Bauxite is hydrated alumina; general formula is hydrated alumina: \(\mathrm{Al_2O_3\cdot nH_2O}\).
Roasting of metal sulfides (e.g. \(2ZnS+3O_2\to2ZnO+2SO_2\)) yields sulfur dioxide, \(\mathrm{SO_2}\), a colourless gas whose aqueous solution is acidic (forms sulfurous acid).
Calcination is thermal decomposition of carbonate/hydroxide in absence of air to give the oxide and CO2. Example: \(\mathrm{MgCO_3\to MgO+CO_2}\). Option (a) is combustion/oxidation; (b) is roasting.
Aluminium oxide (\(\mathrm{Al_2O_3}\)) has very large negative free energy of formation and is too stable to be reduced by carbon; hence carbon reduction is not feasible for alumina.
The Hall–Héroult process is the electrolytic extraction of aluminium from molten alumina dissolved in cryolite.
Roasting converts sulphides to oxides which are easier to reduce; carbon and hydrogen are not generally suitable reducing agents for sulphides (they are used for oxides). Hence statement (d) is not true.
A: Cyanide process → extraction of Au (iv). B: Froth flotation → concentration/dressing of sulphide ores like ZnS (ii). C: Electrolytic reduction → extraction of Al (iii). D: Zone refining → production of ultrapure semiconductors (e.g. Ge) (i). So mapping is (iv,ii,iii,i).
Wolframite (iron–manganese tungstate) is magnetic while cassiterite (tinstone, SnO2) is non‑magnetic; electromagnetic separation exploits this magnetic difference.
Feasible reactions require the metal to be above the ion in the activity series. Zn can reduce Cu2+ (a feasible). Cu cannot reduce Zn2+ (b not feasible). Cu can reduce Ag+ (c feasible). Fe reduces Cu2+ (d feasible).
Highly reactive metals like sodium are obtained by electrolysis (Downs cell) of molten salts; sodium is extracted electrolytically.
Flux combines with gangue to form a fusible (soluble in slag) compound (e.g. silicate) so that impurities are removed as slag. It converts infusible impurities into fusible/soluble slag.
Froth flotation is ideal for concentrating sulphide ores such as galena (PbS).
Cryolite (Na3AlF6) dissolves alumina and lowers its melting point and improves conductivity, making electrolysis of alumina practical.
Zinc oxide is reduced by carbon (coke) at high temperature: \(\mathrm{ZnO + C\to Zn + CO}\).
After cyanide leaching, silver is commonly precipitated by displacement using zinc (Merrill–Crowe process): \(\mathrm{2[Ag(CN)_2^-] + Zn \to 2Ag + [Zn(CN)_4^{2-}] }\).
From Ellingham diagram MgO line yields a more negative total free energy (for the stoichiometry required) than Al2O3; magnesium can reduce alumina thermodynamically whereas Fe, Cu and Zn cannot.
This is the Van Arkel (iodide) process (also called crystal bar process) used to purify Zr (and Ti) via volatile iodides.
Concentration methods include leaching (dissolving unwanted soluble components) and froth flotation (separation of sulphide ores). Roasting is a chemical conversion, not a concentration method.
Gold is leached with dilute sodium cyanide (not sodium chloride). Statements (a),(b),(c) are correct.
In electrolytic refining the impure metal is used as the anode; pure copper is deposited at the cathode while impurities fall off as anode sludge.
Ellingham diagram is a plot of standard Gibbs free energy change of oxide formation against temperature: plot of ΔG° (y-axis) vs T (x-axis). The slope of each line equals −ΔS° for the reaction.
For C + 1/2 O2 → CO the entropy change ΔS° is positive (gas formed), so the slope (−ΔS°) on the Ellingham diagram is negative. Hence ΔG° decreases with T; it may be positive at low T and becomes negative above a certain temperature (~700 °C), so statement (d) is correct.
Reaction (b) is the reverse of (a). Al2O3 is more stable (more negative ΔG°) than Cr2O3, so chromium cannot reduce Al2O3 to Al. Thus (b) is not thermodynamically feasible by Ellingham criteria.
The CO2 formation line is nearly horizontal (i.e., almost parallel to the temperature axis), not parallel to the free energy (vertical) axis. Statements (a), (c) and (d) are correct descriptions of Ellingham behaviour.
