Metals and Metallurgy

Difference between metals and non-metals:

Metals	Non-Metals
1. They exist in a solid-state. ( except mercury which is liquid)	1. They may exist in solid, liquid, and gas states.
2. Usually, metals are hard.	2. They may or may not be hard.
3. They are good conductors of electricity.	3. They are poor conductors of electricity. (except graphite)
4. They are malleable, ductile, and have metallic lusture.	4. They are brittle and do not have metallic lusture.
5. They have low ionization energy, electron affinity, and electronegativity.	5. They have high ionization energy, electron affinity, and electronegativity.

The elements having intermediate properties of metals and non-metals are called metalloids. eg. beryllium, germanium, arsenic, antimony, etc.

Alloys:

The homogeneous mixture of metal with other elements is called an alloy. eg.Brass (Cu+Zn), Bronze (Cu+Sn), Steel (Fe+C), German silver (Cu+Zn+Ni), etc.

The homogeneous mixture of metal with mercury is called amalgam. eg. Sodium amalgum (NaHg), Zinc amalgum (ZnHg), etc.

Minerals and Ores:

The naturally occuring substance in which metal is present is called mineral. eg. rock, clay, etc.

The naturally occuring mineral from which metals can be extracted easily any profitably is called ore.

Every ore is a mineral but every mineral is not an ore. For eg. clay (Al₂O₃.2SiO₃.2H₂O) is a mineral of aluminium but is not an ore because aluminium cannot be easily and profitably extracted from clay. Bauxite (Al₂O₃.2H₂O) is an ore of aluminium from which aluminium can be easily and profitably extracted.

Some important ores:

• Copper pyrite (CuFeS₂)
• Iron pyrite (FeS₂)
• Haematite (Fe₂O₃)
• Cinnabar (HgS)

Metallurgy:

The process of extraction of metals from its ore is called metallurgy.

Types of metallurgy:

1. Pyrometallurgy: It is the method of extraction of metal from its ore by chemical reduction at high temperatures.
2. Electrometallurgy: It is the method of extraction of metal from its ore by electrolytic reduction in the molten or aqueous state.
3. Hydrometallurgy: It is the method of extraction of metal from its ore by dissolving the ore in a suitable chemical reagent and precipitating it with other more active metals.

Process of Metallurgy:

1. Crushing and pulverization:

Big lumps of ores are crushed using jaw crushers to get crushed ore which is pulverized using a pulverizer or stamp mill to get powdered ore.

2. Concentration:

Powdered ores may contain unwanted earthy impurities which are known as gangue or matrix. The process of removal of these earthy impurities from the powdered ore is called concentration (ore dressing). On the basis of the nature of ore, it is done by one or more of the following processes:

i. Hand-picking: If the impurities are different and are in large size, handpicking is used.

ii. Froth floatation process: This process is based on the preferential wetting of ore by oil. It is used for the concentration of sulphide ores like copper pyrite (CuFeS₂), zinc blende (ZnS), cinnabar (HgS), galena (PbS), argentite (Ag₂S), etc.
In this process, powdered ore is taken in a tank containing water and a small amount of pine oil. The mixture is heated by passing a blast of air. Impurities are wetted by water and get collected at the bottom of the tank. Ore particles are wetted by oil and come to the surface as froth. The froth is skimmed off to collect concentrated ore.

iii. Gravity separation process: This process is based on the difference in density of ore and matrix. It is used for the concentration of oxide ores like hematite (Fe₂O₃).
In this process, the powdered ore is washed with an upward stream of water. Heavier particles settle down washing the lighter impurities.

iv. Magnetic separation process: This process is used for the separation of magnetic impurities from non-magnetic ore and vice-versa. For eg. non-magnetic ore, tinstone (SnO2) is separated from magnetic impurity, wolframite (FeWO₄).
In this process, powdered ore is dropped over a belt revolving around the electromagnetic roller. Magnetic particles are attracted by a magnetic roller and form a heap near the belt while non-magnetic particles form a heap away from the magnetic particles.

v. Leaching (a chemical separation process): In this method, the ore is treated with a suitable chemical reagent. The compounds of the ore dissolves whereas the impurities remain unaffected. Examples:
Bauxite is purified by reacting it with hot NaOH to get sodium meta aluminate which on heating with water followed by ignition gives alumina.

