Thursday, August 18, 2011

Crushing and Grinding

The Ore obtained from mining is in the form of big lumps. These are broken into smaller pieces with the help of crushers known as Jaw Crushers. This process is known as Crushing. The crushed ore is then converted into fine powder with the help of stamp mill or ball mill. This process is known as Pulverisation of Ore.

Metallurgical Operations

The branch of science which deals with the extraction of metal from its ore is called Metallurgy. There is no common process available for the extraction of all the metals because they differ in their chemical and physical properties and also the impurities associated with them. However, there are certain operations or procedures common in the metallurgy of all metals. These operations or procedures are called Metallurgical Operations. The various steps involved in metallurgical operations are;
1.   Crushing and Grinding of Ore (Pulverisation)
2.   Concentration of Ore.
3.   Extraction of Metal from Oxide Ore.
4.   Refining of Crude Metal.

Types of Minerals

The Various types of minerals depend upon the nature of the chemical compounds that contains the given metal. These are:
1. OXIDES:  Oxides are those minerals in which a metal occurs in combination with oxygen. Examples of such minerals are,
            (a).       Iron as Hematite (Fe2O3), Magnetite (Fe2O3. FeO), Chromate (Fe3O4Cr2).
            (b).      Aluminium as Bauxite                        (Al2O3 . 2H2O)
            (c).       Manganese as Pyolusite                      (MnO2)
            (d).      Tin as Tin stone or Cassiterite             (SnO2)
            (e).       Titanium as Rutile                               (TiO2)
            (f).       Copper as Cuprite                               (Cu2O)
2. SULPHIDES:  Sulphide are those minerals in which a metal occurs in combination with sulphur.
            For Examples:–        
            (a).       Iron as Iron Pyrites                 (FeS2)
            (b).      Copper as Copper Pyrites       (CuFeS2)
            (c).       Copper as Copper Glance       (Cu2S)
            (d).      Mercury as Cinnabar               (HgS)
            (e).       Zinc as Zinc Sulphide             (ZnS)
            (f).       Lead as Lead Sulphides          (PbS)
3. CARBONATES:  These are those minerals in which a metal occurs as a carbonates.
            For Example:
            (a).       Calcium as Calcite                  (CaCO3)
            (b).       Magnesium as Dolomite         (CaCO3 . MgCO3)
            (c).       Zinc as Calamine                     (ZnCO3)
            (d).      Iron as Siderite                        (FeCO3)          
4. SULPHATES:  The minerals in which a metal occurs as sulphates are sulphate minerals.
            For Example
            (a).       Barium as Barytes                   (BaSO4)
            (b).      Lead as Anglesite                   (PbSO4)
            (c).       Calcium as Gypsum                (CaSO4 . 2H2O)
            (d).      Magnesium as Epsom Salt      (MgSO4 . 7H2O)
            (e).       Sodium as Glaber’s salt          (Na2SO4 . 10H2O)
5. HALIDES:  These are minerals in which a metal occurs in combination with a halogen. Examples of halide minerals are:
            (a).       Sodium as common salt          (NaCl)
            (b).      Aluminium as Cryolite            (Na3AlF6)
            (c).       Calcium as Fluorite                 (CaF2)
            (d).      Silver as Horn Silver               (AgCl)
            (e).       Magnesium as Carnallite         (KCl. MgCl2 . 6H2O)
6. PHOSPHATES:  The minerals in which a metal occurs as a phosphate are called phosphate minerals. Example;
                        Calcium as hydroxyapatite [Ca3(PO4)5(OH)2]
7. SILICATES:  The minerals in which the metal occurs as a silicate are called Silicate Minerals.
            For example.
            (a).       Silicon as Quartz (SiO2)
            (b).      Calcium and Magnesium as Asbestos (CaSiO3.MgSiO3)
            (c).       Aluminium as China Clay (Al2O3.SiO2.2H2O)

Minerals and Ores

Majority of metals are present in earth’s crust in the combined form as oxides, sulphides, carbonates, phosphates etc. The Combined State of the metals is called Minerals. Thus a mineral may be defined as the Combined State in which a metal occurs naturally in the crust of earth. Minerals are generally associated with many earthy and rocky impurities which are called Gangue or Matrix. All the minerals of a particular metal cannot be treated for its isolation, because the process may be very tedious in certain cases and may not be profitable in some other cases. The minerals from which a metal can be profitably and conveniently extracted is called Ore.

