What is The Meaning of Metallurgy

Aug 26, 2022, 16:45 IST

What is the meaning of Metallurgy

Metallurgy It is the process of extracting a metal from its ores. The materials available for making tools and weapons, houses and skyscrapers, computers and lasers have had a profound effect on the development of human civilization. The earth’s crust is the main source of metals. The occurrence of metal in native or in combined state in the earth’s crust along with a number of rocky and other impurities depends upon the chemical nature of metals. Metals having less electropositive character have less affinity for oxygen, moisture and occur in free or metallic or native state. Minerals which are naturally occurring chemical substances in the earth’s crust obtainable by mining. Out of many minerals in which a metal may be found, only a few are viable to be used as sources of that metal. Such minerals are known as ores.

Defination of Metallurgy

Metals normally do not exist in nature in free state. Due to having very high oxidation potential value these elements combine with various elements to acquire stability by completing their octet.

All the steps involved in the extraction of a metal from its naturally occuring form are combinedly called metallurgy.

SOME IMPORTANT TERMS FREQUENTLY USED IN METALLURGY

Minerals-The natural inorganic compounds containing the metal is called minerals.

Ores-The metal-bearing minerals which can be used a source for the commercial extraction of one or more metals are called ores.

Gangue-The foreign materials present in the ore are collectively known as gangue. It includes rock, clay, silica, silicates etc.

Flux -Flux is a substance which is added to the furnace charge to remove the gangue. The selection of the flux depends on the chemical nature of the gangue present; if the gangue is acidic in nature then to remove it basic flux should have been added and vice-versa.

Slag-The mixture of gangue and flux forms a fusible mass which is called slag.

Flux + Gangue = Slag

Calcination-It is the process of heating the ore in absence of air without fusion of the ore. The purpose of the process is to drive out the moisture and volatile impurities from the ore.

Roasting-It is also the process of heating the ore in a plentiful supply of air without fusion. But the temperature involved is relatively higher than that is involved in calcination. The process of roasting also removes the volatile impurities but the main purpose of roasting is to oxidize the ore to the corresponding oxides.

Ores may be divided into four groups

i) Native Ores: These ores contain the metal in free state eg. Silver gold etc.

ii) Oxidised Ores: These ores consist of oxides or oxysalts (eg. carbonates, phosphate) and silicate of metal. Important oxide ore includes, Fe2O3, Al2O3.2H2O etc. and important cabonate ores are limestone (CaCO3), Calamine (ZnCO3) etc.

iii) Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc.e.g iron pyrites (FeS2). galena (PbS), Cinnabar (HgS)

iv) Halide ores: Metallic halides are very few in nature. Chlorides are most common examples include horn silver (AgCl) carnallite KCl. MgCl2.6H2O and fluorspar (CaF2) etc.

parts of Metallurgy

It is the process of extracting a metal from its ores. The following operations are carried out for obtaining the metal in the pure form.

i) Crushing of the ore

ii) Dressing or concentration of the ore

iii) Reduction of metal.

v) Purification or refining of the metal

Ores usually contain soil, sand, stones and other useless silicates. These undesired impurities present in ores are called Gangue or Matrix. The removal of these impurities from the ores is known as concentration. The concentration process involve physical as well as chemical method.

Physical Method used in Metallurgy

a) Gravity separation: The separation is based on the difference in the specific gravities of the gangue particles and ore particles. The powdered ores is agitated with water or washed with a running stream of water. The heavy ore particles of sand, clay etc. are washed away. For this either wilfley table or hydraulic classifier is used.

b) Electro Magnetic Separator: When one component either ore or the impurity is magnetic in nature this method can be used for separation. A magnetic separator consists of a belt moving over two rollers, one of which is magnetic.