Key differences: (1) Definition: mineral = naturally occurring substance (e.g. quartz, hematite); ore = mineral containing metal in sufficient concentration (e.g. hematite Fe2O3 as an iron ore). (2) Economic value: only ores have economic value for metal extraction. (3) Composition: minerals may be pure compounds; ores contain metal plus gangue (impurities). (4) Examples: mineral—quartz (SiO2); ore—bauxite (Al2O3·nH2O) for Al.
Mineral: naturally occurring chemical compound or element; may or may not be of economic value. Ore: a mineral (or mixture) from which a metal can be profitably extracted.
Briefly: (1) Concentration (beneficiation) to remove gangue (e.g. froth flotation, magnetic separation). (2) Calcination/roasting to remove volatile impurities and convert ores to oxides/sulfates. (3) Reduction (chemical or electrolytic) of oxide to metal (e.g. carbon reduction, aluminothermy, electrolysis). (4) Refining to obtain high purity metal (electrolytic refining, Mond process, zone refining etc.).
Concentration → Calcination/Roasting → Reduction/Smelting → Refining.
In the blast furnace CaO reacts with silica (gangue) to form calcium silicate (slag): CaO + SiO2 → CaSiO3. Slag is molten and less dense than iron and is removed, thus separating impurities from the metal.
Quicklime (CaO) acts as a flux to form a removable slag with acidic impurities (silica).
Froth flotation separates hydrophobic sulfide minerals from hydrophilic gangue using collectors (e.g. xanthates) and frothers. Examples: lead ore (galena, PbS) and zinc ore (sphalerite, ZnS).
Sulfide (and other hydrophobic) ores, e.g. galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2).
Impure nickel is treated with CO at 50–60 °C to form volatile nickel tetracarbonyl: Ni + 4CO → Ni(CO)4. The gas is separated and decomposed at 200–250 °C on a heated surface to give pure nickel and CO: Ni(CO)4 → Ni + 4CO. This yields high-purity nickel; CO is recycled.
Mond process (carbonyl refining) is commonly used for nickel.
Principle: impurity distribution coefficient k (= impurity in solid/impurity in melt) is ≪1 for many impurities. A narrow molten zone is passed along a crystalline rod; impurities preferentially enter the melt and are transported to one end. Repeating the process yields extremely pure metal/semiconductor (e.g. Si, Ge).
Zone refining removes impurities by moving a molten zone along a bar; impurities concentrate in the melt and are carried to one end. Example: production of ultrapure silicon for semiconductors.
Ellingham criterion: a metal A will reduce oxide BX if ΔG°(formation of A oxide) is more negative than ΔG°(formation of B oxide) (i.e. A2O3 line lies below BxOy). (A)(i,ii) On standard Ellingham diagrams MgO and Al2O3 lines lie very low; neither Al nor Mg can thermodynamically reduce the other oxide under ordinary temperatures (their oxide lines do not lie above the other). (B) Carbon (as CO) line crosses the Fe-oxide line at temperatures used in blast furnaces, so reduction of Fe2O3 by coke/CO is thermodynamically feasible around ~1200 K.
(A)(i) Aluminium cannot reduce MgO under normal conditions; (ii) Magnesium cannot reduce Al2O3. (B) Yes — Fe2O3 can be reduced by coke (via CO/CO2) near 1200 K.
Uses of zinc include:
- Galvanising iron/steel to prevent corrosion.
- Alloys: brass (Cu–Zn), some bronzes and die-casting alloys.
- Batteries: zinc–carbon and alkaline cells (zinc acts as the anode).
- Sacrificial anodes for corrosion protection of ships and pipelines.
- Zinc oxide (ZnO) in the rubber industry, paints, cosmetics and medicines (e.g., skin ointments).
- Electroplating and as a white pigment (zinc white).
Purified alumina (from Bayer process) is dissolved in molten cryolite (Na3AlF6) to lower melting point and increase conductivity. Electrolysis at ~940–980 °C: at cathode 4e− + Al3+ → Al (liquid), at carbon anode C + O2− → CO/CO2 + electrons. Overall (approx.): 2Al2O3 + 3C → 4Al + 3CO2. Cryolite reduces melting point, increases solubility and conductivity; carbon anodes are consumed and must be replaced.
