Minerals, Ores, Extraction of Metals is Metallurgy
The earth’s crust is the major source of metals. Seawater also contains some soluble salts such as sodium chloride, magnesium chloride, etc. The elements or compounds, which occur naturally in the earth’s crust, are known as minerals. At some places, minerals contain a very high percentage of a particular metal and the metal can be profitably extracted from it. These minerals are called ores.
Steps of metallurgy:
Crushing of the ore.
Concentration of ore also known as Dressing of the ore.
Conversion of concentrated ore to oxide
Reduction of oxide to metal
Refining of metal or Purification
Minerals:- The compounds of an element that occurs naturally in the earth’s crust.
Ores: - The minerals from which an element can be profitably and conveniently extracted.
Enrichment of ores: - It is done to remove earthy impurities like clay, sand etc. from an ore is called enrichment or concentration of the ore.
Gangue: - The earthy impurities that is found with ores.
Native Ores: These ores contain the metal in free state eg. Silver gold etc. These are usually formed in the company of rock or alluvial impurities like clay, sand etc.
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.
Sulphurised Ores: These ores consist of sulfides of metals like iron, lead, mercury etc. Examples are iron pyrites (FeS2). galena (PbS), Cinnabar (HgS)
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.
Concentration Methods
a) Physical Method :
Gravity separation: 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.
Froth Floatation Process: 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.
Electro Magnetic Separator:. 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 impurity falls farther off
b) Chemical Methods
Calcination: Carbonate or hydrated oxide ores are subjected to the action of heat in order of expel water from hydrated oxide and carbon dioxide from a carbonate.
Examples:
ZnCO3 --> ZnO + CO2
CaCO3 --> CaO + CO2
Al2O3×2H2O --> Al2O3 + 2H2O
2Fe2O3×3H2O --> 2Fe2O3 + 3H2O
Roasting: Sulphide ores either are subjected to the action of heat and air at temperatures below their melting points in order to bring about chemical changes in them.
Examples:
2PbS + 3O2 --> 2PbO + 2SO2
PbS + 2O2 --> PbSO4
2ZnS + 3O2 --> 2ZnO + 2SO2
ZnS + 2O2 --> ZnSO4
CuS + 2O2 --> CuSO4
2Cu2S + 3O2 --> 2Cu2O + 2SO2
Leaching: It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain insoluble. The ore is recovered from the solution by suitable chemical method.
Al2O3 + 2NaOH -->2 NaAlO2 + H2O
Extracting metal low in activity series:- The metals low in the reactive series are very less reactive. The oxides of such metals can be reduced to metals by heating alone e.g. mercury is obtained from its ore, cinnabar (HgS), by the process of heating.Extraction of metal using Heat is called Pyrometallurgy.
Heat
2HgS(s) + 3O2 --------------> 2HgO(s) + 2SO2(g)
2HgO(s) --------------> 2Hg(l) + O2(g)
Copper can also be obtained in a similar manner from its sulphide ore (Cu2S).
Heat
2Cu2S + 3O (g) -------------------> 2u2O(s) + 2S2O (g)
Heat
2Cu2O + Cu2S -------------------------> 6Cu(s) + SO2 (g)
Extracting Metals in the Middle of the Activity series
Metals like iron, zinc, lead etc. occur as sulphides or carbonates in nature. First the ore is converted into its oxide form This is done by either roasting or by calcination.
Reduction of Free Metal
a) Smelting: Reduction of a metal from its ore by a process involving melting
Several reducing agents such as sodium, magnesium and aluminium are used for reduction.
The calcinated or roasted ore is mixed with carbon (coal or coke) and heated in a reverberatory or a blast furnace.
Carbon and carbon monoxide produced by incomplete combustion of carbon reduce the oxide to the metal.
PbO + C -> Pb + CO
PbO + CO -> Pb + CO2
SnO2 + 2C -> Sn + 2CO
CrO3 + 2Al -> 2Cr + Al2O3
3Mn3O4 + 8Al -> 9 Mn + 4 Al2O3
b) Flux
The ores even after concentration contain some earthy matter called gangue which is heated combine with this earthy matter to form an easily fusible material. Such a substance is known as flux and the fusible material formed during reduction process is called slag.