Occurrence of Elements

Elements occur in nature in two states; Native State and Combined State.
1. NATIVE STATE   
The Elements are said to be in native state if they are found in their elementary form. Generally, less active elements are found in Native State. Oxygen, nitrogen, noble gases are some non – metals which occur in native state with the common examples of metals are gold, silver, copper, platinum etc.
2. COMBINED STATE   
The elements are said to occur in the Combined State if they are found in nature in the form of their compounds. Generally the reactive elements occur in the form of their compounds. For Example, the reactive metals are found in the form of their oxides, carbonates, Sulphide and silicates etc.

Sea as a Source of Elements

Sea water contains a large number of elements. The most common among them are sodium, potassium, calcium, chlorides and sulphate. The main source of these elements is the chemical weathering of igneous rocks followed by the extraction of soluble salts with water.
It may be noted that although sea water is a treasure house of valuable elements, yet only four elements namely: Chlorine, bromine, magnesium and sodium are commercially recovered from sea water.
In recent years new sources has been discovered on the ocean floor which is called Manganese Nodules. These nodules are rock like objects consisting of oxides of Mn, Fe and small amounts of CO, Cu and N. It is believed that marine organisms play a major role in the formation of such wodules.

Earth as a Source of Elements

Earth is a source of a large number of elements. The different parts of earth contains different elements. Earth can be broadly divided into three main parts; namely Atmosphere, Hydrosphere and Lithosphere.
1.         ATMOSPHERE
It consists of a gaseous mixture surrounding the earth. The main constituents of atmosphere are; nitrogen (» 78%), oxygen (» 21%) and other gases such as CO2, He, Ne, Ar, Kr, Xe (less than 1%).
2.         HYDROSPHERE
It covers about 80% of earth’s surface and constitutes streams, lakes, rivers and oceans. There are large number of elements present in sea water. These are present in the form of their dissolved salts.
3.         LITHOSPHERE
It is a solid phase of the earth. It consists of different type of rocks such as igneous rocks, sedimentary rocks and metamorphic rocks. These rocks from a source of large number of elements.
The distribution of main elements on the surface of the earth in terms of their relative abundance is as follows;


Thursday, August 11, 2011

Preparation of Ozone

2O2 + O2 ↔ 2O3 ; DH = +288KJ

Effect of concentration
On increasing the concentration of oxygen, the equilibrium will shift toward right i.e. forward reaction is favoured. If the concentration of ozone is increased, the equilibrium will shift towards left i.e. backward reaction is favoured.

Effect of Temperature
The formation of ozone is endothermic reaction i.e. forward reaction is endothermic whereas backward reaction is exothermic reaction. So on increasing temperature forward reaction will be favoured and if temperature is decreased, backward reaction is favoured.

Effect of Pressure
On increasing pressure, forward reaction is favoured because it involves decreases in number of moles. If pressure is decreased, the backward reaction is favoured, because it involves increase in number of moles.

Relation Between Kp and Kc


Application of Le–Chatelier’s Principle

HABER’S PROCESS
One of the most important industrial reactions is the synthesis of ammonia from nitrogen and hydrogen, by Haber’s Process.
N2    +   3H2   ↔   2NH3;    DH   =   – 92 KJ/mol.
Effect of Concentration:
According to Le – Chatelier’s Principle increases in concentration of N2 or H2 (reactants) will shift the equilibrium towards right i.e. forward reaction is favourable and hence production of NH3 will be more. But if concentration of NH3 is increased, then equilibrium will shift towards left i.e. backward reaction.
Effect of Temperature:
The above reaction is exothermic in forward direction and hence endothermic in backward direction. Thus according to Le – Chatliers Principle, increase in temperature will favor endothermic reaction and hence reaction will proceed in backward direction i.e. equilibrium will shift towards left. If temperature is lowered, the reaction will proceed in forward direction, because it is exothermic and hence equilibrium will shift towards right.
Effect of Pressure:
The forward reaction i.e. formation of NH3 is accompanied with decreases in number of moles and backward reaction is accompanied with increase in number of moles.
According to Le – Chatelier’s Principle increase in pressure favour that reaction which is accompanied with decrease in number of moles and hence on increasing pressure equilibrium will shift toward right i.e. forward reaction. But if pressure is decreased, the favourable reaction is one which involves increase in number of moles. So on decreasing the pressure the reaction will favour backward reaction i.e. equilibrium will shift to left.