Metallurgy, Chemistry tutorial

Introduction:

Metals frame a main part of the elements in the periodic table. Metals are categorized into ferrous and non-ferrous, having the ferrous metals group being Iron, Manganese, Chromium and their alloys. The non-ferrous metals are of four kinds namely: the heavy metals like Cu, Pb, Sn and Ni, Light metals (that is, alkaline and alkaline earth metals), the Noble or precious metals like Pt, Ir, Os, Pd, Ru, Rh and Ag; and the rare metals. Metals usually are very significant for the structural applications, electrical wires, cooking vessels, decorative items and numerous other purposes. Though, metals have as their main chemical characteristic, the capability to give up electrons and as such nearly all metals in nature are found in ores, joins with non-metals like oxygen, sulphur and the halogens. This thus, requires their separation of such metals from their ores and reduction of the metal ions to the free metals and makes them available for use in an appropriate form. This method is termed as metallurgy. 

Metallurgy, what does it signify?

We are familiar that because of the major chemical property of metals (that is, the capability of giving up electrons); they exist in ores in combination with non-metals. Thus, in order to have such metals in an appropriate and stable form, metallurgy comes into play. Now, what is metallurgy?

Metallurgy is basically the branch of chemistry that deals the process of extraction of metals from their ores and preparation of alloys. 

The extraction of metals can't be carried out via any universal method. Why? This is because extraction of metals needs various methods or techniques of extraction that based on the nature and properties of the metal. Usually, the noble metals like Au, Ag and so on are generally extracted via amalgamation or cyanide process. As well, the active metals like Na, K, Ca, Mg, Al and so on are generally obtained via electrolysis of their chlorides, oxides or hydroxides while the heavy metals are extracted via the use of roasting and smelting processes.

Types of Furnaces used:

In the metallurgical operations, different kinds of furnaces are employed for heating substances to requisite temperatures. The following are the kinds of furnaces used in the metallurgical operations: 

a) Reverberating furnace: This is particularly employed for calcinations, roasting and smelting

b) Blast Furnace: It is mainly employed for the smelting of iron, copper and lead ores.

c) Bessemer converter: It is utilized in the extraction of copper and in the preparation of steel. 

d) Open Hearth Furnace: It is mainly used for the manufacture of steel. This furnace is based on the regenerative system of heat economy. The regenerative system in that in the heat is discharged (that is, heat content of the furnace gases) is utilized for preheating the gaseous fuels employed in the furnace to save cost.

e) Electric Furnace: Mainly used for manufacturing very fine steel.

Steps involved in Metallurgical Operations:

There are fundamentally three main steps comprised in the metallurgical operations (or metallurgy). These are: Ore pretreatment or Ore dressing, Extraction of crude metal and metal purification. The three steps are illustrated below, let us now look into what each of such steps comprise.

1) Ore Dressing:

Ore pretreatment is the technique of eliminating or removing gangue or matrix (that is, non-metallic and rocky materials like quartz, mica, feldspars and other silicates) from a metal ore. This is accomplished by the given methods:

a) Hand Picking:

The gross lumps of rocks might be eliminated from the ore via simple hand picking and the lumps are then broken away by the hammer. An illustration of the ore in which this is applicable is Heamatite (that is, an iron ore).

b) Hydraulic washing or Levigation:

In this, the ore after grinding is washed by a running stream of water. The lighter gangue particles are therefore washed away whereas the heavier ore particles settle down quickly. This technique is in reality based on the difference in the densities of gangue and the mineral particles.

c) Froath floatation:

This is particularly appropriate for the concentration of low grade ores. The method is mainly based on the different wetting features of the ore and gangue particles by water and oil. The ore is preferentially wetted via oil and the gangue particles by water. Ores such as copper pyrites (CuFeS2), galena (PbS) and zinc blende (ZnS) are purified via this process.

d) Magnetic concentration:

This method is employed whenever the mineral is attracted via a magnet, however not the gangue. This is applicable in the case of ferromagnetic ores like iron, tinstone related with wolfram.  

e) Electromagnetic separation:

This process is employed for separating ore from magnetic impurities. In this technique, the crushed or powdered ore is dropped over a belt moving over two rollers, one of which is magnetic. As the mass passes over the electromagnetic roller, the non- magnetic ore falls off and the magnetic impurities are held and moved round the magnetic roller so long as they are attracted by it. Though, whenever there is no more magnetic force of attraction, the impurities fall down to a separate heap. An illustration is tinstone ore which is separated from the magnetic impurity, wolframite (FeWO4).

