Metallurgy is a branch of chemistry which deals with,
1. Extraction of metals from ores
2. Refining of crude metal
3. Producing alloys and the study of their constitution, structure and properties.
4. The relationship of physical and mechanical treatment of metals to alloys.
The extraction of metals cannot be carried out by any universal method because extraction of each metal requires different methods of extraction. This depends upon the nature and preparation of metals. In general, noble metals such as Au, Ag, etc are usually extracted by electrolysis of their chlorides, oxides or hydroxides. Heavy metals, e.g. Cu, Zn, Fe, Pb, Sn, etc., are extracted by making use of roasting and smelting methods.
Roasting is one of the oxidation method where ore is converted into metal oxide. In the process of roasting, the ore either alone or with the addition of suitable material, is subjected to the action of heat in excess of air at temperature below its melting point. Roasting is usually carried out in a reverberatory furnace or in a blast furnace. During roasting
(a) Volatile impurities like S, As, Sb etc. get oxidized and escape out as volatile gases SO2, As2O3 and Sb2O3 (b) The sulphide ores decompose to their oxides evolving SO2 (c) The moisture is removed. Mass becomes porous and thus it can easily be reduced. Roasting may be of many types.
Oxidising Roasting - In this type of roasting S, As, and Sb impurities are removed in the form of their volatile oxides as SO2, As2O3 and Sb2O3 etc. due to combined action of heat and air. The ore is simultaneously converted into its oxides. This type of roasting is carried out for ores like copper pyrites, zinc blende and lead ores (PbS) etc.
2ZnS + 3O2 ® 2ZnO + 2SO2
2PbS + 3O2 ® 2PbO + 2SO2
Another method of conversion of ore into metal oxide (oxidation) is called calcination. It is the process in which the ore is subjected to the action of heat at high temperature in the absence of air below its melting point. The process of calcination is carried out in the case of carbonate and hydrated ore. As a result of calcination (a) The moisture is removed.
(b) Gases may be expelled. (c) Volatile impurities are removed. (d) The mass becomes porous. (e) Thermal decomposition of the ore takes place. For example,
CaCO3 (limestone) ® CaO + CO2 - MgCO3 (Magnesite) ® MgO + CO2 -
MgCO3.CaCO3 (Dolomite) ® MgO + CaO + 2CO2 - CuCO3.Cu(OH)2 (Malachite) ® 2CuO + H2O + CO2 - ZnCO3 (Calamine) ® ZnO + CO2-
2Fe2O3.3H2O (Limonite) ® 2Fe2O3 + 3H2O -
The name calcination originated from the ore calcite which on thermal decomposition gives quick lime. Calcination is usually carried out in reverberatory furnace.
2.Smelting - Reduction
Smelting is one of reduction method where the metal oxide is converted into metal. It is the process used for all operations where the metal is separated by fusion from the ore. The process of smelting is that in which ore is melted with a flux and often with a reducing agent, and it involves, calcination, roasting and reduction. In general, the process of separation of a metal or its sulphide mixture from its ore in fused state is called smelting. Smelting is generally carried out in a blast furnace and high temperature is produced by burning coal or by using electric energy.
In smelting, the roasted or calcined ore is mixed with coke and then heated in a furnace. As a result, (carbon and CO produced by the incomplete combustion) carbon reduces the oxide to the metal. For example, in the extraction of iron, haematite ore(Fe2O3) is smelted with coke and limestone (flux). As a result of reduction, iron is obtained in fused or molten state.
Fe2O3 + 3C - > 2Fe + 3CO CaCO3 - > CaO + CO2 Fe2O3 + 3CO - > 2Fe + 3CO2 CaO + SiO2 - > CaSiO3
Flux + Gangue - > Slag
Similarly, in the extraction of copper from copper pyrites, the ore is mixed with coke and heated in blast furnace (smelted). Infusible impurity FeO is converted to FeSiO3(slag) and is removed. A mixture containing sulphide of copper and iron, called matte is formed in the molten state.
FeO (Gangue) SiO2 (Flux) - > - > FeSiO3 (Slag)
ZnO + C - > Zn + CO
SnO2 + 2C - >Sn + 2CO
MnO2 + 2C - > Mn + 2CO
It is the process used for the manufacture of steel from pig iron. Steel is an alloy of carbon and iron and contains 0.15-1.5% of carbon with traces of sulphur, phosphorus, manganese and silicon as impurities. Depending upon the carbon content, steel are classified into three classes namely mild carbon steel (0.15-0.3%), medium carbon steel (0.3-0.8%) and high carbon steel (0.8-1.50).
The process was discovered by Henry Bessemer in England (1856). The principle involved in this process is that cold air blowed through refractory lined vessel known as converter containing molten pig iron at about 2 atmospheric pressure, oxidizing the impurities and simultaneously converting pig iron to steel.
This process mainly differs in the use of acidic and basic refractory linings of the converters. In this process low phosphorus pig iron (below 0.09%) is treated by acidic Bessemer process and high phosphorus pig iron (more than 1.5%) is treated in basic.
The converter is a pear shaped furnace about 6m high and 3m in diameter. It is made of steel plates and is lined inside with silica or magnesia (MgO), depending upon the nature of impurities present in the pig iron. If the impurities present in the pig iron are basic, e.g., manganese, a lining of silica brick is used and the process is known as acid Bessemer process. If impurities are acidic, e.g., sulphur, phosphorus etc., a basic lining of lime (CaO) or magnesia (MgO) is used in the converter and process is then known as basic Bessemer process. The capacity of the converter is from 10-25 tonnes of charge at a time. The converter is mounted on shafts or trunnions, one of which is hollow and serves as a wind pipe and upon which the converter can rotate in any position. The converter is also provided with a number of holes at the bottom through which a hot blast of air can be introduced.
The molten pig iron is mixed in mixers and then charged into converter. About 15-16 tonnes of iron can be charged at a time. The converter is first set in the horizontal position and after charging the converter is adjusted in vertical position. After charging a blast of cold air is admitted through the hole provided at the bottom at a pressure of about 2-3 kg/cm3. The blast is continued for about 15 minutes during which the impurities are oxidized. Mn is oxidized to MnO and Si is oxidized to SiO2. Carbon is also oxidized to CO. The resulting oxides of Mn and Si (MnO and SiO2) combine together to form slag of manganese silicate Fig..
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