Extraction of metals (Metallurgy)
The art of extracting metal from its ores is called metallurgy. It also includes the art of preparing alloys.
The ores and even the native metals usually contain worthless material (clay, sand, limestone etc.) called Matrix or Gangue. It is, therefore, very essential to remove the gangue before the actual process for the extraction of a metal is taken. These processes are known as Mineral benefication.
Below are given the general methods for the extraction of metals:
- Crushing and Grinding of Ore: (i) by jaw crusher and (ii) by the stamp mill.
- Concentration of the Ore by: (i) Gravity separation (ii) Magnetic separation (iii) Froth floatation process and (iv) Leaching.
- Extraction of metals from the concentrated ore obtained from step (1) above
- Refining of the crude metal obtained from step No. (2) above.
- Crushing and Grinding:
Most of the ores are found in the form of huge lumps. In order to bring about the necessary chemical changes in them, they must be changed into the powder form. Thus, the huge lumps of the ore are broken into small pieces with the help of jaw crushers. The small pieces of the ore thus obtained are further changed into the powder form with the help of a stamp mill or the ball mill. This step is called pulverisation of the ore.
- Concentration of the ore:
First, of all, the main object is to remove maximum gangue. The process is called concentration, In other words, concentration is the process of removing maximum gangue from the ore. The following methods are used for concentration.
(a) Gravity Separation:
It includes: (i) Washing the crushed ore with water, (ii) Separation of less dense material (gangue) from ore or metal with high specific gravity by choosing suitable medium.
Procedure: The crushed ore obtained from preliminary treatment is spread on long tables having transverse ridges, called wilfley tables. The tables are given rocking motion and stream of water flown over them. The gangue being lighter is carried away by water while ore is detained by the ridges.
E.g. take the case of the concentration of Galena (PbS) which occurs in limestone. When a stream of water is flown over crushed ore, the less dense limestone is carried away while galena is left behind.
Note. Generally, oxide and carbonate ore are concentrated by this method, e.g., Haematite, Tin stone, etc.
(b) Froth floatation process:
This method is meant for the concentration of sulphide ores. The reason is that the sulphide ore are preferentially wetted by oil than by water. The reverse is true for the gangue particles. The crushed sulphide ore (PbS, ZnS etc.) is mixed with water and a few drops of pine oil or eucalyptus oil are added to it. A few drops of acid are added which decrease interfacial surface tension. The suspension is vigorously agitated by passing compressed air through a perforated pipe. The froth is formed (due to oil) whose bubbles take the sulphide ore particle to the top of the vessel from where it is transferred to another vessel. Here the ore particles settle at the bottom.
If an ore contains sulphide of two metals, then each metal sulphide can be separated by this method e.g. felspar or quartz contains Galena with Zinc blende mixed with it. The ore is mixed with KCN and slaked lime. The mixture is powdered. Now water, pine oil and a little potassium dichromate (K2COS2) is added and mixture is agitated with air. Due to selective action of KCN and slaked lime, only PbS comes at the top in the form of froth and is removed. Now to the rest of solution, a little copper sulphate, pine oil and (K2COS2) are added and again agitated with air when ZnS comes at the top and is removed. Thus, galena (PbS) and zinc blende (ZnS) are separated.
(c) Magnetic separation:
This method is meant for concentrating such ores which differ from their impurities in magnetic character, For example, the main impurity in tin stone (SnO2) is ferrous tungstate (FeWO4) called wolfram. Here tin stone is diamagnetic in behaviour. The powdered ore is dropped over a belt revolving round the two rollers, one of the rollers being a magnet. The magnetic part of the ore is attracted by the magnetic roller and forms a heap near it, whereas the non-magnetic part of the ore forms a separate heap a little away from the magnetic impurities.
(3) Roasting:
It is a process of heating the concentrated ore strongly in a controlled supply of oxygen in a reverberatory furnace to get metal oxide. This process is generally used to convert sulphide ores into metallic oxides. The main functions of roasting are as follows
- It removes moisture in the form of steam.
- It removes volatile impurities like Arsenic and Antimony compounds.
- It oxidises substances which can be easily oxidised, e.g., ferrous compounds change to ferric ones.
- It decomposes certain compounds, e.g., carbonates of metals other than alkali ones are easily converted to their oxides.
PbS + O2 ——–> PbO + SO2
CuFeS2 + O2 ——–> Cu2S + FeS + SO2
FeCO3 + O2 ———> Fe2O3 + CO2
S + O2 ——–> SO2
As + O2 ——–> As2O3
P4 + O2 ———-> P2O5
The roasting process is carried in a specially designed furnace, called reverberatory furnace. It consists of a floor on which the charge is placed and a hearth where the fuel is burnt. Air enters through the holes meant for the purpose, the waste gases exit through the chimney.
(4) Smelting:
In this method, the roasted ore is mixed with the suitable quantity of coke or charcoal which acts as a reducing agent. The mixture is heated to a high temperature above its melting point. During the process, an additional substance is added to the ore which reacts with the impurities, still present in the ore to form a fusible product. The additional substance is called flux and the fusible product formed is called slag.
Impurities + Flux ———> Fusible Slag
(In the ore)
The selection of the flux depends upon the nature of the impurities. 1f the impurities are basic such as CaO, FeO, MgCO etc., then the flux employed is acidic (i.e., silica).
CaO + SiO2 ————> CaSiO3
Basic impurity Flux Slag
On the other hand, if the impurities are acidic such as SiO2 P2O5 etc., then the flux used is basic (i.e., CaO).
P2O5 + 3CaO ——-> Ca3(PO4)2
Acidic impurity Flux Slag
The metal oxide during the reduction process gets reduced to the metallic state.
PbO + C ——-> Pb + CO
Fe2O3 + 3C ——-> 2Fe + 3CO
The carbon reduction process is usually carried out in a blast furnace. The tall cylindrical blast furnace is made of steel plates and lined inside with heat resisting bricks. The mouth of the furnace is provided with an inlet for the introduction of charge and an outlet for the waste gases. Slag and the ‘molten metal are removed from the base of the tower, It is also provided with tuyers arrangement for the introduction of a hot blast of air. The exact size of the blast furnace depends upon the nature of the ore used.
(5) Refining: The process of the removal of impurities from a crude metal is called refining. e.g. electro-refining.
Electro-refining:
A suitable electrolyte is chosen and put into an electrolytic cell. Now pure metal plate is dipped into the electrolyte and made the cathode. The impure metal is made the anode and a suitable e.m.f. is applied. The pure metal from the electrolyte gets deposited on the cathode while an equivalent quantity of the metal from anode goes into the solution.
The metals, less electropositive than the metals under examination do not go in solution but get separated as anode mud. Metals like Al, Cu, Ag, Zn, Pb and Sn are purified by this method.
