JEE Chemistry General Principles And Processes Of Isolation Of Elements Notes Set 05

Occurrence, Ores and Minerals

A mineral is any naturally occurring compound of a metal; an ore is a mineral from which the metal can be extracted profitably. The earthy impurities in the ore are the gangue (matrix). Reactive metals occur combined, noble metals (Au, Pt) partly native. Main groups: oxides - bauxite \( \ce{Al2O3.2H2O} \), haematite \( \ce{Fe2O3} \), magnetite \( \ce{Fe3O4} \), cuprite \( \ce{Cu2O} \), cassiterite \( \ce{SnO2} \), zincite ZnO; carbonates - calamine \( \ce{ZnCO3} \), siderite \( \ce{FeCO3} \), malachite \( \ce{CuCO3.Cu(OH)2} \), cerussite \( \ce{PbCO3} \), magnesite \( \ce{MgCO3} \), dolomite \( \ce{CaCO3.MgCO3} \); sulphides - galena PbS, zinc blende ZnS, cinnabar HgS, copper glance \( \ce{Cu2S} \), copper pyrites \( \ce{CuFeS2} \), argentite \( \ce{Ag2S} \), iron pyrites \( \ce{FeS2} \); halides - rock salt, sylvine KCl, carnallite \( \ce{KCl.MgCl2.6H2O} \), horn silver AgCl, fluorspar \( \ce{CaF2} \), cryolite \( \ce{Na3AlF6} \); silicates - feldspar, kaolinite, beryl.

Concentration of Ores

Concentration (dressing, benefaction) removes the gangue. Hydraulic washing or levigation: a current of water carries away the lighter gangue and leaves the heavier oxide ore (oxides of Fe, Sn). Magnetic separation: a magnetic ore or gangue is pulled out by a magnet (haematite, chromite, wolframite from cassiterite). Froth flotation: finely ground sulphide ore is stirred with water, pine oil (frothing agent) and a collector such as xanthate; the oil-wetted ore particles rise with air bubbles, while gangue sinks. A depressant prevents one sulphide from floating: sodium cyanide keeps ZnS down so that PbS can be separated from it, because ZnS forms the soluble complex \( \ce{Na2[Zn(CN)4]} \). Leaching is a chemical method: bauxite is digested with hot concentrated NaOH (Baeyer's process) to give soluble sodium aluminate while iron oxide and silica remain behind; Ag and Au ores are leached with dilute NaCN in the presence of air to give \( \ce{[Ag(CN)2]^-} \) and \( \ce{[Au(CN)2]^-} \).

Roasting, Calcination and Smelting

Calcination is heating in limited air or none to drive off water and carbon dioxide from hydrated or carbonate ores, for example \( \ce{CaCO3 -> CaO + CO2} \) and \( \ce{Fe2O3.xH2O -> Fe2O3 + xH2O} \). Roasting is heating in excess air below the melting point to convert a sulphide to oxide (or sulphate), burn off S, As and Sb as volatile oxides and make the mass porous: \( \ce{2ZnS + 3O2 -> 2ZnO + 2SO2} \). It is exothermic. Smelting is reduction of the roasted or calcined ore to metal with carbon in a furnace, along with a flux that combines with the gangue to make a fusible slag. An acidic flux (silica) removes basic gangue (CaO, FeO), while a basic flux (limestone, CaO) removes acidic gangue (silica): \( \ce{CaO + SiO2 -> CaSiO3} \). Slag is lighter than molten metal, melts easily and floats on it, protecting it from oxidation.

Thermodynamic Principle and the Ellingham Diagram

Reduction of an oxide by a reducing agent is possible only if the overall free energy change is negative: \( \Delta G=\Delta H-T\Delta S \). The Ellingham diagram plots \( \Delta G^\circ \) for formation of oxides (per mole of \( \ce{O2} \)) against temperature. Because one mole of gas is consumed, \( \Delta S \) is negative and nearly the same for all metals, so the lines slope upward; the slope changes at the melting or boiling point of the metal. For \( \ce{C + O2 -> CO2} \) the line is flat, for \( \ce{2C + O2 -> 2CO} \) it slopes downward, and for \( \ce{2CO + O2 -> 2CO2} \) it slopes upward. A metal (or carbon) whose line lies below that of another metal oxide can reduce that oxide at that temperature. The intersection of the C-CO line with the ZnO line (about 1200 K) gives the temperature above which carbon reduces ZnO; Al2O3 lies so low that carbon cannot reduce it at practical temperatures, which is why Al is made by electrolysis. Above the \( \Delta G=0 \) line an oxide is unstable and decomposes, which is how Ag2O and HgO decompose on mild heating.

