JEE Chemistry General Principles and Processes of Isolation of Elements Notes Set 06

Minerals, Ores and Types of Metallurgy

A mineral is any natural metal-bearing compound found in the earth's crust, and an ore is the mineral from which a metal can be extracted profitably. The unwanted rocky impurity is called gangue (matrix). Taking the ore out of the earth is mining, and the science of obtaining a metal from its ore is metallurgy.

Types of metallurgy:
1. Pyrometallurgy uses heat (calcination, roasting, smelting) and suits Fe, Zn, Pb, Sn, Cu.
2. Hydrometallurgy uses aqueous solutions, as in the cyanide process for Ag and Au.
3. Electrometallurgy uses electrolysis of fused salts for very electropositive metals (Na, K, Mg, Ca, Al).
4. Ion-exchange methods are used for trans-uranium elements.

Common ores: sulphides: galena PbS, zinc blende ZnS, copper glance Cu₂S, copper pyrites CuFeS₂, cinnabar HgS, argentite Ag₂S. Oxides: haematite Fe₂O₃, magnetite Fe₃O₄, bauxite Al₂O₃·2H₂O, cuprite Cu₂O, pyrolusite MnO₂, zincite ZnO. Carbonates: limestone CaCO₃, magnesite MgCO₃, dolomite CaCO₃·MgCO₃, siderite FeCO₃, calamine ZnCO₃, malachite CuCO₃·Cu(OH)2. Halides: rock salt NaCl, fluorspar CaF₂, cryolite Na₃AlF₆, horn silver AgCl, carnallite KCl·MgCl₂·6H₂O.

Steps in Extraction

1. Crushing and pulverisation of the ore.
2. Concentration (dressing) to remove gangue.
3. Conversion to oxide by calcination or roasting.
4. Reduction of the oxide to the metal.
5. Refining of the crude metal.

Concentration of the Ore

Gravity separation (levigation): a stream of water carries away lighter gangue and leaves the heavier oxide ore, so it suits oxide ores.

Froth flotation: used for sulphide ores, which are wetted by oil while gangue is wetted by water. Pine oil or eucalyptus oil is the frother, sodium ethyl xanthate is the collector, and NaCN acts as a depressant (it prevents ZnS from floating along with PbS) while CuSO₄ acts as an activator.

Magnetic separation: used when either the ore or the gangue is magnetic, for example magnetite, or removal of wolframite from SnO₂.

Leaching: chemical dissolution of the ore. Bauxite is dissolved in hot concentrated NaOH (Baeyer's process, for red bauxite with iron oxide impurity); in Hall's process the ore is fused with Na₂CO₃; in Serpeck's process white bauxite (with silica) is heated with coke in nitrogen to give AlN, which hydrolyses to Al(OH)3 and NH₃.

Calcination and Roasting

Calcination is heating in the absence or limited supply of air below the melting point. It removes water and CO₂ and makes the ore porous:
Al₂O₃·2H₂O → Al₂O₃ + 2H₂O
CaCO₃ → CaO + CO₂
ZnCO₃ → ZnO + CO₂
FeCO₃ → FeO + CO₂

Roasting is heating a sulphide ore in excess air, which converts it to oxide and removes sulphur, arsenic and antimony as volatile oxides:
2ZnS + 3O₂ → 2ZnO + 2SO₂
2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂
2PbS + 3O₂ → 2PbO + 2SO₂

Reduction of the Oxide

Smelting with carbon: oxides of Pb, Zn, Fe, Sn and Cu are heated with coke along with a flux. A flux combines with gangue to form a fusible slag: acidic gangue (SiO₂) is removed by basic flux (CaO from CaCO₃) as CaSiO₃; basic gangue is removed by acidic flux such as SiO₂ or borax.

Self reduction: partly roasted sulphide reacts with unroasted sulphide, as in 2Cu₂O + Cu₂S → 6Cu + SO₂ and 2PbO + PbS → 3Pb + SO₂.

Metal displacement: a more electropositive metal displaces a metal from its solution, for example Zn displaces Ag from Na[Ag(CN)₂].

Electrolytic reduction is used for Na, K, Mg, Ca and Al, whose oxides cannot be reduced by carbon.

Aluminothermy: Cr₂O₃ + 2Al → 2Cr + Al₂O₃. Chromium and manganese oxides are reduced by Al because carbon would give carbides.