2) Extraction of Crude metal:

Subsequent to the ore has been pretreated, the metal is then extracted. This extraction comprises the utilization of the given chemical reaction or processes.

a) Calcination:

Calcination is the technique or process in which the mineral ore is subjected to the action of heat at high temperature in the absence of air however below its melting point. The method of calcination is carried out in the case of carbonate and hydrated ores. Calcination is generally carried out in a reverberatory furnace.

This method yields in the following:

  • Removal of moisture
  • Removal of volatile impurities
  • Expellation of gases might occur
  • The mass becomes porous
  • Thermal decomposition of the ore occurs

Illustrations of ores in which the calcination is used are as follows:

CaCO3 (limestone) → CaO + CO2

MgCO3.CaCO3 (dolomite) → MgO + CaO + 2CO2

CuCO3.Cu(OH)2 (Malachite) →  2CuO + H2O + CO2

ZnCO3 (Calamine) → ZnO + CO2

b) Roasting:

Roasting comprises subjecting either the ore alone or by the addition of appropriate material to the action of heat in surplus of air at temperatures beneath its melting point. This is generally taken out in a reverberatory or blast furnace. Throughout roasting, volatile impurities such as S, As, Sb and so on get oxidized and escape out as volatile gases SO2, As2O3 and Sb2O3. Sulphide ores as well decompose to their oxides evolving SO2. Moisture is taken out and the mass becomes porous therefore making reduction simple. 

There are numerous kinds of roasting namely:

  • Oxidizing roasting
  • Blast roasting
  • Reduction roasting
  • Sulphating roasting
  • Chlorodising roasting

c) Smelting:

This is the method in which an ore is melted by a flux and often by a reducing agent and it comprises calcination, roasting and reduction. Usually, the method of separation of a metal or its sulphide mixture from its ore in a fused state is known as smelting. Smelting is usually carried out in a blast furnace and high temperature is generated by burning coal or via utilizing electric energy.  

In smelting, the roasted or calcined ore is mixed by coke and then heated in the furnace. As an outcome, carbon and carbon monoxide generated by the incomplete combustion of carbon reduce the oxide to the metal. For illustration, in the extraction of iron heamatite ore (Fe2O3) is smelted by coke and limestone (that is, flux). As an outcome of reduction, iron is obtained in fused of molten state.

Fe2O3 + 3C → 2Fe + 3CO

Fe2O3 + 3CO → 2Fe + 3CO

CaCO3 → CaO + CO2

CaO + SiO2 → CaSiO2

(Flux) (Gangue) (Slag)

d)  Reduction:

The extraction of metals from their ore usually signifies reduction and it can be taken out in a number of manners. We shall consider some of these manners.

i) Reduction by heating in air: The metals whose sides are unstable towards heat (that is, the less active metals like Hg, Pb, Cu, Sb and so on) are extracted via air reduction. For illustration: roasting of cinnabar (that is, a sulphide ore of mercury) results the mercury metal and not the oxide.

2HgS + 3O2 → 2HgO + 2SO2

2HgO → 2Hg + O2

2HgO + HgS → 3Hg+ SO2

ii) Auto reduction method: This procedure doesn't comprise any additional reducing method and is identical to the above mentioned manner of reduction. In this process sulphide is heated in air until a portion is transformed into oxide. On further heating in the air, the unchanged sulphide reduces the oxide to metal. Example: 

2PbS + 3O2 → 2PbO + 2SO2

2PbO + PbS → 3Pb + SO2

iii) Reduction with coke or carbon monoxide: In this, the calcined or roasted ore is mixed by coke and heated in appropriate furnace. Carbon and CO (made up by incomplete combustion of carbon) reduce the oxide to the metal. For illustration, concentrated cassiterite (SnO2) is reduced to the metallic tin via heating with coke in the reverberatory or blast furnace.