Acidic fluxes like silica, borax etc are used when the gangue is basic such as lime or other metallic oxides like MnO, FeO, etc
Basic fluxes like CaO, lime stone (CaCO3), magnesite (MgCO3), haematite (Fe2O3) etc are used when the gangue is acidic like silica, P4O10 etc.
Metals are ductile. It is the property that allows the metals to be drawn into wires. Non metals are not ductile.
Metals are lustrous, malleable, ductile and are good conductors of heat and electricity. They are solids at room temperature, except mercury which is a liquid. Metals can form positive ions by losing electrons to non-metals. The extraction of metals from their ores and then refining them for use is known as metallurgy Steps involved in Extraction of metals from ore include dressing or ore, concentration , roasting and calcination and refining. The surface of some metals, such as iron, is corroded when they are exposed to moist air for a long period of time. This phenomenon is known as corrosion. Corrosion can be prevented by barrier protection, sacrificial protection and cathode protection. The arrangement of metals in decreasing order of their reactivity is called the activity or reactivity series of the metals. All metals above hydrogen, in the activity series, reduce hydrogen ions from dilute acids and give out hydrogen.
The sulphide ores are converted into oxides by heating strongly in the presence of excess air. This process is known as roasting.
The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination.
1. Roasting:-
2Zns(s) + 3O2 (g)---- heat --------> 2ZnO +2SO2 (g)
2Pbs (s) + 3O2 (g)---- heat --------> 2PbO +2SO2 (g)
4Fes (s) +7O2 (g)---- heat --------> 2Fe2O3 +4SO2 (g)
2. Calcination:-
ZnCO3(s) -----heat--------> ZnO(s) +CO2(g)
Al2O3.2H20(s) -----> Al2O3(s) + 4 H2O(g)
CaCO3 ---------> CaO + CO2
3. SMELTING
The metal oxide is reduced to metal by using suitable reducing agent such as Carbon.
For example, when zinc oxide is heated with carbon, it is reduced to metallic zinc.
ZnO(s) + C(s) → Zn(s) + CO(g)
Besides using carbon (coke) to reduce metal oxides to metals, sometimes displacement reactions can also be used. The highly reactive metals such as sodium, calcium, aluminium, etc., are used as reducing agents because they can displace metals of lower reactivity from their compounds.
For example, when manganese dioxide is heated with aluminium powder, the following reaction takes place –
3MnO2(s) + 4Al(s) → 3Mn(l) + 2Al2O3(s) + Heat
The reaction of iron(III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the thermit reaction.
Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) + Heat
Extracting metals towards the top of activity series The metals high up in the activity series are very reactive. They cannot obtain by reduction of their oxides by carbon. These metals are obtained by the electrolysis of their molten chlorides e.g. Extraction of Na from NaCl
At cathode Na+ + e- -----àNa
At anode 2Cl- --à Cl2 + 2e-
Sodium Rock salt: NaCl
Feldspar:Na3AlSi3O8 Electrolysis of fused NaCl or NaCl/CaCl2 Sodium is highly reactive, it reacts with water
Copper Copper pyrites:CuFeS2
Malachite:CuCO3.Cu(OH)2
Cuprite: Cu2O
Copper glance: Cu2S Roasting of sulphide partially and reduction
2Cu2O + Cu2S → 6 Cu +SO2 It is self-reduction in a specially designed converter. Sulphuric acid leaching is also employed.
Aluminium Bauxite: AlOx(OH)3-2x where 0<x<1
Cryolite: Na3AlF6
Kaolinite: [Al2(OH)4Si2O5] Electrolysis of Al2O3 dissolved in molten cryolite or in Na3AlCl6 A good source of electricity is needed in the extraction of Al
Zinc Zinc blende or Sphalerite:
ZnS
Zincite: ZnO
Calamine: ZnCO3 Roasting and then reduction with C The metal may be purified by fractional distillation.