The powdered ore is dropped on the belt at the other end. Magnetic portion of the ore is attracted by the magnetic roller and falls near to the roller while the non-magnetic impurities fall farther off.

c) Froth Floatation Process: Metallic sulphides are wetted by certain oil like pine oils and not by water. The finely divided ore is introduced into water containing small quantity of oil (e.g. Pine oil). The mixture is agitated violently with air a froth is formed which carries away along with it the metallic particles on account of the surface tension forces. The froth is transferred to another bath where gangue-free ore settles down. As the heavy metallic material is floated upward with the froth, this process is known as froth floatation process. The method is based upon preferential wetting of surfaces by liquids.

Isolation of Metal and application of Mtallurgy

Extraction of pure metal from its naturally occurring form requires specific techniques and chemical treatments. The concentrated ore is subjected to calcination and roasting. Then the metal is extracted from the ore by one of the following methods.

(i)Electrolytic reduction process:

An electron is the reducing agent in this process. When molten form of an ionic compound of a metal is electrolysed, the metallic ion is reduced at the cathode, producing the metal; Mn++ ne → M. This is an excellent method and gives very pure products. But the method is highly expensive so it is used for the metals which cannot be conveniently reduced by other common reducing agents. Elements of group IA, IIA and IIIA are mainly extracted by this method.

The alkali and alkaline earth metals are too strongly electropositive to be obtained by the electrolysis of the aqueous solution of their salts, because in the electrolysis of their aqueous solution preferential discharge of H+ occurs at the cathode liberating H2 and not the metal.

These metals are mainly obtained by the electrolysis of their fused salts, mainly fused hydroxides (e.g., sodium and potassium) and fused chlorides (e.g., calcium and magnesium). The electrolysis of the fused salt at high temperature often leads to the formation of dispersed metal ‘fogs’. Therefore for carrying out the electrolysis at a relatively lower temperature a mixture of fused salts is often used e.g., mixture of fused sodium chloride and calcium chloride is used for the extraction of sodium in the Downs cell.

Aluminium is also extracted by this method. Fused aluminium oxide dissolved in molten cryolite and fluorspar is used for this process. The presence of cryolite (Na3AlF6) and fluorspar lowers the melting temperature of alumina from 2000°C to 870°C and makes the solution electrically more conductive. Aluminium is set free at cathode through the reaction

Al3+ + 3e → Al.

(ii)Thermit Process: The reduction of the metallic salt into metal can also be carried out by aluminium powder. If the metal is less electropositive than aluminium then metal can be displaced from its salt by the reaction with aluminium.

The process involves making of an intimate mixture of the oxide to be reduced with aluminium powder and then igniting the same by means of a pellet of magnesium powder and barium peroxide. Once started, the reaction is selfsupporting and very strongly exothermic A number of metals, e.g., chromium and manganese, are now obtained by this method in a high state of purity and particularly free from carbon:

Cr2O3 + 2Al → 2Cr + Al2O3; 3Mn3O4 + 8Al → 9Mn + 4Al2O3

This is also the basis of Thermit process, also called Goldschmidt’s ‘aluminothermic’ process or aluminothermy where molten iron is tap out for the purpose of welding through the reaction: Fe2O3 + 2Al = Al2O3 + 2Fe

(iii)Self reduction process: This process is mainly carried out for the extraction of the few metals namely lead and copper from their sulphide ores. In this process, the sulphide ore is first partially roasted in presence of air to convert it into corresponding oxides and then the supply of air is cut off. Concentrated but unroasted ore is now added to this roasted ore where sulphide and oxide react together forming metal.

2PbS + 3O2 → 2 PbO + 2SO2;

2PbO + PbS → 3 Pb + SO2

2Cu2S + 3O2 → 2Cu2O + 2SO2

2Cu2O + Cu2S → 6Cu + SO2

(iv)Carbonreduction process: Carbon can be used to reduce a large number of metallic compounds. Coke is readily available and is not too expensive. Therefore this method is widely used for the extraction of the metals from their salts. But for some metals very high temperature is needed which will make the process highly expensive and at high temperature carbon also reacts with the metals produced forming metallic carbides. Carbon reduction process is mainly used for the extraction metallurgy of Zn, Sn, Hg. The corresponding ore is first converted to oxide (by calcination and roasting) and then treated with coke.