SnO2 + 2C → Sn + 2CO

The other illustrations are as illustrated in the given chemical equations:

Fe2O3 + 3CO → 2Fe + 3CO2

ZnO + C → Zn + CO 

iv) Reduction with hydrogen: The oxides of certain metals like WO3, NiO, Co2O3, In2O3 might be conveniently reduced via means of hydrogen to the corresponding metals as illustrated below:

In2O3 + 3H2 → 2In + 3H2O

Co2O3 + 6H → 2Co + 3H2O

NiO + 2H → Ni + H2O

WO3 + 6H → 3H2O + W

This process is appropriate for metals that are heavier than manganese.

v) Reduction of complex salts: Metals such as gold, silver and so on can be precipitated from their complete salt solutions via more electropositive zinc metal.

2NaAg(CN)2 + Zn → Na2Zn(CN)4 + 2Ag

2KAu(CN) + Zn → K Zn(CN)  + 2Au

Metals like Ti, Zr, Ta and so on are obtained via reducing their complex salts with alkali metals or Aluminum.

K2TiF6 + 4K → 6KF + Ti

K2ZrF6 + 2Al → 2AlF3 +2K + Zr

Other reduction methods comprise reduction by aluminum for some metal oxides which can't be reduced efficiently with carbon or carbon monoxide, reduction with water gas, reduction by silicon or calcium carbide and amalgamation.

The subsequent step after extraction is the purification of metals.

3) Purification of Metals:

The metals obtained by the techniques illustrated above need sufficient refining as the metal is not pure. This refining is accomplished via a large number of physical and chemical methods. Though, the choice of refining method based on the use to which a metal is to be put.

There are some physical processes for refining the metals. These are:

a) Fusion method:

This process is applicable if the metal is related with adsorbed gases. For example, dissolved oxygen is eliminated from silver via this method and as well dissolved SO2 gas from the copper metal.

b) Liquation:

Liquation is used whenever the melting point of the metal is lower than that of the impurities. Therefore, the material should be heated till one of the metals begins to melt and drain away from the other and can be collected. Zinc is separated from lead via this method. 

c) Distillation:

This is employed to refine low boiling point metals like Hg, Cd and Zn. The method comprises heating the impure metal in a retort as a result of which pure metal distils over and collects in the receiver, whereas non-volatile impurities are left behind. The volatile compounds are further separated via similar method to obtain the pure metal.

The other physical methods comprise: fractional crystallization, vacuum arc sublimation method and cage-zone melting method.

Pyrometallurgical oxidation is a chemical method in which the impurities are eliminated from metals by oxidation. The oxidation might be taken out by either of the given methods illustrated below: 

i) Poling or furnace refining: The impurities of reducible oxides can be eliminated from the metals by poling. For illustration, blister copper having traces of cuprous oxide as impurity is refined through melting the impure metal on the hearth of a reverberatory furnace. The melt is stirred by green logs of wood. We might wonder why use green wood? Well, the green wood generates hydrocarbons that reduce the cuprous oxides to copper metal whereas reducing gases like SO2, A2O3 and so on are given off. In order to prevent the copper metal from re-oxidation via air, a layer of charcoal powder is employed to cover the molten copper surface. The other metal refined via poling is tin.

ii) Cupellation: This process is employed to purify silver having lead as impurity and based on the selective oxidation of lead over silver. The impure silver is smelted in the cupel made up of bone ash in a blast of air in reverberatory furnace. The lead is oxidized to lead oxide that is partially blown away from the crucible through blast of air. The remaining portion melts and is absorbed via the bone ash cupel. The completion of the purification method is pointed out by a flash generated by the pure molten silver in the cupel.

iii) Electrolytic refining: This is one of the most suitable and significant process of refining and it provides metal of high purity. This is as well applicable to numerous metals like Cu, Ag, Pb, Ni and so on. In this, the blocks of impure metals form the anode and thin sheets of metal form the cathode whereas the electrolyte is a solution of the salts of pure metals. On passing an electric current via the solution, pure metal dissolves from the anode and deposits on the cathode. At similar time, more of the metal ions enter the electrolyte via the oxidation of the anode. The insoluble impurities dissolve in the electrolyte or fall at the bottom and collect as anode mud. For illustration in the refining of copper, impurities such as Fe and Zn dissolve in the electrolyte whereas Au, Ag and Pt are left behind as anode mud.

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