Extraction of Some Metals

Iron (blast furnace): charge of calcined haematite, coke and limestone. Zones: top (about 500-800 K) \( \ce{3Fe2O3 + CO -> 2Fe3O4 + CO2} \) and \( \ce{Fe3O4 + CO -> 3FeO + CO2} \); middle \( \ce{FeO + CO -> Fe + CO2} \) and \( \ce{CaCO3 -> CaO + CO2} \); lower (above 1500 K) \( \ce{C + CO2 -> 2CO} \) and slag formation \( \ce{CaO + SiO2 -> CaSiO3} \). The product, pig iron (3-4 percent carbon), is tapped from below the slag layer.

Copper (from copper pyrites): froth flotation, roasting (FeS goes to FeO, which is removed as slag \( \ce{FeSiO3} \) with silica), smelting to matte (mostly Cu2S with some FeS), then Bessemerisation: \( \ce{2Cu2S + 3O2 -> 2Cu2O + 2SO2} \) followed by self-reduction \( \ce{2Cu2O + Cu2S -> 6Cu + SO2} \) gives blister copper (about 98 percent Cu). Lead (galena): partial roasting then self-reduction in the same furnace, \( \ce{2PbO + PbS -> 3Pb + SO2} \); limestone is added as flux. Zinc: ZnS is roasted to ZnO and reduced with coke at about 1673 K, and zinc distils out. Excess carbon is used to keep \( \ce{CO2} \) converted to CO. Tin: cassiterite is reduced with carbon; too much lime is avoided because it would form calcium stannate and lose tin. Mercury: cinnabar is roasted, HgS self-reduces to mercury vapour.

Aluminium (Hall-Heroult): pure alumina from bauxite (Baeyer's or Serpeck's process) is dissolved in molten cryolite, with some fluorspar, in a steel tank lined with graphite (cathode) with graphite anodes, at about 1230 K. Cryolite lowers the melting point from over 2300 K and raises conductivity. Cathode: \( \ce{Al^{3+} + 3e^- -> Al} \); anode: carbon is oxidised to CO and CO2, so the anodes are replaced periodically. The net reaction is \( \ce{2Al2O3 + 3C -> 4Al + 3CO2} \). Magnesium (Dow process): carnallite is dehydrated and the molten chlorides, with NaCl and CaCl2 added to lower the melting point and improve conductivity, are electrolysed with an iron cathode and graphite anode. Gold and silver (MacArthur-Forrest): \( \ce{4Au + 8NaCN + 2H2O + O2 -> 4Na[Au(CN)2] + 4NaOH} \) followed by displacement with zinc, \( \ce{2[Au(CN)2]^- + Zn -> [Zn(CN)4]^{2-} + 2Au} \).

Refining of Metals

Liquation: a low-melting metal (Sn, Pb, Bi) flows away from a sloping hearth leaving infusible impurities. Poling: molten impure Cu or Sn is stirred with green wood poles; the reducing gases formed reduce the oxide impurity. Electrolytic refining: impure metal anode, pure metal cathode and a solution of its salt; less active impurities (Ag, Au, Pt) collect as anode mud and more active ones stay in solution (Cu, Pb, Ag, Al by Hoop's process, Zn). Zone refining: impurities concentrate in the molten zone of a moving heater and are swept to one end (Ge, Si, Ga, B). Vapour phase refining: Mond process for nickel, \( \ce{Ni + 4CO ->[330 K] Ni(CO)4 ->[450-470 K] Ni + 4CO} \); Van Arkel method for Ti and Zr, in which the volatile iodide formed at a lower temperature decomposes on a hot tungsten filament. Parke's process uses zinc to extract silver from molten lead, followed by distillation of zinc; cupellation removes traces of lead from silver by oxidising it to PbO in a bone-ash cupel. Chromatography is used when only small quantities are needed.