Lead Galena: PbS Roasting of the sulphide ore and then reduction of the oxide Sulphide ore is concentrated by froth floatation process
Silver Argentite: Ag2S
Sodium cyanide leaching of the sulphide ore and finally replacement of Ag by Zn It involves complex formation and displacement
Gold Native, small amounts in many ores such as those of copper and silver Cyanide leaching, same as in case of silver Gold reacts with cyanide to form complex
Important Ores of Iron:
Haematite Fe2O3 (red oxide of iron)
Limonite Fe2O3×3H2O (hydrated oxide of iron)
Magnetite Fe3O4 (magnetic oxide of iron)
Siderite: FeCO3
Iron pyrites: FeS2 Reduction with the help of CO and coke in blast furnace.
Limestone is added as flux that removes SiO2 as calcium silicate (slag) floats over molten iron and prevents its oxidation. Temperatures approaching 2170 K is required.
Sintering: It is a process of crushing ore to reasonable size before concentration of ore. It is followed by heating of solid particles below its melting point to change it into a single mass.
Concentration of ore: The process of removing unwanted materials like sand, clay, rocks etc. from the ore is known as concentration, ore-dressing or benefaction. It involves several steps that depend upon physical properties of metal compound and impurity (gangue). The type of metal, available facilities and environmental factors are also taken into consideration.
Hydraulic washing (or gravity separation): It is based on difference in densities of ore and gangue particles. Ore is washed with a stream of water under pressure so that lighter impurities are washed away whereas heavy ores are left behind.
Magnetic separation: This method is based on the difference in magnetic and non – magnetic properties of two components of ore (pure and impure). This method is used to remove tungsten ore particles from cassiterite (SnO2). It is also used to concentrate magnetite (Fe3O4), chromite (FeCr2O4) and pyrolusite (MnO2) from unwanted gangue.
Froth Floatation Process: The principle of froth floatation process is that sulphide ores are preferentially wetted by the pine oil, whereas the water wets the gangue particles. Collectors are added to enhance the non-wettability of the mineral particles. Examples are pine oil, fatty acids and xanthates are added to it. Froth stabilizers are added to stabilize the froth. Examples are cresols, aniline. If two sulphide ores are present, it is possible to separate the two sulphide ores by adjusting proportion of oil to water or by adding depressants. For example- In the case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to froth but allows PbS to come with the froth.
Leaching (Chemical separation): It is a process in which ore is treated with suitable solvent that dissolves the ore but not the impurities.
Conversion of ore into oxide: It is easier to reduce oxide than sulphide or carbonate ore. Therefore, the given ore should be converted into oxide by suitable method:
Roasting Calcination
A process in which ore is heated in a regular supply of air at a temperature below melting point of the metal to convert the given ore into oxide ore. It is a process of heating ore in limited supply of air to convert carbonate ores into oxides.
Sulphide ores are converted into oxide by roasting Carbonate ores are converted into oxide by calcination
It is also used to remove impurities as volatile oxides It is also used to remove moisture and volatile impurities
e.g.: 2ZnS+ 3O2→ 2ZnO + 2SO2 E.g.: CaCO3□(→┴heat )CaO+CO2
Reduction of oxide to metal: The process of converting metal oxide into metal is called reduction. It needs a suitable reducing agent depending upon the reactivity or reducing power of metal. The common reducing agents used are carbon or carbon monoxide or any other metals like Al, Mg etc.
Coupled reactions: If reactants and products of two reactions are put together in a system and the net ΔG of two possible reactions is –ve the overall reaction will take place. These reactions are called coupled reactions.
Ellingham diagrams: The plots between ΔfGӨ of formation of oxides of elements vs. temperature are called Ellingham diagrams. It provides a sound idea about selecting a reducing agent in reduction of oxides. Such diagrams help in predicting the feasibility of a thermal reduction of an ore. ΔG must be negative at a given temperature for a reaction to be feasible.
Limitation of Ellingham Diagrams: It does not take kinetics of reduction into consideration, i.e., how fast reduction will take place cannot be determined.
Types of iron:
Pig iron: The iron obtained from blast furnace is called pig iron. It is impure form of iron containing 4% carbon and small amount of S, P, Si and Mn. It can be casted into variety of shapes.
Cast iron: It is made by melting pig iron with scrap iron and coke using hot air blast. It contains about 3% of carbon content. It is extremely hard and brittle.
