Coupon Accepted Successfully!



It involves the use of elevated temperature and changes in the chemical composition of the entire body of ore. Basic processes of pyrometallurgy are as follows: (a) calcinations, (b) roasting, (c) smelting, and (d) distillation (purification).
  1. Calcination
    Description: 52582.png
  2. Roasting: In this process, the ore is heated either alone or in the presence of some substances so that volatile impurities are removed. Some chemical changes also take place during this process. It can be of two types.
    Oxidized roasting: Ores when heated in the presence of O2 get converted into their oxides and impurities are converted into their volatile form, which do escape.
    Description: 52591.png
    Roasting or calcination can be carried out in a reverberatory furnace.
    Chlorinating roasting: This is done especially in the case of silver ore.
    Description: 52602.png
  3. Smelting: The process of extracting metal from its fused (molten) state is called smelting. The roasted or calcined ore containing metal oxide is mixed with a reducing agent and heated to a high temperature. In this case, a less electropositive metal ore of Pb, Zn, Fe, etc., are treated with powerful reducing agents such as C, H2, and CO. Depending on the nature of the oxide and metal, the extraction of metal can be carried out by the following reducing agents.
    Carbon reduction process: Carbon, the cheapest available reducing agent usually in the form of coke, is employed in the extraction of the lead, zinc, iron, and tin. The oxides of the metals (either naturally occurring or obtained by calcination of the naturally occurring carbonates or roasting of the sulphides) are mixed with coke and heated in a suitable furnace. Carbon or carbon monoxide reduces the oxide to free metal.
    For example,
     ZnO + C Zn + CO
     ZnO + CO Zn + CO2ffa
    During reduction, additional substance called flux is also added to the ore. It combined with impurities to form easily fusible product known as slag.
     Impurities + Flux Fusible product (slag)
    Flux is a substance that is added to the ore during smelting (i) to decrease the melting point, (ii) to make the ore conducting, and (iii) to remove all the impurities (basic and acidic).
    The nature of the flux depends on the nature of impurity to be removed. An acidic flux (e.g., sand) is used to remove basic impurities (e.g., metallic oxides such as FeO), while basic flux (e.g., CaO) is used to remove acidic impurities (e.g., sand).
     Description: 52619.png
    Alumina is a bad conductor of electricity, but when cryolite (flux) is added, it becomes a good conductor and the melting point is decreased. Hence, CaF2, KF, cryolite, etc., are neutral flux.
    Reduction by another metal (aluminium): If the temperature needed for carbon to reduce an oxide is too high for economic or practical purposes, the reduction may be affected by another highly electropositive metal such as aluminium, which liberates a large amount of energy (1675 kJ/mol) on oxidation to Al2O3. This process of reduction of a metal oxide to metal with the help of aluminium powder is called aluminothermy or Goldschmidt aluminothermic process or thermite process.
    Self-reduction process: The cations of the less electropositive metals such as Pb, Hg, Sb, and Cu may be reduced without the use of any additional reducing agent. Elevated temperatures and anion or anions associated with the metal may bring about this change.
    For example, in the extraction of mercury, the sulphide ore (cinnabar) is heated in a current of air when the following reactions take place:
    Description: 52626.png (self-reduction reaction)
    Electrolytic reduction: The strongest possible reducing agent is an electron. Any ionic material may be electrolyzed and reduction occurs at the cathode. This is excellent method and gives very pure products, but electricity is quite expensive. Electrolysis may be performed in aqueous solution provided that the products do not react with water.
    This process is mainly used in the extraction of alkali and alkaline earth metals. In the case of highly electropositive metals, isolation by chemical reducing agents is extremely difficult. In such cases, the metal is obtained by electrolysis of fused salts. Under such conditions, the ions readily move to the oppositely charged electrodes and are distinguished and discharged over there. Some other salts may have to be added to lower the melting point of the compound taken.
    Reduction in other solvents: Electrolysis can be carried out in solvents other than water. Fluorine reacts violently with water, and it is produced by electrolysis of KHF2 dissolved in anhydrous HF. The reaction has many technical difficulties: (i) HF is corrosive; (ii) hydrogen produced at the cathode must be kept separate from the fluorine produced at the anode, otherwise explosion may occur; (iii) water must be rigorously excluded; and (iv) fluorine produced attacks the anode and the reaction vessel.
    In fused melts: Elements that react with water are often extracted from fused melts of their ionic salts. These melts are frequently corrosive and involve large fuel bills to maintain the high temperature required. Aluminium is obtained by electrolysis of a fused mixture of Al2O3 and cryolite, Na3[AlF6].
    An example of this is the manufacture of sodium by electrolysis of a fused mixture of sodium and calcium chlorides (Down’s process). The cell and electrodes used should not be affected by the electrolyte or the products. Hence, a steel cell, a graphite anode, and an iron cathode are employed. The various reactions that take place are as follows:
    On fusion, NaCl  Na+ + eNa(reduction)
    On electrolysis,
    Cathode: Na+ + e Na (reduction)
    Anode: Description: 52635.png
  4. Refining or purification of Metals: Metals obtained by reduction processes still contain some objectionable impurities and hence have to be refined.
    1. By poling: This method is employed when the metal contains the impurities present in the form of its oxide. To reduce the impurity level, air is blown through molten metal to which a suitable flux may be added. Impurities are oxidized and escape as gases (CO2) or pass into slag. Finally, the surface of molten metal is covered with low sulphur coke (to prevent oxidation of the metal) and green logs of wood are introduced into the molten metal.
      Hydrocarbon gases reduce any oxide of the metal that might have been formed. This method is used for the purification of Cu and Sn.
    2. By liquation: Readily fusible metals such as Sn, Pb, and Bi are refined by this method. Impure metal in the form of ingots blocks is placed in the upper part of a sloping hearth (usually of a reverberatory furnace) maintained at a temperature slightly above the melting point of the metal.
      The impurities remain behind as dross, while the pure metal melts and flows down into a well at the bottom of the incline.
    3. By cupellation: This is a method employed to purify silver containing lead as an impurity. The impure silver is heated in a shallow vessel made of bone-ash under a blast of air. The lead is easily oxidized to powdery lead monoxide. Most of it is carried away by the blast of air. The rest melts and is absorbed by the bone ash cupel. Pure silver is left behind. Silver itself is not oxidized under these conditions.
    4. By electrolytic refining: This is a very convenient method for refining many impure metals. Most of the metals such as copper, silver, gold, zinc, and chromium are refined electrolytically. The impure metal is made the anode and a thin sheet of the pure metal as cathode. A solution of a salt of the metal serves as the electrolyte. On passing an electric current through the electrolyte, the metal dissolves in the electrolyte by oxidation of the anode and pure metal is deposited at the cathode. The impurities present in the anode either dissolve in the electrolyte or collect below the anode as anode mud. In the electrolytic refining of copper, impurities of iron and zinc are dissolved in the electrolyte, while gold, platinum, and silver are left behind as anode mud.
    5. Van Arkel method: This is used for getting ultra pure metals. The principle involved is to convert the metal into a volatile unstable compound and to subsequently decompose it to give the pure metal. The impurities present should be such as not to be affected. Metals such as titanium and zirconium are purified by using this method.
      Description: 52710.png
    6. Mond’s process: Nickel is purified by this method. Impure nickel is heated with CO at 6080°C when volatile compound [Ni(CO)4] is formed. This compound decomposes at 180°C to form pure Ni and CO.
      Impure Description: 52721.png
Note: Van Arkel and Mond’s processes are collectively called as vapor-phase refining.

Test Your Skills Now!
Take a Quiz now
Reviewer Name