JEE Chemistry General Principles and Processes Of Isolation Of Elements Notes Set 04

Occurrence of Metals

1. A mineral is any natural material in which a metal occurs. An ore is a mineral from which the metal can be extracted conveniently and economically, so every ore is a mineral but not every mineral is an ore.
2. Unreactive metals such as Au, Pt and sometimes Ag, Cu and Hg occur native (free). Reactive metals occur combined as oxides, sulphides, halides, carbonates, sulphates, silicates and nitrates.
3. Oxide ores: haematite Fe₂O₃, magnetite Fe₃O₄, limonite Fe₂O₃.3H₂O, bauxite Al₂O₃.2H₂O, corundum Al₂O₃, cuprite Cu₂O, zincite ZnO, cassiterite SnO₂, pyrolusite MnO₂.
4. Sulphide ores: galena PbS, zinc blende ZnS, cinnabar HgS, argentite Ag₂S, copper pyrites CuFeS₂, copper glance Cu₂S, iron pyrites FeS₂.
5. Carbonate ores: calamine ZnCO₃, siderite FeCO₃, magnesite MgCO₃, dolomite CaCO₃.MgCO₃, malachite CuCO₃.Cu(OH)₂, cerussite PbCO₃. Halide ores: horn silver AgCl, rock salt NaCl, carnallite KCl.MgCl₂.6H₂O, fluorspar CaF₂, cryolite Na₃AlF₆. Sulphates: gypsum, barytes, Epsom salt, anglesite.
6. Aluminium is the most abundant metal in the earth's crust (about 8.3%), followed by iron (about 5.1%).

Concentration of Ores

1. Ore as mined contains earthy impurities called gangue. Removing it is called concentration, dressing or benefaction. The ore is first crushed and ground to a fine powder.
2. Gravity separation (levigation or hydraulic washing) uses the density difference between heavy ore particles and light gangue. It suits heavy oxide ores such as haematite and cassiterite.
3. Electromagnetic separation is used when either the ore or the gangue is magnetic. Example: magnetic wolframite is separated from non-magnetic cassiterite.
4. Froth floatation is used for sulphide ores (galena, zinc blende, copper pyrites). Finely powdered ore is mixed with water, pine oil (frothing agent) and a collector such as xanthate. Air is blown in, and the sulphide particles are wetted by oil and rise in the froth while the gangue sinks.
5. Depressants and activators allow separation of two sulphides. NaCN depresses the floatation of ZnS, so PbS floats alone; adding CuSO₄ (activator) then allows ZnS to float.
6. Leaching is a chemical method: the ore dissolves in a suitable reagent and the impurities remain. Examples: bauxite dissolved in hot concentrated NaOH (Baeyer's process) to give sodium aluminate, and gold or silver dissolved in dilute NaCN solution in air to give soluble cyanide complexes (cyanide process): 4M + 8CN^- + 2H₂O + O₂ gives 4[M(CN)₂]^- + 4OH^-, followed by displacement with zinc.

Roasting, Calcination and Smelting

1. Calcination: heating the ore strongly below its melting point in limited or no air. It removes moisture and volatile matter and converts carbonates and hydrated oxides to oxides: ZnCO₃ gives ZnO + CO₂, Al₂O₃.2H₂O gives Al₂O₃ + 2H₂O.
2. Roasting: heating the ore in excess air below its melting point. Sulphide ores are oxidised to oxides and SO₂, and arsenic and antimony are removed as volatile oxides: 2ZnS + 3O₂ gives 2ZnO + 2SO₂.
3. Smelting: reduction of the roasted or calcined ore to the molten metal by heating with a reducing agent (coke) and a flux in a furnace, such as the blast furnace.
4. Flux: a substance that combines with the infusible gangue to form fusible slag. Acidic gangue (SiO₂) needs a basic flux (CaO, limestone); basic gangue (CaO, FeO) needs an acidic flux (SiO₂). Examples: CaO + SiO₂ gives CaSiO₃ and FeO + SiO₂ gives FeSiO₃.
5. Slag is lighter than the metal, forms a separate layer on top and protects the metal from oxidation. In iron extraction the slag is CaSiO₃, and in copper extraction it is FeSiO₃.
6. Refractory materials (fire clay, magnesia, silica, graphite) line furnaces because they withstand very high temperatures without melting or reacting.