Wrought iron: It is the purest form of commercial iron. It is also called malleable iron. It is prepared by oxidative refining of pig iron in reverberatory furnace lined with haematite thatoxidisescarbon to carbon monoxide.
Electrolytic Reduction (Hall – Heroult Process): Purified bauxite ore is mixed with cryolite (Na3AlF6) or CaF2 that lowers its melting point and increases electrical conductivity. Molten mixture is electrolysed using a number of graphite rods as anode and carbon lining as cathode.
The graphite anode is useful for reduction of metal oxide to metal.
2Al2O3 + 3C → 4Al + 3CO2
Al2O3□( -> electrolysis )2Al3++ 3O2-
At cathode: Al³+ (melt) + 3e-→ Al (l)
At anode: C(s) + O²- (melt) → CO (g) + 2e-
C(s) + 2O²- (melt) → CO2 (g) + 4e-
Graphite rods are burnt forming CO and CO2. The aluminium thus obtained is refined electrolytically using impure Al as anode, pure Al as cathode and molten
cryolite as electrolyte.
At anode: Al → Al³+ + 3e-
(Impure)
At cathode: Al³+ + 3e-→ Al (pure)
Electrolysis of molten NaCl:
NaCl→Na++ Cl-
(Molten)
At cathode: Na++ e-→ Na
At anode: 2Cl-→ Cl2 + 2e-
Thus sodium metal is obtained at cathode and Cl2 (g) is liberated at anode.
Refining: It is the process of converting an impure metal into pure metal depending upon the nature of metal.
The metals obtained by the application of above reduction methods from the concentration ores are usually impure. The impure metal is thus subjected to some purifying process known as refining in order to remove undesired impurities. Various process for this are
a) Liquation process b) Distillation process c) Cupellation d) Poling e) Elecrolytic refining
f) Bessemerisation
Distillation: It is the process used to purify those metals, which have low boiling points, e.g., zinc, mercury, sodium, potassium. Impure metal is heated to convert it into vapours that changes into pure metal on condensation and is obtained as distillate.
Liquation: Those metals that have impurities whose melting points are higher than metal can be purified by this method. In this method, Sn metal can be purified. Tin containing iron as impurities heated on the top of sloping furnace. Tin melts and flows down the sloping surface where iron is left behind and pure tin is obtained.
Electrolytic refining: In this method, impure metal is taken as anode, pure metal is taken as cathode, and a soluble salt of metal is used as electrolyte.
When electric current is passed, impure metal forms metal ions that are discharged at cathode forming pure metal.
At anode: M →Mⁿ+ + ne-
(Impure)
At cathode: Mⁿ++ ne-→ M
(Pure)
Zone refining: It is based on the principle that impurities are more soluble in the melt than in the solid state of the metal.
The impure metal is heated with the help of circular heaters at one end of the rod of impure metal. The molten zone moves forward along with the heater
with impurities, reaches the other end, and is discarded. Pure metal crystallizes out of the melt. The process is repeated several times and heater is moved in the same direction. It is used for purifying semiconductors like B, Ge, Si, Ga and In.
Vapour phase refining: Nickel is purified by Mond’s process. Nickel, when heated in stream of carbon monoxide forms volatile Ni(CO)4 which on further subjecting to higher temperature decomposes to give pure metal.
Ni + 4 CO □(→┴(330-350K) )Ni(CO)4□(→┴(450 – 470K ) ) Ni + 4 CO
(Impure) (Pure)
van-Arkel method: It is used to get ultra-pure metals. This process purifies Zr and Ti. Zr or Ti are heated in iodine vapours at about 870 K to form
volatile ZrI4 or TiI4 which are heated over tungsten filament at 1800K to give pure Zr or Ti.
Ti + 2I2→TiI4→Ti + 2I2
(Impure) (Pure)
Zr + 2I2→ZrI4→Zr + 2I2
(Impure) (Pure)
Chromatographic method: It is based on the principle of separation or purification by chromatography that is based on differential adsorption on an adsorbent. In column chromatography, Al2O3is used as adsorbent. The mixture to be separated is taken in suitable solvent and applied on the column. They are then eluted out with suitable solvent (eluant). The weakly adsorbed component is eluted first. This method is suitable for such elements that are available only in minute quantities and the impurities are not very much different in their chemical behaviour from the element to be purified.
