Mineral

The naturally occurring substances in the form of which the metals occur in the earth’s crust are called minerals.  

Ore  

The mineral from which  a metal can be extracted profitably and conveniently is called an ore.

Gangue

The earthy impurities like sand, rock, etc., that surround the worthy mineral in a ore, are called called gangue.

Metallurgy

The scientific and technological process used for separating metal from its ore is known as metallurgy.

Extraction of metals

The extraction of metals involves three major steps:

(i) Concentration of the ore

(ii) Isolation of the metal from its concentrated ore

(iii) Purification of the metal

Concentration of ores

The process of removal of unwanted materials from the ore is called concentration or benefaction of the ore.

It can be carried out by various steps as stated below:

Hydraulic washing: In this method, the lighter earthy impurities are washed away from the heavier ore particles.  Thus this method of concentration of ore is based on the difference in specific gravities of the ore and gangue particles.

Magnetic separation: This method is based on the magnetic and non-magnetic properties of the ore components.

Froth flotation: This method is quite useful for the purification of the sulphide ores. The mineral particles are wetted by oils and the gangue particles by water. As a result. the ore particles become light and rise to the top in the form of froth while the gangue particles become heavy and settle down. The froth can be stabilised by the addition of stabilisers like aniline or cresols.

Extraction of crude metal from concentrated ore  

Concentrated ore is usually converted to oxide before reduction, as oxides are easier to reduce. Thus, extraction of crude metal from concentrated ore involves two major steps:

(i) Conversion to oxide.

(ii) Reduction of the oxides to metal

Conversion to oxide

It can be carried out by following two methods:

Calcination: It is the process of converting an ore into its oxide by heating it in a limited supply of air or in absence of air, below its melting point. The volatile matter is burnt away and the oxide of the metal is obtained. This process is useful for metal carbonates and hydroxides. For example: CaCO3 → CaO + CO2  and  Al2O3.2H2O → Al2O3 + 2H2O.

Roasting: It is the process of converting an ore into its oxide by heating the ore in excess of oxygen (air). This process is commonly used for suiphide ores.

For example: 2PbS + 3O2 → PbO + 2 SO2 and 2 Cu2S + 3O2 → 2 Cu2O + 2 SO2

Reduction of metal oxides to metal

The metal present in metal oxide can be converted from cationic form to free by supplying electrons, i.e., by reduction of metal oxide. The nature of reducing agent used depends upon the activity of metal.

For example: If the metal is very reactive like Na, K, electrolytic reduction method is used whereas the less reactive metals like Cu, Sn, Fe can be reduced by chemical reducing agents like CO, H2, etc.

Reduction by carbon (Smelting):

The process of using carbon in form of coke, charcoal, CO to reduce metal oxides to respective metals, is termed as smelting.

For example: Fe2O3 + 3 CO → 2 Fe + 3CO2  

Reduction by hydrogen:

Because of highly inflammable nature of H2 it is used as a reducing agent especially for oxides of Tungsten (W) and Molybdenum (Mo).

For example: MO3 + 3H2 → M + 3H2O

Reduction by Aluminium:  

Reduction of metal oxides to respective metals by using aluminium is known as alumino thermite or Gold Schmidt thermite process. It is mainly used to reduce Cr2O3 or Fe2O3

For example: Cr2O3 + 2Al → Al2O3 + 2Cr

Electrolytic refining:

This method is used for Cu, Au, Ag, Pb, zinc, aluminium etc. The impure metal is made anode and pure metal is cathode. The following reaction takes place:

                                       Anode: M → Mn+ + ne‒

                                            Cathode: Mn+ + ne‒ → M

The impurities are collected at the bottom of anode and are called anode mud.

Zone refining method:

It is the method used to get elements of very high purity like Ge, Si, B, Ga. The method is based on the principle that the impurities are more soluble in the melt than in the pure metal. The method is specially useful for producing semiconductors of very high purity.

Distillation:

This method of purification is useful for low boiling metals. Examples are purification of zinc and mercury. Liquation In this method of purification, a low melting metal like tin can be made to flow on a sloping surface and thus separated from higher melting impurities.

General Principles & Processes of Isolation of Metals

Types of Ores:

Ores may be divided into four groups

Native Ores:  These ores contain the metal in free state eg. Silver gold etc. These are usually formed in the company of rock or alluvial impurities like clay, sand etc.

