (MgCO₃ . CaCO₃)

– Epsomite (MgSO_{4 .} 7H₂O)

CALCIUM (Ca)

– Calcite (CaCO₃)

– fluorspar (CaF₂)

 

COPPER (Cu)

– Cuprite (Cu₂O)

– Copper glance or chalcocite (Cu₂S)

– Copper pyrite (CuFeS₂)

– Malachite (Cu(OH)_{2 .} CuCO₃)

– Azurite (Cu(OH)_{2 .} 2CuCO₃)

SILVER (Ag)

– Ruby silver (3Ag₂S . Sb₂S₃)

– Horn silver or silver chloride (AgCl)

– Argentite (Ag₂S)

ZINC (Zn)

– Zinc blende (ZnS)

– Calamine (ZnCO₃)

– Zincite or Zinc oxide (ZnO)

– Willemite (Zn₂SiO₄)

MERCURY (Hg)

– Cinnabar (HgS)

 

GOLD (Au)

– Calaverite (AuTe₂)

– Auric chloride (AuCl₃)

 

LEAD (Pb)

– Galena (PbS)

– Cerrusite (PbCO₃)

 

IRON (Fe)

– Haematite (Fe₂O₃)

– Magnetite (fe₃O₄)

– Siderite (FeCO₃)

– Limonite (2fe₂O₃ . 3H₂O)

– Iron pyrite (FeS₂)

GANGUE– The earthern impurities like soil, sand, etc present in the ores, is called gangue or matrix.

STEPS INVOLVED IN METALLURGY

It involves the following sequence of steps:

CONCENTRATION OF ORES

Removal of the unwanted materials (gangue or impurities) from ore is called ore concentration, dressing or benefaction. It can be done by hand picking but several other techniques can also be used, description of which are as follows:

1- HYDRAULIC WASHING – This is based on the differences in gravities of the ore and the gangue particles. It is, therefore a type of gravity separation. In this process, an upward stream of running water is used to wash the powdered ore. The lighter gangue particles are washed away and the heavier ores are left behind. e.g., Ores of iron (Fe₂O₃ and Fe₃O₄) are concentrated by this process.

2- MAGNETIC SEPARATION – It is based on the differences in magnetic properties of the ore and gangue. e.g., ores of tungsten from cassiterite (SnO₂), Fe₃O_4, Cr₂O_3, etc are concentrated by this method.

3- FROTH FLOATATION METHOD – It is used for removing gangue from sulphide ores. Collectors (pine oil) and froth stabilizers (cresols) are added to the suspension of powdered ore in water. Ore particles become wet by oils and come with froth which is skimmed off.

4- LEACHING – It is used if the ore is soluble in some suitable solvents. It is recovered from the solution while impurities are insoluble. e.g., leaching of bauxite ore is done with conc. NaOH solution, silver and gold are leached with dilute solution of sodium cynaide or potassium cyanide.

EXTRACTION OF CRUDE METALS FROM CONCENTRATED ORES

Isolation of metals from concentrated ores involves two major steps:

1- CONVERSION TO OXIDES

It is acheived by the following two proceses.

• CALCINATION: It involves heating of ore with or without the presence of air when the volatile matter escapes leaving behind the metal oxide. It is done in case of carbonate, hydroxide and oxide ores of metals. These substances get decomposed to respective oxides with the evolution of small molecules.
• ROASTING: It is done only by sulphide ore. The ore is heated in the regular supply of air at a temperature below the melting point of metal, to convert it into oxide ore. During this process the impurities of sulphur, arsenic, etc get oxidised and escape as volatile impurities leaving behind the respective metal oxide.

e.g.,            CuFeS₂ + 3O₂ ——–> CuO + FeO + 2SO₂

2- REDUCTION OF OXIDES TO METALS

SMELTING – It is the reduction of carbon to its oxides. i.e., CO. Metals in the middle of activity series are reduced by this process. It is the process of heating an ore above its melting point with coke and flux is added. Oxides of lead, iron, tin, zinc, etc are reduced by this process. this process is used to fuse the whole mixture and reduce metal oxide to metal.

e.g.,            PbO + CO ———-> Pb (molten lead) + CO₂ ^

FLUX is the extra substance added to the ore during the process of smelting. Infusible materials present in the ore is converted by the flux into a fusible substance.

e.g.,            In smelting of iron, limestone (CaCO₃) is used as a flux.

A flux may be acidic or basic. An acidic flux like SiO₂ is used to remove basic impurities and a basic flux like CaO, FeO, etc is used to remove acidic impurities.

The acidic or basic impurities present in metal oxide ore are converted into fusible substance called slag on addition of flux.

                Flux + Acidic/Basic impurities ——–> Slag

The slag being lighter and fusible floats above the surface of molten metal. e.g., Ore of Fe contains SiO₂ as impurity which combine with flux CaO (formed from CaCO₃ at 1000 ^oC) to form CaSiO₃ (slag).

e.g.,        SiO₂ + CaO ——–> CaSiO₃

• ALUMINOTHERMIC PROCESS – Oxides of metals such as chromium and manganese are reduced by aluminium.

e.g.,       Cr₂O₃ + 2Al ———–> Al₂O₃ + 2Cr

• AUTO REDUCTION – It is used for metals low in the activity series. The oxides of these metals such as Cu, Hg, Pb can be reduced to metals by heating alone.
• ELECTROLYTIC METHOD – It is used for metals high in activity series such as Na, Al, Ca, etc. Their oxides are reduced to metals by this process. Sodium is obtained by electrolysis of Sodium chloride.

REFINING

A metal extracted by any method is usually contaminated with some impurities. The process of extraction of pure metals from their impure or crude form under suitable conditions, is called refining of metals. It depends upon the nature of elements as well as the nature of associated impurities. For obtaining metals of high purity, several techniques are used. These techniques are listed below:

• DISTILLATION– Purification of Zn, Cd, Hg metals is done by this method, as they are the metals low boiling points.

• LIQUATION– Metals such as tin, mercury, lead are purified by this method as they have low melting points.
• ELECTROLYTIC REFINING– Metals such as copper, silver, gold, aluminium and lead are purified by this method. In this method impure metal acts as anode while pure metal acts as cathode and salt of metal acts as electrolyte.
• ZONE REFINING– This method is useful for producing semiconductors and other metals with very high purity. e.g., germanium, silicon, boron, gallium and indium. It is based on the principle that impurities are soluble in the molten than in the solid state of metal.

• VAPOUR PHASE REFINING– In this method the metal is converted into its volatile compound and then it is decomposed to give pure metal.

e.g.,

Mond’s process It is used for Nickel (Ni)

Ni (Impure) + CO ——–>Ni(CO)₄ ———> Ni (Pure) + 4CO

Van Arkel process It is used for Zirconium (Zr) & Titanium (Ti)

Ti (Impure) + 2I₂ ———->TiI₄ ———–> Ti (Pure) + 2I₂

• CHROMATOGRAPHIC METHOD– It is based on the principle that different components of mixture are differently absorbed.

POINTS TO REMEMBER

1- Borax or KNO₃ is used as flux in refining of silver.

2- Wrought or malleable iron is the purest form of commercial iron.

3- Cast iron is used in the manufacture of wrought iron and steel.

4- A mixture of Fe₂O₃ and Al in 3:1 is called thermite mixture and is used for welding railway lines.

5- Alkali and alkaline earth metals are obtained by the electrolysis of their chlorides, oxides or hydroxides.