Give the reaction of magnesium with water
Mg + 2H2O -----------> Mg(OH)2 + H2
Electroplating: Iron and steel are protected from corrosion by coating them with a
layer of tin or chromium metals which are resistant to corrosion. This can be done by electroplating. A thin layer of tin deposited on the inner surface of iron containers makes them safe, for storing food.
Chromium plating protects steel furniture, taps and bicycle handles from corrosion. Coating of chromium not only giveslonger life to the coated objects but also gives a good shining appearance to them.
Important Ores of Aluminium:
Bauxite : Al2O3×2H2OCryolite: Na3AlF6
Feldspar: K2OAl2O3×6SiO2 or KAlSi3O8
Mica: K2O×3Al2O3×6SiO2×2H2O
Corundum: Al2O3
Alumstone or Alunite: K2SO4×Al2(SO4)3×4Al(OH)3
The addition of cryolite (Na3AlF6) and fluorspar (CaF2) makes alumina a good conductor of electricity and lowers its Fusion temperature from 2323 to 1140 K. the reaction taking place during electrolysis.
Refining of Aluminium:
The graphite rods dipped in pure aluminium and Cu–Al alloy rod at the bottom in the impure aluminium work as conductors. On electrolysis, aluminium is deposited at cathode from the middle layer and equivalent amount of aluminium is taken up by the middle layer from the bottom layer (impure aluminium).
Therefore, aluminium is transferred from bottom to the top layer through middle layer while the impurities are left behind. Aluminium thus obtained is 99.98% pure.
Hyrdrometallurgy (solvent extraction)
Solvent extraction is the latest separation technique and has become popular because of its elegance, simplicity and speed. The method is based on preferential solubility principles.
Solvent or liquid-liquid extraction is based on the principle that a solute can distribute itself in a certain ratio between two immiscible solvents, one of which is usually water and the other an organic solvent such as benzene, carbon tetrachloride or chloroform. In certain cases, the solute can be more or less completely transferred into the organic phase
Q.1- What is slag?
A.1- It is easily fusible material fusible material, which is formed when gangue still present in roasted ore combines with the flux. e.g. CaO (flux) +
SiO2 (gangue) →CaSiO3 (slag)
Q.2- Which is better reducing agent at 983K, carbon or CO?
A.2- CO, (above 983K CO being more stable & does not act as a good reducing agent but carbon does.)
Q.3- At which temperature carbon can be used as a reducing agent for Foe ?
A.3- Above 1123K, carbon can reduce FeO to Fe.
Q.4- What is the role of graphite rods in electrometallurgy of aluminium ?
A.4- Graphite rods act as anode, are attacked by oxygen to form CO2 and so to be replace time to time.
Q.5- What is the role of cryolite in electrometallurgy of aluminium?
A.5- alumina cannot be fused easily because of high melting point. Dissolving of alumina in cryolite furnishes Al3+ ions, which can be electrolyzed easily.
Q.6- What are depressants?
A.6- It is possible to separate two sulphide ore by adjusting proportion of oil to water in froth flotationprocess by using a substance known as
depressant. e.g. NaCN is used to separate ZnS and PbS.
Q.7- Copper can be extracted by hydrometallurgy but not Zn. Why?
A.7- The E0 of Zn is lower than that of Cu thus Zn can displace Cu2+ ion from its solution. On other hand side to displace Zn from Zn2+ ion, we need a more reactive metal than it.
Q.8- Give name and formula of important ore of iron .
A.8- Haematite – Fe2O3, Magnetite –Fe3O4, Iron pyrites FeS2.
Q.9- Give name and formula of important ore of Copper .
A.9- Copper pyrites CuFeS2, Malachite CuCO3 . Cu (OH) 2, Cuprite Cu2O.
Q.10- Give name and formula of important ore of Zinc .
A.10- Zinc blende - ZnS, Calamine- ZnCO3, Zincite – ZnO
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