JEE Chemistry General Principles and Processes Of Isolation Of Elements Notes Set 02

Basic Terms

1. A mineral is any naturally occurring compound of a metal. An ore is a mineral from which the metal can be extracted both easily and profitably. Hence every ore is a mineral but not every mineral is an ore.
2. The earthy or rocky impurity mixed with an ore is gangue (matrix). Metallurgy is the whole scientific and technological process of getting a pure metal from its ore.
3. Reactive metals (Li to Mg) occur as halides, sulphates and carbonates, metals from Al to Sn mostly as oxides, metals near or below hydrogen (Pb, Hg, Cu, Ag) as sulphides, and the least reactive (Au, Pt, sometimes Ag and Cu) in the native state.
4. Order of abundance in the earth's crust: O, Si, Al, Fe, Ca, Na, K, Mg.
5. Pyrometallurgy uses high temperature (Fe, Zn, Cu, Sn), hydrometallurgy uses aqueous solutions (Ag, Au, Cu from lean ores) and electrometallurgy uses electricity (Na, K, Ca, Mg, Al).

Common Ores and Their Formulae

Four Stages of Extraction

1. Stage 1 is concentration (dressing or benefaction) of the crushed ore to remove gangue.
2. Stage 2 is converting the concentrated ore into the oxide form by calcination or roasting.
3. Stage 3 is reduction of the oxide to the metal.
4. Stage 4 is refining of the crude metal.

Concentration of the Ore

1. Gravity separation (levigation): the heavy ore particles settle while lighter gangue is washed away by a stream of water. It suits heavy oxide ores such as haematite and cassiterite.
2. Froth flotation is used for sulphide ores. The finely powdered ore is stirred with water, pine oil (frother) and a collector such as sodium ethyl xanthate while air is blown through. The oil-wetted ore particles rise in the froth and the water-wetted gangue sinks. Stabilisers such as cresol steady the froth.
3. Depressants allow two sulphides to be separated. NaCN depresses ZnS but not PbS, so PbS floats and ZnS stays down.
4. Magnetic separation is used when either the ore or the gangue is magnetic. Examples: magnetite from non-magnetic gangue, and magnetic wolframite removed from non-magnetic cassiterite.
5. Leaching uses a chemical that dissolves the ore and leaves the impurity. It is used for Al, Ag and Au ores.

Leaching of Bauxite, Silver and Gold

1. Red bauxite (Fe₂O₃ impurity): Bayer's process dissolves Al₂O₃ in hot concentrated NaOH to give sodium aluminate, which is diluted and seeded to precipitate Al(OH)₃. Hall's process fuses the ore with Na₂CO₃ and then passes CO₂ through the solution of aluminate.
2. White bauxite (SiO₂ impurity): Serpeck's process heats the ore with coke in a stream of nitrogen to form AlN, which water converts to Al(OH)₃ with release of NH₃.
3. Al(OH)₃ is ignited at about 1470 K to give pure Al₂O₃.
4. Cyanide process for silver: Ag₂S + 4NaCN → 2Na[Ag(CN)₂] + Na₂S. Air oxidises the Na₂S so that the reaction goes to completion. Zinc then displaces silver: 2Na[Ag(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Ag.
5. For gold: 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH, followed by displacement with zinc dust.

Calcination and Roasting

1. Calcination is heating the ore below its melting point, usually in limited or no air, to expel water or CO₂. It converts hydrated and carbonate ores to oxides and makes the ore porous. Examples: Al₂O₃·2H₂O → Al₂O₃ + 2H₂O, ZnCO₃ → ZnO + CO₂, CaCO₃ → CaO + CO₂.
2. Roasting is heating a sulphide ore in excess air below its melting point in a reverberatory furnace. The sulphur leaves as SO₂ and volatile oxides of As, Sb and P also escape. Example: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
3. Roasting of iron ore converts FeO to Fe₂O₃, so that iron is not lost as FeSiO₃ slag later.
4. For copper pyrites, roasting is only partial: 2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂, and FeS is oxidised to FeO more readily than Cu₂S is oxidised.

Reduction to the Metal

1. Smelting is reduction of an oxide with carbon (coke) at high temperature, with a flux to remove gangue as slag. It is used for Zn, Pb, Sn, Fe and for the matte stage of copper.
2. Flux: an acidic flux (SiO₂) removes basic gangue like FeO or CaO, and a basic flux (CaO, limestone) removes acidic gangue like SiO₂. Slag is the fusible product, for example FeSiO₃ or CaSiO₃.
3. Thermite (aluminothermic) process: aluminium reduces oxides such as Cr₂O₃ and Mn₃O₄ that carbon cannot reduce cleanly. The Fe₂O₃ + Al reaction is so exothermic that it is used to weld railway tracks.
4. Self (auto) reduction: PbS, Cu₂S and HgS are partly roasted and the oxide formed reacts with the remaining sulphide, for example 2PbO + PbS → 3Pb + SO₂.
5. Displacement: a more electropositive metal removes a less electropositive one from solution, e.g. CuSO₄ + Fe → FeSO₄ + Cu, and 2Na[Ag(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Ag. Kroll's process: TiCl₄ + 2Mg → Ti + 2MgCl₂.
6. Electrolytic reduction is used for highly electropositive metals (Group 1, Group 2, Al). Na is made by electrolysing fused NaCl with CaCl₂ added to lower the melting point.