ZnO + C → Zn + CO

SnO2 + C → Sn + CO2

(v)Carbon monoxide reduction process: Carbon monoxide is a very good reducing agent which is used to extract a few metals from corresponding oxides such as Iron and Copper.

Fe2O3 + 3CO → 2Fe + 3CO2 (Blast Furnace Reaction)

CuO + CO → Cu + CO2

(vi)Hydrometallurgy: Silver and gold are obtained by Mac Aurther Cyanide process. In this method metal is extracted from their ore by allowing the metal to go into aqueous solution forming water soluble salt and then precipitating the metal in the free state using a suitable reducing agent, usually a more electropositive and cost effective metal such as zinc is used in the case of silver which is obtained from its ore Argentite (Ag2S).

Ag¬2S + 2KCN → 2AgCN + K2S.

Silver cyanide dissolves in excess of KCN to give water soluble complex.

AgCN + KCN → K[Ag(CN)2]

Now metallic zinc granules are added.

2K[Ag(CN)2] + Zn → K2[Zn (CN)4] + 2Ag+

Questions based on Metallurgy

Question 1: The actual reducing agent of hematite in blast furnace is

(A) C (B) CO (C) Al (D) CO₂

Solution: C + O₂ → CO₂ ; C + CO₂ → 2CO

CO + Fe₂O₃ → 2FeO + CO₂ ; CO + FeO → CO₂ + Fe

(B)

Question 2: Calamine is an ore of

(A) Hg (B) Zn (C) Cd (D) Ca

Solution: Calamine is an ore of Zn containing ZnCO₃.

(B)

Question 3: Froth floatation process for the concentration of ores is a practical application of

(A) Adsorption (B) Absorption (C) Coagulation (D) Sedimentation

Solution: Gangue particles are wetted by water and adsorbed.

(A)

Question 4: Plumbosolvency refers to

(A) Oxidation of lead to lead oxide

(B) Oxidation of lead to red lead

(D) Making lead wires by forcing heated metal through a die

Solution: Lead dissolves in water containing dissolved air, due to the formation of lead hydroxide. This solvent action of water on lead is called plumbo solvency.

(c)

Question 5: The metal oxide, which cannot be reduced to metal by carbon is

(A) ZnO (B) Fe₂O₃ (C) PbO (D) Al₂O₃

Solution: Al has more affinity for oxygen than carbon.

(d)

Question 6: The material used in semiconductors

(A) Si (B) Sn (C) Ti (D) Cs

Solution: (A)

Question 7: Electrolytic bath electrolytic refining of lead contains

(A) H₂SiF₆ only

(B) PbSiF₆ only

(D) H₂SiF₆ and PbSiF₆ in presence of gelatin

Solution: (D)

Question 8: Which of the following is manufactured by Siemens Martin process?

(A) Pig iron (B) Cast iron (C) wrought iron (D) steel

Solution: (D)

Question 9: The process of producing a hard coating of iron nitride on the surface of steel is called

(A) Annealing (B) Quenching (C) Tempering (D) Nitriding

Solution: (D)

Question 10: Bordeaux mixture is

(A) CuSO₄+MgSO₄ (B) FeSO₄ + CuSO₄

Solution: (C)

Question 11: The main function of roasting is

(A) to remove the volatile impurities (B) oxidation of metal

Solution: (B)

Question 12: Which method of purification is represented by the following equation

Ni + 4CO → Ni(CO)₄ → Ni + 4CO

(A) van Arkel (B) zone refining

Solution: (C)

Question 13: The process of converting hydrated alumina into anhydrous alumina is called

(A) roasting (B) calcination (C) smelting (D) dressing

Solution: (B)

Question 14: Zone refining is a method to obtain

(A) Very high temperature (B) Ultra pure Al

Solution: (C)

Question 15: Thomas slag is -

(A) Ca₃(PO₄)₂ (B) MnSiO₃ (C) CaSiO₃ (D) FeSiO₃

Solution: (A)

What is The Meaning of Metallurgy