3. Calcination and Roasting:

A. Calcination: The process of heating concentrated ore in the absence or limited supply of air below the melting point to convert into oxide is called calcination. Following changes takes place during calcination:

i. Volatile impurities and moisture are removed.
ii. Impurities like S, P, As get oxidized.
S + O₂ → SO₂
P₄ + 5O₂ → 2P₂O₅
4As + 5O₂ → 2As₂O₅

iii. Carbonate or hydroxide ores are decomposed into oxides ores.
ZnCO₃(calamine) → ZnO + CO₂
CuCO₃.Cu(OH)₂(malachite) → 2CuO + CO₂ + H₂O

iv. Water of crystallization is removed.
2Fe₂O₃.3H₂O(limonite) → 2Fe₂O₃ + 3H₂O

B. Roasting: The process of heating concentrated ore in presence of an excess supply of air below the melting point to convert into oxide is called roasting. Following changes takes place: i. Volatile impurities, organic matters, and water of crystallization are removed.
ii. Impurities like S, P, As get oxidized.
iii. Sulphide ores are oxidized into oxide ores.
2ZnS + 3O₂ → 2ZnO + 2SO₂
2PbS + 3O₂ → 2PbO + 2SO₂

Both calcination and roasting are carried out in a reverberatory furnace.

4. Reduction:

It is used for converting metallic oxide into metal. The common reducing agents used in metallurgy are coke (C), CO, H₂, Al, Mg, Zn, etc. The reduction is done by the following processes:

i. Reduction by carbon (Smelting):

It is the process of heating calcined or roasted ore strongly in presence of coke to get metal in the molten state. Ores after calcination or roasting contain some refractory materials as impurities. The materials which do not melt and don’t get volatilized at high temperature are called refractory impurities. The substance used in smelting to convert refractory infusible material into light fusible mass is called flux. The fusible mass is called slag which is obtained by the reaction of refractory material and flux.

Refractory material(infusible material) + Flux → Slag(fusible mass)

Flux may be acidic or basic.

Acidic flux: They are used to remove basic impurities like FeO, MnO, CaO, etc. as their slag. eg. silica (SiO₂), borax (Na₂B₄O₇.10H₂O), etc.
CaO(impurities) + SiO₂(flux) → CaSiO₃(slag)

Basic flux: They are used to remove acidic impurities like SiO₂, P₂O₅, etc. as their slag. eg. CaO, MnO, etc.
SiO₂(impurities) + MgO(flux) → MgSiO₃(slag)

During smelting, roasted or calcined ore is mixed with coke and suitable flux in a blast furnace. The blast of hot air is supplied from the bottom of the furnace. In doing so, metallic oxide gets reduced to metal and flux reacts with infusible impurities to give slag. In the hearth of the furnace, molten metal is obtained at the bottom and slag floats over the molten metal.

ii. Reduction by aluminium (Alumino-thermite reduction):

Some metallic oxides like Cr₂O₃ and Mn₃O₄ cannot be reduced by carbon because oxygen has a high affinity with these metals than with carbon. In such a case, aluminium is used as a reducing agent.
In the alumino-thermite process, the mixture of roasted or calcined ore and aluminium (the mixture is called thermite) is mixed with barium peroxide and suitable flux in a large crucible. Burning magnesium is introduced in the crucible to start ignition. A large amount of energy is released as the reaction is exothermic. The metallic oxide is reduced to metals in the molten form which is collected at the bottom.

Cr₂O₃ + 2Al → 2Cr + Al₂O₃ + heat
3MnO₂ + 4Al → 3Mn + 2Al₂O₃ + heat

iii. Electrolytic reduction:

In this method, the fused state of the ore is subjected to electrolysis. In doing so, metal is obtained at the cathode. eg. In metallurgy of sodium by Down’s process.

5. Purification of metals (refining)

Metals obtained after reduction are associated with many impurities. The methods to purify metals mainly depend upon the nature of metal and impurities. Some methods for the purification of metals are given below.

i. Poling:
This method is used when the metal oxides are present as impurities (Cu, Pb). In this method, molten metal is stirred with poles of greenwood. The hydrocarbon present in the greenwood reduces metal oxide into free metal. eg.
Cu₂O + CH₄ → Cu + CO + H₂O

ii. Electrolytic refining:
Metals like Cu, Zn, Ag, Au, etc. are refined by this method. In this method, impure metal is made anode, a thin sheet of pure metal is made cathode, and a suitable metal salt of the same metal is taken as an electrolyte in an electrolytic cell. When the electric current is passed through the cell, metal from the anode dissolves and deposits as a pure metal in the cathode.

eg. Purification of copper:
The impure copper is taken as anode and a thin sheet of pure copper is taken as a cathode in a large tank. Acidified CuSO₄ solution is taken as an electrolyte. On electrolysis, the anode dissolves and deposited as pure copper in the cathode. The impurities are left behind near the anode as anode mud.

At anode: Cu(s)(impure) → Cu⁺⁺ + 2e^–
At cathode: Cu⁺⁺ + 2e^– → Cu(pure)

Some Important Questions

1. Every ore is a mineral but every mineral is not an ore. Explain.
2. Define metallurgy with its types.
3. Define gangue with examples.
4. Explain the froth floatation process and magnetic separation process.
5. Differentiate calcination and roasting.
6. Write the reactions of each taking place in the smelting and alumino-thermite process.
7. Define amalgam and metalloids with examples.