Extraction of Aluminium (Hall-Heroult Process)

1. Pure alumina melts at about 2323 K and conducts poorly. It is dissolved in molten cryolite Na₃AlF₆ with some fluorspar CaF₂, which lowers the melting point to roughly 1200 K and makes the melt conducting.
2. The cell is a steel vessel lined with carbon, which is the cathode, with graphite rods as anodes.
3. Cathode: Al³⁺ + 3e^- → Al (molten, tapped from the bottom). Anode: C + O^2- → CO + 2e^- and C + 2O^2- → CO₂ + 4e^-.
4. The graphite anodes are slowly burnt away and must be replaced periodically.
5. Aluminium is refined further by Hoope's electrolytic process, using three molten layers: impure Al as anode, a fluoride electrolyte in the middle and pure Al as cathode.

Extraction of Copper from Copper Pyrites

1. Concentrate the ore by froth flotation and roast it moderately in air: the product contains Cu₂S, FeS, Cu₂O and FeO.
2. Smelt it in a blast furnace with coke and silica. FeO + SiO₂ → FeSiO₃ (slag) and FeS + Cu₂O → FeO + Cu₂S. The molten Cu₂S + FeS layer is the copper matte. Coke here is mostly a fuel.
3. In the Bessemer converter, silica is added and air is blown. FeS is oxidised and removed as slag, and then 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ followed by Cu₂S + 2Cu₂O → 6Cu + SO₂. Escaping SO₂ gives blister copper (about 98 percent Cu).
4. Refining: poling with green wood removes dissolved Cu₂O, and electrolytic refining gives 99.9 percent copper with blister copper as anode, pure copper as cathode and CuSO₄ with dilute H₂SO₄ as electrolyte. Ag, Au and Pt collect as anode mud.

Extraction of Iron in the Blast Furnace

1. Ore (haematite, magnetite), coke and limestone are charged from the top while hot air is blown from near the bottom.
2. Upper zone (500 to 800 K): 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂ and Fe₃O₄ + CO → 3FeO + CO₂.
3. Middle zone: FeO + CO → Fe + CO₂, CaCO₃ → CaO + CO₂ and CaO + SiO₂ → CaSiO₃ (slag), with C + CO₂ → 2CO supplying more CO.
4. Lower, hottest zone (about 2000 K): C + O₂ → CO₂ (main heat source) and FeO + C → Fe + CO. Molten iron and slag collect at the bottom, with the lighter slag floating on the iron.
5. Pig or cast iron has about 2.5 to 5 percent carbon, steel has 0.25 to 2 percent and wrought iron has about 0.1 to 0.25 percent. Wrought iron is the purest.

Extraction of Zinc

1. Zinc blende is concentrated by froth flotation and roasted: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
2. ZnO is mixed with powdered coke and heated near 1673 K: ZnO + C → Zn + CO. Zinc is volatile at this temperature, distils over and is condensed.
3. Crude zinc is refined by distillation or electrolysis.

Refining of Metals

1. Liquation: low melting metals (Sn, Pb, Bi) flow away from infusible impurities on a sloping hearth.
2. Distillation: volatile metals such as Zn, Cd and Hg.
3. Zone refining: impurities concentrate in the molten zone as it moves along the rod. Used for ultrapure Ge, Si, Ga and B.
4. Cupellation: used to refine silver containing lead. Lead is oxidised to PbO, which a bone-ash cupel absorbs.
5. Poling: used for Cu and for Sn. Green wood or hydrocarbon gas reduces the oxide impurity.
6. Electrolytic refining: Cu, Ag, Zn, Sn, Pb, Al, Ni, Cr. The impure metal is the anode and pure metal the cathode in a salt solution, and the noble impurities collect as anode mud.
7. Vapour phase refining: Mond process for Ni (Ni + 4CO → Ni(CO)₄ at about 330 to 350 K, then decomposition near 450 to 470 K) and Van Arkel process for Ti and Zr (metal + I₂ → volatile iodide, then decomposition on a hot filament).

Thermodynamics of Reduction and the Ellingham Diagram

1. A reaction is feasible when ΔG is negative. Since ΔG = ΔH − TΔS, plotting ΔG° against T for the formation of oxides gives straight lines of slope −ΔS.
2. For most metals the oxidation M + O₂ → MO consumes gas, so ΔS is negative and the line slopes upward. The line bends upward at the melting or boiling point of the metal, when its entropy changes.
3. The lower a metal's line, the more stable its oxide. A metal can reduce the oxide of any metal whose line lies above its own. At the temperature where two lines cross, the two reactions have the same ΔG, and above it the metal with the lower line is the better reductant.
4. For carbon, the line C to CO₂ is almost flat, C to CO slopes downward (ΔS positive) and CO to CO₂ slopes upward. At about 983 K (near 710 °C) the CO and C lines cross. Below it CO is the better reducing agent, above it carbon is.
5. Al₂O₃ and MgO lie far below the carbon lines, so carbon cannot reduce them in a normal furnace, and their metals are obtained by electrolysis.
6. Cu₂O lies high on the diagram, so copper oxides are reduced easily. Hg and Ag oxides decompose on simple heating.

Alloys