Thermodynamics of Reduction: Ellingham Diagram

For a spontaneous reduction, ΔG must be negative, where ΔG = ΔH − TΔS. When a metal forms its oxide, gaseous O₂ is used up, so ΔS is negative and the plot of ΔG° against T slopes upward for most oxides. A metal reduces the oxide of any metal lying above it in the diagram, because the overall ΔG° becomes more negative.

The line for 2C + O₂ → 2CO slopes downward (ΔS is positive), so it crosses the line for 2CO + O₂ → 2CO₂ near 983 K: below this temperature CO is the better reductant and above it carbon is better.

Limitations: the diagram gives only thermodynamic feasibility, not the rate of reduction, and it assumes that reactants and products are in equilibrium.

Extraction of Copper and Lead

Copper (from CuFeS₂): the ore is concentrated by froth flotation and roasted:
2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂
It is then smelted with coke and silica, where FeO forms FeSiO₃ slag and a molten mixture of Cu₂S and FeS called matte is obtained. In the Bessemer converter silica and air are blown through the matte and self reduction gives blister copper: 2Cu₂O + Cu₂S → 6Cu + SO₂. Electrorefining then uses impure copper as the anode, pure copper as the cathode and acidified CuSO₄ as electrolyte; Ag, Au and Pt collect as anode mud.

Lead (from galena): the concentrated ore is partly roasted and then self-reduced: 2PbS + 3O₂ → 2PbO + 2SO₂; PbS + 2PbO → 3Pb + SO₂. Silver is removed by Parke's process using molten zinc, and lead is electrorefined (Betts process) in lead silicofluoride and hydrofluosilicic acid.

Extraction of Iron and Steel

Haematite is concentrated, roasted, and charged into a blast furnace with coke and limestone. In the upper zone (about 500–975 K) CO reduces the oxide in stages: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂, Fe₃O₄ + CO → 3FeO + CO₂, FeO + CO → Fe + CO₂. In the hotter lower zone C + CO₂ → 2CO, and CaCO₃ → CaO + CO₂ with CaO + SiO₂ → CaSiO₃ slag. Molten iron sinks and slag floats above it.

Forms of iron: pig iron (2.5–5% C), cast iron (2.5–4.3% C), steel (0.25–2% C) and wrought iron (0.12–0.25% C, purest). The melting point order is wrought iron > steel > cast iron.

Steel making: impurities of pig iron (C, Si, Mn, P, S) are oxidised in the Bessemer converter or by the basic oxygen process, in which oxygen is blown through a lance with powdered limestone, giving a slag of calcium phosphate and silicates.

Heat treatment: quenching (sudden cooling) makes steel hard and brittle; annealing (slow cooling) makes it soft; tempering (reheating quenched steel to 500–575 K) removes brittleness. Nitriding (steel in NH₃ at about 1000 K) and case hardening (heating with charcoal) harden the surface.

Extraction of Silver, Gold, Aluminium and Magnesium

Silver and gold (cyanide process): the finely ground concentrated ore is leached with dilute NaCN in the presence of air: Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S; 4Au + 8NaCN + 2H₂O + O₂ → 4Na[Au(CN)₂] + 4NaOH. Air also oxidises Na₂S to Na₂SO₄ so the reaction goes to completion. Scrap zinc then precipitates the metal: 2Na[Ag(CN)₂] + Zn → Na2[Zn(CN)₄] + 2Ag.

Aluminium (Hall-Heroult process): pure Al₂O₃ from bauxite is electrolysed in molten cryolite with fluorspar, which lowers the melting point and viscosity and raises conductivity. Carbon-lined tank is the cathode and graphite rods are anodes; the anodes burn away because the liberated oxygen forms CO and CO₂. Hoop's process gives 99.98% pure aluminium using molten metal layers.

Magnesium: fused MgCl₂ with NaCl and NaF is electrolysed at about 970 K in an iron pot (cathode) with a graphite anode, under coal gas to protect the metal from air.

Refining Methods

1. Liquation: low melting metals (Sn, Pb, Bi) flow away from infusible impurities.
2. Distillation: volatile metals (Zn, Cd, Hg).
3. Parke's process (distribution): removes silver from lead using zinc.
4. Zone refining: Ge, Si, Ga of ultra high purity, since impurities are more soluble in the melt.
5. Oxidation methods: cupellation, puddling and Bessemerisation for impurities that oxidise more easily than the metal.
6. Electrorefining: Cu, Ag, Zn, Sn, Pb, Al, Ni and Cr. Impure metal is the anode and pure metal the cathode.
7. Vapour phase refining: Mond process (Ni through Ni(CO)₄) and Van Arkel process (Ti and Zr through iodides).