Oxidised Ores:These ores consist of oxides or oxysalts (eg. carbonates, phosphate) and silicate of metal. Important oxide ore includes, Fe2O3, Al2O3.2H2O etc. and important cabonate ores are limestone (CaCO3), Calamine (ZnCO3) etc.

Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc. Examples are iron pyrites (FeS2). galena (PbS), Cinnabar (HgS)

Halide ores: Metallic halides are very few in nature. Chlorides are most common examples include horn silver (AgCl) carnallite KCl. MgCl2.6H2O and fluorspar  (CaF2) etc.

Metallurgy:  

It is the process of extracting a metal from its ores. The following operations are carried out for obtaining the metal in the pure form.

Physical Method

Gravity separation: The powdered ores is agitated with water or washed with a running stream of water. The heavy ore particles of sand, clay etc. are washed away.

Froth Floatation Process: The finely divided ore is introduced into water containing small quantity of oil (e.g. Pine Oil). The mixture is agitated violently with air a froth is formed which carries away along with it the metallic particles on account of the surface tension forces. The froth is transferred to another bath where gangue-free ore settles down.  

Chemical Methods

Calcination: Carbonate or hydrated oxide ores are subjected to the action of heat in order of expel water from hydrated oxide and carbon dioxide from a carbonate.

Examples:

2Fe2O3×3H2O --> 2Fe2O3 + 3H2O

Roasting: Sulphide ores either are subjected to the action of heat and air at temperatures below their melting points in order to bring about chemical changes in them.

Examples:

2Cu2S + 3O2 --> 2Cu2O + 2SO­2

Leaching: It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain insoluble. The ore is recovered from the solution by suitable chemical method.    

Al2O3 + 2NaOH  -->2 NaAlO2  + H2O

Reduction of Free Metal:

Smelting:  

Reduction of a metal from its ore by a process involving melting

Several reducing agents such as sodium, magnesium and aluminium are used for reduction.

The calcinated or roasted ore is mixed with carbon (coal or coke) and heated in a reverberatory or a blast furnace.

Carbon and carbon monoxide produced by incomplete combustion of carbon reduce the oxide to the metal.

Flux:

The ores even after concentration contain some earthy matter called gangue which is heated combine with this earthy matter to form an easily fusible material. Such a substance is known as flux and the fusible material formed during  reduction process is called slag.

Acidic fluxes like silica, borax etc are used when the gangue is basic such as lime or other metallic oxides like MnO, FeO, etc

Basic fluxes like CaO, lime stone (CaCO3), magnesite (MgCO3), hematite (Fe2O3) etc  are used when the gangue is acidic like silica, P­4O10 etc.  

Refining

The metals obtained by the application of above reduction methods from the concentration ores are usually impure. The impure metal is thus subjected to some purifying process known as refining in order to remove undesired impurities. Various process for this are

a) Liquation  process              b)   Distillation process

c) Cupellation                         d)   Poling

e) Electrolytic refining              f)   Bessemerisation

Thermodynamic Principles of Metallurgy:

ΔG =ΔH-TS

or ΔG0 =-RT ln K

An element A can reduce element B if ΔG value for oxidation of A to AO is lower than the ΔG value for oxidation of B to BO.

i.e. ΔG(A→AO) < ΔG(B→BO)

Refining of Metals:

Zone refining (Fractional crystallization): This method is employed for preparing extremely pure metals. This method is based upon the principle that when a molten solution of the impure metal is allowed to cool, the pure metal crystalises out while the impurities remain in the melt.

Electro-refining: In this method, the impure metal is converted into a block which forms the anode while cathode is a rod or plate of pure metal. These electrodes are suspended in an electrolyte which is the solution of a soluble salt of the metal usually a double salt of the metal. When electric current is passed, metal ions from the electrolyte are deposited at the cathode in the form of pure metal while an equivalent amount of metal dissolves from the anode and goes into the electrolyte solution as metal ion. The soluble impurities present in the crude metal anode go into the solution while the insoluble impurities settle down below the anode as anode mud.

Van-Arkel Method: In this method, the metal is converted into it volatile unstable compound such as iodide leaving behind the impurities. The unstable compound thus formed is decomposed to get the pure metal.