Thermodynamics of Reduction

1. The Ellingham diagram plots the standard free energy of formation of oxides against temperature. A metal reduces the oxide of another metal if its line lies below the other's line at that temperature, because the net free energy change is then negative.
2. The carbon line slopes downwards (2C + O₂ gives 2CO, entropy increases) and crosses the metal oxide lines at high temperature. This is why carbon reduces ZnO, FeO and SnO₂ above a certain temperature but cannot reduce Al₂O₃, MgO or CaO.
3.
4. Very reactive metals (Na, K, Ca, Mg, Al) are obtained by electrolysis of their fused salts. Cr and Mn oxides, which are hard to reduce by carbon, are reduced by aluminium (thermite or Goldschmidt process): Cr₂O₃ + 2Al gives Al₂O₃ + 2Cr.
5. Self-reduction: in copper extraction, partially roasted Cu₂S reacts with Cu₂O to give copper and SO₂, no external reducing agent needed: 2Cu₂O + Cu₂S gives 6Cu + SO₂.

Extraction of Iron, Copper, Zinc and Aluminium

1. Iron: roasted haematite is mixed with coke and limestone and charged into the blast furnace. Hot air burns coke to CO₂ and CO; in the upper zones Fe₂O₃ is reduced stepwise by CO to Fe. Limestone gives CaO, which forms slag with silica. The product is pig iron, which is purified to wrought iron or steel.
2. Copper: copper pyrites is concentrated by froth floatation and roasted, then smelted with silica to remove iron as FeSiO₃ slag, leaving matte (Cu₂S + FeS). In the Bessemer converter air is blown through the molten matte, FeS is oxidised to slag, and Cu₂S is converted to blister copper by self-reduction.
3. Zinc: zinc blende is concentrated by froth floatation, roasted to ZnO, and ZnO is reduced with coke at about 1673 K. The metal is distilled as vapour and collected.
4. Aluminium: bauxite is purified by Baeyer's (or Hall's) process to pure alumina. Alumina is dissolved in molten cryolite with a little fluorspar (which lowers the melting point and raises conductivity) and electrolysed (Hall-Heroult process) between graphite electrodes at about 1233 K. Aluminium collects at the cathode and carbon anodes burn away to CO and CO₂.
5. In the Hall-Heroult cell the cathode reaction is Al³⁺ + 3e^- gives Al, and the anodic reaction forms O^2- discharged as oxygen that burns the graphite anode.

Refining of Metals

1. Liquation: low-melting metals (Sn, Pb, Bi) flow away from infusible impurities on a sloping hearth. Distillation refines volatile metals (Zn, Hg, Cd).
2. Poling: molten impure metal (blister copper, tin) is stirred with green wooden poles. Hydrocarbon gases from the wood reduce metal oxide impurities (Cu₂O).
3. Electrolytic refining: the impure metal is the anode and a thin strip of pure metal is the cathode in a solution of the metal salt. Pure metal deposits at the cathode and insoluble impurities fall as anode mud. Used for Cu, Ag, Au, Zn, Ni, Sn, Pb and Al (Hoope's process).
4. Cupellation removes lead from silver by heating the alloy in a current of air in a cupel, which converts lead to PbO absorbed by the bone ash.
5. Zone refining gives ultra-pure semiconductors (Si, Ge, Ga) because impurities are more soluble in the molten zone than in the solid metal. Van Arkel method converts the metal (Ti, Zr) into a volatile unstable compound (iodide) that decomposes on a hot tungsten filament to give very pure metal.