explain the process of extraction of metals with help of a suitable diagram
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1. Introduction to the extraction of metals
What methods can be used in extracting metals from mineral ores?
The Earth's crust contains many different rocks.
Rocks are a mixture of minerals and from some we can make useful substances.
A mineral can be a solid metallic or non–metallic element or a compoundfound naturally in the Earth's crust.
Mineral ores are naturally occurring rocks that provide an economic starting point for the extraction and manufacture of metals for a huge variety of purposes ie a metal ore is rock containing sufficient metal to be worth extracting the metal from it.
The simplest definition of an ore is a mixture of a metal containing mineral and other materials ('minerals') from the surrounding rocks, which can be described as impurities with respect to what you want from the ore.
Metal ores are obtained by mining and that this may involve digging up and processing large amounts of rock.
Most ores are mined have to be concentrated before the metal is extracted and purified.
This often results in lots of waste material that must be dealt with from an environment of view.
The social, economic and environmental impacts of exploiting metal oresare discussed on a separate page.
The rock must contain enough of the metal compound, hence enough of the metal, to be worth exploiting the ore reserve and extract the metal by physical and chemical processes.
Low grade ores are usually uneconomic to mine and process, but high grade ores have a high enough metal content to be worth mining and extracting the metal.
The metal ore, a mineral or mixture of minerals from which economically viable amounts of metal can be extracted, i.e. its got to have enough of the metal, or one of its compounds, in it to be worth digging out!
Ores are often oxides, carbonates or sulphides because these non-metals readily combine with many metals.
e.g. iron + oxygen ===> iron oxide ores
Fe + O2(air) ===> Fe2O3 or Fe3O4 (not balanced equations, just the idea!)
This the typical chemistry involved in metal ore formation.
In extracting metals from ores you have to reverse the process e.g. by heating the ore with carbon (coke or charcoal) in a very hot smelting furnace e.g. in a blast furnace to make iron ...
iron oxide ore + carbon ===> free iron + carbon dioxide waste gas
Fe3O4 + 2C ===> 3Fe + 2CO2
The iron oxide is reduced (oxygen loss) and the carbon reducing agent is oxidised (oxygen gain).
This is typical extraction chemistry for less reactive metals.
These ores are all finite resources so we should use them wisely!
What methods can be used in extracting metals from mineral ores?
The Earth's crust contains many different rocks.
Rocks are a mixture of minerals and from some we can make useful substances.
A mineral can be a solid metallic or non–metallic element or a compoundfound naturally in the Earth's crust.
Mineral ores are naturally occurring rocks that provide an economic starting point for the extraction and manufacture of metals for a huge variety of purposes ie a metal ore is rock containing sufficient metal to be worth extracting the metal from it.
The simplest definition of an ore is a mixture of a metal containing mineral and other materials ('minerals') from the surrounding rocks, which can be described as impurities with respect to what you want from the ore.
Metal ores are obtained by mining and that this may involve digging up and processing large amounts of rock.
Most ores are mined have to be concentrated before the metal is extracted and purified.
This often results in lots of waste material that must be dealt with from an environment of view.
The social, economic and environmental impacts of exploiting metal oresare discussed on a separate page.
The rock must contain enough of the metal compound, hence enough of the metal, to be worth exploiting the ore reserve and extract the metal by physical and chemical processes.
Low grade ores are usually uneconomic to mine and process, but high grade ores have a high enough metal content to be worth mining and extracting the metal.
The metal ore, a mineral or mixture of minerals from which economically viable amounts of metal can be extracted, i.e. its got to have enough of the metal, or one of its compounds, in it to be worth digging out!
Ores are often oxides, carbonates or sulphides because these non-metals readily combine with many metals.
e.g. iron + oxygen ===> iron oxide ores
Fe + O2(air) ===> Fe2O3 or Fe3O4 (not balanced equations, just the idea!)
This the typical chemistry involved in metal ore formation.
In extracting metals from ores you have to reverse the process e.g. by heating the ore with carbon (coke or charcoal) in a very hot smelting furnace e.g. in a blast furnace to make iron ...
iron oxide ore + carbon ===> free iron + carbon dioxide waste gas
Fe3O4 + 2C ===> 3Fe + 2CO2
The iron oxide is reduced (oxygen loss) and the carbon reducing agent is oxidised (oxygen gain).
This is typical extraction chemistry for less reactive metals.
These ores are all finite resources so we should use them wisely!
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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.
Catagory IV :Heavy metals such as Fe, Co, Ni , Cu , etc . are extracted by roasting and reduction methods.
Catagory V :Less reactive metals such as Ag, Au, Pt, etc. are extracted by amalgamation or cyanide process.
The various process involved in the extraction of pure metals from their ores are :
Concentration or Benefaction of the ore
Extraction of crude metal from the concentrated ore
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 .
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.
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.
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 :
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
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 .
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
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.
Catagory IV :Heavy metals such as Fe, Co, Ni , Cu , etc . are extracted by roasting and reduction methods.
Catagory V :Less reactive metals such as Ag, Au, Pt, etc. are extracted by amalgamation or cyanide process.
The various process involved in the extraction of pure metals from their ores are :
Concentration or Benefaction of the ore
Extraction of crude metal from the concentrated ore
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 .
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.
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.
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 :
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
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 .
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
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