Hall–Héroult electrolytic process: Al2O3 dissolved in molten cryolite is electrolysed to produce Al.
Gangue: non-metallic impurities associated with an ore (e.g. SiO2, clay). Slag: product formed by reaction of flux (e.g. CaO) with gangue (e.g. SiO2) during smelting; it is molten, less dense than metal and removed. Example in iron extraction: CaO (flux) + SiO2 (gangue) → CaSiO3 (slag).
(i) Gangue: unwanted earthy impurities in an ore (e.g. silica). (ii) Slag: molten flux–gangue compound removed during smelting (e.g. CaSiO3).
Requirements: (1) Metal forms a volatile, reversible compound (e.g. metal halide, carbonyl) at moderate temperature. (2) Impurities either do not form volatile compounds or form ones with different volatility. (3) Volatile compound can be transported and decomposed under different conditions to yield pure metal. Example: Mond process (Ni + CO ⇌ Ni(CO)4).
Metal must form a volatile compound selectively which can be decomposed to give pure metal; impurity must not form the volatile species or decomposes differently.
(i) In copper smelting silica (SiO2) reacts with iron oxide to form fayalite slag (2FeO·SiO2), removing iron impurities. (ii) Cryolite (Na3AlF6) dissolves Al2O3, lowers melting point and increases electrical conductivity in Hall–Héroult cell. (iii) Iodine reacts with impure Zr to form volatile ZrI4 which decomposes on a hot filament to give pure Zr (van Arkel–de Boer). (iv) NaCN is used in froth flotation to depress pyrite and other unwanted sulfides by forming complexes with Fe, improving selectivity for desired sulfide minerals.
(i) Flux to form slag with FeO. (ii) Solvent and flux to lower melting point & increase conductivity. (iii) Forms volatile ZrI4 for zone purification (van Arkel–de Boer). (iv) Depressant/activator to control flotation of sulfide minerals.
In copper refining: anode = impure Cu, cathode = pure Cu starter sheet, electrolyte = CuSO4 + H2SO4. At anode: Cu → Cu2+ + 2e− (impurities fall as sludge). At cathode: Cu2+ + 2e− → Cu (pure). Result: copper dissolves from anode and plates pure at cathode; precious impurities (Au, Ag) collect as anode slime.
Impure metal anode dissolves; pure metal plates at cathode; impurities either remain in solution or collect as anode sludge. Example: electrolytic refining of copper.
Using Ellingham diagram: metal A reduces oxide BxOy if ΔG°(formation of A oxide) < ΔG°(formation of B oxide). Example: carbon (as CO) reduces Fe2O3 to Fe at high T because ΔG° for formation of CO/CO2 makes the overall reaction thermodynamically favorable; carbon cannot reduce Al2O3 because Al2O3 is thermodynamically more stable than CO/CO2 at accessible temperatures.
A reducing agent is chosen such that its oxide is more stable (more negative ΔG°) than the oxide of the metal to be reduced; thus the reduction is thermodynamically downhill.
Limitations: (1) Only standard free energies (ΔG°) — real activities and non‑ideal solutions can change outcomes. (2) Kinetics not considered; a thermodynamically feasible reaction may be slow. (3) Assumes standard pressure (1 bar) and pure phases; partial pressures (e.g. of O2, CO) alter lines. (4) Does not include formation of intermediate species or complex oxides, chlorides, etc. (5) Phase changes cause line breaks and some curves may shift with composition/pressure.
Ellingham diagrams give only thermodynamic feasibility (ΔG°) not kinetics; assume pure substances and standard states; ignore activities/non‑ideal behaviour, partial pressures, and complex formation.
Principles: metal reduction/oxidation are viewed as electrode processes with characteristic potentials. The feasibility of electrolysis is given by the cell EMF (related to ΔG° by ΔG° = −nFE°). Electrolytic refining/electrowinning apply an external voltage greater than decomposition potential to reduce metal ions at the cathode. Comparison of standard potentials helps choose conditions and predict whether a metal can be extracted electrolytically. Examples: copper electrorefining, Hall–Héroult aluminium production, and electrolysis for metals less reactive than hydrogen under appropriate conditions.
Electrochemical metallurgy uses electrode potentials and cell EMFs to predict and carry out extraction/refining of metals by electrolysis or electrowinning.